Chloride in cement : study of the system CaO-Al2O3-CaCl2-H2O
Tn this work, the system CaO-Al2O3-CaCl2-H2O is investigated at 20 2 C. The subsystem, CaO-CaCl2-H2O, is studied by hydrating mixtures of CaO and CaCl2 in various proportions. Four hydrates are found to exist in the system at 20 2 C, viz. : Ca(OH)2, 3CaO.CaCl2.15H2O, CaO.CaCl2.2H2O and CaCl2.6H2O. The hydrate CaO.CaCl2.2H2O is not stable at temperatures below 20 C : transforms to 3CaO.CaCl2.15H2O. Both CaO.CaCl2.2H2O and 3CaO.CaCl2.15H2O are stable in contact with aqueous solutions only at high chloride concentrations. 3CaO.CaCl2.15H2O could only be prepared by hydrating a (1:1) mixture of CaO and CaCl2 at water/solid ?1.0. A good method for synthesis of CaO.CaCl2.2H2O is found to be dissolving 3CaO.CaCl2 15H2O in a saturated aqueous CaCl2 solution. Both 3CaO.CaCl2.15H2O and CaO.CaCl2.2H2O are efflorescent. Friedel's salt, C3A.CaCl2.10H2O, was synthesised by hydrating an equimolar mixture of C3A and CaCl2 in a filtered saturated Ca(OH)2 solution at solution/solid ratio of 1.0. Friedel's salt was found to dissolve congruently in water. Its solubility product is calculated as 3.467 x 10-28. Chloride-ettringite was prepared by hydrating a stoichiometric mixture of CaO and CaCl2 at water/solid ratio of 1.0. However the compound was found to be unstable at 20 2 C : it transforms to Friedel's salt. The transformation also occurs when drying and when washing with alcohol or water. Formation of solid solution series in the system CaO-Al2O3-CaCl2-H2O is studied by hydrating mixtures of C3A, CaCl2 and CaO at CaCl2/C3A molar ratio ranging from zero to 1.0. A series of solid solutions of the general formula C3A.(1-x)CaCl2.xCa(OH)2 (10+8x)H2O formed with a miscibility gap about x = 0.75 whereby the solid 3CaO.Al2O3.0.25CaCl2.0.75Ca(OH)2.11.5H2O coexists with C4AH19. The cell dimensions of the solid solutions increases with decreasing chloride content.