Comparative studies of the carbonation of hydrated cements

A method is described to determine the depth of carbonation of cement paste cylinders using a thermoanalytical method. Depth-profiled thermogravimetry has been used to determine the extent and kinetics of reaction with moist CO₂, The procedures are described and the results are compared with those obtained using chemical indicators. The two methods give broadly comparable results, but, whereas phenolphthalein shows an apparently sharp interface between carbonated and uncarbonated material, thermogravimetry discloses that mineralogical zonation occurs and that isolated regions of high pH persist behind the interface. It is suggested that the existence of a zone of transitional pH may be a potential cause of pitting corrosion of steel in partially carbonated concrete.