CO2 emissions of various cement binders as a function of OPC content. Display omitted
► Alkali activated slag and ash cement has 80% less CO2 emissions than OPC cement. ► Geopolymers face regulatory, ...supply chain, product trust and technical barriers. ► Projects using geopolymers compensate partially for lack of 100+ year track record. ► Relationship between geopolymer microstructure and durability shown by research. ► Colloid science, particle technology and rheology are key to geopolymer knowledge.
If formulated optimally, geopolymer cement made from fly ash, metallurgical slags and natural pozzolans could reduce by 80% the CO2 emissions associated with the manufacturing of cement. However, almost all standards and design codes governing the use of cementitious binders and concrete in construction are based on the use of Portland cement. The 100+ year track record of in-service application of Portland cement is inherently assumed to validate the protocols used for accelerated durability testing. Moreover, the entire supply chain associated with cementitious materials is based on the production of Portland cement. The geopolymerisation of aluminosilicates constitutes a radical change in construction materials chemistry and synthesis pathways compared with the calcium silicate hydrate chemistry which underpins Portland cement. Consequently, there are regulatory, supply chain, product confidence and technical barriers which must be overcome before geopolymer cement could be widely adopted. High profile demonstration projects in Australia have highlighted the complex regulatory, asset management, liability and industry stakeholder engagement process required to commercialise geopolymer cement. While the scale-up from the laboratory to the real-world is technically challenging, the core challenge is the scale-up of industry participation and acceptance of geopolymer cement. Demand pull by a carbon conscious market continues to be the key driver for the short term adoption of geopolymer cement. In the absence of an in-service track record comparable in scale and longevity to Portland cement, research is essential to validate durability testing methodology and improve geopolymer cement technology. Colloid and interface science, gel chemistry, phase formation, reaction kinetics, transport phenomena, comminution, particle packing and rheology, which are familiar concepts to minerals engineers, are also key building blocks in the development of geopolymer knowledge. Analysis of the nanostructure of geopolymer gels has enabled the tailored selection of geopolymer precursors and the design of alkali activator composition, aiding in establishing the relationship between geopolymer gel microstructure and durability.
Binders formed through alkali-activation of slags and fly ashes, including ‘fly ash geopolymers’, provide appealing properties as binders for low-emissions concrete production. However, the changes ...in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C–A–S–H and N–A–S–H) are formed; under accelerated carbonation, the N–A–S–H gel behaves comparably to fly ash-based systems, while the C–A–S–H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies.
•C-A-S-H gel in alkali-activated slag decalcifies during accelerated carbonation.•Alkali-activated fly ash gel changes much less under CO2 exposure.•Blended slag-fly ash binder contains two coexisting gel types.•These two gels respond differently to carbonation.•Understanding of carbonation mechanisms is essential in developing test methods.
The carbonation resistance of alkali-activated binders is often tested via accelerated test protocols designed for Portland cements, without questioning whether the tests replicate the mechanisms ...observed in service. Thus, validation of accelerated methods is required to enable realistic prediction of material performance. Changes in pore solution equilibria cause the formation of sodium bicarbonates during accelerated carbonation, compared with hydrous sodium carbonates in natural carbonation. This shifts the carbonation mechanism to favour more rapid reaction progress, to give a higher apparent degree of acceleration (compared to natural conditions) than in Portland cements. The pore solution pH under accelerated carbonation is significantly lower than at natural CO2 concentrations, leading to a falsely short predicted service life (time to expected corrosion of embedded steel), as natural CO2 concentrations appear not to reduce the pH below 10. Thus, accelerated carbonation testing is unduly aggressive towards alkali-activated binders, and test results must be cautiously interpreted.
The potential position of and drivers for inorganic polymers (“geopolymers”) as an element of the push for a sustainable concrete industry are discussed. These materials are alkali-activated ...aluminosilicates, with a much smaller CO
2 footprint than traditional Portland cements, and display very good strength and chemical resistance properties as well as a variety of other potentially valuable characteristics. It is widely known that the widespread uptake of geopolymer technology is hindered by a number of factors, in particular issues to do with a lack of long-term (20+ years) durability data in this relatively young research field. There are also difficulties in compliance with some regulatory standards in Europe and North America, specifically those defining minimum clinker content levels or chemical compositions in cements. Work on resolving these issues is ongoing, with accelerated durability testing showing highly promising results with regard to salt scaling and freeze–thaw cycling. Geopolymer concrete compliance with performance-based standards is comparable to that of most other high-strength concretes. Issues to do with the distinction between geopolymers synthesised for cement replacement applications and those tailored for niche ceramic applications are also discussed. Particular attention is paid to the role of free alkali and silicate in poorly-formulated systems and its deleterious effects on concrete performance, which necessitates a more complete understanding of the chemistry of geopolymerisation for the technology to be successfully applied. The relationship between CO
2 footprint and composition in comparison with Portland-based cements is quantified.
