Relationship between macro-properties and amorphous gel of FA-based AAMs with different curing conditions after elevated temperature

This study investigated the effect of curing conditions (40 degrees C, 60 degrees C, and 80 degrees C curing temperature, 3 h, 5 h, and 8 h curing time) on the macro-properties (compressive strength, surface properties, and length change) of alkali-activated fly ash materials (FA-based AAM) at elevated temperatures (300 degrees C, 600 degrees C, 800 degrees C, and 1000 degrees C). The microstructure properties of samples before/after high-temperature treatment were studied by the FTIR, XRD, XRD-Rietveld, SEM-EDS, and 29Si NMR experiments. The results show that the compressive strength of samples shows a decreasing trend beyond elevated 600 degrees C. A similar phenomenon was found in the surface and length properties. Unlike samples with lower curing temperatures (40 degrees C) exit the lower residual mechanical properties beyond 600 degrees C, samples with higher curing temperatures have stable performance in the compressive strength, surface properties, and length change. And high curing temperature is favorable for the formation of amorphous gels with a Si-rich phase in FA-based AAM. Lower diffusion coefficient of water and high skeleton stability were obtained in the higher Si/Al ratio of N-A-S-H amorphous gel decreasing the evaporative rate of water to exac-erbate the damage of microstructure at a low elevated temperature, while increasing the stability at high temperature.