Numerical Analysis of Mineralogical and Mechanical Deterioration of Cementitious Materials under Various Leaching and External Sulfate Attack

Abstract

The excellent performance of cementitious materials in normal service environment has long been recognised. However, cementitious materials may suffer from serious deterioration when subjected to aggressive conditions. In this thesis, the material deterioration under two of the most notorious durability issues, i.e. leaching and external sulfate attack (ESA), are investigated. The research background and aims are introduced first, followed by a state-of-the-art review on the modelling of the leaching and ESA. Both analytical and numerical methods are developed in this thesis. To carry out the simulations, a module-oriented numerical framework is established, which includes three main modules, i.e. an ionic transportation module, a chemical reaction module and a damage evaluation module. Since the chemical degradations on cementitious materials are mainly diffusion-controlled, a mixed effective medium theory (MEMT) is first developed to estimate the materials diffusivity. The MEMT integrates the advantages of the conventional EMTs, and the influences of both inclusion geometry and finite capillary connectivity on the effective diffusivity are considered. Employing the numerical framework, the deterioration under the combined leaching and ESA is then modelled. A modified Poisson-Nernst-Planck (MPNP) model is developed to describe the ionic transportation in the cracked materials. Moreover, a simplified method of modelling the decalcification of the C-S-H as multiple phases in equilibrium is proposed. The module-oriented method is further developed in the modelling of accelerated leaching. A novel method of coupling chemical kinetics and chemical thermodynamics is developed to model the complex chemical system under the accelerated leaching. By studying the detailed mechanism of the accelerated leaching, the nature for the widely scattered acceleration factor is revealed, and a simple linear relation for predicting the acceleration factor is proposed. Furthermore, the deterioration of flexural strength of cement mortar under the ESA is investigated. An analytical model is developed for estimating the flexural strength for a medium undergoing internal pressure, which simulates the case of expansion caused by precipitations of sulfate-bearing hydrates during the ESA. The significant influence of the internal pressure on the deterioration of flexural strength during the ESA is also discussed.