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

The effect of temperature (10°C, 27°C and 50°C) on the hydration kinetics, phase assemblages and strength development of calcined clay blends (CCBs) was investigated at two replacement levels. The temperature sensitivity of the CCBs significantly increased when clinker was replaced with calcined clay, even at a 30% replacement level. The compressive strength development of the CCBs was higher than that of ordinary Portland cement at a lower temperature, perhaps due to the formation of low-density products such as stratlingite rather than enhanced clinker hydration. The reduced compressive strength development of the CCBs cured at 50°C could be due to multiple reasons (e.g. lower clinker hydration, reduced pozzolanic reaction, reduced stability of carboaluminates and ettringite). However, merely a 24-h lag in temperature exposure reversed this temperature effect, mainly due to the enhanced stability of carboaluminate and ettringite phases. The reduction in hydration was mainly in the belite phase and could be due to large changes in calcium alumina silicate hydrate having a high alumina substitution. It is hypothesised that the adverse effect of high-temperature exposure on CCBs is due to the formation of a physical barrier of highly polymerised calcium silicate hydrate on the surface of 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.