Impact of slag-fly ash cementitious system on thermal controls and durability of high-strength mass concrete

The rise in temperature due to the heat of hydration in the high-strength mass concrete poses serious issues such as thermal cracking and delayed ettringite formation. The present experimental investigation is to optimize the proportions of SCMs like fly ash (FA) and ground granulated blast-furnace slag (GGBS) for the same grade of high-strength mass concrete with a focus on controlling the peak temperature rise and improving sustainability. For this purpose, one 100% ordinary Portland cement (OPC)-based sample (T1) and five SCMs-based samples viz. T2, T3, T4, T5, and T6 in which OPC was replaced by 20% FA, 35% FA, 50% GGBS, 70% GGBS, and 33%FA + 33% GGBS, respectively, were cast in the insulated blocks having one cubic meter volume. Analyzing critically, it is observed that for the same grade of concrete, the right blend of SCMs greatly reduces the peak core temperature (similar to 33.2% and similar to 39.4% for T5 and T6 samples, respectively) than the sample prepared with 100% OPC. This is due to the reduction in the total heat of hydration in the SCMs-based system than that of the OPC-based system at early age. The amount of hydration products formed at early age varies for different percentages of SCMs, which is investigated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Field emission scanning electron microscopy (FESEM) analyses. Additionally, compressive strength analysis reveals that all the samples achieved almost similar strength (71.4, 74.6, 73.5, 71.5, 69.2, and 68.2 MPa, respectively, for T1 to T6 samples) at 56 days. Moreover, the rapid chloride permeability test (RCPT) values of the T2 to T5 samples are measured to be 597, 461, 672, 584, and 621 coulombs; while, the same is measured to be 2637, coulombs for the T1 sample, indicating higher durability of SCMs-based samples. Finally, based on the analyses, a plausible model has been presented to explain the phenomenon, and industrial viability analysis is done to adjudge its suitability for an onsite application.