Chemo-mechanical study of cement paste degradation subjected to external weak sulfate attacks in geological nuclear waste disposals
Résumé
This study assesses the containment durability of cementitiousmaterial in the context of deep geological nuclear wasterepositories. Concrete would be used as a buffer material locatedaround steel waste packages or as a mechanical support ofsealing systems. The geological groundwater in contact withwaste packages or sealings contains sulfate ions in low quantitieswhich may diffuse in concrete porous media, leading to externalweak sulfate attacks. Low concentration sulfate attacks arecharacterized by the precipitation of secondary ettringite andgypsum that affects chemical and microstructural properties. Asignificant precipitation of these minerals in the porous mediaeventually leads to both swelling and cracking by differentialstrain mechanisms. The novelty of this work is to study thecoupled chemo-mechanical evolution of CEM I cement pastesunder such conditions, by combining mineralogical (DRX, SEMEDS, microtomography, autoradiography) and mechanicalanalyses (microindentation) with reactive transport andmechanical modeling.The experimental setup consisted in immersing CEM Ihydrated cement pastes (w/c = 0.5) in a low-concentrationsodium sulfate solution (30 mmol/L), representative of realisticservice conditions, which were removed at specific timeframesfor analysis (from 15 days to 6 months). Different C3A ratioswere used to boost either ettringite or gypsum formation duringthe chemical attack. An initial crack was also generated in onesample to estimate whether mineral precipitation in cracks mayaccelerate its propagation. Results showed sample decalcificationcontributing to the weakening of the material. Furthermore,apparition of cracks located in the calcium-depleted area wasrevealed, induced by swelling of the paste due to secondaryettringite and gypsum formation.Chemical degradation was modeled with the reactive transportcode HYTEC to estimate mineralogy and porosity evolution ofsamples over time. These modeling results were then used toestimate the local evolution of mechanical properties byanalytical homogenization techniques. The calculated chemicalfronts and Young Modulus values fitted well with experimentalresults, highlighting a decrease of the material mechanicalproperties related to the paste decalcification. HYTEC modelingwas then applied to the pre-cracked sample, which corroboratedthe enhanced propagation of the sulfate attack along the crack.