This paper presents a brief review of the role of particle technology in the development of low-CO2 aluminosilicate ‘geopolymer’ binders and concretes as an alternative to traditional Portland ...cement-based materials. The role of particle shape in particular is highlighted, both in the context of its effect on paste rheology and on water demand. The spherical particles of fly ash and the platy particles of metakaolin show opposite effects in each of these areas, and this must be understood and controlled if an effective geopolymer concrete is to be designed. The angular particles of blast furnace slag are also important in determining paste rheology and porosity. The selection of the correct combination of aggregate gradings is critical in maximising concrete durability, as the ability of aggregates to pack sufficiently densely in a hardened concrete product then hinders the ability of aggressive external agents to migrate into the concrete and cause structural damage to either the binder or the embedded steel reinforcing.
A mechanistic model accounting for reduced structural reorganization and densification in the microstructure of geopolymer gels with high concentrations of soluble silicon in the activating solution ...has been proposed. The mechanical strength and Young's modulus of geopolymers synthesized by the alkali activation of metakaolin with Si/Al ratio between 1.15 and 2.15 are correlated with their respective microstructures through SEM analysis. The microstructure of specimens is observed to be highly porous for Si/Al ratios ≤1.40 but largely homogeneous for Si/Al ≥1.65, and mechanistic arguments explaining the change in microstructure based on speciation of the alkali silicate activating solutions are presented. All specimens with a homogeneous microstructure exhibit an almost identical Young's modulus, suggesting that the Young's modulus of geopolymers is determined largely by the microstructure rather than simply through compositional effects as has been previously assumed. The strength of geopolymers is maximized at Si/Al
=
1.90. Specimens with higher Si/Al ratio exhibit reduced strength, contrary to predictions based on compositional arguments alone. The decrease in strength with higher silica content has been linked to the amount of unreacted material in the specimens, which act as defect sites. This work demonstrates that the microstructures of geopolymers can be tailored for specific applications.
The physical evolution of materials during heating is a critical factor in determining their suitability and performance for applications ranging from construction to refractories and adhesives. The ...effect of different cations (sodium and potassium) on the physical evolution of geopolymeric materials derived from metakaolin is investigated for a range of specimens with Si/Al ratios between 1.15 and 2.15. It is observed that the effect of potassium is to reduce the thermal shrinkage, while thermal shrinkage increases with increasing Si/Al ratio in the presence of each alkali type. The thermal shrinkage behavior of mixed-alkali specimens is observed to change from a mean of the sodium and potassium specimens at low Si/Al ratio to behave similarly to sodium specimens at high Si/Al ratios. It is clear from this investigation that alkali cations only have a significant effect on thermal shrinkage of geopolymer at low Si/Al ratios (⩽1.65), while both Si/Al ratio and alkali cation have little effect on the extent of thermal shrinkage at Si/Al⩾1.65.
This paper presents a discussion of the ability to design raw materials for use in geopolymers. To provide a “green” material to complement existing cement binders, as well as in the interests of ...waste beneficiation, various potential means of tailoring geopolymer precursor chemistry and particle behavior are outlined. The opportunities presented by the development of one‐part “just add water” geopolymer formulations are identified as exceeding the potential of the traditional two‐part (solid plus alkaline activator solution) mix design. The key roles played by network‐modifying (alkali and alkaline earth) cations and alumina in rendering glassy phases “ideal” for geopolymerization are discussed, and the potential value of ASTM Class C ashes in synthesis of high‐performance geopolymers becomes evident. This provides a significant step toward the development of international standards for the application of geopolymer binders in the construction industry worldwide, and raises a number of important challenges for researchers in the field of geopolymer and cement technology.
A model describing the distribution of silicate sites in concentrated alkali metal silicate solutions used in the synthesis of geopolymeric gels is presented. The model is based on a free energy ...minimization algorithm, using free energy of formation data obtained from the literature. A simplified form of the Pitzer method (Pitzer, K. S. J. Phys. Chem. 1973, 77, 26) is used to calculate activity coefficients, with interaction parameters calculated from known values for ions of similar size and charge type. Ion pairing effects are incorporated into the model formulation by a simple approximation, whereby larger cyclic or cagelike ions form strong ion pairs with dissolved alkali metal cations, and smaller monomeric (Q0) or end-group (Q) sites pair less strongly with the cations. The model is able to describe accurately the speciation data obtained from a systematic 29Si NMR investigation of sodium, potassium, and mixed (1:1) sodium/potassium silicate solutions. The model is also tested against a large data set from the literature on sodium silicate solutions with a wide range of compositions. The model provides understanding of the speciation of silicate solutions and a basis for further understanding and modeling of the geopolymerization process.
The correlation between mechanical and dilatometric properties of aluminosilicate geopolymer binders is highlighted by analysis of a set of samples synthesised from a single ash source using ...different activating solution compositions and liquid/solid ratios. The geopolymers which display the best strength performance also show a small expansion in the temperature range 700–800
°C, which is identified as corresponding to the swelling of a high-silica phase present as pockets within the geopolymeric gel structure. Systems in which this phase is absent (made using hydroxide activating solutions or very low liquid/solid ratios) generally show a low extent of binder formation and do not achieve high strength, while systems in which the expansive phase dominates have a sub-optimally structured geopolymer phase and also correspondingly reduced strength.
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