Comparing pozzolanic activity from thermal-activated water-washed and coal-series kaolin in Portland cement mortar

Abstract

In this paper, we investigated the pozzolanic activity of thermal-activated coal-series kaolin and two types of water-washed kaolin, which were calcined at temperatures ranging from 600 to 700 °C for 3 h. Chapelle test and strength activity index were used to evaluate the pozzolanic activity of thermal-treated kaolin. Results indicated that thermal-activated water-washed kaolin presented a higher pozzolanic activity when calcined at same temperature, which is due to its well-defined morphology, relatively high specific surface area, and high amounts of active aluminum. The incorporation of 10 wt% of calcined kaolin significantly increased the amount of hydrate water and promoted strength development at 7 and 28 days. Moreover, the crystallinity of kaolinite showed obvious impacts on the dehydroxylation temperature and aluminum structure. The optimum dehydroxylation temperature is mainly dependent on the crystallinity of the kaolinite; in addition, the aluminium structure is closely related to the kaolinite crystallinity. Well-crystallized kaolin showed higher pozzolanic activity than poorly crystallized kaolin at the same calcination condition.