fluoride mineralization of portland cement

Interdisciplinary Nanoscience Center, 2011

FACULTY OF SCIENCE

AARHUS UNIVERSITY

Fluoride Mineralization of Portland cem

entPhD

Thesis 2011Thuan T. Tran

Front cover illustration. Stacked plot of a 2D 29Si19F HETCOR NMR spectrum for

a synthetic sample of cuspidine (Ca4Si2O7F2).

PhD Thesis

THUAN T. TRAN

Fluoride Mineralization of Portland cement

Applications of Double-Resonance NMR Spectroscopyin Structural Investigations of Guest Ions in Cement Phases

Thuan Thai Tran

PhD Thesis

Instrument Centre for Solid-State NMR Spectroscopy

Interdisciplinary Nanoscience Center (iNANO)

Department of Chemistry, Faculty of Science and Technology

Aarhus University, Denmark

August 2011

Preface

This thesis presents the results obtained during my PhD study from August 2007 to July

2011 at the Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and

the Interdisciplinary Nanoscience Center (iNANO) at Aarhus University. The PhD study has

been a part of the FUTURECEM project (2007 2010), which is a joint collaboration between

Aalborg Portland A/S, iNANO (Aarhus and Aalborg Universities) and the Geological Survey of

Denmark and Greenland (GEUS), Copenhagen, with partly financial support from the Danish

National Advanced Technology Foundation (“Højteknologifonden”).

Acknowledgments

It is a pleasure for me to express my gratitude to all the people

who have contributed with valuable assistances to my PhD project.

First and foremost, I would like to thank my supervisor Associate Professor Jørgen

Skibsted for his excellent support and scientific guidance, ever since I joined the Instrument

Centre for Solid-State NMR Spectroscopy as a young, undergraduate student for almost eight

years ago. He introduced me to solid-state NMR as well as cement research and gave me the

opportunity to extend my studies at the Instrument Centre. His ongoing fruitful discussions on

both subjects have been a great source of inspiration to this PhD project.

Professor Hans Jørgen Jakobsen and Associate Professor Henrik Bildsøe are

acknowledged for their general interest in the project and several fruitful discussions about

experimental as well as theoretical solid-state NMR problems. They are also thanked for being

available when there have been problems with the spectrometers or the computers. Mrs. Rigmor

Søeberg Johansen and Mrs. Anne Birgitte Bundgaard Johannsen are thanked for their valuable

assistance in conducting numerous of NMR experiments during my time at the Instrument

Center. I also want to acknowledge all the members at the NMR laboratory, including PhD Søren

Lundsted Poulsen and Phd students Søren Sørensen, Tine Fly Sevelsted, Maiken Rabøl

Jørgensen and Nicklas Bromose Kolman, for creating a good scientific and social atmosphere.

I would like to thank the researchers at the R&D, Quality and Technical Sales Support at

Aalborg Portland A/S: Project Managers Lise Frank Kirkegaard and Mette Steenberg Glyø and

Chief Scientist Duncan Herfort for a good collaboration and several interesting discussions on

the cement chemistry.

Finally, the Danish National Advanced Technology Foundation and the Faculty of

Science and Technology, Aarhus University are acknowledged for their financial support of this

PhD project.

Abstract

The increasing applications of alternative raw materials, alternative fuels and

supplementary cementitious materials (SCMs) in today’s cement production lead to a significant

amount of impurities (e.g. fluoride, SO3, P2O5 etc.) in the final cement materials. These so-called

minor components may also be added to the raw meal of starting materials to modify the

clinkering processes. They introduce several important effects on the formation of the cement

clinker phases as well as on their hydrational properties. In general, the structural

characterization of the minor components is rather difficult due to their low quantities in cement.

This PhD thesis focuses on two main subjects: (i) modification of the Portland clinker

composition with the aim of increasing the clinker reactivity, enabling a 30 % replacement of the

Portland clinkers by SCMs without a significant reduction in the strength properties of the

blended cement, and (ii) development and implementation of solid-state double-resonance

Nuclear Magnetic Resonance (NMR) techniques including Cross Polarization (CP), Rotational-

Echo Double-Resonance (REDOR) and Rotational-Echo Adiabatic-Passage Double-Resonance

(REAPDOR) experiments in the structural characterization of cementitious materials.

The mineralizing properties of calcium fluoride have been investigated in detail. The

mechanism for fluoride mineralization is probed using different solid-state 19F, 27Al and 29Si

NMR techniques. Furthermore, the impacts of Al3+ and Fe3+ ions on the fluoride mineralization

and their site preferences in the calcium silicate phases of Portland cement are investigated. It is

found that the reactivity of Portland cement and its strength properties may be improved

substantially by the addition of fluoride, Al2O3 and Fe2O3 in appropriate amounts.

A comprehensive study of the incorporation of fluoride ions in the calcium silicate

hydrate (CSH) phases of hydrated Portland cement has been conducted. This work provides

important structural information about the fluoride guest ions in the CSH, contributing to a

further understanding of the retarding effect of fluoride on the hydration of Portland cement.

Furthermore, a new solid-state NMR pulse sequence (Forth and Back Cross Polarization) has

been developed during this work. This experiment provides a selective detection of 19F

resonances that are dipolar coupled to 29Si which is used to investigate 19F29Si connectivities in

the CSH structure.

Finally, AlOSi connectivities in the open framework structures of alkali-activated

materials and strätlingite (2CaO·Al2O3·SiO2·8H2O) have been investigated using 29Si27Al

REAPDOR experiments. These materials are of interest in recent cement research since they

have the potential of being alternative binders in concrete.

Resumé

Den stigende anvendelse af alternative råmaterialer og brændstoffer i moderne Portland

cement produktion tilfører en signifikant mængde af de såkaldte fremmede komponenter, såsom

SO3, P2O5 og fluorider, i den fremstillede cement. Disse oxider kan fremme dannelsen af

cement-klinker faserne og deres hydrauliske egenskaber, men de kan også have en negativ effekt

ved at hæmme dannelsen af alit, som er den vigtigste hydrauliske komponent i Portland cement.

Dette Ph.d. projekt har to hovedformål, hvor det første sigter mod en 30 % erstatning af

Portland cement-klinker med alternative materialer, som har hydrauliske egenskaber, uden at

reducere cementens styrke. Dette opnås bl.a. ved at optimere klinkernes kemiske

sammensætning, således at der opnås en forøget reaktivitet for alit-fasen. Det andet mål i

projektet har været at udvikle og implementere faststof dobbelt-resonans kerne magnetisk

resonans (NMR) eksperimenter til strukturkarakterisering af cement-materialer. I projektet er der

bl.a. blevet anvendt Cross Polarization (CP), Rotational-Echo DOuble-Resonance (REDOR) og

Rotational-Echo Adiabatic-Passage DOuble-Resonance (REAPDOR) NMR eksperimenter til

strukturkarakterisering af gæst ioner i calciumsilikat-faserne (alit og belit) i Portland cement.

I studiet af optimeringen af Portland cement-klinkers kemiske sammensætning har der

været særlig fokus på mineraliseringsegenskaberne af calciumfluorid. Mineraliseringseffekten af

fluorid-ionerne er blevet undersøgt med forskellige faststof 19F, 27Al og 29Si NMR teknikker.

Disse undersøgelser har også inkluderet indflydelsen af trivalente metal-ioner (såsom Al3+ og

Fe3+) på flourid-mineraliseringsprocessen og deres strukturelle omgivelser i calciumsilikat-

faserne. Hoved-konklusionen er, at tilsætningen af fluorid, Al2O3 og Fe2O3 i passende mængder

kan signifikant forøge Portland cements reaktivitet og trykstyrke. PhD afhandlingen inkluderer

også et omfattende studium af indbygningen af fluorid-ioner i calcium-silikat-hydrat (CSH)

fasen dannet ved hydratisering af Portland cement. Resultaterne herfra bidrager med vigtig

strukturel information om fluorid-ionernes placering i CSH fasen og afslører mekanismen for

den retarderende effekt af fluorid-ioner på hydratiseringen af Portland cement. I disse studiet

blev der udviklet et nyt faststof NMR eksperiment kaldet ”Forth and Back Cross Polarization”.

Dette eksperiment blev anvendt til at studere SiF sammenkædninger i CSH stukturen. Til

sidst er der foretaget en karakterisering af AlOSi netværkstrukturerne i alkali-aktiverede

materialer og i den uordnede struktur for strätlingit (2CaO·Al2O3·SiO2·8H2O) ved hjælp af 29Si27Al REAPDOR eksperimentet. Alkali-aktiverede materialer har i øjeblikket en stor

forskningsmæssig bevågenhed, da de potentielt delvist kan erstatte Portland cement-klinker,

primært med henblik på at reducere CO2 emissionen fra cement produktionen.

Table of content

Futurecem ……………………………………………………………………………………..1

Chapter 1: Cement Chemistry ................................................................................................. 3

1.1. Portland cement manufacture .................................................................................. 4

1.1.1. Quarrying and preparing of raw materials ............................................................... 4

1.1.2. Clinker production in the rotary kiln ....................................................................... 5

1.1.3. Cement grinding ...................................................................................................... 6

1.2. The main constituent phases of Portland clinker ..................................................... 8

1.2.1. Tricalcium silicate ................................................................................................... 8

1.2.2. Dicalcium silicate .................................................................................................... 9

1.2.3. Tricalcium aluminate, 3CaOAl2O3 ....................................................................... 10

1.2.4. Ferrite, Ca2(AlxFe1-x)O5 ......................................................................................... 11

1.3. Minor components ................................................................................................. 12

1.3.1. Flux ........................................................................................................................ 13

1.3.2. Mineralizers ........................................................................................................... 14

1.4. Hydration chemistry for Portland cement ............................................................. 14

1.4.1. Hydration of calcium silicate phases ..................................................................... 14

1.4.2. Hydration of tricalcium aluminate ......................................................................... 15

1.4.3. Hydration of Portland cement ............................................................................... 16

1.5. Structure of the main hydration product CSH ................................................... 18

1.5.1. Tobermorite 14-Å, Ca5Si6O16(OH)27H2O ............................................................ 19

1.5.2. Jennite, Ca9Si6O18(OH)68H2O .............................................................................. 20

1.5.3. Portlandite, Ca(OH)2 ............................................................................................. 22

Chapter 2: Applications of solid-state NMR in cement research ....................................... 23

2.1. NMR theory ........................................................................................................... 24

2.1.1. Chemical shift interaction ...................................................................................... 25

2.1.2. Quadrupolar coupling interaction .......................................................................... 26

2.1.3. Dipolar coupling interactions ................................................................................ 27

2.2. Solid-state NMR techniques in cement research ................................................... 29

2.2.1. 29Si MAS NMR ..................................................................................................... 29

2.2.2. 27Al MAS NMR ..................................................................................................... 32

2.2.3. 19F MAS NMR ...................................................................................................... 32

2.2.4. 43Ca MAS NMR .................................................................................................... 34

2.2.5. Inversion-Recovery (IR) MAS NMR .................................................................... 36

2.2.6. Cross Polarization (CP) ......................................................................................... 37

2.2.7. Forth and Back Cross Polarization (FBCP) ........................................................... 38

2.2.8. Rotational-Echo Double-Resonance (REDOR) .................................................... 41

2.2.9. Rotational-Echo Adiabatic-Passage Double Resonance (REAPDOR) ................. 42

2.2.10. Multiple-Quantum (MQ) MAS NMR ................................................................... 46

Chapter 3: Fluoride Mineralization ...................................................................................... 49

3.1. Site preferences of F in the calcium silicate phases of Portland cement ............. 50

3.1.1. 19F MAS NMR ...................................................................................................... 50

3.1.2. 29Si19F and 27Al19F CP/MAS NMR ............................................................... 51

3.1.3. 29Si19F CP-REDOR NMR.................................................................................. 53

3.2. Mineralizing effects of calcium fluoride and calcium sulphate ............................ 57

3.3. Secondary effects of fluoride mineralization......................................................... 60

3.4. Incorporation of Fe3+ ions in the calcium silicate phases ...................................... 65

3.5. Hydration of fluoride-mineralized Portland cement .............................................. 71

3.6. Application of fluoride mineralization .................................................................. 76

3.6.1. Preparation of test clinkers .................................................................................... 76

3.6.2. Strength performances of fluoride-mineralized Portland cement.......................... 77

3.7. Summary ................................................................................................................ 79

Chapter 4: Fluoride-ion environments in CSH ................................................................ 81

4.1. A brief description of the CSH structure from the T/J viewpoint. .................... 82

4.2. Fluoride-ion environments in synthetic CSH and hydrated Portland cement ... 84

4.2.1. Site preferences of F ions in the CSH structure from 19F MAS NMR ............ 86

4.2.2. Influence of fluoride ions on the CSH structure ............................................... 89

4.3. Influence of fluoride ions on the hydration of Portland cement ............................ 95

4.3.1. Identification of CaF2 in hydrated Portland cement by 19F MAS NMR ............... 95

4.3.2. Hydration of fluoride-mineralized Portland cement from 19F MAS NMR ........... 96

4.3.3. Retarding mechanism of fluoride ions on the hydration of Portland cement ...... 101

4.4. Summary .............................................................................................................. 103

Chapter 5: Framework structures of alumino silicates from NMR ................................. 105

5.1. SiOAl connectivities in alkali-activated materials .......................................... 106

5.1.1. Effects of Si/Al and Na/Al molar ratios .............................................................. 108

5.1.2. 29Si 27Al REAPDOR NMR .............................................................................. 111

5.2. Disorder in the double tetrahedral layer structure of Strätlingite ........................ 115

5.2.1. 29Si MAS NMR ................................................................................................... 116

5.2.2. 29Si27Al REAPDOR NMR ............................................................................... 117

5.3. Summary .............................................................................................................. 119

Conclusions …………….....………………………………………………………………....120

Reference ………………………………………………………………………………….123

Appendix 1 Sample preparations...………………………………………………………….143

Appendix 2 NMR Measurements and other analytical techniques …………………………146

Paper I Site Preferences of Fluoride Guest Ions in the Calcium Silicate Phases............151

Paper II Incorporation of Fluoride Guest Ion in the Calcium Silicate Phases..……...…155

Paper III Characterization of Guest-Ion Incorporation…………………………..….……163

Paper IV Characterization of Guest-Ion Incorporation (in russian)……………..…….…163

Paper V Site Preferences of F, Al3+ and Fe3+ Guest Ions in the Calcium Silicate……..173

Paper VI Characterization of the network structure of alkali-activated aluminosilicate..181

Manuscript I Characterization of the aluminosilicate network………………………………..191

Manuscript II The distordered structure of Strätlingite.........………………………………..225

Manuscript III Fluoride mineralization of Portland cement…………………………………..…251

Introduction 1

FUTURECEM

Sustainable cement for the future

Cement is the essential glue in concrete, which is today’s fundamental building material

with an annual consumption only surpassed by water. The cement manufacturing process is

however associated with a severe CO2 emission. With an annual worldwide cement production

of about 3.3 billion tonnes in 2010[1], the cement industry is responsible for about 5 % of the

man-made CO2 emissions[2-4]. In fact, according to the increasing demand for concrete, as

growth and modernization take place in the developing countries, the cement production is

forecasted to double by the year 2050. Thus, identifying technology to reduce the CO2 emission

intensity from cement production is urgent. The Getting the Number Right (GNR) Database

System[5], which has registered data from 844 cement installations worldwide from 1999 to 2008,

reveals that about 60 % of the direct CO2 emission originates from the chemical reactions during

cement clinker formation, e.g. the calcination process, CaCO3 CaO + CO2. The fuel

combustion to reach temperatures of 1450 C in the cement kilns, necessarily for the formation

of clinker phases, is responsible for the remaining 40 %. Indirect emissions (e.g., from electric

power consumption) only contribute about 10 % to the overall CO2 emissions. Based on current

knowledge, the International Energy Agency (IEA) and the World Business Council for

Sustainable Development (WBCSD) Cement Sustainability Initiative (CSI) have recently

presented the Cement Technology Roadmap 2009[6], focusing on four distinct reduction levers

available in cement production: (i) thermal and electric efficiency, (ii) alternative fuel use, (iii)

clinker substitution and (iv) carbon capture and storage (CCS). The roadmap also emphasizes

that none of the options alone can yield the necessary reduction in CO2 emissions. However,

from an economical viewpoint, option (i) is less attractive since an optimization of the energy

efficiency usually requires installation of new plants or upgrading of the old plants. CCS is a

relatively new technology and it has not yet been utilized at the industrial scale in cement

production. Thus, most of the ongoing developments and innovations focus on the use of

alternative fuels and clinker substitutions by other minerals with hydraulic and/or pozzolanic

properties[7,8]; those are usually referred to as Supplementary Cementitious Materials (SCMs).

However, the use of SCMs is complicated by several factors, e.g. high water demand, poor

workability, retention and a significant reduction in the early strength of the blended cement.

Therefore, the clinker substitution is typically limited to an average clinker factor of 0.78, i.e., 22

% of the clinker is replaced by SCMs including gypsum[6].

Introduction 2

The FUTURECEM project was established in 2007 for a four year period (2007 - 2010)

with the main purpose to develop new building materials produced from readily available Danish

raw materials of low cost and low energy consumption, which should have the same

performance as today’s concrete materials. The project is a joint collaboration between Aalborg

Portland A/S, iNANO at the Aarhus and Aalborg University, and the Geological Survey of

Denmark and Greenland (GEUS), with partly financial support from the Danish National

Advanced Technology Foundation. The first principal goal is to obtain a 30 % replacement of

the clinkers in concrete production by SCMs, resulting in an approx. 30 % reduction of the total

CO2 emission. On a longer time scale, the FUTURECEM project also targets a larger

replacement of clinkers utilizing geopolymeric materials, such as alkali-activated metakaolin,

which possibly may result in a 70-80 % CO2 reduction.

The research program for this PhD project is a part of FUTURECEM and focuses on

modifications of the Portland cement clinker composition with the aim of increasing the clinker

reactivity, which may enable a higher degree of clinker replacement. Particularly, the

mineralizing effects of calcium fluoride, which is widely used in today’s Portland clinker

production, are investigated. The presence of fluoride ions has shown several important impacts

on the formation of the cement clinker phases as well as their hydrational reactivities[9,10]. The

optimization of the mineralizing properties for fluoride, which seems to be dependent on the

chemical composition of the clinkers, requires knowledge about the location of the fluoride ions

and their potential couplings to other guest ions in the anhydrous as well as hydrated phases of

Portland cement. Thus, another major goal of this project is to investigate the feasibility of solid-

state Nuclear Magnetic Resonance (NMR) spectroscopy in obtaining such information for

cementitious materials, which otherwise is difficult to be measured by other techniques.

The present dissertation consists of five chapters. The first two chapters give a brief

introduction to the relevant cement chemistry and the basic theory for NMR spectroscopy,

respectively. Chapter 3 is devoted to the investigation of fluoride mineralization of Portland

clinker while Chapter 4 concerns the influence of fluoride ions on the hydration of Portland

cement and their site preferences in the calcium silicate hydrate phases. Chapter 5 demonstrates

the potential applications of solid-state NMR in structural investigations of alumina-rich

cementitious systems such as alkali-activated materials and strätlingite. The dissertation includes

applications of NMR techniques such as Cross Polarization (CP), Rotational-Echo DOuble-

Resonance (REDOR) and Rotational-Echo Adiabatic-Passage DOuble-Resonance (REAPDOR)

on different nuclear-spin isotopes, including 19F, 27Al, 29Si and 43Ca, to obtain complementary

structural information for the studied cement phases.

Chapter 1. Cement Chemistry 3

1. Chapter

Cement Chemistry

Portland cement was patented by Joseph Aspdin in 1824[11]. The name refers to the

similarity in the color and strength properties of the cement and Portland stone, i.e. a limestone

quarried in Dorset, south west England. However, the mineralogy and hydrational properties of

Aspdin’s cement differ significantly from today’s Portland cement. Nowadays, several cement

types are produced throughout the world for a wide range of applications, although the majority

has been developed for general constructional use[12]. Cements are named in accordance with

standard specifications, which define the chemical composition, physical properties and

performance requirements for the cement. The standard specifications also set the upper limits

for the content of each of the minor components. For example, the upper limit for the MgO

content in Portland cement is typically set to 5 %w because MgO levels above this limit result in

uncombined MgO (i.e. MgO that is not incorporated in the clinker phases), which can cause

destructive expansion in hardened concrete[13]. In fact, the standard specifications used by

countries around the world can be slightly different and therefore, various names may also be

given to the same class of cements[14]. For example, “Portland cement” in current European and

British standards corresponds to “Types I and II Portland cement” in the American standard, i.e

American Society for Testing and Materials (ASTM).


1.1. Portland cement manufacture

Portland cement is generally made from calcareous materials such as limestone,

marlstone or chalk. Furthermore, argillaceous materials including sand, clays or bauxite and iron

ores may be needed to provide additional SiO2, Al2O3 and Fe2O3. Typically, the raw meal has a

chemical composition of 67 %w CaO, 22 %w SiO2, 5 %w Al2O3, 3 %w Fe2O3 and 3 %w of

minor components such as MgO, SO3, alkali oxides and halogenides[13]. Various engineering

technologies are available for the Portland cement production. However, the short kiln with a

multistage of preheater and a cooling system is the current most efficient technology[15,16]. The

basic steps of the process will be briefly described below (for further reading see references [17

and 18]).

1.1.1. Quarrying and preparation of raw materials

Cement plants are often located close to naturally occurring calcareous and argillaceous

deposits to minimize the transport cost. The first step of the dry process involves crushing of the

quarried lime stone and clay into small pieces. In order to ensure a high reactivity for the raw

materials, they are milled together to produce a raw meal with more than 85 % of the particles

having a diameter below 90 m. The quality of the cement depends largely on the chemical

composition of the raw meal. Three important parameters[13] are widely used in clinker

production to design the chemical composition of the raw meal

)OFe 0.65(%w)OAl 1.2(%w)SiO 2.8(%w

CaO %wLSF

32322 (1.1)

3232

2OFe %wOAl %w

SiO %wSR

(1.2)

32

32

OFe %w

OAl %wAR (1.3)

The proportion of silicate phases in Portland clinker is determined by the silica modulus (SR)

while the lime saturation factor (LSF) mainly governs the alite (Ca3SiO5) to belite (Ca2SiO4)

ratio. For the pure CaOSiO2Al2O3Fe2O3 system, a LSF of 1.0 or above indicates that the

formation of alite from belite and lime is saturated. For most productions, LSF is set in the range


of 0.92 – 0.98 to minimize the amount of uncombined CaO, i.e. the free CaO which are not

incorporated in the principal clinker phases. The alumina modulus (AR) is important for the

quantity of the clinker melt which is the only transport medium for the raw material in the rotary

kiln. In general, the silica and alumina modulus are in the ranges of 2.0 3.0 and 1.0 4.0,

respectively. However, this does not include special clinker types such as white Portland clinker

which has much lower aluminum and iron oxide contents. Variations in LSF, SR and AR can

significantly change the “burnability”, which is defined as the ease of combining belite and lime

into alite. The burnability may also be affected by several other factors, including the quantity of

the minor components and the grain size of the raw material. It is usually expressed by the

quantity of free CaO present in the final clinker.

1.1.2. Clinker production in the rotary kiln

The fine-grained raw meal is dried in a preheater tower, which consists of a series of

vertical cyclones, before it is fed into the rotary kiln. As the particles fall into the combustion

chamber, which is placed at the bottom of the preheater above the kiln, they have typically

reached a temperature of 800 – 900 C. In this temperature zone, approximately 90 % of the

limestone decomposes to lime in accordance with the chemical reaction

CaCO3(s) CaO(s) + CO2(g) (1.4)

This process is usually referred to as calcination and is responsible for 60 65 % of the direct

CO2 emissions from clinker production. The remaining ~40 % originates from the combustion of

fuel, which is fired directly into the discharge end of the kiln. As the tilted kiln slowly rotates,

about 3 5 rotations per minute, the calcined material slides and tumbles towards the flame. The

material is heated to partial fusion and several chemical reactions occur as a kiln temperature of

about 1450 C is progressively reached. Due to the rotation, the clinker leaves the kiln as

roughly spherical nodules of 1 3 cm in diameter. The relative proportions of clinker minerals

formed as the material moves into different kiln-temperature zones are graphically illustrated in

Figure 1.1. The chemical reactions for the clinker formation can be divided into three groups:

(1) The most important reactions occurring below ~1300 C are calcination, decomposition

of the clay minerals and formation of belite (2CaOSiO2), aluminate and ferrite. The

major phases at the end of this stage are belite, lime, aluminate and ferrite. Only a small


amount of clinker melt is formed at this temperature. The formation of belite, which is the

second most occurring phase in Portland clinker, can be expressed as

2CaO(s) + SiO2(s) 2CaOSiO2(s) (1.5)

(2) As the kiln temperature is raised above 1300 C, a melt phase is formed, consisting

mainly of tricalcium aluminate (3CaOAl2O3) and ferrite (Ca2(AlxFe1x)2O5, where 0 < x

< 0.7). The pure CaO SiO2 Al2O3 Fe2O3 system has a eutectic point, which occurs at

1338 C for an alumina modulus of AR = 1.38. A deviation of the alumina modulus from

this value requires much higher temperature to preserve the same level of clinker melt

content. In the temperature zone of 1300 – 1450 C, the clinker melt is the only transport

medium for belite and lime. Therefore, the quantity and properties (e.g., viscosity and

surface tensions) of the clinker melt are decisively important for the formation of alite.

Lime and belite are brought into reaction within the melt phase to form alite, in

accordance with the chemical reaction

CaO(s) + 2CaOSiO2(s) 3CaOSiO2(s) (1.6)

To some extent, the presence of the minor components such as SO3, P2O5, F and alkali

oxides may contribute to the quantity as well as the properties of the clinker melt. These

minor components originate from either the quarried raw materials or the combustion

fuel. However, they may also be deliberately added to the raw meal to modify the clinker

towards certain application requirements.

(3) The hot clinker falls into a cooler where it is immediately quenched either by incoming

air or by water. Upon cooling, the clinker melt re-crystallizes to aluminate and ferrite.

Depending on the cooling rate, different polymorphic transitions will also occur for alite

and belite.

1.1.3. Cement grinding

To produce cement, the clinker is mixed and finely ground with other hydraulic minerals

and usually 4 5 %w gypsum. Depending on the application requirements, the clinker content

can vary from 10 %w to 95 %w. For example, the clinker factor in blastfurnace cement CEM III

(C) is only 0.5 0.19, although this cement type is only used for a few special constructions

such as roads and tunnels.


Figure 1.1 Schematic illustration of the typical proportions of phases for the formation of Portland clinker minerals

as a function of the progressive kiln temperature. The figure is adapted from the work by Wolter[19].


1.2. The main constituent phases of Portland clinker

The properties of Portland cement are mainly determined by the proportion of its four

principal clinker phases which are the impure forms of Ca3SiO5 (alite), Ca2SiO4 (belite),

Ca3Al2O6 (tricalcium aluminate) and Ca2(AlxFe1-x)2O5 (ferrite). Other phases such as periclase

(MgO), quartz (SiO2), free lime (CaO), etc. may also be present in minor quantities, usually less

than 1 %w.

1.2.1. Tricalcium silicate

Alite is the most important constituent clinker component and typically constitutes 50

70 %w of Portland clinker. This highly hydraulic clinker mineral is an impure form of tricalcium

silicate, which can occur in seven crystal modifications: three triclinic[20] (T), three

monoclinic[21] (M) and one rhombohedral[22] (R) polymorphs. These alite structures are built of

Ca2+, O2 and SiO44 ions. They are all structurally similar with respect to the positions of Ca, Si

and O atoms, although they differ significantly in the orientation of the SiO4 tetrahedra.

Furthermore, the mean coordination number of the Ca2+ ions is different for the polymorphs. For

example, it is 5.66 in the R form, but 6.21 in TI polymorph. On heating, tricalcium silicate

undergoes a series of reversible phase transitions[23]

RMMMTTT C 1060III

C 1050II

C 990I

C 980III

C 920II

C 600I

oooooo

In fact, pure tricalcium silicate is only thermodynamically stable at temperatures above 1300 C.

On cooling, it becomes unstable between 1300 C and 1000 C with a maximum rate of

decomposition into belite and lime at 1175 C. At room temperature, pure tricalcium silicate is

meta-stable only in its TI form. The crystal structure for tricalcium silicate[20] in its triclinic form

is shown in Figure 1.2; the unit cell corresponds to the formula unit Ca27Si9(Ob)36(Oi)9, where b

and i denote the oxygen sites involved in covalent SiO bonds and the interstitial oxygen sites,

respectively. The high-temperature polymorphs of tricalcium silicate can be stabilized at room

temperature by the incorporation of a sufficient amount of impurities in its structure[24-27]. For

example, alite is stabilized as the MIII polymorph by the incorporation of Mg2+ ions[26] in the

Ca2+ sites while the rhombohedral form is stabilized by Zn2+ incorporation[28]. The impurities are

often referred to as substituent oxides and their ions as guest ions. In Portland clinkers, alite

occurs mainly in the MIII and/or MI forms, containing roughly 4 %w substituent oxides[29-31]. The


typical chemical compositions, including the most common encountered substituent oxides, for

Portland clinkers are summarized in Table 1.1.

