Synergistic valorization of agricultural and industrial solid wastes in a cement-free binder: Strength development, microstructural refinement, and carbon–cost trade-offs

This study develops a cement-free cementitious binder derived entirely from solid wastes, integrating combining carbide slag (CS), rice straw ash (RSA), phosphogypsum (PG), and ground granulated blast furnace slag (GGBS). In this quaternary system, CS supplies alkalinity, PG liberates sulfate ions, RSA contributes reactive silica, and GGBS acts as the primary aluminosilicate precursor. Fifteen mixture proportions were systematically designed to evaluate the influence of CS (1–5%), RSA (10–25%), and PG (0–15%) on workability, mechanical strength, hydration chemistry, microstructural densification, pore characteristics, and environmental-economic performance, with GGBS content adjusted to maintain a constant binder mass. At 28 days, compressive strength ranged from 15.70 to 42.13 MPa, while flow diameter varied between 160 and 220 mm, with the optimal formulation (SA2: 1% CS, 15% RSA, 10% PG, 74% GGBS) achieving the highest strength of 42.13 MPa and a flow diameter of 185 mm. The superior performance of SA2 was attributed to a balanced interaction among the four constituents: CS supplied alkalinity to promote GGBS dissolution, PG provided sulfate ions to facilitate AFt formation, RSA contributed reactive silica to enhance C-(A)-S-H gel formation and polymerization, and GGBS served as the primary aluminosilicate precursor. This balanced combination favored the concurrent formation of AFt and C-(A)-S-H gel, contributing to matrix densification and pore refinement. Multi-technique characterization—including XRD, FTIR, and TG–DTG—revealed that strength development across the mixtures was governed by the combined effects of alkali activation, sulfate activation, and pozzolanic reactions, whereas excessive dosage of any single component disrupted this balance and impaired hydration product formation. In the optimal mixture SA2, this combined activation was most effectively achieved, as evidenced by the coexistence of well-crystallized ettringite and a highly polymerized C-(A)-S-H gel, together with a pronounced blue shift in the Si–O–T band indicating an enhanced degree of silicate polymerization. SEM–EDS further revealed that SA2 developed a dense "crystal skeleton–gel filling" microstructure with Ca/Si ratios between 0.80 and 1.65, while LF NMR confirmed that SA2 possessed the finest and most uniform pore architecture, with small capillary pores occupying 50.90% and macropores only 2.02% of total pore area—collectively accounting for its superior mechanical performance. Importantly, all mixtures reduced CO₂ emissions by over 90% relative to ordinary Portland cement. SA2 attained a carbon intensity index of 1.65 kg CO₂/(t·MPa) and an economic efficiency index of 6.19 RMB/(t·MPa), demonstrating a favorable trade-off among mechanical performance, environmental footprint, and production cost. The clinker-free route and potential applications in precast components, road base layers, and non-structural or semi-structural elements further support the scale-up potential of the proposed binder.