Interfacial transition zones in recycled aggregate concrete with different mixing approaches

Interfacial Transition Zone (ITZ) is an important phase that influences the mechanical properties of Recycled Aggregate Concrete (RAC). This paper is devoted to an investigation of the effect of different mixing approaches on the properties of ITZs in RAC. Nanoindentation and Scanning Electron Microscopy (SEM) were implemented to characterize nanomechanical properties and microstructures of the old and new ITZs. The results indicate that the indentation modulus of ITZs have different distribution trends when using different mixing approaches. It reveals that the Two-Stage Mixing Approach (TSMA) can improve the nanomechanical properties of the new ITZ by reducing the volume fraction of voids and Calcium Hydroxide (CH). The SEM observation also indicates that an obviously denser and more homogeneous microstructure was achieved in the new ITZ prepared by TSMA compared to a Normal Mixing Approach (NMA). Correlating the nanoindentation and SEM results with the compressive strength, it can be concluded that TSMA has a beneficial influence on the mechanical properties of RAC by improving the properties of the new ITZ.