combining plasticizers/retarders and accelerators · high shear rate (i.e. relevant for mixing) and...
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Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen
Departement Burgerlijke Bouwkunde
Norwegian University of Science and Technology
Faculty of Natural Sciences and Technology
Department of Materials Science and Engineering
Combining Plasticizers/Retarders
And Accelerators
E2006
Promotor: prof. dr. H. Justnes
prof. dr. ir. D. Van Gemert
Klaartje De Weerdt
Dirk Reynders
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Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen
Academiejaar: 2005-2006
Departement: Burgerlijke Bouwkunde
Adres en telefoon: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54
Naam en voornaam studenten: De Weerdt Klaartje
Reynders Dirk
Titel eindwerk: Combineren van plastificeerders/vertragers en versnellers
Korte inhoud eindwerk:
De combinatie van plastificeerders/vertragers en versnellers werd bestudeerd met drie
mogelijke toepassingen in het achterhoofd: 1) het tegengaan van het vertragend effect van
plastificeerders zonder de reologie sterk te wijzigen, 2) de activatie van vertraagd beton op de
werf na veilig transport in warme streken of steden met onvoorspelbaar verkeer en 3) het
oververtragen van overschotten aan vers beton gevolgd door activatie na één of meerdere
dagen.
De experimenten werden grotendeels uitgevoerd op cementpasta. Een Paar-Physica MCR 300
rheometer werd gebruikt ter bepaling van de reologie en een TAM Air isotherme calorimeter
ter bepaling van de hydratiecurves.
Er werd vastgesteld voor toepassing 1) dat calciumnitraat het vertragend effect van natrium en
calcium lignosulfonaat sterk terugschroeft en in het geval van polyacrylaat zelfs volledig
wegneemt terwijl de combinaties werken als plastificeerders, voor toepassing 2) dat de
combinatie natriumgluconaat/calciumnitraat een mogelijk werkend systeem is en voor
toepassing 3) dat de combinatie citroenzuur/calciumnitraat het hergebruik van overschotten
aan vers beton op een later tijdstip mogelijk maakt.
Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessoren: prof. dr. ir. L. Vandewalle – ir. G. Heirman
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Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen
Year: 2005-2006
Department: Burgerlijke Bouwkunde
Address en tel.: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54
Name and surname students: De Weerdt Klaartje
Reynders Dirk
Title of thesis: Combining plasticizers/retarders and accelerators
Summary of thesis:
The combination of plasticizers/retarders with accelerators has been studied in view of three
potential concrete applications: 1) counteracting retardation of plasticizers without negatively
affecting rheology too much, 2) activating retarded concrete at site after safe transport in hot
climate or cities with unpredictable traffic and 3) over-retarding residual fresh concrete one
day and activating it next day or after several days.
The experimental work is largely carried out on cement paste using a Paar-Physica MCR 300
rheometer to determine flow curves and gel strength and a TAM Air isothermal calorimeter
for determination of heat of hydration curves.
It has been found for application 1) that calcium nitrate strongly reduces retardation of sodium
and calcium lignosulphonates and even cancels retardation of polyacrylates, whereas the
blend also has plasticizing effects, for 2) that sodium gluconate/calcium nitrate is a potentially
effective system and for 3) that citric acid/calcium nitrate may facilitate later use of residual
fresh concrete.
Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessors: prof. dr. ir. L. Vandewalle – ir. G. Heirman
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Table of Contents
1 Introduction 1
2 Background on cement, cement hydration, rheology and admixtures 4
2.1 Cement .................................................................................................................. 4
2.2 Cement hydration .................................................................................................. 5
2.3 Rheology ............................................................................................................... 9
2.4 Plasticizers/retarders ............................................................................................. 13
2.5 Calcium nitrate ...................................................................................................... 22
3 Materials and apparatus 24
3.1 Materials................................................................................................................ 24
3.2 Apparatus .............................................................................................................. 27
4 Counteracting plasticizer retardation 34
4.1 Introduction ........................................................................................................... 34
4.2 Calorimetric and rheological measurements......................................................... 35
4.3 Mortar measurements............................................................................................ 75
4.4 General conclusion................................................................................................ 80
5 Long transport of fresh concrete 81
5.1 Introduction ........................................................................................................... 81
5.2 Sodium lignosulphonate........................................................................................ 81
5.3 Citric acid .............................................................................................................. 95
5.4 Lead nitrate............................................................................................................ 98
5.5 Sodium gluconate.................................................................................................. 101
5.6 General conclusion................................................................................................ 110
6 Reutilizing residual fresh concrete 111
6.1 Introduction ........................................................................................................... 111
6.2 Phase I – Screening of retarders............................................................................ 111
6.3 Phase II – Determination of required retarder dosage .......................................... 114
6.4 Phase III – Activation using calcium nitrate ......................................................... 116
6.5 Phase IV – Strength measurements....................................................................... 120
6.6 General conclusion................................................................................................ 126
7 Conclusions 127
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1
Chapter 1
Introduction
This thesis continues a long tradition of Erasmus exchanges between the “Katholieke
Universiteit Leuven” (Belgium) and the “Norges Teknisk-Naturvitenskapelige Universitet i
Trondheim” (Norway). For many years students have been studying advanced aspects of
cementitious materials. Thys, A. and Vanparijs, F. ([1]) studied the longterm performance of
concrete with calcium nitrate, Ardoullie, B. and Hendrix, E. ([2]) focused on the chemical
shrinkage of cementitious pastes and mortars, Clemmens, F. and Depuydt, P. ([3])
investigated early hydration of Portland cements, the thesis of Van Dooren, M. ([4])
concerned the factors influencing the workability of fresh concrete, and Brouwers, K. ([5])
studied a number of cold weather accelerators.
In this thesis the combination of plasticizers/retarders and accelerators has been investigated
in view of three different potential concrete applications.
The first application, which made up the major part of this study, focused on the fact that
plasticizers that are used to increase flow for cementitious materials at equal water-to-cement
ratio also to a variable extent retard setting as a side effect. The objective was to find an
accelerator that at least partially would counteract this retardation without negatively affecting
the rheology too much. Whereas earlier studies on this topic focused on plastic viscosity at
high shear rate (i.e. relevant for mixing) and relatively low dosages of plasticizer, the study
reported here focused on the lower shear rate range (i.e. relevant for pouring concrete) and
higher dosages of plasticizer. The results of this study are presented in Chapter 4. These
results are valuable elements in evaluating the combined use of plasticizers and accelerators,
as it was e.g. applied during construction of Statoil’s Troll platform (Figure 1.1), a huge gas
platform located 80 km north-west of Bergen (Norway) that reaches 303 m below the surface
of the sea. During the construction of its 350 m tall base an accelerator has been used to speed
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Chapter 1: Introduction 2
up the slip forming process of the plasticized concrete as construction works were behind
schedule.
The second application concerns long transport of fresh concrete. The preliminary study was
largely carried out on paste. It was investigated if a concrete mix from a ready mix plant after
being deliberately over-retarded for long transport in for instance hot climate or cities with
unpredictable traffic (e.g. traffic jam) could be activated by adding an accelerator in the
revolving drum close to the construction site before pumping the concrete in place. Results
are discussed in Chapter 5.
The third potential application, presented in Chapter 6, concerns the search for a system to
preserve residual fresh concrete for a few days (e.g. over a weekend) followed by activation
before use. However, it might also be used as an overnight concept. Whereas recently a
freezing preservation technique has been proposed as method for reutilizing left-over
concrete, this study concentrated on a technique consisting of over-retardation of residual
fresh concrete followed by later activation using an accelerator.
Figure 1.1 Troll gas platform (1996)
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Chapter 1: Introduction 3
The necessary background on cement, cement hydration, rheology and admixtures is given in
Chapter 2. Chapter 3 introduces and describes the materials and the apparatus that have been
used throughout this work.
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4
Chapter 2
Background on cement, cement
hydration, rheology and admixtures
2.1 Cement
Cement chemists use in general a short hand notation, C = CaO, S = SiO2, A =Al2O3,
F = Fe2O3 and S = SO3, for the main elements in the chemical analyses of cement, in
addition to H = H2O to describe hydration processes. The elements are determined by
X-ray fluorescence or analytical chemistry and given as the corresponding oxides.
Assuming that the only minerals in the cement are alite (C3S), belite (C2S), aluminate
phase (C3A), ferrite phase (C4AF) and anhydrite ( SC ) the content of these minerals
may be calculated through mass balances. The first four minerals are formed during
equilibrium conditions in the burning of the cement clinker, while the latter mineral
(or gypsum, 2HSC ) is added to the mill when clinker is ground to cement. In
specification sheets, the content of other oxides is also given: N (Na2O), K (K2O) and
M (MgO). “Free lime” is the content of free CaO due to insufficient burning or due to
the decomposition of C3S into C2S and “free lime” if the cooling rate is too low.
The specific surface area (m2/kg) of cement is commonly determined directly by an
air permeability method called the Blaine method. In addition to the specific area, the
particle size is of importance for the hydration rate of cement, since the hydration
takes place at the interface between the cement grain and the water phase. However, it
is important to realise that the surface of a cement grain is inhomogeneous. The
distribution of C3S/C2S- and C3A/C4AF-domains are determined by the milling
process and the difference in resistance against fracture. Since cement grains are
composite grains with possibly all 4 major phases in one grain, efforts to simulate
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Chapter 2: Background 5
cement by adding corresponding amounts of individual minerals will therefore fail.
