Geopolymer cements

There is often confusion between the meaning of the two terms "geopolymer cement" and "geopolymer concrete". Cement is a binder, while concrete is the composite material resulting from the addition of cement to aggregates. In other words, to produce concrete, cement (usually Portland or geopolymer cement) is purchased and added to the concrete batch. This article deals only with geopolymer cements.

Comparing the chemistry of Portland and geopolymer shows profound differences, explaining their properties. Portland hardens by simple hydration of C ₃ S to CSH and Ca(OH) ₂ , whereas geopolymer cement sets by polycondensation of potassium or sodium oligo-(sialate-siloxo) to poly(sialate-siloxo). NMR spectroscopy of the ²⁹ Si clearly shows the difference between the two systems: on the one hand a matrix made of small molecules (oligomers), this is the CSH, on the other a three-dimensional polymerized structure, the geopolymer.

Geopolymer cement is manufactured using :

• an aluminosilicate raw material: MK-750 calcined clays, calcined or non-calcined rocks, fly ash, blast furnace slag;

• a harmless alkaline reagent (soluble sodium or potassium silicates with a molar ratio SiO ₂ :M ₂ O > 1.45, M being Na or K) ;

• water.

This article presents the four categories of geopolymer cements currently in use:

• slag-based geopolymer cement/MK-750: matrix (K,Na,Ca)-poly(sialate-siloxo) Si:Al = 2 ;

• geopolymer cement based on slag/rock: matrix (K,Na,Ca)-poly(sialate-disiloxo) Si:Al = 3 ;

• geopolymer cement based on slag/fly ash class F: matrix (K,Na,Ca)-poly(sialate-siloxo) Si:Al = 2 ;

• slag/ferrosialate-based geopolymer cement (under development).

Each category is the subject of a scientific study involving NMR and electron microscopy. The article also presents all the physical (shrinkage, expansion, heat of reaction, fire resistance), physico-chemical (alkali-aggregate reaction, etc.) and mechanical (compressive strength, flexural strength) characteristics. These are essentially a function of the geopolymer's molecular structure. In particular, by varying the slag/MK-750 weight ratio, users can choose between high compressive strength and low stability (chemical corrosion), or optimum long-term strength and corrosion stability. For...