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The geologic disposal of radioactive waste could lead to confined conditions in which cementitious materials impose a moderate alkaline pH, in which the rocks supply sulfate ions at rather low concentrations. In this context, the purpose of this work was to study the degradation of cement pastes under such conditions using a non-renewed 30 mmol/L Na2SO4 solution without pH regulation. Samples were investigated through laboratory testing and numerical modeling. XRD, SEM–EDS, and micro-indentation acquisitions were performed to follow the evolution of chemical, mineralogical, and mechanical properties during the weak external sulfate attack. Reactive transport modeling was performed with the HYTEC code. Based on these results, the Young's moduli of the degraded zones were estimated using analytical homogenization. Decalcification occurred despite the high pH value of the solution and significantly affected the mechanical properties. Macroscopic swelling and cracking were caused by the formation of expansive sulfate minerals after 60 days, despite the low sulfate content of the tank solution. The modeling supported the discussion on the evolution of the mineral fronts (ettringite, portlandite, and gypsum). [ABSTRACT FROM AUTHOR]

Durability of concrete exposed to an external sulfate attack is a great concern for the long-term reliability of nuclear waste containment. This paper presents experimental results obtained on cement paste subjected to a low concentration (30 ×10−3 mol/L) external sulfate attack. Samples were characterized with complementary methods (XRD, SEM-EDS, microtomography, microindentation, autoradiography) to follow the evolution of chemical, mineralogical, microstructural and mechanical properties during 8 months. Cement pastes of different C 3 A contents were used to boost either ettringite or gypsum formation during the sulfate attack to assess their respective impact on the degradation. Cracks parallel to the attacked surface occurred in the zone of gypsum formation and portlandite dissolution, which suggests that gypsum has a key effect on the expansion. Reactive transport modeling supported the discussion on the competition between hydrolysis, gypsum and ettringite precipitation as well as the analytical homogenization to estimate Young's moduli. • Multiple characterization tools were used including a synchrotron radiation CT. • Contributions of hydrolysis and sulfate attack on damage were discriminated. • Expansion occurred even for low C3A content and groundwater SO 4 concentration. • Cracking was initiated in the gypsum formation zone. • Ettringite formation was identified by microindentation and homogenization. [ABSTRACT FROM AUTHOR]