As　the　demand　for　alkali-activated　cementitious　materials　(AACM)　increases,　reliance　on　blast　furnace　slag　(BFS)　raises　concerns　about　potential　shortages.　This　study　evaluates　the　substitution　of　BFS　with　diatomaceous　earth　(DE)　in　AACM,　using　three　DE　types—raw　(RDE),　calcined　(CDE),　and　spent　(SDE)—at　substitution　rates　of　0–30　%.　The　impact　of　these　substitutions　on　fluidity,　setting　time,　and　compressive/flexural　strength　of　the　prepared　mortar　was　assessed,　along　with　an　analysis　of　the　mechanisms　through　hydration　heat,　product　composition,　and　pore　structure.　Results　indicate　that　DE,　characterized　by　the　porous　structure　and　high　content　of　amorphous　silica,　enhances　early　hydration,　reduces　fluidity,　accelerates　setting　times,　and　refines　pore　structure,　thereby　improving　strength.　CDE,　with　its　low　organic　content　and　large　surface　area,　exhibited　the　best　performance,　while　SDE　was　effective　despite　impurities,　and　RDE　performed　the　poorest.　Increasing　DE　substitution　raised　water　absorption,　impacting　fluidity　and　setting　times,　with　20　%　identified　as　the　optimal　substitution　rate,　balancing　strength　and　workability.　The　entropy　method-analytic　hierarchy　process　(EM-AHP)　confirmed　this　rate,　with　CDE　as　the　optimal　variant.　Life　cycle　assessment　(LCA)　showed　that　while　SDE　reduces　environmental　impacts　significantly,　CDE　achieves　a　greater　reduction　when　considering　both　environmental　and　mechanical　performance.　In　conclusion,　CDE　and　SDE　are　viable　BFS　substitutes　in　AACM,　offering　enhanced　performance　and　reduced　environmental　impacts.　©　2024　Elsevier　Ltd