Investigation of internal curing effects on microstructure and permeability of interface transition zones in cement mortar with SEM imaging, transport simulation and hydration modeling techniques

• Apply SEM imaging and 3D digital sample reconstruction techniques.
• Conduct transport simulation with heterogeneous pores for permeability analysis.
• Internal curing can reduce pore permeability at interface transition zone.
• Conduct multiphysical hydration simulation of interface microstructure development.
• Combine image-based analysis and hydration simulation for internal curing effects.

This study investigates the internal curing effects on the microstructure and permeability of interface transition zones (ITZ) in cement mortar samples with SEM imaging, transport simulation, and hydration modeling techniques. Two types of mortar samples were prepared with saturated lightweight aggregates (with internal curing) and regular sands (without internal curing). The scanning electron microscope (SEM) techniques were applied to characterize the ITZ microstructure of both mortar samples. The internal curing introduced by lightweight aggregates (LWA) is found to have significant impacts on ITZ microstructure development in mortar/concrete samples. 3D image reconstruction techniques were used to generate the 3D microstructure images for transport property analysis. The permeability solver code (developed in National Institute of Standards and Technology) was used to calculate the total porosity, percolated porosity, and permeability of the 3D digital samples. The characterization and simulation results indicate that the permeability of ITZ section in samples with internal curing were smaller due to less percolated porosity and smaller characteristic pore sizes, compared to the samples without internal curing. To further understand the dynamic process of ITZ formation, a meso scale chemo-thermo-hydraulic model is developed to simulate the development of ITZ zone, including processes such as migration of water, production of heat, and growth of CSH, that are involved in cement hydration. Results indicate that the interactions of cement and aggregate, which is responsible for the development of ITZ zone, can be described from computational simulations. The combination of advanced imaging (which captures the microstructure of ITZ at a certain time) and holistic simulations (which describes the time evolution of ITZ in the meso scale) provides a promising way to understand the influence of internal curing on the behaviors and evolution of ITZ.