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 a constrained simplex-centroid mixture design, the synergistic effects of solid waste proportions and sodium silicate dosages on the flowability, unconfined compressive strength (UCS), and microstructure were systematically investigated. The solidification mechanism was revealed through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy, and the carbon emissions and economic costs of the optimal mixture were evaluated. In the mixture design, the CS and FA contents were varied from 5% to 25%, whereas the GGBS content was automatically determined to be within 70%–90% under the condition that the sum of the three components was unity. Sodium silicate with a modulus of 2.4 was used as the activator, and its dosage was set at 15%, 20%, and 25% of the total binder mass. The results showed that the proposed system effectively solidifies the pile-foundation waste slurry with an initial water content of 140%. The prepared controlled low-strength material exhibited flowability values ranging from 91 to 262 mm, indicating that both the activator dosage and precursor composition strongly affect the fresh-state performance. In most cases, increasing the sodium silicate dosage reduced the flowability because the stronger alkaline environment accelerated the dissolution of reactive phases and the early formation of gel products, thereby increasing the slurry viscosity, although this effect was not monotonic for all the mixtures. The 28-d UCS demonstrated significant nonlinear responses to component variations with a peak value of 2970.5 kPa. An optimal sodium silicate dosage of 15% was identified, under which the formulation of CS∶GGBS∶FA = 5%∶90%∶5% achieved peak strength. When the dosage was increased to 20% and 25%, the system stability improved but the strength ceiling decreased, indicating that excessive alkalinity induced rapid early-age reactions and microstructural defects. Component synergy plays a decisive role in the development of performance. GGBS, which is rich in reactive calcium-rich glassy phases, rapidly dissolves under alkaline conditions to form abundant C–(A)–S–H gels, which serve as primary strength-contributing components. CS functions as both an alkaline activator and a calcium source. Appropriate dosages (5%–10%) increase the pH of the pore solution and supply Ca²⁺ ions, promoting precursor dissolution and accelerating gel nucleation, whereas excessive dosages dilute reactive precursors and lead to residual plate-like Ca(OH)₂ crystals that form weak interfacial zones and hinder later strength development. FA primarily supplies aluminosilicate components but exhibits relatively low reactivity; therefore, increasing its content reduced the effective gel-forming fraction and led to a lower strength. Microstructural analysis revealed a progressive evolution from the original loose quartz-dominated skeleton of the raw slurry to a dense composite matrix dominated by C–(A)–S–H gels. High slag content combined with optimal sodium silicate dosage favored continuous gel formation and enhanced structural densification, whereas high FA or excessive CS content led to the accumulation of unreacted particles and crystalline phases with increased porosity, consistent with the observed “first increase then decrease” strength development pattern. Compared with conventional Portland cement solidification, the optimal formulation (CS∶FA∶GGBS = 5%∶5%∶90% with 15% sodium silicate) reduced carbon emissions per unit strength by 87.9% and material cost per unit strength by 74.1%, demonstrating substantial environmental and economic advantages. Therefore, the proposed CS–GGBS–FA alkali-activated ternary solid waste system provides a promising low-carbon approach for the flowable solidification of high-water-content waste slurry and offers an effective pathway for the coordinated resource utilization of waste slurry and industrial solid waste.