The role and management of free water in the production of durable radioactive waste products using hydraulic cements

Water is a necessary component in the production of encapsulated wastes based on hydraulic cements which are widely used for immobilization of intermediate and low level waste (ILW) and (LLW). Apart from providing the fluidity required to readily transport slurry wastes, it plays an essential role in hydrating the cement. Too low a water content prevents homogeneous mixing of the cement binder and waste and does not provide the fluidity needed for effective infilling of solid wastes. The water left after hydration creates a porous network that allows egress of gaseous corrosion/radiolytic degradation products such as hydrogen. A broad envelope (i.e. range) of acceptable water/binder ratios is essential for effective process control, particularly for the encapsulation of slurry wastes which have widely varying water contents. Nevertheless, the presence of large amounts of free water in the pore system of the hardened matrix allows easy transport of soluble ions such as hydroxide, which can lead to metal corrosion, and the increased permeability of the system increases the teachability. Therefore effective management of the 'free' water content of a waste product will allow optimisation of both the encapsulation process and the product quality and durability. This paper describes a range of innovative approaches to 'water management', including the use of alternative hydraulic cements, modification of powder characteristics and use of super-plasticised composite OPC grouts and examines the contribution of 1H NMR relaxometry in providing improved understanding of the distribution of water within the pores of the hardened cement matrix. (authors)