Immobilization of co %28ii%29 ions in cement pastes and their effects on the hydration characteristics

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J. Mater. Sci. Technol., 2011, 27(1), 74-80.

Immobilization of Co (II) Ions in Cement Pastes and Their Effects on the Hydration Characteristics
Eisa Hekal1)† , Essam Kishar2) , Wafaa Hegazi2) and Maha Mohamed2)
1) Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt 2) Chemistry Department, Women s College for Arts, Science and Education, Ain Shams University, Cairo,Egypt
[Manuscript received May 14, 2009, in revised form January 30, 2010]

The immobilization of Co (II) in various cement matrices was investigated by using the solidification/stabilization (S/S) technique. The different cement pastes used in this study were ordinary Portland cement in absence and presence of water reducing- and water repelling-admixtures as well as blended cement with kaolin.Two ratios of Co (II) were used (0.5% and 1.0% by weight of the solid binder). The hydration characteristics of the used cement pastes were tested via the determination of the combined water content, phase composition and compressive strength at different time intervals up to 180 d. The degree of immobilization of the added heavy metal ions was evaluated by determining the leached ion concentrationafter time intervals extended up to 180 d. The leachability experiments were carried out by using two modes: the static and the semi-dynamic leaching processes. It was noticed that the concentration of the leached Co2+ ions in the static mode of leachability was lower than the solubility of its hydroxide in all the investigated cement pastes. KEY WORDS: Hydration characteristic of cement;Admixtures; Immobilization; Heavy metals

1. Introduction The protection of public health and environment from hazardous pollutants is a major concern of industrialized nations. As the quantity of hazardous industrial wastes increases significantly owing to rapid industrialization, its appropriate management is required to reduce adverse impacts on humans and ecosystems[1,2] . Various technologies havebeen developed to render a waste non-toxic or to reduce the potential for the release of toxic species into the environment. One of these technologies is solidification/stabilization (S/S) by cementitious binders[3–5] . Portland cement is the most commonly used binder for this process due to its low cost[6] . High pH is effective in immobilizing many toxic metals, by precipitation and sorptionreaction[7] . The leaching tests determine the total amounts of leachable contaminants (leaching rates). The leaching rate is extremely dependent on the concentration
† Corresponding author. Ph.D.; eisa− hekal@yahoo.com (E. Hekal). E-mail address:

and solubility of the contaminates and on the permeability of the matrix[8] . A study on the long-term performance of the cement-based S/S of heavy metalwastes such as Pb2+ , Zn2+ , Cu2+ , Ni2+ (positive ions as nitrates), and Cr(6) (a negative ion as potassium dichromate) using a flow-through leaching test method was described[9] . S/S of hazardous sludge from steel processing plant was studied. It was found that mechanical strength decreased with increasing waste content. Pb, Zn, Cu, Fe and Mn could be considerably immobilized by the S/S process.The elements least immobilized were Na, K, and Cl. Leaching of heavy metals can be considered as pH dependent and corresponding metal hydroxide solubility controlled process[10] . Therefore, the aim of this study is to examine the immobilization of some heavy metals (as chloride or nitrate) in various cement pastes and the investigation of the effect of these metal salts on the physicochemicalproperties of cement.

E. Hekal et al.: J. Mater. Sci. Technol., 2011, 27(1), 74–80

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Table 1 Chemical analysis of the used Portland cement and Kaolinite SiO2 Al2 O3 Fe2 O3 CaO MgO Na2 O K2 O Cl− SO3 LOI OPC/% 20.18 4.78 3.88 63.27 1.34 0.38 0.19 0.01 2.37 1.90 Kaolinite/% 46.02 39.44 0.40 0.30 0.26 – – – – 13.5

processes, i.e. static and semi-dynamic processes. 2.3.1 Chemically...
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