Effect of temperature and clinker content on hydration and strength development of calcined clay blends

Abstract

The effect of curing temperatures: 10°C, 27°C and 50°C, on hydration kinetics, phase assemblages and strength development of blends with calcined clay was investigated at two replacement levels. The temperature sensitivity of the blends significantly increased when the clinker is replaced with calcined clay, even at a 30% replacement level. The compressive strength development in calcined clay blends was higher than that of OPC at a lower temperature, perhaps due to formation of low-density products such as stratlingite, rather than enhanced clinker hydration. The reduced compressive strength development at 50°C for calcined clay blends could be due to multiple reasons such as lower clinker hydration, reduced pozzolanic reaction, and reduced stability of carboaluminates and ettringite. However, merely a 24-hour lag in this temperature exposure reverses this temperature effect, mainly due to the enhanced stability of carboaluminates and ettringite phases. It is seen that the reduction in hydration is mainly in the belite phase and could be due to the large changes in the C-A-S-H having a high alumina substitution. It is hypothesised that the adverse effect of high-temperature exposure in calcined clay blends is due to the formation of a physical barrier of highly polymerised C-S-H on the surface of the unhydrated clinker. The improved performance due to the delay in high-temperature exposure could be due to the enhanced stability of ettringite and carboaluminate phases.