Solubility reactions of cement components with NaCl solutions: I. Ca(OH)2 and C-S-H

Abstract

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.