Phosphogypsum-ground granulated blast-furnace slag-Fly ash system for sustainable green grouting materials:...

Industrial solid wastes provide an environmentally friendly alternative to Portland cement. This work presents the development of a green, fully solid-waste grouting material (PGGM) composed of phosphogypsum (PG), ground granulated blast-furnace slag (GGBFS) and fly ash (FA). Multi-objective RSM-BBD was used to optimise the mix, with regression models revealing how component ratios govern property development. Multi-scale characterization (XRD, FTIR, SEM-EDS) tracked the crystalline phase assemblage and microstructural development of PGGM. Optimum PGGM was injected into porous media under both saturated and unsaturated states to create solidified specimens and grouting models, enabling assessment of engineering performance, diffusion kinetics and filling efficiency. Life-cycle environmental impacts and economic viability of PGGM were subsequently appraised. Experimental results revealed that PG, GGBFS and FA reacted in an alkaline environment to generate zeolite-like phases and C-S-H gel, producing a dense three-dimensional network. The optimised mix proportion of PGGM was 51.75% PG (wt% of binder solids: PG + GGBFS + FA), 4.17% alkaline activator (wt% of binder solids) and a water-to-binder ratio of 0.34. These proportions delivered 22.8 MPa compressive strength at 28 d, 135 min initial set and 266 mm flow, satisfying GB 50448-2015 specifications. Solidified matrices exhibited 6.6- and 7.5-fold strength gains under unsaturated and saturated states, with permeability reduced to 0.75 × 10^-7 cm/s. Transparent models demonstrate stabilization within 30 min and water-blocking exceeding ninety-five percent. Life-cycle assessment indicates 84.9% lower carbon emissions and 62.5% material cost savings compared with cement grout, while consuming 1620 tons of waste per project. The findings validate large-scale valorization of solid wastes for low-carbon geotechnical applications.