The coupled chemo-mechanical degradation of cement-based materials

Abstract

The thesis is devoted to study the degradation mechanisms of cement-based materials in contact with aggressive aqueous solutions, including the process of chemical reactions between cement constituents and pore solution with intruded ion spices, the subsequent variation of elastic moduli due to constituents transformation and the overall mechanical behaviour based on the nonlinearity of chemically degraded material. The proposed methodology presents the integrated solution for cement-based materials under requirements of various serving conditions, long duration lifetime and multi failure criteria. The reactive transport model is employed to reproduce the dissolution and precipitation of cement constituents with thermodynamic equilibrium and kinetic laws. The Mori-Tanaka micromechanical model accommodates solid phases of cement-based materials with micromechanical behaviour and evaluate the elastic properties of the material in the present study. The post-peak behaviour of the material due to microcracks propagation is captured with the displacement based non-local damage model, and subsequently the overall flexural performance. The process of chemical degradation of ordinary Portland cement (OPC) is predominated by the portlandite dissolution and calcium silicate hydrate (C-S-H) decalcification, and the diffusion-controlled progress is slow in real service conditions. The long-term performance of cement-based materials exposed to aqueous solutions is simulated up to 1000 years by reactive transport model with the calibrated parameters. The chemical reactions between cement constituents and diffused ion species result in porosity change, which is adopted to determine the relation of the corrosion depth versus square root of time. The corrosion rate can be reduced by the precipitation of calcium carbonate in cement matrix exposed to carbonate ions enriched solutions. The dissolution and precipitation of cement constituents render the variation of material microstructure, and subsequently the relevant change of elasticity. The Mori-Tanaka scheme is employed to evaluate the elastic moduli of chemically degraded cement-based materials, which can cause the microcracks under external loading due to the stiffness reduction. The propagation of mechanical damage is evaluated by the non-local continuum damage model, and converted into porosity change of the material, which in turn promotes the progress of chemical degradation. The fully coupled chemo-mechanical simulation is able to demonstrate the instantaneous interaction between chemical degradation and mechanical damage, which can result in marked differences with the non-coupled case. The calcium carbonate precipitation is a common phenomenon in cement-based materials, and plays an important role in chemo-mechanical degradation. Magnesium ions can be incorporated into the amorphous calcium carbonate reducing its solubility. The formation of amorphous calcium magnesium carbonate is observed on the surface of cement-based materials exposed to aqueous solution rich in magnesium, of which the sealing effect is demonstrated by both experiments and numerical simulations. The calcium carbonate layer forms within cement matrix without the presence of magnesium in the leaching solutions, and can cease the progress of chemical degradation by clogging the pore space. The interaction of pH and CO₂ concentration causes the dissolution/re-precipitation of calcium carbonate, presenting a shifting calcium carbonate layer towards the interior of the material. The influences of cement constituents transformation, including the precipitated calcium carbonate, is evaluated by the aforementioned methodology, which is subsequently applied to investigate the overall mechanical performance. The proposed methodology accommodates the dissolution/precipitation of cement constituents and relevant mechanical behaviour, is suggested to be applied to other chemo-mechanical related projects in future studies.