Figure 1.2 Graphical illustration of the crystal structure for the triclinic form of tricalcium silicate. The unit cell

includes 9 non-equivalent SiO4-tetrahedra, 9 interstitial oxygens (Oi) sites and 36 oxygens (Ob) bonded directly to

the Si atoms. () Ca2+, (): interstitial oxygen O2 and (blue tetrahedra): SiO44-. The crystal structure data is adapted

from reference [20].

1.2.2. Dicalcium silicate

The second major constituent phase of Portland clinkers is the impure form of dicalcium

silicate, denoted belite. Its content in Portland clinkers is typically 5 – 30 %w. The structure of

dicalcium silicate is built of Ca2+ and SiO44+ ions and can occur in five different polymorph

modifications: one , one and three forms[32]. At room temperature, dicalcium silicate occurs

mostly in the form. On heating, the mineral undergoes phase transitions to form the high

temperature polymorphs

α α α β γ C 1425'High

C 1160''Low

C 680630C 500 oooo


The structure of -Ca2SiO4, which is similar to that of olivine, (Mg,Fe)2SiO4, is much less dense

as compared to the other polymorphs. Therefore, the transition from the high temperature

polymorphs to on cooling causes the sintered material to crack and fall to a more voluminous

powder. The transition can be avoided by the incorporation of a sufficient amount of guest

ions in the dicalcium silicate structure. It can also be prevented if the -Ca2SiO4 crystallites are

sufficiently small. The arrangements of Ca2+ and SiO44+ ions in the high temperature phases are

very similar. However, they differ significantly from the form. Clinker belite typically contains

4 6 %w substituent oxides and occurs largely in the form [29-31]. An illustration of the

dicalcium silicate crystal structure is shown in Figure 1.3; the asymmetric unit contains a single

tetrahedral SiO4 site and two distinct Ca2+ sites, which have seven and eight oxygen atoms

within the first coordination sphere.

Figure 1.3 Graphical illustration of the crystal structure for the beta form of dicalcium silicate [33]. The asymmetric

unit only includes a single SiO4-tetrahedral site and two distinct Ca2+ sites. (): Ca2+ and (blue tetrahedra): SiO44-.

1.2.3. Tricalcium aluminate

Pure tricalcium aluminate (3CaOAl2O3) has a cubic structure and does not exhibit

polymorphism[34]; it is built of Ca2+ ions and rings of six AlO4 tetrahedra. However, a rather

large amount of guest ions can be incorporated in the Ca2+ as well as the Al3+ sites, resulting in a

series of different structures[35]. In general, the tricalcium aluminate phase of Portland clinker

exhibits cubic and/or orthorhombic structures. These impure forms contains up to 13 %w and 20

%w substituent oxides, respectively. The most important guest-ion incorporations are Si4+ and

Fe3+ substituted in the tetrahedral Al3+ sites. A large amount of other guest ions such as Na+, K+

and Mg2+ may also be incorporated in the Ca2+ sites. The tricalcium aluminate content in

ordinary Portland clinker is in the range of 5 10 %w whereas it is much lower in white

Portland clinkers.


1.2.4. Ferrite

The ferrite (Ca2(AlxFe1-x)O5) content in ordinary Portland clinker is usually 5 15 %w,

but much lower in white Portland clinker. The grey color of ordinary Portland cement is mainly

due to the presence of this iron-rich phase. In clinkers, ferrite is typically found closely mixed

with tricalcium aluminate. Due to the similarity in the cell parameters for the tricalcium

aluminate and ferrite phases, oriented intergrowth can occur for these phases on re-

crystallization from the clinker melt[36]. The structure of ferrite can be derived from that of

perovskite (CaTiO3). It can occur with any of the compositions in the solid-solution series

Ca2(AlxFe1-x)2O5, where 0 < x < 0.7[13]. Each Ca2+ coordinates to seven oxygen atoms.

Furthermore, Al3+ and Fe3+ are both distributed between octahedral and tetrahedral sites. Clinker

ferrite contains about 10 %w of substituent oxides, mainly SiO2 and MgO. Other ions such as

Na+, K+ and S6+ are also present in minor concentrations. The guest ions are mainly incorporated

by substituting for either the Al3+ or Fe3+ sites whereas Ca2+ exhibits almost no substitution [29,37].

Table 1.1 Typical chemical compositions for the four principal clinker phases of Portland cement [13].

CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 P2O5 TiO2 Mn2O3

Alite 71.6 25.2 1.0 0.7 1.1 0.1 0.1 0.1 0.1 0.0 0.0

Belite 63.5 31.5 2.1 0.9 0.5 0.1 0.9 0.3 0.1 0.2 0.0

Aluminate 56.6 3.7 31.3 5.1 1.4 1.0 0.7 0.0 0.0 0.2 0.0

Ferrite 47.5 3.6 21.9 21.4 3.0 0.1 0.2 0.0 0.0 1.6 0.7


1.3. Minor components

In addition to CaO, SiO2, Al2O3 and Fe2O3, Portland clinker contains approximately 3

%w of other components such as MgO, SO3 and alkali oxides[13]; those are usually referred to as

minor component in cement chemistry. They originate mainly from the quarried raw materials

and the combustion fuel. The most encountered minor components in Portland clinker can be

divided into two groups according to their general influences on the clinkering process [24,25]:

(1) Flux agents include compounds that modify the temperature of liquid formation, the

properties of the clinker melt and the crystal morphology of the principal clinker

minerals.

(2) Mineralizer agents include compounds that influence the thermodynamic stability of the

principal clinker minerals. However, in many cases, it is only used to categorize the

compounds that influence the reaction of alite formation, i.e. Ca2SiO4 + CaO Ca3SiO5,

by modifying the thermodynamic properties of one or both calcium silicates.

Furthermore, some minor components can modify the hydration properties of Portland cement to

a significant extent. They typically involve one of the processes:

(1) Introduction of structural defects by forming solid solution within the principal clinker

mineral phases.

(2) Stabilization of clinker minerals in their high temperature polymorphs.

(3) Effects arising during cement hydration such as coating of the cement grains by insoluble

basic salts.

In general, the first two effects increase the hydrational reactivity of the clinker phases whereas

the last prevents the hydration of the anhydrous cement phases. Another important distinction

that also requires careful attention is the volatility of the minor components. Appreciably volatile

components typically have a higher concentration in the kiln atmosphere than that in the raw

material. Furthermore, they tend to be deposited in the cooler part at the end of the kiln due to

the recycling of air to reduce heat loss. For laboratory synthesis, the volatile components have a

rather high loss on ignition and thus, their contents in the clinkers can be much lower than that

added to the raw meal. SO3, K2O, Na2O, ZnO, and Cl2 are very volatile while F, V2O5 and As2O3


have low volatility. Components such as MgO, P2O5, TiO2 and NiO are essentially non-volatile

in the clinkering process.

1.3.1. Flux

Since almost all of the clinker melt is formed immediately at the eutectic temperature, the

reactants, e.g. lime and belite, are limited within “local volumes” by various proportions.

Therefore, the ease of combining lime and belite to form alite is strongly controlled by the

transport of the reactants through the clinker melt between such local volumes. The transport rate

is dependent on the quantity, viscosity and surface tension of the clinker melt. Most of the minor

components, when present in the raw meal, decrease the temperature for the first liquid

formation and contribute to the quantity of the clinker melt by approximately the same amount

as their content. Their influence on the viscosity and surface tension of the clinker melt is

somewhat complicated. In general, ions of strongly electropositive elements such as alkali metal

increase the viscosity while those of electronegative elements like Cl and F have a reverse

effect[25]. The influence of the p-block elements on the surface tension also follows a similar

trend, where the surface tension is decreased for an increase in the electronegativity. On the

other hand, the surface tension is increased as the electronegativity of the s-block elements is

increased.

The application of minor components for controlling the morphology of alite and belite

plays a very important role in clinker production. It is apparent from Table 1.1 that Mg2+, Al3+

and Fe3+ ions constitute the major part of guest ions in the calcium silicate phases. However,

compounds including ions such as S6+, P5+, K+ and Na+ have an increased interest in modern

Portland clinker production. For example, Mg2+ ions are incorporated in the Ca2+ sites and

stabilize alite in its MIII polymorph[26]. S6+ ions, on the other hand, substitute for the tetrahedral

Si4+ sites and promote the formation of alite in its MI form. Furthermore, S6+ ions, when

incorporated in belite, stabilize this phase in its form[38]. Alkali ions and P5+ are preferentially

incorporated in belite, stabilizing the ’-belite polymorph, and they substitute for the Ca2+ and

Si4+ sites [24,39,40], respectively. The guest ions may also compete for specific structural sites in

the clinker phases and therefore, limit or increase the substitution level for one another. For

example, Na+, K+ and Mg2+ ions substitute preferentially for the Ca2+ sites while Al3+, P5+ and

S6+ ions compete for the tetrahedral Si4+ sites.


1.3.2. Mineralizers

According to the restricted definition of a mineralizer which only includes compounds

that influence the reaction of alite formation[25], the free lime content in the clinker can be used

to monitor the mineralizing effect. For example, the substitution of Si4+ by Al3+ ions in the

calcium silicate phases effectively increases the bulk SiO2 content and therefore, increases the

belite content. However, this is a mass-balance effect and Al3+ is not a mineralizer since the free

lime content remains low. On the other hand, S6+ ion is a strong mineralizing agent on the

formation of belite. It increases the thermodynamic stability of belite by forming solid solutions

in this phase[38]. Therefore, the presence of a SO3 source in the raw meal simultaneously

increases the amount of belite and the free lime content for the SO3-mineralized clinker. In

modern Portland cement production, CaF2 and CaSO4 are by far the most used mineralizers,

where F strongly facilitates alite formation and SO3 mineralizes belite [41-45].

1.4. Hydration chemistry for Portland cement

The hydration of Portland cement is a process that includes many simultaneous chemical

reactions between the clinker minerals, gypsum and water, where the water/solid ratio required

in the cement mixture (paste) is typically 0.3 0.6 by weight[13]. Furthermore, the addition of

supplementary cementitious materials (SCM) such as natural pozzolans may also have important

impacts on the entire hydration process[46-50]. This section gives a brief description of the basic

hydration reactions for the clinker minerals as individual phases, which is fundamental for the

hydration of Portland cement in general. The description is based on references [13 and 51].

1.4.1. Hydration of calcium silicate phases

The main products formed from the hydration of alite and belite are calcium silicate

hydrates (CSH = xCaOSiO2yH2O), which constitute approx. 60 %w of the entire hydration

product of Portland cement. The relative proportions of phases formed from the hydration of

Portland cement are graphically illustrated in Figure 1.4. Typically, about 40 % of alite has

reacted within 1 day after mixing with water, 70 % within 28 days and virtually all in one year.

For a given grain-size distribution and water/solid (w/s) ratio, alite sets and hardens in a similar

manner to Portland cement. In this context, the term setting (or set) is used for stiffening without


increase in compressive strength while hardening (or harden) is the process of significant

strength development. Furthermore, hydration times up to 28 days are usually denoted early

hydration periods whereas the later hydration is referred to as long-term hydration. The

hydration of alite can be divided into three principle stages[52]:

(1) Within the first four hours, alite is attacked by water and a honeycomb-like product of

calcium silicate hydrates (CSH) is formed on the surface of the alite grains. Some

portlandite, i.e., Ca(OH)2, is also formed.

22222 x)Ca(OH)(nOyHSiOxCaOOy)Hx(nSiOnCaO (1.7)

(2) During the middle stage, i.e. 4 24 hours, CSH and portlandite are rapidly formed.

The morphology of CSH formed at this stage strongly depends on the available space

for growth. Fibrous CSH has been observed as the outer product whereas the

honeycomb-like material grows in the surface fractures of the alite grains. As the

hydration proceeds after 24 hours, the inner product of CSH is also formed, which is

characterized by a very fine pore structure.

Belite follows the same hydration process as alite, however, with a much slower hydration rate;

approx. 30 % of belite has reacted during the early hydration period and about 90 % in one year.

Thus, the hydration of belite only contributes little to the early strength development, but it is

mainly responsible for the strength development during the long-term hydration.

1.4.2. Hydration of tricalcium aluminate

Tricalcium aluminate (C3A)1 possesses a much higher reactivity as compared to alite and

belite. Initially, the meta-stable calcium aluminate hydrate phases, C4AH13 and C2AH8, are

rapidly formed from the paste-liquid[53]. Subsequently, these phases are slowly converted to the

thermodynamically stable C3AH6 phase. The overall hydration of C3A has finished within a few

hours after the mixing. This excessive hydration of C3A results in a severe stiffening, which

cannot be dispelled by remixing. This phenomenon is usually referred to as flash setting. In order

to retard the hydration of C3A, gypsum is normally added to the Portland clinker at the end of the

1 The nomeclature C = CaO, S = SiO2, A = Al2O3, F = Fe2O3 and H = H2O is commonly used in cement chemistry.

However, it is only used to write the hydration reactions of tricalcium aluminate in this section and to abbreviate the

calcium silicate phases throughout the thesis.


clinkering process[54]. In the presence of sulphate ions, the C3A phase reacts rapidly to form

ettringite, which is an AFt phase.

3236Fast

23 HSAC16HHS3CAC (1.8)

124Slow

32363 HS3C4HHSACA2C (1.9)

When the sulphate ions are consumed by this reaction, ettringite slowly reacts with the

exceeding aluminate phase to form monosulphates (AFm), equation (1.9). The general formula

for the AFm (Al2O3F2O3mono) and AFt (Al2O3F2O3tri) phases are

Ca2(Al,Fe)(OH)6XwH2O and [Ca3(Al,Fe)(OH)612H2O]2X3wH2O, respectively[13,51]. Here, X

in AFm denotes one formula unit of a singly charged anion such as OH or half a formula unit of

a double-charged anion such as SO42 or CO3

2. On the other hand, in AFt it denotes one formula

unit of a double-charged anion or two formula units of a single-charged anion. In addition to

setting regulation properties, the sulphate ions have important impacts on the hydration rate of

alite and the volume stability of the hydration products. The optimum content of gypsum,

however, depends on several factors, such as the cement composition, hydration time and other

conditions of hydration. The minimum content of SO3 required to control setting is approx. 2

%w for an ordinary Portland cement.

1.4.3. Hydration of Portland cement

The microstructure of hydrated Portland cement develops in a similar manner to that formed

from the hydration of tricalcium silicate[55,56]. Soon after mixing, a gel-like layer containing

amorphous alumina, silica, calcium and sulphate in varying amounts is formed on the cement

grain surfaces. The AFt phase (e.g. ettringite) starts to precipitate within about 10 minutes after

mixing. After about four hours, the cement grains are completely covered by a thickening layer

of CSH. As the hydration proceeds, the CSH shells grow outward and inter-grow with

surrounding adjacent grains. This happens about 12 hours after mixing and usually it is referred

to as the cohesion point, where the setting has completed. A space filled with a highly

concentrated ion solution is observed between the shell and the inside of the anhydrous material;

the space can be up to 0.5 m wide. At this point, the CSH shell is still sufficiently porous and

ions can migrate between the inner and outer space through the shell wall. The hydration at this


stage is driven by dissolution and precipitation mechanisms. At a later stage (>24 hours), the

permeability of the shell is decreased significantly, preventing ion migration. CSH is formed

and deposits on the inside wall. The concentration of SO42 ions inside the shells also drops

rapidly and AFt slowly reacts with the remaining aluminate phase to form AFm. The strength

development of Portland cement is mainly related to the formation of CSH, where the SiO4

monomers released from the hydration of alite and belite polymerize into dimers and longer

chains of silicate tetrahedra.

Figure 1.4 Schematic representation of the hydration for the main Portland clinker phases and their resulting

hydration products. The areas of the boxes represent roughly the typical relative proportions of these phases as

reported in reference [13].


1.5. Structure of the main hydration product CSH

A calcium silicate hydrate (CSH) phase is the principal binding phases in hardened

cement[13,51]. The nanostructure of CSH exhibits several similarities to the structure of the

crystalline minerals tobermorite and jennite. However, CSH occurs with a wide range of

compositions, morphology and degree of structural order. The dashes indicate that no particular

composition is implied. The Ca/Si molar ratio in CSH formed from a nearly saturated alite

paste can possess a value in the range from about 1.2 to 2.1[57]. The average Ca/Si ratio is,

however, typically ~1.75[57-59]. An increase in the quantity of belite and/or SCMs results in a

decrease in the Ca/Si molar ratio, which typically enhances the long-term strength and durability

of the hydrated Portland cement, where the lower limit for the Ca/Si ratio in hydrated Portland

cement is 0.7[57]. Several models have been proposed for the nanostructure of CSH phases that

are formed from the hydration of calcium silicates, cement or related materials[60-64]. Many of

them exhibit several similar features, although they provide different degree of flexibility and

complexity. The models fall typically into two categories, which are derived from the structure

of tobermorite 14-Å. The first category, denoted T/CH, includes building blocks of tobermorite-

like structure (T) intermixed with layers of calcium hydroxide (CH). The second group, which is

usually denoted as T/J, contains elements of tobermorite and jennite (J) structures. The structures

of tobermorite, jennite and Ca(OH)2 are described in Sections (1.5.1) (1.5.3), respectively.

Richardson and Groves introduced a generalized model for CSH[62,65], which includes

formulations that could be interpreted from both T/CH and T/J structural viewpoints. The model

proposes the following general composition for the CSH

OHCaOHOSiHCa 2

2

229132 mynynwnnwn (1.10)

The number of silanol (SiOH) groups is given by w and the degree of protonation of the silicate

chains by w/n. The Ca/Si ratio can be obtained as

132

4Ca/Si

n

yn (1.11)

The chemical formula for the main layer is given by the braces. The main layer contains 2n Ca2+

ions surrounded by dreierketten silicate chains of mean length 3n 1. The charge balance is

preserved by n (w/2) interlayer Ca2+ ions of the (ny)/2 given outside the braces. From the

T/CH viewpoint, the remainder of the (ny)/2 Ca2+ ions occur in Ca(OH)2 layers which


corresponds to the interlayer of tobermorite whereas, from the T/J viewpoint, they form a part of

the main layer resulting in regions with jennite-like structure. This flexible structure model

allows the Ca/Si ratio to vary from 1.0 to 2.5 by

(1) omission of part of the bridging SiO4 tetrahedra in the dreierketten silicate chains,

resulting in an increased Ca/Si ratio. Ca2+ may also be substituted into the bridging sites

leading to a further increase in the Ca/Si ratio,

(2) varying the degree of protonation, w/n; since, the increased Ca2+ content can be balanced

by a decrease in the SiOH content and vice versa,

(3) incorporation of additional Ca2+ ions can be charge-balanced by additional OH. For the

T-based CSH structure the OH ions are located in the interlayer whereas they are

incorporated in the main calcium layer of the J-based part.

1.5.1. Tobermorite 14-Å

Tobermorite 14-Å, Ca5Si6O16(OH)27H2O, is the most hydrated phase of the tobermorite

mineral family[66]. The name refers to the basal spacing between the principal layers which

exhibit a chemical formula of [Ca4Si6O16(OH)2(H2O)2]2. The principal layer includes two

structurally nonequivalent calcium ions in sevenfold coordination and three tetrahedral Si4+ sites.

The SiO4 tetrahedra form dreierketten chains with periodicity of three silicon atoms, two Si on

the pair sites and one Si in the bridging site. The Ca atoms are coordinated to six oxygen atoms

and a hydroxyl group of the bridging SiO4 tetrahedron forming a central Ca-O sheet, where the

dreierketten silicate chains are running on both sides. The CaO7 and SiO4 polyhedra form the

principal layer by sharing their vertices and apices as illustrated in Figure 1.5. The principal

layers are separated by an interlayer, which contains Ca2+, OH and a large amount of water

molecules. The layers are held together by strong hydrogen bonds which are formed between the

oxygen atoms from the principal layer and the hydrogen atoms from water molecules present in

the interlayer.


Figure 1.5 Illustration of the layer structure for tobermorite 14-Å, Ca5Si6O16(OH)27H2O. The calcium oxide in the

main layers is shown by green polyhedra. The dreierketten silicate chains shown in blue are formed from sequences

of three SiO4 tetrahedra, one bridging and two pair sites. The interlayer consists of Ca2+ and OH ions, which are

depicted in grey and red, respectively. The crystal structure data is adapted from reference [66].

1.5.2. Jennite

The crystal structure of jennite[67], Ca9Si6O18(OH)68H2O, is classified in the space group

P-1. It exhibits a triclinic unit cell with a = 10.756 Å, b = 7.265 Å, c = 10.931 Å, = 101.30,

= 96.98 and = 109.65. The structure, Figure 1.6, is built of three distinct modules: ribbons

of two calcium octahedra sharing edges, dreierketten silicate chains and calcium octahedra on

inversion centers. The ribbons are connected to each other by sharing vertices. This results in a

zigzag layer of calcium layers, containing two types of ribbons: the first is sharing all its vertices

with other polyhedra while in the second ribbon, the apices on both sides of the zigzag layer are


corresponded to water molecules. The ribbons are further firmly linked through the dreierketten

silicate chains, which are present on both sides of the zigzag calcium layer. Furthermore, the

silicon atoms on the bridging sites are strongly coupled to the protons on water molecules. Both

calcium ribbons and silicate chains are running along the b axis. The composite octahedral-

tetrahedral layers are further connected through the additional calcium atoms in octahedral

coordination occurring on the inversion centers.

Figure 1.6 Illustration of the layer structure for jennite[67], Ca9Si6O18(OH)68H2O. The calcium oxides in the main

layers are shown by green polyhedra. The dreierketten silicate chains shown in blue are formed from sequences of

three SiO4 tetrahedra, one bridging and two pair sites. The main layers are connected through the additional calcium

atoms in octahedral coordination occurring on the inversion center.


1.5.3. Portlandite

Ca(OH)2 has a layer structure[68] (Figure 1.7) and belongs to the space group P-3m1. The

layers consist of one Ca2+ sheet which is sandwiched by two sheets of hydroxyl groups (OH).

Each calcium ion coordinates to six hydroxides forming a perfect octahedron with a CaO bond

length of 2.37 Å. Under ideal conditions, Ca(OH)2 crystallizes with euhedral hexagonal shape. In

hydrated Portland cement, however, it exhibits a more massive and indeterminate shape.

Furthermore, a small amount of SiO2 can be incorporated in the calcium layer. Ca(OH)2

constitutes roughly 20 %w of the hydration products from Portland cement and this phase is

usually referred to as portlandite.

Figure 1.7 The layered crystal structure for Ca(OH)2. The calcium ions, shown in grey, are coordinated to six

hydroxyl groups. The oxygen atoms are shown in red and hydrogen atoms in blue. The crystal structure data is

adapted from reference [68].

Chapter 2. Applications of solid-state NMR in cement research 23

2. Chapter

Applications of Solid-State NMR in Cement Research

The main advantage of solid-state NMR is related to its applicability in structural

investigations of crystalline as well as amorphous or poorly crystalline materials.

Complimentary to conventional analytical methods in cement research such as powder X-ray

diffraction (XRD), which generally detect the bulk or long-range order structures, different

NMR techniques can be used to obtain specific information about local structures. Generally, the

individual NMR-active spin isotopes exhibit different resonance frequencies and their NMR

interactions show a strong dependence on their specific site locations. Another important benefit

of NMR in cement research is its high sensitivity for specific NMR isotopes, making it possible

to investigate the structure of guest ions, which otherwise is very difficult by other techniques.

This Chapter gives an introduction to the basic solid-state NMR theory and its

applications in cement research. Furthermore, it includes relevant details for the NMR

techniques, which have been employed in this PhD project.


2.1. NMR theory

For nuclear-spin isotopes with a non-zero spin-quantum number (I), when subjected to an

external static magnetic field (B0), their nuclear magnetic moments orient either parallel or

antiparallel with respect to the direction of 0B 0, 0,B . The interaction between the nuclear

spin and the externally applied static magnetic field is the so-called Zeeman interaction, which

splits the degenerated energy states of the nuclear spin into (2I + 1) states. The detected NMR

resonance frequency () is related to the energy difference (E2 E1) between these Zeeman

states by

eff

12 BΔEE

ν

γm (2.1)

Here, is the Planck’s constant (h/2) and m is the magnetic quantum number, which can take

any of the values from I to I. In NMR, only single-quantum coherences, i.e. transitions with m

= 1, can be detected directly. However, multiple-quantum coherences such as zero- and double-

quantum coherences of a two spin system or triple-quantum coherences for I ≥ 3/2 quadrupolar

nuclei may be indirectly detected in advanced NMR experiments. is the gyromagnetic ratio,

which is a specific constant for each of the nuclear-spin isotopes in the periodic table. Beff is the

effective magnetic field whose magnitude is determined by the externally applied magnetic field

and the local electronic and magnetic environments at the specific nuclear site. In essence, the

state of such a system is governed by quantum mechanics and can be fully described by the time-

dependent Schrödinger equation. However, solving the Schrödinger equation for a NMR

experiment, which often includes a very large number of spins (> 1015), can be rather difficult

and time consuming. Alternatively, the spin state in the NMR experiment is more conveniently

expressed by a spin-density matrix formalism using the Liouville-Von Neumann equation, which

in the absence of relaxation is given by[69]

t,ttt

ρiρ H

(2.2)

JDσQrfZ HHHHHHH (2.3)

where i is the complex imaginary unit. The density matrix, (t), is the statistical average for an

ensemble of spins. H(t) is the nuclear-spin Hamiltonian describing the external and internal spin


interactions, where the external terms (HZ and Hrf) are applied to modify the nuclear-spin state

whereas the internal interactions (HQ, H, HD and HJ) are nuclear-spin dependent and sensitive

to the local environment of the observed nuclear spin. HZ = LIZ is the Hamiltonian for the

Zeeman interaction. For a given static magnetic field, B0, the Larmor frequency L = B0 is

specific for each nuclear-spin isotope. The Hrf term in equation (2.3) represents the pulsed

radiation, applying an external rf field perpendicular to B, which in the laboratory-fixed frame

may be expressed as

xcarr1rf Iφtcos2 H (2.4)

where 1 = –B1/2 is the rf-field strength, carr (L) is the carrier frequency of the pulse and

is its phase relative to the direction of B1 . The J-coupling (HJ) is the direct spin-spin

interaction, i.e. a through bond coupling, which is usually small and has not been of importance

in the present study. A short description of the remaining internal interactions, i.e. the chemical

shift interaction (H), dipolar couplings (HD) and the quadrupolar coupling interaction (HQ), is

provided below. In general, these interactions are very important in solid-state NMR of inorganic

diamagnetic solids.

2.1.1. Chemical shift interaction

The chemical shift interaction, including the isotropic chemical shift and chemical shift

anisotropy (CSA), originates from the chemical shielding of the external magnetic field caused

by the local electron distribution surrounding the observed nuclear spin. The Hamiltonian for the

chemical shift interaction between the nuclear spin I and the external magnetic field ( B) is

given by BI = H , where is the chemical shift tensor, which in the Principal-Axis

System (PAS) can be expressed as[70]

iso

σiso

σσiso

zz

yy

xx

σ00

01σ0

001σ

σ00

0σ0

00σ

σ ½½

(2.5)

Here, the PAS elements fulfill the condition |zz iso| |xx iso| |yy iso| and the other

parameters are defined according to


zzyyxxiso σσσ3

1 zzisoσ σσ

σ

yyxxσ

σσ

η (2.6)

iso = iso is the isotropic component, where the chemical shift () is positive to high frequency

(i.e. down-field) and the absolute shielding () is positive to low frequency (i.e. up-field). and

give the magnitude and the asymmetry of the chemical shift interaction, respectively.

Alternatively, the CSA can be characterized by the span ( = 33 11) and skew ( = 3(iso

22)/) parameters, where the PAS elements are chosen as 11 22 33 and iso = 1/3(11 +

22 + 33). In general, the NMR resonances are referenced by their chemical shift values quoted

in parts per million (ppm)

6

ref

refsamplesample 10

ν

νν

(2.7)

This normalization of NMR resonances by standard reference samples (ref) makes it possible to

compare data independent of the externally applied static magnetic field. In liquid-state NMR,

iso is the only detectable component of the chemical shift interaction since the orientation-

dependent CSA is averaged out by molecular tumbling. A similar averaging can be achieved for

solids using Magic-Angle Spinning (MAS)[71]. In this technique, the powder sample is rotated

around an angle of 2arctanθ (~54.736) with respect to the external magnetic field. The

geometrical dependency of the CSA becomes time dependent, where the time-dependent terms

can be averaged out by employing a spinning frequency (R) larger than the width of the

resonance in a static-powder experiment. For lower spinning speeds, the CSA results in a

characteristic spinning sideband (ssb) pattern with an envelope that resembles the lineshape in a

static NMR spectrum[72]; the positions of the spinning sidebands are separated by the value of R.

The magnitude of and , which reflect the local electronic structure of the probed spins, can

be obtained from a simulation of the ssb pattern[73].

