Studies of Zinc, Cadmium and Mercury Stabilization in OPC/PFA Mixtures
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STUDIES OF ZINC, CADMIUM AND MERCURY STABILIZATION IN OPC/PFA MIXTURES C.S. POON* and R. PERRY** *Department of Metallurgy and Science of Materials, Oxford University, Parks Road, Oxford OXl 3PH, U.K. **Department of Civil Engineering, Imperial College, London, U.K. Received 5 October,
1986; refereed
ABSTRACT The utilization of pulverized fuel ash (PFA) (fly ash) for the stabilization of heavy metal waste is described. Solutions of the group IIB elements (zinc, cadmium and mercury) are used as model materials because of their significance as industrial wastes. The study included aqueous chemistry determinations, a leaching test, the use of SEM to examine microstructure, and compressive strength measurements. The use of PFA in a cementitious matrix lowers the alkalinity of the overall system and thus improves the immobilization of the amphoteric metal such as zinc. The interaction between mercuric solution and PFA plays an important role in improving the retention of the blended system for mercury. SEM results show that the microstructure of the ordinary Portland cement (OPC)/PFA blended system is significantly modified by the incorporation of the waste material. The advantages of using the blended system over a pure OPC system are described in physical and chemical terms. INTRODUCTION Ordinary Portland cement (OPC), pulverized fuel ash (PFA), slag, silicates and other cementitious materials have long been utilized for the disposal of low level radioactive waste [1]. The growing environmental pressure has recently required the generally regarded less hazardous materials, such as heavy metal wastes, to be stabilized to assure proper disposal. Various commercial processes have been developed during the past two decades for such purposes. A lime-pozzolana process is widely used in the U.S. [2]. In the U.K., however, an OPC-PFA process has been used for a number of years [3]. The objective of the present paper is to increase the basic understanding of the chemistry and mechanism involved between the waste and the stabilizing agents in the OPC/PFA process using solutions of Zn, Cd and Hg as models of heavy metal wastes. MATERIALS AND METHODS Materials Zinc nitrate, cadmium nitrate and mercury chloride and nitrate were "AR" grade and were obtained in the U.K. from BDH. The chemical compositions of OPC and PFA used in this study are presented in Table I. The soluble silicate was a 40% sodium metasilicate (Na2 SiO3 ) solution. The heavy metal content in these matrix materials is insignificant when compared to the concentration of the simulated waste solutions. Aqueous Chemistry The aqueous chemistry of OPC, PFA and OPC/PFA in water and in solutions containing the heavy metals was studied in a N2 atmosphere. A volume of 200 mL of water or of solutions containing 2000 ppm (0.2 wt.%) of Zn, Cd or Hg were mixed with 40 g of solid (OPC, PFA, and OPC/PFA in a 3:1 ratio) and
Mat. Res. Soc. Symp. Proc. Vol. 86. '1987 Materials Research Society
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TABLE I Composition (as wt.% oxides) of OPC and PFA
OPC PFA
Si0 2
A120 3
Fe
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