Porosity characterization of ITZ in cementitious composites: Concentric expansion and overflow criterion

The interfacial transition zone (ITZ) needs to be assessed since its excess of porosity could bring a negative impact on the overall material behavior. An intention of current research is to investigate porosity characteristics of ITZ in hardened Portland cement-based composites, by means of backscattered electron–scanning electron microscope (BSE–SEM). Emphasis is laid on presenting an elaborative illustration of image analysis with regard to ITZ porosity characteristic. Specifically, a new method named ‘concentric expansion’ is proposed to process the strip delineation for analyzing porosity profile from aggregate surface. The overflow criterion is employed to determine the upper threshold for pore segmentation. Such procedure was applied on two kinds of mortars with 10% and 50% sand volume fractions. Image analysis results indicated that a typical excess of porosity took place in the vicinity of aggregate for designed mortars, which is considered as a sign for ITZ in conventional cementitious composites. Furthermore, the average porosity level (36.32%) of ITZ in mortar with 10% sand content was higher than the value (21.37%) in mortar with 50% sand content. Such discrepancy came from varied effective water binder ratio and air content.