Different formulations of Blast Furnace Slag blended with Ordinary Portland Cements are used in encapsulation of Intermediate Level Waste containing magnesium and aluminium alloys. A set of different candidate formulations are investigated to determine the corrosion rates of encapsulated materials, and the structural and chemical properties of the matrices which influence their suitability as encapsulant materials. The microstructural and mechanical properties of the cements tested by performing compressive strength, porosity and permeability measurements. The cements were found to display increased porosity with increasing initial water content, with a consequent decrease in the compressive strength. Pore Water Expression was used to extract the electrolyte present in the porous structure of the cements. The pH of the samples was found to be decreased by increasing BFS addition and increasing water content. The CSA sample was found to have similar pore water pH to the blended cements. Electrochemical Impedance characterisation of samples of the different Mg and Al containing samples in the different cement systems were performed. The EIS results for aluminium showed two time constant present in the system, either of which could possible have been related to the charge transfer process, but both of which evolved in the same proportion with time. The EIS results for magnesium showed only one time constant, allowing the corrosion rates to be calculated unambiguously. Tomographic analysis of the cement/metal interface, clearly shows the effects of hydrogen bubble formation of the surface area of the samples. This allows a correction factor to be applied to the corrosion rates found by EIS. Conclusions are presented in which the fitness of BFS: OPC blends for use as encapsulants is confirmed, and the merits of CSA as an encapsulant for aluminium, but not for Magnox are described.