Thermal behavior up to 800 °C of geopolymers based on rice husk ash and metakaolin

Through a multi-technique strategy this study analyzes, the thermal stability, structural, microstructural and mechanical behavior of rice husk ash and metakaolin based geopolymers, subjected to heat treatment up to 800 ◦C. Thermogravimetry and differential thermal analysis identified two main regions of mass loss: between 30 and 200 ◦C, related to the removal of physically adsorbed water, and between 200 and 600 ◦C, associated with the dehydroxylation of phases such as zeolites and carbonates. X-ray diffraction and fourier-transform infraredspectroscopy showed structural transformations related to the rearrangement of aluminosilicate phases and the progressive transition toward a vitreous phase. Dilatometric analysis revealed a significant expansion at 800 ◦C, attributed to a swelling phenomenon due to the formation of a glassy phase simultaneously with gas release.Scanning electron microscopy images corroborate this microstructural transformation from a dense matrix to a porous and partially vitrified structure, with a substantial increase in porosity (up to 83 % at 800 ◦C) and a volumetric expansion of 52 %. The compressive strength decreased with firing temperature from 4.9 to 2.6 MPa, due to porosity development. The results highlight a critical thermal threshold at 600 ◦C, below which structural integrity is maintained. At 800 ◦C, the microstructural transformation places these materials as promising candidates for thermal insulation applications.