review · Ceramics International
Geopolymerisation chemically dissolves amorphous, semi-crystalline, and crystalline phases in raw materials using alkaline or acidic activation below 100 degrees Celsius, producing an inorganic binder resembling artificial stone. While research predominantly focuses on amorphous precursors, certain semi-crystalline and crystalline minerals also dissolve partially or completely, generating zeolitic networks and new minerals such as sodalite or zeolite A. Higher synthesis temperatures enhance this dissolution and accelerate network formation. However, current characterisation predominantly relies on qualitative comparisons of X-ray diffraction patterns rather than rigorous phase quantification. Consequently, the extent of phase dissolution, the precise degree of geopolymerisation, and the proportion of unreacted amorphous material remain poorly understood. To establish dependable relationships between mineralogical changes and the mechanical performance of geopolymer materials, quantitative methods such as Rietveld refinement with internal standards, alongside complementary techniques like thin sections, are needed.
Geopolymers offer potential alternatives to conventional binders for artificial stone and construction products. Understanding how different mineral phases dissolve and transform is essential for controlling material quality. Establishing standardised, quantitative characterisation techniques will help researchers accurately monitor the chemical transformation, enabling more consistent and durable formulations derived from diverse precursor materials.
The work is foundational and early stage, providing diagnostic recommendations rather than a market-ready product. Developers and manufacturers of geopolymer building materials could use rigorous phase-quantification methods, such as Rietveld refinement, to better predict structural performance from varying raw feedstocks. Moving closer to commercial deployment will require resolving inconsistencies in how unreacted phases and binder networks are quantified during production.
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The geopolymerization reaction is a chemical process involving the dissolution of amorphous, semi-crystalline and crystalline phases following alkaline or acid attack at ambient or at low temperature (T˂100 °C). Together with precondensed entities of the used activator solution the dissolved parts form a geopolymer network which glues together all remaining unreacted parts as ingredients of the artificial stone. The fate of the mineralogical phases present in the precursors used in the synthesis of alkaline and acid geopolymers is reviewed in this article. The different starting materials used in the synthesis of geopolymers and the different techniques used to modify the reactivity of these materials are reported. The mineralogical evolution of the amorphous, semi-crystalline and crystalline phases after activation at ambient and at low temperatures (T < 100 °C) is also reported. This study shows that for the mineralogical investigation of raw material, precursors and geopolymers, XRD is the most widely used analytical method. The most commonly used method to confirm the participation of crystalline phases in geopolymerization is the comparison of the diffractograms of the raw material to those of the products. It also indicates that, in addition to the amorphous aluminosilicate phases, certain semi-crystalline, and crystalline phases participate in this geopolymerization dynamic either by undergoing total or partial dissolution to form an inorganic macromolecule with an amorphous structure of the zeolitic type and other new minerals. The composition of the phases formed depends on the base material. Increasing the synthesis temperature promotes the dissolution of phases, the formation of geopolymer networks, and in some cases, the formation of new phases such as sodalities, zeolite A and zeolite P. Little work has been done to quantify the mineralogical phases before and after precursor activation, making it impossible to assess dissolution, the degree of geopolymerization, and mineral formation. The exact structure of the phases formed is often questionable, as most of the papers do not mention their reference structure number. There is also a lack of quantification of the amorphous phase resulting from the hardening of the activating solution and the evaluation of the fraction of the amorphous phase contained in the precursor remains unreacted. The mineralogical investigation must also extend to the quantification of phases using Rietveld refinement with internal standards to correlate these with the physical and mechanical properties of geopolymer products. Additional mineralogical study techniques, such as thin sections, should be used in future work for in-depth mineralogical investigation of geopolymers.
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DOI: 10.1016/j.ceramint.2024.07.024
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