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A model for the CaO—SiO2—H2O system at 25°C is proposed, and based upon this, more complex systems including the effect of SO42-, Cl-, CO32-, Na+ and K+ on phase relations and solubilities in the title system have been calculated by thermodynamic modelling. At sea-water concentrations of NaCl, mixed sea-water-pore fluid compositions enhance the solubility of calcium from Ca(OH)2 and C-S-H but do not otherwise affect the stabilities of these phases. Carbonate and sulfate, on the other hand, react strongly with cement substances both with and without sodium being present; the complex solubility and reaction paths are quantified. Comparison of calculated reactions with literature data are made where possible; generally good agreement is obtained.

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The ability of NaCI and MgSO 4 to impair the performance of Portland cement, blended cements containing slag and fly ash and of a permeable backfill have been measured. Performance is determined by decrease in pH, changes in mineralogy and loss of physical coherence. Experiments have been made at 250, 550 and 85'C and extensively backed up by chemical models of cement performance. NaCI, up to 1.5M, has a comparatively slight impact on performance but MgSO 4 rapidly and almost quantitatively reacts, lowering system pH's to < 10, conditioned by mixtures of Mg(OH) 2 and magnesium silicates with gypsum.