Geopolymers: An Environmental Alternative to Carbon Dioxide Producing Ordinary Portland Cement

Erin McNulty

Senior Comprehensive Paper

Department of Chemistry

The Catholic University of America

April 2009

Geopolymers 

Abstract

Current concerns about global warming due to the buildup of greenhouse gases in our

atmosphere have prompted the cement industry to investigate alternatives to ordinary Portland

cement (OPC). One ton of carbon dioxide is produced for every ton of OPC cement. This is due

not only to the emitted carbon dioxide but also the burning of fossil fuels used to heat the raw

materials to a temperature sufficient for the chemical reaction to take place. Geopolymers are

low CO2 producing cements that provide an alternative to ordinary Portland cement.

Geopolymers produce no CO2 in chemical reactions and less emitted carbon dioxide due to

manufacturing techniques.

Recent research has resulted in knowledge about the structure and properties of the

alternative cement. X-ray diffraction and both 27Al and 29Si MAS-NMR data provide insight

into the aluminosiliate structure. Results from the X-ray diffraction showed a diffuse halo peak at

27-29° 2θmax indicating a semi-crystalline amorphous structure. The 27Al MAS-NMR showed a

single peak at 55ppm indicating a single tetrahedrally coordinated aluminum complex. 29Si

MAS-NMR showed peaks at 91ppm, 87ppm and 84ppm indicating three types of Si-Al

coordination and a peak at 79ppm showing the presence of orthosilcates.

Knowledge of the structure has allowed scientists to develop the most effective method

of geopolymer cement formation with properties comparable or better than those of ordinary

Portland cement. Problems still exist with the production of geopolymer cements which prevent

them from becoming widely used in the building and construction industries. Further research

and acceptance on the part of these industries could lead to a more environmentally friendly

method of cement production and major reductions in CO2 emissions.

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Geopolymers 

Introduction

Efforts aimed at environmental preservation and sustainable design have become

necessary for all industries. The cement industry is no exception. One of the most pressing

concerns for this industry is global warming. Global warming is causing an increase in the

earth’s near-surface temperature that results from the change in the atmospheric composition.

Human production of carbon dioxide and other greenhouse gases (GHG) has led to a

considerable increase in the earth’s temperature. The cement industry contributes 5% of the

carbon dioxide emitted into the atmosphere every year and is the second fast growing source of

CO2 emissions. Therefore, major reductions in CO2 emissions are necessary.1 These reductions

will be achieved not only as a result of modifications to existing cement production methods, but

through development of alternative cement binders. These binders will need to show comparable

or better properties and costs compared with the existing ordinary Portland cement.

The cement industry, realizing the need to reduce carbon emissions, began an initiative to

bring down the industry’s contribution to GHG. The Cement Sustainability Initiative is an

environmental program proposed by a group of the largest cement producers in the world. It

suggests that by the year 2020, improvements in cement production could reduce CO2 emissions

by 30%.3 The immediate objective was to modify the existing method for cement production.

This is currently being done by increasing energy efficiency, using alternative starting materials

and low carbon fuels. After the year 2020 further CO2 reductions will need to be implemented.

Ordinary Portland cement

The cement industry contributes to carbon dioxide emissions in two ways. Both the

manufacturing process and resulting by-products of the chemical reaction that produces ordinary

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Geopolymers 

Portland cement (OPC) contribute to total carbon dioxide emissions. About 40% of this CO2 is

produced by burning of fossil fuels to make OPC clinker, about 5% is the result of transporting

the clinker, and 5% comes from electricity used in manufacturing.2 The cement industry is taking

steps toward reducing CO2 emissions by working on alternative energy sources and methods of

making OPC. However, these steps will not be enough meet the industry goal of reduced CO2

emissions.

A major problem in reaching this goal is the current chemical process used to produce

cement clinker. The current production process is based on the following chemical reaction from

limestone and silica into the calcium silicates and carbon dioxide products.

5CaCO3 +2SiO2 → Ca3SiO5+Ca2SiO4 + 5CO2

This reaction produces roughly 597kg of CO2 gas for every ton of cement produced. The

chemical process accounts for 50% of the total CO2 emissions of the cement industry (Figure 1).

