Understanding the long-term evolution of C-S-H phases present in cement backfills

In the UK, cement grouts are used to encapsulate some intermediate level radioactive wastes (ILW) and as a potential backfill around waste packages in a Geological Disposal Facility (GDF). The main binding phase in these cementitious backfills is calcium silicate hydrate (C S H) which changes with time. Hence, to ensure the long-term performance of these backfills, it is essential to study how the main phases in the hardened cement evolve with time. This PhD project aims to characterize various backfill materials and then to synthesize and characterize the analogues of the main binding C S H present in the backfill material. This study also investigates the structural and morphological evolution of C-S-H phases due to leaching and hydrothermal ageing. Single phase synthetic C-S-H phases without the use of C3S or alkalis are synthesized via various methods with high Ca/Si ratios in the range 1.5 – 2.0 and analysed using techniques like NMR, TEM – EDX, STA and XRD to understand the morphology, chemical composition and silicate structure. The results obtained indicate that the C-S-H morphology of samples synthesized via silica – lime reactions with Ca/Si of ~ 2.0 is fibrous. The morphology of C-S-H in samples synthesized by constant lime concentration method depends on the lime concentration in the solution. As the lime concentration is increased, the morphology is found to change from foil-like to fibrous with Ca/Si ratios increasing from 1.5 – 1.8. On artificial ageing (leaching and hydrothermal ageing) of these high Ca/Si ratio C S H phases, the morphology, composition and silicate structure changed as a result of degradation of the C-S-H phase.