Clay Calcination Methods and Composition Impacts on Calcined Clay Properties

Calcined clay is a well-established Supplementary Cementitious Material (SCM) in the cement industry. Its high pozzolanic reactivity – upon thermal treatment – and widespread availability offer the potential to reduce the clinker factor substantially. While extensively explored, research has primarily focused on muffle furnace calcination, with limited studies available on understanding the influence of varying calcination methods and scalability. Thus, it is crucial to generate new research data that allow for optimizing different calcination methods. In this study, two clays – with different kaolinite and iron impurity contents – calcined under three laboratory methods, namely a muffle furnace, a rotary kiln, and an entrained flow reactor are investigated. The calcined samples are characterized by TGA/DSC, XRD, and the R³ bound water test to assess: a) dehydroxylation degree, b) recrystallization peak formation, and c) pozzolanic reactivity. In addition, PSD and BET are used to explore structural changes in the calcined samples regarding agglomeration and specific surface area, while touching on deposit formation and color control of flash calcined clays. The results show that both soak and flash calcination techniques can deliver optimal pozzolanic reactivity for both clays. DSC has shown a good correlation by accurately indicating the recrystallization. Agglomeration and specific surface area rates were significantly affected by flash calcination, requiring aftergrinding. The clay containing higher iron content demonstrated a grey color after flash calcination, while at high temperatures deposit formations were limiting the material’s collection from the entrained flow reactor. Ongoing research of samples with varying iron contents will further evaluate this occurrence.