The　increasing　demand　for　sustainable　construction　materials　has　spurred　interest　in　seawater　sea-sand　concrete　(SWSSC)　as　a　substitute　for　natural　resources.　However,　SWSSC＇s　durability　faces　challenges　from　aggressive　chloride　and　sulfate　ions　existing　in　seawater,　causing　structural　degradation.　Therefore,　this　study　has　prepared　seven　SWSSC　formulations　with　the　different　ultrafine　metakaolin　(UMK)　and　nano-TiO2　(NT)　dosages　using　untreated　seawater　and　sea　sand.　The　SWSSC　specimens　fabricated　using　these　formulations　were　evaluated　through　the　wet-dry　sulfate　cycling,　chloride　permeability　and　water　permeability　tests　to　assess　their　durability　performance.　The　advanced　microstructural　analyses,　including　the　Fourier　transform　infrared　spectroscopy,　the　scanning　electron　microscopy　and　the　X-ray　diffraction,　were　also　employed　to　examine　the　effects　of　the　UMK　and　NT　on　the　pore　refinement,　the　phase　evolution　and　the　functional　group　changes　within　the　concrete　matrix.　The　test　results　have　revealed　that　a　combined　addition　of　15　wt%　UMK　and　0.5　wt%　NT　significantly　reduced　the　chloride　ion　permeation　and　the　water　permeability,　enhancing　the　initial　impermeability　of　the　modified　concrete.　However,　after　the　60　cycles　of　sulfate　exposure,　the　specimens　with　the　15　wt%　UMK　addition　(with　or　without　NT)　lost　their　strengths,　while　the　unmodified　concrete　specimens　retained　the　higher　residual　strengths.　The　formulation　with　the　addition　of　5　wt%　UMK　and　0.5　wt%　NT　demonstrated　the　resistance　improvement　up　to　the　60　cycles,　although　the　specimen　strength　was　slightly　lower　than　that　of　unmodified　SWSSC　specimen.　These　findings　highlighted　the　need　for　optimizing　the　UMK　and　NT　dosages　in　order　to　balance　initial　impermeability　and　long-term　durability　of　SWSSC.　©　2025　Elsevier　Ltd