Impact of different mixing temperature on the microstructural evolution and properties of metakaolin-based geopolymers

Temperature has an important influence on geopolymerization. While existing studies have mainly focused on curing temperature effects, research on the influence of mixing temperature remains limited. This study investigated the effects of mixing temperature between alkali activators and metakaolin (MK) on the physical properties, microstructure, and reaction kinetics of metakaolin-based geopolymers (MKGs). The results demonstrated that different mixing temperatures led to distinct geopolymerization processes and had significant impacts on mechanical performance. With increasing mixing temperature, compressive strength first increased and then decreased. Geopolymers prepared at 50 °C (MKG50) exhibited a denser structure and the highest compressive strength, reaching 46.47 MPa at 7 days. Elevated mixing temperatures accelerated geopolymerization, promoting rapid dissolution of raw materials and coagulation of the paste. As the temperature increased from 25 °C to 90 °C, the final setting time was shortened from 351 to 6 min. Extreme mixing temperatures (5 °C and 90 °C) both induced rapid setting within 15 min but through distinct mechanisms. At 90 °C, excessively accelerated geopolymerization produced gel structures encapsulating incompletely decomposed raw materials, hindering subsequent reactions and deteriorating the structure and strength. In contrast, at 5 °C, rapid setting was caused by hydrated sodium silicate crystal inclusions, which did not negatively affect the final structure or performance, as the final setting was only 15 min, with an initial compressive strength of 2.66 MPa and achieved 36.80 MPa after 7 days. This suggests that low-temperature reaction can be used as an effective method for preparing rapid-setting geopolymers.