Effects of Unconventional Supplementary Cementitious Materials on the Mechanical Strength and Environmental Benefits of Cement-Based Materials—Multi-Objective Optimization for Separately Modified Mortars

This study experimentally investigates the mechanical properties and development patterns of mortars incorporating steel slag (SS), coal gangue (CG), and biochar (BC) and establishes a multi-objective optimization framework for sustainable mix design. The investigation systematically evaluates the individual effects of these materials on mechanical properties, economic cost, and environmental impact, providing a comparative basis for single-SCM mix design. Experimental results demonstrate distinct material-specific behaviors: SS substitution (10–40%) exhibits behavior consistent with latent hydraulic activity, mitigating early-age strength loss and showing a relative 28-day strength recovery from 71.4% to 80.5% at 30% substitution. CG acts primarily as a filler-type component under the tested conditions, causing a progressive, dosage-dependent strength reduction, with 28-day relative strength declining to 46.4% at 40% substitution. In contrast, BC at low dosages (1–2%) functions as a performance-enhancing cement replacement, improving 28-day compressive strength by up to 11.7% within the tested curing period through physical effects supported by the previous literature. Preliminary environmental screening indicates that both SS and CG significantly reduce cost (down to 2421.6 and 2410.2 CNY/m3, respectively), energy consumption (down to 3645.3 and 3028.6 MJ/m3), and carbon footprint (down to 492.2 and 438.0 kg CO2-eq/m3) at 40% substitution. Conversely, BC increases cost and energy demand but reduces carbon emissions (619.8 kg CO2-eq/m3 at 4% replacement) under the adopted carbon sequestration assumption. The optimization model successfully identifies Pareto-optimal mixtures that balance the trade-offs between superior mechanical performance, cost-effectiveness, and reduced environmental impact, providing viable strategies for developing sustainable construction materials through the separate utilization of industrial and agricultural waste streams.