Dependence of the geopolymerization process and end-products to the nature of solid precursors: Challenge of the sustainability

This review retrieves the determinant role of the solid precursor on the definition, chemistry, processing and applications of geopolymers. It is demonstrated that the process, the alkaline solution, the curing conditions as well as the orientation of the end-products in term of performance and potential application are governed by the intrinsic nature and characteristics of the aluminosilicate precursors. In particular, the amorphous fraction and the available Al and Si cations governs the geopolymerization. Solid precursors with relatively large amount of amorphous phase are easily activated with standard alkaline solution (6–8 M) leading to the formation of a gel, designated as cement or binder. Al-rich gels are efficient for the design of high strength concretes or composites. Solid precursors with low amorphous fraction need relatively high concentrated alkaline solution (>8 M), particularly aluminosilicates with crystalline habitus such as fly ash, volcanic ash, feldspars, granites, nepheline, etc. In these cases, both the dissolution and the curing steps require temperature above ambient. The pastes produced are dominated by non-reacted or incongruently dissolved particles more addressed for mortars and precast. When high concentrated alkaline solution is used, some additions of Si-rich or Al-rich reactive materials are needed to stabilize the extra alkali present within the matrix: steam or long-term curing are, therefore, required. Regarding the curing conditions, the temperature and relative humidity to be applied are directly linked to the nature of the solid precursor. This paper should be viewed as a significant contribution for the understanding and classification of geopolymer cement and composites as well as the science and technology of the geopolymers.