The effects of calcium hydroxide and activator chemistry on alkali-activated metakaolin pastes exposed to high temperatures

Having a detailed understanding of the high temperature behavior of alkali-activated metakaolin (AAMK) is important for use of this material in fire resistance applications. Here, we assess the impact of a small amount of calcium hydroxide (10 wt% replacement for metakaolin) on the phase formation, pore structure and microstructure of AAMK exposed to temperatures of up to 900 °C. 5 and 10 M NaOH-activated pastes, and equivalent molarity Na₂SiO₃-activated pastes, with and without calcium hydroxide are characterized using Fourier transform infrared spectroscopy and X-ray diffraction to identify the crystalline phases that form on heating and the likely location of calcium within the material. Mercury intrusion porosimetry and scanning electron microscopy provide a detailed understanding of the evolution of the pore network on heating and degree of densification of the samples. It is found that the calcium addition enhances the formation of nepheline on heating and reduces the concentration-dependent differences seen between the 5 and 10 M Na₂SiO₃-activated systems.