Effects of carbonation and calcium leaching on microstructure and transport properties of cement pastes

During the past decades, a significant progress has been made in the development of concrete materials. With the development of high performance and self-compacting concrete, the design of concrete structures for hundred-year service life becomes possible. In recent years, concrete has been considered as a useful material for facilities with extremely long-term service life such as radioactive waste repositories. Therefore, the assessment of the long-term performance of such concrete structures is of utmost importance. Within its service environment, these structures undergo chemical degradation processes which are very slow but they significantly change the physical integrity (e.g. transport and mechanical properties) and the chemical condition (e.g. pH) of the structures in the long-term. Chemical degradation is typically the result of alteration of the cement matrix mineralogy caused by the interaction with environmental conditions. The interaction disturbs the equilibrium between the pore solution of the cementitious materials and the solid phases of the cement matrix which results in dissolution and/or precipitation of minerals. The chemical degradation of cementitious materials is mostly followed by alteration of the microstructure and, thereby, transport properties. The transport properties such as permeability and diffusivity are the key parameters to evaluate whether the concrete still retains its function as a barrier against the transport of radionuclides and other hazardous products out of the disposal system. Although a lot of effort has been spent on studies concerning the use of cement-based materials in such structures, the evolution of the microstructure and transport properties under chemical degradation over long time periods (up to thousands of years) is still unclear due to the limited experimental timeframe available to capture these processes. This thesis presents a comprehensive study of the consequences of exposure of cementitious materials to carbonation and calcium leaching. Due to the extremely slow nature of these degradation processes, accelerated methods are needed to reach a certain degradation stage in order to study the behaviour of concrete representative for the long-term. In the present work, an ammonium nitrate solution was used to accelerate the Ca-leaching degradation kinetics, while pure CO2 at high pressure was applied to speed up the carbonation. The changes in permeability and diffusivity of the degraded materials were measured by novel methods. Microstructural and mineralogical alterations were qualitatively and quantitatively examined by a variety of complementary techniques including SEM/SEM-EDX, MIP, TGA, N2-adsorption, XRD/QXRD and ion chromatography. The experiments were performed on cement pastes with different water/powder ratios and limestone filler replacement. In parallel, phenomenological models were also developed to better understand the transient state of degradation and to predict the evolution of the cementitious materials during the degrading processes, which is difficult to capture with experiments. By using the accelerated techniques, it was able to obtain degraded mateials which are representative for long-term degradation states and as such it allowed for studying the microstructure and hence its relation to the transport properties. The novel methods to measure transport properties proposed in this study are promising in terms of the required experimental time, the control of parameters (pressure, flow, concentration) and reliability. Moe importantly, the techniques allowed for a convenient capturing of the changes in transport properties of the degraded materials, thanks to their high compatibility with the accelerated degradation techiques. Results showed that leaching and carbonation significantly changed the microstructure and transport properties of cementitious materials but in different manners. The leaching significantly altered the microstructure of the cement paste to a material with a higher specific surface area, increased total porosity and a shift to larger pore sizes resulting in a significant increase in transport properties depending on the degradation state. In contrast, carbonation led to a porosity reduction, shift of pore size distribution to smaller ranges and lower permeability and diffusivity. However, both leaching and carbonation processes induced a lower pH with possible loss of beneficial condition for waste package integrity (e.g. rebar corrosion). The phenomenological models developed enabled us to simulate and predict the evolution of the microstructure and related transport properties (permeability, diffusivity). In combination with accelerated experiments, it provides us the possibility to assess the long-term performance of cement-based materials used in disposal systems.