Introduction Application of silica nanoparticles in concrete has attracted much attention. Its unique filling characteristics and high volcanic ash characteristics can improve the pore structure of concrete and promote hydration reaction, having a positive effect on the mechanical properties of concrete. After concrete is subjected at a high temperature, the pore pressure inside the structure, the deterioration of the transition zone of the slurry aggregate interface and the change of phase are all reasons for the decline of the mechanical properties of concrete at a high temperature. Silica nanoparticles have a filling effect, increasing the compactness of the material, while silica nanoparticles can undergo a volcanic ash reaction to generate more hydrated calcium silicate (C-S-H) gel, improving the high-temperature resistance of concrete. The inside of alkali slag mortar is alkaline environment and its hydration reaction is completely different from concrete, and the influence of silica nanoparticles on the high-temperature resistance of alkali slag cementitious material is different from that of ordinary concrete. Therefore, silica nanoparticles can improve the high-temperature mechanical properties of cement-based materials, but the influence of silica nanoparticles on the high-temperature resistance of seawater sea sand alkali slag mortar is more complicated. This paper was to investigate the effect of silica nanoparticles on the high-temperature resistance of seawater sea sand alkali slag mortar.Methods Simulated seawater and sea sand with a fineness modulus of 2.3 were used as mixing water and fine aggregates. Liquid sodium silicate and solid sodium hydroxide were formulated as alkali initiators in a molar ratio of 1.7 for SiO2 and Na2O. Silica nanoparticles with the sizes of 20-50 nm and a specific surface area of 670 m2/g as an admixture were supplied by Taihong Shengda New Materials Co., Ltd., China. Four sets of alkali slag mortar with different concentrations of silica nanoparticles (i.e., 0%, 1%, 2% and 3%) were produced. After mold removal, it was placed in a curing room at (20±2) ℃ and a relative humidity of > 90% for 27 d, and then dried naturally for 1 d. The changes of mass and length in specimens were recorded by scales and vernier calipers before and after high-temperature heating. The flexural and compressive strengths were determined on seawater sea sand alkali slag mortar specimens (40 mm×40 mm×160 mm) at different high temperatures (i.e., 200, 400, 600 ℃ and 800 ℃).Results and discussion At room temperature, silica nanoparticles have little effect on the compressive strength of seawater sea sand alkali slag mortar, and the compressive strength of the specimen is only increased by 1.5% and 2.3% at the concentrations of silica nanoparticles of 2% and 3%, respectively. Also, silica nanoparticles can reduce the pH value of alkali initiator, which is unstable in salt solutions where Mg2+ is > 0.1% (in mass fraction). Monomeric silicon ([SiOn(OH)4-n]n-) is formed when silica nanoparticles are added into the base initiator, improving the SiO2/Na2O ratio of the solution. At 200 ℃, compared with ordinary concrete, the addition of silica nanoparticles can increase the compressive strength of seawater sea sand alkali slag mortar. The compressive strength of the specimen with silica nanoparticles content of 1%-3% is 103%-123% of the normal temperature state. This may be related to the superior dispersion of silica nanoparticles in alkaline environments. OH- can convert the silicon hydroxyl group on the surface of silica nanoparticles to Si-O- groups, so that the surface of silica nanoparticle generates a negative charge and increases the dispersion. The deformation results at a high temperature show that the residual deformation of the specimen can be effectively alleviated when the silica nanoparticles content is 3%. Combined with the analysis of the results of mass loss, the addition of silica nanoparticles can reduce the content of evaporable water in the specimen. This decreases the pressure generated by the evaporation of partially pore water, thereby reducing the partial crack generation.Conclusions The addition of silica nanoparticles improved the compressive strength of seawater sea sand alkali slag mortar at 200 ℃. This could be related to the dispersion of silica nanoparticles in alkaline environments rather than in neutral environments. Silica nanoparticles could reduce the evaporable water content inside seawater sea sand alkali slag mortar. At the silica nanoparticles content of 3%, the deformation and cracks of the specimen at a high temperature were effectively inhibited.