AbstractThis paper aims to develop an optimal design mixture for concrete containing lightweight expanded clay aggregates (LECA) and silica fume with a holistic view in which the mechanical properties, strength retention at elevated temperatures, costs, and environmental impacts are all considered. Following the response-surface methodology, 30 concrete mixtures were developed, which contained different water-to-cement ratios (0.25–0.5), total cementitious materials contents (300–700  kg/m3 of concrete), and different amounts of LECA (0%–100% of the total aggregate volume) and silica fume (5%–15% of weight of total cementitious material). The mixtures were used to fabricate a total of 240 concrete cylinders, half of which remained at the ambient temperature, whereas the other half were exposed to an elevated temperature of 750°C for 2 h. The results of mechanical tests on specimens were combined with the economic and environmental performance indices to find the optimal mix design. The result showed that using a mixture containing 0.33 water-to-cement ratio, 300  kg/m3 cement content, 14.97% silica fume and 84% total aggregate replacement by LECA may lead to the concrete that performs the best in terms of mechanical characteristics, strength retention during fire, and economic and environmental impacts.

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