Influence of initial curing on sulphate resistance of blended cement concrete

Abstract

The paper presents results of an investigation on the effect of initial curing conditions on the sulphate resistance of concrete made with ordinary portland cement and using pfa, silica fume and ground granulated blast furnace slag for partial replacement of cement. In addition, porosity and pore structure analysis of representative pastes was carried out to examine the relationship between these properties and sulphate resistance of concrete. The depth of carbonation in specimens of pastes was also determined. Three different initial curing conditions immediately after casting of specimens were adopted, namely: WET/AIR CURED at 45°C, 25% RH; AIR CURED at 45°C, 25% RH; AIR CURED at 20°C, 55% RH. The results show that pore volume and pore structure of the paste bear no direct relationship with the sulphate resistance of concrete. The presence of a carbonated layer on the surface is generally accompanied by superior sulphate resistance—there are, however, important exceptions. Low humidity curing at high temperature (45°C) results in higher depths of carbonation but lower sulphate resistance than similar curing at 20°C. The sulphate resistance of concrete increases with the replacement of cement with 22% pfa, 9% silica fume and 80% ggb slag. The sulphate resistance also increases due to drying out of concrete during early curing at low relative humidity and due to carbonation. The possible common factor which leads to this improved sulphate resistance is the reduced Ca(OH) 2 content which leads to smaller volume of the expansive reaction products with sulphate ions. The effect of initial curing at high temperature (45°C) is significantly harmful to the sulphate resistance of plain concrete but much less so to the blended cement concretes.