Effect of nano-SiO₂/CaCO₃ compound incorporation on the properties of fully recycled coarse aggregate concrete

Compared with natural coarse aggregate, the basic physical properties of recycled coarse aggregate are relatively poor due to the inevitable damage in the crushing process and the attached old mortar, and the mechanical properties and durability of the prepared recycled aggregate concrete (RAC) are worse due to its formed multiple interface structures. To further improve the performance of RAC, based on this, the mechanical properties and water absorption behavior of fully recycled coarse aggregate concrete (FRAC) modified by different nano-SiO₂ (NS) and nano-CaCO₃ (NC) single-mixed or compound-mixed were respectively investigated. The NS content (S) was 1%, 2% and 3%, and the NC content (C) was 1%, 3% and 5%. The modification effect of single/compound-mixed nanomaterials on the micromorphology was characterized by SEM observation. The results show that the compressive strength and splitting tensile strength of RAC increase with the addition of nanomaterials within the appropriate incorporation range no matter the single-mixed and multiple-mixed forms. However, when the NC content reaches up to 5%, excessive low-carbon hydrated calcium carboaluminate will be generated, and the RAC strength will decrease to a certain extent after agglomeration, but the overall strength is still higher than that before modification, the effect of 3%NS single admixture can be achieved when the complex admixture ratio is 1%NS and 3%NC. The cumulative mass of capillary water absorption and water absorption rate of RAC decrease with the increase of nanomaterial content. In general, the improvement effect of compound mixing is better than that of single mixing, and the optimal content of nanomaterial is 3% in this study. The results of SEM observation also further confirm that when the mixture amount of the two nanomaterials is 3% (S3C3), the least internal pores and microcracks and the denser interfacial transition zone (ITZ) in RAC indicate the better modification effect of the nanomaterials.