Nano/microcharacterization and image analysis on bonding behaviour of ITZs in recycled concrete enhanced with waste glass powder

Abstract

Waste glass powder (WGP) has been shown to help improve concrete properties. However, the underlying mechanism needs to be explored at the micro level, specifically on the interfacial transition zones (ITZs). This study aims to investigate the microtopography and bonding behaviours of ITZs using modelled recycled aggregate concrete (MRAC). To highlight the differences, two sets of MEAC were prepared, consisting of old and new mortars with water-cement ratios of 0.45 and 0.40, respectively. WGP was incorporaed in the new mortar, replacing 20 wt% of cement to enhance the performance of the new ITZ. Two groups of MRAC with three types of ITZs were successfully fabricated and studied. The property differences between the old ITZ, new ITZ without WGP, and new ITZ with WGP were comprehensively evaluated and compared based on phase distribution, chemical composition, and micromechanical properties. The results show that the new ITZ without WGP exhibited the highest porosity (avg. 7.29%) but a higher calcium silicate hydrate (C-S-H) elastic modulus (avg. 11.89 GPa) compared to the old ITZ (avg. 4.33% and 10.37 GPa, respectively). WGP effectively reduced the volume fraction of pores and cracks to 4.30% by reacting with calcium hydroxide (CH) within the new ITZ and the adjacent old mortar, resulting in generation of a significant amount of C-S-H gel with a Si/Ca ratio of approximately 1.5. To obtian more accurate results regarding the bonding strength between the old and new mortars, it is necessary to further evaluate the cohesion of C-S-H and the compactness of the new ITZ in a comprehensive manner.