Low-temperature modified phosphogypsum-based self-leveling mortars: multi-component mix design to frost resistance and life cycle assessment

Phosphogypsum-based self-leveling mortar (PGSM) offers notable advantages over traditional cement-based self-leveling mortars due to its superior thermal insulation and low shrinkage characteristics. However, its relatively low strength and poor durability have limited its broader application in engineering. This study systematically investigates the effects of different mix proportions and types of retarders on the mechanical properties, microstructure, and durability of PGSM, aiming to optimize the mix design for improved strength and durability.The results show that under the optimal formulation, the mortar achieves flexural and compressive strengths of 7.5 MPa and 13.7 MPa at 24 h, respectively, and reaches 11.6 MPa and 33.3 MPa after 28 days of drying—exceeding the G25-grade requirements specified in JC/T 1023–2021: Gypsum-Based Self-Leveling Mortar. Increased PG content promotes the formation of C–S–H gel and Ettringite (AFt), thereby significantly enhancing the strength. In contrast, quartz sand and calcium carbonate serve only as fillers; excessive amounts reduce strength. The addition of cement does not improve performance linearly. Excessive cement may produce early C–S–H gel that envelops hemihydrate gypsum particles, hindering their hydration and leading to reduced workability and strength. Durability tests indicate that while the G25-grade mortar shows some strength degradation under outdoor exposure, it still meets the frost resistance requirements of the Chinese standard for Test Methods of Basic Properties of Building Mortar (JGJ/T 70–2009). Furthermore, SimaPro-based life cycle assessment suggests a relatively low environmental burden of PSLM. However, further evaluation of potential health impacts during the resource utilization process is needed. This research provides a theoretical foundation for the performance optimization and practical engineering application of PGSM.