(Justnes, H., [6], p.10)
2.2. Cement hydration
In the discussion of rheology of cement paste and the interaction with plasticizing
admixtures and retarders, it is of importance to know something about the hydration
until setting. It is sometimes believed that no hydration takes place in the so-called
“dormant” period between water addition and initial setting, while actually a
substantial growth of hydration products takes place on the surface of the cement
grains. (Justnes, H., [6], p.10)
2.2.1 The interstitial phases C3A/C4AF
In the absence of calcium sulphates the first hydration product of C3A which appears
to grow at the C3A surface is gel-like. Later this material transforms into hexagonal
crystals corresponding to the phases C2AH8 and C4AH19. The formation of the
hexagonal phases slows down further hydration of C3A as they function as a hydration
barrier. Finally the hexagonal phases convert to the thermodynamically stable cubic
phase C3AH6 disrupting the diffusion barrier, after which the hydration proceeds with
a fairly high speed. The overall hydration process may thus be written as
phase) (cubic phases) (hexagonal
H 15 AHC 2 AHC AHC H 27 A C 2 63194823 +→+→+
In the presence of calcium sulphate (as in a Portland cement) the amount of hydration
of C3A in the initial state of hydration is distinctly reduced when compared to that
consumed in the absence of SC . Needle-shaped crystals of ettringite are formed as the
main hydration product:
3 2 6 3 32C A 3 CSH 26 H C AS H+ + →
Minor amounts of the monosulphate 124 HSAC or even 194AHC may also be formed
if an imbalance exists between the reactivity of C3A and the dissolution rate of
calcium sulphate, resulting in an insufficient supply of -24SO - ions.
Then ettringite formation is accompanied by a significant liberation of heat. After a
rapid initial reaction, the hydration rate is slowed down significantly. The length of
this dormant period may vary and increases with increasing amounts of calcium
sulphate in the original paste.
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Chapter 2: Background 6
A faster hydration, associated with a second heat release maximum, gets under way
after all the available amount of calcium sulphate has been consumed. Under these
conditions the ettringite, formed initially, reacts with additional amounts of tricalcium
aluminate, resulting in the formation of calcium aluminate monosulphate hydrate
(monosulphate):
12433236 HSAC 3 H 4 A C 2 HSAC →++
As ettringite is gradually consumed, hexagonal calcium aluminate hydrate ( 194AHC )
also starts to form. It may be present in the form of a solid solution with 124 HSAC or
as separate crystals.
The origin of the dormant period, characterised by a distinctly reduced hydration rate,
is not obvious and several theories have been forwarded to explain it. The theory most
widely accepted assumes the build-up of a layer of ettringite at the surface of C3A that
acts as a barrier responsible for slowing down the hydration. Ettringite is formed in a
through-solution reaction and precipitates at the surface of C3A due to its limited
solubility in the presence of sulphates. The validity of this theory has been questioned
arguing that the deposited ettringite crystals are not dense enough to account for the
retardation of hydration. The four proceeding alternative theories have been proposed:
i) The impervious layer consists of water-deficient hexagonal hydrate
stabilised by incorporation of -24SO . It is formed on the surface of C3A and
becomes covered by ettringite.
ii) C3A dissolves incongruently in the liquid phase, leaving an aluminate rich
layer on the surface. Ca2+
- ions are adsorbed on it, thus reducing the
number of active dissolution sites and thereby the rate of C3A dissolution.
A subsequent adsorption of sulphate ions results in a further reduction of
the dissolution rate.
iii) -24SO - ions are adsorbed on the surface of C3A forming a barrier. Contrary
to this theory it has been found that C3A is not slowed down if the calcium
sulphate is replaced by sodium sulphate.
iv) Formation of an amorphous layer at the C3A surface that acts as an
osmotic membrane and slows down the hydration of C3A.
The termination of the dormant period appears to be due to a breakdown of the
protective layer, as the added calcium sulphate becomes consumed and ettringite is
converted to monosulphate. In this through-solution reaction both C3A and ettringite
dissolve and monosulphate is precipitated from the liquid phase in the matrix.
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Chapter 2: Background 7
The composition of the calcium aluminoferrite phase (ferrite phase), usually written
as C4AF, may vary between about C4A1.4F0.6 and C4A0.6F1.4. Under comparable
conditions the hydration products formed in the hydration of the ferrite phase are in
many aspects similar to those formed by the hydration of C3A although the rates differ
and the aluminium in the products is partially substituted by ferric ions. The reactivity
of the ferrite may vary over a wide range, but seems to increase with increasing A/F –
ratio.
2.2.2 The main mineral alite C3S
The hydration of alite can be divided into 4 periods:
a) Pre-induction period: Immediately after contact with water, an intense, but
short-lived hydration of C3S gets under way. An intense liberation of heat may
be observed in this stage of hydration. The duration of this period is typically
no more than a few minutes.
b) Induction (dormant) period: The pre-induction period is followed by a period
in which the rate of reaction slows down significantly. At the same time the
liberation of heat is significantly reduced. This period lasts typically a few
hours.
c) Acceleration (post-induction) period: After several hours the rate of hydration
accelerates suddenly and reaches a maximum within about 5 to 10 hours. The
beginning of the acceleration period coincides roughly with the beginning of
the second main heat evolution peak. The Ca(OH)2 concentration in the liquid
phase attains a maximum at this time and begins to decline. Crystalline
calcium hydroxide (portlandite) starts to precipitate. The initial set as
determined by Vicat-needle is often just after the start of this period and the
final setting time just before the ending of it.
d) Deceleration period: After reaching a maximum the rate of hydration starts to
slow down gradually, however, a measurable reaction may still persist even
after months of curing. The reason for this is that the hydration reaction
becomes diffusion controlled due to hydration products growing around the
unhydrated cement core in increasingly thickness.
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Chapter 2: Background 8
The overall alite hydration reaction may ideally be written as
CH 3 HSC H 7 SC 2 4233 +→+
The calcium hydroxide, CH, is crystalline, while the calcium silicate hydrate is
amorphous with a variable composition and therefore often simply denoted CSH-gel.
2.2.3 Hydration and setting of ordinary Portland cement
The overall hydration of ordinary Portland cement is basically a combination of the
description of the interstitial phase with gypsum and alite as discussed in the
preceding sections. Which of the two dominates the setting is still a matter of
discussion and probably depends on the cement composition
The hydration of Portland cement can be associated with the liberation of hydration
heat. Figure 2.1 shows the heat evolution curve for a typical Portland cement.
Figure 2.1 Hydration heat evolution of an ordinary Portland cement. (Justnes, H., [6],
p. 10)
In cements containing at least a fraction of the K+ in the form of potassium sulphate,
the hydration process may be marked by a distinct initial endothermic peak
immediately after mixing which is due to the dissolution of this cement constituent in
the mixing water. A rather intense liberation of heat with a maximum within a few
Dissolution Ettringite and CSH gel Formation
Induction Period Increase in Ca2+ and OH- Concentration
Initial Set
Final Set
Rapid Formationof CSH and CH
Diffusion-Controlled Reactions
Formation ofMonosulfate
Min Hours Days
Rate
of
Hea
t E
vo
luti
on
Time of Hydration
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Chapter 2: Background 9
minutes is due to the initial rapid hydration of C3S and C3A. Hydration of calcium
sulphate hemihydrate to dehydrate may also contribute to this exothermic peak. After
a distinct minimum, due to the existence of a dormant period in which the overall rate
of hydration is slowed down, a second, mean exothermic peak, with a maximum after
a few hours, becomes apparent. It is mainly due to the hydration of C3S and the
formation of the CSH phase and portlandite. After that, the rate of heat release slows
down gradually and reaches very low values within a few days. In most but not all
cements, a shoulder or small peak may be observed at the descending branch of the
main peak, which is probably due to renewed ettringite formation, there may even be
a second shoulder which is attributed to ettringite-monosulphate conversion. (Hewlett,
P., [7], p. 270-271)
2.3 Rheology
2.3.1 General viscosity
In his “Principa” published in 1687, Isaac Newton formulated the following
hypothesis about steady simple shearing flow: “The resistance which arises from the
lack of slipperiness of the parts of the liquid, other things being equal, is proportional
to the velocity with which the parts of the liquid are separated from each other”. This
is shown in Figure 2.2.
Figure 2.2 Steady simple shearing flow. (Justnes, H., [6], p. 3)
This lack of slipperiness is what we now call “viscosity”. It is synonymous with
“internal friction” and is a measure of “resistance to flow”. The force per unit area
required to produce the motion F/A is denoted shear stress (τ ) and is proportional to
the “velocity gradient” U/d (or “shear rate”, γɺ ). The constant of proportionality, η ,
is called the shear viscosity (also called “apparent” viscosity):
γ
τηɺ
=
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Chapter 2: Background 10
The simplest rheological behaviour for liquids is the Newtonian viscous flow and
Hooke’s law for solid materials. Ideal viscous (or Newtonian) flow behaviour is
described using Newton’s law
γητ ɺ⋅=
Examples of ideal viscous materials are low molecular liquids such as water, solvents,
mineral oils, etc. and they are often called Newtonian liquids.
