A novel mineralogy-controlled non-caustic phosphate geopolymer for sustainable stabilisation of collapsible loess using serpentine mine waste and lead–zinc tailings

This study presents a novel non-caustic magnesium–phosphate geopolymer for stabilising collapsible loess using serpentine mine waste (SMW), lead–zinc tailings (LZW), and waste diatomite (WD)activated by a monoammonium phosphate solution (APA, NH4H2PO4). The main objective was to develop a mineralogy-controlled stabilisation system and evaluate its hydro-mechanical and microstructural performance. Optimisation results identified 3% WD and 40% APA as the optimum binder configuration, while 15% SMW and 15% LZW were selected as optimal precursor dosages. The hybrid formulation (50SMW–50LZW) exhibited the highest unconfined compressive strength (UCS) of 2.22 MPa at 90 days, significantly outperforming single-precursor systems. IcCollapseIndex and wetting-induced settlement were reduced by up to 82% and 89.7%, respectively, compared to untreated loess (Ic( collapse index) = 8.9%). Soil–water retention curves showed increased air-entry value and improved water retention in hybrid specimens. Durability assessment indicated low mass loss (<4.8%) and high strength retention (>80%) after 10 wet–dry cycles. Microstructural analyses (SEM, XRD, FTIR, MIP, and EDS) confirmed progressive pore refinement and formation of Mg–P and Ca–Fe–P gels. The results demonstrate a clear mineralogy-controlled geopolymerization mechanism governing strength development and long-term stability. The proposed system offers a sustainable alternative for large-scale stabilisation of collapsible soils.