Cementitious Grouts for ILW Encapsulation – Composition, Hydration and Performance.

The preferred route to treatment of the vast majority of intermediate level nuclear waste (ILW) within the UK is via encapsulation within a composite cement system. The integrity of these conditioned waste packages must be maintained for hundreds to thousands of years since they will eventually be stored deep below ground in a geological disposal facility (GDF), with this expected to be a permanent route to disposal. A thorough and clear understanding of the hydration, microstructural development, and hence performance, of grouting materials is essential in providing confidence in the suitability of the technology and ensuring that structural integrity is maintained. This project comes at a time of significant uncertainty for the cement industry, as well as the steel industry which has significant ramifications on the availability of blastfurnace slag (BFS, hereafter referred to as slag). Through quantifying the ramifications of changes to supply of either of these materials it will be possible to determine the resilience of the technique to chemical and physical variations, in an effort to futureproof supply. Within this study, grouts prepared with slag and ordinary Portland cement (OPC), at a ratio of 3:1 slag: OPC at a water to binder ratio (w/b) of 0.35, were analysed at 20°C. The impact of OPC composition, slag composition and slag fineness on rate and degree of hydration were assessed. Microstructural development was followed by a number of techniques in 2 and 3 dimensions, with the engineering performance of samples also quantified via a range of testing protocols. Resilience to potential fire scenarios was also investigated through simulated heat-testing of samples and subsequent analysis. Slag fineness is the most significant factor in controlling rate and degree of its hydration within both young and mature pastes at these high replacement levels. Availability of pore space into which hydrates may grow appears to the limiting factor in continued hydration; significant quantities of CH remain after 1 year of hydration, intermixed with C-S-H in a densely filled microstructure.