Hooke’s law states that the shear force acting on a solid is proportional to the resulting
deformation
γτ ⋅= G
where G is the “rigidity modulus”.
Many materials – especially those of colloidal nature – show a mechanic behaviour in
between these to border lines (Hooke’s an Newton’s laws), i.e. they have both plastic
and elastic properties and are called viscoelastic.
Samples with a yield point only begin to flow when the external forces acting on the
material are larger than the internal structural forces. Below the yield point, the
material shows elastic behaviour, i.e. it behaves like a rigid solid that under load
displays only a very small degree of deformation that does not remain after removing
the load. To describe the rheology of samples showing a yield point the Bingham
model is often used. The Bingham model was extended by Herschel/Bulkley to
include samples with apparent yield point due to shear thinning or thickening:
p
p γµττ ɺ⋅+= 0
p = 1 for samples with Bingham behaviour (true yield point)
p < 1 for samples exhibiting shear thinning (apparent yield point)
p > 1 for samples with shear thickening behaviour
Shear thinning is a reduction of viscosity with increasing shear rate in steady flow.
Samples with shear thinning behaviour can be macromolecule solutions or melts
where the individual molecules are entangled. Under high shear load the
macromolecules will stretch out and may be disentangled, causing a reduction of the
viscosity. Furthermore, in dispersions or suspensions shearing can cause particles to
orient in the flow direction, agglomerates to disintegrate or particles to change their
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Chapter 2: Background 11
form. During this process the interaction forces between the particles usually decrease
and this also lowers the flow resistance.
Shear thickening is an increase of viscosity with increasing shear rate. Shear
thickening flow behaviour occurs in concentrated chemically unlinked polymers due
to mechanical entanglements between the mostly branched molecule chains. The
higher the shear load the more the molecule chains prevent each other from moving.
If, during the shear process with highly concentrated suspensions, the particles touch
each other more and more the consequences are similar: the resistance to flow
increases.
Cement paste has shear thinning properties due to both agglomerates of cement grains
and growth of needle-shaped ettringite in the fresh state. An extreme case of
“particles” that will change shape under shear load easily are entrained air bubbles.
There is often more air in concrete than in cement paste, and this may make it difficult
to correlate the concrete rheological properties with those of the “same” paste using
the particle-matrix model. Note that concrete with 5 volume percentage air
corresponds to 15 – 20 volume percentage air in the matrix, something that clearly
will affect the matrix rheology.
2.3.2 Flow resistance
Numerous rheological models have been proposed to describe cementitious materials.
The Bingham model has become very popular due to its simplicity and ability to
describe cementitious flow. The model describes the shear stress (τ ) as a function of
yield stress ( 0τ̂ ), plastic viscosity ( pµ ) and shear rate (γɺ ) as
0 pˆτ = τ + µ ⋅ γɺ
The concept of yield stress is sometimes a very good approximation for practical
purposes. It is however clear that the Bingham model often only applies for limited
parts of the flow curve if the tested material has shear thinning or shear thickening
flow behaviour. The Bingham model is dependent on the shear rate range for shear
thickening materials. The shear thickening behaviour results furthermore in negative
yield stress values at the high shear rate, which has no physical meaning (see Figure
2.3). There is a similar strong effect of the shear rate range on the flow parameters of
a shear thinning paste.
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Chapter 2: Background 12
Figure 2.3 Shear thickening behaviour resulting in negative yield stress values when
using the Bingham model.
The Hershel/Buckley equation p0 pˆτ = τ + µ ⋅ γɺ can be used to fit flow curves of pastes
showing shear thinning or shear thickening behaviour. However, it may be difficult to
compare viscosities (p
µ ) for different mixes with different
p-factors. Negative yield stress values ( 0̂τ ) with no physical meaning can sometimes
also be obtained using the Hershel/Buckley equation. Therefore the area under the
flow curve (Vikan, H. and Justnes, H., [8]) was chosen as a measure of “flow
resistance” (Figure 2.4). This parameter, from here on referred to as “flow resistance”,
shall be used throughout to work to describe the flow curve. The flow resistance will
always be a positive value and not depend on curve shape.
Figure 2.4 Flow resistance.
γɺ
τ
0τ̂
pµ
γɺ
τ
flow resistance
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Chapter 2: Background 13
Furthermore, the choice between two parameters for correlation, as for the Bingham
model, can be omitted. It can be shown (Vikan, H. and Justnes, H., [8]) that the area
under the flow curve represents something more “physical” than an “apparent” yield
stress from Bingham modeling. In a parallel plate set-up with shear area, A [m2], and
gap h [m] between the plates:
A
F=τ [N/m2 or Pa]
h
v∆=∆γɺ [m/s.m or s-1]
where F [N] is the force used to rotate the upper plate and v [m/s] the velocity.
Area under the curve V
vF
hA
vF
h
v
A
F ∆⋅=
⋅
∆⋅=
∆⋅
=∆⋅= γτ ɺ
where V [m3] is the volume of the sample. The unit of the area under the curve is then
[N.m/m3.s or J/m
3.s or W/m
3]. It is in other words the power required to make a unit
volume of the paste flow with the prescribed rate in the selected range. The power,
P [W], required to mix concrete for a certain time interval is actually sometimes
measured by simply monitoring voltage (U [V]) and current (I [A]) driving the
electrical motor of the mixer, since P = U.I.
2.4 Plasticizers/retarders
2.4.1. Introduction
Water-reducing admixtures or plasticizers are all hydrophilic surfactants which, when
dissolved in water, deflocculate and disperse particles of cement. By preventing the
formation of conglomerates of cement particles in suspension, less water is required to
produce a paste of a given consistency or concrete of particular workability.
Maintaining low water contents whilst achieving an acceptable level of workability
results in higher strengths for given cement content as well as lower permeability and
reduced shrinkage. An important consequence of the reduction in the permeability is a
major enhancement of its durability. The permeability of concrete to gases (oxygen,
CO2), and water (carrying chlorides, sulfates, acids and carbonates) is of major
importance with respect to its durability.
Retarding admixtures, which extend the hydration induction period and thereby
lengthening the setting times, are often treated together with plasticizing admixtures
as the main components used for retarding mixtures are also present in water-reducing
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Chapter 2: Background 14
admixtures. As a result, many retarders tend to reduce mixing water and many water
reducers tend to retard the setting of concrete.
A much greater reduction in the volume of mixing water can be achieved using so-
called superplasticizers or high-range water-reducing admixtures in case of concretes
of normal workability. Normal water reducers are capable of reducing water
requirement by about 10-15%. Further reductions can be obtained at higher dosages
but this may result in undesirable effect on setting, air content, bleeding, segregation
and hardening characteristics of concrete. Superplasticizers are capable of reducing
water contents by about 30%. (Ramachandran, V.S., [9], p. 211)
Much of the following is based on ‘Rheology of Cement based Binders – State-of-the-
Art’ by H. Justnes ([6]).
2.4.2. Common plasticizer types
There are four generations of plasticizers/water reducers in terms of time of
discovery/use:
1. Salts of hydrocarboxylic acids with strong retarding effects
2. Calcium or sodium lignosulphonate (denoted CLS or NLS) as by-products
from pulping industry with medium retarding properties.
3. Synthetic compounds like naphtalene-sulphonate-formaldehyde condensates
(SNF) and sulphonated melamine-formaldehyde condensates (SMF) with
small retarding properties.
4. Synthetic polyacrylates with grafted polyether side chains (PA) with small
retarding properties.
The first generation plasticizers, the salts of organic hydroxycarboxylic acids, are
mostly used for their dominating retarding behavior. As the name implies, the
hydrocarboxylic acids have several hydroxyl (OH) groups and either one or two
terminal carboxylic acids (COOH) groups attached to a relatively short carbon chain.
Figure 2.5 illustrates some typical hydroxycarboxylic acids which can be used as
water reducing or retarding admixtures. Gluconic acid is perhaps the most widely
used admixture. Citric, tartaric, mucic, malic, salicylic, heptonic, saccharic and tannic
acid can also be used for the same purpose. Usually they are synthetized chemically
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Chapter 2: Background 15
Figure 2.5 Typical hydrocarboxylic acids used in water reducing admixtures.
(Ramachandran, V.S., [9], p.126)
and have a very high degree of purity as they are used as raw materials by
pharmaceutical and food industries. Some aliphatic hydrocarboxylic acids, however,
can also be produced from fermentation or oxidation of carbohydrates and for this
reason are also called sugar acids. Hydrocarboxylic acids can be used alone as
retarders or water-reducing and retarding admixtures. For use as normal and
accelerating water reducers they must be mixed with an accelerator. (Ramachandran,
V.S., [9], p. 125)
The second generation plasticizers, the lignosulphonates, are still the most widely
used raw material in the production of water reducing admixtures. Lignosulphonates
are sulphonated macromolecules from partial decomposition of lignin by calcium
hydrogen sulphite. Under sulphite pulping, lignin is sulphonated and rendered water
soluble. The spent sulphite liquor contains sulphonated lignin fragments of different
molecular sizes and sugar monomers after removing the pulp. It can be further
purified by fermentation to remove hexoses and by ultrafiltration to enrich larger
molecular fractions. In addition to chemical modification of functional groups for
special applications, simple treatment by sodium sulphate will ion exchange calcium
-
Chapter 2: Background 16
through formation of gypsum that is removed. A fragment of a lignosulphonate is
illustrated in Figure 2.6. Fractionation to enrich larger molecular fractions increases
the effectiveness of lignosulphonate as a dispersant for cement in water and reduces
the retarding effect. Sodium lignosulphonates retard in general less than calcium
lignosulphonates.
