High temperature resistance of a very high volume fly ash cement paste

The physical and chemical changes taking place in a very high volume fly ash cement paste (FAN-4) following exposure to temperatures up to 1000 °C are presented. Tests were repeated with commercially available CEM II/A-M Portland composite-cement (MS). FAN-4 pastes showed impressive residual strengths after heat exposure which increased dramatically when heated previously at 800 or 1000 °C. This was in stark contrast to the MS paste, which showed a continual decrease in residual strength following heating and subsequent cooling. The increase in residual strength with FAN-4 paste coincided with a major shrinkage event, which was associated with sintering and the formation of the new mineral phases anorthite, gehlenite, wollastonite, diopside and albite. The MS cement formed generally non-hydraulic calcium silicate phases upon heating at ⩾800 °C. The differences in phase formation were linked to different starting elemental compositions. The FAN-4 paste was considerably richer in Si, Al, Fe and alkalis but poorer in Ca. The elemental composition of the binder phase will be an important factor to take into account when determining the high-temperature performance of future cements and concretes, which are likely to contain significantly higher contents of supplementary cementitious materials than is presently the case.