Hydration of Calcium Aluminates (3:1, 12:7, 1:1, 1:2 and 1:6) in Phosphoric Acid Solutions at Various Temperatures

Abstract

Hydration of the compounds of the system CaO-Al₂O₃ (i.e. CA_n; C₃A, C₁₂A₇, CA, CA₂ and CA₆) in phosphoric acid solutions was studied at 35°C under a saturated pressure and at 100°C and 200°C by hydro thermal techniques under the pressure of 10 MPa. At 35°C, C₃A hydrated to form C₃AH₆ within 1 h in water, while C₁₂A₇, CA and CA₂ yielded C₂AH₈ after 1 day, which changed gradually into C₃AH₆ in water.

The addition of phosphoric acid into water retarded both the hydration of CA_n and the conversion of C₂AH₈ to C₃AH₆. A more addition of phosphoric acid stopped the hydration of CA_n even for 30 days. It seems that phosphoric acid forms an insoluble layer of amorphous aluminium phosphates on the surface of each CA_n grain. The hydration region, where C₃AH₆ and AH₃ or AH produced seemed to extend to higher H₃PO₄ content sides with higher temperatures and with longer duration in hydrothermal treatments, but CA₆ and α-A could hydrate neither at 100°C nor at 200°C.

The crystalline phosphate phases produced were only CaHPO₄•2H₂O at 35°C, CaHPO₄, Ca₁₀(PO₄)₆(OH)₂ and three types of A1PO₄•2H₂O at 100°C, and CaHPO₄, Ca₁₀(PO₄)₆(OH)₂ and three types of A1PO₄ at 200°C, respectively. Their formation depended upon the contents of H₃PO₄ and CA_n, and also upon the duration.

The standard cement nomenclature is used throughout CaO=C, Al₂O₃=A, H₂O=H, compounds of the system CaO-Al₂O₃=CA_n.