Figure 2.6 Fragment of lignosulphonate. (Justnes, H., [6], p. 30)
Due to the size of the molecule, it cannot be ruled out that lignosulphonates disperse
cement both through electrostatic repulsion and steric hindrance. The average
molecular weight of common lignosulphonates used as plasticizers for cement may be
about 5,000-10,000. It is assumed that the structure of lignosulphonates in solution
consists of a mainly hydrophobic hydrocarbon core with sulphonic groups positioned
at the surface. The bulk of the model is assumed to be made up of cross linked, poly-
aromatic chains which are randomly coiled. The negatively charged groups are
positioned mainly on the surface or near the surface of the particle, and a double layer
-
Chapter 2: Background 17
of counter ions is present in the solvent. The lignosulphonate molecules behave as
expanding polyelectrolytes as they expand at low and contract at high salt
concentrations.
The third generation plasticizers, the synthesized polymers with sulphonated groups,
are not covered here as they were not used in this work.
The fourth generation of plasticizers is based on a polyacrylate (PA) backbone that is
obtained by free radical polymerization of different vinyl monomers. This backbone
may vary widely in composition depending on the choice of monomers as shown in
Figure 2.7. The next step is to graft on side chains of polyether (polyethylene oxide).
Variations in the nature and relative proportions of the different monomers in the
copolymer yield a group of products having broad ranges of physico-chemical and
functional properties. Since some of the polyacrylates seem to enhance the
segregation tendencies, they are often combined with viscosifiers to counteract this
effect.
Figure 2.7 Illustration of a generic group of polyacrylate copolymers where R1 equals
H or CH3, R2 is a poly-ether side chain (e.g., polyethylene oxide) and X is a polar
(e.g., CN) or ionic (e.g., SO3) group. (Ramachandran, V.S. et al., [10], p.52)
2.4.3. Mechanisms of dispersion
There are generally two main mechanisms which explain how plasticizers disperse
particles in a suspension: electrostatic repulsion and steric hindrance. These two
mechanisms are sketched Figure 2.8 and Figure 2.9 respectively. Since its ionic lattice
is cut, any fractured mineral particle will have domains of positive and negative
charged sites. Negatively charged polymers (common feature of most plasticizers)
will absorb to the positive charged sites and render the total particle surface negatively
charged. As negatively charged particles approach each other there will be an
electrostatic repulsion preventing them from getting close and attach to form
-
Chapter 2: Background 18
Figure 2.8 Sketch of how negative charged polymers may adsorb to both positively
and negatively charged domains of particles. The resulting overall negative charge of
the particles will prevent them to form agglomerates by electrostatic repulsion and
they will stay dispersed. The electrostatic repulsion effect increases with increasing
charge density of the adsorbed molecule. (Ramachandran, V.S. et al, [10], p.200)
Figure 2.9 Sketch of branched macromolecules adsorbing on the surface of grains
that will create steric hindrance for them to get close enough to form agglomerates.
The size effect of steric hindrance increases with increasing molecular weight (or
actual size) of the adsorbed molecule. (Ramachandran, V.S. et al, [10], p.201)
agglomerates. The latest generation of grafted polymers may also have some negative
charges on their backbone that can co-ordinate on the positive sites but it should be
noted that the ester group of acrylates may co-ordinate strongly to calcium anyway
without any charge. The grafted polyether chains perpendicular to the backbone may
stretch out and hinder the particles to get close enough to form agglomerates. This
so-called steric hindrance is based on the size of the adsorbed molecules
perpendicular to the particle surface. This is shown in Figure 2.10.
-
Chapter 2: Background 19
Figure 2.10 Idealized model on how a grafted polymer will lead to steric hindrance
by adsorbing the polymer backbone to the surface and stretching the grafted side
chains into the water phase. (Justnes, H., [6], p. 26)
The model of the grafted polymer dispersing according to steric hindrance in Figure
2.10 may be a simplification. It would then be necessary for all the intermolecular
bonds (van der Waals type hydrogen bonds) to break and unwind the polyether chains
to let them stretch out into the water phase (even though the hydrophilic nature of
polyethers may aid in stabilizing such configuration). Alternatively, the molecules
may stay unwound as polymeric balls or “micelles” that equally well will lead to
steric hindrance (see Figure 2.11).
While the first three generations of plasticizers are said to rely on electrostatic
repulsion as mechanism for their dispersion of cement agglomerates, the fourth
generation is the first to be designed to function through steric hindrance.
Figure 2.11 Model of how macromolecules with strong intramolecular forces still
may disperse through steric hindrance as polymer “balls” or “micelles” (after Justnes,
H., [6], p. 26)
Another effect that will prevent agglomerates formation is called depletion as
sketched in Figure 2.12. The mechanism of this is that surplus polymer will not be
adsorbed and will stay in the water phase between the particles and for this reason
prevents them from getting close enough to form agglomerates.
Macromolecular micelles
Cement surface
-
Chapter 2: Background 20
Figure 2.12 Surplus polymer in the water phase (not adsorbed) may prevent the
cement particles to get close enough to form agglomerates. This depletion effect will
not disperse by itself, but rather help stabilize dispersions by preventing flocculation.
(after Justnes, H., [6], p. 27)
Rheology may also be improved by a tribology effect as sketched in Figure 2.13.
Tribology is the science of friction, abrasion and lubrication. Low molecular weight
compounds may reduce the friction between particles and also reduce the surface
tension of the water face.
Figure 2.13 Low molecular compounds in the water phase may improve rheology of
particle suspensions by lubrication and by lowering the surface tension of the water
phase, which may be denoted as a tribology effect. (after Justnes, H., [6], p. 27)
Initial rheology of cement paste is also governed by early hydration, unlike inert
particles suspensions (e.g. limestone). Thus, there are other mechanisms of how
plasticizers may improve rheology of cement pastes. One is adsorption to active sites
cementparticles
cementparticles
Low molecular weight compound
cementparticles
cementparticles
polymer
-
Chapter 2: Background 21
and retardation of the formation of hydration products (see Figure 2.14), another is
changing the morphology of the hydration products formed by reducing growth (see
Figure 2.15) or by intercalation in the hydration products (see Figure 2.16).
Figure 2.14 Rheology in cement pastes may improve due to less hydration caused by
adsorbed polymers co-ordinating to active sites (■). The effect increases with
decreasing size of the molecules. LMW = low molecular weight and HMW = high
molecular weight. (Ramachandran, V.S. et al, [10], p.201)
Figure 2.15 Schematic illustration of hydration nucleation and growth inhibition by
adsorbed molecules. Selective adsorption on crystal planes can give morphology
changes. (Ramachandran, V.S. et al, [10], p.208)
-
Chapter 2: Background 22
Figure 2.16 Intercalation of plasticizer in hydration product with structural alteration
(e.g. lignosulphonates with hydration products of C3A). (Ramachandran, V.S. et al,
[10], p.209)
2.5 Calcium nitrate
This section is based on the paper Setting Accelerator Calcium Nitrate,
Fundamentals, Performance and Applications by Justnes, H. and Nygaard, E. ([11]).
In the past a growing concern about the chloride-induced corrosion of reinforcing bars
embedded in Portland cement concrete has led to the development of a number of
chloride-free set accelerating admixtures to replace the widely used calcium chloride
accelerator. In 1981, calcium nitrate, Ca(NO3)2, was proposed as a basic component
of a set accelerating admixture. Calcium nitrate, denoted as CN, works as a pure set
accelerator (see Figure 2.17), and not as a strength development accelerator. The pure
set accelerating effect is beneficial in preventing any increase in maximum
temperature in massive constructions due to the heat of hydration. In spite of this, an
increase in long term compressive strength is often observed, probably due to binder
morphology changes.
Figure 2.17 Difference between set and hardening accelerators.
Reference
Setting
Hardening
-
Chapter 2: Background 23
The effectiveness of CN as a setting accelerator for cement is dependent on the
cement type. The set accelerating efficiency appeared to be correlated with the belite,
C2S, content, while no correlation between set accelerating efficiency and C3A has
been found. In order to find the reason for the linear correlation between accelerator
efficiency and belite content, and possibly the mechanism of CN as set accelerator for
cement, Justnes and Nygaard undertook a thorough analysis of the water in cement
pastes from mixing to paste setting for two different cement types (HS65 and P30).
For both cement pastes the most noticeable change when
1.55 % CN by weight of the cement was added, was that the calcium concentration
increased and the sulphate concentration decreased. Thus, the mechanism for
accelerated setting is twofold:
i) an increased calcium concentration leads to a faster super-saturation of the
fluid with respect to calcium hydroxide, Ca(OH)2, while
ii) a lower sulphate concentration will lead to slower/less formation of ettringite
which will shorten the onset of aluminate, C3A, hydration.
