title: Phase-Field Modeling for Self-Healing of Mineral-Based Materials
Creators: name: Yang, Sha
Publisher: name: UNSPECIFIED
Year: 2022

description: Concrete is the most widely used building material in the world. The low raw materials cost, its high compressive strength and the simplicity of the production process makes it an enormous attractive and easy to apply material for the construction and building sector. However, when applied, concrete suffers from cracks, which are inevitable and are the result of various environmental and loading impacts such as traffic load, freeze-thaw cycles, but it also depends on the construction quality. These cracks provide harmful elements such as chloride, carbon dioxide or sulphur ions a pathway, which may induce steel corrosion of reinforced concrete structures. It is a mechanism that will seriously threaten the service life of a concrete structure, while causing significant maintenance costs. Mitigating this phenomenon has led to a worldwide development on self-healing methods for crack closure. In the last few years, research efforts on self-healing methods have mainly concentrated on experimental work, where only a limited number of numerical models have been reported in literature. These models treat the boundaries, i.e. interfaces, between different the surfaces of components with a zero thickness. In fact, such interface describes the kinetics of a phase transformation from a non-equilibrium to an equilibrium state. This problem requires the diffusion equations to be solved at the interface under moving boundary conditions, which, although feasible for the evolution of simple geometries, becomes rather impossible for higher-dimensional systems and/or complicated interfaces. For a more accurate description of the above problem, this PhD study presents a novel approach for self-healing of cementitious materials by means of a phase-field (PF) method. Unlike the traditional sharp interface models, a PF method provides a convenient way to numerically deal with free moving boundaries, where the interface is implicitly expressed as a time- and space-dependent function, representing the phase state, and is defined over the entire domain. In this work, the diffusion-controlled isotropic dissolution of minerals is first investigated from a mesoscale phase transition point of view. Based on earlier formulations by Kim and co-workers [1], an expression of interface mobility under diffusion-controlled conditions is proposed. Using sodium chloride dissolution as an example, the results of their PF method are compared with that of analytical models and experiments, while extending the application of a PF method to the field of mineral dissolution. Based on this, the evolution of a carbonation front, which separates the dissolution zone from the carbonation fraction, is modelled on a thermodynamic basis, while mimicking the self-healing carbonation reaction in cementitious materials. Physical-chemical aspects are used to construct the free energy functions for incorporating dissolution and precipitation systems. Moreover, the dissolution model determines the local concentration fields of the active species in the PF. The model parameters were experimentally calibrated on a single mineral, i.e. the carbonation of calcium hydroxide. As a novel feature, the evolution of multiple interfaces is investigated and demonstrated by an experimental case of self-healing with calcium hydroxide carbonation. Good qualitative agreement was achieved between the model results and the experimental data and the evolution of the crack morphology was demonstrated. This PhD study showed the potential of a PF method as a predictive tool to estimate self-healing in cementitious materials.