2.1.2. Quadrupolar coupling interaction

NMR experiments for nuclear spin isotopes with I > 1/2 exhibit an additional interaction,

the quadrupolar coupling interaction ( IQIQ H ), which is the coupling between the non-

spherical charge distribution of the nucleus and the electric-field gradients (EFGs) at the nuclear


sites created by its surrounding electron density[71]. The quadrupolar coupling tensor in its PAS

is given as

100

01½0

001½

122V00

0V0

00V

122Q

QQ

zz

yy

xx

QhII

C

hII

eQ

(2.8)

where V(x,y,z) is the electric field gradient tensor which in its PAS fulfils |Vzz| |Vxx| |Vyy| and

Vzz = eq is used for the principal EFG tensor element. CQ = e2qQ/h is the quadrupolar coupling

constant, where eQ is the nuclear quadrupole moment, and Q = (Vxx Vyy)/Vzz is the EFG

asymmetry parameter (0 Q 1.0). The quadrupolar coupling can possess a magnitude of

several MHz and thus, it cannot be averaged out by MAS. In general, it is sufficient to describe

the quadrupolar coupling interaction by the two lowest orders of its effective Hamiltonian

obtained by the Magnus expansion[69]

~)(

~)(

~

HHH21

QQeffQ (2.9)

The two terms are usually referred to as the first-order and second-order quadrupolar coupling

interactions. For quadrupolar nuclei with half-integer spin-quantum number, the first-order term

introduces a frequency modulation on the satellite transitions (m ↔ m – 1, where m 1/2)

resulting in a manifold of ssbs while it does not affect the central transition (1/2 ↔ 1/2). The

first-order interaction can in principle be averaged out by MAS, although the achievable

spinning speed are often much to low for a complete averaging. On the other hand, all transitions

are affected by the second-order quadrupolar coupling interaction, which is only reduced by

MAS. For strong quadrupolar couplings, the centerband for the central transition exhibits a

characteristic lineshape with singularities, from which the quadrupolar coupling parameters CQ

and Q can be determined. The quadrupolar coupling parameters may also be obtained from a

simulation of the full spectrum of ssbs observed for the satellite transitions[74].

2.1.3. Dipolar coupling interactions

The dipolar coupling is a through space interaction between two adjacent nuclear spins (I

and S), including two possible cases: (i) homo-nuclear dipolar couplings where I and S are


identical spin nuclei and hetero-nuclear dipolar couplings where I and S are different spins. The

Hamiltonian for these interactions in a tensorial form is given as[71]

SDID H (2.10)

where the second-rank tensor for the dipolar coupling takes the following form in its PAS

d

d

d

00

020

002

D00

0D0

00D

D

zz

yy

xx

//

(2.11)

Here, d 0hIS82r3is the dipolar coupling constant, where 0 is the permeability in vacuum

and r is the internuclear distance for the IS spin pair. It is apparent from equation (2.11) that the

tensor is traceless with the largest PAS element (Dzz) oriented along the dipolar coupling axis.

The absence of an isotropic component implies that dipolar couplings cannot be observed

directly in liquids.

In solid-state NMR, the first-order Hamiltonian for the dipolar coupling, when considered

as a perturbation to the Zeeman interaction, takes the form

SI3SI3νt zzD DH (2.12)

where zzyyxx SISISISI and D is the frequency modulation caused by the dipolar

coupling interaction. For a pair of hetero-nuclear spins (i.e. I S), the Hamiltonian can be further

reduced to

zzD SI2νt H (2.13)

assuming that (L,I – L,S) >> D. This non-zero dipolar coupling interaction results in a

frequency modulation, which in the magic-angle spinning axis system is given by

2βsin αtνcos 2βsin αtν2cos 62

ν R2

RD

d

(2.14)

Here, and are the azimuthal and polar angles describing the orientation of the internuclear

vector between I and S within the MAS frame. As the dipolar coupling constant is inversely

proportional to the cube of the distance between the spins (1/rIS3), the condition of R > D is


often achievable for hetero-nuclear dipolar couplings. However, the dipolar couplings between

high- spins such as 1H and 19F and an observed nuclear spin of lower Larmor frequency can be

very strong and therefore, they are not necessarily averaged out by MAS. Alternatively, different

homo- and hetero-nuclear decoupling sequences can be applied to eliminate or partly reduce the

line-broadening caused by the dipolar couplings on the observed resonances[75-77].

2.2. Solid-state NMR techniques in cement research

2.2.1. 29Si MAS NMR

29Si is probably the most studied nuclear spin for cementitious materials, owing to the

high bulk SiO2 content and the decisive role of the silicate phases in anhydrous as well as

hydrated cements. In general, 29Si MAS NMR experiments are rather time-consuming due to the

low natural abundance of the 29Si spin of 4.7 %. Furthermore, when present in cement phases,

the 29Si spins may exhibit a long spin-lattice relaxation time[78], which usually requires a

relaxation delay of 30 s at 7.1 T. From pioneering 29Si MAS NMR studies of zeolites and

amorphous materials such as glasses and cements[79-82], it has been demonstrated that the 29Si

isotropic chemical shift for silicate mainly depends on the condensation of SiO4 tetrahedra

(Figure 2.1), where an increased condensation (Qi, 0 i 4) corresponds to an up-field shift, i.e.

more negative (29Si) value. In addition, the 29Si isotropic chemical shift reflects the number of

Al atoms incorporated in the second-coordination sphere of the probed silicon sites, denoted

Qi(nAl, 0 n i), for which each Si4+ Al3+ substitution leads to a down-field shift of

approximately 5 ppm. However, this implies that the chemical shift regions for different Qi units

overlap with each other, complicating the interpretation of the detected 29Si resonances.

In accordance with the structure of triclinic tricalcium silicate determined by XRD, the 29Si MAS NMR spectrum of a synthetic sample of this mineral identifies nine distinct silicon

environments with isotropic chemical shifts in the region for isolated SiO4 tetrahedra (Q0)[49],

Figure 2.2. For the monoclinic forms MI and MIII, which are the commonly encountered forms of

alite in Portland cement, 29Si MAS NMR shows broadened lineshapes from the overlapping

resonances of their 18 silicon sites[83], covering the same spectral region. As it can be seen from

Figure 2.2, the MI and MIII forms of alite are distinguishable by their characteristic 29Si MAS

NMR lineshapes. In addition to the alite resonances, the 29Si MAS NMR spectrum of anhydrous

Portland cement contains a narrow resonance at (29Si) = 71.33 ppm from belite in its


form[84]. The isotropic chemical shifts for the Q0 tetrahedra of the alite and belite structures can

be correlated with their corresponding mean SiO bond lengths, according to the linear

relationship[85]

3445731629 .. ÅdSi OSi (2.15)

Figure 2.1 Schematic representation of the chemical shift regions for tetrahedrally coordinated silicon in

silicates[79]. Q0 corresponds to the silicate monomers, Q1 denotes the dimers and chain-end groups, Q2 are the middle

groups in a silicate chain while Q3 are chain-branching silicates and Q4 are cross-linked framework silicates.

The fact that the 29Si resonances for SiO4 tetrahedra with different degrees of

condensation appear in distinguishable spectral regions can be utilized to follow the hydration

process for the silicate phases of Portland cement using 29Si MAS NMR. Generally, the

hydration processes involve condensation of the Q0 units to form dimer (Q1) and polymer (Q2)

silicates. The degrees of hydration for the individual phases (e.g. alite and belite) at each

hydration stage can be obtained by


0

1I

tItH (2.16)

where I0 and I(t) are the normalized 29Si intensities of the actual phase in the anhydrous and

hydrated samples, respectively.

The fraction of each individual phase (e.g. alite, belite and CSH) can be quantified

from a deconvolution of the 29Si MAS NMR spectra[83]. In this project, the characteristic

lineshapes for alite in its monoclinic forms (Figure 2.2) have been simulated satisfactorily by

including nine 29Si resonances. For the CSH phases covering the spectral region from –75

ppm to –90 ppm, the deconvolution includes two resonances at 76 ppm and 79 ppm for the Q1

components and two resonances for the Q2(1Al) and Q2 units at –81 ppm and –85 ppm,

respectively. Additionally, a resonance at 83 ppm has been included, accounting for protonated

Q2 units, which have been identified for CSH samples cured in a CO2 atmosphere (Chapter 4).

Figure 2.2 29Si MAS NMR spectra (7.1 T and R = 7.0 kHz) of synthetic samples of alite in the triclinic (a) and

monoclinic MI (b) and MIII (c) forms. Nine resonances have been observed for the triclinic Ca3SiO5, for which the

rather broad peak at –69 ppm includes two overlapping resonances while the intensity of the peak at –73.44 ppm is

twice as large as the remaining resonances. The diamond () at 71.33 ppm indicates an impurity of belite.


2.2.2. 27Al MAS NMR

27Al MAS NMR is another very commonly applied tool in cement research. In contrast to 29Si, 27Al has a quadrupolar nuclear spin with the spin-quantum number I = 5/2, which in the

presence of an external magnetic field gives rise to six non-degenerated energy levels. The 27Al

MAS NMR spectrum may be rather complex if all transitions are observed; the satellite

transitions of the 27Al spin are substantially broadened by the first-order quadrupolar coupling

interaction. On the other hand, the central transition is only perturbed by the second-order

quadrupolar coupling interaction. Thus, in general, only the 27Al MAS NMR spectrum for the

centerband of the central transition is considered in the analysis of the 27Al MAS NMR spectra.

In ordinary Portland cement, the main part of the bulk Al2O3 content is present in

tetrahedral coordination, which exhibits 27Al resonances in the spectral region from about 40

ppm to 90 ppm at 14.1 T[82,86]. In general, the main aluminate phase (i.e. tricalcium aluminate)

appears as a broad resonance covering from about 40 ppm to 85 ppm. As a result of the high

natural abundance for the 27Al spin of 100 %, it is also possible to detect Al3+ guest ions that are

incorporated in the calcium silicate phases, despite their rather low concentration. The Al3+ guest

ions from alite and belite appear as a rather narrow centerband at about 82 ppm with a shoulder

at 84 ppm, respectively. A small amount of Al2O3 is also present in the ferrite phase, but these 27Al spins cannot be observed due to their strong dipolar coupling with the electron spins of the

Fe3+ ions.

Generally, the hydration of Portland cement involves a conversion of tetrahedral AlO4

into octahedrally coordinated species (e.g. ettringite and monosulphate) and therefore, it may

easily be followed by 27Al MAS NMR[82]. The 27Al resonances from ettringite and monosulphate

appear at approx. 13.0 ppm and 9.0 ppm at 14.1 T, respectively. Furthermore, a resonance from

the tetrahedral Al3+ guest ions in the CSH phases has been observed at about 60 ppm.

2.2.3. 19F MAS NMR

The increasing application of fluoride mineralization in Portland cement production has

been the major motivation for the application of 19F MAS NMR in structural investigations of

cementitious materials in this project. Although several 19F MAS NMR studies are available in

the literature for fluoride structures in cement related materials such as glasses[87-90], only a

single study on Portland cement[91] has been reported earlier. 19F is a spin I = 1/2 nucleus with a

natural abundance of 100 %. Due to the high gyromagnetic ratio of the 19F spin, close to the


value of 1H, the method is very sensitive, enabling fluorine to be detected even when present in

very small concentrations.

Previous studies of several different metal fluoride compounds and fluorine minerals have

demonstrated that the (19F) chemical shift is strongly affected by the type of counter ions[92],

and distributed over a spectral region of about 200 ppm with -PbF2 at high frequency (20

ppm) and NaF on the other low-frequency side at approximately 222 ppm. The (19F) chemical

shifts for different SiF and AlF covalent bonds of silicon and aluminum in octahedral,

tetrahedral and five-fold coordination are predicted to appear in distinct spectral regions[90].

However, the presence of different types of counter ions, such as Ca2+ and Na+, may introduce

additional frequency shifts to the observed 19F nucleus, merging the chemical shift regions for

the different fluorine environments of the central cation coordination states. This is illustrated in

Figure 2.3 by 19F MAS NMR spectra for fluoride with different counter ions.

Figure 2.3 19F MAS NMR spectra (7.1 T, R = 8.0 12.0 kHz) of NaF (a), SrF2 (b), Na2SiF6 (c) and MgSiF6 (d).

Spectra (a) and (b) demonstrate the effect of counter ions on the chemical shift of fluoride ions while (c) and (d)

show the effect of counter ions on fluorine in covalent SiF bonds with the silicon atom in octahedral coordination

state. The asterisk shown in (a) indicates an artifact.


2.2.4. 43Ca MAS NMR

The application of 43Ca NMR spectroscopy has been very limited since just about twenty 43Ca NMR studies of organic as well as inorganic compounds have been reported[93-101]. Only

few of these studies concerns the application of 43Ca MAS NMR in structural investigations of

Portland cement or its related materials[93,94]. The major difficulties in obtaining 43Ca MAS NMR

spectra with an acceptable signal to noise (S/N) ratio are related to the low natural abundance of

the 43Ca spin isotope (0.135 %) and its low gyromagnetic ratio, e.g. its Larmor frequency is only

40.39 MHz at 14.1 T[95]. Furthermore, since 43Ca is a quadrupolar spin (I = 7/2), when present in

asymmetric environments such as in Ca(OH)2, its centerband from the central transition is

severely broadened by the strong second-order quadrupolar interaction. These difficulties are

illustrated in Figure 2.4 for a sample of Ca(OH)2. The 43Ca MAS NMR spectrum was recorded

at 14.1 T using a 7.5 mm PSZ rotor (450 l sample volume). To get a decent signal to noise ratio

(S/N = 11.9), the experiment employed 196,608 scans with a relaxation delay of 2 s,

corresponding to a spectrometer time of 111 hours. The quadrupole coupling parameters ((43Ca)

= 63 ppm, CQ = 2.65 MHz and Q = 0.22) for the 43Ca spins in Ca(OH)2 were determined from a

spectral simulation of the second-order quadrupolar lineshape (Figure 2.4) using the STARS

software package[74]. This strong quadrupolar coupling leads to a broad resonance for the

centerband of the central transition, covering a region of nearly 40 ppm.

Figure 2.4 43Ca MAS NMR spectra (14.1 T) of Ca(OH)2 (a, R = 5.0 kHz) and a synthetic sample of cuspidine

Ca4Si2O7F2 (b, R = 3.0 kHz). The spectra were obtained using a solid /2-pulse of 2 s, a relaxation delay of 2 s

and 196,608 and 128,000 scans, respectively. The simulation of the second-order lineshape for the 43Ca spins in

Ca(OH)2 is shown below the spectrum (a).


Another example, shown in Figure 2.4, is for a synthetic sample of cuspidine

(Ca4Si2O7F2), in which calcium is distributed among four distinct crystallographic sites. The 43Ca MAS NMR spectrum (14.1 T, R = 3.0 kHz) was acquired over 74 hours, i.e. 128,000 scans

for a relaxation delay of 2 s. The cuspidine sample was packed in a thin wall 7 mm Si3N4 rotor

(310 l sample volume). This experiment results in a 43Ca MAS NMR spectrum with S/N =

10.6. Due to the severe overlap of resonances, covering a spectral region from approximately

30 ppm to 30 ppm, in conjunction with the low S/N ratio, the different Ca environments in the

cuspidine structure cannot be distinguished.

Alternatively, for Ca sites with weak quadrupolar couplings such as in CaF2, the Cross-

Polarization sequence may be applied to enhance the sensitivity in the 43Ca NMR experiment.

This is demonstrated in Figure 2.5 for a synthetic sample of CaF2. The standard single-pulse 43Ca

MAS NMR spectrum was recorded with 17,280 scans, corresponding to 73 hours and the

spectrum has a S/N ratio of 58.8. However, a significant increase in the S/N ratio is obtained for

the 43Ca19F CP/MAS spectrum, which has S/N = 140.3 and was recorded with only 6,144

scans (~26 hours).

Figure 2.5 43Ca MAS (a) and 43Ca19F CP/MAS NMR spectra (14.1 T, R = 3.0 kHz) of a synthetic sample of

CaF2. The sample was doped with a small amount of NiO in order to reduce the spin-relaxation time for the 19F and 43Ca spins. The spectra were recorded using a 15-s relaxation delay and 17,280 and 6,144 scans, respectively. The

signal to noise (S/N) ratios in the spectra are 58.8 and 140.3, respectively. The vertical scale is expanded by a factor

of 11.6 in (a) relative to (b).


2.2.5. Inversion-Recovery (IR) MAS NMR

A reliable quantification of the intensities in NMR spectra generally requires that the

spectra are recorded with a relaxation delay of at least d1 5T1, where T1 is the spin-lattice

relaxation time. Determination of the magnitude for T1 can be obtained by the Inversion-

Recovery (IR) pulse scheme[102] (Figure 2.6), where a systematic increase of the recovery time

( results in a series of spectra, for which the observed intensity is governed by the exponential

relation[103]

10 exp11

T

tMtM (2.17)

Here, M0 is the equilibrium magnetization, i.e. M(t = ), and is a constant related to the pulse

imperfections, where = 1 for ideal pulses.

Figure 2.6 Illustration of the Inversion-Recovery (IR) pulse scheme. Systematic increase of the recovery time (

results in a series of spectra, from which the T1 or T1’ relaxation time can be determined from a fit of the

experimental data to equation (2.17) or (2.18).

The IR experiment can also be used to study paramagnetic ions indirectly. For dilute spin

systems, the spin-lattice relaxation time constant of the observed spin (I) may be dominated by

the strong dipolar interactions between the I spin and the free electrons spin (S) of the

paramagnetic ions. The inversion-recovery magnetization for such systems can be fitted to a

‘stretch exponential’ equation[103,104]

'

10 exp11

T

tMtM (2.18)

where the spin-lattice relaxation time, usually denoted by T1’, which in the absence of spin

diffusion is related to the average concentration of paramagnetic ions (NP) by


eI

eII2P

3

1 11

5

2 ,N

9

161

eI SSCC

T)(' (2.19)

I and I are the gyromagnetic ratio and the Larmor frequency of the I spins while e and e are

the gyromagnetic ratio and the spin-lattice relaxation time of the electron spins, respectively.

2.2.6. Cross Polarization (CP)

The CP experiment (Figure 2.7) involves a magnetization transfer from one spin (I) to

another (S) via the IS dipolar coupling obtained by matching the rf-field strengths applied on

the I and S spins[105]. In structural investigations of cementitious materials, the CP experiment

may be useful as a result of two main features[106]. First of all, according to the fact that the

dipolar coupling is inversely proportional to the cube of the internuclear distance (1/r3IS), the CP

pulse sequence can be used as a filter which only detects IS spin pairs with internuclear

distances of usually less than 5 Å. Secondly, CP may be used to enhance the sensitivity for NMR

experiments of nuclei (S) with low gyromagnetic ratio, low natural abundance and/or long spin-

lattice T1 relaxation times, such as 29Si and 13C[107,108]. In this case, the magnetization from an I

spin, typically with a high natural abundance, large gyromagnetic ratio and short T1 relaxation

time (e.g. 1H and 19F), is transferred to the dilute S spin. The sensitivity for the detection of the S

spins in such an experiment is enhanced by a theoretical factor of I/S since the magnetization is

initiated from the I spins. Furthermore, as the relaxation delay depends on the T1 relaxation time

of the I spins, a substantial reduction in the spectrometer time may be achieved.

The magnetization transfer requires that the rf-field strengths ( applied on the I and S

spins in the spin-lock period fulfill the Hartmann-Hahn (HH) matching condition[109,110]:

R1S1I νν11ν11 n)m(m)S(S)m´(m´)I(I (2.20)

where n = 1, 2, R is the spinning frequency, I and S are the spin-quantum numbers of the I

and S spins, and m are the magnetic-quantum numbers defining the transitions between the

Zeeman states (e.g. m m 1).

The HH-matching condition is simply reduced to 1I = 1S nR for spin-systems where

both I and S are spin-1/2 nuclei. If quadrupolar nuclei (S > ½) are involved in the spin system,

the CP transfer during the spin-lock period become rather complex since the satellite transitions

of S are substantially perturbed by the first-order quadrupolar interaction[111]. Moreover, MAS


causes the quadrupolar frequency to oscillate back and forth between Q. Depending on the

magnitude of Q = 3CQ/[2S(2S 1)], the quadrupolar frequency experiences two or four zero-

crossings per rotor cycle. These oscillations result in a population interchange between the

2S + 1 Zeeman states of the quadrupolar nucleus and thereby, a modulation of the HH-matching.

Consequently, the CP transfer from the I to S spins is only efficient for the central transition of

the quadrupolar nucleus. Three regimes have been observed for the CP behavior during the spin-

lock period[112]: adiabatic passage (>> 1), intermediate passage (~ 1) and sudden passage

(<< 1), with the parameter defined by )ν/(νν RQ21Sα . Efficient spin-lock can be achieved

both in the adiabatic and sudden passage regimes, while it is not possible for the intermediate

passage. For I = 1/2, the Hartmann-Hahn matching condition in the adiabatic and sudden passage

regimes are (i) R11I ν½)ν(ν nS S for Q >> 1S and (ii) R11I ννν nS for Q << 1S,

respectively.

Figure 2.7 Cross Polarisation (CP) pulse sequence. The dashed line illustrates the magnetization transfer from the I-

spins to the S-spins during the spin-lock period when the Hartmann-Hahn matching condition is fulfilled.

2.2.7. Forth and Back Cross Polarization (FBCP)

In practice, the standard CP experiment (Figure 2.7) requires that the initiated

magnetization originates from a highly abundant I spin with large since the detection of the

reverse processes from a low- nucleus may require unreasonable long spectrometer time. For

spin systems where both I and S are low- spins, the magnetization can be initiated from a third

nuclear spin, typically 1H, as employed in the Double Cross-Polarization (DCP) experiment[113].

The DCP pulse scheme, which is a modification of the standard CP, consists of two cross

polarization periods: the first period is from the 1H spins to one of the two low-nuclear spins

and the second is cross polarization between the low- nuclear spins.


Figure 2.8 Forth and Back Cross Polarization (FBCP) pulse scheme. In this experiment, the initial 19F

magnetization is transferred to 29Si during the first CP period. This magnetization is stored along the x-axis and

transferred back to the 19F spins by the second CP period. The removal of artifacts is obtained by phase-cycling the

pulses in accordance to the CYCLOPS method. 1 = x –x x –x, 2 = x –x –x x, 3 = x x –x x, 4 = –x x –x x, rec = x

y –x –y.

Figure 2.8 shows an alternative pulse scheme, which can be applied to obtain cross

polarization from a low- nuclear spin to a high- spin. The design of this pulse scheme is

inspired by the work of Wilhelm et al.[114], who utilizes the 1H13C dipolar coupling to study the 1H diffusions for alanine and polyethylene in a 13C1H 2D CP/MAS NMR experiment. The

Forth and Back Cross Polarization (FBCP) experiment was developed to selectively detect the

fluoride guest ions incorporated in the CSH phase (cf. Chapter 4). In analogy to the DCP

experiment, the FBCP experiment consists of two cross polarization periods, although only two

different types of nuclear spins are involved, e.g. 19F and 29Si. As in standard CP, the initial

magnetization is created by employing a /2-pulse on the high-nuclear spin (e.g. 19F). 29Si

magnetization is built up during the first CP period and is being spin-locked along the x-axis for

the rest of the pulse scheme. The transverse 19F magnetization remaining after the first CP period

is converted into longitudinal magnetization using a /2 pulse which is phase shifted by 90

relative to the spin-lock rf field applied on the 19F channel. A short delay () of typically 0.5 ms

is applied after the /2 pulse to ensure a complete dephasing of the transverse 19F magnetization.

During the second CP period, the magnetization is transferred back to 19F from the 29Si spins.

The HH-matching condition for the two CP periods is identical since the same 19F29Si spin

pairs are involved. The experiment is further optimized using the phase cycle given in Figure

2.7.

The 19F29Si FBCP/MAS NMR experiment has been tested on a powder mixture of a

synthetic sample of cuspidine and Teflon. Cuspidine is a rare natural abundant mineral with the

general formula of Ca4Si2O7F2[73,115], which has also been found in steelmaking slags. Its unit

cell consists of two crystallographically distinct SiO4 sites, forming a Si2O7 unit, and two


distinct F sites. Both F ions are located in the vicinity of the Si2O7 unit with SiF distances of

about 4.0 Å. On the other hand, Teflon is a polymer of tetrafluoroethylene with a general

formula of [CF2CF2]n. A single-pulse 19F MAS NMR spectrum of this powder mixture is

shown in Figure 2.9 (a). The 19F resonances originating from cuspidine appear at (19F) = –101.6

ppm and (19F) = –106.1 ppm while the 19FC sites from Teflon exhibit a chemical shift of

(19F) = –121.7 ppm. On the other hand, the 19F29Si FBCP/MAS NMR spectrum recorded for

the same powder mixture only shows the 19F resonances from the dipolar-coupled 19F29Si spin

pairs of cuspidine, Figure 2.9 (b). In order to verify that these resonances are built up due to the

second 19F29Si CP period, a control experiment has been conducted in which the rf-irradiation

on the 19F spins during the second CP period is removed. In this case, the 19F resonances from

Teflon as well as cuspidine are absent in the resulting spectrum, which strongly validates the

forth and back cross-polarization property of the 19F29Si FBCP/MAS NMR experiment.

Figure 2.9 19F MAS NMR (a) and 19F29Si FBCP/MAS NMR (b) spectra of a powder mixture of a synthetic

sample of cuspidine and Teflon. The experiments were performed at 7.1 T using a spinning speed R = 10.0 kHz, a

relaxation delay of 8.0 s, and 4 scans and 18571 scans, respectively. The single-pulse experiment allows 19F

resonances from both cuspidine (–101.6 and –106.1 ppm) and Teflon (–121.7 ppm) to be detected. On the other

hand, the 19F29Si FBCP/MAS NMR experiment provides a selective detection of the 19F spins from the dipolar-

coupled 19F29Si spin pairs of cuspidine.


2.2.8. Rotational-Echo Double-Resonance NMR (REDOR)

Magic-angle spinning is a very important technique in solid-state NMR spectroscopy.

The spinning of the powder sample at the magic angle (54.736 º) can averages out several

anisotropic interactions including the CSA and hetero-nuclear dipolar interactions, providing

high-resolution NMR spectra. However, the anisotropic interactions may contain valuable

information about the local environment of the probed nuclear spins. Thus, several double-

resonance NMR pulse sequences[116-118], including REDOR, have been developed with the aim

of retaining the anisotropic interactions and at the same time preserve the high resolution

provided by MAS.

If the condition of R > D is fulfilled, the modulated dipolar frequency given by equation

(2.14) becomes zero when integrated over a full rotor period (Tr) of MAS. In the REDOR

experiment[117,119], the -pulses applied in the middle of each rotor cycle result in a sign-reversal

of the dipolar frequency in the following half-rotor cycle (Figure 2.10). Consequently, the

effective dipolar coupling becomes finite and gives rise to a dipolar dephasing of the

magnetization; the attenuated signal is denoted by S. The dipolar coupling between the nuclear

spins is monitored through a series of different experiments where the evolution period is

increased systematically in steps of nTr. To account for T2 relaxation, a spin-echo experiment,

i.e. when the I-channel (19F in Figure 2.10) is turned off, is performed on the S-spins (29Si) to

produce the full signal (S0) for each evolution period. The REDOR fraction is obtained by

subtracting the attenuated signal from the full signal (S = S0 – S). A plot of S/S0 as a function

of the evolution period (nTr) gives the universal REDOR curves with distinct slopes and

modulations, reflecting the dipolar coupling constant for the probed I-S spin pairs. The analytical

solution for the dipolar dephasing of an isolated spin-pair, in which both I and S are spin-½

nuclei, can be expressed by an expansion using Bessel functions Jk(x)

αββdS

ΔSsin cos sinnT24cos1 r

0

(2.21)

1k

2 rk2

2 r0

0

nT2J116k

1 2nT2J1 dd

S

ΔS (2.22)

Here, k is the order of the Bessel function[120]. The appearance of the REDOR curve becomes

very complicated when multiple-spin systems (InS) are considered[121]. Furthermore, the

REDOR sequence is only appropriate for studying nuclei with spin-quantum number of 1/2 since


the -pulses are inefficient on the satellite transitions for quadrupolar spins[122,123]. The

quadrupolar coupling also introduces an oscillation of the REDOR fraction as a function of the

evolution time.

The REDOR experiment has been applied with success in structural investigations of a

wide range of crystalline as well as amorphous materials[98,124-126]. This experiment is frequently

used to identify and distinguish between independent InS connectivities. Furthermore, it

emerges as a useful technique for determination of the IS distance of isolated two-spin systems.

Figure 2.10 29Si19F CP-REDOR pulse scheme. The 29Si19F dipolar coupling is re-introduced by a string of -

pulses which are phase-cycled in accordance with the xy-4 sequence[127,128], i.e., (xyxy)n. The -pulses are separated

exactly by one half rotor period. The initial CP period[108] is applied to enhance the sensitivity of the experiment and

to selectively detect the 29Si19F spin pairs.

2.2.9. Rotational-Echo Adiabatic-Passage Double Resonance (REAPDOR)

The REAPDOR pulse scheme[129] (Figure 2.11) is somewhat similar to the REDOR

sequence. However, this experiment has been developed for spin systems that involve

quadrupolar spins. In the experiment, it is utilized that the population of the 2S + 1 Zeeman

states can be interchanged due to the oscillations of the quadrupolar frequency during MAS, cf.

