Microscopic Investigation on Interfacial Transition Zone in Fly Ash Cement Hydrates Using Roof-Tile Waste Aggregate

Previous studies reported that the strength reduction in fly ash concrete (FAC) can be improved by internal curing with roof-tile waste aggregate (RWA). However, how much the internal curing by RWA improved the compressive strength of FAC was different, and there was no clear explanation on it. The purpose of this study is to clarify the effect of the internal curing in the FAC by examining the range of the internal curing effect with RWA in the interfacial transition zone (ITZ) around coarse aggregate. Paste and concrete specimens in which part of cement was replaced with fly ash were prepared. For paste specimens, one aggregate particle was placed in the center of the specimen. Vickers hardness around the aggregate in the paste specimen and the compressive strength of the concrete specimen were investigated. As a result, the range of Vickers hardness improvement around RWA changed depending on the replacement ratio of fly ash. This indicates that the internal curing effect differs depending on the mixture proportion of paste matrix such as the replacement ratio of fly ash. Furthermore, it was observed that the compressive strength of concrete increased as the thickness of ITZ around RWA decreased.