TECHNICAL PAPERS

New Test Method to Obtain pH Profiles due to Carbonation of Concretes Containing Supplementary Cementitious Materials

Abstract

The writers have carried out an investigation to develop apparent pH profiles of concretes for a variety of cement blends viz. normal portland cement containing by mass 30% pulverized fuel ash, 50% ground granulated blast-furnace slag, 10% metakaolin, and 10% microsilica, chosen to replicate common replacement levels, along with 100% normal portland cement (OPC) mix. The samples were exposed in an accelerated carbonation environment (5% CO2) for 6weeks during which pH profiles were obtained every week as the concrete carbonated. Measurement of air permeability, carbonation depth, resistivity, and calcium hydroxide content were performed to assist in interpretation of the results. The nature of the pH profiles obtained depended on both the type of binder and the duration of exposure to the carbonation environment. Utilizing the pH profiles, a rate of carbonation was determined, which was found to depend on the type of binder. Both the rate of carbonation and the depth of carbonation after 6weeks of exposure indicated that OPC concrete performed better than concretes containing supplementary cementitious materials. It was also determined that the gas permeability alone cannot provide an accurate indication of the likely rate of carbonation. The thermogravimetric analysis suggests the existence of a relationship between calcium hydroxide content and the apparent pH of carbonated concretes. On the basis of the results in this paper, it can be concluded that the pH profiles, using the technique described in this paper, can be used for measuring the carbonation resistance of concretes containing supplementary cementitious materials.