A comparison between the carbonation of OPC and PFA concrete in a mass concrete structure

The carbonation of an OPC and equivalent PFA concrete (with the same cementitious content) taken from a 30 year old mass concrete buttress dam have been investigated. The results presented compare the determination of the carbonation front made by spraying a freshly broken surface with phenolphthalein, with those determined from calcium hydroxide determination by thermogravimetry (TG), and from the analysis of silicate anions in calcium silicate hydrate (CSH) using trimethylsilylation (TMS). TG and TMS analyses gave similar results for OPC and PFA concrete and indicated in both cases the presence of a carbonating zone of finite width, although the results of using these methods for the OPC concrete showed carbonation to a greater depth than is indicated by phenolphthalein. For the PFA concrete, however, the different methods gave comparable results. Carbonation affected calcium hydroxide and CSH simultaneously within the carbonating zone. A residue of partially carbonated CSH remained in the carbonated zone, indicated by phenolphthalein, although the calcium hydroxide had completely carbonated. It is concluded that the chemical processes of carbonation are very similar for OPC and PFA concrete.