Abstract: To reduce the volume of pile-foundation waste slurry and promote the high-value utilization of industrial solid wastes, this study developed an alkali-activated geopolymer solidification system based on carbide slag (CS), ground granulated blast-furnace slag (GGBS), and fly ash (FA), activated with sodium silicate, to produce a controllable low-strength flowable solidified material suitable for pile-hole and pipeline trench backfilling. Based on simplex centroid design, the synergistic effects of solid waste proportions and sodium silicate dosages on flowability, unconfined compressive strength (UCS), and microstructure are systematically investigated. The solidification mechanism is revealed through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), while carbon emissions and economic costs are also evaluated. Results show that the proposed system effectively solidifies pile foundation waste slurry with an initial water content of 140%. The prepared controlled low-strength material (CLSM) exhibits slump values generally exceeding 200 mm, reaching a maximum of 262 mm, satisfying the flowability requirements for self-compacting backfill materials. The 28-day UCS demonstrates significant nonlinear responses to component variations, with a peak value of 2,970.5 kPa. An optimal sodium silicate dosage of 15% is identified, which maximizes the activation of the ternary solid waste system and achieves the peak strength. When the dosage increases to 20% and 25%, system stability improves but the strength ceiling decreases, indicating that excessive alkalinity induces rapid early-age reactions and microstructural defects. Component synergy plays a decisive role in performance development. GGBS, rich in reactive calcium-rich glassy phases, rapidly dissolves under alkaline conditions to form abundant C-(A)-S-H gels, serving as the primary strength-contributing component. CS functions as both an alkaline activator and calcium source: appropriate dosages (5%–10%) elevate pore solution pH and supply Ca2? ions, promoting precursor dissolution and accelerating gel nucleation, while excessive dosages dilute reactive precursors and lead to residual plate-like Ca(OH)2 crystals that form weak interfacial zones. FA primarily supplies aluminosilicate components but exhibits relatively low reactivity; increasing its content dilutes the gel network without proportionally contributing to strength gain. Microstructural analysis reveals a progressive evolution from the original loose quartz-dominated skeleton of raw slurry to a dense composite matrix dominated by C-(A)-S-H gels. High slag content combined with optimal sodium silicate dosage favors continuous gel formation and enhanced structural densification, whereas high FA or excessive CS content leads to the accumulation of unreacted particles and crystalline phases with increased porosity, consistent with the observed "first increase then decrease" strength development pattern. Compared to conventional Portland cement (OPC) solidification, the optimal formulation (CS: FA: GGBS = 5%:5%:90% with 15% sodium silicate) reduces carbon emission per unit strength by 87.9% and material cost per unit strength by 74.1%, demonstrating substantial environmental and economic advantages.