Section (2.2.6). This transfer process is facilitated by a single adiabatic-pulse of length , which

should be shorter than one rotor period (Tr) to keep the oscillations of the quadrupolar frequency

at a low number of zero-crossings[122,130]. In our studies, the REAPDOR experiments have

employed an adiabatic pulse of length TR/3, which maximizes the number of odd zero-crossings

for the quadrupolar frequency. Consequently, the oscillations caused by the quadrupolar

coupling are almost eliminated from the REAPDOR fraction. A recent study of the REAPDOR

sequence shows that the appearance of its S/S0 curve is determined by the spin-quantum

number of the probed quadrupolar nucleus[122]. In general, the dipolar dephasing, reflected by the


REDOR and REAPDOR curves, becomes much complicated for multiple-spin systems.

However, for small dipolar dephasing, S/S0 0.3, the slope of the curves can be approximated

by a second-order polynomial[131,132]

22

2r

0 π

1nT M

IIIS

ΔS

(2.23)

M2 is the dipolar second moment, which reflects the strength of the dipolar coupling

11

π415

4

I

6IS

20222

2

II

r

μγγM

nSI (2.24)

where I is the spin-quantum number of the non-observed nuclear spin and nI is the number of I

spins that contribute significantly to the I-S dipolar coupling. The remaining parameters used in

equation (2.24) have their usual meanings according to standard abbreviations in NMR

spectroscopy[133].

Figure 2.11 29Si27Al REAPDOR pulse scheme. The 29Si27Al dipolar coupling is re-introduced by the adiabatic

pulse on the 27Al channel using a pulse length of 1/3 Tr. The experiment employs 1H decoupling during the dipolar

evolution and the detection periods in order to remove (or reduce) strong hetero-nuclear dipolar couplings between

the observed nuclei and the 1H atoms present in the system.

As mentioned in Section (2.2.1), the assignment of 29Si resonances from standard single-

pulse 29Si MAS NMR may be somewhat uncertain due to the overlap of chemical shifts for

different SiO4 environments caused by the Si4+ Al3+ substitution in their second-coordination

sphere. In this context, the REAPDOR experiment may be an appropriate tool to distinguish


between different SiOAl connectivities and thereby, provides new insight to the interpretation

of the 29Si MAS NMR spectrum[134,135]. From the general structural features of

aluminosilicates[136], it is reasonable to consider an average SiAl internuclear distance (rSi-Al)

for SiOAl bonds in the Q4 network structures since the bond lengths and bond angles of SiO4

and AlO4 tetrahedra only vary slightly for different SiOAl connectivities. This assumption

reduces equation (2.23) to

2r0

nT AlnkS

ΔS (2.25)

where k represents all the constant parameters, including rAl-Si, and nAl is the number of Al atoms

substituted in the second coordination sphere of the observed silicon site.

The applicability of the 29Si27Al REAPDOR experiment is tested for a synthetic sample

of NaY zeolite. The 29Si MAS NMR spectrum of the aluminosilicate network structure of the

zeolite shows four well-separated resonances in the spectral region from –80 ppm to –105 ppm

(Figure 2.12). The difference 29Si27Al REAPDOR spectrum clearly demonstrates that the

resonance at (29Si) = –100.0 ppm is unaffected by the reintroduction of the 29Si27Al dipolar

coupling and therefore, it is assigned to a Q4(0Al) site. The remaining resonances exhibit

REAPDOR curves with distinct slopes, which are illustrated in Figure 2.13. The values of knAl

for each of the resonances were obtained from a fit of the REAPDOR fractions, S/S0 0.3, to

equation (2.24). The resulting values are knAl = 0.16, 0.30 and 0.44 for the resonances at –94.8

ppm, 89.2 ppm and –83.9 ppm, respectively. However, it should be noted that it was necessary

to include a data point for S/S0 > 0.3 in the curve fit for the resonance at –83.9 ppm since it has

only one data point that fulfils the condition S/S0 ≤ 0.3. The corresponding number of Al atoms

in the second-coordination sphere for each silicon site (i.e. nAl = 1, 2 and 3) are obtained by

taking the relative ratios of their knAl values, since k is approximately constant for all Qi(nAl)

components in the network structure. Thus, according to their isotropic chemical shift values in

conjunction with the number of Al in their second-coordination sphere, the resonances at –94.8

ppm, 89.2 ppm and –83.9 ppm are unambiguously assigned to the Q4(1Al), Q4(2Al) and

Q4(3Al) sites, respectively. This assignment is consistent with the structure of NaY zeolite

proposed from earlier studies[80,137].


Figure 2.12 29Si27Al REAPDOR spectra (14.1 T, R = 10.0 kHz, nTr = 1.2 ms) for a synthetic sample of NaY

zeolite: (a) reference spectrum (S0), (b) REAPDOR spectrum (S) in which the 29Si−27Al dipolar couplings have been

reintroduced and (c) the difference spectrum (S0 − S).

Figure 2.13 29Si27Al REAPDOR curves for a synthetic sample of NaY zeolite. The experimental REAPDOR

fractions S/S0 as a function of evolution times (Tr = 0.1 ms) are shown for the resonances at () 83.9 ppm, ()

89.2 ppm, () 94.8 ppm and (♦) 100.0 ppm. The curve fits of the REAPDOR fractions for small dephasing

(S/S0 0.3) using the function S/S0 = knAl(nTr)2 are shown as solid lines.


2.2.10. Multiple-Quantum (MQ) MAS NMR

The MQMAS NMR experiment[138] plays a very important role in several recent

investigations of half-integer spin quadrupolar nuclei. In general, the analysis of NMR spectra

for the central transition of such spin nuclei is complicated by a strong second-order quadrupolar

interaction. This interaction may introduce a severe line-broadening of the observed signals,

causing an overlap of resonances from structurally different sites. On the other hand, in a two-

dimensional (2D) MQMAS NMR experiment, the second-order quadrupolar line-broadening is

effectively removed in the isotropic dimension (F1) whereas it is retained in the anisotropic

dimension (F2). The elimination of the second-order quadrupolar broadening in the MQMAS

experiment significantly enhances the NMR spectral resolution for quadrupolar nuclei, enabling

more complex quadrupolar spin systems to be studied. Several modifications of the MQMAS

experiment have been developed with the aim of improving the sensitivity of the experiment[139-

143]. It has also been applied with success in combination with other rf pulse schemes such as in

the CP-MQMAS to obtain a high-resolution hetero-nuclear correlation (HETCOR) spectrum[144].

This section gives a brief description of the basic two-pulse sequence for the triple-quantum

MQMAS experiment (Figure 2.14). However, the concepts may be applied to any other multi-

quantum excitation schemes.

The triple-quantum MQMAS experiment utilizes the fact that both the single- (1/2

1/2) and the triple- (3/2 3/2) quantum transitions are affected by the second-order

quadrupolar interaction but unaffected by the strong first-order quadrupolar interaction. In the

MAS axis system, the time averaged precession frequency due to the quadrupolar interaction for

such symmetric transitions (m m) is governed by[145]

θ cosβα,ν

θ cosβα,ννθ,,ν

4(4)I

(4)Q

2(2)I

(2)Q

(0)I

(0)QQ

P

P

mC

mCmCmI

(2.26)

1θ3cos2

1θ cos 2

2 P

3θ30cosθ35cos8

1θ cos 24

4 P

where is the angle between the spinning axis and B, and C(i)I are constant factors which

depend of m and the spin-quantum number I. and are the Euler angles describing the

orientation of the quadrupolar PAS relative to the rotor-axis MAS system. P2(cos) and


P4(cos) are the second-order and fourth-order Legéndre polynomials. It can be seen from

equation (2.26) that the geometrical dependency (3cos2 1) of the second-order Legéndre

function can be removed by applying 2arctanθ , i.e. the magic angle. Furthermore, the term

depending on the fourth-order Legéndre polynomial may be eliminated by correlation of two

different symmetric transitions that fulfils the condition

0224

I114

I )t(mC)t(mC )()( (2.27)

In practice, the MQMAS NMR experiment employs an increment of t1 and the signal is acquired

as a function of t2. Therefore, m2 must be 1/2 whereas m1 can be freely selected by applying the

rules of phases cycling. Figure 2.14 shows the basic MQMAS pulse sequence for triple-quantum

excitation (i.e. m1 = 3) with the corresponding coherence transfer pathway for a half-integer

spin quadrupolar nucleus. It can be seen from equation (2.27) that the spin evolution as triple-

quantum coherence under the effect of the second-order quadrupolar interaction for a time t1 is

refocused after a time t2 = t1k (where k = 7/9 and 19/12 for I = 3/2 and 5/2, respectively) after

the conversion of the triple-quantum coherence to single-quantum coherence.

Figure 2.14 Schematic illustration of the pulse sequence and the coherence transfer pathway for a triple-quantum

MQMAS NMR experiment. The selection of the triple- and single-quantum coherences are obtained by the

excitation and mixing pulses, which are phase-cycled in accordance with i = 2/2N, where is the phase of the

pulse and N is a multiple of p. The triple-quantum coherences excited by the first pulse are evolved for a time t1

before these transitions are converted to single-quantum coherence by the second pulse. The acquisition begins

immediately after the second pulse to gain the best S/N ratio, although the echo occurs at a time t2 = t1k (where k =

7/9 and 19/12 for I = 3/2 and 5/2, respectively) after the second pulse. Furthermore, the phase sensitivity in the

indirect dimension F1 is conducted by the hyper-complex phase-cycling method[139].


The advantage of the MQMAS NMR experiment is illustrated for a sample of gibbsite, -

Al(OH)3 (Figure 2.15). The projection in the F2 dimension, under ideal conditions, corresponds

to a standard single-pulse 27Al MAS NMR spectrum, in which the second-order quadrupolar

line-broadening has not been removed. The interpretation of this spectrum can be somewhat

uncertain since the observed second-order lineshape can potentially be misinterpreted as an

overlap of several resonances. On the other hand, the projection of the F1 dimension in

conjunction with the contour plot clearly identify two well-separated resonances, where the

shoulders at 4 ppm, –3.5 ppm and 9 ppm seen in the F2 dimension correspond to the

singularities and shoulders for the Al site with the strongest quadrupolar coupling. The

observation of two distinct crystallographic Al sites by MQMAS NMR is consistent with the

crystal structure for gibbsite determined by XRD[146,147].

Figure 2.15 27Al MQMAS NMR spectrum (9.4 T, R = 10.0 kHz) of gibbsite, -Al(OH)3. The spectrum was

recorded using the three-pulse z-filter sequence and a 144-steps phase cycle. Furthermore, 1H decoupling (TPPM)

was employed during both the evolution of the triple-quantum coherence (t1) and the detection (t2) periods. The

asterisks (*) indicate spinning side bands in the F1 dimension. Projections onto F1 and F2 dimensions,

corresponding to summations, are shown on the left side and above the 2D spectrum, respectively.

Chapter 3. Fluoride Mineralization 49

3. Chapter

Fluoride Mineralization

Since its first application in clinker preparation, probably in the late 1800s by Michaelis,

fluoride mineralization has got much attention in cement research[41,42,148]. Due to its high natural

abundance, CaF2 (e.g., fluorspar) is by far the most widely used fluoride mineralizing agent in

modern cement production. However, other fluoride-containing compounds such as NaF2, MgF2,

Na2SiF6 and MgSiF6 have also shown similar properties[25,149]. In addition to the mineralizing

properties, fluoride also exhibits fluxing properties and seems to be involved in several chemical

reactions during clinker formation. At a kiln temperature of about 1100 C, fluoride reacts with

the raw meal components forming a clinker melt[150,151], largely consisting of

11CaO7Al2O3CaF2. As belite and lime are brought together within the liquid phase, alite is

slowly formed. The alite formation accelerates rapidly as the kiln temperature is raised just

above 1200 C; approximately 40 % of alite is already formed at 1200 C and about 60 % at

1300 C. At kiln temperatures above 1300 C, 11CaO7Al2O3CaF2 decomposes to tricalcium

aluminate and CaF2. It has been demonstrated in earlier studies of synthetic alite that a small

amount of fluoride can be incorporated in this phase, most likely in accordance with an O2

F substitution. Charge balance can be preserved by the incorporation of a similar amount of

Al3+ ions substituting for Si4+ in the alite structure, tentatively forming a solid solution with the

chemical formula Ca3[Si1-xAlx][O5-xFx], where the upper limit for x is approximately 0.15[9].

This chapter presents the results from a study of the mineralizing effect of fluoride on the

formation of the calcium silicate phases of Portland cement. Furthermore, the site preference of

F and its coupled substitution mechanism with Al3+ and Fe3+ ions have been investigated using

solid-state NMR. Finally, the results are applied in an optimization of the alite content in

Portland clinker.


3.1. Site preferences of F ions in the calcium silicate phases of Portland cement

3.1.1. 19F MAS NMR

The 19F MAS NMR spectrum of a commercial white Portland clinker (wPc) containing

only 0.04 %w F is shown in Figure 3.1. The spectrum exhibits a characteristic lineshape

covering a spectral region from about –100 ppm to –125 ppm, which can only be satisfactorily

simulated by including at least three 19F resonances with (19F) = –122.1 ppm, –116.5 ppm and

–111.4 ppm (Figure 3.1 b-c). 19F MAS NMR spectra of modified clinkers prepared from wPc,

but containing higher F and Al2O3 contents, show somewhat similar lineshape features with a

centre of gravity at (19F) 114.9 ppm, cf. Figure 3.10. This indicates that all F atoms occur in

similar structural environments, but in a less-ordered arrangement. Since the values of (19F) for

fluoride ions cannot be distinguished from those for SiF and AlF covalent bonds, due to the

strong effect of different types of counter ions on the 19F chemical shiftss, an exact identification

of the fluorine environments in Portland clinker cannot be extracted from the 19F chemical shift

alone. However, their chemical shifts exclude them from being considered as the crystalline

phases Ca4Si2O7F2 ((19F) = –101.6 ppm and –106.1 ppm), CaF2 ((19F) = –105.9 ppm) or the

alumina-rich phase 11CaO7Al2O3CaF2, which exhibits two different 19F resonances with a

large spinning sideband intensities resulting from the CSA interactions. Furthermore, recent

studies of aluminosilicate glasses demonstrated that AlF and SiF covalent bonds are only

found in glasses with much higher F contents as compared to that in Portland clinker[152,153].

Figure 3.1 19F MAS NMR spectrum (7.05 T, R = 10.0 kHz) of a commercial white Portland clinker (wPc)

containing 0.04 %w F. The experimental spectrum (a) can be simulated, shown in (b) and (c), by three resonances at

(19F) = –122.1 ppm, –116.5 ppm and –111.4 ppm.


3.1.2. 29Si19F and 27Al19F CP/MAS NMR

The incorporation of fluoride ions in the calcium silicate phases of Portland clinker has

been investigated for a series of fluoride-mineralized clinkers using 29Si19F and 27Al19F

CP/MAS NMR. Since the magnetization is transferred through the dipolar coupling (d 1/rIS3),

the CP period acts as a filter which only allows the 29Si and 27Al spins that are located in the near

vicinity to a 19F spin to be detected. The studied clinkers were modified from a commercial

white Portland clinker to have a high Al2O3 bulk content of 4.3 %w, a lime saturation factor

(LSF) of 0.90 and fluoride contents in the range of 0.04 1.0 wt. % F. The bulk fluoride

contents in selected samples have been measured with an ion-selective electrode, after the

samples was dissolved in a mixture of HCl and KAl(SO4)2. These clinkers are used as reference

samples in the quantification of the fluoride content from 19F MAS NMR experiments for the

remaining samples. Furthermore, the free lime content in all modified clinkers has been

measured. The experimental details for sample preparations and analyses are given in Appendix

1 and 2, respectively.

Figure 3.2 29Si19F CP/MAS NMR spectra (7.1 T) of three selected fluoride-mineralized clinkers containing 0.23

%w F (a), 0.47 %w F (b) and 0.77 %w F (c). The spectra were recorded with a spinning speed of R = 3.0 kHz and a

2.0 ms CP contact time. The overall lineshape observed in these spectra resembles closely the 29Si MAS NMR

spectrum of alite in its monoclinic MIII form, see Figure 2.


29Si19F CP/MAS NMR spectra of selected modified clinkers are shown in Figure 3.2.

The absence of the belite resonance at (29Si) = 71.33 ppm in these spectra demonstrates that

the fluoride ions are only incorporated in the alite phase. Furthermore, they all exhibit broadened

lineshapes with features resembling closely that of alite in the MIII form. In contrast to this work,

most of the previous studies on the structure of fluoride-mineralized alite propose that this phase

is stabilized in its rhombohedral form[9,154,155]. However, the polymorphic form of the alite phase

when present in real Portland cement may as well be controlled by several other factors[26] such

as the presence of other impurities, the cooling rate, etc.

As considered earlier, the O2 F substitution should be charge-balanced by a similar

amount of Al3+ guest ions substituting into the tetrahedral silicon sites of alite. The location of

these Al3+ guest ions are, of course, important for the thermodynamic properties of the fluoride

mineralization since a random distribution of F and Al3+ guest ions in alite yields the highest

possible number of atomic configurations and thereby, maximizes the entropy of mixing for this

phase. However, a random distribution of F and Al3+ guest ions will create local regions with

formula units (Ca3SiO4F)+ and (Ca3AlO5), which are probably energetically unfavorable. On the

other hand, the charge balance can be preserved locally if Al3+ guest ions are located in the

proximity to the fluoride ions. Consequently, the incorporation of these Al3+ guest ions will not

affect the entropy of mixing for the alite phase significantly. 27Al MAS and 27Al19F CP/MAS

NMR spectra of a modified clinker containing 0.77 %w F are shown in Figure 3.3. The single-

pulse spectrum (Figure 3.3) shows a sharp 27Al resonance at approximately 75 ppm, which

originates from the tetrahedrally coordinated Al3+ guest ions in the calcium silicate phases. In

addition to the fact that fluoride is only incorporated in the alite structure, the appearance of the

resonance at (27Al) ~75 ppm in the 27Al19F CP/MAS spectrum (Figure 3.3 b), reveals that the

fluoride ions are located in the near vicinity of the Al3+ guest ions. Furthermore, the HH-

matching condition of F = 3Al + R applied in this experiment to achieve magnetization

transfer reflects that the observed 27Al spins possess a large quadrupole coupling constant, which

is consistent with the values reported previously for Al3+ guest ions in the alite phase. The

observation clearly clarifies the essential role of an aluminum source in fluoride mineralization

as proposed by Shame and Glasser[9], since charge balance is achieved locally by a coupled

incorporation of F and Al3+ ions.


Figure 3.3 (a) 27Al MAS and (b) 27Al19F CP/MAS NMR spectra (7.1 T and R = 5.0 kHz) of a fluoride-

mineralized clinker containing 0.77 %w F. The magnetization transfer from the 19F spins to the 27Al spins is

obtained using a Hartmann-Hahn matching condition of F = 3Al + R and a contact time of 1.5 ms. The asterisks

(*) indicate spinning sidebands.

3.1.3. 29Si19F CP-REDOR NMR

The structures of the alite polymorphs include two different crystallographic oxygen sites,

as described in section 1.2.1: the covalently bonded oxygen in SiO4 tetrahedra (SiOb) and the

interstitial oxygen ions (SiOi). Their corresponding average SiO internuclear distances (rSi-O)

are 1.63 and 4.32 Å, respectively, for the monoclinic MIII structure. The mineralizing properties

of fluoride ions may strongly depend on which types of those oxygen sites that are accessible for

O2 F substitution. In order to elucidate the site preference of fluoride ions in the alite

structure, the average internuclear distance of the SiF spin pairs has been measured using 29Si19F CP-REDOR NMR. The initial CP period acts as a 29Si19F filter by only detecting the 29Si nuclear spins which are dipolar coupled to 19F, i.e., roughly rSiF < 5 Å. Subsequently, the

dipolar re-coupling ability of the REDOR sequence allow a measurement of the 29Si19F dipolar

coupling constant, from which the average SiF internuclear distance can be calculated.

Furthermore, the CP period enhances the sensitivity of the experiment since the magnetization is

initiated from the 19F spins, which have a high gyromagnetic ratio and a short relaxation delay of

only 8 s.


Figure 3.4 29Si magnetization (Mx(t) given in arbitrary units) as a function of the CP contact time obtained from

29Si19F CP/MAS NMR experiments (7.1 T, R = 5.0 kHz) for a modified clinker containing 0.77 wt.% F () and

for cuspidine (). The experiments employed rf fields of SiB1/2 = 47.5 kHz and FB2/2 = 42.1 kHz, and a

relaxation delay of 8 s. The solid curves illustrate the optimum fits to the experimental data for the clinker and

cuspidine to equation (3.1) and (3.2), respectively.

The cross-polarization dynamics curves for a modified clinker containing 0.77 %w F and

a synthetic sample of cuspidine are shown in Figure 3.4. Considering the cross-polarization

curves for the applied contact times (t 8.0 ms), the cross-polarization time constant (TSiF),

which reflect the strength of the SiF dipolar couplings, may be obtained by fitting the

experimental data to[106]

Mx(t) = [M0/ TSiF/T1,F exp(tcp/T1,F) [1 exp((1/ T1,F 1/TSiF) tcp)] (3.1)

and

Mx(t) = M0[1 exp( tcp/TSiF)] (3.2)

for the clinker and cuspidine, respectively. In equation (3.1), it is assumed that the 29Si rotating-

frame relaxation time is at least an order of magnitude larger than the corresponding value for 19F (i.e., T1,Si >> T1,F). On the other hand, equation (3.2) assumes that both T1,Si and T1,F are

much larger than TSiF. M0 is the 19F magnetization immediately after the initial 90º pulse on the 19F-channel and tcp is the CP contact time. An optimized fit corresponding to the parameters TSiF


= 4.61 ms and T1,F = 4.57 ms has been obtained for the modified clinker. On the other hand, the

SiF2 three-spin spin system of cuspidine, with SiF distances of 3.88 Å and 4.00 Å, exhibits a

CP time constant of only 1.20 ms. Owing to the low natural abundance of the 29Si nuclear spins

(4.7 %) and the low fluoride content (0.77 %w) of the Portland clinker, it is reasonably to

consider the 29Si19F spin-pairs as isolated. In the absence of spin diffusion, the SiF spin

system may be approximated by a rigid lattice model, where the cross-polarization time constant

TSiF becomes inversely proportional to the dipolar second moment M2, whose magnitude

depends on the SiF distance ( 6SiFr ) and the number of F atoms involved in the dipole coupling

(see equation 2.24). According to this assumption, the factor of nearly four between the values of

the CP time constant for the SiF spin system in Portland clinker and the three-spin system,

SiF2, in cuspidine indicates that the rSiF distances in alite is slightly longer than that for SiF in

cuspidine structure.

Results from the 29Si19F CP-REDOR experiment are summarized in Figure 3.5 and 3.6.

The experimental data are plotted along with their optimized simulation and two simulated

universal REDOR curves for the average SiOb and SiOi distances in monoclinic alite (Figure

3.6). As a result of the severe overlap of resonances and the low signal to noise ratio of the

REDOR spectra (Figure 3.5), it has not been possible to distinguish between the different 29Si

resonances from the eighteen crystallographic silicon sites of monoclinic alite. Therefore, they

have been considered to be equal in the analysis of the REDOR spectra. A dipolar coupling

constant of d = 285 Hz, corresponding to an average internuclear distance of 4.29 Å for the

isolated SiF spin-pair in alite, was obtained from a numerical fitting of the experimental

REDOR fractions to equation (2.21), see Appendix 2. The magnitude of the SiF internuclear

distance as well as the slopes of the REDOR curves shown in Figure 3.6 clearly demonstrate that

the fluoride anions are located in the interstitial oxygen site, i.e., the fluoride anions not involved

in covalent SiF bonds. These data are obtained for an optimized CP contact time of 3.0 ms

(Figure 3.4). However, selected REDOR experiments for CP contact times of 1.0 and 7.0 ms

give S/S0 fractions of the same order of magnitude as observed for the 3.0 ms contact time.

The experimental observation for the fluoride environments in the alite phase of Portland

cement is consistent with a recent theoretical investigation of the coupled incorporation of F and

Al3+ guest ions in the alite phase applying Density Functional Theory (DFT) calculation and

structure optimizations[156]. These calculations of energies for the Si4+ + O2 Al3+ + F

substitution propose that the F ion is preferentially substituted for the interstitial oxygen site,

since it requires a much lower substitution energy as compared to the value for the formation of

SiF covalent bonds.


Figure 3.5 29Si19F CP-REDOR NMR spectra (7.1 T, R = 10.0 kHz) of a fluoride-mineralized Portland cement

containing 0.77 %w F. (Left) Full signal (S0), which is not affected by the SiF dipolar coupling. (Right) The

attenuated signals (S), reflecting the intensity reduction caused by the SiF dipolar couplings. The evolution periods

are (a): 4 Tr, (b): 16 Tr, (c): 28 Tr and (d) 40 Tr, where Tr = 0.1 ms.

Figure 3.6 Experimental REDOR fractions (S/S0) for the 29Si–19F spin-pairs in the alite phase of a modified

Portland clinker (0.77 %w F). The experiments employ CP contact times of 1.0 ms (o), 3.0 ms () and 7.0 ms (x).

Numerical fitting of the experimental data (solid line) results in a rSi-F distance of 4.29 Å. The numerical simulations

for rSi-F of 1.6 Å and 4.3 Å are illustrated by the dashed lines, corresponding to the mean Si–O bond length of the

SiO4 tetrahedra and the mean distance for the interstitial oxygen atoms (Oi) to their nearest Si atoms, respectively.


3.2. Mineralizing effects of calcium fluoride and calcium sulphate

The main effect of fluoride mineralization may be addressed to the fact that the coupled

incorporation of F and Al3+ ions increases the entropy of mixing ( )ln(kmix ΩS ) for the alite

phase. However, due to the local charge-preserving effect of F and Al3+, only the concentration

of F contributes effectively to the increase in mixing entropy, where the total number of

possible atomic configurations[157] is given by )!!/(!Ω OFalinterstiti nnn ; ninterstitial is the total

number of the interstitial sites, i.e. nine in the triclinic unit cell, Ca27Si9(Ob)36(Oi)9, while nF and

nO are the numbers of the interstitial sites occupied by F and O2, respectively. Thus, the

entropy of mixing associated with fluoride mineralization for the alite phase has its maximum

when nF = 4.5, corresponding to the incorporation of 4.1 %w F in alite, or x = 0.1 for the formula

Ca3[Si1-xAlx][O5-xFx]. However, this content is much lower as compared to the fluoride level of x

= 0.15, reported in a previous study using the selective chemical dissolution method[9]. One

possible explanation for this discrepancy is that at high fluoride contents, the F atoms may also

form AlF and/or SiF covalent bonds. At sufficiently low fluoride levels, the ethalpy (H) for

the alite phase may not be affected by the presence of fluoride ions. On the other hand, the Gibbs

free energy (G) for alite formation from belite and lime will be more negative[157]

TΔΔΔ SHG (3.3)

SHG , , Δ limebelitealite ,

where T is the temperature at which the reaction takes place. Alternatively, for complete reaction

between belite and lime, the amount of uncombined CaO, i.e. the free lime content, can be used

to monitor the thermodynamic stability of alite relative to belite. For a given burning condition, a

high free lime content indicates that belite is more favorable as compared to alite whereas a low

free lime content indicates the reverse.

Calcium sulphate is another widely use mineralizing agent in Portland clinker

production[42,148,158], either alone or in combination with CaF2. In contrast to F, the S6+ ions are

preferentially incorporated in the belite structure[38], in accordance with the tentative coupled

substitution mechanism, 3Si4+ S6+ + 2Al3+. The mineralizing effect of CaSO4 has been

reexamined for a series of modified clinkers prepared from raw meals including 4.3 %w Al2O3

(~12 %w tricalcium aluminate), a LSF of 1.0 and SO3 contents in the range 0 3 %w. The 29Si

MAS NMR spectra for these samples are shown in Figure 3.7. It is apparent from the spectra that

the belite resonance exhibits an increasing line-broadening as a function of the SO3 content,


supporting the assumption of an increased incorporation of S6+ in the belite structure.

Furthermore, in addition to an increase in the belite intensity with increasing SO3 content (Figure

3.7), the amount of uncombined CaO is increased rapidly as a function of the SO3 content,

Figure 3.8. This clearly demonstrates that the presence of a SO3 source in the raw meal increases

the stability of the belite phase and thereby, reduces the quantity of alite. It is also apparent from

Figure 3.8 that SO3 has a rather high volatility since the actual SO3 contents are much lower than

the SO3 amount added to the corresponding raw meals.

In a similar way, the mineralizing effect of calcium fluoride is demonstrated for a series

of modified clinkers prepared from raw meals including a LSF of 1.0, 4.3 %w Al2O3, 3.0 %w

SO3 (the actual SO3 content in the clinkers is approximately 2.3 %w) and fluoride contents in the

range 0.04 1.0 %w F. The strong mineralizing effect of calcium fluoride is recognized by the

rapid decrease in the amount of uncombined CaO as a function of the fluoride content, Figure

3.8. It can be seen that the stability of SO3-mineralized belite is effectively reduced by the

addition of calcium fluoride. It only requires 0.6 %w F to retain the free CaO content at ~0.4

%w, i.e. the same level of uncombined CaO as in the clinker without using any mineralizing

agents. The mineralizing effects of fluoride and SO3 on alite and belite, respectively, are

graphically illustrated in Figure 3.9.

