Synergistic activation of coal bottom ash in a ternary GGBS– desulphurisation gypsum binder system: Microstructural evolution and phase assembly in high-water-content matrices

The development of low-carbon alternatives to Portland cement is critical for sustainable construction, particularly for applications involving difficult high-water-content waste soils. This study investigates a ternary alkali-activated binder comprising Coal Bottom Ash (CBA), Ground Granulated Blast Furnace Slag (GGBS), and Flue Gas Desulphurisation Gypsum (DG) for the stabilisation of high-water-content excavated soil. While CBA is chemically similar to fly ash, its lower reactivity and coarser morphology have historically limited its application as a primary precursor. This research addresses these limitations through mechanical grinding, alkaline activation (NaOH + Na₂SiO₃), and sulphate activation via DG. The binder was evaluated within a siliceous, high-water-content matrix (80% moisture) to simulate a difficult dispersed system. Results indicate that while binary CBA–GGBS binders achieve moderate strength, the ternary CBA–GGBS–DG system performs better in the dispersed matrix, with DG promoting AFt formation. Together with low-Ca aluminosilicate and Ca-bearing gel products, AFt provides a combined mechanism of particle bonding and pore filling. The study identifies an optimal activator dosage of 30%, yielding 28-day UCS values of 4.13 MPa (30% precursor) and 6.66 MPa (40% precursor). XRD and SEM/EDS support the coexistence of aluminosilicate- and calcium-bearing reaction products and show that excessive alkalinity (>50%) leads to microstructural degradation. These findings indicate that CBA–GGBS–DG ternary systems have potential as low-carbon binder systems for controlled low-strength fill materials, while long-term durability and environmental safety remain to be verified. • Ternary CBA-GGBS-DG binder activates low-reactivity coal bottom ash. • DG-driven ettringite fills pores in wet soil but damages dense paste. • Optimal 30% activator dosage yields 6.66 MPa UCS at 28 days. • Hybrid N-A-S-H and C-(A)-S-H gels confirmed by SEM-EDS analysis. • Clinker-free binder system offers qualitative carbon advantage vs OPC.