Heat resistance and sorptivity of an air-entrained concrete containing mineral admixtures and CBA

Purpose The purpose of this work is to improve the air entrainment capacity of a concrete by using fine mineral admixtures such as fly ash (FA) and silica fume (SF) as cement substitute, and coal bottom ash (CBA) as fine aggregate substitute. Air entrainment capacity has been studied indirectly as a measure of heat resistance of concrete. Literature has suggested that mineral admixtures improve the air absorption in the paste component of the concrete, on the one hand, whereas they perform pore and grain size refinement, on the other, thereby reducing the air entrainment. CBA, which being porous, creates the possibility of air adsorption by the aggregate component. Therefore, the study finds out whether a double benefit of adding both of these materials will be achieved, or CBA will try to improve the deficiency in the air entrainment created by the mineral admixtures. Design/methodology/approach Air-entrained concrete (AEC) mixes were constituted in three groups. First group represents mixes with natural fine aggregates only, and second with 25% fine aggregates substituted by CBA. Progressively, the third group has 50% fine aggregates substituted with CBA. In all the three groups, cement was substituted with FA and SF @ 0%, 20% and 40%, and 0%, 5% and 10%, respectively, thereby creating four binary and four ternary mixes corresponding to each group. Compressive and flexural strength tests were conducted at 28 days on the concrete mixes pre and post high-temperature heat treatment, i.e. 100°C, 200°C and 400°C, respectively. This study also examines the microstructure characteristics of AEC after 14 days of curing via X-ray diffraction. Sorptivity test was also conducted to estimate the capillary and air-entrained voids in concrete. Findings It was found that a concrete mix containing 20% FA and 10% SF along with 50% CBA could give similar post-heated strength to a normal (without mineral admixtures) AEC. In AECs where only CBA is present and cement paste is not substituted, both of the pre- and post-heated strengths of concrete reduce. Also, some mixtures containing large amounts of mineral admixtures in concrete with nil CBA show a high reduction in post-heated strength though they show good pre-heated strength. Therefore, mineral admixtures and CBA complement each other in improving the post-heated strength. Air pore structure found from sorptivity test also verifies these results. Originality/value AEC is very helpful for insulation of buildings during summer season by absorbing heat waves. AEC containing FA and CBA reduces carbon footprint because of substitution of cement and it also helps to conserve natural resources by the use of CBA in place of fine aggregates.