Optimization of mix design for controlled low-strength material produced from red-bed mudstone

Converting red-bed mudstone waste into controlled low-strength material (CLSM) provides a viable path for waste valorization. However, conventional mixes often require high water content to achieve sufficient flowability, compromising the hardened properties. To meet strength demands, cement content is usually increased, reducing environmental and economic benefits. Polycarboxylate superplasticizer (PCE) was incorporated alongside red-bed mudstone, cement, and water to reduce water and cement usage while maintaining or improving performance. A regression-based orthogonal experiment was conducted to quantitatively assess the effects of cement-to-soil ratio (C), water-to-solid ratio (W), and polycarboxylate superplasticizer content (P) on CLSM performance, based on which the mix design was optimized. Results indicate that C significantly affects compressive strength, setting time, and water stability; W primarily influences flowability, bleeding, subsidence, and drying shrinkage; while P significantly impacts all properties except strength. In optimized mixes, 1 kg of PCE reduced water and cement usage by approximately 27 kg and 35 kg. For PCE-incorporated specimens, at flowability levels of 180 mm and 210 mm, setting time increased by 12.5 % and 17.9 %, bleeding rate dropped by 75 % and 74.29 %, subsidence rate fell by 66.67 % and 80 %, drying shrinkage rate decreased by 1.53 % and 3.73 %, and residual strength after wet-dry cycles improved by 9.29 % and 6.17 %. When W required to meet flowability exceeds 51 % without PCE, bleeding often exceeds the 5 % threshold. In such cases, PCE is recommended to reduce W and maintain bleeding within acceptable limits. These findings provide practical guidance for waste soil utilization and CLSM optimization, promoting sustainability and improving environmental and economic performance.