In　recent　years,　research　into　the　properties　of　seawater　and　sea　sand　concrete　(SWSSC)　has　emerged　as　a　prominent　area　of　investigation,　and　the　alkali　silica　reaction　(ASR)　of　SWSSC　is　an　urgent　issue　to　be　solved.　However,　little　attention　has　been　paid　to　the　effect　of　K/Na　on　the　ASR　of　SWSSC.　In　order　to　fill　this　gap,　the　effects　of　different　K/Na　on　ASR　products,　pore　structure,　pH　and　alkali　ion　content,　and　expansion　of　SWSSC　were　measured.　The　findings　demonstrated　that　the　composition　of　the　amorphous　product　ASR-P1　(K0.52Ca1.16Si4O8(OH)2.84–1.5H2O)　of　SWSSC　exhibited　an　inverse　relationship　with　K/Na,　whereas　the　crystalline　product　K-shlykovite　(NaCaSi4O8(OH)3–2.3H2O)　displayed　a　direct　correlation　with　K/Na.　The　increase　of　K+　concentration　leads　to　the　decrease　of　silica　dissolution,　which　is　the　main　reason　for　the　lowest　ASR　degree　in　the　high　K/Na　group.　The　transformation　of　ASR-P1　to　K-shlykovite　also　resulted　in　different　main　ASR　products　in　different　K/Na　groups.　The　experimental　data　show　that　increasing　the　K/Na　from　0.85　to　1.85　reduced　the　14d　expansion　from　0.248　to　0.22%　and　28d　ASR　expansion　from　0.415　to　0.350%.　Notably,　the　most　significant　suppression　occurred　in　the　high　K/Na　group,　which　exhibited　the　lowest　expansion　values　at　all　ages.　The　findings　of　this　study　provide　a　foundation　for　the　theoretical　application　of　SWSSC　in　the　field　of　ocean　engineering.　These　results　suggest　that　adjusting　K/Na　ratios　could　serve　as　a　viable　strategy　to　mitigate　ASR　induced　damage　in　marine　concrete　structures.　For　such　structures,　this　implies　that　material　selection　should　prioritize　potassium-rich　binders,　such　as　blended　cements　or　SCMs　like　potassium　feldspar,　to　naturally　elevate　the　K/Na　ratio.　In　mix　design,　controlled　additions　of　KOH　during　mixing　can　adjust　the　alkali　balance,　while　leveraging　seawater　with　inherently　higher　K/Na　ratios　or　supplementing　seawater　with　potassium　salts　can　help　achieve　the　target　K/Na　ratio.　The　findings　of　this　study　provide　a　foundation　for　the　theoretical　application　of　SWSSC　in　the　field　of　ocean　engineering　to　enhance　long-term　durability　in　ocean　engineering　applications.　©　The　Author(s),　under　exclusive　licence　to　RILEM　2025.