Effects of nano-particles on failure process and microstructural properties of recycled aggregate concrete

- Failure process of RAC was influenced by the relative strength between new and old cement mortars.
- Colloidal NS could enter into ITZ regions in RAC to increased hydration products by secondary chemical reaction.
- Powdered NL could not enhance the compressive strength of RAC due to the particles agglomeration.
- Colloidal NS could effectively reduced porosity and water absorption of RAC.

The effects of nano-particles including nano-silica (NS) and nano-limestone (NL) on the crack propagation and microstructure properties of recycled aggregate concrete (RAC) were experimentally investigated in this study. The crack initiation and propagation of nano-particles modified RAC with different nano-particle modification were evaluated using digital image correlation technique (DIC). The microstructures and porosity of interfacial transition zones (ITZ) in nano-modified RAC were also examined using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). It was found that the micro-cracks were typically derived from relatively weak ITZs in RAC, and then progressively propagated along the compressive loading direction. The meso-crack developments eventually led to final splitting failure. The results indicated that compared to NL, NS was more effective in improving the microstructure properties and enhance the mechanical strength of RAC. The porosity and water absorption of RAC were obviously reduced by the NS incorporation. However, due to particles agglomeration, NL could not effectively improve the microstructure of RAC for further enhancing the RAC mechanical properties. Furthermore, in terms of severe particles agglomeration, NL was even detrimental to the mechanical strength of RAC especially at the late-age.

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