Strength formation mechanism and environmental impact assessment of low-carbon sustainable mortar based on multi-source solid waste

The stockpiling of steel slag (SS), ground granulated blast furnace slag (GGBS), fly ash (FA), and graphite tailings (GTS) causes environmental pollution, while the scarcity of river sand and high carbon emissions of cement hinder sustainable construction. A new type of building mortar (NTBM) was developed using GGBS, SS, FA, and GTS. The novelty of this study is threefold: (1) elucidation of the synergistic reaction mechanisms among the three binders; (2) quantitative characterization of pore structure evolution via fractal dimension analysis based on the Neimark thermodynamic model; and (3) comprehensive environmental assessment integrating cost analysis, heavy metal immobilization, and carbon footprint quantification. The optimal mix ratio was determined by response surface design, and the strength formation mechanism was revealed by multiple microscopic testing methods. The results showed that NTBM achieved optimal performance at an aggregate-binder ratio of 1.03, mass concentration of 79.3%, and alkali dosage of 4.48%. GGBS, SS, and FA exhibited significant reaction superposition effects, and their combination promoted reaction completeness. With increasing curing time, the fractal dimension of pore regions II and IV increased, and gel pores in the calcium silicate hydrate (C-S-H) structure grew, densifying the matrix and enhancing strength. The main hydration products were calcium aluminum silicate hydrate (C-A-S-H) and C-S-H gels, which inhibited shrinkage-induced cracking. Furthermore, NTBM effectively immobilized heavy metals and reduced costs by 23.5% and CO 2 emissions by 50.6% compared with ordinary Portland cement mortar. These findings demonstrate that NTBM is a promising low-carbon sustainable building material for large-scale applications.