Impact of a 70°C temperature on an ordinary Portland cement paste/claystone interface: An in situ experiment

International audienceRadioactive wastes in future underground disposal sites will induce a temperature increase at the interface between the cementitious materials and the host rock. To understand the evolution of Portland cement in this environment, an in situ specific device was developed in the Underground Research Laboratory in Tournemire (France). OPC cement paste was put into contact with clayey rock under water-saturated conditions at 70°C. The initial temperature increase led to ettringite dissolution and siliceous katoite precipitation, without monosulfoaluminate formation. After one year of interaction, partial decalcification and diffuse carbonation (calcite precipitation) was observed over 800 μm in the cement paste. At the interface, a layer constituted of phillipsite (zeolite), tobermorite (well-crystallised C-S-H), and C-(A)-S-H had formed. Globally, porosity decreased at both sides of the interface. Geochemical modelling supports the experimental results, especially the coexistence of tobermorite and phillipsite at 70°C, minerals never observed before in concrete/clay interface experiments

Influence d'une température de 70°C sur la géochimie, la microstructure et la diffusion aux interfaces béton/argile : expérimentations en laboratoire, in situ et modélisation

Radioactive wastes in future deep geological disposals will generate heat and locally increase temperature in the engineered barriers and host-rock. In the French design of disposal cells, temperature may reach 70°C in cementitious materials and at their contact with the clayey host-rock. The impact of temperature under such disposal conditions is still poorly known, especially regarding the geochemical and physical evolution at the interface between these two materials.Two experimental devices are designed. The first involves creating interfaces between OPC paste and argillite of Tournemire in diffusion cells. The evolution of solutions and materials are analysed over time. The second device involves creating OPC paste / argillite interfaces at 70°C under in situ conditions in the underground laboratory of Tournemire (France). This device, more representative of a deep disposal, is dismantled after one year. Prior to interface study, behaviour of the OPC paste after a temperature increase from 20 and 70°C was analysed and simulated. Reactive transport modelling supports the experimental results in order to better understand the physico-chemical evolutions at the interface.Neoformation of tobermorite (well-crystallised C-S-H), phillipsite (only in situ), C-A-S-H and calcite formed a layer at the interface. A kinetic of tobermorite precipitation is evaluated. Significant decalcification and carbonation were noticed in the cement paste. Total porosity decreases in the cement paste despite an opening of the macroporosity due to portlandite dissolution. Argillite seems to be weakly altered even if alkaline plume goes deeply through it. Porosity changes do not alter significantly diffusive properties at the studied time scale.Dans le concept actuel du stockage géologique des déchets radioactifs en France, les interfaces entre la roche encaissante, une argilite, et les matériaux cimentaires utilisés pour les bouchons de scellement et les corps des alvéoles de stockage pourraient subir une température de 70°C due à l’activité exothermique de déchets. Les évolutions minéralogiques, microstructurales et leurs conséquences sur les propriétés de transport à ces interfaces sont mal connues dans ces conditions de température.Deux dispositifs expérimentaux sont conçus. Le premier consiste à créer des interfaces pâte de ciment CEM I / argilite de Tournemire en cellules de diffusion. La chimie des solutions est suivie dans le temps et quatre échéances permettent d’étudier l’évolution temporelle des matériaux. Le second dispositif consiste à créer de telles interfaces in situ à 70°C dans le laboratoire souterrain de Tournemire. Ce dispositif, plus représentatif des conditions de stockage, est démantelé après un an d’interaction. Au préalable, le comportement de la pâte de ciment CEM I à l’issue d’une augmentation de température de 20 à 70°C est analysé. La modélisation en transport réactif (Hytec) est utilisée en support à la compréhension des évolutions physico-chimiques.La néoformation de tobermorite, de phillipsite (in situ uniquement), de C-A-S-H et de calcite formant un ruban à l’interface est avérée. Une cinétique de précipitation de la tobermorite a ainsi pu être évaluée. La pâte de ciment est décalcifiée et carbonatée. La porosité totale diminue dans la pâte de ciment, malgré une ouverture de la macroporosité par dissolution de portlandite. L’argilite semble être peu altérée. La température accélère la diffusion, tandis que les variations de porosité et le ruban ne changent pas significativement les propriétés de diffusion sur une année

70°C impact on geochemistry, microstructure and diffusion at concrete / clay interfaces : in situ and laboratory experiments, modelling

