Evaluating Test Methods for Rapidly Assessing Fly Ash Reactivity for Use in Concrete

In addition to various properties, including fineness and loss-on-ignition, used to assess fly ash, standards covering the material as an addition in concrete normally include a measure of its pozzolanic activity. While variations exist, approaches commonly used involve testing the compressive strength of standard fly ash mortar, at different ages, and comparisons with the corresponding Portland cement reference. Given the nature of the material, these can be carried out over extended periods (e.g. 90 days for Activity Index in BS EN 450-1). If the timescale was reduced, this could offer benefits with regard to fly ash production testing and subsequent use. The paper will review various options in the literature towards achieving this and describe an investigation carried out to examine the potential of selected property measurements/techniques in the role. These include (i) fly ash characteristics: fineness, bulk chemistry and amorphous content; (ii) accelerated curing (elevated temperature) conditions for Activity Index and hydrated lime/fly ash mortars; and (iii) wet chemical tests involving lime-consumption. The methods were considered for a range of Class F fly ashes covering the effects of different sources, seasonal variations and co-combustion, with evaluations made through comparisons against the Activity Index to BS EN 450-1. The paper will identify their relative effectiveness in assessing fly ash reactivity and explore how they relate to concrete.