Influence of fineness of raw clay and calcination temperature on the performance of calcined clay-limestone blended cements

Abstract Workability and strength performance were investigated for blended cements with 35% mass replacement for two calcined clays (mostly containing 2:1 clay minerals) and mixtures with limestone. Raw clays were flash calcined at different temperatures and with different raw meal fineness, but ground to similar 45 μm residues after calcination. The results showed that both calcination temperature and fineness of the raw clay had a noticeable effect on the morphology, kinetics and long-term reaction degree of the resulting calcined particles, which was evident in data on the workability and strength development of the blended cements in mortar. Workability was especially influenced by the calcined clay content and raw clay fineness, to a greater extent when calcined at low temperatures. In contrast, workability progressively improved with the increase of the calcination temperature due to resulting lower specific surface area, as a consequence of increased clay transformation, which gave a spherical shape to some of the calcined clay particles. Strength performance was maximized for finely ground raw clays calcined at temperatures up to 850 °C. Raw clays with higher contents of montmorillonite showed further clay transformation at calcination temperature of 925 °C. These phases reacted more slowly with calcium hydroxide on hydration of a cementitious system than the amorphous phase, resulting in lower performance at 28 days than achieved at optimum calcination temperature, but equivalent performance after 90 days of hydration.