Effects of elevated temperatures on properties of hybrid alkaline-belite cement with high level of fly ash

Abstract

This paper aimed to study the physical and chemical changes taking places in a hybrid alkaline-belite cement (HYP, with a high level of fly ash, 47.5%) paste at high temperature. To investigate the influence of chemical and mineralogical differences on the behavior of cementitious materials when exposed to high temperature, an alkaline-activated fly ash cement (FAN) paste was also parallelly studied. The stress–strain behavior of HYP was compared with that of FAN under exposure to high temperatures, and its compressive strength was determined. Additionally, the mechanical strength of both materials was assessed by performing post-thermal treatment tests after high-temperature exposure. Mineralogical and microstructural variations in the binders with temperature were examined by X-ray diffractometer, thermogravimetric/differential thermal analysis, mercury intrusion porosimetry, scanning electron microscopy with energy dispersive X-ray spectroscopy and 29Si and 27Al magic angle spinning with nuclear magnetic resonance. The different types of hydration gels lead to the following. First, softening phenomenon did not occur in the matrix of HYP compared to FAN under high temperature; Second, an increase in total porosity with temperature but different pore size fractions, 1 μm may be a critical pore size to determine the mechanical strength after high temperature exposure; Third, different stress–strain behaviors of HYP and FAN during exposure, at 600 °C, the stain for FAN and HYP are 12% and 3.5%, respectively; Fourth, different pathways for phase recrystallization (>800 °C) contributing to high-temperature resistance.