Figure 3.7 29Si MAS NMR spectra (7.1 T, R = 7.0 kHz) of SO3-mineralized clinkers prepared from a commercial

white Portland clinker. The actual SO3 content in the clinkers is 0.48 %w SO3 (a), 1.09 %w SO3 (b) and 1.91 %w

SO3 (c). Furthermore, the preparation of the clinkers used a LSF of ~1.0 and a bulk Al2O3 content of ~4.3 %w.


Figure 3.8 Mineralizing effects of CaSO4 and CaF2 reflected by the quantity of uncombined CaO (free lime) in the

clinkers as a function of the SO3 and F contents, respectively. (Left) The graph shows the free CaO content () and

the actual SO3 content (♦) in the mineralized clinkers (LSF ~1.0, 0.04 %w F) as a function of the SO3 quantity added

to the raw meal. (Right) Free lime content as a function of the actual fluoride content in the mineralized clinkers

(LSF ~1.0, 2.3 %w SO3). The actual SO3 contents of the clinkers have been determined by X-ray fluorescence.

Figure 3.9 Graphical illustration of the effects of fluoride and SO3 mineralization on the thermodynamic stability of

alite and belite, respectively.


3.3. Secondary effects of fluoride mineralization

In order to explore non-thermodynamic effects of fluoride mineralization, a series of

clinkers prepared from raw meals containing 4.3 Al2O3 (~12 %w tricalcium aluminate), a LSF of

0.9 and fluoride contents in the range 0.04 1.0 %w have been investigated. Experimental

details including the sample preparation and analysis are given in Appendices 1 and 2. The 19F MAS NMR spectra of the clinkers show a single broad resonance with a centre of

gravity at 114.9 ppm, Figure 3.10. However, the lineshape exhibits some variation as a result of

the increased fluoride content. This observation confirms that for low concentrations (> 0.77

%w) the fluoride ions are only incorporated in the interstitial oxygen sites of alite, which is

consistent with earlier discussed investigations[10,150], since excess phases of CaF2 ((19F) =

–105.9 ppm), cuspidine ((19F) = –101.6 ppm and –106.1 ppm) or 7CaO11Al2O3CaF2 (two 19F

sites with large CSA’s) are not formed in these systems. However, due to the moderate volatility

of CaF2, the actual fluoride contents in the clinkers are somewhat lower than that used in the

clinker preparation.

Figure 3.10 19F MAS NMR spectra (7.1 T, R = 10 kHz) of selected fluoride-mineralized clinkers modified from a

commercial white Portland clinker to have a high bulk aluminate content of 4.3 %w (~12 %w tricalcium aluminate)

and fluoride contents of 0.04 (a), 0.23 (b), 0.36 (c) and 0.77 %w F (d). The broad resonance with a centre of gravity

at (19F) 114.9 ppm indicates that the fluoride anions have nearly identical structural environments, but they are

present in a less-ordered arrangement.


Figure 3.11 27Al MAS NMR spectra (14.1 T, R = 13.0 kHz) of selected fluoride-mineralized clinkers modified

from a commercial white Portland clinker (0.04 %w F, 2.13 %w Al2O3) to have a relatively high bulk aluminate

content of 4.3 %w (~12 %w tricalcium aluminate) and fluoride contents of 0.04 (a), 0.32 (b), 0.48 (c), 0.69 (d) and

0.77 %w F (e). The asterisks (*) indicate the spinning sidebands for spectrum (a) while the diamond () marks the 27Al centerband from an AFt phase (ettringite), which most likely reflects a small degree of pre-hydration.

The correlation between the incorporation of fluoride and aluminum ions in alite is

apparent from the 27Al MAS NMR spectra of the selected samples shown in Figure 3.11. The

increasing intensity for the 27Al resonance at ~82 ppm illustrates that the increased incorporation

of fluoride ions in alite is accompanied by an increase in the quantity of Al3+ guest ions in this

phase. On the other hand, the 27Al resonance from the tricalcium aluminate phase, which appears

as a broad lineshape from approximately 30 ppm to 80 ppm, exhibits a decrease in its intensity


as the fluoride content is increased, indicating that this phase is partially consumed by the

increased incorporation of Al3+ ions in alite. Therefore, the coupled substitution of F and Al3+

ions in alite may be described by the reactions

Ca27Si9(Ob)36(Oi)9 + 0.5xCaF2 [Ca27Si9(Ob)36(Oi)9-x(Fi)x]x+ + 0.5xCaO (3.4)

[Ca27Si9(Ob)36(Oi)9-x(Fi)x]x+ + 0.5x3CaOAl2O3

Ca27Si9-xAlx(Ob)36(Oi)9-x(Fi)x + xSiO2 + 1.5xCaO (3.5)

xSiO2 + 2xCaO x2CaOSiO2 (3.6)

The reactions only consider the incorporation of F and Al3+ guest ions, assuming that no other

guest ions are present in the solid solution. The alite phase is expressed with the formula for one

unit cell of its triclinic polymorph[20], in which it is discriminated between the covalently bonded

(SiO) and interstitial (non-bonded) oxygen sites. In fact, the presence of fluoride may also

results in incorporation of Al3+ ions in the alite phase by other mechanisms since Figure 3.11d

indicates that the majority of the bulk Al2O3 content of 4.4 %w (~ 8.610-2 mol Al3+) is

incorporated in this phase. On the other hand, the fluoride content in this sample only

corresponds to 1.810–2 mol F.

The distribution of Si atoms in the calcium silicate phases, quantified by the molar ratio

of Si in alite and belite, nSi[alite/belite], is achieved from the 29Si MAS NMR spectra using a

deconvolution procedure described previously. Although the broad sub-spectra for MI and MIII

alite originate from an overlap of resonances from their eighteen different crystallographic

silicon sites, a satisfactory simulation can be achieved by only including nine different 29Si

resonances, Figure 3.12. In this work, the MI lineshape has only been employed for the reference

clinker shown in Figure 3.12, which contains 0.04 %w F. The deconvolutions of the 29Si MAS

NMR spectra for the remaining fluoride-mineralized clinkers include a sub-spectrum of alite in

its MIII form, with small modifications only at the tails of the overall peak. The nSi[C3S/C2S]

molar ratio, resulting from the deconvolutions, as a function of the actual fluoride content is

illustrated in Figure 3.13. This graph clearly demonstrates that for a fixed chemical composition

an increased incorporation of fluoride ions in alite tends to decrease the alite to belite molar

ratio. However, this decrease in nSi[alite/belite] does not reflect a decrease in the actual alite

content since the quantity of uncombined CaO in these clinkers is rather constant, i.e. approx.

0.4 %w free CaO.


Figure 3.12 Simulations of 29Si MAS NMR spectra for two modified clinkers with LSF ~0.9, 4.3 %w Al2O3 and

0.04 %w F (left) and 0.32 %w F (right), prepared from a commercial white Portland clinker. (Left) The spectrum

simulation (b) includes one resonance at (29Si) = 71.33 ppm for the belite phase (c) and a broad sub-spectrum of

alite in its MI form (d). (Right) For the fluoride-mineralized clinker (a), a sub-spectrum of the MIII form of alite is

employed (d). Both simulations include only nine different 29Si resonances for the sub-spectra of alite.

The results from the 19F, 27Al and 29Si NMR experiments demonstrate first of all that the

incorporation of fluoride ions in the interstitial oxygen sites of alite increases the amount of Al3+

guest ions substituting for Si4+ on the tetrahedral sites, for which the tricalcium aluminate phase

acts as the aluminate source for the fluoride mineralization, equation (3.4) and (3.5). The

coupled substitution of F and Al3+ guest ions stabilizes local regions with the composition

Ca27Si9-xAlx(Ob)36(Oi)9-x(Fi)x in the alite structure. Secondly, belite is formed from the formally

released quantities of SiO2 and CaO, equation (3.6), according to the fact that “free SiO2” has not

been observed in the 29Si MAS NMR experiments and that the free CaO content in the clinkers is

constant. Thus, the decreased nSi[alite/belite] molar ratio as a function of the fluoride content

may account for the Si4+ Al3+ substitution on the tetrahedral sites of alite and an increase in

the quantity of belite formed from the released SiO2 at the same time. Moreover, equations (3.4

3.6) reveal that the alite content is almost unaffected by the quantity of fluoride whereas the

chemical composition of this phase is significantly changed by the fluoride mineralization. This

secondary effect may be compensated by additional CaO according to the fact that fluoride

mineralization increases the thermodynamic stability for alite only, Figure 3.8.


Figure 3.13 The molar ratio of Si atoms in the alite and belite phases, nSi[alite/belite], for the fluoride-mineralized

clinkers. The graph is based on data obtained from deconvolutions of the 29Si MAS NMR spectra.


3.4. Incorporation of Fe3+ ions in the calcium silicate phases of Portland cement

The iron content in ordinary Portland cement is typically around 3 %w Fe2O3, but much

lower in white Portland cement. The main constituent iron phase is ferrite (Ca2(AlxFe1-x)2O5),

although a small amount of Fe3+ may also be present as guest ions in tricalcium aluminate and

the calcium silicate phases[29,31,159]. Thus, these Fe3+ guest ions may be involved in the fluoride

mineralizing process in a similar manner as the Al3+ ions. Considering the 27Al MAS NMR

spectrum of the modified white clinkers shown in Figure 3.11 (e), it can be seen that the majority

of Al2O3 must be incorporated in the alite phase, partly associated with the fluoride

mineralization. Following this result, a series of fluoride-mineralized clinkers has been prepared

from raw meals containing 1.0 %w F and a LSF of 0.9. Furthermore, the bulk Al2O3 content of

4.3 %w is partly replaced by additional Fe2O3, targeting Fe/Al molar ratios in the range of 0.05

0.8. The corresponding bulk Fe2O3 contents are in the range of 0.3 4.0 %w. The potential of

Fe3+ ions in replacing Al3+ in the fluoride mineralizing process has been investigated for this

series of modified clinkers using 29Si MAS NMR, 29Si IR NMR and XRD. This study also

reveals the influences of an increased incorporation of Fe3+ ions in the calcium silicate phases of

Portland cement, with the main findings presented below.

Figure 3.14 29Si MAS NMR spectra (7.05 T, R = 5.0 kHz) of two modified clinkers (LSF ~0.9, 0.9 %w F)

containing 0.38 %w Fe2O3 (a) and 3.33 %w Fe2O3 (b). The spectra demonstrate that the 29Si resonances from belite

as well as alite are strongly affected by the increased bulk Fe2O3.


The 29Si MAS NMR spectra of two modified clinkers with different bulk Fe2O3 contents,

shown in Figure 3.14, clearly demonstrate that the presence of paramagnetic Fe3+ ions in

Portland cement introduces a significant line-broadening and intensity loss on the observed

nuclear spins. Recently, it has been demonstrated that the spin-lattice relaxation process for the 29Si spins in the calcium silicate phases of ordinary Portland cements is predominantly affected

by the Fe3+ ions incorporated as guest ions in these phases[78], rather than by the Fe3+ ions present

in the separate ferrite phase as proposed earlier[160]. This observation been reexamined for two

modified Portland clinkers containing 4.3 %w Al2O3, LSF ~ 1.0 and Fe2O3 contents of 1.5 %w

and 2.0 %w. The aluminate and ferrite phases were removed from the clinkers using a selective

dissolution method[161]. The validity of this method is supported by the absence of the

2reflexions at ~33.2 and ~33.8 in the XRD pattern of the clinkers after selective dissolution,

Figure 3.15. On the other hand, the 29Si IR NMR spectra shown in Figure 3.16 for the same

clinkers before and after selective dissolution show no significant differences, which

demonstrates that the ferrite and aluminate phases only have a small effect on the 29Si relaxation.

0

2000

4000

6000

8000

10000

12000

14000

16000

28 29 30 31 32 33 34 35

2

Arb

itra

ry U

nit

C3S

C3A

C4A

F

C3S

C3S C

3S

C3S

Figure 3.15 XRD diffractograms of a modified clinker (4.3 %w Al2O3, ~2.0 %w Fe2O3 and LSF ≈ 1.0). The XRD

pattern for the original clinker is shown by the solid line while the dashed line corresponds to the pattern after

selective dissolution of the aluminate (C3A) and ferrite (C4AF) phases.


Figure 3.16 29Si IR NMR spectra (7.1 T, R = 5.0 kHz) of a modified clinker (4.3 %w Al2O3, ~2.0 %w Fe2O3 and

LSF ≈ 1.0). The 29Si IR NMR spectra of the same sample before and after selective dissolution are shown in parts

(a) and (b), respectively, where the recovery times (in seconds) are shown below the individual spectra.

Figure 3.17 shows the results from the 29Si IR NMR experiments, including experimental

data together with their best fits to equation (2.18), for the alite and belite phases of three

clinkers with bulk Fe2O3 contents of 0.38, 0.93 and 3.91 %w. The Mz(t) values for the alite and

belite phases have been obtained for each of the 29Si IR NMR spectra employing the

deconvolution procedure described earlier. It shows clearly that the relaxation times for the 29Si


resonances from alite as well as belite are affected by the increase in the bulk Fe2O3 content,

reflecting an increased incorporation of the paramagnetic Fe3+ ions in the alite and belite

structures. However, the actual concentration of Fe3+ ions incorporated in the alite and belite

phases can not be obtained from these experiments since the spin-lattice relaxation time for the

electron spins has not been determined, cf. equation (2.19). It is evident from the plot of 29Si

spin-lattice relaxation times (T´1) for alite and belite as a function of the bulk Fe2O3 content,

Figure 3.18, that the increase in the amount of Fe3+ guest ions (1/T´1 NP2) is very similar for

these two phases at low bulk Fe2O3 concentrations (i.e. < 0.8 %w Fe2O3), indicating that the

presence of F ions in alite has no significant impact on the Fe3+ guest-ion incorporation in the

calcium silicate phases. Therefore, Fe3+ seems not to be involved in the fluoride mineralization.

The slight change in slope for 1/T´1 for bulk Fe2O3 contents above 0.8 %w suggests that different

amounts of Fe3+ ions are incorporated in alite and belite, potentially in different sites as

mentioned by Taylor[13] who suggests that the Fe3+ ions enter the tetrahedral sites in belite by

substituting for Si4+ ions, but occupy the octahedrally coordinated sites in alite obtained by

substitution for Ca2+ ions.

Figure 3.17 Plots of Mz(t) values for alite (left) and belite (right) as a function of the recovery time obtained from

the 29Si IR NMR experiments for three modified clinkers containing 0.38 %w Fe2O3 (), 0.93 %w Fe2O3 (♦) and

3.91 %w Fe2O3 (). The best fits of the experimental data to equation 2.13 are shown as solid lines.


Figure 3.18 A plot of the relaxation rate (1/T´1) for the 29Si spins in alite () and belite () versus the bulk Fe2O3

content for a series of modified clinkers. The clinkers were prepared from a commercial white Portland cement,

targeting LSF ~0.9, 0.9 %w F and Fe/Al molar ratios in the range of 0.05 0.8. The relaxation time constant T´1 for

each clinker was obtained from 29Si IR NMR experiments.

The proposed Ca2+ Fe3+ substitution is further supported by an evaluation of the alite and

belite content for each of the studied modified clinkers by a plot of the molar ratio of silicon in

alite and belite (nSi(alite/belite)) as a function of the bulk Fe2O3 content, Figure 3.19. For small

concentrations (< 0.8 %w Fe2O3), the incorporation of Fe3+ ions in the calcium silicate phases

results in an increase of the alite content, i.e. the nSi(alite/belite) ratio, for a fixed clinker

composition apart from the variation in the Fe/Al molar ratio. Since the amount of free CaO is

nearly constant (~0.4 %w) for all clinkers, the increase in the alite content may potentially reflect

that the Fe3+ ions preferentially substitute for the Ca2+ sites in alite, corresponding to an effective

increase in the bulk CaO content. For a further increase of the bulk Fe2O3 content above ~0.8

%w, the alite content decreases as a result of the formation of the ferrite phase which reduces the

amount of CaO available for alite formation. This is supported by the observation of the ferrite

phase for the clinkers with bulk Fe2O3 contents above 0.8 %w by the 2 reflection at 33.8 in the

XRD diffractograms (not shown).


Figure 3.19 Plot of the molar fractions of silicon in alite and belite (), nSi(alite/belite), as a function of the bulk

Fe2O3 content. The nSi(alite/belite) ratios were obtained for each of the modified clinkers by deconvolution of the 29Si MAS NMR spectra.


3.5. Hydration of fluoride-mineralized Portland cement

In addition to the strong mineralizing effect, the quantity of fluoride ions also shows an

important impact on the hydrational properties of the mineralized cement. It seems to have an

adverse effect on the reactivity of the calcium silicates leading to a retarding effect on the

hydration[154,162]. However, in combination with an optimized amount of SO3 containing

mineralizers (e.g. CaSO4), fluorine shows improved effects on the early hydration process[10,41]

with an optimum fluoride content, as monitored by strength measurements after one day, at

about 0.2 %w for an ordinary Portland cement. Higher fluoride contents will reduce the

compressive strength measured after one day while the long-term hydration, i.e. > 28 days, is

almost unaffected.

Figure 3.20 Experimental (a) and simulated (b) 29Si MAS NMR spectra of a fluoride-mineralized cement hydrated

for 28 days. The cement is prepared from a clinker containing 0.36 %w F, 4.3 %w Al2O3 and with a LSF of 0.9. The

deconvolution of the spectrum includes the sub-spectrum for the MIII alite and resonances for belite and the CSH

phase. Alite and belite are shown in (c) while the CSH spectrum (d) includes five 29Si resonances at (29Si) = –

76.7 ppm, –79.1 ppm, –81.5 ppm, –83.3 ppm and –85.4 ppm.


To investigate the effects of fluoride ions on the hydration process, hydrated cement

samples have been prepared (water/solid = 0.5, nSO3/nAl2O3 = 1.3 and a relative humidity of RH =

100 %) for clinkers produced from raw meals including 4.3 %w Al2O3, a LSF of 0.9 and fluoride

content in the range 0.04 1.0 %w. The hydration has been stopped and studied by 27Al and 29Si

MAS NMR for each series at 1, 2, 7, and 28 days. The degrees of hydration, given by equation

2.11, for alite and belite at each hydration time were estimated from 29Si MAS NMR spectra

using the common deconvolution procedure. Based on the observations for synthetic CSH

presented in Chapter 4, five 29Si resonances at (29Si) = –76.7 ppm, –79.1 ppm, –81.5 ppm,

–83.3 ppm and –85.4 ppm have been included in addition to the resonances from belite and alite

in its MIII form in the deconvolution of 29Si MAS NMR spectra the for hydrated samples. An

example is shown in Figure 3.20 for a modified cement (0.36 %w F) hydrated for 28 days.

A plot of the degree of hydration as a function of the fluoride content for alite and belite

is shown in Figure 3.20. This graph clearly demonstrates that the quantity of fluoride ions has an

important impact on the hydration rate of the calcium silicate phases. A critical fluoride content

seems to exist around 0.36 %w F. Below this value, the fluoride ions enhance the hydration rate

for the alite phase, Figure 3.21. An increase in the fluoride content above 0.36 %w F

significantly retards the early hydration rate of alite. However, the effects from an increased

fluoride content on the hydration of alite are less apparent as the hydration proceeds and nearly

cancel out at 28 days of hydration. The degree of hydration for belite at 28 days of hydration also

tends to be reduced slightly by fluoride levels above 0.36 %w, Figure 3.21.

27Al MAS NMR spectra of selected samples, investigating the effects of fluoride ions

on the hydration process of the Al-containing phases, are shown in Figure 3.22. It is apparent

from these spectra that the quantity of fluoride ions strongly influences the hydration of the

aluminate phases after one day of hydration. However, the effect from fluoride anions is less

apparent as the hydration proceeds to seven day of hydration, Figure 3.21, and it nearly cancels

out after hydration for 28 days (not shown). Considering the centers of gravity for the

centerbands at cg(27Al) 13 and 9 ppm in Figure 3.21, it can be seen that the AFt phase

(ettringite) is rapidly converted into an AFm phase (monosulphate) as the fluoride content is

increased up to 0.36 %w. Finally, when considering the intensity of the 27Al resonance at ~82

ppm (i.e., Al3+ guest-ions in alite), the fluoride content of 0.36 %w F also appears to be a critical

limit for the reactivity of alite. Below this value, an increase in the fluoride content tends to

enhance the hydration rate of the alite phases.


Figure 3.21 The degree of hydration for alite and belite in fluoride-mineralized cements prepared from clinkers

containing different quantities of fluoride ions. The relative quantities of the silicate species are obtained from 29Si

MAS NMR spectra (7.1 T, R = 7.0 kHz, 30-s relaxation delay) of the hydrated samples. (Top) the total degree of

hydration for alite after 1 day (), 2 days (), 7 days () and 28 days () of hydration. (Bottom) the degree of

hydration for belite () after 28 days of hydration. The lines are shown as a guide to the eyes.


The hydration experiments demonstrate that the quantity of fluoride ions in Portland

cement has an important impact on the early hydration, most significantly after one and two days

of hydration. In agreement with the previous investigation for the effects of fluoride ions on the

compressive strength of Portland cement[10], the 27Al and 29Si MAS NMR experiments of the

hydrated samples have shown the existence of a critical fluoride content, although for this series

of clinkers of the critical fluoride content is of ~0.36 %w F. Below the critical value, an increase

in the fluoride content tends to improve the hydration properties of the hydrated Portland

cement. Most importantly, the increasing fluoride content up to 0.36 %w F increases the

reactivity of the alite phase. Hence, a larger amount of aluminate ions may be released from the

hydration of alite into the paste-liquid, which probably is the reason for the rapid conversion of

the AFt (ettringite) into a AFm phase (monosulphate) after one day of hydration, see Figure

3.22. Above the critical value of ~0.36 %w F, the fluoride ions reduce the degree of hydration

for alite up to 28 days, although the most pronounced reduction is observed after one and two

days of hydration. One possible reason for the existence of this critical fluoride content is that

calcium fluoride has a very low solubility product of ksp = 3.71011 M3, which causes CaF2 to

precipitate immediately on the cement grain surface as the fluoride ions are released from the

hydration of alite (cf. Chapter 4). Above the critical fluoride content, the precipitating CaF2

forms a protective layer on the cement grain surface reducing the cement hydration. This is

consistent with the observation by 29Si and 27Al MAS NMR revealing that above the critical

concentration, fluoride ions reduce the degree of hydration of alite and belite, and they affect the

formation of all hydrated phases. Thus, the presence of fluoride ions in high concentrations may

result in a decrease in the quantity of CSH phases and therefore, the early strength

development for the cement is reduced. Furthermore, it also retains a large amount of aluminium

ions in the anhydrous alite phase leading to a retarding effect on the conversion of AFt

(ettringite) into AFm phases. The critical fluoride content may, however, vary slightly depending

on the quantity and reactivity of the alite phase. Another effect that also needs consideration is

the decreased SiO2 content in fluoride-mineralized alite as a consequence of the coupled

incorporation of F and Al3+ ions (section 3.2). Obviously, this results in a reduction in the

quantity of the CSH phase formed during the early hydration period, since only a small

amount of belite has reacted with water at this time. As the hydration process proceeds, the

cement grain will expand due to the gradual hydration of alite and belite. Hence, the protecting

layer of CaF2 is broken and the retarding effects from the fluoride ions become less apparent

with time.


Figure 3.22 27Al MAS NMR spectra (14.1 T, R = 13.0 kHz) of selected hydrated fluoride-mineralized cements

after one (left column) and seven (right column) days of hydration. The cements were prepared from clinkers

containing 0.04 %w F (a), 0.23 %w F (b), 0.36 %w F (c) and 0.65 %w F (d). The spectra demonstrate that the

quantity of fluoride significantly affects the formation of the AFt (ettringite) phase and its conversion into AFm

phases (monosulphate). The asterisks (*) indicate the spinning sidebands from the AFt phases.


3.6. Application of fluoride mineralization

3.6.1. Preparation of test clinkers

Based on the results presented in the previous sections, a fluoride-mineralized clinker,

denoted test clinker, was prepared using a commercial white Portland clinker (HKL) as the main

source. The chemical compositions of the original and modified clinkers are shown in Table 3.1.

As it is shown by the table, only the contents of SiO2, Al2O3, Fe2O3 and F have been modified.

The modification of the chemical composition leads to an increase in the lime saturation factor

from 0.95 for HKL to approximately 1.00, although the actual CaO content is nearly constant for

the clinkers. Furthermore, the actual fluoride content in the test clinker is 0.28 %w rather than

0.36 %w which is the optimum fluoride content observed in the hydration experiments (cf.

section 3.5). This is due to the fact that CaF2 has a moderate volatility, which makes it difficult

to control the actual content of fluoride in the final clinker. Quantitative Rietveld analysis of the

test clinker for batch 1 reveals that it consists of 77 %w alite, 21 %w belite and 2 %w tricalcium

aluminate whereas the unmodified HKL contains 69 %w alite, 24 %w belite and 5 % tricalcium

aluminate. In fact, the alite content may be further increased by additional CaO, if it is desired;

the saturation condition defined by equation (1.1), i.e. LSF = 1.0, may not be applied to this

clinker type since the majority of aluminum is not present in tricalcium aluminate but rather as

guest ions in the alit phase.

Table 3.1 Chemical composition of the original and the modified clinkers from XRF experiments. The bulk fluoride

content and the quantity of free CaO for the clinkers were measured using the methods described in Appendix 2.

HKL Test clinker

Batch 1 Batch 2

CaO 70.3 70.4 70.3

SiO2 25.4 22.8 22.5

Al2O3 2.13 4.49 4.40

Fe2O3 0.37 0.78 0.73

F 0.04 0.28 0.29

SO3 0.12 0.36 0.40

MgO 0.63 0.56 0.73

Na2O 0.19 0.10 0.07

K2O 0.07 0.00 0.02

Free CaO 1.7 0.4 0.4


3.6.2. Strength performances of fluoride-mineralized Portland cement

The clinkers were milled together with gypsum to produce cements with a Blaine fineness

of 480 m2/kg. The SO3 content for the clinkers was optimized by following the heat evolution of

the hydration process up to eighteen hours after mixing for a series of cements with different

SO3 contents. For white Portland cement (wPc) produced from the unmodified HKL, the

optimum SO3 content is 2.1 %w whereas the test cements required a SO3 content of 4.2 %w to

avoid flash setting, due to their rather high bulk Al2O3 content. Strength tests were performed for

the white Portland cement, an ordinary Portland cement (oPc) and the two test cements (from

batch 1 and 2) in accordance with the mini-RILEM method[163]. The compressive strengths for

the hydrated cements were measured at 1, 2, 28 and 90 days of hydration with the results

presented in Figure 3.23. A comparison of the hydration experiments for wPc and oPc clearly

demonstrates the effect of the quantities of alite and belite on the strength development. For one

and two days of hydration, the high alite content in oPc results in higher compressive strengths

as compared to wPc. On the other hand, as the hydration proceeds the strength development is

taken over by the hydration of belite and therefore, wPc exhibits higher strengths at 28-days and

later hydration times.

After one day of hydration, the test cements exhibit comparable strengths as the oPc due

to their high alite content. However, the high belite content of the test cements as compared to

the oPc results in a much faster strength development during the long-term hydration. Moreover,

in comparison with the unmodified wPc, the test cements have shown improved compressive

strengths for all hydration stages; after one day of hydration, the test cements exhibit an increase

in the compressive strength corresponding to an average value of 36 % and approximately 10 %

after long-term hydration.

Strength tests have also been conducted for a series of blended cements produced from

the test clinker of batch 1, employing a 30 %w replacement of the clinker by limestone filler

(CaCO3), heat-treated bentonite or glass particles. The heat-treated bentonite and glass are

developed in two other PhD co-projects of the FUTURECEM project. After one and two days of

hydration, the test cement shows the best performance with limestone filler alone, Figure 3.24.

However, its compressive strength is somewhat lower than that for pure wPc. At 28 days of

hydration and later, the partly replacement by 10 %w CaCO3 and 20 %w glass results in

compressive strengths which are roughly 5 % lower than that of pure wPc. The replacement of

the clinker by heat-treated bentonite results in the lowest strength for all samples, except for that

after one-day of hydration.


0.0

20.0

40.0

60.0

80.0

100.0

120.0

1 2 28 90 days

Co

mp

res

sive

str

eng

th (

MP

a)

Figure 3.23 Compressive strength of blended cements measured in accordance with the mini-RILEM method. The

test cements (, ) and white Portland cement () have a Blaine fineness of 480 m2/kg whereas it is 575 m2/kg for

the ordinary Portland cement oPc (). Thus, to be able to compare the results of test cements with oPc, the actual

compressive strength of oPc has been adjusted relating to the difference in their fineness[18].

