Introduction　With　the　rapid　development　of　assembled　buildings,　the　low-carbon　and　high-efficient　production　of　precast　concrete　components　becomes　a　hotspot　for　the　industry.　Portland　cement　(PC),　as　one　of　the　most　important　cementitious　materials　for　the　preparation　of　precast　components,　has　the　disadvantages　of　insufficient　early　strength　development　and　late　volumetric　shrinkage.　To　realize　the　volume　stability　of　non-steam-cured　precast　components,　expansion　agents　are　often　introduced　into　Portland　cement.　Sulphoaluminate　cement　(CSA)　with　a　large　amount　of　Ye’elimite　and　gypsum　is　often　used　to　improve　the　early　strength　and　volumetric　stability　of　PC.　Different　types　of　CSA　have　different　expansion　capacities　due　to　the　different　contents　of　Ye’elimite　and　gypsum,　affecting　the　composition　of　the　hydration　products　with　the　saturation　of　ettringite.　In　this　paper,　the　mechanical　properties　and　volumetric　stability　of　the　specimens　were　investigated　via　varying　the　relative　contents　of　C4A3S̅　and　CS̅　in　Portland　cement　(i.e.,　CS̅　/C4A3S̅　molar　ratio　and　C4A3S̅　-CS̅　content).　The　hydration　process,　phases　composition,　and　the　microstructure　of　system　were　characterized　to　analyze　the　effect　of　C4A3S̅　-CS̅　on　the　hydration　of　Portland　cement.　Methods　Portland　cement　clinker　(Tangshan　Jidong　Cement　Sanyou　Co.,　Ltd.,　China),　and　anhydrite　(Tangshan　Polar　Bear　Building　Materials　Co.,　China)　were　used　as　raw　materials.　CaCO3,　Al2O3,　and　CaSO4-2H2O　mixed　in　a　molar　ratio　of　3:3:1　were　heat-treated　in　a　high-temperature　muffle　furnace　for　2　h　to　obtain　the　C4A3S̅　mineral.　Four　different　CS̅　/C4A3S̅　molar　ratios　of　M,　2,　5,　8　and　11　were　designed,　while　five　C4A3S̅　-CS̅　content　levels　of　5%,　10%,　15%,　20%　and　25%　were　set　up　in　a　total　of　20　sets　of　crossover　experiments.　The　10%-M2　represents　the　content　of　this　sample　of　C4A3S̅　-CS̅　in　Portland　cement　clinker　is　10%　at　the　CS̅　/C4A3S̅　molar　ratio　M　of　2.　The　water-cement　ratio　was　0.35,　and　the　paste　was　molded　as　a　net　paste　test　block　with　the　sizes　of　20　mm×20　mm×20　mm.　The　compressive　strength　at　the　corresponding　age　was　determined.　The　phase　compositions　of　the　hydration　products　were　analyzed　by　a　model　D/Max-RB　X-ray　diffractometer　and　a　model　STA　449F3　thermal　analyzer.　The　pore　structure　of　the　specimens　was　characterized　by　a　model　AutoPore　IV　9500　V1.09　instrument.　Results　and　discussion　C4A3S̅　-CS̅　has　an　effect　on　the　free　expansion　rate　of　Portland　cement,　and　it　can　significantly　increase　the　early　compressive　strength　of　Portland　cement　clinker.　When　the　C4A3S̅　-CS̅　content　of　≤15%　in　the　sample,　the　free　expansion　rate　is　small,　the　volumetric　stability　is　better.　When　hydration　time　is　14　d,　the　free　expansion　rate　remains　stabilized,　and　the　expansion　rate　of　the　specimen　does　not　change　at　28　d.　The　free　expansion　rate　of　the　sample　is　lower　than　that　of　the　sample　with　C4A3S̅　-CS̅　.　The　effect　of　C4A3S̅　-CS̅　on　the　expansion　properties　of　Portland　cement　belongs　to　the　ettringite-type　expansion.　The　key　reason　of　the　volume　stability　and　early　strength　development　of　the　C4A3S̅　-CS̅　-PC　system　is　affected　by　the　amount　of　ettringte.　Ettringite　growth　in　the　molding　paste　is　limited　as　the　crystal　grows,　the　pressure　of　ettringite　on　the　surrounding　paste　environment　gradually　increases,　reaching　a　certain　threshold　that　makes　the　specimen　gradually　collapse.　As　a　result,　the　amount　of　ettringite　generation　is　strictly　controlled.　At　ettringite　content　of　less　than　8.51%,　the　compressive　strength　increases　with　the　increase　of　ettringite　content.　Ettringite　plays　a　role　of　dense　pore.　However,　at　the　ettringite　content　of　more　than　8.51%,　the　expansion　increases　with　increasing　ettringite　content,　and　the　mechanical　properties　of　the　cement　begin　to　decrease.　The　most　available　pore　sizes　of　the　specimens　10%-M11　were　smaller　than　those　of　the　specimens　5%-M11,　while　the　specimens　10%-M11　have　a　narrower　distribution　of　pore　sizes　and　smaller　total　porosity,　which　can　account　for　the　maximum　mechanical　properties.　The　location　and　the　crystallization　situation　of　hydration　products　both　have　an　influence　on　the　expansion　properties　of　Portland　cement.　Ettringite　fills　the　pores　of　the　paste,　and　the　C-S-H　gel　can　adhere　other　hydration　products　such　as　ettringite　to　enhance　the　performance　of　the　cement,　and　avoid　the　ettringite　abnormal　growth.　Conclusions　The　10%　content　of　C4A3S̅　-CS̅　appropriately　increased　the　expansion　deformation　of　Portland　cement,　limited　its　late　volumetric　deformation　and　stablized　the　expansion　rate.　The　content　of　C4A3S̅　-CS̅　also　improved　the　early　mechanical　properties　of　Portland　cement,　having　a　positive　effect　on　the　development　of　mechanical　properties.　Under　the　condition　of　10%　C4A3S̅　-CS̅　and　the　molar　ratio　of　CS̅　/C4A3S̅　of　11,　the　compressive　strength　of　Portland　cement　at　12　h　was　increased　by　102%,　and　the　free　expansion　rate　at　28　d　was　0.279%.　There　were　three　main　reasons　of　high　early　strength　in　the　C4A3S̅　-CS̅　-PC　system,　i.e.,　hydration　of　C4A3S̅　-CS̅,　hydration　of　C3A　with　CS̅,　and　rapid　hydration　of　C3S.　The　increased　content　of　C4A3S̅　-CS̅　facilitated　the　formation　of　ettringite,　and　hydration　consumed　Ca(OH)2　in　the　hydration　product　to　generate　ettringite　to　cause　the　expansion,　accelerating　the　hydration　rate　of　the　silicate　phase　and　improving　the　cement　hydration　degree.　The　ettringite　generated　by　the　hydration　of　C4A3S̅　-CS̅　-PC　could　compensate　for　the　self-shrinkage　of　Portland　cement,　and　the　short　and　fine　needle-like　ettringite　grew　on　the　surface　of　the　hardened　cement　paste　and　in　the　pores,　and　the　staggered　lapping　filled　the　pores,　reducing　the　total　porosity　of　the　cement　and　the　most　available　pores.　An　appropriate　amount　of　C4A3S̅　-CS̅　could　optimize　the　distribution　of　ettringite　in　the　system,　and　avoid　the　expansion　caused　by　the　abnormal　growth　of　ettringite　crystal　size.　©　2024　Chinese　Ceramic　Society.　All　rights　reserved.