Strength　of　soil　stabilized　by　the　composite　stabilizers　which　generate　both　cementitious　hydrates　(hydrated　calcium　silicate,　C-S-H)　and　expansive　hydrates　(ettringite,　AFt)　may　be　much　higher　or　lower　than　that　of　soil　stabilized　by　cement　which　generates　mainly　C-S-H,　the　reason　of　which　remains　unexplained.　In　this　study,　three　types　of　composite　stabilizers　with　different　AFt　formation　rates　were　used　to　stabilize　two　kinds　of　soils.　Unconfined　compressive　strength　of　the　stabilized　soils　was　measured,　and　X-ray　diffraction　and　differential　scanning　calorimetry-thermogravimetry　measurements　were　performed　to　determine　the　type　and　amount　of　the　major　hydrates　in　the　stabilized　soils　at　different　curing　time.　The　results　indicated　that　the　volume　expansion　of　AFt　could　eliminate　the　pores　in　the　soil,　and　might　also　damage　the　cementation　structure　formed　by　C-S-H.　During　the　AFt　formation,　if　the　amount　of　C-　S-H　was　rather　little　and　the　cementation　structure　formed　by　the　C-S-H　was　still　weak,　then　the　effect　of　AFt　volume　expansion　filling　the　pores　in　stabilized　soil　was　in　dominate,　therefore　provided　the　stabilized　soil　strength　increment.　In　contrast,　the　AFt　volume　expansion　would　destroy　the　cementation　structure　and　decrease　the　strength　if　C-S-H　formed　rapidly　and　made　most　of　the　AFt　form　in　significant　restriction　by　the　cementation　structure　formed　by　the　C-S-H.　The　weaker　the　cementation　structure　formed　by　the　C-S-H　during　the　AFt　growing,　the　more　remarkable　the　enhancement　produced　by　the　AFt　expansion.　Decreasing　the　formation　rate　of　cementitious　hydrates　and/or　increasing　the　formation　rate　of　expansive　hydrates　should　result　in　higher　strength　of　stabilized　soil.　An　example　was　given　to　show　that　with　an　expansive　component　with　slow　AFt　formation　rate,　remarkable　strength　increment　of　the　stabilized　soil　stabilized　by　composite　stabilizers　were　gained　by　adjusting　the　cementitious　components　to　slow　down　the　C-S-H　formation　rate.　©　2017　Nova　Science　Publishers,　Inc.