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 28 90 Days

Co

mp

ress

ive

str

en

gth

(M

Pa

)

Figure 3.24 Compressive strength of blended cements measured in accordance with the mini-RILEM method. The

cements were prepared with a clinker substitution level of 30 %w, i.e. () 30 %w limestone filler, () 10 %w

limestone filler and 20 %w pre-treated betonite and () 10 %w limestone filler and 20 %w glass particles. The

unmodified wPc is shown in red ().


3.7. Summary

Site preferences for F and Al3+ guest ions in the calcium silicate phases of Portland

clinker have been investigated using 19F, 27Al, and 29Si MAS NMR combined with double-

resonance experiments such as 27Al19F and 29Si19F CP/MAS, and 29Si19F CP-REDOR. It

is demonstrated that fluoride mineralization can be represented by the coupled substitution

Si4+ + O2 Al3+ + F in alite, where fluoride ions substitute for the oxygen on interstitial sites

and Al3+ ions replace silicon on tetrahedral sites in the near vicinity of the F ions. The coupled

substitution of F and Al3+ stabilizes local regions with the chemical composition

Ca27Si9-xAlx(Ob)36(Oi)9-x(Fi)x, where the upper limit of x is 4.5.

A series of laboratory-synthesized fluoride-mineralized Portland clinkers have been

prepared and studied using solid-state NMR and other analytical tools such as XRD, XRF, free

lime measurement, etc. This work demonstrates that the use of fluoride-mineralizing agents in

clinker production promotes the formation of alite by two different mechanisms. The most

important effect is that the incorporation of fluoride in alite results in an increase in the entropy

of mixing, which corresponds to a reduction in the Gibbs free energy for this phase. Assuming

that the enthalpy of alite is unaffected by the incorporation of fluoride, the optimum fluoride

content in Portland cement is approx. 4.1 % by the weight of alite. Another important effect

arises from the mass balance of the coupled substitution Si4+ + O2 Al3+ + F in alite, which

results in an increase in the quantity of belite and thereby, further facilitates the forward reaction

CaO + belite alite.

The incorporation of Fe3+ ions in calcium silicate phases of Portland clinker has been

investigated using 29Si IR NMR. It is evident that small amounts of Fe3+ ions are incorporated

into the alite as well as belite structures. However, the Fe3+ ions do not appear to be involved in

fluoride mineralization since the presence of a high F content in alite does not promote the

incorporation of Fe3+ ions in this phase. This study also demonstrates that the Fe3+ ions tend to

be incorporated on the octahedral sites of alite by substituting for the Ca2+ ions.

The hydration experiments for fluoride-mineralized cements have proven the existence of

a critical bulk fluoride content of 0.36 %w. Below this value, an increase in the fluoride content

tends to improve the hydrational properties of Portland cement, e.g. it promotes the hydration of

alite and the conversion of AFt (ettringite) into AFm (monosulphate) phases. A further increase

in the fluoride content above this critical value significantly reduces the degree of hydration for

alite up to 28 days. Furthermore, it exhibits a retarding effect on the conversion of AFt into


AFm, although this is nearly cancelled out after seven days of hydration. The high fluoride level

also seems to affect the degree of hydration for the belite phase.

A test clinker has been prepared based on the results from the optimization of the fluoride

content in Portland clinker. Although the CaO content in the test clinker is comparable to the

unmodified white Portland clinker, i.e. 70 %w CaO, the test clinker has an increase in the alite

content by approximately 8 %w whereas the content of belite and tricalcium aluminate is

decreased with a similar amount.

Tests on the strength performances of fluoride-mineralized cements prepared from the

test clinker have been conducted in accordance with the mini-RILEM method. The test cements

exhibit an increase in its one-day compressive strength by 36 % as compared to the unmodified

white Portland cement. For longer hydration times, the test cement shows an increase of

approximately 10 %.

Blended cements with a 30% clinker replacement have been prepared based on the test

clinkers. Three systems have been studied: (i) 30 %w lime, (ii) 10 %w limestone and 20 %w

glass particles and (iii) 10 %w limestone and 20 %w pre-treated clay. The blended cement (i)

shows the best early strength performance whereas system (ii) has the best performance after

long-term hydration. In both cases, the blended cements exhibit a reduction of about 5 % as

compared to the compressive strength of the unmodified white Portland cement.

Chapter 4. Fluoride-ion environments in CSH 81

4. Chapter

Fluoride-ion environments in synthetic

CSH and hydrated Portland cement

The main aim of this work is to investigate the structural environments of fluoride guest

ions in the calcium silicate hydrate (CSH) phases and their influence on the hydration of

Portland cement. As it is demonstrated in a number of previous studies, in addition to the strong

mineralizing effects, fluoride ions also affect the hydration properties of Portland cement; they

lead to an extended setting time and a reduction in the early strength of hydrated Portland

cement[10,41,154,162]. Despite the obvious important effects of fluoride, the mechanism by which

fluoride ions influence the hydration of Portland cement is only poorly understood. Two main

proposals have been presented. The first considers the retarding effect as an intrinsic property of

fluoride-mineralized alite[155], which is possibly stabilized in its rhombohedral form. This

polymorph has proven a prolonged dormant time but it also hydrates much faster than the

remaining alite forms. The second proposal explains the retarding effect of fluoride as a result of

the precipitation of CaF2 salt on the cement-grain surface, preventing the cement to hydrate[164].


This explanation seems to be more appropriate for the reduction in the degree of hydration of

Portland cement with fluoride contents above the critical value (cf. Section 3.5). However,

experimental identification of CaF2 forming from the hydrating Portland cement has not yet been

possible, due to the very low fluoride content.

This chapter is divided into two parts. The first focuses on the incorporation of fluoride

and its influence on the structure of synthetic calcium silicate hydrates while the second part

presents the results from a study on the fluoride environments in hydrated Portland cement. The

observation contributes to a further understanding of the mechanism by which fluoride strongly

modifies the early hydration properties of Portland cement.

4.1. A brief description of the CSH structure from the T/J viewpoint.

Figure 4.1 shows a schematic two-dimensional representation of the principal layer of

CSH structure based on the tobermorite (T) structure[62,66], consisting of a CaO layer

sandwiched between two sheets of dreierketten silicate chains. The upper part of the figure

represents an infinite silicate chain with periodicity of one bridging Q2B and two Q2

P sites. If the

principal layers are not moved too far away from each other, such as in tobermorite 11-Å, the

Q2B silicate units from two adjacent layers can inter-link forming Q3 units[165]. Different types of

guest ions may be present in the CSH structure[39,65,166,167]. For example, the substitution of

Si4+ by Al3+ ions on the bridging sites is usually accompanied by incorporation of monovalent

alkali cations or Ca2+ in the interlayer, preserving the charge balance. As it is shown in the lower

part of Figure 4.2, each Al3+ incorporated in the bridging sites results in two Q2(1Al) units. The

principal layer consists of highly disordered finite silicate chains of length 3n 1, where n is an

integer whose value depends on the Ca/Si molar ratio and the degree of protonation (w/n). The

tetrahedral silicate units on the bridging sites and the defect sites are charge-balanced partly or

fully by the H atoms present in the interlayer region, forming the silanonl SiOH groups. For

high Ca/Si molar ratios, the bridging SiO4 tetrahedra may also be replaced by calcium ions,

which are coordinated to seven oxygen atoms originating either from water molecules or

hydroxyl groups, Figure 4.1. The figure is only for the purpose of demonstrating the distinctions

between different SiO4 connectivities and the distribution of OH in the CSH structures; it is

not meant to accurately represent the actual arrangement of the atoms.


Figure 4.1 Schematic two-dimensional representations of the CSH structure derived from the tobermorite/jennite

(T/J) viewpoint. The upper part in (a) shows an infinite dreierketten silicate chain with SiO referring to as SiOSi

and SiO groups that are charge-balanced by Ca2+ present in the interlayer (not shown). The lower part of (a)

shows the incorporation of Al3+ ions on the bridging site leading to two Q2(1Al) sites for each Al3+ for Si4+

substitution. The diagram shown in (b) illustrates the distribution of hydroxyl groups between tobermorite- and

jennite-based dimer structures. For the T-based CSH structure, OH occurs in the interlayer (i.e. the lower part)

whereas it takes part of the principal layer (i.e. the upper part) for the jennite-based part.


4.2. Fluoride-ion environments in synthetic CSH and hydrated Portland cement

29Si MAS and 29Si19F CP/MAS NMR spectra of a fluoride-mineralized Portland

cement and two synthetic CSH samples, revealing the presence of F guest ions in the CSH

phase, are shown in Figure 4.2 and 4.3, respectively. For the hydrated Portland cement sample

(Figure 4.2), the 29Si resonances identifying SiF connectivities from remainders of alite appear

in the spectral region from about 65 ppm to 75 ppm while for the CSH phase, they are

observed from 75 ppm to 90 ppm[82]. The 29Si19F CP/MAS NMR spectrum clearly shows

the presence of the Q1 and Q2 components. However, a clear identification of the remaining

silicate environments (such as Q2(1Al) and Q2B*) cannot be extracted from the spectrum due to

the severe overlap of resonances from the disordered CSH structure formed by the hydration

of Portland.

Figure 4.2 29Si MAS (7.1 T, R = 7.0 kHz) and 29Si19F CP/MAS NMR spectra (7.1 T, R = 10.0 kHz) of a

Portland cement hydrated for 28 days, prepared from a fluoride-mineralized clinker containing 0.32 %w F. The

spectra were recorded with relaxation delays of 30 s for (a) and 4 s for (b). The total numbers of scans for the

experiments are 2048 scans and 62400 scans, respectively. Furthermore, the 29Si19F CP/MAS NMR experiment

used a CP contact time of 5.0 ms.


Figure 4.3 (a, c) 29Si MAS (7.1 T, R = 7.0 kHz) and (b, d) 29Si19F CP/MAS NMR spectra (7.1 T, R = 10.0 kHz)

of two synthetic CSH samples prepared with Ca/Si molar ratios of 1.25 and 0.83, respectively. The F/Si molar

ratio in both samples is 0.3. The spectra were recorded with relaxation delays of 30 s for (a, c) and 10 s for (b, d).

The total numbers of scans for the experiments are 2048 scans and 22400 scans, respectively. Furthermore, the 29Si19F CP/MAS NMR experiment used a CP contact time of 5.0 ms.

In contrast to the hydrated Portland cement sample, the synthetic CSH samples exhibit 29Si MAS NMR spectra with three well-resolved peaks at 79.2 ppm, 83.3 ppm and 85.2 ppm,

Figure 4.3. Furthermore, the broad resonance covering a spectral region from 90 ppm to 95

ppm, Figure 4.3 (c), may be assigned to a Q3 site, which is the characteristic chain-branching

silicate of the tobermorite structure[165,166,168]. According to previous studies, the resonances at

85.2 ppm may account for both Q2B and Q2

P units while the resonance with (29Si) = 79.2

ppm should be attributed to the chain-end Q1 or dimeric silicate species. The resonance at about

83 ppm has been identified in several studies[63,168,169] and it is assigned to the single protonated

bridging silicate, referred to as Q2B*

in Figure 4.1; however, this assignment has not yet been

fully proven experimentally. Additionally, the 29Si19F CP/MAS NMR spectra reveal a

resonance of low intensity at 73.9 ppm, which may originate from monomeric silicate species,

Q0(H), as a result of the edging effect[170].

To achieve further information about the site preference of the fluoride guest ions and

their influence on the formation of the CSH structures, a series of synthetic CSH samples

including molar ratios Ca/(Si + Al) = 0.83 1.75, where Al/Si = 0 and 0.05, and F/Si = 0 0.5

have been studied by 19F and 29Si MAS NMR, with the main findings presented below.


4.2.1. Site preferences of F ions in the CSH structure from 19F MAS NMR

19F MAS NMR spectra of a synthetic CSH sample recorded at two different magnetic

fields, i.e. 7.1 T and 14.1 T, are shown in Figure 4.4. The spectra reveal two distinct fluoride

environments with isotropic chemical shifts at 122.0 ppm and 101.4 ppm. In fact, the same

resonances are observed for all of the synthetic CSH samples, although they exhibit different

spectral intensities as a result of the variation in the Ca/Si molar ratio. The narrow peak at

122.0 ppm may originate from an ordered fluoride structure which exhibits a large CSA pattern

covering a spectral width of nearly 250 ppm. This CSA pattern can be simulated by including

two different 19F sites with identical isotropic chemical shift of 122.0 ppm where one has a

large chemical shift anisotropy () and the other possesses a small CSA. However, the

optimized simulations for the spectra recorded at 7.1 T and 14.1 T result in two distinct sets of

CSA parameters, Figure 4.4. This observation indicates that these fluoride ions are located in

nearly identical local structures with an ordered arrangement but with a slight distribution in

their CSA tensors. The second 19F resonance observed at 101.4 ppm is shifted slightly towards

higher frequency relative to the resonance observed for CaF2, (19F) = –105.9 ppm. Furthermore,

it exhibits a symmetric spinning sideband pattern covering a spectral region from 180 ppm to

20 ppm (Figure 4.5), similar to that for CaF2, indicating that this resonance is affected by strong

dipolar couplings such as 19F19F or 1H19F. Furthermore, the rather large line width of the

centerband for this resonance indicates that these fluoride ions occur in a disordered local

environment.

According to the structure model for CSH proposed by Richardson and Groves[62,170],

two distinct types of hydroxyl groups are available for replacement with F ions, i.e. the

covalently bonded hydroxyl groups of the Q1 and Q2B sites and the free OH ions distributed in

the interlayer of the T-based part or in the principal layer of the J-based part of CSH structure;

those are sketched in Figure 4.1. In order to obtain information about the SiF connectivities in

the CSH structure, the 29Si19F CP hetero-nuclear correlation (HETCOR) experiment was

applied to a synthetic CSH sample containing Ca/Si = 1.25 and F/Si = 0.3. However, a

reconstruction of the 2D spectrum has not been possible due to a short T2 relaxation for the 19F

spins, which causes the 19F magnetization to dephase completely within very short time under

the t1 evolution. Alternatively, the 29Si19F FBCP experiment, described in section (2.2.7.), was

applied to identify 19FSi connectivities. Since this experiment utilizes the dipolar coupling to

transfer magnetization between the 19F and 29Si spins, the absence of a resonance at 122.0 ppm

in the 29Si19F FBCP spectrum, Figure 4.5 (b), indicates that these fluoride ions are located far


away from the dreierketten silicate chains. Thus, they are tentatively assigned to F ions that

substitute for the OH groups in the principal layer of the J-based CSH structure. This

assignment is supported by the large chemical shift anisotropy observed for this resonance,

which may be a result of the ordered local structure in the principal layer.

Figure 4.4 19F MAS NMR spectra of a synthetic CSH sample prepared with a Ca/Si molar ratio of 1.75 and F/Si

= 0.3. Spectrum (a) was recorded at 7.1 T using a spinning speed R of 10.0 kHz, a 10-s relaxation delay and 128

scans. Spectrum (b) was recorded 14.1 T with R = 13.0 kHz, a 15-s relaxation delay and 512 scans. The

corresponding simulated spectra using the STARS program are shown in (b) and (d), respectively. The simulations

included three 19F sites one with isotropic chemical shift at 101.4 ppm ( = 0) and two with identical isotropic

chemical shifts of 122.0 ppm. However, the refined simulations (root-mean-square, rms < 1 %) of the spectra

result in two different set of CSA parameters; these are = 132 ppm and 48 ppm, = 0.02 and 0.08 with relative

intensity ratio of 1.0 : 3.1 for (b) and = 112 ppm and 57 ppm, = 0.13 and 0.34 with a relative intensity ratio

of 1.0 : 2.4 for (d).


Figure 4.5 19F MAS (a) and 19F29Si FBCP MAS (b) NMR spectra (7.05 T, R = 10.0 kHz) of a synthetic CSH

sample prepared using a Ca/Si molar ratio of 1.25 and F/Si = 0.3. Both spectra were recorded using a 10-s relaxation

delay. The spectrum in (a) is achieved with 512 scans while (b) is recorded with 58482 scans. Furthermore, the 19F29Si cross polarization transfers were achieved using an 8-ms CP contact time.

The resonance at (19F) = 101.4 ppm, which exhibit a 19F29Si connectivity (Figure 4.5),

may originate either from F atoms that substitute for the covalently bonded OH groups of the Q1

and Q2B silicates or from the fluoride ions distributed in the interlayer of the T-based CSH

structure. In this particular case, the 29Si19F CP-REDOR experiment (Section 2.2.8) may be

applied to determine the average SiF distances for each of the Q units, facilitating an

unambiguous assignment. However, due to the limited time available for this project, such

experiments have not been conducted. Nevertheless, recent theoretical calculations as well as

experimental investigations of the incorporation of fluoride in bioactive calcium/alkali-silicate

glasses demonstrate that SiF covalent bonds in tetrahedral environments is unlikely to form in

such systems, which is consistent with the absence of resonance from the Q2B*

component

((29Si) = 83.3 ppm) in the 29Si19F CP/MAS NMR spectra shown in Figure 4.3. Therefore,

the 19F resonance at 101.4 ppm is most likely associated with the fluoride ions distributed in the

interlayer of the T-based CSH structure. This assignment is supported by the observation of a

large linewidth and a large symmetric spinning sideband pattern for the 19F resonance, which

may result from the disordered structure of the interlayer and the strong dipolar couplings

between these fluoride ions and H atoms from the OH groups or the water molecules.


4.2.2. Influence of fluoride guest ions on the CSH structure

29Si MAS NMR spectra for a series of synthetic CSH samples, illustrating the effect of

fluoride on the polymerization of the silicate chains, are shown in Figure 4.6. The CSH

samples were prepared from raw mixes including a Ca/Si molar ratio of 1.25 and F/Si molar

ratios in the range 0 0.5. The 19F MAS NMR experiments for these samples only show the two

resonances at 101.4 ppm and 122.0 ppm which indicates that at the studied concentrations,

fluoride are exclusively incorporated in the CSH phase. However, the distribution of fluoride

ions between these two fluoride environments exhibits variations with increasing F/Si molar

ratio, Figure 4.7. Overall, the increased bulk fluoride content seems to increase the amount of

fluoride guest ions in the T-based CSH structure, reflected by the decreased I[-122 ppm]/I[101 ppm]

ratio. However, a slight increase is observed at F/Si = 0.5.

Figure 4.6 29Si MAS NMR spectra (7.1 T, R = 7.0 kHz) of synthetic CSH samples synthesized with a Ca/Si

molar ratio of 1.25 and F/Si molar ratios of (a) 0, (b) 0.1, (c) 0.2, (d) 0.3 and (e) 0.5. The spectra were recorded

using a 30-s relaxation delay and typically 2048 scans.


As it can be seen from the overall features of the 29Si MAS NMR spectra shown in Figure 4.6,

the increased bulk fluoride content leads to a slight decrease in the line width for all 29Si

resonances indicating that a CSH with a higher locally ordered structure is formed. Moreover,

it significantly increases the amount of the Q2 components at the expense of Q1 units. The

average silicate chain length, <CL> = (Q2 + Q1)/½Q1, as a function of the F/Si molar ratios is

determined from deconvolutions of the 29Si MAS NMR spectra and plotted in Figure 4.7. For all

spectra, in order to obtain acceptable lineshape simulations, it includes two Q1 resonances (79

ppm and 80 ppm) and two Q2 resonances (83 ppm and 85 ppm) whereas the Q0(H) resonance

at 74 ppm has been neglected. The plot of <CL> versus the F/Si molar ratio clearly shows that

for a fixed Ca/Si molar ratio an increased incorporation of fluoride ions in the CSH phase

tends to promote the polymerization of the SiO4 tetrahedra. In addition, the chemical shifts of the

Q1 and Q2 sites experience a small shift towards more negative ppm values with increasing F/Si

molar ratio, Figure 4.8.

Figure 4.7 (Left) Plot of the I[122 ppm]/I[101 ppm] ratio, where I is the spectral intensities of the 19F resonances at 122

ppm and 101 ppm, as a function of the F/Si molar ratio. The plot reflects the distribution of fluoride ions in the T-

based part relative to J-based part of CSH structure. (Right) Plot of the average silicate chain length (i.e. <CL> =

(Q1 + Q2)/½Q1) for the CSH versus the F/Si molar ratio. The data are obtained from 19F MAS and 29Si MAS

NMR experiments at 7.1 T, respectively.


Figure 4.8 Plots of 29Si isotropic chemical shifts (iso) for the Q1 and Q2 sites as a function of the F/Si molar ratio

(left) and the Ca/(Si + Al) molar ratio (right). The values of (29Si) are determined from 29Si MAS NMR

experiments at 7.1 T. Furthermore, the graph shown on the right includes 29Si chemical shifts for two series of

synthetic CSH samples, i.e. with (Al/Si = 0.05) and without including Al2O3 which are shown in red and blue,

respectively.

Similar trends are observed for two other series of synthetic CSH samples, which are

synthesized with and without including an aluminum source (Al/Si = 0.05). Furthermore, the

syntheses used a fixed F/Si molar ratio of 0.3 but with Ca/Si ratios varying from 1.75 to 0.83; the

Ca/Si ratio of 1.75 is the typical average value for CSH in mature cement pastes while 0.83

and 1.5 are the ideal values for tobermorite 14-Å and jennite, respectively. The results from the 19F MAS and 29Si MAS NMR experiments for these samples are summarized in Figure 4.8 and

Figure 4.9. As it can be seen from Figure 4.8, the increase in the Ca/Si molar ratio slightly shifts

the Q1 and Q2 resonances towards positive ppm values. In fact, the same trend is reported in

previous investigations[169,171,172], although for CSH without including fluoride. Evidence for

the incorporation of Al3+ in the tetrahedrally coordinated environment is obtained from 27Al

MAS NMR experiment on a synthetic CSH sample (Ca/Si = 1.25), Figure 4.10. The spectrum

clearly shows two resonances in the spectral region for Al3+ ions in tetrahedral coordination, i.e.

67 ppm and 73 ppm. However, their total spectral intensity only account for half of the bulk

Al2O3 content. The remainding Al2O3 occurs as octahedrally coordinated Al species such as the

third aluminate phase[173] (4 ppm) and AFm phases (9 ppm). A minor amount of aluminum also

occurs in penta-coordinated environments (36 ppm). As the 29Si chemical shift primarily reflects

local structural features within the first and second coordination sphere, this Al3+ incorporation


in the bridging sites only introduces a shift of approximately 3 ppm towards positive ppm value

on the two Q2P silicates, which are referred to as Q2(1Al) in Figure 4.1, whereas the remaining Q1

and Q2 resonances (including Q2B and Q2

P) are nearly unaffected by presence of the Al3+ guest

ions, Figure 4.8.

A comparison of the reciprocal average silicate chain length for the two series of samples

with a previously reported data set[174] for a series of CSH sample without fluoride ions is

shown in Figure 4.9. Consistent with earlier observations, this figure shows that a decrease in the

Ca/(Si + Al) molar ratio results in an increase in the average silicate chain length. However, in

the presence of fluoride guest ions, CSH phases with much longer average silicate chains are

formed. The effect of fluoride guest ions on the average chain length seems to be increased with

decreasing Ca/(Si + Al) molar ratio. For Ca/(Si + Al) ratios above 1.25, the average silicate chain

length of the fluoride-containing CSH phases tends to be increased by the presence of an

aluminum source whereas it is reduced below this value.

The distribution of fluoride ions between the J-based and T-based CSH structures is

also affected by the presence of Al3+ guest ions, although a significant difference is only

observed for samples with Ca/(Si + Al) ratios of 1.25 and 1.5. For all samples, only two 19F

resonances with isotropic chemical shifts at 101.4 ppm and 122.0 ppm are observed.

However, the distribution of fluoride over these two sites has shown a strong dependency on the

molar ratios Ca/(Si + Al), Al/Si as well as F/Si. It is apparent from a plot of the I[-122 pm]/I[-101 ppm]

ratio as a function of the Ca/(Si + Al) molar ratio that the ordered fluoride environment is

predominant in samples with high bulk CaO content. As the Ca/(Si + Al) ratio is decreased, the

resonance at 101.4 ppm exhibits an increasing intensity. The most significant change is

observed when the Ca/(Si + Al) ratio is reduced just below 1.5, i.e. the ideal Ca/Si molar ratio of

jennite. This observation strongly supports the assignment of the 19F resonances, (19F) = 101.4

ppm and 122.0 ppm, to fluoride ions incorporated in the T-based and J-based CSH

structures, respectively. In the absence of an aluminum source, the intensity of the resonance at

101.4 ppm becomes much larger than that at 122.0 ppm as the Ca/Si molar ratio is decreased

to 1.25. However, below this value, the I[101 ppm]/I[122 ppm] ratio is slightly increased and the

distribution of fluoride in the two different environments become almost equal as the Ca/Si

molar ratio is further reduced to 0.83. This observation is consistent with the fact that the J-based

CSH structure is predominant at high Ca/Si ratios and therefore, a larger amount of fluoride

ions are incorporated into this phase. As the Ca/Si ratio decreases, a larger amount of T-based

structure will be formed within the CSH phase leading to an increased intensity for the

fluoride resonance at 101.4 ppm.


Figure 4.9 (Left) Plot of the reciprocal average silicate chain length, <CL>1, as a function of the Ca/(Si + Al) molar

ratio for three series of synthetic CSH samples: (♦) F/Si = 0.3 and Al/Si = 0, () F/Si = 0.3 and Al/Si = 0.05, ()

F/Si = 0 and Al/Si = 0.05. (Right) Plot of the I[-122 ppm]/I[-101 ppm] ratio, where I is the spectral intensitiy of the 19F

resonances at 122.0 ppm and 101.4 ppm, as a function of the Ca(Si + Al) molar ratio.

A possible explanation for the similarity in the shifting effects on the Q1 and Q2

resonances as a result of an increase in the fluoride content and the decreased Ca/Si molar ratios

(Figure 4.8) is that the increased incorporation of fluoride in the T-based CSH structure is

accompanied by a similar increase in the quantity of Ca2+ ions in the interlayer of this phase.

This effectively reduces the Ca/Si molar ratio for the main calcium layer and therefore, it

increases the average silicate chain and promotes the formation of the T-based CSH structure

at the same time. Furthermore, the absence of the Q2B* resonance in the 29Si19F CP/MAS

spectra shown in Figures 4.2 and 4.3 indicates that these fluoride guest ions most likely

substitute for the OH groups associated with CaOH on the bridging sites of the silicate chains

rather than distributed uniformly in the interlayer. Thus, a large amount of fluoride ions may be

incorporated in the dimeric T-based CSH structure, which may predominate for Ca/Si ratios

just below the ideal value of the jennite structure (i.e. 1.5). As the Ca/Si molar ratio is further

decreased, SiO4 dimers polymerizes to form longer silicate chains (Figure 4.9), reducing the

amount of Ca2+ ions on the bridging sites and the quantity of fluoride guest ions in the T-based

CSH structure at the same time. A similar effect may result from the incorporation of Al3+ in

the bridging sites. Furthermore, as it is apparent from Figure 4.10, the formation of calcium

aluminate hydrate phases may reduce the Ca/(Si + Al) molar ratio for the CSH phase and


therefore, it affects the average silicate chain length as well as the distribution of fluoride

between the T-based and J-based CSH structures.

Figure 4.10 27Al MAS NMR spectrum (14.1 T, R = 13.0 kHz) of a synthetic CSH sample prepared using a

Ca/(Si + Al) molar ratio of 1.25 and Al/Si = 0.05. The fluoride content used in the synthesis corresponds to F/Si =

0.3. The spectrum was recorded using a 0.5-s excitation pulse, a 2-s relaxation delay and 27520 scans. The

tetrahedrally coordinated Al3+ guest ions of the CSH phase are identified by the two resonances at about 73 ppm

and 67 ppm. The Al3+ ions in penta-coordination appear at about 36 ppm while the octrahedral aluminum species,

the third aluminate phase and AFm, are observed as a sharp peak at 4 ppm and a shoulder at 9 ppm, respectively.


4.3. Influence of fluoride on the hydration of Portland cement

Based on the observations from the 29Si and 27Al NMR experiments of the hydration of

fluoride-mineralized Portland cements presented in Section (3.5), the hydration of the

unmodified white Portland cement (0.04 %w F) and three selected fluoride-mineralized cements,

containing approximately 0.04, 0.29 and 0.9 %w F, have been examined using 19F, 27Al and 29Si

MAS NMR. Hydrated samples with ages from 6 hours up to 200 days have been prepared using

the procedure described in Appendix 1.

4.3.1. Identification of CaF2 in hydrated Portland cement by 19F MAS NMR

The ability to identify CaF2 formation during the early hydration of Portland cement may

be decisive for the understanding of the retarding mechanism of fluoride. The 43Ca19F

CP/MAS NMR experiment, described in Section (2.2.4.), may provide a clear distinction for the

resonance from Ca2+ ions of the insoluble CaF2 salt from the remaining calcium-containing

phases. However, due to the very low CaF2 content in combination with the low natural

abundance of the 43Ca nuclear spin isotope, it has so far not been possible to detect the 43Ca

signal from this phase for hydrated Portland cement samples.

