Z. Xu | TU Delft Repository

An experimental and numerical study of alkali-activated fly ash paste

From dissolution kinetics to microstructure formation

Journal article (2025) - Yun Chen, Jiayi Chen, Mayank Gupta, Xuhui Liang, Luiz Miranda de Lima, Zhiyuan Xu, Yibing Zuo, Suhong Yin, Qijun Yu, Guang Ye

This study presents an extended numerical approach based on GeoMicro3D to simulate the reaction kinetics and three-dimensional (3D) microstructure evolution of alkali-activated fly ash (AAFA). Dissolution experiments were conducted under varying NaOH concentrations and temperatures to formulate predictive rate functions for Si and Al release. These experimentally derived kinetic functions, alongside a thermodynamic dataset for N-(C-)A-S-H gels, were incorporated into the GeoMicro3D model to capture the chemical reactions and 3D microstructure evolution of AAFA. The model well captured reaction degree of fly ash, formation of solid products, evolution of pore solution compositions, and porosity over time. Notably, it is the first to predict the time-dependent spatial distribution of phases within the 3D AAFA microstructure by integrating kinetic and microstructural modeling. Dual validation using both dissolution data and microstructural metrics demonstrates the model's reliability and robustness. This integrated framework provides new insights into the coupled chemical–microstructural evolution of alkali-activated materials. ...

Understanding Chloride Diffusion Coefficient in Cementitious Materials

Journal article (2023) - Zhiyuan Xu, Guang Ye

One of the key problems that affect the durability of reinforced concrete structures is the corrosion of rebar induced by chloride. Despite the complicated transport mechanism of chloride ions in cementitious materials, diffusion is still the key mechanism of chloride ingress. The determination of the chloride diffusion coefficient will help to predict the chloride profile inside the cementitious materials and estimate the service life with regard to chloride-induced corrosion. However, this paper shows that the chloride diffusion coefficient in the literature is sometimes misunderstood. Such a misunderstanding results in the overestimation of the chloride resistance of cementitious materials. To clarify the chloride diffusion coefficient, this paper first presents the steady- and non-steady-state diffusion equations in cementitious materials. The factors that influence the diffusive flux are identified. The effective and apparent diffusion coefficients are then clearly explained and properly defined. We also point out the obscure definitions of the effective diffusion coefficient in the literature. The varied definitions of the effective diffusion coefficient are the result of the consideration of different factors affecting the diffusion process. Subsequently, this paper discusses two natural diffusion test methods that are frequently employed in cementitious materials to measure the chloride diffusion coefficient. The influencing factors considered by the measured diffusion coefficients are analyzed in detail. Then, the diffusion coefficients determined in some of the studies are reviewed. It is shown that three typical errors could occur when numerically determining the diffusion coefficients. ...

Lattice Boltzmann simulation of chloride transport in alkali-activated slag

Conference paper (2021) - Z. Xu, Y. Zuo, G. Ye

In this work, a numerical model is proposed to study chloride transport in alkali-activated materials. This model is based on multiple-relaxation-time lattice Boltzmann method, where particle distribution function is introduced to simulate the chloride binding and diffusion. This model takes into account diffusion, homogenous reaction between chloride ions and diffusive solid, and heterogeneous reaction between chloride ions and non-diffusive solid. The accuracy of the model is confirmed by a benchmark simulation of transient reactive transport problem. As a demonstration, this model is then applied to simulate chloride transport in alkali-activated slag paste with varying alkaline activators. The influence of alkali content, silica content, curing age and chloride binding on the chloride transport property is briefly discussed. ...

A dissolution model of alite coupling surface topography and ions transport under different hydrodynamics conditions at microscale

Journal article (2021) - Jiayi Chen, Pablo Martin, Zhiyuan Xu, Hegoi Manzano, Jorge S. Dolado, Guang Ye

Portland cement is the most produced material in the world. The hydration process of cement consists of a group of complex chemical reactions. In order to investigate the mechanism of cement hydration, it is vital to study the hydration of each phase separately. An integrated model is proposed in this paper to simulate the dissolution of alite under different hydrodynamic conditions at microscale, coupling Kinetic Monte Carlo model (KMC), Lattice Boltzmann method (LBM) and diffusion boundary layer (DBL). The dissolution of alite is initialised with KMC. Two Multiple-relaxation-time (MRT) LB models are used to simulate the fluid flow and transport of ions, respectively. For solid-liquid interface, DBL is adapted to calculate the concentration gradient and dissolution flux. The model is validated with experiment from literature. The simulation results show good agreements with the results published in the literature. ...