Pozzolanic activity of mechanochemically and thermally activated coal-series kaolin in cement-based materials

The shortage of good quality supplementary cementitious materials (SCMs) for producing cement concrete eagerly demands an alternative pozzolan material. Thanks to the rich silica-alumina contents and large reserves in China, coal-series kaolin (CK) (a by-product of coal mining) is promising to be used as an effective SCMs in cement-based materials. This study investigated the effectiveness of using thermally activated CK coupled with mechanical grinding in ordinary Portland cement paste and mortar. The mineralogy, physicochemical structures, thermal decomposition, pozzolanic reactivity of this novel treated CK were evaluated and compared with two water-washed high-purity kaolinitic clays. The results showed that the large change of Al coordination environment in the thermally activated CK at a relatively low temperature (650 °C) rendered a high pozzolanic reactivity. The high content of kaolinite and reduced particle size by mechanical treatment were beneficial to enhancing the reactivity of thermally activated CK. With the use of thermoactivated CK, the early hydration of cement paste was significantly accelerated. The high degree of dihydroxylation, large specific surface area and high pozzolanic reaction of thermoactivated CK contributed to refining the pore structure and improving the compressive strength of blended mortar. The results of this study provided an implication of using treated CK as an alternative SCMs.