While alternative energy sources can lower the CO2 produced by the overall process, the

necessary reductions will come not through modification of the method of producing OPC, but

rather an evolution of the industry including hybrid-energy cement facilities, carbon capture and

sequestration, and non-limestone based binders. Early research on alternative cement binders has

shown them to be a very promising method of CO2 emission reduction.2

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Geopolymers 

Figure 1. Global CO2 emission breakdown overall and for cement industry from the year 2000.1

Alternative cement binders

OPC is used today because of the widespread abundance and affordability of the

limestone that is its major component. A suitable alternative cement binder would need to

maintain the availability and quantities necessary for the industry to continue production at rates

and costs that are comparable to the OPC production. The following materials could provide a

realistic alternative based on a data from a US Geological survey, clay (Al and Si), calcium

sulfates, iron oxides, silica, coal ash, and sodium carbonate or sodium chloride. The emissions of

CO2 during the production of the various binders made from these alternative materials are found

in Table 1.1 Four major types of cement have lower CO2 emissions. These types include calcium

(sulfo) aluminate cements, calcium sulfate-based cements, magnesia cements, and alkali

activated cements. This last type shows increased appeal because of its utilization of industrial

by-products like fly ash and blast furnace slag in addition to the smaller carbon footprint.4

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Geopolymers 

Table 1. The comparison of quantities of CO2 produced from the conversion of raw materials to OPC and to alternative cement compounds.1

Alkali Activated Geopolymers

Alkali activated cements contain a number of subgroups, all characterized by an

alumino-silicate bonding phase in which the alumina-rich source materials and alkali silicate

solution interact. The subgroups include geopolymers, alkali activated fly ash and aluminate

cements4. Geopolymers have specifically drawn the attention of scientists and engineers because

of their lower carbon foot print and potential superiority over OPC in terms of their chemical and

mechanical properties. Increased research in the past decade has lead to greater knowledge of the

benefits of this alternative cement.

The production of geopolymers reduces CO2 emissions by as much as 80% when

compared with OPC. This reduction results from two major differences between the formation of

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Geopolymers 

geopolymer cements and OPC clinker. First, geopolymerization has no calcination step like the

one found in OPC clinker formation. The calcination step causes 40% of CO2 emissions, which

occur upon burning fossil fuel to reach the high temperatures necessary for this reaction.

Secondly, the calcium carbonate or limestone itself produces 50% of CO2 emissions that occur

during the reaction of calcium carbonate or limestone to form OPC. Such CO2 generation is

absent in geopolymer cement formation. Instead, geopolymerization follows the general equation

seen in Figure 2 does not lead to the formation of CO2. A more detailed mechanism for the

polycondensation reaction that leads to geopolymer structure will be discussed later.

Figure 2. Geopolymers reaction in two steps

One common source of the aluminum and silica in geopolymers is metakaolin.

Metakaolin is a common pozzolan, or additive to OPC mixtures that increases the long terms

strength of the cement. This material starts as an aluminum silicate earth mineral like feldspar.

When feldspar is chemical weathered it forms kaolinite, which is hydrated aluminum disilicate,

Al2Si2O5(OH)4. The kaolinite is heated to 500-800°C in order to dehydroxylate the substance.

This reaction of kaolinite to metakaolin takes place by the following reaction.

2(Si2O5·Al2(OH)4)n 2(Si2O5·Al2O2)n + 4n H2O (2)

The ratio of silica to alumina present during the reaction is of great importance for the

resulting material. Therefore, other silica and alumina sources include recycled fly ash and blast

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Geopolymers 

furnace slag. Fly ash, a byproduct of burning coal for energy, commonly contains silica dioxide

and blast furnace slag, a byproduct of iron manufacturing, can contain both aluminum and silicon

oxides. To obtain the desired 2:1 silica to alumina ratio, these products will be added to the dry

cement mixture before the addition of the alkali activator.9 The alkali activator is most

commonly an alkali hydroxide or alkali silicate solution. The alkali metal is commonly sodium

of potassium and is a necessary component in the geopolymer structure.

Geopolymers Structure

Geopolymers result from alkali activation of the aluminosilicate minerals to produce

amorphous macromolecules. The geopolymer is characterized by Davidovits as aluminosilicates

containing AlO4- and SiO4 as tetrahedral subunits. These subunits are of the three types depicted

in Figure 3. The different types are abbreviated PS for poly(sialate), PSS for poly(sialate-siloxo),

and PSDS for poly(sialate-disiloxo).5 The subunits alternate between Si and Al units and

covalently share oxygen atom in order to make the larger macromolecules called geopolymers.

