Chemical and Mineralogical Changes at the Interface between Cementitious Materials and Groundwater

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

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear waste repositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rock bolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremely important to understand the long-term material behaviour in repository environments. A reference concrete and mortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed and characterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges, SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strength tests). The reference concrete enabled comparison of results from different institutes and experimental techniques, unifying the individual results to more comprehensive body. The Cebama reference concrete and mortar were designed to have high durability and compatible formulation with respect to an engineered barrier system in clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This work presents main results regarding their characterization and alteration in contact with representative waters present in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. The main hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of 0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearing solid phases were identified. Connected porosity and pore size distribution was characterized by MIP observing that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to the nanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa, corresponding to traditional high-performance concrete. Degradation of these materials in contact with different waters mainly produce their decalcification and enrichment in Mg for waters containing high amount of this element, like the clay waters.•The performance of “low-pH” mix designs containing slag exceeds the performance of traditional Portland cement concretes.•That slag is able to replace fly ash when producing ”low-pH” mixtures, if needed due to material supply or quality problems.•Eight experimental techniques have provided mechanical, hydraulic and geochemical properties of new designed materials.•Micro-mortar in contact with clay and bentonite waters, shows a Mg enrichment and calcium depletion at the reaction front

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear wasterepositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rockbolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremelyimportant to understand the long-term material behaviour in repository environments. A reference concrete andmortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed andcharacterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges,SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strengthtests). The reference concrete enabled comparison of results from different institutes and experimental techniques,unifying the individual results to more comprehensive body. The Cebama reference concrete and mortarwere designed to have high durability and compatible formulation with respect to an engineered barrier systemin clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This workpresents main results regarding their characterization and alteration in contact with representative waterspresent in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. Themain hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearingsolid phases were identified. Connected porosity and pore size distribution was characterized by MIPobserving that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to thenanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa,corresponding to traditional high-performance concrete. Degradation of these materials in contact with differentwaters mainly produce their decalcification and enrichment in Mg for waters containing high amount of thiselement, like the clay waters

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear waste repositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rock bolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremely important to understand the long-term material behaviour in repository environments. A reference concrete and mortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed and characterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges, SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strength tests). The reference concrete enabled comparison of results from different institutes and experimental techniques, unifying the individual results to more comprehensive body. The Cebama reference concrete and mortar were designed to have high durability and compatible formulation with respect to an engineered barrier system in clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This work presents main results regarding their characterization and alteration in contact with representative waters present in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. The main hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of 0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearing solid phases were identified. Connected porosity and pore size distribution was characterized by MIP observing that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to the nanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa, corresponding to traditional high-performance concrete. Degradation of these materials in contact with different waters mainly produce their decalcification and enrichment in Mg for waters containing high amount of this element, like the clay waters.</p

Cebama reference mix design for low-pH concrete and paste, preliminary characterisation

Trabajo presentado al Second Workshop of the HORIZON 2020 CEBAMA Project, celebrado en Espoo (Finlandia), del 16 al 18 de mayo de 2017.A reference low-pH concrete and paste mix were manufactured within the Cebama-project. Reference mixtures were casted at VTT in March 2016 and were distributed among some of the Cebama partners. Reference mix designs will be used by different partners as a common material to study their behaviour in contact with waters of different composition and interaction with radionuclides. Additionally, these materials will be used for model calibration. This article provides a summary of the characterization methods used by different partners and gives an overview of the experiments which will be made in the future with the reference materials.The research leading to these results has received funding from the European Union's Horizon 2020 Reasearch and Training Programme of the European Atomic Energy Community (EURATOM) (H2020-NFRP-2014/2015) under grant agreement n° 662147 (CEBAMA)