Synthetic calcium silicate hydrogels, of Ca/Si mole ratios 0·85, 1·10, 1·40 and 1·80, were prepared and homogenised at 25°C. Solubilities of Ca(OH)2 and of C-S-H were measured in initially pure water at 25, 55 and 85°C. Equilibration times ranged from a few days up to 9 months: data are presented for the longer reaction times, 4·5 to 9 months. These solids were also equilibrated with NaCl solutions, approximately 0·5, 1·0 and 1·5 mol/l, at 25, 55 and 85°C. The expected solubility trends emerge in initially pure water: C-S-H dissolved incongruently and Ca solubilities decreased with either rising temperature or at lower Ca : Si molar ratios. The presence of NaCl enhanced Ca solubilities, especially in the range 0–0·5mol/l NaCl. However pH remained almost unaffected, ± 0·1 unit, as a function of NaCl concentration, other factors remaining constant. The C-S-H gel partially crystallised in the course of solubility determinations made at 85°C. Sorption of NaCl on C-S-H was negligible. Comments on the experimental procedures and quality of data are made. The long-term durability of cement exposed to dissolution in NaCl solutions is discussed.

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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.