Investigation of the interphase between recycled aggregates and cementitious binding materials using integrated microstructural-nanomechanical-chemical characterization

The mixtures including crushed recycled aggregates have multiple complex aggregate/paste interphase regions compared to conventional concrete mixtures, which brings significant technical challenges in understanding and characterization of their properties. To gain a better understanding of such complex material organization, this study adopted multiscale experimental methods by using nanoindentation test-analysis, laser scanning microscopy, and energy dispersive spectroscopy. The multiscale methods were applied to two different composites in which the same recycled aggregates were mixed with two different cementitious binders: a fly ash-based geopolymer and conventional Portland cement. The test-analysis results demonstrate that, in cement concrete mixtures with recycled aggregates (CCRA), the pre-existing incomplete interphase within the recycled aggregares was observed, although new paste was relatively well-bonded to the old recycled aggregate paste by having an approximately 20-μm thick interfacial transition zone. In geopolymer concrete mixtures with recycled aggregates (GCRA), both the old and new interphase appeared dense. More interestingly, the pre-existing incomplete interphase within the recycled aggregates was filled in the GCRA, which was not the case observed from the CCRA. Further analysis using energy dispersive spectroscopy suggests that geopolymeric materials can reach the pre-existing incomplete interphase and create hydration-geopolymerization products that combine calcium-silicate-hydrate (C-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gel. The resulting cementitious composite is expected to show enhanced mechanical properties owing to a better interphase region.