Statistical and experimental analysis of concrete using calcined lime sludge and calcined clay as cementitious substitutes

The increasing demand for sustainable construction materials has encouraged the utilization of industrial by-products as partial replacements for cement in concrete. This study investigates the performance of M40 grade concrete incorporating calcined clay (CC) and calcined lime sludge (CLS) as supplementary cementitious materials. Different concrete mixes were prepared by replacing cement with CC and CLS at proportions of 5%–20%. The fresh and hardened properties of concrete were evaluated through workability, compressive strength, split tensile strength, flexural strength, and water absorption tests at different curing ages. The results indicated that concrete workability decreased with increasing replacement levels due to the higher specific surface areas of CC and CLS. However, the mechanical properties improved at moderate replacement levels. The optimum mix (15% CC + 10% CLS) showed ∼15%–20% higher compressive strength compared to the control at 90 days, with reduced water absorption. The mix containing 15% CC and 10% CLS exhibited the highest split tensile, and flexural strengths compared with the control mix, indicating a synergistic effect between the pozzolanic activity of CC and the calcium-rich composition of lime sludge. Beyond this replacement level, a reduction in strength and an increase in water absorption were observed. Response surface methodology effectively identified the optimum mix of 15% CC and 10% CLS, with a curing period of 90 days, demonstrating higher predictive capability for optimizing the mechanical and durability characteristics of concrete. This study demonstrates that the combined use of CC and CLS can enhance the mechanical performance of concrete while reducing cement consumption and promoting the sustainable utilization of industrial waste materials, supporting SDGs 9, 11, and 13.