Compressive Strength and Microstructure of All-Solid-Waste Concretes Modified with Basalt Fiber or Biochar: A Comparative Study of Carbide Slag-Based and Steel Slag-Based Systems

To support the national dual-carbon strategy, this study developed low-carbon solid-waste-based concretes, in which carbide slag or steel slag, together with ground granulated blast-furnace slag and desulfurization gypsum, fully replaced ordinary Portland cement, and compared the mechanical properties and microstructures of the two systems. The carbide slag binder was modified with chopped basalt fiber (0–0.30% by mass) for added toughness, with a 1% or 3% biochar as a basal component, whereas the steel slag binder was modified with corn-stalk biochar (1–7% by mass) as a partial fine-aggregate replacement for carbon sequestration. Compressive strength tests, together with XRD, SEM, TG/DTG and MIP, characterized both concretes. When tested at 7 days, the carbide slag concrete showed a first-rising-then-declining strength response to basalt fiber, strength increased from 3.24 MPa at 0.10% fiber to a maximum of 4.91 MPa at 0.25%, then dropped to 4.68 MPa at 0.30%, while the no-fiber baseline reached 3.17 MPa. The steel slag concrete was markedly stronger, the 1%-biochar fiber-free mix reaching 11.0 MPa at 7 days, larger than the 3.17 MPa recorded for the carbide slag binder under the matched condition, and climbing to 38.7 MPa after 28 days. Increasing biochar dosage penalized strength, the 3% mix reaching only 8.5 MPa at 7 days and the 5% and 7% mixes being too weak for valid testing; with further curing, however, all four steel slag mixes developed measurable strengths by 28 days, at 38.7, 32.6, 22.5 and 18.2 MPa for 1%, 3%, 5% and 7% biochar, respectively. Microstructurally, the steel slag system hydrated more fully, Ca(OH)2 being largely consumed to form C-S-H and calcite (CaCO3); TG/DTG gave a calcium carbonate content of about 24.8%, higher than the 16.6% measured for the carbide slag binder, and MIP revealed a markedly refined pore network, with the dominant pore size shifting from about 40 nm to about 10 nm and the sub-10 nm pore fraction rising from 2.2% to 4.0%, even though its total porosity was slightly higher, at 27.7% compared with 24.9% in the carbide slag binder. Both solid-waste concretes delivered adequate mechanical performance, with the steel slag system outperforming; basalt fiber and biochar acted as auxiliary modifiers that improved toughness and sequestered carbon, respectively, and their dosages, therefore, require careful control.