The　compositions　and　contents　of　phases　in　cement　paste　soaked　in　sodium　sulfate　solution　changing　with　time　were　investigated　in　this　study.　First　of　all,　the　porosities　in　cement　paste　specimens　over　time　were　measured　by　an　in　situ　X-ray　computed　tomography　(X-ray　CT)　test.　Cement　hydration　and　sulfate　erosion　occur　simultaneously　during　the　soaking　process　of　cement　paste　specimens　in　sodium　sulfate　solution.　Therefore,　the　hydration　behaviour　and　the　erosion　of　sulfates　in　cement　paste　were　combined　in　a　new　way　to　establish　an　X-CT-hydration-deterioration　model,　which　was　used　to　calculate　the　variations　of　the　volume　fractions　of　the　phases　in　cement　paste.　Subsequently,　the　volume　fractions　of　each　phase　were　used　to　calculate　the　elastic　moduli　of　cement　paste　specimens　with　the　bulk　modulus,　shear　modulus　of　each　phase　and　a　self-consistent　(SC)　method.　Afterward,　the　calculated　elastic　moduli　were　compared　with　the　experimental　results　obtained　by　uniaxial　compression　test　and　the　error　values　were　1%　for　28　d,　2.5%　for　88　d　and　5%　for　148　d,　which　were　all　within　5%,　indicating　that　the　advanced　X-CT-hydration-deterioration　model　combined　with　the　SC　method　can　successfully　predict　the　elastic　modulus　of　cement　paste　eroded　in　sodium　sulfate　solution.