The difference between the two cements was that P30 contained much more of the
mineral aphthitalite, K3Na(SO4)2, which leads to a high initial sulphate concentration
in the fluid. When CN was added, much of the calcium precipitated as sparingly
soluble gypsum. Even when 1.55 % CN was added to the P30 paste, the sulphate
concentration in the fluid was higher than in the water of HS65 paste without CN. At
the same time, the calcium concentration in the fluid of P30 with CN was only
slightly higher than for HS65 without CN. The Ca2+
concentration in the water of
HS65 paste, on the other hand, was increased with about 4 times when 1.55 % CN
was added. Thus, the reason why CN did not accelerate the setting of P30 was that it
contained a very soluble alkali sulphate originating from the clinker process.
The correlation between belite content and set accelerating efficiency is
understandable since belite can incorporate a portion of the total alkalies in its
structure and consequently prevent them from taking part in the early fluid chemistry
since belite is a slow reacting mineral. Hence, for a series of cements, with about
equal total alkali content and increasing belite content, it is expected that the set
accelerating efficiency of CN will increase. On the other hand, in an investigation of
calcium acetate, chloride and nitrate on belite hydration, it has been found that after 1
day, the chemically bound water was 6 times larger when 2 % CN was mixed in the
water, while 2 % calcium acetate and 2 % calcium chloride only increased the 1 day
chemically bound water by 30 % compared with the reference. Therefore, a special
influence of CN on β-C2S can not be excluded.
-
24
Chapter 3
Materials and apparatus
The purpose of this chapter is to introduce and describe the materials and the apparatus that
have been used frequently throughout this work.
3.1 Materials
3.1.1. Cements
Two Portland cements have been used in this thesis. Their physical characteristics are given
in Table 3.1, chemical analysis according to producer and minerals by Bogue estimation is
given in Table 3.2 and the mineralogy of the cements determined by multicomponent Rietveld
analyses of XRD profiles, specific surface determined by the Blaine method and content of
easily soluble alkalis determined by plasmaemissionspectrometry are given in Table 3.3.
Table 3.1 Physical characteristics of Portland cements according to EN 196
Cement type CEM I
52.5 R - LA
CEM I
42.5 RR*
Fineness:
Grains + 90 µm
Grains + 64 µm
Grains – 24 µm
Grains – 30 µm
Blaine (m2/kg)
1.7%
4.1%
66.3%
75.6%
359
0.1%
0.5%
89.2%
94.8%
546
Water demand 26.7% 32.0%
Le Chatelier 0.5 mm 0 mm
Initial set time 145 min. 115 min.
σσσσc (MPa) at 1 day
2 days
7 days
28 days
17.1
27.5
42.5
58.6
32.7
39.9
49.3
58.9
-
Chapter 3: Materials and apparatus 25
Table 3.2 Chemical analysis (%) of the Portland cements according to producer and minerals
(%) by Bogue estimation.
Cement
type
CEM I
52.5 R - LA
CEM I
42.5 RR*
Chemical
analyses
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Free CaO
K2O
Na2O
Equiv. Na2O
Cr6+
(ppm)
Carbon
Chloride
LOI
Fly Ash
63.71
20.92
4.21
3.49
2.67
1.87
0.84
0.46
0.19
0.49
0.30
0.17
0.02
1.72
-
61.98
20.15
4.99
3.36
3.55
2.36
1.23
1.08
0.42
1.13
0.00
0.04
0.03
1.34
-
Minerals
by Bogue
C3S
C2S
C3A
C4AF
CS
50.4
22.0
5.3
10.6
5.8
50.7
19.5
7.5
10.2
7.7
(* The RR term refers to the Norwegian standard NS 3086 (2003) where RR means extra
demands to 1 and 2 day strength compared to R. 42.5 RR should then have characteristic 1
day strength ≥ 20.0 MPa and 2 day strength ≥ 30.0 MPa.)
It can be seen that the CEM I 42.5 RR cement had a higher alkali and C3A content and a
higher specific surface than the CEM I 52.5 R LA cement and, as a consequence of the latter
two, had a higher water demand. CEM I 42.5 RR cement pastes were therefore prepared with
a w/c ratio of 0.50, whereas CEM I 52.5 R LA cement pastes were prepared with a w/c ratio
of 0.40 throughout this work.
-
Chapter 3: Materials and apparatus 26
Table 3.3 Mineral composition (%) and alkali content of Portland cements obtained by
QXRD and plasmaemissionspectrometry
Cement
type
CEM I
52.5 R - LA
CEM I
42.5 RR
Alite
Belite
Ferrite
Cubic
aluminate
Orthorombic
aluminate
Lime
Periclase
Gypsum
Hemihydrate
Anhydrite
Calcite
Portlandite
Quartz
Arcanite
Mullite
Amporhous
65.0
12.9
9.6
0.5
3.0
0.6
0.3
1.4
1.5
0.4
4.0
0.3
0.4
0.0
-
-
64.7
14.8
7.5
5.9
1.1
1.0
1.6
0.0
1.8
0.6
0.5
0.3
0.0
0.3
-
-
Blaine 364 546
K (%)
Na (%)
Naeqv (%)
0.32
0.74
0.26
0.92
0.22
0.76
3.1.2. Plasticizers/retarders
Borregaard Lignotech, Sarpsborg, Norway delivered two lignosulphonate powders denoted as
Ultrazine Na and Ultrazine Ca. Ultrazine Ca (CLS) was sugar reduced and large molecular
size enriched by ultra filtration of the basic calcium lignosulphonate obtained in the sulfite
process on spruce. In Ultrazine Na (NLS) the calcium in Ultrazine Ca has been ion exchanged
with sodium. Solutions with 30% dry matter were prepared before use.
A polyether grafted polyacrylate water solution containing 18% solids and a viscosifying
agent has also been used as a plasticizer. The molecular weight of the polyacrylate was
220,000.
A number of substances were used as retarders. They were all of analytical laboratory grade:
- citric acid (C6H8O7 ⋅ H2O )
- sodium salt of gluconic acid (C6H11NaO7)
- sodium salt of tartaric acid (Na2C4H4O6 ⋅ 2H2O, right-turning form)
- lead nitrate (Pb(NO3)2)
- zinc acetate (Zn(CH3OO)2 ⋅ 2H2O)
- sucrose (C12H22O11)
-
Chapter 3: Materials and apparatus 27
The trisodiumphosphate (Na3PO4 ⋅ 12H2O) used in this work was from technical quality.
Household sugar was also used as a retarder.
3.1.3. Accelerator
Technical calcium nitrate (CN) was used as an accelerator. Its formula may be written as
xNH4NO3 ⋅ yCa(NO3)2 ⋅ zH2O, and named xyz CN according to short hand practice. The CN
used in the present work had x = 0.092, y = 0.500 and z = 0.826, or in other words 19.00%
Ca2+
, 1.57% +4NH , 64.68% -
3NO and 14.10% H2O. The CN was delivered in the form of
granules by Yara, Porsgrunn, Norway.
Calcium nitrate was also used in the form of a 50% aqueous solution of pure calcium nitrate
Ca(NO3)2, also obtained from Yara. The fluid is colourless, viscous and can easily be blended
into the mixing water.
3.2. Apparatus
3.2.1. Mixer
The cement pastes were blended in a high shear mixer by Braun (MR5550CA) and by Tefal
(Rondo 500) as illustrated in Figure 3.1. The mixers had a rotational speed of approximately
800 rpm. It will be notified which of the blenders has been used in each chapter. The blending
was performed by adding cement to the water and mixing for ½ minute, resting for 5 minutes
and blending again for 1 minute.
Figure 3.1 High shear blenders from Braun (left) and Tefal (right)
-
Chapter 3: Materials and apparatus 28
3.2.2. Rheometer
Rheological measurements have been performed with a MCR 300 rheometer produced by
Paar Physica (Figure 3.2). A parallel-plate measuring system was used as illustrated in Figure
3.3. This measuring system consisted of two plates. The surfaces of both the bob and the
motionless plate were flat, but the upper plate had a serrated surface of 150 µm depth to avoid
slippage.
Figure 3.2 MCR 300 rheometer by Paar Physica
Figure 3.3 The parallel plate measuring system (Mezger T., [12], p. 177)
The geometry of the upper plate is determined by the plate radius R being 2.5 cm. The
distance H between the two parallel plates must be much smaller than the radius R and has
been recommended to be at least 10 times larger than the largest of the particles of the sample
(Mezger T., [12], p. 177-179). The average particle size of unhydrated cement being
-
Chapter 3: Materials and apparatus 29
approximately 10 µm (Taylor, [13]), the gap between the plates was set to 1 mm for all
measurements. The temperature controlled bottom plate was set to 20° C.
The parallel plate measuring system makes it possible to measure dispersions containing
relatively large particles as well as samples with three-dimensional structures. The measuring
system has however also a number of disadvantages. There is no constant shear gradient in
the measurement gap because the shear rate (or shear deformation) increases in value from
zero at the center of the plate to the maximum at the edge. Furthermore, several unwanted
phenomena can occur at the edge of the plate: inhomogeneities, emptying of the gap, flowing-
off and spreading of the sample, evaporation of water, or skin formation (Mezger T., [12], p.
