Sustainable seawater-sea sand cement mortar with municipal solid waste incineration ash: Mechanical performance, microstructure, and environmental benefits

• Ecological seawater-sea sand cement mortar was developed. • Fluidity, mechanical properties and durability of ecological mortar were studied. • Hydration process and microstructural evolution of ecological mortar were revealed. • EM can effectively solidify heavy metals in municipal solid waste incineration ash. • 1 m³ EM can reduce carbon emission and cost by 8.99% and 32.07%, respectively. Given the increasing scarcity of natural river sand resources and the persistently high carbon emissions from the cement industry. This study proposes a cleaner alternative: ecological seawater-sea sand cement mortar (EM) using seawater, sea sand, municipal solid waste incineration fly ash (IFA), and bottom ash (IBA). Orthogonal experiments were carried out to determine the optimum dosage of IFA and IBA, followed by an evaluation of the flowability, mechanical properties and durability of EM, as well as analysing the hydration kinetics, the microstructural evolution and the ability to solidified heavy metals, and an assessment of the environmental and economic benefits. Results showed that seawater and sea sand reduced EM’s fluidity and exhibited dual mechanical effects: early strength enhancement (3 rd day) due to accelerated hydration but later strength (28 th day) reduction slightly, compressive and flexural strength reduction by 1.16%–6.66% and 11.20%–21.5%, respectively. Drying shrinkage increased by 20.29%–33.74% and the chloride migration coefficient increases by 12.28%–22.84%. Microanalysis showed that Cl⁻ accelerated early hydration but increased porosity and worsened pore structure. Despite this, EM solidified heavy metals with excellent results, with Cu and Zn leaching concentrations well below regulatory limits. The material sustainability index showed that 1 m³ of EM reduced energy consumption by 7.83%, carbon emissions by 8.99%, and costs by 32.07%, highlighting its ecological and economic benefits. This study provides an economic and green alternative building material for sustainable construction in resource poor and garbage polluted coastal areas.