Durability and erosion mechanism of biomass-coal mixed combustion ash geopolymer backfill under compound erosion

To explore the durability and erosion mechanism of biomass-coal mixed combustion ash geopolymer backfill (BCCGB) under compound salt erosion, the appearance, mass loss, unconfined compressive strength (UCS), microscopic morphology and erosion products of BCCGB under compound salt erosion were studied by means of dry and wet (WD) cycle tests, microscopic tests and molecular dynamics, and the erosion mechanism of SO42-, Cl- and Mg2+ was revealed at the ionic scale. The results showed that after 30 WD cycles, BCCGB remained solid, no large amount of aggregate was exposed, and produced Na2SO4·10H2O crystals. The mass loss rate was 0.93 %, and the UCS decreased by 33.2 %. The erosion products were CaSO4·2H2O (gypsum), ettringite (AFt), calcium chloro-aluminate hydrate (Friedel's salt) and magnesium silicate hydrate (M-S-H). SO42-, Cl- and Mg2+ bind or replace Ca2+ in calcium silicate hydrate (C-S-H), Cl- erosion was mainly aimed at free calcium, SO42- and Mg2+ erosion were mainly aimed at structural calcium. In this process, changes in Coulomb forces and van der Waals forces occur, which is an endothermic and exothermic energy dynamic equilibrium process, and the presence of Cl- inhibits the diffusion of SO42- and Mg2+, and the order of the diffusion coefficient was Cl- >SO42- >Mg2+. Stability of erosion products: Friedel salt > Gypsum and AFt > M-S-H. [Display omitted] •The mechanism of compound salt erosion is revealed from nanometric scale.•The interaction between ions in the process of compound erosion is analyzed.•Cl- erosion targeted free calcium, SO42- and Mg2+ erosion targeted structural calcium.•It is found that compound erosion is an energy dynamic equilibrium process.