The Thermal Behaviour and Carbonation of Portlandite Ca(OH)2: An X-Ray Diffraction Study

Portlandite, Ca(OH)2, is a cornerstone material in cement chemistry, carbonation, and calcium-looping technology. The carbonation and thermal stability of calcium hydroxide have been studied using thermogravimetric (TG) and X-ray diffraction (XRD) analysis under different atmospheres. Portlandite decomposition is a typical time–temperature-dependent reaction that starts at temperatures as low as 280 °C. Isothermal XRD experiments reveal a single reaction mechanism involving direct transformation to CaO, which is complete in less than an hour at 400 °C. The formed CaO, however, is partly transformed to CaCO3 when CO2 is available even in small quantities. An empirical activation energy of 140(4) kJ/mol was extracted for the decomposition reaction in a dry-air atmosphere. In a reactive Ar/CO2 atmosphere, direct carbonation of Ca(OH)2 to CaCO3 is more favourable, and the formation of CaO is completely suppressed. Room-temperature carbonation of Ca(OH)2 is distinctly affected by relative humidity. For fast transformation within 24 h, a relative humidity of at least 75% is necessary; the reaction is distinctly slowed at 53% RH and is almost completely inhibited below 30% RH.