Abstract: Embankment structures are continuously subjected to complex environmental stressors, including hydraulic pressure, freeze–thaw cycles, and differential settlement, which frequently lead to cracking and seepage. These defects compromise structural integrity and pose significant safety risks. Conventional repair materials such as cement–soil often exhibit shortcomings in such scenarios, including slow strength development, susceptibility to shrinkage cracking, limited bonding with existing substrates, and a relatively high carbon footprint. To address these limitations, this research investigates an innovative and sustainable alternative: a solidified soil composite using a geopolymer binder system incorporating recycled glass fiber reinforced polymer (GFRP) powder and slag. This approach aims not only to enhance engineering performance but also to promote the beneficial utilization of composite waste. The study systematically designed and implemented an orthogonal experimental array to evaluate the effects of four key mix design parameters on the principal properties of the solidified soil. The investigated factors included: (1) the content of recycled GFRP powder (a waste-derived filler and potential reactive component), (2) the total content of the geopolymer binder (composed of GFRP powder and slag), (3) the concentration of the sodium hydroxide–based alkaline activator (expressed in molarity, mol/L), and (4) the liquid-to-solid ratio. Performance was assessed primarily through unconfined compressive strength (UCS) tests, reflecting mechanical resistance, and permeability coefficient tests, evaluating impermeability—a critical property for seepage control. To elucidate the underlying mechanisms, microstructural characterization was conducted using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), analyzing morphological features, reaction products, and elemental distribution. Statistical analysis of the orthogonal experiment results identified the geopolymer binder content as the most significant factor influencing both UCS and the permeability coefficient. The synergy between GFRP powder and slag within the geopolymer system proved critical. An optimal mix proportion was determined: 20% GFRP powder content, 30% total geopolymer binder content, an alkaline activator concentration of 10.2 mol/L, and a liquid-to-solid ratio of 1.4. This formulation achieved a favorable balance among workability, reaction kinetics, and final microstructure, resulting in enhanced strength and reduced permeability. Microstructural investigation provided clear evidence of the chemical processes responsible for performance improvement. Under strong alkaline activation, calcium ions (Ca²⁺) from slag and sodium ions (Na⁺) from the activator promoted the dissolution of silicon (Si) and aluminum (Al) species from both slag and clay particles. Subsequent polycondensation reactions led to the formation of a dense, interwoven matrix of cementitious gels, primarily N-A-S-H (sodium aluminosilicate hydrate) and C-A-S-H (calcium aluminosilicate hydrate). These gels effectively coated soil particles, filled interparticle voids, and bridged microcracks, producing a significantly densified and more uniform structure. This microstructural refinement directly corresponded to macroscopic gains in strength and impermeability. For practical validation, the optimal mixture was applied to a simulated embankment repair section using the deep mixing method. Core samples were extracted from the treated soil after curing for performance verification. The results were highly encouraging: the developed material exhibited rapid early strength development. After only 7 days of curing, the measured UCS and permeability coefficient met—and in some cases exceeded—the typical design requirements for conventional cement–soil at 28 days. This rapid performance development represents a major advantage for emergency repairs and time-critical reinforcement projects, reducing downtime and disruption. In summary, this study successfully developed and validated a high-performance, environmentally friendly material for embankment maintenance. The recycled GFRP powder–geopolymer solidified soil demonstrated strong potential for rapid seepage repair and structural reinforcement. Its key attributes—industrial waste utilization, rapid strength gain, low permeability, and compatibility with common construction techniques such as deep mixing—position it as a sustainable alternative to traditional cement-based grouts and solidified soils for urgent infrastructure rehabilitation. Future work should focus on long-term durability under diverse environmental conditions and further optimization for large-scale applications.