The effect of rough vs. smooth aggregate surfaces on the characteristics of the interfacial transition zone

Due to its higher porosity, the interfacial transition zone (ITZ) between cement paste and aggregates in cementitious composites is often viewed as a weak link in the microstructure. Sectional plane analysis based on back-scattered electron scanning electron microscope (SEM-BSE) images is used to quantify the ITZ porosity gradient, yet not much research has focused on the effect of aggregate surface morphology on the ITZ composition. In this paper, a model concrete specimen with a single spherical ceramic particle acting as an aggregate is used to study the properties of the ITZ and its uneven distribution around the aggregate based on quantitative analysis of SEM-BSE images. A careful treatment of the statistics of the ITZ was employed. The average porosity of the ITZ at a smooth part of the aggregate surface was smaller than that found at a rough part of the aggregate surface. A method is proposed to describe the local surface roughness (SR) at the pixel level and relate this quantity to other ITZ properties. The relationship between the surface roughness and the ITZ porosity gradient within 30 pm of the aggregate surface was analyzed with the "K-means" clustering method. The role of gravity was seen, since more porous ITZ regions tended to form underneath the aggregate than above the aggregate, with intermediate results at the aggregate sides.