The presence of the an alkali metal as a positive ion is necessary to the geopolymer structure

because it balances the negatively charged aluminate in IV-fold coordination.5

The molecular structure of these geopolymers usually takes the form of a chain or a ring

and can range from amorphous to semi-crystalline. In highly crystalline polymers, the monomer

chains line up in ordered rows with no unordered areas. Amorphous polymers are non-uniform

polymer chains that do not line up. Geopolymers have both areas of ordered and unordered

polymer structures and are therefore called semi-crystalline.

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Geopolymers 

Figure 3. Geopolymer types with aluminosilicate structure6

The mechanism by which the geopolymer structure forms is thought to involve three

major stages. For the purposes of this paper, the geopolymerization of metakaolin MK-750 will

be explored to provide an example of the general alkaline reaction by which all geopolymers are

formed. This mechanism is seen in Figure 4. The first of the three stages involves the

depolymerization of the existing poly(siloxo) layer of the kaolinite. This is followed by the

formation of (OH)3-Si-O-Al-(OH)3. Finally, there is polycodensation to higher oligimers and

polymers that come together to make up the overall structure.9

The mechanism takes place in about seven steps. The alkalination takes place in the first

step of the mechanism which results in the tetravalent aluminum group formation. The hydroxide

then attacks the attached silicon in the second step, taking it to a penta-covalent state and a

negatively charged central Si atom.9 This leads to the cleavage of the second Si as a silanol (Si-

OH) group. The third step of the mechanism leads eventually to the production of the ortho-

sialate molecule shown as the product in the fourth step of the mechanism. The ortho-sialate

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Geopolymers 

molecule is the major subunit of geopolymerization. In the fifth step, the cation bonds with the

basic siloxo, Si-O-, to form a terminal bond.9

The following steps show the polymerization of these subunits into the larger amorphous

units that make up the final structure of the Na-poly(sialate-disiloxo). As shown in step six, the

condensation results in a complex frame work with a cyclo-tri-sialate and ortho-sialate-disiloxo

structures that does not easily line up in perfectly crystalline rows. However, the structure in step

seven shows the ordered nature of the final geopolymers that give the final product its

amorphous to semi-crystalline nature. This mechanism is the same for both the Na and K forms

of the poly(sialate-disiloxo) geopolymer structures.9

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Geopolymers 

11  

Figure 3. Geopolymer mechanism with metakaolin MK-750 with alumoxyl Al(V) –Al=O 9

Geopolymers 

X-ray Diffraction

There is evidence to support this proposed mechanism in the form of experimental data

and comparison to similar compounds that have better known structures. X-ray diffraction is one

method of determining the structure of materials. This technique is a non-destructive process in

which an x-ray beam is directed at the material. Using the incident and scattering intensities the

structure of the material can be determined. The distribution of the atoms on the surface of the

sample is determined by the interference of diffracted x-rays. If the surface atoms are evenly

spaced, as is the case in crystalline materials, then the resulting spectrum will contain sharp

peaks at points where constructive interference takes place and the structure is then determined

using Bragg’s Law. This law states that for a given lattice, where d is the interlattice distance,

the condition for a peak is given by 2dsinq=nλ, where λ is the wavelength, q is the scattering

angle and n is an integer representing the diffraction peak order.7

Geopolymers are not crystalline, and therefore this is not the most accurate method for

determining their structure. However, it can confirm the amorphous structure of the material.

Several samples of geopolymer were tested and found to have defuse halo peaks at 27-29° 2θmax

corresponding to d of around 3.05-3.30A.5 The comparison of these substances to materials such

as feldspatic glass shows that the structure is similar. Both materials are almost completely

amorphous with areas of short-range order.

This near amorphous structure is desirable in cementinous substances. As stress is

applied to a crystalline substance, a single fracture in the overall structure can cause the whole

structure to weaken and shatter. An more amorphous material reacts differently under applied

stress. If a fracture occurs in the material it will likely break off a small piece of the structure but

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Geopolymers 

leave most of it intact. While x-ray diffraction reveals the overall amorphous to crystalline

qualities of the cement, other techniques show more detail about the atomic arrangement.

