Uncover the thermal behavior of geopolymer: insights from in-situ high temperature exposure

The understanding of geopolymers' behavior at elevated temperatures is lacking due to the most focuses on post-situ research, leading to unsubstantiated expectations of in-situ thermal performance. This work systematically investigates the in-situ thermal behavior of geopolymers, including phase changes, deformation, and mechanical performance, following a comparison between in-situ and ex-situ properties. The results reveal a notable discrepancy between the in-situ and ex-situ thermal performance of geopolymers. During heating, geopolymers shift from a brittle to a ductile state by physicochemical transformation, facilitating accommodation of thermal incompatibilities. As we observed, the in-situ mechanical strength and creep strain increase until partial melting, with higher Na₂O% accelerating melting of geopolymer. During cooling, geopolymers undergo matrix shrinkage and cracking, which impairs ex-situ performance. A denser matrix provides superior in-situ strength, while its high stiffness negatively impacts structural integrity during cooling, further reducing residual strength. These findings highlight the limitations of ex-situ experiments in estimating high-temperature performance of geopolymers. To accurately predict the in-situ thermal performance, future ex-situ research must account for partial melting during heating and deterioration induced by cooling.