Effect of cellulose nanofiber addition on the microstructure characterization and nano-mechanical behavior of interfacial transition zones in recycled concrete

The aggregate-matrix interfacial transition zone (ITZ) is known as the weakest link in recycled concrete. Cellulose nanofiber (CNF), as a green and renewable nano-reinforcement material, proven effective in modifying the mechanical properties of cementitious composites, is considered for incorporation into recycled concrete for the first time. This study investigates the effect of CNF addition on the microstructure characterization and nano-mechanical behavior of ITZs in recycled concrete. The elastic modulus distribution and 2D microstructure topology of the old/new aggregate-matrix interfaces are characterized through statistic nanoindentation technique for different CNF dosages. The results show that the width of the new ITZ is between 40 μm and 60 μm, while the width of the old ITZ is between 30 μm and 50 μm. The old ITZ has better mechanical properties than the new ITZ, where a remarkable low elastic modulus area at the new ITZ is observed. The incorporation of an appropriate amount of CNF can significantly improve the nano-properties of the new ITZ due to the increased hydration degree of cement particles at the recycled aggregate-matrix interface. Moreover, a dense network structure around the cement hydration products is formed with improved compactness, resulting in an increase in the overall mechanical strength of the recycled aggregate concrete material. The research outcome provides a viable solution for improving the performance of concrete containing recycled aggregates and urges the green and sustainable development in concrete industry.