TY - CONF A1 - Gluth, Gregor A1 - Henning, Ricky A1 - Sturm, Patrick A1 - Keßler, S. T1 - Chemical resistance of hybrid cements against aggressive saline solutions N2 - The safe storage of radioactive material in nuclear waste repositories is a major task of our societies. The sealing function of such repositories is depending on the ability of the applied construction materials to form and maintain a geotechnical barrier with the surrounding rock. Corrosion of the construction material can lead to leakage of radioactive compounds and must therefore be avoided or minimized. Since concretes based on conventional Portland cement have not been found suitable in this context, alternative binders must be evaluated. One such class of binders are hybrid cements, which are blends of low fractions of Portland cement or Portland clinker and high fractions of supplementary cementitious materials, such as blast furnace slag and fly ash, activated by an alkali salt. Besides a low heat of hydration and a sufficient early compressive strength [1], chemical similarities with ancient Roman concrete suggest an excellent durability in saline environments [2], which makes these cements potentially suitable for applications in nuclear waste repositories in evaporite rock, such as the Morsleben repository in Germany. In the present study, two previously designed and characterized [3] hybrid cements, an alkali-activated slag/fly ash blend, and an OPC paste were studied regarding their resistance against corrosion in an aggressive saline solution. The saline solution was designed by the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) [Association for Facility and Reactor Safety] as a reference representing a solution forming as the result of contact of surface water with evaporite rock. The cement pastes were exposed to the saline solution up to 70 days and characterized by X-ray diffraction, thermogravimetric analysis and spatially resolved X-ray fluorescence spectroscopy. In addition, thermodynamic modelling was performed to simulate the alterations of the phase assemblage with increasing exposure to the saline solution and, thus, provide indications on the long-term durability of the cement pastes. The experimental results revealed a correlation between the Portland clinker content of the cements and the resistance of the cement pastes against attack by the saline solution. This outcome was related to the formation of portlandite when sufficient clinker was available, which maintained the pore solution pH at ~12.5, and thus prevented the dissolution of cementitious phases. Once portlandite was consumed, C-N-A-S-H and ettringite dissolved and released calcium, aluminium, and hydroxide ions in the solution, maintaining the pH at ~10. In this pH range, the formation Cl-AFm phases was observed. At lower pH values, i.e., extended exposure durations, gypsum was the major corrosion product. The pH-dependent dissolution and formation of phases lead to pronounced zonation in the exposed cement pastes. Thermodynamic modelling indicated that the dissolved silicon from C-N-A-S-H reacts with magnesium ions in the saline solution to form M-S-H, and that long-term exposure eventually leads to a material rich in amorphous silica and brucite. T2 - Joint 6th International Workshop on Mechanisms and Modelling of Waste / Cement Interactions & EURAD - WP CORI Workshop CY - Prague, Czech Republic DA - 20.11.2023 KW - Hybrid cement KW - Corrosion KW - Salt solution KW - Magnesium chloride KW - Evaporite rock PY - 2023 AN - OPUS4-58885 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -