Effect of the moisture states of artificial fly ash aggregate as a partial river sand replacement on bulk density and mechanical strengths of hardened concrete

In this study, artificial fly ash aggregate (FAA) under oven-dry (OD) and natural (N) states was employed for partially replacing natural river sand (RS) to investigate effect of moisture states of FAA on slump of fresh concrete, bulk density and mechanical strengths (i.e., flexural strength and compressive strength) of hardened concrete. The FAA with particle sizes ranging from 1.25 to 5 mm was made from 85% Class-F fly ash, 15% Porland cement, a water-to-binder ratio of 0.21, and cured for 1 day in air and 13 days in water condition. Compared with RS, density and bulk density of FAA were lower by 22.4 and 32.5%, respectively, while water absorption was significantly higher. Based on the particle size distribution results of fine aggregate mixtures (including FAA and RS), replacing RS with FAA at volume ratios of 20 and 40% were selected for producing concrete. The slump of fresh concrete with 20 and 40% replacements of FAA under the N state was higher than that with corresponding replacement of FAA in the OD state and the fresh control concrete without FAA had the highest slump. The use of FAA changed insignificantly bulk density at 28 days of hardened concrete, regardless of FAA moisture states. In contrast, the flexural strength at 28 days of hardened concrete declined when FAA content increased, and the difference in flexural strength of hardened concretes using FAA under different moisture states was in a range from 9.3 to 13.6%. Meanwhile, the compressive strength at 3, 7, and 28 days of hardened concrete tended to increase when using FAA in the OD state and decreased when using FAA in the N state. Consequently, 40% replacement of RS with FAA in OD state can be suggested for the concrete production to limit the exploitation of RS and utilize the most fly ash released from coal-fired power plants, towards sustainable development for the concrete industry.