Abstract
Interaction between groundwater and cement is important to understand for the purpose of radioactive waste disposal within geological disposal facilities. This interaction causes chemical and mineralogical changes to both phases of the system. The spatial and temporal alteration was studied with the use of diffusion experiments. Four cement blends, CEMI, GGBS:OPC, NRVB and CEBAMA, were used in the form of cured cement monoliths and placed in contact with three synthetic groundwaters that emulate granitic, clay and saline host rock environments. Different solid to liquid ratio experiments were set-up; one with an excess of groundwater compared to the volume of cement and the other with equal volumes of groundwater and cement. Solution analysis, pH measurements, ion chromatography, microwave plasma-atomic emission spectroscopy and inductively coupled plasma-mass spectrometry, were used to determine the composition of the groundwaters following interaction with the cement. To complement the solution data, solid characterisation was completed on the cement blocks to establish alteration to crystalline phases (X-ray diffraction analysis) and the uptake and leaching of groundwater analytes within the cement block (scanning electron microscopy-energy dispersive spectroscopy).
From these experiments, it was identified that the cements altered the groundwater compositions differently. One such example is the higher alkaline pH of groundwater achieved following interaction with CEMI and NRVB blends (greater than pH 12) compared with GGBS:OPC and CEBAMA blends (between pH 10 and 11.5). The greatest enrichment of sodium, potassium and calcium within the three groundwaters was observed following interaction with CEMI and NRVB blends. Leaching of sodium, potassium and calcium hydroxides from the cement was the cause of the concentration increase. Uptake of chloride and sulphate by CEMI was reported following interaction with clay and saline groundwater, with the formation of Friedel’s salt and ettringite identified within the XRD patterns. Conversely, enrichment of sulphate within groundwaters that had interacted with GGBS:OPC and CEBAMA blends was identified with the use of ion chromatography. The data obtained showed sulphate enrichment within granitic groundwater up to 400 % following interaction with GGBS:OPC. The enrichment is based on the greater sulphur content within the starting composition for these blends. Carbonation occurred within the cement-groundwater systems, as a calcium carbonate layer was identified on the surface of the cement blocks with the use of SEM-EDS. Calcium carbonate precipitate was also recovered from CEMI and NRVB experimental containers, independent of the groundwater used for interaction. Solid characterisation of the cement blocks identified that there was uptake and depletion of the groundwater ions within the cement. Significant changes include the leaching of calcium from CEMI and NRVB and the magnesium uptake by GGBS:OPC and CEBAMA reference blend observed by SEM-EDS. Overall, the combinations of cement and groundwater will need to be considered depending on the cement materials used and the host rock that the geological disposal facility will be placed in.