Calcination and Reactivity of Marlstones: A Comparison Between Palygorskite and Smectite Bearing Marlstones

In the race to reduce greenhouse gas emissions, the cement industry appears to be a bad pupil and is responsible for non-negligible pollution, particularly due to clinkerization. Calcined clays are among the most promising Supplementary Cementitious Materials (SCMs) and allow a significant reduction in the environmental footprint of cement manufacturing. Many secondary resources, which are considered as waste by the mining industry, contain clay phases, and are therefore potentially recoverable as SCMs once calcined. This study focuses on the calcination and reactivity of two marlstones (MS1 and MS2) from a phosphate mine. The evolution of the phases as well as the physico-chemical modifications induced by calcination and hydration were characterised by X-ray diffraction (XRD) and Solide State Nuclear Magnetic Resonance (MAS NMR). The two materials are relatively similar and mainly composed of dolomite, associated with clay phases (smectite and biotite), quartz and other minority phases. The main difference between the two materials is the occurrence of palygorskite - a particular TOT clay mineral - in the MS1 sample. The calcination of these two materials resulted in a significant dehydroxylation of the clay phases, a dissociation of the dolomite into lime and periclase and the formation of dicalcium silicate. The self-reactivity of the samples in the presence of water was then tested in order to determine the evolution of the reactive phases (lime, periclase, dicalcium silicate) and to test the pozzolanic reactivity of the calcined clay phases. The results show a strong reactivity of the MS1 sample (mainly pozzolanic) while MS2 exhibits a lower reactivity. The presence of palygorskite in the MS1 material improves its reactivity once calcined, it thus seems that this unconventional clay has a promising pozzolanic reactivity, which opens new ways of valorization for this type of secondary resources.