Although　both　Portland　slag　cement　(PSC)　and　supersulfated　cement　(SSC)　exhibit　excellent　sulfate　resistance,　their　underlying　mechanisms　remain　obscure　due　to　fragmented　studies　on　composition.　This　work　employs　a　contour-optimized　ternary　system　of　slag　-　anhydrite　-　clinker　to　decode　the　Al2O3-SO3-portlandite　interdependencies　that　govern　sulfate　attack　resistance.　The　system　was　tested　under　full　immersion　and　drying-wetting　cycles　in　a　5　%　Na2SO4　solution.　The　hydration　and　erosion　products　were　characterized　by　XRD,　TG-DTG,　NMR　and　SEM　methods.　The　primary　products　of　PSC　were　C-A-H,　C-(A)-S-H,　ettringite　(AFt),　and　portlandite.　Its　deterioration　mainly　stemmed　from　expansion　due　to　AFt　and　gypsum　crystallization.　In　contrast,　SSC　produced　AFt,　C-(A)-S-H,　and　gypsum　as　main　hydration　products.　The　deterioration　of　SSC　primarily　resulted　from　AFt　formation.　To　mitigate　sulfate　attack　in　PSC,　increase　the　slag　content　and　reduce　the　clinker　content.　This　suppressed　CH　formation　and　enriched　the　formation　of　C(A)-S-H.　Additionally,　introducing　a　controlled　quantity　of　anhydrite　helps　regulate　the　conversion　of　C-A-H,　AFm　and　AFt,　stabilizing　the　AFt　equilibrium.　For　SSC,　maintaining　a　high　slag　content　is　essential　to　ensure　adequate　aluminum　availability　for　sulfate　resistance.　Meanwhile,　moderately　reducing　the　anhydrite　dosage　helps　control　the　sulfate　balance.　This　approach,　combined　with　an　optimal　clinker　addition　to　activate　slag　reactivity,　promotes　the　synergistic　formation　of　AFt　and　C-(A)-S-H　gel　phases.　The　optimized　compositions　for　superior　sulfate　resistance　are:　PSC　(slag　60-75　%,　anhydrite　5-10　%,　clinker　20-30　%)　and　SSC　(slag　70-85　%,　anhydrite　10-20　%,　clinker　5-10　%).