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.

In order to reduce heat loss and diffusion of underground heating pipelines, this research incorporated phase change material (PCM) into the controlled low-strength material (CLSM) to prepare a pipeline backfill material with temperature control performance. In response to the problem that PCM leaks easily, a new type of paraffin-rice husk ash composite PCM (PR-PCM) was obtained by adsorbing melted paraffin into rice husk ash. Through mixing PR-PCM with dredged sediment (DS) and ordinary Portland cement (OPC), a controlled low-strength material (CLSM) with temperature control performance was prepared. The flowability, mechanical properties, microscopic characteristics, thermal characteristics, and durability of CLSM were analyzed through flowability, unconfined compressive strength (UCS), X-ray diffraction (XRD), scanning electronic microscopy (SEM), differential scanning calorimetry (DSC), and phase change cycle tests. The results show that when water consumption is constant, as the PR-PCM content increases, the flowability of CLSM increases, and the strength decreases. The CLSM has an obvious paraffin diffraction peak in the XRD pattern, and its microstructure is dense with few pores. The melting point of CLSM is 50.65 degrees C and the latent heat is 4.10 J/g. Compared with CLSM without PR-PCM, the maximum temperature difference during the heating process can reach 3.40 degrees C, and the heat storage performance is improved by 4.1%. The strength of CLSM increases and the melting point decreases after phase change cycles. CLSM containing PR-PCM has the characteristics of phase change temperature control, which plays a positive role in reducing heat loss by heating pipelines and temperature change in backfill areas.