Effect of calcium chloride on the hydration and leaching of lead-retarded cement

The effect of adding CaCl2 to ordinary portland cement containing 10% (by w eight) of a synthetic lead hydroxide waste has been investigated using diff erential thermal analysis, conduction calorimetry, and by analysing solutio ns extracted from mixes during the early stages of hydration. Lead wastes a re known to retard initial cement hydration reactions, and for the waste lo ading used in these experiments the peak in the heat of hydration curve was delayed from 8.7 to 172 h. The addition of CaCl2 reduced this retarding ef fect but was associated with a decrease in the overall extent of cement hyd ration, particularly at high CaCl2 concentrations. The acceleration induced is associated with lower concentrations of hydroxyl ions in solution. This reduces dissolution and subsequent adsorption of Pb(OH)(3)(-) ions onto C- S-H surfaces forming around hydrating cement particles. Samples containing CaCl2 show significantly higher weight loss during leaching in 0.05 N aceti c acid solutions. This observation, combined with the overall reduced level of hydration, suggests that addition of CaCl2 does not produce improved ma terials for disposal. (C) 1999 Elsevier Science Ltd. All rights reserved.