Micromechanical characterization of the interfacial transition zone in recycled brick concrete by means of combined nanoindentation and SEM

The interfacial transition zone (ITZ), which is the weakest component of concrete, is inherently complex due to its spatially varying heterogeneity.It is therefore essential to gain a deeper understanding of the relationship between the microstructural characteristics and the macroscopic mechanical properties.In this context, the quality and resolution of both microscopic and micromechanical testing techniques have been improved, and nanoindentation has become an accepted measurement method for determining the micromechanical properties of materials.In the present study, nanoindentation is combined with scanning electron microscopy (SEM) to investigate the ITZ of recycled concrete made from recycled brick aggregate.The measurement procedure was adapted as follows: three suitable areas of ITZ and one additional area of bulk paste were selected for further analysis using optical light microscope.These areas were subsequently analyzed using SEM to identify and characterize the ITZ.Based on the image analysis, the distribution of porosity was determined.Nanoindentation grids with 5 m spacing were then applied to the areas.In addition, deconvolution analysis of the probability density function obtained from more than 700 indentation results in each ITZ and bulk paste allows the quantitative characterization of the identified phases.The porosity analysis shows that the ITZ thickness is 65 m, and the porosity of the ITZ is twice that of the bulk paste.The nanoindentation result shows that the porosity, low density calcium silicate hydrate, and calcium hydroxide are higher in the ITZ, while high density calcium silicate hydrate is higher in the bulk paste.The results provide a fundamental basis for the wider use of recycled brick aggregates in construction.