Strength development in concrete incorporating PFA and metakaolin

Abstract

The work presented in this paper forms part of an ongoing investigation examining the potential of using metakaolin pulverized fuel ash (MK–PFA) blends for cements in concrete. The programme of research involves the examination of the effects of the blends on the workability, strength development and factors affecting durability, including chloride penetration, carbonation and water transport properties. The influence on the workability was reported previously and this paper gives the results for compressive-strength development. Several blend compositions were employed at water : binder ratios of 0·4, 0·5 and 0·6 to produce concretes, the compressive strengths of which were evaluated at 7, 28 and 90 days. It is found that the contrasting influences on the strength, particularly at early curing times, effected by PFA and MK when used in isolation as blends for cement can be combined effectively by the employment of various ratios of MK and PFA. Early strengths surpassing that of the control are obtained with low partial cement replacements and high MK/PFA ratios. Higher cement replacements with low MK/PFA ratios result in 90 day strengths that are also greater than the control strength. Careful choice of composition can enable reduced cement content without reduction in the compressive strength. The reduction in cement content leads to environmental benefits, and with increased utilization of these blends the relative cost of the pozzolanic blend will reduce and overall savings will result.