Ceramics-Silikáty 58, (4) 282 - 287 (2014)

The article continues the research of input reduction of phosphogypsum preparation for the production of building materials. Desiccated apatite (2.18 % P₂O₅ in gypsum) makes the least changes in the properties of Portland cement: it prolongs the cement setting times and reduces the compressive strength about 10 %. The apatite formation in the pastes of soluble orthophosphate-cement occurs during the formation of X-ray amorphous colloidal calcium orthophosphate hydrate, which prolongs cement setting time 2 - 3 times; next crystallization of CaHPO₄.2H₂O is involved in the formation of cement stone; therefore, it does not affect the strength of CEM I 42.5R Portland cement after 28 days of curing; however, due to the decomposition of these crystals in the alkaline medium during the further hydration of cement and formation of apatite, the strength is reduced to 20 % after 2 months. Destruction additive of insoluble calcium orthophosphate hydrates (molar ratio CaO/P₂O₅ = 2 - 3) by lime and formation of apatite beginning after mixing cement with water and continue about 30 days; it reduces the strength of Portland cement to 27 % after 28 days of curing. After 3 months of cement hydration, the strength of Portland cement with a maximum (2.18 % P₂O₅ in gypsum) amount of additive of various orthophosphates level off and remains about 10 % less than control. Adding the apatite suspension to the cement of temperature 80 - 110^oC increase the setting times up to 30 % and reduce the strength to 5 - 8 % as compared with the additive of desiccated apatite.