180-181). To reduce evaporation both upper and lower plates were covered with a plastic ring
and a metallic lid while a water trap attached to the upper plate was filled with water to ensure
saturated water pressure.
The following measuring sequence was used to determine the flow resistance (area under the
(down) flow curve in the range from 2 to 50 1/s), the gel strength after 10 seconds of resting
and the gel strength after 10 minutes of resting:
1. 1 minute with constant shear rate (γɺ ) of 100 1/s to stir up the paste
2. 1 minute resting
3. Stress (τ ) – shear rate (γɺ ) curve with linear sweep of γɺ from 2 up to 200 1/s in 30
points lasting 6 s each (up curve)
4. Stress (τ ) – shear rate (γɺ ) curve with linear sweep of γɺ from 200 down to 2 1/s in 30
points lasting 6 s each (down curve)
5. 10 s resting
6. Shear rate (γɺ ) – stress (τ ) curve with logarithmic sweep of τ from 1 to 100 Pa in 30
points lasting 6 s each to measure the gel strength after 10 s rest
7. 10 minutes resting
8. Shear rate (γɺ ) – stress (τ ) curve with logarithmic sweep of τ from 1 to 400 Pa in 70
points lasting 6 s each to measure the gel strength after 10 minutes rest
The recording of the shear rate (γɺ ) – stress (τ ) curves was stopped whenever the shear rate
(γɺ ) exceeded 300 1/s to prevent the sample from being lost from the measurement gap.
A flow chart of the mixing and measurement sequence is shown in Figure 3.4.
-
Chapter 3: Materials and apparatus 30
The reproducibility of the rheological measurements was investigated for two different
cement pastes. The cement pastes were made with distilled water. The plasticizer was added
to the water. Cement paste 1 was prepared with CEM I 52.5 R LA cement and 0.30% sodium
lignosulphonate by weight and a w/c ratio of 0.40. Paste 2 was prepared with CEM I 42.5 RR
cement and 0.50% sodium lignosulphonate by weight and a w/c ratio of 0.50. Total paste
volume was approximately 250 ml.
Each of the two cement pastes was prepared 5 times. The rheological data has been
transformed into flow resistance (area under the flow curve in the range from 2 to 50 1/s), gel
strength after 10 seconds of rest and gel strength after 10 minutes of rest. The results are
shown in Table 3.3 for cement paste 1 and Table 3.5 for paste 2.
The data show that the reproducibility of the flow resistance is reasonable. Measurements of
the gel strength show higher deviations, especially for the 10 minute gel strength of the CEM
I 52.5 R LA cement pastes which had a standard deviation of 27%.
Shear rate
Time
mixing
½ minute
mixing
1 minute
5 minutes
rest
8 ½ minutes
1 minute
rest
10 seconds
rest
10 minutes
rest
1 minute
at 100 1/s
up
curve
down
curve
gel
strength
gel
strength
transfer to
rheometer
Figure 3.4 Flow chart of the mixing and measurement sequence
-
Chapter 3: Materials and apparatus 31
Table 3.4 Reproducibility of rheological measurements for cement paste 1
(w/c=0.40 – CEM I 52.5 R LA – 0.30% Ultrazine Na)
Gel strength [Pa] Flow resistance
[Pa/s] 10 sec. 10 min.
391 2.4 14.2
383 2.4 13.0
394 2.8 9.2
419 2.8 10.0
PASTE 1
384 2.8 7.1
Average 394 2.7 10.7
Standard deviation 15 0.2 3
% standard dev. 4% 9% 27%
Table 3.5 Reproducibility of rheological measurements for cement paste 2
(w/c=0.50 – CEM I 42.5 RR – 0.50% Ultrazine Na)
Gel strength [Pa] Flow resistance
[Pa/s] 10 sec. 10 min.
2119 22.2 36.8
2375 22.2 36.8
2455 26.1 40.1
2343 22.2 40.1
PASTE 2
2392 22.2 36.8
Average 2337 2.7 38.1
Standard deviation 128 1.7 2
% standard dev. 5% 7% 5%
3.2.3. Calorimeter
An eight-channel TAM Air Isothermal Calorimeter from Thermometric AB, Sweden was
used for the heat of hydration measurements (Figure 3.5). The calorimeter was calibrated at
20° C. The hydration heat was measured by weighing 6 to 7 grams of cement paste into a
glass ampoule after which the ampoule was sealed and loaded into the calorimeter. The
ampoules were wiped with a paper tissue to make sure that they were perfectly clean and dry
when they were inserted into the calorimeter.
When studying the heat of hydration measurements it should be kept in mind that when an
ampoule is loaded into the calorimeter the temperature of the calorimeter will be disturbed. If
the temperature of the ampoule is 2 degrees higher than the thermostat temperature, an
exothermic heat flow, showing an exponential decay, of roughly 400 mW is observed. This
phenomenon explains the exponential decay in specific heat which is observed in the first
hour after mixing.
-
Chapter 3: Materials and apparatus 32
Figure 3.5 TAM Air Isothermal Calorimeter
3.2.4. Adsorption of plasticizers
To measure the consumed amount of lignosulfonate on the cement a UV Spectrophotometer
from Thermo Spectronic was used as illustrated in Figure 3.6. The adsorption measurements
in this work utilized a wavelength of 285 nm. Pore solutions were extracted from the cement
pastes by filtering the pastes through 0.45 µm filter paper on a Büchner funnel using low
vacuum 15 minutes after water addition. They were then diluted 25, 50 or 100 times with a
solution of ‘artificial pore water’ (NaOH and KOH with a K/Na molar ratio equal to 2 and pH
= 13.2). The amount of plasticizer in the water phase was read from calibration curves which
had been made with a dilution series of each of the two lignosulfonates being used in this
work. The difference between the added and the measured content of plasticizer gave the
bound portion.
Figure 3.6 UV Spectrophotometer from Thermo Spectronic
-
Chapter 3: Materials and apparatus 33
The consumption of polyacrylate on cement was determined by measuring Total Organic
Carbon (TOC) left in the pore water with a Shimadzu TOC Analyzer 5000A. The Shimadzu
TOC 5000A works by converting organic matter to carbon dioxide by combustion with a
catalyst that promotes the redox reaction with oxygen. The reaction takes place at a
temperature of 680° C. The amount of carbon dioxide formed is measured to determine the
carbon content. The amount of plasticizer bound to the cement is given by the difference
between the added and the measured content of organic carbon.
-
34
Chapter 4
Counteracting plasticizer retardation
4.1 Introduction
Plasticizers are used to increase flow for cementitious materials at equal water-to-cement
ratio, but will also to a variable extent retard cement setting as a side effect. The objective was
to find an accelerator that at least partially would counteract this retardation without
negatively affecting the rheology too much. Earlier papers (Justnes, H., Petersen, B.G., [14]
and [15]) focusing on this topic studied rheological properties at high shear rate (i.e. relevant
for mixing) for relatively low dosages of plasticizer, whereas the study reported in this
chapter focused on the lower shear rate range (i.e. relevant for pouring concrete) and higher
dosages of plasticizer. Three different plasticizers were tested in the present study, but the
accelerator was chosen to be calcium nitrate.
The experimental work is largely carried out on cement paste using a Physica MCR 300
rheometer to determine flow curves and gel strength and an isothermal calorimeter for
determination of heat of hydration curves.
Two promising admixture blends were also tried out in mortar.
-
Chapter 4: Counteracting plasticizer retardation 35
4.2 Calorimetric and rheological measurements
4.2.1. Experimental
The investigated cement pastes were made with distilled water. Plasticizer and accelerator
were added to the water before mixing, except for one series of pastes marked with DA
(delayed addition), where the plasticizer was added 5 minutes after the start of initial blending
in a 30% aqueous solution. Both a CEM I 52.5 R LA and a CEM I 42.5 RR Portland cement
were used. Three different plasticizers were studied: a sodium lignosulphonate (NLS), a
calcium lignosulphonate (CLS) and a polyether grafted polyacrylate (PA). The setting
accelerator calcium nitrate (CN), available in a 50% aqueous solution, was used to counteract
the retardation. A more detailed description of both plasticizers and accelerator can be found
in Chapter 3. Table 4.1 provides an overview of the experimental program.
Table 4.1 Experimental program
Cement type Plasticizer Accelerator
CEM I 52.5 R LA Reference (0%)
(w/c = 0.40) 0.15% NLS*
0.15% NLS DA*
0.30% NLS
0.50% NLS
0.30% CLS
0.50% CLS
0.10% PA
CEM I 42.5 RR Reference (0%)
(w/c = 0.50) 0.50% NLS
1.00% NLS
0.50% CLS
1.00% CLS
0.10% PA
0.00% CN
0.25% CN
0.50% CN
0.75% CN
1.00% CN
(* The 1.00% CN dosage was not studied for these series.)
In Chapter 3 it was pointed out that the CEM I 42.5 RR cement had a higher alkali and C3A
content and a higher specific surface than the CEM I 52.5 R LA cement and, as a consequence
of the latter two, had a higher water demand. CEM I 42.5 RR cement pastes were therefore
prepared with a w/c ratio of 0.50, whereas CEM I 52.5 R LA cement pastes were prepared
with a w/c ratio of 0.40 throughout this work. Total paste volume was approximately 250 ml.
