▎ 摘　　要

There is a constant drive to improve the properties of recycled concrete owing to its inferior strength and fracture toughness compared to normal concrete and recent progress in graphene oxide (GO) nanomaterials impelling nanosized reinforcements to recycled concrete. Here, GO-modified natural sand (NS)- or recycled sand (RS)-based mortars (GONMs or GORMs) with six GO fractions (w(GO)s) were fabricated to explore their 28 d mechanical strengths (f(t)(28), f(c)(28)), fracture toughness (K-IC, delta(c)), and microhardness (H-v), as well as their crystal phases (using X-ray powder diffraction) and microstructures (using scanning electronic microscopy). Results reveal, greater enhancements in mechanical strengths (4.50% and 10.61% in f(t)(28), 4.76% and 13.87% in f(c)(28)), fracture toughness (16.49% and 38.17% in K-IC, 160.14% and 286.59% in delta(c)), and microhardness (21.02% and 52.70% in H-v) of GORM with just 0.025 wt parts per thousand and 0.05 wt parts per thousand GO, respectively, with respect to the control are achieved when comparing with those of GONM with the same w(GO). More zigzag surfaces, more irregular weak interface slips, and the relatively lower strengths of RS bring the superiority of the template and reshaping effects of GO into full play in GORM rather than in GONM. These outcomes benefit a wide range of applications of recycled concrete products.