Advanced processing of high Ca fly ash for enhanced reactivity and improved high value-added application possibilities

The main purpose of the research was to examine the processing possibilities of a low-grade high Ca fly ash (FA) to increase its future utilisation potential. Size separation via sieving and mechanical activation with the use of a planetary ball mill was examined to compare the results and complement the application of the various mechanical processes. Based on the preliminary chemical analysis, the composition of the FA below and above 32 µm showed significant variations, thus, the < 32 µm size fraction was separated and the coarse fraction was utilised due to the higher Si and Al content. After milling the coarse FA for various grinding times, the frequency distribution of the < 1 µm particles showed a considerable increase but aggregation and agglomeration were also observed after 30 min, with a 5.49 µm average particle size. The pozzolanic activity almost doubled with only 5 min grinding, reaching the optimal grinding time with the highest CaO uptake value at ∼20–30 min with over 160 mg/g absorbed CaO at only 20 days. According to the FT-IR analysis of the mechanically activated samples, important structural changes took place in the FA framework. Considering the geopolymerization behaviour of the samples, applying only classification can significantly reduce the compressive strength by up to a 33% decrease. However, with the mechanical activation of the coarse fraction, an over five times increase was observed in the compressive strength with the optimal grinding time.