The blending was performed in a high shear mixer of Braun (see 3.2.1) by adding the cement
to the water containing plasticizer and/or accelerator and mixing for ½ minute, resting for 5
-
Chapter 4: Counteracting plasticizer retardation 36
minutes and blending again for 1 minute. The cement pastes containing 0.15% sodium
lignosulphonate were mixed with a high shear mixer by Tefal using the same blending
sequence.
The heat of hydration versus time curves were measured by accurately weighing 6 to 7 grams
of cement paste into a glass ampoule after which the ampoule was sealed and loaded into the
calorimeter.
The rheological properties were studied by performing the measurement sequence discussed
in section 3.2.2 on the cement pastes 15 minutes after the start of the blending:
To measure the consumed (adsorbed and intercalated) amount of plasticizer by cement, pore
solutions were extracted from the cement pastes by filtering the pastes through 0.45 µm filter
paper on a Büchner funnel using low vacuum 15 minutes after water addition.
The consumed amount of lignosulphonate was determined using a UV Spectrophotometer
from Thermo Spectronic. The adsorption measurements in this work utilized a wavelength of
285 nm. The pore solutions were diluted 25, 50 or 100 times with a solution of ‘artificial pore
water’ (NaOH and KOH with a K/Na molar ratio equal to 2 and pH = 13.5). The amount of
plasticizer in the water phase was read from calibration curves which had been made with a
dilution series of each of the two lignosulphonates being used in this work. The calibration
curves for NLS and CLS are given in Figure 4.1 and Figure 4.2 respectively. The difference
between the added and the measured content of plasticizer gave the consumed amount.
The consumption of polyacrylate by cement was determined by measuring Total Organic
Carbon (TOC) left in the pore water with a Shimadzu TOC Analyzer 5000A.
-
Chapter 4: Counteracting plasticizer retardation 37
Calibration curve, NLS
y = 137.5844x
R2 = 0.9992
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007
% Added
Ab
so
rba
nc
e
Figure 4.1 Calibration curve for adsorbance of sodium lignosulphonate (NLS).
Calibration curve, CLS
y = 137.2718x
R2 = 0.9997
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007
% Added
Ab
so
rba
nc
e
Figure 4.2 Calibration curve for adsorbance of calcium lignosulphonate (CLS).
Prior to discussing the results, we shall provide an overview of the way in which the read outs
from the rheometer were converted into flow resistance (area under the flow curve in the
range from 2 to 50 1/s, see also Chapter 2), gel strength after 10 seconds of rest and gel
strength after 10 minutes of rest. The measurements on the cement paste made with
CEM I 52.5 R LA cement without any admixtures shall be used to illustrate this:
-
Chapter 4: Counteracting plasticizer retardation 38
1. The flow resistance is defined as the area under the down flow curve in the range from
2 to 50 1/s. The down curve for the paste made with CEM I 52.5 R LA cement is
shown in Figure 4.3. Table 4.2 shows the read outs from the rheometer. The area
under the curve was determined by calculating the average of the shear stresses for
every two consecutive measuring points in the range from 2 to 50 1/s and multiplying
this by the difference in shear rate for these points. In this case a value of 2283 Pa/s
was found for the flow resistance.
Table 4.2 Rheometer read outs for the down curve.
Meas. Pt. Shear Rate [1/s] Shear Stress [Pa]
1 200 98.2
2 193 96.9
3 186 95.7
4 180 94.6
5 173 93.4
6 166 92.9
7 159 91.1
8 152 89.9
9 145 88.8
10 139 87.5
11 132 86.4
12 125 85.1
13 118 83.8
14 111 82.5
15 104 81.1
16 97.6 79.5
17 90.8 77.7
18 83.9 75.7
19 77.1 73.6
20 70.3 71.4
21 63.4 69.4
22 56.6 67.3
23 49.8 64.7
24 43.0 61.4
25 36.1 58.0
26 29.3 53.3
27 22.5 47.3
28 15.7 40.1
29 8.83 30.8
30 2.01 22.2
-
Chapter 4: Counteracting plasticizer retardation 39
Down Curve
0
20
40
60
80
100
120
0 50 100 150 200
Shear Rate [1/s]
Sh
ea
r S
tre
ss
[P
a]
Figure 4.3 Down curve.
2. The 10 sec. gel strength can be derived from the shear rate (γɺ ) – stress (τ ) curve with
logarithmic sweep of τ from 1 to 100 Pa in 30 points lasting 6 s each. The curve is
plotted in Figure 4.4. The rheometer read outs are given in Table 4.3. The 10 sec. gel
strength was calculated by taking the average of the shear stresses of measuring points
19 and 20 (Table 4.3) as the breakthrough happened somewhere in between. That way
a value of 19 Pa was found for the 10 sec. gel strength.
10 sec. gel strength
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100
Shear Stress [Pa]
Sh
ear
Rate
[1/s
]
Figure 4.4 Shear rate – stress curve to determine the 10 sec. gel strength.
gel strength
-
Chapter 4: Counteracting plasticizer retardation 40
Table 4.3 Rheometer read outs to determine the 10 sec gel strength.
Meas. Pt. Shear Rate [1/s] Shear Stress [Pa]
1 0.00 1.00
2 0.00 1.17
3 0.00 1.37
4 0.00 1.61
5 0.00 1.89
6 0.00 2.21
7 0.00 2.59
8 0.00 3.04
9 0.00 3.56
10 0.00 4.18
11 0.00 4.89
12 0.00 5.74
13 0.00 6.72
14 0.00 7.88
15 0.00 9.24
16 0.00 10.8
17 0.00 12.7
18 0.00 14.9
19 0.00 17.4
20 3.44 20.4
21 9.32 24.0
22 12.9 28.1
23 16.0 32.9
24 21.3 38.6
25 26.1 45.2
26 35.0 53.0
27 48.7 62.1
28 73.6 72.8
29 110 85.3
30 155 100
3. The calculation of the 10 min. gel strength is completely similar to that of the 10 sec.
gel strength and shall therefore not be treated.
-
Chapter 4: Counteracting plasticizer retardation 41
4.2.2. Results and discussion for reference pastes
Figure 4.5 shows the flow resistances for both CEM I 52.5 R LA and CEM I 42.5 RR
reference cement pastes. The flow resistance of the CEM I 42.5 RR cement paste (w/c = 0.50)
is higher than the CEM I 52.5 R LA paste (w/c = 0.40) in spite of the higher water-to-cement
ratio. This is due to the higher specific surface and the content of cubic C3A. Addition of
calcium nitrate appeared to have no effect on the flow resistance of these pastes.
reference
0
500
1000
1500
2000
2500
3000
3500
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
Flo
w r
esis
tan
ce (
Pa/s
)
CEM I 52.5 R LA
CEM I 42.5 RR
Figure 4.5 Flow resistance for CEM I 52.5 R LA (w/c = 0.40) and CEM I 42.5 RR reference
cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
The gel strengths after 10 seconds of rest are depicted in Figure 4.6. In case of CEM I 52.5 R
LA cement paste, an increasing 10 seconds gel strength was observed for increasing calcium
nitrate dosages up to 0.50%. Figure 4.7 shows the gel strengths after 10 minutes of rest. For
both cement types an increasing (albeit less pronounced in case of CEM I 42.5 RR cement)
gel strength can be seen for increasing calcium nitrate dosages.
-
Chapter 4: Counteracting plasticizer retardation 42
reference
0
5
10
15
20
25
30
35
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
10
se
c.
ge
l s
tre
ng
th (
Pa
)
CEM I 52.5 R LA
CEM I 42.5 RR
Figure 4.6 Gel strength after 10 seconds of rest for CEM I 52.5 R LA (w/c = 0.40) and
CEM I 42.5 RR reference cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
reference
0
50
100
150
200
250
300
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
10 m
in. g
el str
en
gth
(P
a)
CEM I 52.5 R LA
CEM I 42.5 RR
Figure 4.7 Gel strength after 10 minutes of rest for CEM I 52.5 R LA (w/c = 0.40) and
CEM I 42.5 RR reference cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
-
Chapter 4: Counteracting plasticizer retardation 43
The heat of hydration curves are shown in Figure 4.8 and Figure 4.9. It can be seen that
calcium nitrate speeded up hydration with approximately two hours for both cement types.
The peak in the hydration curve for the pastes without calcium nitrate was seen at about 9
hours after water addition.
CEM I 52.5 R LA - w/c = 0.40 - reference
0
0.5
1
1.5
2
2.5
1 3 5 7 9 11 13 15 17 19 21 23 25
Time (hours)
Ra
te o
f h
yd
rati
on
hea
t (m
W/g
)
1.00 % CN
0.75 % CN
0.50 % CN
0.25 % CN
0.00 % CN
Figure 4.8 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40) for
different dosages of calcium nitrate.
CEM I 42.5 RR - w/c = 0.50 - reference
0
0.5
1
1.5
2
2.5
3
3.5
4
1 3 5 7 9 11 13 15 17 19
Time (hours)
Ra
te o
f h
yd
rati
on
hea
t (m
W/g
)
1.00 % CN
0.75 % CN
0.00 % CN
0.25 % CN
0.50 % CN
Figure 4.9 Heat of hydration curves for CEM I 42.5 RR cement pastes (w/c = 0.50) for
different dosages of calcium nitrate.
