Metakaolin as supplementary cementitious material - Optimization of kaolin to metakaolin conversion

In this paper the optimization of the kaolin calcination is studied, aiming at using the produced metakaolin as supplementary cementitious material. Representative samples of poor Greek kaolin (Milos island) and a high purity commercial kaolin were tested. Samples were heated at different temperatures during different times. The optimization of calcination conditions was studied by DTA-TG and XRD analysis of the raw and thermal treated kaolin samples, by pozzolanic activity analysis of metakaolins and finally by strength development analysis of cement-metakaolin mixtures. This approach showed that heating at 650 degrees C for 3 h is efficient to convert poor kaolins with low alunite content to highly reactive metakaolins. However in the case of kaolin with a high alunite content, thermal treatment at 850 degrees C for 3 h is required in order to remove undesirable SO3. Evidence was found that poor kaolins can be efficiently used for the production of highly reactive metakaolins.