Sodium　fluosilicate　(Na2SiF6)　is　prepared　from　by-product　of　phosphate　fertilizer　production.　According　to　a　certain　percentage　of　Na2SiF6　mixed　in　the　cement,　the　influence　of　different　dosages　on　the　setting　time　of　cement　under　different　water-cement　ratio　was　discussed,　and　the　effect　of　different　forms　of　fluoride　compounds　on　the　setting　time　of　cement　were　compared.　The　effect　and　mechanism　of　sodium　fluosilicate　on　the　hydration　products　of　cement　were　researched　by　using　the　X-ray　diffraction　analysis　and　micro　electron　microscope.　Experimental　results　show　that　the　retarding　effect　of　sodium　fluosilicate　on　the　coagulation　time　had　a　critical　effect,　and　beyond　a　certain　value,　it　sharply　decreased　the　setting　time.　The　critical　dosage　increased　with　the　increase　of　water-cecment　ratio.　Compared　with　the　retarding　effect　of　Na2SiF6,　MgSiF6,　NaF　and　CaF2　on　the　coagulation　time　of　cement,　Na2SiF6　was　the　best.　The　incorporation　of　Na2SiF6　does　not　change　the　types　of　the　hydration　products,　the　main　function　of　FN　is　hydrolyzed　with　Ca2+　ions　to　form　CaF2　with　a　very　low　solubility.　©　2017　Trans　Tech　Publications,　Switzerland.