Dolomitic lime: thermal decomposition of nesquehonite

Nesquehonite (MgCO3·3H2O) (N) can be obtained from a dolomitic quicklime paste in a CO2-rich atmosphere. Thermal decomposition of this synthetic nesquehonite has been studied by TG-DTA analysis. It is very similar to the hydromagnesite (HY) thermal decomposition, as it show similarities in the decarbonations at 440 and 550 °C, and an exothermic phenomenon at 510 °C. It has been reported in the literature that some intermediate in N→HY transformation occur, but this process cannot be directly detected during the heating. Samples were heated at 115, 230, 280, 370, 460, 520, 600, 800 and 1000 °C and XRD and FT-IR were used in order to determine the structural changes in nesquehonite and the intermediate phases formed. Results show that nesquehonite transforms at lower temperatures (115 °C) into a stable amorphous magnesium carbonate with chemical composition very close to that of HY. Thermal decomposition of nesquehonite, during a gradual temperature increase, proceeds via the formation of this compound. At higher temperatures (460 °C/short heating times), nesquehonite transforms into HY. The occurrence of an exothermic peak at 510 °C has been also discussed.