Figure 4.11 demonstrates an alternative approach for the identification of CaF2 in

hydrated Portland cement using 19F MAS NMR. The experiment utilizes the fact the 19F nuclear

spins of CaF2 has a much longer spin-lattice relaxation time than that for F ions present in the

CSH phase; the required repetition time for complete spin-lattice relaxation of the 19F spins in

pure CaF2 is about 120 s at 14.1 T while it is only 15 s for the fluoride guest ions in CSH.

Thus, the 19F MAS NMR spectrum recorded using a 120-s relaxation delay may contain 19F

resonances from CaF2 ((19F) = 106 ppm) as well as the CSH phases ((19F) = 103 ppm

and –122 ppm). In a second experiment where the 19F MAS NMR spectrum is recorded using a

relaxation delay of only 15 s, the 19F resonance of CaF2 only contributes to the first few scans

and will be partly saturated afterwards. As demonstrated in Figure 4.11 for a Portland cement

(0.9 %w) which has been hydrated for 200 days, the difference spectrum corresponds to the

spectrum of CaF2. For this particular sample, the subtraction of spectra recorded with different

relaxation delay seems not to be necessary. However, as demonstrated in the following section,

the CaF2 content in samples corresponding to one-day of hydration is much lower and therefore,

the subtracting procedure is necessary in order to identify the 19F resonance from CaF2.


Figure 4.11 19F MAS NMR spectra (14.1 T, R = 13.0 kHz and 2048 scans) of a Portland cement (0.9 %w F)

hydrated for 200 days. The spectrum shown in (a) was recorded using a 120-s relaxation delay whereas it was 15 s

in (b). The difference spectrum (c) shows the 19F resonance from the insoluble CaF2 salt. The asterisks (*) indicates

the spinning sidebands from the CaF2 phase.

4.3.2. Hydration of fluoride-mineralized Portland cement studied by 19F MAS NMR

As it is demonstrated in Figure 4.12 – 4.15, the high sensitivity of 19F MAS NMR makes

it possible to follow the hydration of fluoride ions in Portland cements, even for very small

fluoride concentration of only 0.04 %w F, within reasonable spectrometer times; the spectra

presented in Figure 4.12 and 4.13 was obtained using a 8-s relaxation delay and typically 8190

scans which corresponds to approximately 18 hours whereas the spectra in Figure 4.14 and 4.15

can be obtained within two hours applying a repetition time of 8 s and typically 512 scans. Since

fluoride is entirely incorporated in the alite phase, the hydration of fluoride may somehow reflect

the hydration of this phase. In general, the 19F MAS spectra of hydrated Portland cements

contain resonances from the anhydrous phase centered at 114 ppm and two resonances at 122

ppm and 103 ppm, which have been identified as fluoride guest ions of the CSH phases as

described above. These resonances experience variation in their intensities as the hydration

proceeds. Furthermore, it can be seen that the release of fluoride ions from the anhydrous phase

is nearly complete after 28 days of hydration. In addition, the insoluble CaF2 salt ((19F) = 106


ppm) has been identified in samples prepared from the modified cements containing 0.29 and 0.9

%w F after hydration for one day, Figure 4.16.

Considering the two series of samples shown in Figure 4.12 and 4.13, they contain

approximately the same fluoride concentration, but differ significantly in their Ca/Si ratios and

Al2O3 contents; the unmodified white Portland clinker has a fluoride content of 0.04 %w, 70.30

%w CaO, 25.42 %w SiO2 and 2.13 %w Al2O3 while the modified clinker has 0.04 %w, 68.40

%w CaO, 24.98 %w SiO2 and 4.43 %w Al2O3. Thus, apart from the difference in the distribution

of fluoride guest ions between the T- and J-based CSH structures, which is possibly due to the

difference in their Ca/Si ratios, the fluoride species in the two cements show very similar

hydrational properties. Both cements show clear hydrational activities at 12 hours after mixing

with water, indicated by the decrease in intensity of the resonance at 114 ppm and the

appearance of the resonance at 122 ppm. At hydration ages up to seven days, the resonance at

122 ppm exhibits the predominant intensity while the fluoride guest ions of the T-based CSH

structure is only present in small amount. As the hydration proceeds, the resonance at about

102 ppm exhibits an increasing intensity with time whereas the fluoride guest ions of the J-

based CSH structure experience variation in quantity and finally decrease to nearly zero after

200 days of hydration. This is consistent with earlier observations for the CSH structures

formed from the hydration of Portland cement[58,175], which identify tobermorite- as well as

jennite-like structures for the CSH formed during the early hydration period while after longer

hydration times, the CSH phase turns out to be suitably described by the T/CH viewpoint. For

both cements, the fluoride guest ions in alite are nearly consummed after 28 days of hydration.


Figure 4.12 19F MAS NMR spectra (7.1 T, R = 10.0 kHz) of hydrated samples prepared from the original white

Portland clinker (0.04 %w F) with the hydration times given below each spectrum. The spectra were recorded using

an 8-s relaxation delay and typically 8192 scans. The 19F resonance from the anhydrous phase is observed at –114

ppm whereas the fluoride guest ions from the CSH phases appear at –102 ppm and –122 ppm.

Figure 4.13 19F MAS NMR spectra (7.1 T, R = 10.0 kHz) of hydrated samples prepared from a modified white

Portland clinker (0.04 %w F and 4.3 %w Al2O3) with the hydration times given below each individual spectrum.

The spectra were recorded using a 8-s relaxation delay and typically 8192 scans.



Portland clinker (0.29 %w F and 4.3 %w Al2O3) with the hydration times given below each individual spectrum.

The spectra were recorded using a 8-s relaxation delay and typically 512 scans.


Portland clinker (0.9 %w F and 4.3 %w Al2O3) with the hydration times given below each individual spectrum. The

spectra were recorded using a 8-s relaxation delay and typically 512 scans.


Fluoride in the cement prepared from the clinker containing 0.29 %w F exhibits a

somewhat slower hydration rate within the first 12 hours after mixing with water; only minor

changes are observed in the 19F MAS NMR spectra recorded for the hydrated sample stopped

after 12 hours as compared to that of anhydrous cement. For longer hydration times, it shows

similar hydration features to the two cements described above. After 24 hours of hydration, the

samples clearly show a large proportion of fluoride ions incorporated in both the T- and J-based

CSH structures, i.e. the clear appearances of resonances at 102 ppm and 122 ppm,

respectively. Moreover, a minor amount of CaF2 salt is formed at this hydration stage, identified

by the appearance of the 19F resonance at –106 ppm, Figure 4.14. It is clearly evidenced from the

experiments that the quantity of CaF2 observed after one day of hydration increases with

increasing fluoride content in the anhydrous cement.

As the fluoride content in the cement is increased much above the critical value (i.e. 0.36

%w F in cement clinker), the alite phase seems not to react with water before 12 hours; the 19F

MAS NMR spectrum of this sample closely resembles that of the anhydrous cement, Figure

4.15. Moreover, instead of being incorporated in the CSH phase, a large amount of fluoride

ions precipitates as CaF2 within the first 12 24 hours after mixing, Figure 4.16. Furthermore,

the absence of 19F resonances at 122 and 102 ppm reflects that only a minor amount of

fluoride ions is incorporated in the CSH phase at this hydration stage.

Figure 4.16 19F MAS NMR spectra (7.1 T, R = 10.0 kHz) of a synthetic CSH sample (Ca/Si = 1.25 and F/Si =

0.5) and samples hydrated for one day, prepared from the three modified cements with fluoride contents shown

below their corresponding spectra. The difference spectra clearly demonstrate that CaF2 is only formed in the

hydrated modified cements, where the corresponding clinkers contain 0.29 %w F and 0.90 %w F, respectively.


4.3.3. Retarding mechanism of fluoride ions on the hydration of Portland cement

An understanding of the retarding mechanism of fluoride on the hydration of Portland

cement may be derived when evaluating the results from the 19F, 27Al and 29Si MAS NMR

experiments together with observations from previous investigations[155,162,164]. It is apparent

from the 27Al MAS NMR spectra shown in Figure 4.18 that a significant amount of AFt phases

(e.g. ettringite) is already formed within the first six hours after mixing, regardless the fluoride

content. From six to twelve hours after mixing, the hydration of the aluminum species in

cements with fluoride contents below the critical value (i.e. 0.36 %w F) proceeds, although with

a very slow hydration rate. The conversion of AFt into AFm phases is already observed for the

modified cement containing 0.04 %w F after 12 hours of hydration while in the modified cement

containing 0.29 %w F, the formation of ettringite is still proceeding. On the other hand, the

fraction of aluminum in the anhydrous and hydrated phases of the modified cement containing

0.9 %w F exhibits no significant changes from six to twelve hours of hydration. In conjunction

with the observations by 19F MAS NMR, summarized in Figure 4.15, this strongly indicate that

the alite phase in the cement containing 0.9 %w F is intact within this hydration period. This

retarding effect might be an intrinsic property of the fluoride-mineralized alite[155] as proposed

earlier since fluoride is not yet released from the alite phase and thus, CaF2 cannot be formed at

this hydration stage.

Figure 4.17 The degree of hydration for alite and belite in fluoride-mineralized cements containing different

quantities of fluoride: () 0.04 %w F, () 0.29 %w F and (♦) 0.90 %w F. The relative fractions of the silicate

species are obtained from deconvolutions of the 29Si MAS NMR spectra (7.1 T, R = 7.0 kHz, 30-s relaxation delay)

of the hydrated samples, stopped at different hydration times up to 100 days.


Figure 4.18 27Al MAS NMR spectra (14.1 T, R = 13.0 kHz) of selected hydrated samples for three modified

cements with the fluoride contents shown below the spectra. The tetrahedrally coordinated Al3+ guest ions of the

alite and belite phases are identified by the resonance at about 82 ppm with a shoulder at 84 ppm, respectively. The

tricalcium aluminate phase appears as a broad resonance from 30 to 85 ppm. The hydrated phases including the AFt

and AFm phases are observed at 13 ppm and 9 ppm, respectively. The asterisks (*) indicate spinning sideband from

the AFt phase.

For one day of hydration and later, the presence of fluoride in quantities above the critical

value tends to affect the hydration of the entire cement. As demonstrated in Section 3.5, the

quantity of fluoride ions strongly modifies the release of Al3+ from the alite phase and the

conversion of AFt into AFm phases. The examination of the effect of fluoride on the hydration

properties of the alite and belite phases of Portland cement using 29Si MAS NMR also

demonstrates that below the critical value, the presence of fluoride ions only slightly affects the

hydration of alite and belite, Figure 4.17; up to seven days, the presence of fluoride seems to

increase the reactivity of alite, but it slightly decreases the degree of hydration of belite. When

present in concentrations above the critical value, fluoride has shown a substantial retarding

effect on the hydration of alite as well as belite. It significantly reduces the degree of hydration


of alite up to 28 days. However, for longer hydration times, the quantity of fluoride has only a

marginal effect on the hydration of alite. According to the fact that fluoride is only incorporated

in the alite phase but seems to affect the hydration of the entire cement when present in

quantities above the critical content, a plausible explanation for this is the formation a protective

layer of insoluble CaF2 salt on the cement grain surface, which prevents the cement to

hydrate[162,164]. The formation of such layer is supported by the observation of CaF2 in the

hydrated fluoride-mineralized cement, Figure 4.16.

4.4. Summary

Site preferences of fluoride ions and their influence on the structure of CSH has been

investigated for synthetic CSH and hydrated fluoride-mineralized Portland cements using 19F, 27Al and 29Si MAS NMR. These studies demonstrate that 19F MAS NMR represents a unique

tool for the structural characterization of fluoride ions in Portland cement. The high sensitivity of

the 19F spins makes it possible to follow the hydration of fluoride species even when they are

present in quantities of as low as 0.04 %w F.

From the study of synthetic CSH, it is demonstrated that for the studied fluoride

contents (F/Si = 0 – 0.5), fluoride ions are almost exclusively incorporated in the CSH

structure. The insoluble CaF2 salt is not formed in such systems. The 19F MAS NMR

experiments reveal two different structural environments for the fluoride guest ions of the

CSH phases. The first exhibits a 19F resonance at (19F) = 122.0 ppm with a large CSA

spinning sideband pattern, which reflects an ordered local structure and therefore, it has been

assigned to fluoride ions distributed in the principal layer of the jennite-based CSH structure.

The second type of fluoride ions appear at (19F) = 101.4 ppm. It is evident from the 19F29Si

FBCP/MAS experiment that this resonance is dipolar-coupled to the 29Si spins from the

dreierketten silicate chains and therefore, it most likely originates from the fluoride ions

incorporated in the interlayer for the tobermorite-based CSH structure. This is supported by

the observation of a symmetric spinning side band pattern for this resonance, which covers

nearly 160 ppm, indicating that these fluoride ions possess strong 1H19F dipolar couplings,

possibly to the H atoms from the OH groups or the water molecules.

The incorporation of fluoride ions in the CSH phase results in an increase in its

average silicate chain length. This effect of fluoride tends to increase with decreasing Ca/Si

ratios. Furthermore, it also seems to be affected by the simultaneous incorporation of Al3+ ions in

the bridging site of the silicate chain. For Ca/Si ratios above the ideal Ca/Si ratio of the jennite


structure (i.e. Ca/Si = 1.5), the presence of Al3+ guest ions appears to increase the effect from

fluoride ions on the average silicate chain length whereas below this value, it reduces the

average silicate chain length of the fluoride-containing CSH structure. It is also observed that

the increased incorporation of fluoride ions introduces a slight high-field shift, i.e. towards more

negative (29Si) value, of the 29Si isotropic chemical shifts for the Q1 and Q2 silicates.

The hydration of fluoride ions in Portland cement has been followed by 19F MAS NMR

for four cements: a commercial white Portland cement and three modified cements prepared

from clinker with a high bulk aluminium content (~ 4.3 %w Al2O3) and fluoride contents of

0.04, 0.29 and 0.90 %w F. The experiments show that the majority of fluoride is incorporated in

the CSH phases, although the fraction of fluoride distributed between the jennite-based and

the tobermorite-based CSH structures experience variation with hydration time. For hydration

ages up to seven days, the fluoride ions of the jennite-based CSH is predominant while on

longer hydration times, the fluoride ions are preferentially incorporated in the tobermorite-based

CSH structure. For the cements prepared from the modified clinker with fluoride contents of

0.29 and 0.9 %w F, the CaF2 salt is formed already after one day of hydration. The content of

this phase tends to increase as the hydration proceeds. However, it only constitutes a minor part

of the bulk fluoride content.

The hydration of the cements has also been followed by 27Al and 29Si MAS NMR. The

experiments for samples with hydration ages up to 12 hours reveal that the prolonged setting

time might be an intrinsic property of the fluoride-mineralized alite, since the fluoride is not yet

released at this hydration stage and therefore, the CaF2 cannot be formed. On the other hand,

after one day of hydration, a significant amount of CaF2 is detected by 19F MAS NMR for the

cement containing 0.9 %w F. Moreover, the 27Al and 29Si MAS NMR experiments demonstrate

that the hydration of all the present phases are affected by the presence of fluoride, despite the

fact that fluoride ions are only incorporated in the alite phase. Therefore, the consideration of the

formation of a protective layer of CaF2 is plausible for Portland cements with a fluoride content

above the critical value of 0.36 %w F (cf. Chapter 3). This layer prevents the cement grains to

react with water and therefore, it reduces the degree of hydration for all cement phases during

the early hydration period.

Chapter 5. Framework structures of aluminosilicate binders 105

5. Chapter

Framework structures of alumino-

silicate binders from solid-state

NMR spectroscopy

This chapter includes two applications of the 29Si27Al REAPDOR NMR experiment in

structural investigations of the network structure of aluminosilicate binders. The first part

concerns a study of the SiOAl connectivities in geopolymeric materials formed from alkali

activation of metakaolin samples. The second part presents the results from a reexamination of

the disordered layer structure of strätlingite using solid-state NMR. More details on both studies

are presented in manuscripts 1 and 2, respectively.


5.1. SiOAl connectivities in alkali-activated materials

Recent developments of alternative building materials have resulted in a renewed interest

in alkali-activated materials[176-179], often denoted geopolymers, as alternative binders, with

emphasis to the generally much lower energy consumption and CO2 emission required for

production of these materials as compared to conventional Portland cement[180]. Additionally, the

materials provide other advantageous properties like high early strength development, long-term

durability, fire-resistance and storage of hazardous inorganic waste[181-184]. The process of

forming the aluminosilicate networks requires activation by a relatively high concentration of

alkali hydroxides, typically NaOH and/or KOH. It involves three main steps[177,178,185]

(1) Release of monomeric Si(OH)4 and Al(OH)4- units from the starting materials activated

by alkali hydroxide.

(2) Reorganization and diffusion of ions to form small coagulated structures.

(3) Termination by poly-condensation, i.e., the formation of SiOAl linkages to form a 3-

dimensional framework structure.

The structure of geopolymeric materials as well as their performance is dependent on several

factors, including the concentration of alkali hydroxides, the bulk chemical composition and the

quantity of soluble silicates and aluminates dissolved from the precursors[186-188]. Alternatively,

the alkali-activation process can be applied to blended cements[189,190], in which the clinker

substitution level can be increased above the current limit of approximately 30 % by weight.

However, geopolymer cements have only been commercialized in small-scale facilities so far,

but not in large-scale applications where the strength is critical.

In the generally accepted structural model[185], geopolymers are described by a backbone

structure of poly(sialates) with the empirical formula of Mn(SiO2)z AlO2nwH2O, where the

SiO4 and AlO4 tetrahedra are linked by sharing all their oxygen atoms, corresponding to Q4

units. The presence of cations M such as Na+, K+, Li+, Ca2+, Ba2+, NH4+ and H3O

+ located in the

framework cavities is necessary to preserve charge balance of the incorporated aluminium ions

in four-fold coordination. So far, the detailed characterization of those materials has been a

major challenge, owing to the lack of long-range periodicity in their open-framework structures.

Thus, alkali-activated materials are often considered as ‘X-ray amorphous’ and their XRD

patterns provide very little structural information[177,185].


Figure 5.1 Experimental (a) and simulated (b) 29Si MAS NMR spectra of an alkali-activated metakaolin sample

(molar Si/Al = 2.0, Na/Al = 1.5). The experimental spectrum (a) was recorded at 7.1 T using a spinning speed of R

= 7.0 kHz and a 30-s relaxation delay. Spectrum (b) is a simulation of (a), in which the eight different resonances

shown in (c) and (d) are included.

According to the general ability of NMR spectroscopy to probe local structural features

independent of long-rang order, 29Si MAS NMR spectroscopy appears to be a useful tool for

investigations of the open framework structures of alkali-activated materials. However, a

limitation of the single-pulse 29Si MAS NMR experiment is apparent when a number Al3+ ions is

substituted into the second-coordination sphere of the probed SiO4 site; each Al3+ for Si4+


substitution in the second-coordination sphere leads to a resonance shift of about 5 ppm towards

higher frequency[79], causing resonances from different types of SiO4 condensation to overlap

and preventing a straight-forward interpretation of the observed chemical shifts[191-194]. For

example, the chemical shifts of Q3(0Al) falls in the same spectral region as those from Q4(2Al)

and Q4(3Al) sites. This is illustrated for an alkali-activated metakaolin sample in Figure 5.1; the

sample was synthesized using a Si/Al molar ratio of 2.0 and Na/Al molar ratio of 1.5. It is

apparent from the deconvolution, shown in Figure 5.1 (b-d), that the 29Si MAS NMR spectrum

consists of eight resonances having chemical shifts within the region from –115 to –75 ppm. The

assignment of these resonances from single-pulse 29Si MAS NMR experiment is somewhat

uncertain, since this chemical shift region may include any of the Q4(nAl) and Q3(nAl)

components. In such cases, the 29Si27Al REAPDOR experiment described in section 2.2.10

appears to be an appropriate tool for probing the SiOAl network. An unambiguous assignment

of the 29Si resonances may be achieved from a combination of the 29Si chemical shifts and the

corresponding number of Al atoms in their second-coordination spheres, obtained from 29Si27Al REAPDOR experiments.

5.1.1. Effects of Si/Al and Na/Al molar ratios

29Si MAS NMR spectra for a series of alkali-activated metakaoline samples, synthesized

using Si/Al = 1.0 3.0 and Na/Al = 1.0, are shown in Figure 5.2. As it can be seen from the

spectra, the 29Si resonances appear in the region from about 115 ppm to 75 ppm, which, in

principel, can be assigned to all types of tetrahedral SiO4 species. Considering these 29Si MAS

NMR spectra, a very intense peak at about 85.0 ppm has been observed for the sample prepared

using a Si/Al molar ratio of 1.0. However, four additional 29Si resonances with isotropic

chemical shifts (29Si) at 78.6 ppm, 83.2 ppm, 87.5 ppm and 92.2 ppm can be identified

from a deconvolution of this spectrum. As the Si/Al molar ratio is increased from 1.0 to 2.0, five

well-separated 29Si resonances are observed at 87.5 ppm, 92.2 ppm, 97.0 ppm, 102.5 ppm

and 107.1 ppm. A further increase in the Si/Al molar ratio results in locally disordered network

structures, which is indicated by the smooth, broadened lineshape of the 29Si MAS NMR spectra

shown in Figure 5.2 (d) and (e). Finally, when the Si/Al molar ratio becomes larger than 3.0, a

significant amount of non-reacted metakaolin have been detected by the 29Si as well as the 27Al

MAS NMR experiments, cf. manuscript 1.


Figure 5.2 29Si MAS NMR spectra (7.05 T, R = 7.0 kHz) of alkali-activated metakaoline samples, prepared using a

Na/Al molar ratio of 1.0 and Si/Al = 1.0 (a), 1.5 (b), 2.0 (c), 2.5 (d) and 3.0 (e). The spectra were recorded

employing a 30-s relaxation delay and typically 2048 scans.

As it is stated above, the use of alkali in a correct amount is vital for the synthesis of

aluminosilicate network structure since the substitution of Al3+ for Si4+ on the tetrahedral sites

has to be charge-balanced. Thus, deficient or excessive amounts of alkali may affect the

formation of the aluminosilicate structures substantially, leading to several different SiO4

environments within the network structure. This is illustrated in Figure 5.3 showing selected 29Si

MAS NMR spectra for another series of alkali-activated samples, for which the Si/Al molar ratio

is 2.0 and the Na/Al molar ratio is varied from 0.8 to 2.15. However, only samples with Na/Al

1.5 have been studied; the alkali-activated product becomes gel-like and is very difficult to pack

in a NMR rotor as the Na/Al molar ratio is raised above 1.5. As it is apparent from Figure 5.3,

the spectra consist of partly overlapping 29Si resonances with isotropic chemical shifts in the


range from 107 to 79 ppm. Furthermore, the 29Si resonances that appear at 85.0, 83.2 and

78.6 ppm are only observed with significant intensity for samples including an excessive

amount of Na+ ions (i.e. Na/Al > 1.0).

Altogether, when changing the raw mix composition (e.g. Na/Al and Si/Al molar ratios)

used in the syntheses, eight 29Si resonances within a spectral region from –75 ppm to –110 ppm,

i.e. the chemical shift region for silicon in tetrahedral coordination, have been observed for the

alkali-activated metakaolin samples. The assignment of these resonances from their chemical

shifts is somewhat uncertain since this chemical-shift region may include any of the Q1, Q2(nAl),

Q3(nAl) and Q4(nAl) components.

Figure 5.3 29Si MAS NMR spectra (7.05 T, R = 7.0 kHz) of alkali-activated metakaolin samples, prepared using a

Si/Al molar ratio of 2.0 and Na/Al = 0.8 (a), 10.0 (b), 1.3 (c) and 1.5 (d). The spectra were recorded employing a 30-

s relaxation delay and typically 2048 scans.


5.1.2. 29Si 27Al REAPDOR NMR

In order to assign the eight 29Si resonances, observed for alkali-activated metakaolin

samples presented above, 29Si27Al REAPDOR NMR has been applied to determine the

number of Al atoms substituted in the second-coordination sphere for each of the resonances.

This study involves three selected alkali-activated metakaolin samples corresponding to the

spectra shown in Figures 5.2 (a), 5.3 (c) and 5.3 (d).

The first sample considered in this study contains Si/Al and Na/Al molar ratios of 2.0 and

1.3, respectively. Its 29Si MAS NMR spectrum, Figure 5.3, clearly shows five distinct resonances

separated by approx. 5 ppm. Additionally, the 29Si1H CP/MAS NMR experiment (not shown)

of this sample reveals a broad resonance of low intensity at about 85.0 ppm. However, this

resonance has been neglected in the deconvolution of the 29Si27Al REAPDOR spectra owing

to its very low intensity. It appears from the 29Si27Al REAPDOR spectra shown in Figure 5.4

that only four of the 29Si resonances are affected by the re-introduced SiAl dipolar couplings.

They experience different rates of Si–Al dipolar dephase, reflecting that the numbers of Al atoms

incorporated in their second-coordination sphere are different. On the other hand, the resonance

at –107.1 ppm shows no significant change in its intensity (S0 ~ S) and therefore, it may be

assigned to a Q4(0Al) unit. The individual 29Si27Al REAPDOR spectra were deconvolved to

assess the intensities, S0 and S, for the different sites. From a plot of the REAPDOR fractions

versus the evolution time, Figure 5.5, it is confirmed that only four 29Si resonances are affected

by the Si–Al dipolar couplings. The knAl values associated with their dipolar dephases are

obtained from the curve fits shown in Figure 5.5 (right). However, it has been necessary to

include S/S0 > 0.3 in the curve fit for the resonance at –87.5 ppm, since it has only one data

point that fulfils the condition S/S0 ≤ 0.3. The knAl values are summarized in Table 5.1.

Complementary results have been achieved for another alkali-activated metakaolin

sample corresponding to that with the 29Si MAS NMR spectrum shown in Figure 5.3(d). From

the spectral deconvolution shown in Figure 5.1, it can be seen that the sample includes eight

non-equivalent 29Si sites: six 29Si resonances having the same isotropic chemical shifts as

observed for the spectrum in Figure 5.3(c) and two additional resonances at 78.6 and –83.2

ppm with low intensities. The last two resonances have been neglected in the deconvolution of

the 29Si27Al REAPDOR spectra for this sample due two their rather low intensities. The knAl

values obtained from curve fits of S/S0 > 0.3 for the remaining 29Si resonances are listed in

Table 5.1.


Figure 5.4 29Si27Al REAPDOR (14.1 T, R = 10.0 kHz) spectra of an alkali-activated metakaolin sample with

Si/Al and Na/Al molar ratios of 2.0 and 1.3, respectively. (Left) Full signal, S0, which is not affected by the SiAl

dipolar couplings. (Right) The attenuated signals, S, reflecting the SiAl dipolar couplings. The evolution periods

are (a): 4 Tr, (b): 8 Tr, (c): 12 Tr and (d) 16 Tr, where Tr = 0.1 ms.

Figure 5.5 29Si27Al REAPDOR curves for an alkali-activated metakaolin sample with Si/Al and Na/Al molar

ratios of 2.0 and 1.3, respectively: (♦) 87.5, () 92.4, () 97.2 and ( ) 102.5 ppm. (Left) Experimental

REAPDOR fractions, S/S0, as a function of the evolution times (Tr = 0.1 ms). (Right) Curve fits for the

REAPDOR fractions at short dephasing (S/S0 0.3), using the function S/S0 = at2, corresponding to equation

(2.21).


Finally, in order to assign the 29Si resonance at (29Si) = 83.2 ppm, the alkali-activated

metakaoline sample containing Si/Al = 1.0 and Na/Al = 1.0 has been studied by 29Si27Al

REAPDOR. It is evident from the deconvolution of its 29Si MAS NMR spectrum, Figure 5.6,

that the sample contains six non-equivalent 29Si environments. However, the resonances at about

79 and 97 ppm, Figure 5.6 (e), have been neglected in the deconvolution of the REAPDOR

spectra due to their rather low intensity. Furthermore, the dipolar dephasing for the resonances at

87.5 ppm and 85.0 ppm were considered together. This assumption is reliable since both

resonances are derived from the Q4(4Al) components and have shown to have similar

magnitudes of dipolar dephasing, Table 5.1. The knAl values for the resonances obtained from a

fit of their REAPDOR fractions are summarized in Table 5.1. It shows clearly that the resonance

at (29Si) = 83.2 ppm possesses a dipolar dephase and an isotropic chemical shift corresponding

to a Q3(2Al) component.

Figure 5.6 Experimental (a) and simulated (b) 29Si MAS NMR spectra of an alkali-activated metakaolin sample

(Si/Al = 1.0, Na/Al = 1.0). The experimental spectrum (a) was recorded at 7.05 T using a spinning speed of R = 7.0

kHz, a 30-s relaxation delay and 2048 scans. The deconvolution of the spectrum reveals six different 29Si

resonances, which are shown in (c), (d) and (e).


As it is apparent from the data in Table 5.2, the eight 29Si resonances observed in Figure

5.1 can be assigned unambiguously by considering their (29Si) values together with the knAl

values obtained from the 29Si27Al REAPDOR experiments. So far, it has not been possible to

assign the 29Si resonance at (29Si) = 78.6 ppm owing to its very low 29Si intensity in the

studied sample. However, its isotropic chemical shift implies that it may originate from a non-

fully condensed tetrahedrally coordinated silicon environment.

