Utilizing recycled waste organic muck and phosphogypsum in controlled low-strength materials: An experimental study on engineering properties and environmental safety

This study develops a novel organic muck (OM)-phosphogypsum (PG)-based Controlled Low-Strength Material (OP-CLSM) to enable large-scale reuse of these two wastes. It also systematically evaluates the feasibility of using OM-PG mixtures as fine aggregates for CLSM preparation. In this formulation, OM and PG are combined with Ordinary Portland cement (OPC), fly ash (FA), and various admixtures, including polycarboxylate superplasticizer (PCE), calcium chloride (CaCl₂), and triethanolamine (TEA). A total of 18 groups of specimens are prepared with varying mix proportions to investigate the effects of the OM: PG mass mixing ratio (M OM : M PG ), the organic matter content ( w u ) of OM, the OPC: FA mixing ratio ( a OPC : a FA ), and the incorporation of different admixtures. Systematic laboratory experiments are conducted to evaluate the fresh-state properties, including flowability, bleeding rate, and setting time, as well as the hardened-state property of unconfined compressive strength (UCS). The mineral composition and microstructural characteristics of OP-CLSM are analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furthermore, the environmental safety of OP-CLSM is assessed by measuring its pH and examining the Leachability of potentially hazardous substances. Results indicate that, with appropriate mix proportions, OP-CLSM achieves high flowability (>150 mm), an acceptable bleeding rate (0.7–7.0 %), a short setting time (<24 h), and a moderate UCS (0.3–1.8 MPa). The pH ranges from 10 to 12, and the leached concentrations of potentially hazardous substances comply with regulatory standards. Among the tested admixtures, PCE demonstrates superior performance in enhancing flowability, reducing bleeding rate and setting time, and improving UCS. With an optimized mix design, OP-CLSM exhibits promising mechanical and environmental performance, providing a sustainable solution for the reuse of waste materials.