Effects of calcium ions on the alkali–silica reaction of seawater–sea sand concrete

To promote the application of seawater–sea sand concrete (SSC) in marine engineering and investigate the influence of ions in the marine environment on the alkali–silica reaction (ASR) in concrete, the impact of calcium ions on the formation and evolution of ASR products was explored. By incorporating varying amounts of calcium oxide, the characteristics of ASR products (composition and microstructure) in SSC under different calcium/silicon molar ratios (CSRs) were systematically examined, elucidating the influence of the CSR on ASR pathways and product stability. The results showed that, with an increase in the CSR from 0.30 to 0.55, excess Ca2+ ions progressively facilitated the staged transformation of sodium shlykovite into the intermediate phase ASR-P1, followed by further conversion to calcium silicate hydrate gel. Concurrently, the pore solution exhibited significant dilution of Na+ and K+ ions (concentrations reduced to 26.13 mg/l and 7.41 mg/l, respectively), effectively suppressing the formation of deleterious alkali silicate gels. Specimen expansion rates at 14 and 28 days decreased from 0.212% and 0.345% to 0.085% and 0.109% (reductions exceeding 60%), respectively, with 14-day expansion rates remaining below 0.1% at CSR ≥ 0.50. This study proposes a CSR optimisation strategy for ASR mitigation, providing theoretical foundations for the engineering application of SSC in marine environments.