Red-bed mudstone from civil excavation is often treated as waste due to its poor water stability and tendency to disintegrate. This study proposes a sustainable approach for its utilization in controlled low-strength material (CLSM) by blending it with cement and water. Laboratory tests evaluated the fresh properties (i.e., flowability, bleeding rate, setting time, and subsidence rate) and hardened properties (i.e., compressive strength, drying shrinkage, and wet-dry durability) of the CLSM. The analysis focused on two main parameters: cement-to-soil ratio (C/S) and water-to-solid ratio (W/S). The results show that increasing W/S significantly improves flowability, while increasing C/S also contributes positively. Flowability decreased exponentially over time, with an approximately 30% loss recorded after 3 h. Bleeding and subsidence rates rose sharply with higher W/S but were only marginally affected by C/S. To meet performance requirements, W/S should be kept below 52%. In addition, the setting times remained within 24 h for all mixtures tested. Compressive strength showed a negative correlation with W/S and a positive correlation with C/S. When C/S ranged from 8% to 16% and W/S from 44% to 56%, the compressive strengths ranged from 0.3 MPa to 1.22 MPa, meeting typical backfilling needs. Drying shrinkage was correlated positively with water loss, and it decreased with greater C/S. Notably, cement's addition significantly enhanced water stability. At a C/S of 12%, the specimens remained intact after 13 wet-dry cycles, retaining over 80% of their initial strength. Based on these findings, predictive models for strength and flowability were developed, and a mix design procedure was proposed. This resulted in two optimized proportions suitable for confined backfilling. This study provides a scientific basis for the resource-oriented reuse of red-bed mudstone in civil engineering projects.

Civil excavation projects frequently produce significant amounts of excess spoil. Repurposing this spoil into usable backfill material instead of disposing of it offers economic and environmental benefits. This study explores the prospect of converting red-bed mudstone construction waste, a type of soil frequently found at shallow depths, into a ready-mixed soil material (RMSM). It assesses the fresh mixture's workability characteristics (initial flowability, bleeding rate, and density) and the hardened material's mechanical properties (compressive strength and stress-strain relationship) by adjusting the water-to-solid ratio (W/S) and cement-to-soil ratio (C/S). The study investigates the impact of W/S, C/S and time on RMSM's flowability loss and proposes an empirical formula to provide a scientific reference for RMSM's flowability design in engineering applications. Findings highlight the significant influence of W/S on flowability, bleeding rate, and compressive strength, while showing C/S has a limited effect on flowability and bleeding. A negative exponential relationship is observed between flowability and time for all mixes, with the flowability loss ratio increasing over time, ranging from 22.9% to 35.6% after 1 h and stabilizing after 3 h. These insights are crucial to optimize RMSM's performance and suggest the need to further improve the flowability retention of RMSM. Furthermore, in comparison to soil cement and concrete, RMSM reduces backfill costs by 30.8% and 80.0%, respectively, while also achieving a reduction in CO2 emissions by 25.9% and 69.2%. Therefore, RMSM presents as an economically and environmentally friendly alternative for backfill applications.

In this study, the axial swelling strain of red-bed mudstone under different vertical stresses are measured by swell-under-load method, and the microstructure of mudstone after hygroscopic swelling is studied by mercury intrusion porosimetry (MIP). The weakening coefficient and Weibull distribution function are introduced into the coupling model of mudstone moisture diffusion-swelling deformation-fracture based on finite-discrete element method (FDEM). The weakening effect of moisture on mudstone's mechanical parameters, as well as the heterogeneity of swelling deformation and stress distribution, is considered. The microcrack behavior and energy evolution of mudstone during hygroscopic swelling deformation under different vertical stresses are studied. The results show that the axial swelling strain of mudstone decreases with increase of the vertical stress. At low vertical stresses, moisture absorption in mudstone leads to formation of cracks caused by hydration-induced expansion. Under high vertical stresses, a muddy sealing zone forms on the mudstone surface, preventing further water infiltration. The simulation results of mudstone swelling deformation also demonstrate that it involves both swelling of the mudstone matrix and swelling caused by crack expansion. Notably, crack expansion plays a dominant role in mudstone swelling. With increasing vertical stress, the cracks in mudstone change from tensile cracks to shear cracks, resulting in a significant reduction in the total number of cracks. While the evolution of mudstone kinetic energy shows similarities under different vertical stresses, the evolution of strain energy varies significantly due to the presence of different types of cracks in the mudstone. The findings provide a theoretical basis for understanding the hygroscopic swelling deformation mechanism of red-bed mudstone at various depths. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).