Dans le concept actuel du stockage géologique des déchets radioactifs en France, les interfaces entre la roche encaissante, une argilite, et les matériaux cimentaires utilisés pour les bouchons de scellement et les corps des alvéoles de stockage pourraient subir une température de 70°C due à l’activité exothermique de déchets. Les évolutions minéralogiques, microstructurales et leurs conséquences sur les propriétés de transport à ces interfaces sont mal connues dans ces conditions de température.Deux dispositifs expérimentaux sont conçus. Le premier consiste à créer des interfaces pâte de ciment CEM I / argilite de Tournemire en cellules de diffusion. La chimie des solutions est suivie dans le temps et quatre échéances permettent d’étudier l’évolution temporelle des matériaux. Le second dispositif consiste à créer de telles interfaces in situ à 70°C dans le laboratoire souterrain de Tournemire. Ce dispositif, plus représentatif des conditions de stockage, est démantelé après un an d’interaction. Au préalable, le comportement de la pâte de ciment CEM I à l’issue d’une augmentation de température de 20 à 70°C est analysé. La modélisation en transport réactif (Hytec) est utilisée en support à la compréhension des évolutions physico-chimiques.La néoformation de tobermorite, de phillipsite (in situ uniquement), de C-A-S-H et de calcite formant un ruban à l’interface est avérée. Une cinétique de précipitation de la tobermorite a ainsi pu être évaluée. La pâte de ciment est décalcifiée et carbonatée. La porosité totale diminue dans la pâte de ciment, malgré une ouverture de la macroporosité par dissolution de portlandite. L’argilite semble être peu altérée. La température accélère la diffusion, tandis que les variations de porosité et le ruban ne changent pas significativement les propriétés de diffusion sur une année.Radioactive wastes in future deep geological disposals will generate heat and locally increase temperature in the engineered barriers and host-rock. In the French design of disposal cells, temperature may reach 70°C in cementitious materials and at their contact with the clayey host-rock. The impact of temperature under such disposal conditions is still poorly known, especially regarding the geochemical and physical evolution at the interface between these two materials.Two experimental devices are designed. The first involves creating interfaces between OPC paste and argillite of Tournemire in diffusion cells. The evolution of solutions and materials are analysed over time. The second device involves creating OPC paste / argillite interfaces at 70°C under in situ conditions in the underground laboratory of Tournemire (France). This device, more representative of a deep disposal, is dismantled after one year. Prior to interface study, behaviour of the OPC paste after a temperature increase from 20 and 70°C was analysed and simulated. Reactive transport modelling supports the experimental results in order to better understand the physico-chemical evolutions at the interface.Neoformation of tobermorite (well-crystallised C-S-H), phillipsite (only in situ), C-A-S-H and calcite formed a layer at the interface. A kinetic of tobermorite precipitation is evaluated. Significant decalcification and carbonation were noticed in the cement paste. Total porosity decreases in the cement paste despite an opening of the macroporosity due to portlandite dissolution. Argillite seems to be weakly altered even if alkaline plume goes deeply through it. Porosity changes do not alter significantly diffusive properties at the studied time scale

Temperature effect on the Portland cement/argillite interface – Main results gained from the CEMTEX project

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Modelling of temperature impacts on cement paste in clayey environment constrained by field experiments

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Temperature effect on CEM-I and low-pH hydrated cement pastes in a clayey environment

International audienceThe CEMTEX project in the Tournemire URL studied the temperature effect on the in situ evolution of CEM-I and low-pH cement pastes in contact with argillite after 1, 2 and 5 years, in the context of radioactive waste disposals. Reactive transport modeling was used as a tool to support the most significant experimental results, e.g. occurrence of zeolite and well-crystallized C-S-H at the interface and reduction of cement alteration depth with temperature for CEM-I, but M-S-H precipitation and increase of alteration depth with temperature for low-pH cement

Geochemical evolution of cementitious materials in contact with a clayey rock at 70 C in the Tournemire underground research laboratory

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Evolution of the argillite / CEM I interface at 70°C: in situ tests and modelling results

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Geochemistry of claystone/cement paste interface at 70 °C: an in situ experiment

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Mineralogical and microstructural evolution of Portland cement paste/argillite interfaces at 70 °C – Considerations for diffusion and porosity properties

International audienceA Portland cement (CEM I) paste was poured onto an argillite disk in diffusion cells with reservoirs filled by alkaline water and argillite pore water. The system evolution was followed over the course of 415 days. The imposed temperature of 70 °C affected the mineralogy (precipitation of crystallized C-S-H) and mechanical strength of the interface that became brittle over time. The interface consisted of a calcite/tobermorite/C-A-S-H layer, whose thickness increased at a growth rate of 0.3 μm/d. Contrary to calcite crusts that formed in immersion tests or when hardened cement was placed in contact with argillite, this layer had no significant effect on the diffusion properties during the one-year duration of the experiment due to its microporous structure and rather small thickness (100 μm). The argillite mineralogy was altered over 100 μm. In the cement paste, the total porosity decreased because carbonation was enhanced with temperature, which counterbalanced the effect of decalcification over 400 μm