Performance of cement concrete with mineral admixtures

Abstract

The scope of manufacturing blended cements in India and the resultant saving in energy consumption were identified. The characterisation of the two main mineral admixtures, namely fly ash (FA) and blast furnace slag (BFS), was based on their chemical and mineralogical composition and their particle characteristics. It was found that all the three factors are important from the point of view of selection of appropriate mineral admixture for blending with cement. The durability performance of concrete with blended cements was improved in terms of its resistance to the three deteriorating factors, namely alkali±silica reaction (ASR), sulphate attack and the corrosion of steel reinforcement. The improved resistance towards the ASR was due to the higher retention of alkalis and the refinement of pore structure, and is due to formation of secondary calcium silicate hydrate (C-SH) during the hydration of FA or BFS components in cement. Its resistance towards the Na2SO4 expansion is on account of lower content of free Ca(OH)2, besides the refinement of pore structure. Its lower resistance towards the strength reduction in MgSO4 environment, like ordinary portland cement (PC), is attributed to the deleterious reaction of Mg++ cations with C-S-H. In general, blended cements having tricalcium aluminate (C3A) contents in the range of 5–8% and the FA content of 25–30% or the BFS content above 60%, show moderate to good sulphate resistance. The higher corrosion resistance is attributed to the refinement of pore structure resulting in lower chloride and oxygen diffusivity, greater chloride binding capacity and higher electrical resistivity of the concrete made with blended cements. In comparison to concrete with OPC only, early and extended curing are required to obtain higher long term strength and better durability performance: the economic considerations show that the use of blended cement in concrete is the cheapest alternative available to control reinforcement corrosion.