Impact of calcination technology on properties of calcined clays

Calcined kaolinitic clays are known to be very reactive pozzolans, and combined with limestone can enable significant clinker substitution in cementitious systems. Thermal activation of kaolinitic clays takes place when the hydroxyl groups are removed, leading to formation of an amorphous reactive structure. There are several technologies for clay activation, but the most used at industrial scale are flash and stationary soak calcination. The objective of this paper is to investigate the impact of the calcination regime on the properties of the calcined product. It presents the results of an experimental program carried out with a kaolinitic clay calcined at a flash calciner and at a laboratory furnace. In both cases calcination brings about a drop in specific surface, and an increase of average diameter due to agglomeration, an effect more pronounced in stationary soak calcination. No major differences were found at the heat of hydration, CH consumption and phase assemblage for the fully dehydroxylated material. The flash calcined material had slightly better results mainly due to a finer PSD compared with the one soak calcined. No major difference was found in water demand and compressive strength for both regimes. In conclusion, for the experimental set and the clay used, calcination regime appears to have little influence on the properties of the calcined clay, except for the degree of agglomeration of the calcined material.