Magnesium-silicate-hydrate cements for encapsulating problematic aluminium containing wastes

Low pH cement systems for encapsulating legacy nuclear industry wastes containing aluminium metal are being developed using blends of MgO and silica fume (SF). The rheology of the binary system is significantly improved by addition of sodium hexa-metaphosphate (Na-HMP) which reduces the required w/s ratio. The addition of MgCO3 further controls the initial pH of the cement system and sand filler reduces shrinkage cracking. The physical, chemical and mechanical properties of the optimised MgO/SF cement system in terms of pH, setting time, microstructure, compressive strength, flexural strength and heat of hydration are reported and compared to the PC/BFS blended system currently used for waste encapsulation. The optimised MgO/SF cement and PC/BFS cement are also compared by monitoring the volume of H2 generated from binder samples containing Al metal strips. Al metal was firmly bound into MgO/SF samples and H2 gas was not detected, indicating that any corrosion reactions were very limited. It is concluded that the MgO/SF cement system has significant potential for encapsulating certain types of problematic wastes containing Al metal and may have uses in a range of other applications.