Influences of conditioned pulverized-fuel ash as a cement component on the properties of concrete

Abstract

This paper describes a laboratory-based study carried out to examine the use of conditioned pulverized fuel-ash (PFA) as a cement component in concrete. A range of conditioning variables, including moisture content (10 and 20%), storage period (up to 18 months) and PFA properties (fineness and low and high lime contents), were investigated. Tests were carried out for heat of hydration and calcium hydroxide content of paste to examine Portland cement/PFA reactions, and workability, strength development and permeation properties of concrete to consider influences on practical performance. The results indicate that conditioning of PFA leads to slight changes in rates of heat evolution in fresh and hydrating paste, with reduced pozzolanic activity apparent after setting, compared to dry PFA. In concrete, conditioning of PFA gave slightly reduced slump, but early strength development (to 28 days) and permeation properties were either unchanged or slightly improved. At later test ages, slightly poorer properties, compared to dry PFA concrete, were generally noted, particularly with storage periods of six months or greater. While PFA fineness and conditioning moisture content did not influence these trends, the effects described increased with material having high total/free lime contents. A parallel series of tests with material conditioned and stored at power stations (i.e. exposed to an external environment) showed general agreement with laboratory data. The practical implications of the study covering the use of conditioned PFA are considered.