Calcined clays are gaining increasing interest as future supplementary cementitious materials for the production of blended cements. Besides the mineralogy, the right production conditions can affect the pozzolanic activity of calcined clays. In this paper, the pozzolanic reactivity of two calcined natural clays in dependence of burning temperature, residence time in the furnace, cooling conditions and particle size of the final product is investigated. The highest pozzolanic reactivity was found at calcination temperatures between 600 and 800°C. While different cooling conditions had no identified effect on reactivity, decreased particle size and residence time increased the reactivity.
This study investigates the feasibility of using bauxite residue (BR) as supplementary cementitious material (SCM) for the cement and concrete industry. It is shown from pastes of BR and calcium hydroxide, that BR is highly pozzolanic in nature. The early hydration of cement pastes with BR is accelerated while long-term strength is reduced probably due to the alkaline nature of BR. To be used as cement replacement material in concrete, attempts have been made to reduce the alkali content of BR, in particular to reduce the chance of alkali-aggregate reactions. Co-calcination of BR with kaolin or washing and cooking of BR with calcium hydroxide or calcium hydroxide and gypsum resulted in considerable reduction of alkali content; up to 75%. At the same time the reactivity of the BR was reduced but still being higher compared to fly ash already used in the cement industry.