TG/DTG-DSC and high temperature in-situ XRD analysis of natural thaumasite

Highlights: • The thaumasite decomposition when heated to 1673 K, includes the dehydration, dexydroxylation, decarbonation and desulphuration stages. • During the thermal decomposition in Ar and Air, the intermediate phases (calcium carbonate, ternesite, spurrite, and anhydrite) are produced. • In an Ar, the thermal thaumasite decomposition is carried out with a higher degree of conversion and with mass losses close to theoretical. • Larnite is the main phase in the solid residue after heating up to 1673 K in Ar and Air. • The thaumasite thermal decomposition scheme has a practical application in cement materials, and chemical archaeology. This paper investigated thermal properties of natural thaumasite, such as phase composition and reaction mechanism of thermal decomposition using simultaneous TG/DTG-DSC in Ar and Air medium up to 1673 K, coupled with masspectrometer for analysis of evolving gases, and in-situ powder X-ray diffraction measurements. The transitional solid phases, grown with increasing of temperature at thaumasite thermal decomposition, are calcium hydrogen carbonate (Ca(HCO₃)₂) and hydrogen sulphate (Ca(HSO₄)₂), calcite, anhydrite, calcium silicates (wolastonite and larnite), calcium silico-carbonate (spurrite), and calcium silico-sulphate (ternesite). The thermal decomposition in both gaseous media includes the stages of dehydration, dehydroxylation, dacarbonation and desulphuration with obtaining a solid residue of varying degrees of crystallinity. The main solid phase, grown at the highest temperatures, is larnite. Based on the obtained results it was proposed the scheme of chemical reactions, which presents the reaction mechanism of thaumasite thermal decomposition. The defined scheme has both fundamental importance by adding new details of reference data, and practical application for thaumasite identification in chemical archaeology, and in the chemistry of cement and cement-based materials.