Investigation of the deterioration of blended cement concrete under sulfate attack in terms of interfacial transition zone

The importance of the porous interfacial transition zone to the chemical aggression of concrete is obvious when one considers the relations existing between porosity, permeability, chemical composition and the sulfate attack. In this study, the effect of ITZ quantity through varying aggregate content on the deterioration of blended cement concrete under sulfate attack, was determined to understand better the relationship between sulfate ions and concrete microstructure. The ITZ quantity was directly proportional to the aggregate volume fraction. Therefore, the effect of ITZ on sulfate resistance ability of concrete made with pure OPC and blended binders was evaluated by a comparison among mortars with systematically varied aggregate volume fraction. The porosity distribution with the ITZ was determined by using a quantitative backscattered electron microscopy (BSE) image analysis. It was found that the incorporation of moderate amount of Limestone filler is able to compact the microstructure of both ITZ and bulk matrix by filling effect and nucleation sites effect. The effects of slag on the porosity of ITZ were dependent on the replacement rate. The degree of deterioration had a slight tendency to increase for the samples prepared with higher aggregate volume content, which means high ITZ volume fraction. For the sulfate to reach the interior of the samples, it must move through the bulk cement matrix. The effect of aggregate and ITZ can only be notable when the interior structure was exposed to the sulfate ions. The presence of ITZ was normally accompanied by a denser bulk cement matrix. This could limit the ingress of sulfate ions and delay the formation of expansive products in initial stage. After the sulfate penetrates into the interior of the samples, the inner structure was expected to exert more significant influences on the deterioration.