Abstract
Reactions of the compounds of the system CaO-Al2O3 in phosphoric acid solutions were studied by hydrothermal techniques at 100°C under the pressure of 10MPa. C3A, C12A7, CA and CA2 hydrated in pure water or phosphoric acid solutions to form C3AH6 and AH3 (gibbsite). The hydration reactivity of Calucium aluminates decreased with the decrease of its CaO/Al2O3 ratio and the hydration of CA and CA2 could not proceed even after 30 days by the addition of a phosphoric acid. It seemed that phosphoric acid formed AlPO4-like insoluble materials on the surface of CA and CA2 to retard the hydration. CA6 and α-A hydrated neither in water nor in phosphoric acid solutions.
H3PO4 preferentially reacted with CaO components to form Ca10(PO4)6(OH)2 or CaHPO4. The formation region of C3AH6 appeared to extend to the line from CaHPO4 to Al2O3 in the system CaO-Al2O3-H3PO4(H2O). AH3 was formed together with AlPO4⋅2H2O below the line of H3PO4/CaO+Al2O3+H3PO4=50mol%.
The formation of various phosphates were strongly dependent on the pH of solutions. The products were Ca10(PO4)6(OH)2 in alkaline solutions, Ca10(PO4)6(OH)2, CaHPO4, AlPO4⋅2H2O (Messbach-type) or AlPO4⋅2H2O (Lucin-type) in neutral or weak acid solutions, CaHPO4, AlPO4⋅2H2O (Messbach-type), AlPO4⋅2H2O (Lucin-type) or weak AlPO4⋅2H2O (Metavariscite) in acid solutions and Ca(H2PO4)2⋅H2O or AlPO4⋅2H2O (Metavariscite) in strongly acidic solutions.
