Workability of concrete incorporating pulverized fuel ash and metakaolin

Abstract

Workability of concrete incorporating combinations of pulverized fuel ash (PFA) and metakaolin (MK) as partial replacements for Portland cement (PC) has been investigated. The aim of the research work is to explore the potential of using PFA and MK as blends with PC in terms of the flow properties of the resulting concrete. Ninety mixtures containing 0, 10, 20, 30 and 40% total replacement of cement with combinations of metakaolin (0–15%) and PFA (0–40%) for concretes with water to binder ratios of 0·4, 0·5 and 0·6 were prepared. The workability of the concrete was measured by the slump, compacting factor and Vebe time tests. Substantial reductions in the workability were obtained when the mixtures contained MK only, with greater reductions being experienced as the MK replacement level increased. In contrast, the mixtures with PFA showed considerable improvement in workability with increase in replacement level. It was found that, for concretes containing blends of PC, PFA and MK, the loss in workability due to the presence of MK can be compensated for by the incorporation of PFA. However, at low cement replacement levels, improvement in workability by the PFA can be achieved only if a superplasticizer is used. The degree of restoration of workability provided by PFA is influenced by the level of cement replacement, MK/PFA ratio and water to binder (w/b) ratio. In particular at low w/b ratio (= 0·4), where superplasticizer is used, there is a critical MKIPFA ratio (∼0·4) above which workability decreases and below which workability increases with increasing replacement level. In general, the incorporation of PFA not only compensates for the loss of workability resulting from the presence of MK but also provides economic benefits.