Display omitted •Long term micro function of modified-SWSSC at various influential factors was studied.•Gradual improvement of microstructural zonation in tidal exposed SWSSC due to SF,•pH reduction, SF reaction, magnesium sulphate decomposed chloride-containing phases.•MgO and CaO in SCMs reacted with CO2, forming dolomite CaMg(CO3)2, SWSSC strength loss.•C-S-H physical properties increased the chloride binding capacity of MK&SF-mixed SWSSC. In tidal zones, the transfer of conductive ions accelerates concrete deterioration due to dry-wet cycles, temperature, and humidity gradients over time. Thus, this study investigates the effects of ground granulated blast furnace slag (GGBS), silica fume (SF), and metakaolin (MK) on the deterioration rate of seawater SeaSand concrete (SWSSC) exposed to tidal conditions for four years caused by carbonation, sulphate and chloride penetration. Results showed continuous exposure to MgSO4, reduced Fe content and converted AFm into AFt, contributing to an increase in total SWSSC porosity of 12.9% while reduced to 10.2%, 7.4% and 7.3% with the optimal addition of GGBS, SF and MK. In combined sulphate and chloride-rich conditions, enhanced gel phase physical properties were more influential than AFm phases in increasing chloride binding capacity. SF suppressed AFt expansion, formed dense C-S-H with optimal Ca/Si ratio and physically increased the chloride binding capacity of SWSSC. Similarly, despite its high alumina content, MK restricted chloride mobility mainly through C-S-H and, subsequently, AFm phases. Additionally, through its initial acceleration of calcium-hydroxide consumption, SF reduced alkalinity and minimized the formation of CaCO3, outperforming MK in mitigating carbonation. Higher CaO and MgO in GGBS-mixed samples led to dolomite formation in severely carbonated zones, causing long-term mechanical strength reduction. In summary, SF significantly enhanced SWSSC durability and served as an effective additive in tidal-exposed SWSSC.