Thermal conversion and microstructural evaluation of geopolymers or "Alkali Bonded Ceramics" (ABCs)

Geopolymers are a class of cementitious materials that do not depend on the presence of calcium. They are amorphous, inorganic materials that are formed by the mixing of alkali-silicate solutions with aluminosilicate minerals or glasses. Two processing routes for the thermal conversion of potassium-based geopolymers into ceramics were carried out for this study. A low water content geopolymer, having a molar composition of K₂O · Al ₂O₃ · 4 SiO₂ · 7.5 H ₂O and reinforced with amorphous 500 nm silica spheres, was thermally converted into crystalline leucite of the same dehydrated composition, by heating above 900°C. Upon thermal conversion, the dehydrated geopolymer exhibited macroscopic cracking. The microstructure consisted of a glazed surface covered with spherical voids, caused by entrapped air. In the second processing route, ceramic powders derived by crushing geopolymers having a molar composition of K₂O · Al₂O₃ · 4 SiO₂ · 7.5 H₂O were die-pressed into pellets and sintered at 1200°C for 1 hour to form leucite. The resulting pellets exhibited no cracking upon conversion into leucite. SEM analysis also showed a similar glazed surface. The interior of the sample was compromised of ∼3 μm size phase, which according to XRD were leucite grains, dispersed in an intergranular, amorphous phase. TEM of the amorphous matrix showed a two-phase microstructure, a lighter continuous matrix, surrounding darker 5 nm diameter inclusions. The microstructure was reminiscent of spinodal decomposition such as might be formed in a thermodynamic miscibility gap. Selected area diffraction patterns of the continuous matrix phase suggested that it was mostly amorphous, with some incipient nanocrystals forming, presumably of leucite. Further indepth TEM studies are warranted. It is suggested that geopolymers may be thought of as an inorganic gel, and a type of chemically bonded ceramic, more specifically, as "alkali-bonded ceramics" or "ABCs".