Characterisation of Hardened Cements Incorporating Simulant Intermediate Level Waste

Blended cement grouts are used in the UK for the encapsulation of various intermediate level nuclear wastes (ILW). To ensure the long term confidence in the durability of these grouts detailed knowledge of the exact nature of the hardened cement and its interaction with the waste material is essential. This PhD project aims to characterise various aged blended cement grouts utilised in the encapsulation of radioactive wastes. The grouts have been modified by the incorporation of simulant wastes produced to allow comparison with current in-service waste formulations. The grouts were analysed using a multi-technique approach to assess the reactions which have occurred within the cement matrix and their significance for successful encapsulation of the waste. The samples underwent investigation after long-term storage using a variety of techniques including thermal analysis, X-ray diffraction, electron microscopy and nuclear magnetic resonance spectroscopy. This suite of techniques allowed for the identification of various crystalline and amorphous phases produced during the hydration process and thereafter. Comparison was then made with similar samples containing no simulant waste to contrast the findings and possible effects upon the cement matrix and understand the implications for long-term storage. The blended cement samples showed a varying level of modification due to the inclusion of the simulant wastes. It was shown a major controlling factor was the relative solubility of the waste-form. A magnesium corrosion based simulant showed very little reaction in cement matrix, however aluminium based simulants typically displayed a wide range of reactions. Waste carbonate systems also displayed a high level of reaction, with the formation of new phases causing the destabilisation of AFt and encapsulation of the carbonate waste within AFm. The requirement for detailed characterisation of the C-S-H phase was confirmed with substantial changes being observed for samples containing waste flocs. Analysis of this phase proposed a high degree of substitution into the silicate chain lengths with the likely incorporation of additional metal cations into the structure via charge balancing. It was found all samples showed characteristics supporting the potential successful physical encapsulation of radioactive wastes. In addition to this various chemical immobilisation process have been studied, confirmed or proposed.