Cementand ConcreteResearch, Vol. 27, No. 8, pp. 1249-1260, 1997 Copyright 0 1997 Elsetier Science Ltd Printedin the USA. All rights reserved OOOS-8846/97 $17.00 + .OO PI1 s0008-?3846(97)00109-9 SOLUTION CHEMISTRY DURING CEMENT HYDRATION IN THE PRESENCE OF METAL HYDROXIDE WASTES S. Asavapisit, G. Fowler and ‘C.R. Cheeseman Centre for Environmental Control and Waste Management, Imperial College of Science Technology and Medicine, London, SW7 2BU, UK (Refereed) (Received July 5,1996; in final form June 2,1997) ABSTRACT Experiments were completed to compare the effects of 10% additions of Pb, Zn, and Cd hydroxide synthetic wastes on the early hydration of ordinary Portland cement, by analysing changes in the composition of solutions ex- tracted during hydration, and using conduction calorimetry, and differential thermal analysis. Pb’+was initially present at high concentrations, although this rapidly falls when the rate of heat evolution increases, despite the high pH environment. The Pb waste also delayed SO? removal from solution compared to OPC control data. Zn*’ was present in extracts at low concentra- tions but suppressed the heat evolution associated with normal cement hydra- tion. SO:- was rapidly removed from solution suggesting CaZnz(OH)6.Hz0 precipitates coat the surfaces of gypsum as well as cement clinker particles. Cd was present in the waste as crystalline Cd(OH)2 and was not detected in solutions extracted from hydrating cement/Cd waste. 0 1997 Elsevier Science Ltd Introduction Stabilization/solidification is a pre-landfill waste treatment process which has been used for many types of industrial wastes, but is particularly suited to those containing heavy metals (1,2). The production of a solidified waste reduces the surface area available for leaching, and the highly alkaline environment produced by cementitious binders ensures that most heavy metals are effectively immobilized. Commercial solidification processes use cement or other readily available pozzolanic binders as solidification additives. The effects of wastes on the hydration reactions of the binders are important, as these influence the microstructure and therefore the performance of the treated product. Commercial stabilization/solidification operators often accept solid and liquid wastes from a range of different industries. These are then chemically stabilized by controlling the pH of the mixed wastes to between 8 and 9.5. Most transition series elements and heavy metals are likely to precipitate under these conditions as insoluble hydroxides, although a ‘To whom all correspondence should be sent. 1249