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

THE INFLUENCE OF ORTHOPHOSPHATES ON THE PROPERTIES OF PORTLAND CEMENT

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°C increase the setting times up to 30 % and reduce the strength to 5 - 8 % as compared with the additive of desiccated apatite.

Keywords: Portland cement, Orthophosphates, Strength, pH kinetics