Characteristic analysis of innermost layer of ITZ of cement paste based on modelling the spatial distribution of calcium hydroxide

• Modelling confirms CH redistribution and reveals microstructural features of the innermost ITZ. • CH deposition increases in the 2.5–5.0 μm zone near aggregate due to high distance field values. • The wall effect reduces CH deposition and increases porosity in the innermost ITZ layer. The innermost layer of the interfacial transition zone (ITZ) between cement paste and aggregate has remained poorly understood due to insufficient experimental characterisation and numerical modelling. In this study, the microstructure of ITZ was simulated by modelling the spatial distribution of calcium hydroxide (CH) based on the mechanism of crystal redistribution in porous media. The modelling results showed that CH concentrated adjacent to the innermost layer (i.e. 2.5–5.0 μm from the aggregate) due to the large distance map values in this region. However, in the innermost layer ((i.e. within 2.5 μm from the aggregate), the distance map values decreased sharply, thereby reducing CH deposition. Consequently, the innermost layer of the ITZ exhibited an extremely loose microstructure. For the ITZ with the water to cement (W/C) ratio of 0.4, the innermost layer (0.5 μm from the aggregate) reached a porosity of 76.6 % at 28 days.