Effect of Artificial Coarse Aggregate From Fly Ash Activated by Calcium Sulfate Dihydrate on Compressive Strength and Water Absorption of High-Strength Concrete

The present study focuses on investigating effect of artificial aggregates from fly ash activated by calcium sulfate dihydrate as a partial crushed stone replacement, on bulk density, compressive strength, and water absorption of high-strength concrete (HSC). The artificial fly ash aggregate (AFAA) was made from a mixture consisting of 85% Class-F fly ash, 15% Portland cement, and 0 or 2% calcium sulfate dihydrate (CaSO4‧2H2O) as an activator and a low water-to-binder ratio of 0.20. The replacements of crushed stone by the AFAA were 0, 20, 40, and 60% by volume. All the mixture proportions of HSC had a low water-to-cement ratio of 0.37. All concrete specimens were cured in water at a temperature of 28 ± 2 ℃. The results showed that although bulk density of hardened concrete containing AFAA was lower than that of the control concrete without AFAA, the value was in a range from 2229 to 2429 kg/m3. The partial replacement of crushed stone by AFAA in the range of 20–60% decreased the compressive strength of hardened concrete by 7.9–31.4% when compared with the control concrete at the age of 28 days. The AFAA replacement increased the water absorption of all the concrete samples by 15.7–65.9% compared with control concrete. The hardened concretes with 20% replacement of AFAA activated by 2% CaSO4 2H2O had the highest compressive strength at all ages and the lowest water absorption at 28 days among the ones with AFAA replacements.