Abstract: Alkali residue is a byproduct of the ammonia-soda process used to produce soda, characterized by high production volume, low utilization efficiency, high moisture content, high porosity, and fine particle size. The primary disposal methods for alkali residue include surface stacking (e.g., constructing tailing dams) and direct discharge into water bodies such as rivers or seas. The hazards associated with surface stacking include land resource occupation, reduced agricultural yield, groundwater and air contamination, soil pollution, adverse effects on vegetation growth, ecological imbalance, and the formation of saline-alkali land. Additionally, the discharge of alkali residue into rivers or seas can lead to water pollution, threatening the sustainability of aquatic ecosystems. Sedimentation may also occur, potentially blocking river channels, reducing flow cross-sections, and significantly impairing the river’s flood discharge capacity. These challenges have resulted in a large-scale accumulation of alkali residue, severely constraining the development of the soda industry. Therefore, there is an urgent need to accelerate its large-scale application. This study provides a comprehensive review of the latest research findings on the fundamental properties and engineering applications of alkali residue. The results indicate that similar to soil, alkali residue exhibits a three-phase system, where the solid phase, composed of various mineral components, forms a skeletal structure. In contrast, the liquid and gas phases fill the pores, creating a porous medium. The properties of alkali residue can be characterized using soil indicators. Its mineral composition includes CaCO₃, CaSO₄, CaCl₂, and NaCl, where CaCO₃ and CaSO₄ are insoluble salts, while CaCl₂ and NaCl readily dissolve in water. The CaCO₃ content ranges from 32.52% to 64.00%, while the chemical composition is dominated by CaO, accounting for 32.25% to 74.20%. Alkali residue solutions are slightly alkaline, with pH values typically ranging from 8 to 12. As a general industrial solid waste, alkali residue contains heavy metals such as copper, zinc, cadmium, lead, total chromium, and chromium, all of which meet environmental standards. Alkali residue has been explored for various engineering applications, including the remediation of bioleached heavy metal-laden sediment, sludge, clay, expansive soil, contaminated soil, shield tunneling slag, weathered mudstone, and coal gangue, as well as for backfill materials and the preparation of alkali residue-based soil and composite cementitious materials. However, current application methods suffer from low alkali residue utilization efficiency, limited application scenarios, challenges in Cl^- solidification, potential steel reinforcement corrosion, and risks of secondary pollution. To address these challenges, the author systematically investigates the fundamental properties of alkali residue from Lianyungang and proposes a method for producing alkali residue-based lightweight soil (A-LS) by combining alkali residue, cement, and granulated blast furnace slag (GGBS). A-LS was utilized as a roadbed filler in the Lianyungang—Suqian expressway, demonstrating compressive strength, California bearing ratio (CBR), rebound modulus, and deflection that met design and specification requirements. The material exhibited strong road performance, high resistance to wet-dry cycling, freeze-thaw cycling, and sulfate corrosion, with a durability coefficient ranging from 0.71 to 1.51. Furthermore, A-LS offers several advantages, including high strength, low density, a simple production process, high efficiency, and low cost. With a 28-day compressive strength ranging from 0.96 to 4.27 MPa, A-LS is suitable as a subgrade filling material. The alkali residue content in A-LS ranges from 87.01 to 164.35 kg·m^?3, facilitating large-scale disposal and high-value utilization of alkali residue.