Calcium sulphate formation during the heat-up period: some essential parameters

The appearance of white deposits on some ceramic products is known as efflorescence. In this study, efflorescence is principally made up of hydrated calcium sulphate. This salt is mainly formed during the firing of the material and is affected by various parameters: firing cycle, material texture and mineralogical and chemical composition of the raw materials. Evidence for this formation is found by analyzing gases evolved during firing: SO2 (from pyrite oxidation) and CO2 (from decomposition of CaCO3). The retention of SO2 is essential in the CaSO4 formation. Fourier transform infrared (FTIR) analysis of CaCO3 reacting with SO2, as a function of time and temperature, has allowed to determine the formation mechanism. Determination of the pore size distribution of the ware during the firing has revealed that porosity is a crucial property for the oxidation of pyrite. This detailed analysis has finally led to recommendations to limit the formation of CaSO4 during the firing cycle.