-
Chapter 4: Counteracting plasticizer retardation 44
4.2.3. Results and discussion for sodium lignosulphonate
Flow resistances, gel strengths after 10 seconds and 10 minutes of rest measured on
CEM I 52.5 R LA cement pastes (w/c = 0.40) are listed in Table 4.4, Table 4.5 and Table 4.6,
respectively. Those measured on CEM I 42.5 RR cement pastes (w/c = 0.50) are listed in
Table 4.7, Table 4.8 and Table 4.9.
Table 4.4 Flow resistance (Pa/s) for CEM I 52.5 R LA cement paste (w/c=0.40).
Calcium nitrate [%] Flow resistance
[Pa/s] 0.00 0.25 0.50 0.75 1.00
Reference 2283 2253 2515 2418 2372
0.15% NLS 1552 1973 1815 2060
0.15% NLS DA 683 618 727 839
0.30% NLS 353 651 819 1030 1201
0.50% NLS 147 287 528 671 881
Table 4.5 Gel strength after 10 seconds of rest (Pa) for CEM I 52.5 R LA cement paste
(w/c=0.40).
Calcium nitrate [%] 10 sec. gel
strength [Pa] 0.00 0.25 0.50 0.75 1.00
Reference 18.9 22.2 30.5 30.5 30.5
0.15% NLS 22.2 35.8 30.5 35.8
0.15% NLS DA 5.3 3.9 4.5 6.2
0.30% NLS 2.4 4.5 6.2 8.6 13.1
0.50% NLS < 1 3.3 6.2 7.3 10.0
Table 4.6 Gel strength after 10 minutes of rest (Pa) for CEM I 52.5 R LA cement paste
(w/c=0.40).
Calcium nitrate [%] 10 min. gel
strength [Pa] 0.00 0.25 0.50 0.75 1.00
Reference 73.6 104 114 161 271
0.15% NLS 52.1 87.6 95.6 35.6
0.15% NLS DA 20.1 20.1 26.0 30.9
0.30% NLS 7.1 15.5 30.9 52.1 67.5
0.50% NLS 3.9 11.9 36.8 47.7 73.6
-
Chapter 4: Counteracting plasticizer retardation 45
Table 4.7 Flow resistance (Pa/s) for CEM I 42.5 RR cement paste (w/c=0.50).
Calcium nitrate [%] Flow resistance
[Pa/s] 0.00 0.25 0.50 0.75 1.00
Reference 2788 3161 2644 3099 3160
0.50% NLS 2138 2884 2614 2542 2364
1.00% NLS 231 425 416 492 581
Table 4.8 Gel strength after 10 seconds of rest (Pa) for CEM I 42.5 RR cement paste
(w/c=0.50).
Calcium nitrate [%] 10 sec. gel
strength [Pa] 0.00 0.25 0.50 0.75 1.00
Reference 22.2 30.5 22.2 26.1 26.1
0.50% NLS 22.2 30.5 26.1 30.5 26.1
1.00% NLS < 1 8.6 10.0 10.0 13.1
Table 4.9 Gel strength after 10 minutes of rest (Pa) for CEM I 42.5 RR cement paste
(w/c=0.50).
Calcium nitrate [%] 10 min. gel
strength [Pa] 0.00 0.25 0.50 0.75 1.00
Reference 67.5 104 80.3 104 95.6
0.50% NLS 33.7 52.1 52.1 56.8 61.9
1.00% NLS 7.7 14.2 16.9 5.9 33.7
The flow resistances for CEM I 52.5 R LA cement pastes are also shown in Figure 4.10. In
case of the reference no significant influence of the addition of calcium nitrate on the flow
resistance could be measured. When sodium lignosulphonate (NLS) was added, however,
calcium nitrate had a clear increasing effect on the flow resistance as can be seen in Figure
4.10. The values found for the flow resistance are nevertheless still far below those of the
reference. From Figure 4.11, which shows the increase in flow resistance relative to the flow
resistance of the respective reference without calcium nitrate, it can be seen that the increasing
effect of calcium nitrate on the flow resistance became more pronounced when higher dosages
of sodium lignosulphonate were used. An interesting observation for the flow resistance was
that simply delayed addition of 0.15% sodium lignosulphonate makes it in excess of 50%
more effective as plasticizer than when it is added with the mixing water. This effect is
attributed to less intercalation of lignosulphonate in the early hydration products of cement,
leaving more lignosulphonate available to function as plasticizer through physical absorption
on the grain surface.
-
Chapter 4: Counteracting plasticizer retardation 46
CEM I 52.5 R LA - w/c = 0.40
0
500
1000
1500
2000
2500
3000
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
Flo
w r
esis
tan
ce (
Pa/s
)
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
Figure 4.10 Flow resistance for CEM I 52.5 R LA cement pastes (w/c = 0.40) for different
dosages of calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40
0
100
200
300
400
500
600
700
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
Flo
w r
esis
tan
ce (
%)
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
Figure 4.11 Increase in flow resistance relative to the flow resistance of a reference without
calcium nitrate for CEM I 52.5 R LA cement pastes (w/c = 0.40) for different dosages of
calcium nitrate.
-
Chapter 4: Counteracting plasticizer retardation 47
Figure 4.12 shows the flow resistances for CEM I 42.5 RR cement pastes. Neither clear
increasing nor decreasing effect of calcium nitrate on the flow resistance could be denoted in
case of the reference or in case of the pastes prepared with 0.50% sodium lignosulphonate.
The pastes prepared with 1.00% sodium lignosulphonate, however, again show the increasing
trend also observed for the CEM I 52.5 R LA pastes.
When comparing CEM I 42.5 RR cement paste (w/c = 0.50) and CEM I 52.5 R LA paste
(w/c = 0.40), one can see that higher dosages of plasticizer were required to achieve
comparable reductions in flow resistance in spite of the higher water-to-cement ratio. The
tendency of increasing flow resistance with increasing calcium nitrate dosage is less
pronounced for CEM I 42.5 RR than for CEM I 52.5 R LA paste. This may be associated with
calcium nitrate being a less effective accelerator for this cement compared to the other
according to the mineralogy: CEM I 42.5 RR cement has a lower belite and a higher alkali
content than CEM I 52.5 LA cement (see section 2.5 and Table 3.2).
As only a limited number of plasticizer concentrations were studied in case of CEM I 42.5 RR
and as the effect of 0.50% sodium lignosulphonate on the flow resistance was rather small, no
noteworthy conclusions can be drawn from Figure 4.13, which shows the increase in flow
resistance relative to the flow resistance of the respective reference without calcium nitrate.
CEM I 42.5 RR - w/c = 0.50
0
500
1000
1500
2000
2500
3000
3500
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
Flo
w r
es
ista
nc
e (
Pa
/s)
Reference
0.50% NLS
1.00% NLS
Figure 4.12 Flow resistance for CEM I 42.5 RR cement pastes (w/c = 0.50) for different
dosages of calcium nitrate.
-
Chapter 4: Counteracting plasticizer retardation 48
CEM I 42.5 RR - w/c = 0.50
0
50
100
150
200
250
300
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
Flo
w r
es
ista
nc
e (
%)
Reference
0.50% NLS
1.00% NLS
Figure 4.13 Increase in flow resistance relative to the flow resistance of a reference without
calcium nitrate for CEM I 42.5 RR cement pastes (w/c = 0.50) for different dosages of
calcium nitrate.
Figure 4.14 shows the gel strengths after 10 seconds of rest for CEM I 52.5 R LA cement
pastes. An increasing effect of calcium nitrate on the gel strength can be seen. This increasing
effect on the gelling tendency may be beneficial in some cases since tendencies to segregation
will be reduced.
-
Chapter 4: Counteracting plasticizer retardation 49
CEM I 52.5 R LA - w/c = 0.40
0
5
10
15
20
25
30
35
40
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
10 s
ec. g
el str
en
gth
(P
a)
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
Figure 4.14 Gel strength after 10 seconds of rest for CEM I 52.5 R LA cement pastes (w/c =
0.40) for different dosages of calcium nitrate.
The gel strengths after 10 seconds of rest for CEM I 42.5 RR cement pastes are depicted in
Figure 4.15. Only in case of the pastes prepared with 1.00% sodium lignosulphonate a clear,
increasing, trend can be seen.
Figure 4.16 and Figure 4.17 show the gel strengths after 10 minutes of rest for
CEM I 52.5 R LA and CEM I 42.5 RR cement pastes, respectively. For all mixtures an
increasing effect of calcium nitrate on the 10 minutes gel strength was measured.
-
Chapter 4: Counteracting plasticizer retardation 50
CEM I 42.5 RR - w/c = 0.50
0
5
10
15
20
25
30
35
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
10
se
c.
ge
l s
tre
ng
th (
Pa
)
Reference
0.50% NLS
1.00% NLS
Figure 4.15 Gel strength after 10 seconds of rest for CEM I 42.5 RR cement pastes (w/c =
0.50) for different dosages of calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40
0
50
100
150
200
250
300
0.00 0.25 0.50 0.75 1.00
Calcium nitrate (%)
10 m
in. g
el str
en
gth
(P
a)
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
Figu