Recently, master's student Mr. Xuan Gao from the Department of Civil Engineering proposed a new theoretical framework to explain the formation process of the interfacial transition zone (ITZ) in concrete. This framework quantifies the formation and evolution of ITZ under multi-mechanism interactions from a microscopic perspective. The relevant work is published in the internationally renowned journals Cement and Concrete Research (“A new model for investigating the formation of interfacial transition zone in cement-based materials”), Construction and Building Materials (“Effect of global aggregate distribution on interfacial transition zones in cement-based materials: An analytical-numerical study”), and Journal of Building Materials (“Numerical study on microscopic characteristics of interfacial transition zone between cement paste and aggregate”). The first author is Xuan Gao, and the corresponding author is Prof. Qingfeng Liu.

Interfacial issues have always been a major concern in material science, especially for composite materials like concrete. At the mesoscopic scale, concrete consists of aggregate, cement paste, and the ITZ located between them. The ITZ is loose and porous, which provides an easy pathway for the erosion of aggressive substances and the development of microcracks, and thus is the first to be damaged during service. Therefore, ITZ is often regarded as the weak link in concrete, and its formation theory and microscopic characteristics have attracted a lot of attention.
Currently, the widely accepted theory for the formation of ITZ is known as the “wall effect”, which can describe the loose distribution of cement particles in the ITZ. However, this theory cannot explain many recent experimental findings, such as the enrichment of calcium hydroxide near the aggregate. For this purpose, Xuan Gao and Prof. Liu have proposed the hypothesis of “ion transport in local interfacial zone”, which attributes the above enrichment phenomenon to the migration and redistribution of calcium hydroxide during the cement hydration. On this basis, they interpreted the ITZ formation process as a joint result of five mechanisms (including the wall effect, hydration process, ion transport, aggregate features, and filtration effect), which is the first time to comprehensively explain the ITZ formation at the theoretical level. Furthermore, they quantified these mechanisms and their interactions individually by using micro-geometric modeling and math-physics equations, and then established a model that can simulate the whole process of ITZ formation and evolution. The reliability of this model is also verified experimentally from multiple perspectives.

This new model not only solves the challenge of mathematical characterization of the multi-mechanism coupling process at the microscopic scale, but also accurately predicts the local and global characteristics of ITZ based on the raw material's proportion. These breakthrough results have received a lot of attention from domestic and international peers, e.g., invited by the Edwards Chair Prof. Jason Weiss at Oregon State University, to give a keynote lecture at the well-known DU-Re webinar series, just after the publication of articles. This work provides a new perspective for studying the interfacial issues in material science, and also provides an important theoretical basis for improving the performance of ITZ and concrete, which can help ensure the long-term service of concrete structures.

This research was funded by the National Natural Science Foundation of China, the National Science Foundation for Distinguished Young Scholars of Chongqing, and the Natural Science Foundation of Shanghai.