Improving Engineering Performance of Controlled Low-Strength Materials Produced from Recycled Red-Bed Mudstone by Incorporating Polycarboxylate Superplasticizer

Abstract The preparation of controlled low-strength material (CLSM) with waste red-bed mudstone provides an effective approach to utilize solid waste and improve backfilling quality. However, achieving sufficient flowability often requires high water content, which reduces volume stability, weakens strength, deteriorates durability, and increases cement consumption. In this study, polycarboxylate superplasticizer (PCE) was introduced to reduce water demand, and its effects on the fresh and hardened properties of CLSM were systematically evaluated. CLSM was prepared using red-bed mudstone, cement, water, and PCE, and tests were conducted on flowability, bleeding rate, setting time, subsidence rate, compressive strength, drying shrinkage rate, and wet–dry cycles. The results showed that the improvement of flowability by PCE was significantly influenced by the water-to-solid ratio, with the effect being optimal when the ratio was 43%–55%. At the same flowability (165–340 mm), 0.2% PCE reduced water demand by 6.69%–10.17%, decreased bleeding by 2.0%–5.1%, extended setting time by 0.5–1.5 h, and lowered subsidence by 1.0%–4.0%. In the hardened stage, 0.2% PCE increased the 7–90 day compressive strength by 0.112–0.647 MPa, and the residual strength after 13 wet–dry cycles increased by about 10%–38%. Although drying shrinkage rose slightly in the first 10 days, it decreased by 0.11%–0.14% at 56 days. Overall, incorporating PCE reduced the water demand of CLSM, improved its stability and mechanical performance, and provided a practical reference for the engineering application of red-bed mudstone CLSM.