Table 5.1 Isotropic chemical shifts and corresponding knAl values for the eight 29Si resonances observed for alkali-

activated metakaolin samples. The data are obtained from curve fits of the REAPDOR fractions at short dipolar

dephase (S/S0 0.3), using the function S/S0 = at2, corresponding to equation (2.21).

29Si isotropic chemical shift in ppm

102.5 97.2 92.0 87.5 85.0 83.2

Figure 5.2(a) 0.39 0.51 0.25

Figure 5.3(c) 0.16 0.31 0.43 0.73

Figure 5.3(d) 0.19 0.29 0.44 0.67 0.69

Table 5.2 An assignment of the eight 29Si resonances observed for the alkali-activated metakaolin samples. The

assignment is made in accordance with the results achieved from the 29Si27Al REAPDOR experiments.

29Si isotropic chemical shift in ppm

-107.1 -102.5 -97.2 -92.0 -87.5 -85.0 -83.2

Figure 5.2(a) - - - Q4(3Al) Q4(4Al) Q4(4Al) Q3(2Al)

Figure 5.3(c) Q4(0Al) Q4(1Al) Q4(2Al) Q4(3Al) Q4(4Al) - -

Figure 5.3(d) Q4(0Al) Q4(1Al) Q4(2Al) Q4(3Al) Q4(4Al) Q4(4Al) -


5.2. Disorder in the double tetrahedral layer structure of Strätlingite

Strätlingite is a potential hydration product of aluminate-rich cements[195]. More recently,

this mineral has also been identified in alkali-activated blended cements, in which alkali

hydroxides are used to activate the hydration of supplementary cementitious materials[196,197].

Strätlingite exhibits the ideal composition 2CaO·Al2O3·SiO2·8H2O and crystallizes in the trigonal

space group R3m. A previous single-crystal X-Ray diffraction (XRD) study[198] of a mineral

sample revealed that the strätlingite structure consists of an octahedral brucite-type layer

[Ca2Al(OH)6·2H2O]+ and a double tetrahedral layer [(T,)4(OH,O)8· 0.25H2O]–. The octahedral

sites are fully occupied by Al3+ while approx. 45% of the tetrahedral sites are vacant (). The

remaining 55 % are occupied by either Si4+ or Al3+ with an overall Si/Al molar ratio of 1:1.

Supplementary information about the double tetrahedral layer of strätlingite was provided by

another study[199] using 27Al and 29Si MAS NMR. That study identified four different tetrahedral 29Si resonances from the 29Si MAS NMR experiments (7.1 T), indicating four different SiO4

environments of the double tetrahedral layer. Based on their isotropic chemical shifts, the three

almost equally intense 29Si resonances in the region from –88.0 ppm to –81.0 ppm were assigned

to Q2(0Al), Q2(1Al) and Q2(2Al) components and a low intensity resonance at −110 ppm was

ascribed to the Q4(0Al) unit. This assignment is, however, in contrast to the basic structure of

strätlingite determined by XRD, which suggests that Al3+ and Si4+ occur mainly as Q3 units. In

the same NMR study, a single tetrahedral AlO4 resonance from the double tetrahedral layer and

an AlO6 resonance from the brucite-type layer were observed. The resonances occur at 60.4 and

8.4 ppm in the 27Al MAS NMR spectrum (7.05 T), respectively.

In this work the local structure of silicon and aluminum of the octahedral brucite-type

layer and the double tetrahedral layer have been reexamined using single-pulse 29Si and 27Al

MAS, 29Si1H and 27Al1H CP/MAS, 27Al 3QMAS and 29Si27Al REAPDOR NMR

experiments. However, this section only presents the results from the 29Si27Al REAPDOR

NMR experiments for a synthetic sample of strätlingite, from which the 29Si resonances are

assigned. In order to enhance the 29Si sensitivity, a 29Si1H CP/MAS period was applied prior

to the 29Si27Al REAPDOR sequence as demonstrated by Figure 2.11 in Section 2.2.9. A

detailed discussion of the disordered layer structure of strätlingite is provided in manuscript 2.


5.2.1. 29Si MAS NMR

The 29Si resonances of strätlingite shown in Figure 5.7 appear in the region from about

75 to 95 ppm, i.e. the isotropic chemical shift region for tetrahedral SiO4 environments.

However, the overall lineshape of the spectrum differs somewhat from that presented in the

previous study of strätlingite. A 29Si1H CP/MAS NMR experiment of the same sample clearly

resolves the lineshape into four well-separated 29Si resonances with isotropic chemical shifts at

(29Si) = 90.8 ppm, 86.4 ppm, 82.1 ppm and 79.7 ppm. Furthermore, a comparison of

Figure 5 (a) and (b) reveals an additional resonance at 84.9 ppm, which has to be included to

obtain satisfactory simulations of the spectra. As discussed above, the assignment of these

resonances from their isotropic chemical shifts alone appears to be uncertain, since the chemical

shift region from 75 ppm to 95 ppm may include any of the Q2(nAl) and Q3(nAl) structural

units. Furthermore, fully condensed Q4 sites with a high number of Al incorporated in the

second-coordination sphere such as Q4(4Al) and Q4(3Al) may also exhibit isotropic chemical

shifts within this spectral region.

Figure 5.7 29Si MAS (a) and 29Si1H CP/MAS (b) NMR spectra (9.4 T) of a synthetic sample of alkali-containing

strätlingite. The spectra were recorded using spinning speeds R of 6.0 and 3.0 kHz, respectively. Furthermore, the 29Si1H CP/MAS NMR experiment was employed a contact time of 5.0 ms


5.2.2. 29Si27Al REAPDOR NMR

The assignment of the 29Si resonances from the double tetrahedral layer of strätlingite has

been facilitated by supplementary information obtained using 29Si27Al REAPDOR NMR

experiments. The ability of the REAPDOR experiment to retain SiAl dipolar couplings is

utilized to determine the number of Al atoms in the second-coordination sphere for each of the 29Si resonances, as demonstrated in the previous sections. A plot of the REAPDOR fractions as a

function of evolution time for a synthetic sample of strätlingite is shown in Figure 5.8. The plot

clearly demonstrates that all 29Si resonances are affected by the re-introduced SiAl dipolar

couplings. The corresponding knAl values are determined from curve fits of REAPDOR

fractions to equation (2.21) and are summarized in Table 5.3. According to their isotropic

chemical shifts together with the knAl values, the five 29Si resonances representing five different

SiOAl environments of the double tetrahedral layer of strätlingite can be assigned to Q3(1Al):

90.8 ppm, Q3(2Al): 86.4 ppm, Q3(3Al): 83.8 ppm, Q2(1Al): 82.1 ppm and Q2(2Al): 79.7

ppm. This assignment is consistent with the fact that the resonances at 79.7 and 82.1 ppm

experience a more efficient 29Si1H cross-polarization transfer than the remaining resonances

(cf. Figure 5.7), since they are linked to a larger number of OH groups.

Figure 5.8 29Si27Al REAPDOR curves for a synthetic sample of strätlingite: () 90.8 ppm, () 86.4 ppm, ()

83.8 ppm, () 82.1ppm and (♦) 79.7 ppm. (Left) Experimental REAPDOR fractions, S/S0, as a function of

evolution time (Tr = 0.1 ms). (Right) Curve fits for the REAPDOR fractions at short dephasing (S/S0 0.3), using

the function S/S0 = at2, corresponding to equation (2.21).


Figure 5.9 Experimental (a) and simulated (b) 29Si MAS NMR spectra of a synthetic sample of strätlingite. The

experimental spectrum (a) was recorded at 9.4 T using a spinning speed of R = 6.0 kHz, a 30-s relaxation delay and

2048 scans. The deconvolution of the spectrum reveals five different resonances, which are shown in (c) and (d)

with isotropic chemical shifts at (29Si) = 90.8 ppm, 86.4 ppm, 83.8 ppm, 82.1 ppm and 79.7 ppm.

Table 2.3 Isotropic chemical shifts, knAl values determined from 29Si27Al REAPDOR experiments and relative

intensities achieved from a deconvolution of the single-pulse 29Si MAS NMR spectrum for a synthetic sample of

strätlingite.

Site assignment (29Si) knAl Normalized intensity

29Si: Q3(1Al) 90.8 ppm 0.21 2.8 %

29Si: Q3(2Al) 86.4 ppm 0.44 55.2 %

29Si: Q3(3Al) –83.8 ppm 0.65 10.3 %

29Si: Q2(1Al) 82.1 ppm 0.25 22.7 %

29Si: Q2(2Al) 79.7 ppm 0.48 8.9 %


The distribution of silicon on the five different SiO4 sites, reflected by their intensities in

the 29Si MAS NMR spectrum, is obtained from a deconvolution, Figure 5.9; their normalized

intensities relative to the total intensity of the spectrum are Q3(1Al): 2.8 % , Q3(2Al): 55.2 %,

Q3(3Al): 10.3 %, Q2(1Al): 22.7 % and Q2(2Al) : 8.9 %. The result clearly demonstrates that the

double tetrahedral layer consists largely of Q3 components, where Q represents either Si or Al in

tetrahedral coordination with three QOQ and one QOH bonds, forming a 2-dimensional

layer structure, which is consistent with the layer structure of 6-membered rings for strätlingite

reported from XRD[198]. Furthermore, this alumino-silicate network is disrupted by site vacancies

leading to approx. 30 % Q2 units.

5.3. Summary

It has been demonstrated that the double-resonance REAPDOR sequence represents a

valuable tool in structural characterisation of cementitious materials. The 29Si27Al REAPDOR

experiment has been applied to estimate the number of Al atoms incorporated in the second-

coordination sphere for SiO4 tetrahedra and it has been shown that it provides a clear

discrimination between different Qi(nAl) environments. Unambiguous assignment of the 29Si

resonances has been achieved by this approach for alkali-activated metakaolin samples and for

strätlingite.

Conclusions 120

Conclusion

In this project, the effects of fluoride ions on the formation of Portland clinker and its

hydration have been investigated. The modified clinkers were prepared using a white Portland

clinker from Aalborg Portland A/S as a main source. It is found that the mineralizing effect of

fluoride on the clinker formation associates with a coupled substitution Si4+ + O2 Al3+ + F in

the alite phase, promoting the formation of this phase by two different mechanisms. The first is a

thermodynamical effect, where the coupled incorporation of F and Al3+ stabilizes local regions

with the chemical composition Ca27Si9-xAlx(Ob)36(Oi)9-x(Fi)x in the alite structure, where i and b

denote the interstitial and covalently bonded oxygen atoms, respectively. The second effect

arises from the mass balance of the coupled substitution, resulting in formal increase in the

quantity of belite which facilitates the reaction, CaO + belite alite. For the studied fluoride

contents (i.e. 0.04 0.77 %w), the mineralizing effects are increased with increasing fluoride

quantity.

For the hydration of fluoride-mineralized Portland cement, the existence of a critical bulk

fluoride content of about 0.36 %w F has been demonstrated. Below this value, an increase in the

fluoride content tends to increase the degree of hydration for the alite phase. However, it is

found that above the critical value, fluoride substantially affects the early hydration (i.e. the

hydration up to 28 days) for all cement phases by two tentative mechanisms. Firstly, the retarded

setting during the first 24 hours after mixing seems to be an intrinsic property of the fluoride-

mineralized alite; its hydrational activity during this period is very small. Secondly, at 24 hours

after mixing, the precipitation of the rather insoluble CaF2 salt has been identified for the

fluoride-mineralized cements, where the CaF2 quantity is increased with an increased fluoride

content in the anhydrous cements. This observation supports the model where the insoluble CaF2

salt precipitates on the cement grain surface. At sufficiently high concentrations (i.e. for fluoride

contents above the critical value), a protective layer of CaF2 may be formed, which retards the

cement hydration. However, this effect seems to be leveled out after 28 days of hydration.

From the optimizations of the Al2O3, Fe2O3 and fluoride contents in Portland clinker, it is

found that the alite content can be increased by about 8 %w without additional CaO, which

implies that the CO2 emission from the chemical reactions of the clinker formation is remained

at the same level as that of the unmodified white Portland clinker. Furthermore, hydration

experiments of the cement produced from this modified clinker show an increase by 36 % in its

Conclusions 121

one-day compressive strength as compared to the unmodified white Portland cement. For longer

hydration times, an increase of about 10 % in the compressive strength is observed for the

modified cement. The strength performance of blended cements produced from the modified

clinker using a 30 % replacement by different supplementary cementitious materials has also

been investigated. A remarkable reduction in the one-day compressive strength relative to that of

wPc is observed for all blended cements. However, for the long-term hydration (i.e. after 28 days

of hydration), the blended cement containing 70 % modified clinker, 10 % limestone filler and

20 % glass particles (developed in another PhD project of FUTURECEM) shows a reduction in

strength of only 5 % as compared to the white Portland cement.

Solid-state NMR has proven to be a valuable tool for structural investigation of the guest

ions (such as F, Al3+ and Fe3+) in anhydrous as well as hydrated phases of Portland cement. The

high sensitivity of the 19F spins makes it possible to study the local structure environments of the

fluoride ions and their hydration processes in Portland cement. Moreover, it has been

demonstrated in this project that the couplings of fluoride to other guest ions may be investigated

using a combination of different advanced solid-state NMR techniques. In particular, the site

preferences for F and Al3+ guest ions in the calcium silicate phases of Portland clinker have

been investigated using 27Al19F and 29Si19F CP/MAS, and 29Si19F CP-REDOR

experiments. It is found for the studied fluoride contents (> 0.77 %w F) that fluoride ions

substitute for the interstitial oxygen sites of the alite structure only. Furthermore, the

incorporation of fluoride ions in the alite structure is charge-balanced by a Si4+ Al3+

substitution on the tetrahedral site in the near vicinity of F. This observation unambiguously

reveals the coupled substitution mechanism for the incorporation of F and Al3+ ions in the alite

structure, i.e. Si4+ + O2 Al3+ + F.

A new solid-state NMR pulse scheme (Forth and Back Cross Polarization), which is a

modification of the Cross-Polarization experiment, has been developed for studying the site

preferences of F ions in the calcium silicate hydrate (CSH) phases of Portland cement. This

experiment utilizes the dipolar couplings to transfer the magnetization from the 19F spins to 29Si

spins and subsequently back to the 19F spins, making it possible to selectively detect 19F

resonances from 29Si19F connectivities in the CSH structure within reasonable spectrometer

times. In this study, it is observed that a rather large amount of fluoride ions may be incorporated

in the CSH structure. The fluoride ions are either incorporated in the principal layer of the

jennite-like part or distributed in the interlayer of the tobermorite-like part of the CSH

structure. The incorporation of fluoride ions in the CSH structure tends to increase the average

silicate chain length for this phase.

Conclusions 122

The incorporation of the paramagnetic Fe3+ ions in the calcium silicate phases of Portland

clinker has been investigated using the 29Si Inversion-Recovery NMR experiment and X-ray

powder diffraction. These experiments show clear evidence for the presence of paramagnetic

Fe3+ ions in the alite as well as the belite structures. It is also demonstrated that the Fe3+ ions tend

to be incorporated in the octahedral sites of alite by substitution for the Ca2+ ions.

Finally, the applicability of the 29Si27Al REAPDOR experiment in structural

characterizations of the aluminosilicate networks structure has been demonstrated in this project

for a series of alkali-activated metakaoline samples and a synthetic sample of strätlingite. This

NMR experiment allows a determination of the number of Al atoms incorporated in the second-

coordination sphere for the probed silicon atoms and thereby, discriminating between different

SiOAl connectivities.

References 123

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Appendixes 143

Appendix 1

Sample preparations

Precursors

The white Portland clinker (HKL) was received from Aalborg Portland A/S, Denmark, and has

the following chemical composition: 70.30 wt. % CaO, 25.42 wt. % SiO2, 2.13 wt. % Al2O3,

0.37 wt. % Fe2O3, 0.63 wt. % MgO, 0.41 wt. % P2O5, 0.12 wt. % SO3, 0.04 wt. % F, 0.065 wt. %

K2O, 0.19 wt. % Na2O, 0.01 wt. % Cl, 0.094 wt. % TiO2, 0.0033 wt. % Cr2O3, 0.03 wt. % C and

a Blaine fineness of approx. 395 m2/kg.

Ca(OH)2: Sigma-Aldrich Laborchemikalien GmbH, Seelze, Germany.

Gypsum: VWR International Ltd., Poole, England.

CaF2: Sigma-Aldrich Laborchemikalien GmbH, Seelze, Germany.

Na2SiF6: Merck KGaA, Darmstadt, Germany.

AlOOH: Boemitte, CONDEA Vista Company, Houston, Texas.

FeOOH: Riedel-De Haën AG, Seelze – Hannover.

SiO2: Silicagel, Fluka Chemie GmbH, Buchs, Switzerland.

Kaolinite: Fluka Chemie GmbH, Buchs, Switzerland.

NaAlO2: Strem Chemicals, New Buryport, USA.

NaSiO39H2O: Fisher Chemicals, Fair Lawn, New Jersey.

10 M NaOH solution: Merck KGaA, Darmstadt, Germany.

KOH: Sigma-Aldrich Laborchemikalien GmbH, Seelze, Germany.

Sucrose: Merck KGaA, Darmstadt, Germany.

Fluoride-mineralized Portland clinker/cement

Clinker preparation

The fluoride-mineralized clinkers were synthesized using a white Portland clinker as the

main source. The desired bulk CaO, Al2O3, Fe2O3, SO3 and F contents were achieved by adding

small quantities of Ca(OH)2, AlOOH, FeOOH, CaSO4 and CaF2. Nodules with diameters of

about 1 cm were formed by hand from mixtures of typically 20 g powder and 7 ml water,

corresponding to a water/solid (w/s) ratio of 0.35. The wet nodules were dried at 200 ºC for one

day.

Appendixes 144

The clinkers were prepared by burning the dried nodules at 1450 ( 10) ºC for one hour in

a high-temperature furnace. Subsequently, the clinkers were cooled to 1250 – 1300 ºC and

immediately, quenched in water at ~25 ºC. Finally, the resulting clinkers were dried at 100 ºC for

one day and afterwards stored in closed containers.

Preparation of hydrated samples

The fluoride-mineralized clinkers were ground and sifted to a grain size below 40 m

prior to the hydration experiments. The hydrated samples were prepared from a mixture of water

(w/c = 0.5), gypsum (SO3/Al2O3 = 1.3) and typically 10 g clinker. The paste was mixed by hand

for 15 minutes and subsequently stored over water (RH = 100 %) in a sealed desiccator at room

temperature.

The hydration was stopped after 6h, 12h, 1, 2, 7, 28, 90 and 200 days. Initially, a small

piece (~1.0 g) was knocked off the paste cylinder and ground to a fine powder. Subsequently, the

powder was suspended in acetone for 15 minutes under slow rotation using a magnetic stirrer.

Finally, the powder was separated from the acetone and dried at room temperature in a

desiccator for 24 hours. The samples were stored in small sealed specimen tubes to avoid

reactions with moisture and CO2.

Selective dissolution

A solution containing 15 g KOH, 15 g sucrose and 150 ml water was heated to 90 C. A

quantity of 5 g clinker was added to this solution and stirred for 1 minute. The residue was then

washed in 50 ml water and 100 ml methanol and dried at 60 C.

Preparation of CSH samples

The CSH samples were prepared from typically 2 g powder mixture of Ca(OH)2, SiO2,

AlOOH and Na2SiF6. The solids, with the desired composition, were mixed with three-times

distilled water using a water/solid ratio of 45. The mixture was stored at room temperature for

three weeks in closed glass tubes with continuous stirring using a magnetic stirrer. The reaction

was stopped by filtering the suspensions and the precipitates were washed with three-time

distilled water. The resulting products were dried in a desiccator over silica gel at room

temperature.

Appendixes 145

Alkali-activated metakaolin

Metakaolin was prepared by heat treatment of kaolinite at 600 oC for 4 hours. The powder

was cooled to room temperature in a desiccator before adding the alkali-activating solution. The

desired molar Si/Al and Na/Al ratios were obtained by adjusting the quantities of metakaolin,

silica and the amount of 10 M NaOH solution. Silica was mixed with the 10 M NaOH solution

prior to the addition of metakaolin. The resulting mixtures were cured at 80 oC in closed glass

tubes and the reactions were stopped after one week by suspending the mixtures in acetone for

ca. 15 minutes. Finally, the samples were dried in a desiccator, ground, and stored in a closed

container to avoid reactions with moisture and CO2.

Synthetic strätlingite

Strätlingite were prepared from a mixture of NaSiO39H2O, Ca(OH)2 and NaAlO2 in

accordance with its stoichiometry of 2CaO·Al2O3·SiO2·8H2O. The powder was homogenously

mixed with water using a w/s ratio of 10. Subsequently, the mixture was cured in an ultrasound

bath for 53 days at room temperature. Finally, the powder was filtrated from the solution and

dried at 100 ºC for one day.

Appendixes 146

Appendix 2

NMR measurements and other analytical techniques

The Instrument Centre for Solid-State NMR Spectroscopy has four Varian NMR

spectrometers with different magnetic fields: Unity INOVA-200 (4.7 T), Unity INOVA-300 (7.1

T), Unity INOVA-400 (9.4 T) and Direct Drive VNMRS-600 (14.1 T).

29Si MAS NMR experiments

The 29Si MAS NMR spectra (7.1 T) were recorded using a home-built CP/MAS probe for

7 mm o.d. rotors, a pulse width of 3 s for an rf-field strength of SiSiB1/2 = 58 kHz, a

spinning speed of R = 7.0 kHz, a 30-s relaxation delay and typically 2048 scans.

The 29Si19F CP-REDOR NMR experiments (7.1 T) for the fluoride-mineralized

Portland clinkers used a home-built CP/MAS probe for 5 mm o.d. rotors, R = 10.0 kHz, a 8-s

relaxation delay and 10560 scans. The HH-matching was obtained for Si = 38 kHz, F = 47 kHz

and the CP part used a decreasing linear ramped amplitude (RAMP) on the 19F channel with an

amplitude variation of 10 kHz and a CP contact time of 3.0 ms. The rf-field strengths (rf =

Brf/2 applied in the REDOR sequence are Si = 38 kHz and F = 67 kHz, corresponding to -

pulses of 13.2 and 7.5 s, respectively. The 29Si19F CP/MAS experiments for this series of

samples were performed with the same conditions but used a spinning speed of 5.0 kHz, Si =

47.5 kHz and F = 42.1 kHz.

The 29Si19F CP/MAS NMR spectra (7.1 T) of the synthetic CSH samples were

recorded using a home-built CP/MAS probe for 7 mm o.d. rotors, R = 10.0 kHz, a 10-s

relaxation delay and 16,384 – 22,400 scans. The HH-matching was obtained for Si = 41.0 kHz,

F = 31.2 kHz and the CP part used a RAMP pulse of 10 kHz on the 19F channel for a CP contact

time of 5.0 ms.

The 29Si27Al REAPDOR NMR experiments (14.1 T) used a triple-resonance MAS

probe for 5 mm o.d. rotors from DOTY Scientific Inc. The spectra were acquired using rf-field

strengths of Si = 49 kHz (/2-pulse = 5.1 s and -pulse = 10.2 s) and Al = 50 kHz, a

spinning speed of R = 10.0 kHz, a 30-s relaxation delay and typically 960 scans for the

individual spectra without (S0) and with (S) 27Al irradiation.

Appendixes 147

27Al MAS NMR experiment

27Al MAS NMR spectra (14.1 T) were recorded using a home-built CP/MAS probe for 4

mm o.d. rotors, a pulse width of 0.5 s for an rf-field strength of Al = 60 kHz to ensure

quantitative reliability of the intensities observed for the 27Al central transitions for sites

experiencing different quadrupole couplings. The 27Al NMR experiments typically employed 1H

decoupling with H = 50 kHz, a spinning speed of 13.0 kHz, a 2-s relaxation delay and ~3,200

scans. Furthermore, the 27Al MAS NMR spectrum of the probe itself with an empty spinning

PSZ rotor showed a broadened resonance, which was subtracted from the 27Al MAS NMR

spectra of the samples prior to the evaluation of spectra.

The triple-quantum 27Al MQMAS experiment (9.4 T) was performed using the three-

pulse z-filter sequence with an rf-field strength of B1/2 = 60 kHz for the first and second

pulses and B1/2 = 25 kHz for third selective 90 pulse. Furthermore, 1H decoupling (B2/2 =

40 kHz) was employed in the evolution and detection periods.

The 27Al19F CP/MAS NMR experiment (7.1 T) was performed using a home-built

CP/MAS probe for 7 mm o.d. rotors. The Hartmann-Hahn (HH) match was obtained for Al =

12.5 kHz and F = 35.8 kHz using a CP contact time of 1.5 ms, R = 5.0 kHz, a relaxation delay

of 4 s and 72,384 scans. The CP experiment was further improved using a decreasing linear

ramped amplitude on the 19F channel with an amplitude variation of 15 kHz on the 19F channel

during the CP contact time.

19F MAS NMR MAS experiments

The 19F MAS NMR spectra (7.1 T) were recorded using a home-built CP/MAS probe for

7 mm o.d. rotors, a pulse width of 5 s for an rf-field strength of F = 50 kHz, R = 10.0 kHz, a

10-s relaxation delay and typically 512 scans for the anhydrous samples, 2048 – 8192 for

hydrated Portland cements and 128 for the synthetic CSH.

The 19F MAS NMR spectra for the synthetic CSH samples recorded at 14.1 T used a

home-built CP/MAS probe for 4 mm o.d. rotors, a pulse width of 5.6 s for an rf-field strength

of 48 kHz, a R = 13.0 kHz, a 15-s relaxation delay and typically 512 scans.

Appendixes 148

43Ca MAS NMR experiments

The 43Ca MAS NMR experiments were performed at 14.1 T used two different probes: a

home-built CP/MAS probe for 7 mm o.d. rotors and a Chemmagnetic probe for 7.5 o.d. rotor.

The Hartmann-Hahn (HH) match for the 43Ca19F CP/MAS NMR spectrum was obtained for

AlB1/2 = 13 kHz and FB2/2 = 36 kHz using a CP contact time of 1.5 ms, R = 5.0 kHz, a

relaxation delay of 4 s and 72,384 scans.

The 19F, 27Al, 29Si and 43Ca chemical shifts were referenced to external samples of neat

tetramethylsilane (TMS) using a secondary reference sample of -Ca2SiO4 (–71.33 ppm), a 1.0

M AlCl3.6H2O solution, CFCl3 using a secondary reference sample of Na2SiF6 (-149.3 ppm) and

a 1.0 M CaCl2 solution, respectively.

Other analyses including measurements of the free lime content, the bulk fluoride content

and the chemical compositions by XRF and XRD Rietveld analyses have been conducted at the

chemical laboratory at Aalborg Portland A/S.

Free lime content measurement

The free lime content was measured by dissolving the clinker in ethylene glycol.

Subsequently, the solution was titrated with hydrochloric acid.

Bulk fluoride content measurement

The clinkers were dissolved in a solution of water, HCl and alum KAl(SO4)2. The

fluoride concentration in the resulting mixture was measured by an ion selective electrode. The

solvent were prepared by dissolving 16 g alum in 800 ml water and 800 ml conc. HCl and

finally, diluted with water to 2.0 litres.

Appendixes 149

Procedure for fitting REDOR data in Mathematica

Data

data = x1,y1,x2,y2...,xn,yn

g1 = 2.518148*10^8

g2 = 5.3190*10^7

u = 4Pi*10^(-7)

h = 1.0546*10^(-34)

spin = 10000

k =

Fitting

<< Statistics`NonlinearFit`

sred[n_,dtr_,k_] := 1. - BesselJ[0,Sqrt[2.]*n*dtr]^2. + Sum[2.BesselJ[i,Sqrt[2.]*n*dtr]^2./(16. i^2. - 1.0),

i, 1, k]

besselfit[k_,init_] := NonlinearFit[data, sred[n,dtr,k], n, dtr, init, AccuracyGoal\[Rule]Automatic,

PrecisionGoal\[Rule]Automatic, ShowProgress\[Rule]True]

besselfit[k,0.12]

Plotting

f[n_]:= 1. - BesselJ[0,Sqrt[2.]*n*dtr]^2. + Sum[2. BesselJ[i,

n*Sqrt[2.0]*dtr]^2. /((16. i^2. - 1.0)),i,1,k])

pnkt=ListPlot[data,PlotStyle\[Rule]RGBColor[1,0,0],Thickness[1]]

graf1=Plot[f[n],n,0,80,PlotStyle\[Rule]RGBColor[0,0,1],Frame\[Rule]True,FrameLabel\

[Rule]"nTr",\[CapitalDelta]S/Subscript[S,0],RotateLabel\[Rule]False,PlotRange\[Rule]0,1.1,

TextStyle\[Rule]FontSize\[Rule]20]

Show[graf1,pnkt]

Export["image.bmp",Show[graf1,pnkt],ImageSize\[Rule]1000]

Calculating the distance

dtr=Dipol=dtr/(1/spin)

r=(g1*g2*h*u/(2Pi*4Pi*Dipol))^(1/3)

Appendixes 150

fluoride mineralization of portland cement

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