Exposure of Portland cement to multiple trace metal loadings

This paper examines the influence of combinations of lead, zinc and copper on the hydration of cement and the extent to which these metal ions are able to penetrate the surface of a cement paste in contact with solutions containing them. The overall aim of these experiments was to establish the likely outcome of placing fresh concrete in contact with soil contaminated with these metals. Isothermal conduction calorimetry was used to examine the manner in which combinations of lead, zinc and copper in solution affect the kinetics of cement hydration. Zinc and lead ions have the effect of retarding cement hydration. The influence of copper on hydration is much more complex. Penetration of metal ions into fresh cement pastes is generally dependent on the total mass of metal ions in solution. At depths greater than 7 mm from the cement paste surface it was found that metal concentrations remained very low. The results of geochemical modelling support the Authors' proposed mechanism of large-scale precipitation of metals at the surface layer which severely limits further penetration.