Mechanical properties and micro-mechanisms of tunnel spoil roadbed improved by calcium carbide slag and phosphogypsum

To solve the problems of land resource waste and potential environmental threats caused by the stockpiling of abandoned tunnel spoils and industrial solid wastes, an attempt was made to synergistically improve abandoned tunnel spoils using industrial solid wastes, namely calcium carbide slag and phosphogypsum, to transform them into high-quality engineering fillers. A series of laboratory tests were conducted to investigate the influence patterns of different solid waste ratios on the physical and mechanical properties of the improved soil, and the optimal ratio was selected. Microscopic tests were performed to analyze the microscopic improvement mechanisms and action mechanisms. Road mechanical property tests were carried out to verify the feasibility of using the improved soil as roadbed fillers. The results indicate that the combined addition of calcium carbide slag and phosphogypsum exhibits a significant synergistic effect, especially in enhancing the mechanical properties of abandoned spoils, with an effect far exceeding that of adding calcium carbide slag alone (taking a total mass fraction of 4% as an example, the average compressive strength of the C3P1 improved soil is 1.35 times that of the soil with calcium carbide slag alone). The optimal ratio of calcium carbide slag to phosphogypsum is 3∶1, under which the improvement effect on abandoned spoils is the most significant, and both the mechanical properties and stability of the improved soil reach the optimal state. Hydration reactions and ion exchange reactions occur among calcium carbide slag, phosphogypsum, and the active components in the soil, generating cementitious products such as calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), and ettringite (AFt). These products significantly increase the compactness of the soil through filling, bonding, and adsorbing soil particles, thereby enhancing the macroscopic mechanical properties of the improved soil. Attributed to the double-edged nature of the expansion effect of the product ettringite and the combined action of other cementitious products, under the optimal ratio (3∶‍1), the mechanical properties of the improved soil do not show a linear growth trend with the increase of the total additive content. The improved soil with an appropriate proportion of calcium carbide slag and phosphogypsum meets the requirements of highway subgrade design specifications, preliminarily verifying the feasibility of using the improved soil as roadbed fillers.