*Article* **Studying the C–H Crystals and Mechanical Properties of Sustainable Concrete Containing Recycled Coarse Aggregate with Used Nano-Silica**

**Shahriar Shahbazpanahi 1, Moslem Khalili Tajara 1, Rabar H. Faraj <sup>2</sup> and Amir Mosavi 3,4,5,6,7,\***


**Abstract:** The present study aims to replace 30%, 40%, and 50% of the natural coarse aggregate (NCA) of concrete with recycled coarse aggregate containing used nano-silica (RCA-UNS) to produce a new sustainable concrete. Three groups of concrete are made and their mechanical properties and microstructure are studied. In the first group, which was the control group, normal concrete was used. In the second group, 30%, 40%, and 50% of the NCA were replaced with coarse aggregate obtained from crushed concrete of the control samples and with 0.5% nano-silica as filler. In the third group, 30%, 40%, and 50% of the concrete samples' NCA were replaced with aggregates obtained from 90-day crushed samples of the second group. Water absorption, fresh concrete slump, and compressive strength of the three groups were investigated and compared through scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) tests. The results show that the third group's compressive strengths increased by 12.8%, 10.9%, and 10% with replacing 30%, 40%, and 50% of NAC with RCA-NS at 28 days compared to the control samples, respectively. This could be due to the secondary production of calcium silicate hydrate due to the presence of new cement paste. The third group's microstructure was also improved due to the change in the C–H and the production of extra C–S–H. Therefore, the hydration of cement with water produces C–H crystals while reactions are induced by recycled aggregate containing used nano-silica.

**Keywords:** sustainable concrete; sustainable materials; recycled coarse aggregate; concrete; used nano-silica; compressive strength; sustainable construction materials; recycled concrete; materials design; composite materials
