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Cement pastes prepared from a “typical” ASTM Type I portland cement at a water/cement ratio of 0.50 were mixed with various calcium chloride (CaCl2) admixture treatment levels, hydrated for various periods, and then subjected to pore solution expression. The expressed pore solutions were then analyzed for chloride and hydroxide ion concentrations. It was found that the concentration of chloride ions drops rapidly but reaches an equilibrium level by about ten days. The equilibrium concentrations attained varied between 20 and 50% of the original concentration in the mix water, the latter value being for the highest CaCl2 treatment level, 2% by weight of the cement. Substituting sodium chloride (NaCl) for CaCl2 as the admixture produced only minor differences in equilibrium concentration. The equilibrium Cl−/OH− ratios for treatments of 0.5% CaCl2 and above were found to be beyond that considered to be the boundary for depassivation, slightly above 0.3. X-ray diffraction results were ambiguous but differential thermal analysis (DTA) trials provided some evidence for the formation of Friedel's salt in these pastes. However, it was considered likely that at least some of the chloride not retained in solution is associated with calcium-silicate-hydrate (C-S-H) gel.
Professor, School of Civil Engineering, Purdue University, West Lafayette, IN
Stock #: CCA10062J