MAS-NMR

Magic angle spinning nuclear magnetic resonance or MAS-NMR is one technique used to

determine the actual arrangement of the atoms within a substance. This is a specific type of solid

state nuclear magnetic resonance spectroscopy. A sample is placed into the NMR at a magic

angle ,θ, with respect to the magnetic field. This magic angle allows the reduction or elimination

of dipolar coupling or magnetic dipole-dipole interaction. By reducing the dipolar coupling, the

resolution on the NMR spectrum is greatly improved. The chemical shift that results from the

interactions of nuclei in the material can help to determine the environment of the atom and the

surrounding substances.8 In the case of geopolymers, the most useful information about the

structure comes from the 27Al and 29Si MAS-NMR spectrum. Using this information, the

geopolymer structure is determined and as a result its resulting chemical and physical properties

are better understood.

27Al MAS-NMR

The 27Al MAS-NMR gives useful data when it is looked at in the context of other Al-

coordinated species. Prior research has shown the chemical shifts for a number of silico-

aluminate species. The four coordinated species of aluminum resonates at 50±20ppm while six

coordinated aluminum resonates at about 0±10ppm.5 These values for silico-aluminate species

are shown in Table 2. Using these known chemical shifts, the chemical shifts of geopolymer

samples can be compared and from this structural information can be gained.

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Geopolymers 

Table 2: Al-coordination in silico-aluminates and their 27Al chemical shifts

5

The results for the Na,K-PSS and Na-PSS species from the Davitovits’ research showed a

chemical shift around 55ppm as shown in the spectrum seen in Figure 4.5 This information

shows that the aluminum in geopolymers is a IV-coordinated species that is surrounded by

oxygen in a tetrahedral formation. This is the only peak found in the spectrum, and therefore this

is the only aluminum species present in the geopolymers studied. This supports the idea of

separate aluminum and silicon coordinated units.5 This single narrow peak also excludes the

possibility of other smaller building units like dimers or trimers.6

gure 4: 27Al MAS-NMR Spectrum for K-PSS Figure 5: 29Si MAS-NMR spectrum for K-PSS Fi

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Geopolymers 

29Si MAS-NMR

The 29Si MAS-NMR gives more information about the geopolymer structure by

29

NMR a spectrum of potassium poly(sialate-siloxo) or K-PSS was determined in Figure 5. The

figure shows what appears to be a broad peak at 94ppm. This is actually the blending of three

separate signals at -85ppm, -92ppm, and -99ppm.5,6 These three peaks represent three different

environments of the silicon atoms. These species include silicon surrounded by three, two, or

differentiating between the PS, PSS, and PSDS species. In the Davitovits study, the Si MAS-

one aluminum ion, respectively. These possible silicon species and their chemical shift ranges

re shown in Figure 6.

a

Figure 6: 29Si chemical shifts for Si(nAl) bu ding blocks in framework aluminosilicates. il

There is also a small peak at 79ppm which represents the presence of nesosilicates.

Nesosilicates are isolated silicon ions tetraherdrally coordinated with four oxygen atoms. These

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Geopolymers 

nesosilicates are held in the structure by an ion link to the interstitial cations that balance th

inherent negative charge. These results, when compared with those obtained for other silico-

aluminates, can provide even more information about the structure of geopolymer cement

binders. In fact, it appears that the structure of the geopolymers very closely resembles that

other silico-aluminates. The MAS-NMR spectrum shows nearly identical chemical shift va

for the K-PSS and leu

e

of

lues

cite, which supports a trigonal dipyramidal structure as in Figure 7.6

Leucite is a rock-forming mineral composed of potassium and aluminum silicate with the

formula, KAlSi2O6.

Figure 6: Model for K-PSS structure based on 29Si MAS-NMR.

Properties of Geopolymers

The structure of these geopolymers contributes to advantages in the physical properties of

the cement itself. Tests have shown that geopolymer cements are resistant to many chemically

aggressive species known to cause deterioration in cement, such as chloride solutions (including

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Geopolymers 

sea water), acid, alkali and sulfate. Early research of geopolymer cements shows a fire resista

of up to 1000 °C with few or no toxic fumes emitted. The strength retention of two type of OPC

is compared with two geopolymer cements in Figure 8. This extends applications to fire safe

building materials rapid setting materials, and other types of construction materials.10 The freez

thaw resistance, even in the presence of salt, leads to possibilities in road and bridge con

for many parts of the world that experience changing weather conditions. Unlike other cemen

mixtures geopolymers set rapidly without loss of long term compressive strength.1 The

compressive strength of OPC and geopolymer cement during the first 12 hours of setting ar

compared in Figure 9. Finally, the structure of the geopolymer matrix does not bind directly to

water so that

nce

e-

struction

t

e

water loss from concrete is less likely to cause damage to the overall cement

structure.10 The cement properties stem from the chemical structure and allow for improvements

over OPC.

