Acid-based geopolymers: Understanding of the structural evolutions during consolidation and after thermal treatments

Materials such as ceramic matrix composites are developed for mechanical applications at high temperature, but their cost remains a limitation. Consequently, the use of acid-based geopolymer matrices may be an alternative to reduce costs. In this study, the sample was prepared from metakaolin and phosphoric acid. Fourier Transform InfraRed and Nuclear Magnetic Resonance spectroscopies, X-Ray Diffraction and thermal measurements were used to understand the structural evolution of acid-based geopolymers (binders) during consolidation and after thermal treatments. According to the results, the consolidation of the binder has been divided into four steps: the dissolution of the metakaolin, the polycondensation reactions forming AlPO4 entities and hydrated phases, the breakdown of Si-O-Al bonds with formation of various hydrated silica networks and finally the completion of the networks. After a thermal treatment at 1000 degrees C, the binder consists of AlPO4 phases, quartz and vitreous silica.