Samples of aged Nirex Reference Vault Backfill (NRVB) were investigated to study the nature and effects of leaching and hydrothermal treatment on C-A-S-H phases. NRVB is a potential backfill material in the UK, that is composed of CEM I, limestone, hydrated lime and water with w/s ratio of 0.61. The C-A-S-H present in NRVB aged for between 1 and 16 years had high Ca/Si ratio and fine-fibrillar morphology in the outer product region. Leaching resulted in: removal of portlandite and monocarboaluminate; reduced Ca/Si ratio of the C-A-S-H together with a change from fibrillar to foil-like morphology and lengthening of the aluminosilicate chains; followed by destruction of some of the C-A-S-H with the formation of a low-Ca aluminosilicate gel. C-A-S-H(I) was formed upon hydrothermal treatment of a leached sample
