Synergistic effect of fly ash and bentonite as partial replacement of cement in mass concrete

This research focuses on the effects of using fly ash and bentonite as partial replacement of ordinary Portland cement (OPC) in mass concrete. Replacement of OPC with supplementary cementing materials such as fly ash or bentonite is one of the promising ways to mitigate thermal cracking due to temperature differentials in mass concrete. In this study, three types of concrete mixes with varying amount of partial replacement of OPC were investigated, 25% bentonite, 25% fly ash, and 12.5% bentonite and 12.5% fly ash. These mixtures were named BM, FM and BF, respectively. A fourth control mixture (CM) with no OPC replacement was also studied for comparison purposes. From each mix, one large specimen (having 600 × 800 × 1000 mm overall dimensions) representing mass concrete, and concrete cylinders of standard size representing normal structural concrete were prepared. Various tests were performed on these specimens at ages of 7, 14, 28, 56 and 91 days. Comparatively, BF was the most effective in controlling temperature rise in mass concrete. Moreover, this mix resulted in the highest values of compressive strength at 91 days and higher early-age strength. Ultrasonic pulse velocity (UPV) tests were conducted on mass concrete samples as well as on cores extracted from concrete blocks. Results of these tests revealed that mass concrete blocks made of BF and FM resulted in more uniform properties, or in other words, suffered least from large concrete placement by reducing the heat of hydration.