Figure 8: Strength retentions of different concretes at elevated temperatures.5

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Geopolymers 

Figure 9: Comparison of OPC and geopolymer cement compressive strength during the first 12 hours of setting.5

Geopolymer Problems

There are a number of potential difficulties when considering geopolymers as an

alternative to OPC. For example, the geopolymer production uses fly ash or slag raw materials

which can differ depending on their source. This difference results in the need to develop a

separate process for each source, which can be time consuming and expensive. Furthermore,

metakaolin has proven to be a better precursor material but it is also more expensive than its fly

ash and slag counterparts. In addition, uncertainty concerning the reaction upon adding the

activator leads to varying results. Though geopolymers can produce good results, better than

OPC, lack of consistent results makes them undesirable for use in the building industry whose

main focus is safety.10

The cement and building industries play a major role in bringing about the widespread

use of geopolymer cements. However, geopolymer cement does not contain any OPC and most

building codes allow for only a certain amount of non-OPC binder material at this time.10 The

geopolymer cement concept and research is relatively new compared with the decades of OPC

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Geopolymers 

use. It will take time for geopolymer cement to prove its durability and consistency in the

industry. Until then, the research about this promising cement alternative will continue.

Conclusion

The need for CO2 reductions is a concern for many industries including the cement

industry. The production of CO2 can be reduced by changing production methods but it is

necessary to find alternative binders to replace the high CO2 producing OPC that exists today as

the major product of cement production. Geopolymers present an alternative to OPC production

in which nearly one ton of CO2 is produced for every ton of cement. The geopolymer cements

could nearly eliminate carbon emissions for the cement industry by eliminating the limestone

processing. These geopolymers show comparable and better properties when compared with

OPC.

While geopolymers are a relatively new concept, much research over the past decade has

shown that the structure of geopolymers provide unique benefits in their applications to the

cement industry. Geopolymers are amorphous materials suitable for many applications of

cement. This amorphous character is confirmed by X-ray diffraction and MAS-NMR techniques.

The results showed that the tetrahedral aluminum and silicon building blocks make up the

amorphous structure. The structure has been linked to known silico-aluminates and a theoretical

mechanism has been determined based on the structural data.

The structure of the geopolymer leads to physical and chemical properties that are

comparable or better than OPC in many aspects. The initial research on geopolymers shows them

to be useful materials especially as alternative cement binders. However, some problems exist

with the specific implementation of geopolymer cements in the building and construction

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Geopolymers 

industries. The lack of codes applying to these new materials indicates that additional research

and testing of these materials is necessary. With further pressure from the government and

environmentalists, there is no doubt that geopolymer cements have a future as a low CO2

alternative to OPC.

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Geopolymers 

References

(1) Gartner, E. Cement and Concrete Research 2004, 34, 1489-1498.

(2) World Business Council for Sustainable Development 2002. Towards a Sustainable

Cement Industry: Climate Change.

http://wbcsd.org/DocRoot/oSQWu2tWbWX7giNjAmwb/final_report8.pdf (accessed

January 3, 2009).

(3) World Council for Sustainable Development 2002. The Cement Sustainability Initiative:

Our Agenda for Action. http://www.wbcsdcement.org/pdf/agenda.pdf (accessed October

20, 2008).

(4) Phair, J.W. Green Chemistry 2003, 8, 763-780.

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(6) Davitovits, J. Journal of Materials Education 1994, 16, 91-139.

(7) Materials Research Laboratory, Introduction to X-Ray Diffraction.

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January 3, 2009).

(8) Hornak, J.P. The Basics of NMR http://www.cis.rit.edu/htbooks/nmr/inside.htm

(accessed January 3, 2009).

(9) The Geopolymer Institue http://www.geopolymer.org/science/about-

geopolymerization/2 (accessed January 3, 2009).

(10) Duxson, P.; Provis, J.; Lucky, G.; van Deventer, J. Cement and Concrete Research 2007,

37, 1590-1597.

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(11) Kosmatka, Steven H.; Kerkhoff, Beatrix; Panarese, William C. The Design and Control

of Concrete Mixtures. Portland Cement Association: Skokie, Illinois, 2002.

(12) Duxson, P.; Fernandez-Jimenez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; Van

Deventer, J.S.J. Journal of Material Science. 2007, 42, 2927-2933.