Radioactive waste immobilisation in cement-zeolite and other cement-based matrices
The ability of zeolites, particularly clinoptilolite, to immobilise Cs arising as an intermediate level radioactive waste is studied. The zeolites are incorporated into Portland cement blends containing additives such as blast furnace slag, pulverised fuel ash and silica fume, and high alumina cement blends. Desorption of Cs+ from the zeolite by ion-exchange cement pore fluid species, mainly K+, Na+ and Ca is studied and ion-exchange isotherms are presented. The Cs distribution between clinoptilolite and cement pore fluid is studied by X-ray diffractometry (XRD) and by chemical analysis of the pore fluid. Some Cs is released into the pore fluid, mainly by ion-exchange with K. The kinetics and mechanism of the pozzolanic reaction between clinoptilolite and Portland cement is studied at various temperatures, using a selective dissolution method, as well as XRD, thermogravimetric analysis and analytical electron microscopy. A model is developed, whereby long-term predictions of clinoptilolite reactivity can be made. Methods of limiting the pozzolanic reaction to ensure the long-term persistence of clinoptilolite in cement by blending are investigated. Slag cements show lowest reactivity. Leach tests are carried out on cement-clinoptilolite cylinders (45x80mm) using a standard leaching method. These confirm the importance of ion-exchange and chemical reactivity in determining leach rate. Additionally, the effect of factors such as clinoptilolite particle size, Cs -loading level, clinoptilolite-cement ratio, quantity and type of cement additive, curing temperature, curing time, leaching temperature and mercury porosity on leach rate are investigated. An optimal cement-clinoptilolite blend is suggested. A method is described for the measurement of oxidation-reduction potential (Eh) of cement pore fluids extracted under pressure, and of measuring the poising capacity of solid and aqueous phases. OPC is mildly oxidising, whereas slag cements provide a reducing environment due to the presence of S-containing species. The implications of E. and pH in terms of radwaste immobilisation are discussed.