Sodium silicate-based alkali-activated slag mortars Part II. The retarding effect of additions of sodium chloride or malic acid

In this paper we investigate the hydration of alkali-activated slag cements in the presence of heavy contamination with chloride (up to 13% by weight as NaCl with respect to slag). Reference samples were made by activating slag with sodium silicate solution. Companion samples in which a high level of sodium chloride was added to the mix water were found, based on strength development and calorimetry, to be severely retarded. When the known retarder malic acid was also added to the mixes, the mix that also contained sodium chloride showed no significant heat development in the first 10 days. Pore expression experiments indicated that the bulk of the chloride remained dissolved in the pore solution. XRD indicated that no new crystalline hydrates were formed when NaCl was added to the system. In the systems with added malic acid or sodium chloride, the same calorimetry peaks were observed, albeit retarded, but with similar integrated heat outputs, suggesting that the basic mechanism of hydration was not altered by the retarding additions. (C) 2000 Elsevier Science Ltd. All rights reserved.