Under　the　background　of　shrinkage　cracking　of　cement　and　concrete,　ettringite　is　regarded　as　the　most　effective　expansion　source　because　of　its　outstanding　expansion　characteristics.　In　order　to　study　the　expansion　mechanism　of　ettringite,　the　growth　process　and　formation　conditions　of　ettringite　in　composite　cement　system　were　studied　by　means　of　SEM,　MIP　and　ICP.　The　effects　of　pore　structure　and　ion　concentration　of　pore　solution　on　the　morphology　and　expansion　properties　of　ettringite　were　also　analysed　in　this　paper.　The　results　show　that　the　pore　structure　of　cement　paste　directly　affects　the　expansion　properties　of　ettringite.　It　is　verified　that　there　is　no　obvious　linear　relationship　between　the　expansion　rate　of　ettringite　and　its　quantity.　The　concentration　of　SO42-　and　Al(OH)4-　in　pore　solution　is　the　determinant　of　ettringite　formation　rate　and　quantity　in　composite　cement　system　at　the　initial　stage　of　hydration　reaction.　The　change　of　crystallinity　of　ettringite　will　be　directly　caused　by　pH.　©　2020　Trans　Tech　Publications　Ltd,　Switzerland.