Stability of cementitious materials in saline environments
The complexity of cementitious matrices and their application in the immobilisation of radioactive waste has led to detailed examination of their ability to condition permeating water to high pH by both experimental and thermodynamic studies. This thesis considers the stability and solubility of pure hydrate phases: Ca(OH)2; CaO-SiO2-H2O gel, Ca:Si = 0.85, 1.1, 1.4, 1.8; 3CaO.Al2O3.6HzO; 3CaO.Al2O3.CaSO4.12H2O and 3CaO.Al2O3.3CaSO4.32H2O, and the phase formation and stability within CaO-SiO2-CaCO3-H2O and CaO-Al2O3-SiO2-H2O compositions aged in saline solutions, up to 1.5M NaCl and 0.05M MgSo4, at 25°, 55° and 85°C. The two main high pH conditioning phases of cementitious systems are Ca(OH)2 and C-S-H gel. Sodium chloride enhances the solubility of Ca(OH)2 and causes a slight reduction in the Ca:Si ratio of C-S-H gels by the progressive leaching of calcium. Silicate polymerisation within C-S-H phases is inhibited by sodium chloride though it is uncertain how this alters the crystallisation kinetics. The pH buffering capacity is maintained when aged in sodium chloride concentrations 0.5, 1.0 and 1.5M at 25°, 55° and 85°C. The stability of calcium sulfoaluminate aged in sodium chloride is greater than of 3CaO.Al2O3.6H2O, which is unstable with respect to 3CaO.Al2O3.CaCl2.10H2O in NaCl < 0.5M. These phases undergo a progressive phase change to the 3CaO.Al2O3.0.5CaSO4.0.5CaCl2.10-12H2O and 3CaO.Al2O3.CaCl2.10H2O at increasing aqueous Cl:SO4 ratios. The formation of a limited solid solution region within 3CaO.Al2O3.xCaSO4.l-xCaCl2.yH2O: 0.00 ≤ SO4:Cl ≤ 0.06, was characterised. In magnesium sulfate, 5 - 50m.mol/l, calcium within hydrate phases is progressively replaced by magnesium with formation of Mg(OH)2, MgO-SiO2-H2O gel, 4MgO.Al2O3.xH2O and gypsum. The pH conditioned by the resultant solid assembly decreases to less than that desirable for containment of radioactive waste, to < 9. Consideration of the phase formation and persistence within the CaO-SiO2-CaCO3-H2O and CaO-Al2O3-SiO2-H2O systems was examined in solutions containing both sodium chloride and magnesium sulfate. The chemical interactions observed were dominated by the replacement of calcium by magnesium within the solid phases with the formation and persistence of mixtures of Mg(OH)2, MgO-SiO2-H2O gel and gypsum. At low Mg:Ca-CO3 ratios the persistent stability of gehlenite hydrate at 25°C was observed in appropriate samples. The chemistry of the aqueous phase is dependent on the Mg:Ca-CaCO3 ratio as well as the Ca:Si ratio. At high Mg:Ca-CaCO3 ratios the high pH conditioning properties are destroyed and buffering occurs at a value below pH 9.