Hydration Characteristics of Different Cementitious Materials in presence of Some Heavy Metals

Abstract

Due to increasing concerns about environmental pollutions especially caused by heavy metal wastes, many authors studied the immobilization of heavy metals. This was carried out by using the solidification / stabilization (S / S) method inside the solid matrix to minimize escaping of such metals via leaching. Portland cement and blended Portland cement are the most popular solidification agents for inorganic wastes. Therefore, the aim of this study is to investigate the inertization of some heavy metal salts by using Portland cement paste and some blended Portland cement pastes as well as the hydration characteristics measurements.

The heavy metal salts used in this study were nickel chloride (NiCl2) and chromium chloride (CrCl3). The ratio used of the heavy metal ions was 1.0% of the solid. These salts were used as solutions in the mixing water. The blended materials used in this study were bentonite and burnt bentonite at 250, 500 and 800˚C for 3 hours with a ratio of 20% by weight of solid. In addition, kaolinite and metakaolinite were used as blending materials with OPC by using various ratios, namely; 20, 40 and 80% by weight of solid. Metakaolin was obtained by heat treatment of kaolin at 800˚C for 3 hours. The mixtures of Portland cement with each of the blending materials were first mechanically mixed in the dry state in ball mill to attain complete homogeneity. The cement pastes were prepared by mixing each dry mixture with the suitable amount of water, 0.35 water/solid ratios for most of the pastes, except the pastes containing 80% MK (mixes XI, XIA and XIB) were mixed with W / S ratio of 0.40 in presence or absence of heavy metal salts. Superplasticizer was used in some pastes to attain suitable workability. After complete mixing the resulted pastes were molded into one inch cubic moulds. The moulds containing the pastes were then cured at about 100% relative humidity for the first 24 hours at room temperature (25 ˚C ± 1) in order to attain the final setting of the specimens. The hardened cement pastes were then removed from the moulds and cured at ~ 100% RH up to 90 days. The applied time intervals were 3, 7, 28 and 90 days. After 7 days from hydration, the samples were divided into two sets; the first was cured at ~ 100% RH up to 90 days for examining the physicochemical and mechanical characteristics and the other set was used for the leaching measurements.