Mechanical performance of sustainable concrete containing industrial waste admixtures of SF, FA, CCI, and crushed quartz at different water–binder ratios

The present study examined the mechanical performance of sustainable concrete with silica fume (SF), fly ash (FA), cooled cast iron (CCI), and crushed quartz powder (CQ) used as partial cement replacements at three water-to-binder ratios (0.36, 0.41, and 0.46). Fifteen concrete mixtures were cast and tested for compressive, splitting tensile, and flexural strengths at 7, 28, and 90 days. Results indicated that the lowest water-binder ratio of 0.36 produced the highest strengths, consistent with values of 78 MPa compressive strength at 28 days and 87.8 MPa at 90 days, which were better than those obtained from mixes having the other two ratios by up to 34% in early ages and by up to 33% in later ages. The partial replacement of SF by FA lowered the early-age strength, but very significant long-term gains were recorded; replacing three-quarters of SF with FA increased the 90-day compressive strength by 5.9%, 30.4%, and 30.1% for groups 0.36, 0.41, and 0.46, respectively, compared to their corresponding mixes with SF only. Incorporating CQ led to reductions in strength at 28 days of 9.3-13.4%; however, reductions at 90 days remained modest, 8.1-13.6%, indicating improved long-term pozzolanic contributions. It is noted that tensile-to-compressive strength ratios for all mixtures ranged from 9.8 to 12.2%, and flexural-to-compressive ratios ranged from 13.6 to 14.7%, which are better than the ACI318 predictive correlations for concretes using industrial by-products. The results confirm that optimized SCM blending higher fly-ash proportions at lower water-binder ratios enhances long-term mechanical performance while enabling significant cement reduction, supporting more environmentally sustainable high-strength concrete.