Ceramics-Silikáty 57 (4) 297-304 (2013)

CALCIUM ORTHOPHOSPHATES HYDRATES: FORMATION, STABILITY AND INFLUENCE ON STANDARD PROPERTIES OF PORTLAND CEMENT

Preparation of phosphogypsum to produce the binders requires a much higher input than preparation of natural gypsum stone. This makes it uncompetitive material. The investigations presented therein are meant to reduce this input by looking for the ways of rendering impurities harmless. Soluble acid orthophosphates are the main harmful impurity of phosphogypsum. The studies show that dry insoluble calcium orthophosphates hydrates (1.09 % and 2.18 % P₂O₅ in gypsum) have little effect on W/C, setting times and soundness of Portland cement pastes. Insoluble calcium orthophosphates hydrates {CaHPO₄∙2H₂O, Ca₈(HPO₄)₂(PO₄)₄∙5H₂O and Ca₉(HPO₄)(PO₄)₅(OH)∙4H₂O} formed in acidic medium (pH = 4.2 - 5.9) have been destroyed in alkaline medium and reduce standard compressive strength of cement up to 28 %. Calcium orthophosphates hydrates of hydroxyapatite group are stable in alcaline medium, while in dry state they reduce the standard compressive strength of cement until 10 %, but their suspensions prolong setting times of Portland cement as soluble orthophosphates – 2 - 3 times. Alkalis in cement increase pH of paste, but do not change the process of formation of calcium orthophosphates hydrates of hydroxyapatite group: it takes place through an intermediate phase - CaHPO₄·2H₂O, whose transformation into apatite lasts for 2 - 3 months.

Keywords: Calcium orthophosphates hydrates, pH kinetics, Stability, Portland cement