Evolution of the interfacial transition zone of alkali-activated concrete with aging

The interfacial transition zone (ITZ), as a weak link in the mechanical properties of alkali-activated concrete, serving as a significant pathway for the penetration of water and solutes, as well as a favorable route for the migration of aggressive ions. However, there is still a lack of systematic studies on the properties of the ITZ in alkali-activated concrete. Therefore, this study, the changes in microhardness, tensile strength, micromorphology, chemical composition, and pore distribution of corresponding ITZ pairs in samples with age were tested by alkali-activated net mortar specimens while considering the effects of water-binder ratio (W/B), slag content (SC), and the positions of the upper and lower surfaces of aggregates. The aim is to reveal the mechanism of their evolutionary law, and, at the same time, a correction factor for the ITZ splitting tensile strength of alkaliactivated concrete is proposed. The results indicate that an increase in slag content and the increase of age promotes the generation of hydration products. When the slag content was held constant, the microhardness and splitting tensile strength of ITZ on the upper surface were significantly higher than those on the lower surface under the high water-binder ratio condition, while it was reversed under low water-binder ratio conditions due to the influence of bleeding. The established splitting tensile strength formula of the ITZ can comprehensively consider the effects of age, slag content and interface location. The results of this research provide scientific basis for optimizing interfacial properties and enhancing long-term durability.