Evaluating the use of harvested fly ash as a sustainable alternative in cementitious mixtures

Abstract

This study investigated the feasibility of utilizing harvested fly ash from landfills and ponds as a sustainable alternative to traditional fly ash in cementitious mixtures. The research examined the fresh properties, compressive strength, dimensional stability, transport properties, alkali-silica reaction, and sulfate resistance of pastes, mortars, and concretes made with traditional class F fly ash (TFA) and two sources of class F harvested fly ash (HFA1 & HFA2). The results showed that the cementitious mixtures containing both traditional and harvested fly ash consistently exhibited superior performance across all properties compared to the reference mixture. Amongst the fly ash samples, the mixtures containing traditional fly ash (TFA) demonstrated the best performance, followed by HFA1 and HFA2.At 28 days, reference mortar showed 8–39 % higher compressive strength than fly ash mortars, while TFA-15, HFA1–15, and HFA2–15 mortars surpassed the reference strength by over 10 % at 90 days. TFA-15 mortar showed drying shrinkage of 259 microstrains, which was 10 % and 13.7 % lower than HFA1 and HFA2, respectively. At 30 % replacement, TFA’s shrinkage was 19 % and 26.4 % lower than HFA1 and HFA2. Water absorption in TFA, HFA1, and HFA2 mortars at 15 % replacement was 52 %, 41 %, and 21 % lower than the reference (C100) at 28 days. At 90 days, TFA-15 exhibited 44–49 % and TFA-30 showed 35–27 % reductions in water absorption before and after boiling, respectively, compared to 28 days. For resistance to chloride ingress, TFA, HFA1, and HFA2 concretes at 15 % replacement showed 228 %, 133 %, and 119 % higher resistance compared to C100, respectively. At 30 % replacement, TFA showed 142 % higher resistance, while HFA1 and HFA2 showed 39 % and 30 % higher resistance, respectively. In terms of sulfate resistance (SR) at 28 days, fly ash-containing concrete at 15 % cement substitution showed over 100 % higher SR than the reference concrete (C100). TFA-15 exhibited 26 % and 48 % higher SR than HFA1–15 and HFA2–15, while TFA-30 showed 35 % and 74 % higher SR than HFA1–30 and HFA2–30, respectively. In alkali-silica reaction tests, TFA-20 and HFA1–20 mortars reduced 14-day expansion by 86 % and 83 % below the 0.1 % threshold, and by 84 % and 76 % at 28 days, respectively, staying below the 0.28 % failure limit. TFA-15 reduced expansion by over 34 % and 40 % compared to HFA1–15 and HFA2–15 at 28 days, and by 55 % and 49 % at 30 % replacement.