Geological disposal is the main preferable option for the long-term management of radioactive waste by many countries, including the United Kingdom (UK). A Geological Disposal Facility (GDF) is based on a multi-barrier concept, which utilises a series of engineered barriers that provide physical and chemical containment for radioactive waste, mitigating the potential for radionuclide release to the geoand bio-spheres. Cementitious materials are used for many different parts of the multi-barrier GDF, having a wide range of purposes, for example as waste encapsulate, as backfill or as a sealant. Therefore, understanding the mineralogical evolutions of these cementitious materials, especially when in contact with groundwater, is key for the development of a robust safety case. In this Thesis, investigation of the main mineralogical and microstructure characteristics of two different cementitious materials, a high-pH cement considered to be used as a backfill material in one of the UK GDF conceptual scenarios for the disposal of intermediate level waste (Nirex Reference Vault Backfill, NRVB), and a low-pH cement considered to use by many European countries (called Cebama reference cement), was performed. Extended experiments were also carried out to help develop a detailed understanding of the interactions of these cements with three different groundwater compositions (granitic, saline and clay). Long-term evolution of the phase assemblage of both cements showed that hydration of NRVB was completed within 2 months, whereas for the Cebama reference cement paste, hydration was incomplete even after 1.5 years of curing. Differences between the two cementitious materials were also observed in terms of phase assemblage. Additionally, NRVB presented a higher porosity when compared to the Cebama reference cement paste, which was found to be a very dense material with very low porosity. The work presented in this Thesis demonstrates key differences between the two cementitious materials when in contact with groundwater. In the case of NRVB, the pH of the solutions was mainly buffered through the dissolution of portlandite, C-S-H and ettringite. Moreover, precipitation of secondary phases was observed due to interactions with the different groundwaters, affecting the overall porosity of NRVB. In the Cebama reference cement paste, the edge of the sample was the main area affected by the groundwater interactions. In this degraded area, decalcification of C-S-H and ettringite was observed, being these hydrate phases the main responsible for the buffering of the solutions. In addition, the formation of a protective layer was observed when carbonates were present in the groundwater composition, reducing in this way further groundwater interactions.