Microstructure Modeling Of Cementitious Material Subjected To External Sulfate Attack

Sulfate attack has been considered as one of the major concerns that threatens the durability of cemen-titious materials. Previous experimental studies have made fruitful improvement on understanding the ex-pansion mechanisms happened during sulfate attack, and theoretical calculations have also promoted the predictions on the overall expansion and degradation of cement, mortar and concrete on a macro or meso scale. However, few of literatures have addressed the simulation of sulfate attack on the microstructure lev-el, which is of significance on understanding how these materials are degrading fundamentally. Therefore, this study focuses on the microstructure modelling of cement paste, aiming to simulate how cement hydrates change chemically and the consequent microstructure evolution. The other objective of this thesis is to link the macro properties and the microstructure details of cement paste so that the former can be predicted and insights of the sulfate attack mechanisms can be proposed by this model as well.Sulfate attack can be regarded as being attacked by sulfate solution with low pH values. Therefore, this thesis starts with a 3D microstructure model which simulates microstructure evolution when it is flushed with water of low pH but without any sulfate concentration. The novel aspect of the model enables it to predict not only the well-known phase instability of calcium hydroxide at the onset of leaching, but also the detailed compositional and volumetric changes of C-S-H gel and other calcium, alumianate and sulfate phases. Be-sides tracking the compositional and microstructural changes, we use the evolving microstructure as input to calculate changes in the relative diffusivity and effective Young’s modulus of the binder using established finite difference and finite element models. The results are broadly consistent with previous experimental and modeling investigations of leaching.To have a better understanding of the stability of expansive product (AFt) and chemical reactant (AFm) during sulfate attack, a relatively simple Ca-Al-S-O system is examined using thermodynamic equi-librium calculations. The equilibrium phase assemblages that have been predicted in previous studies at 25℃ are revisited. Phase diagrams for equilibrium in aqueous solutions are constructed using Gibbs free energy minimization as implemented in GEM-Selektor software coupled to the Nagra-PSI thermodynamic database, with no other constraints placed on phase stability. The results agree with recent modeling and experimental studies when the same conditions are used. The influences of alkali ions on the stability of AFm and AFt are then examined and found to significantly shift the regions of phase stability in composition space. Calculated effects of hydrogarnet solubility on AFm stability are discussed to resolve seemingly contradictory reports in the literature on the stability of calcium monosulfoaluminate. Finally, the calculations indicate that SO4- rich and OH-rich AFm phases have well-defined pH ranges of stability that are functions of the carbonate concentration, with carbonates being particularly effective in lower pH environments for stabilizing AFt and reducing the amounts of all AFm phases except the carbonate-based forms.Later on, the microstructre model is coupled with linear thermoelastic finite element model to predic localized deformaion and the onset of damage caused by ettringite grown in confined places with misfit ex-pansion strain and it is applied to simulate near-surface portland cement paste degradation subjected to a sodium sulfate solution. The results show that constrained ettringite growth happens prmiarily at the expense of calcium monosulfoalumiante, carboaluminate and aluminum-rich hydrotalcite, if any, respectively. Ex-pansion and damage can be mitigated by chemically increasing carbonate and magnesium concentrations and by a finer dispersion of monosulfate microstructurally.Finally, a 1D simultaneous diffusion-chemical reaction model is developed. This model first time explic-itly incorporates the estimation of local volume deformation based on crystallization theory. By the coupling of linear thermoelastic finite element model again, it enables the prediction of stress and strain field and the subsequent onset of damage. With the microstructure model, the elastic properties and transport properties are calculated directly and locally for all the microstructures at different depths. Then the updated concentration profiles for ions of interest are estimated on a higher length scale by using full finite difference model without any homogenisation process. The mechanisms of sulfate attack is disscussed in the end by seperating the influences of sulfate concentration and the reduction of pH. It is found that when cement paste is immersed in a low sulfate sodium solution, the overall expansion of cement paste is initialized by the reduction of pH, rather than the increase of sulfate concentration.