1. Introduction
Construction waste shows a fast-increasing trend with urbanization and industrialization, and the annual output of construction waste has climbed up to 3.5 billion tons, causing a non-negligible threat to the environment [
1,
2,
3]. Recycling research and technology on construction waste not only conserves many natural aggregate resources but also reduces construction waste pollution, which effectively conforms to the requirements for the sustainable development of the construction industry [
4]. Extensive and intensive research on the performance of recycled concrete by incorporating this waste after crushing as a recycled aggregate and/or admixture has been explored all around the world, and its workability, mechanical properties, and durability are found to be generally inferior to those of normal concrete [
3,
4,
5,
6,
7]. Indeed, owing to recycled concrete being a type of heterogeneous material with multi-phases and multi-interfaces, its microstructure is more complicated than that of normal concrete, and the existence of multiple but weak interfaces is the main reason for the low compressive strength and long-term property of recycled concrete [
5,
8,
9,
10]. Therefore, it is necessary to further refine the surface features of recycled aggregate and the microstructures of recycled concrete for the corresponding recycled concrete products with enhanced macro-performances to achieve wider applications. Zhao et al. [
11], Sallenhan et al. [
12], and Zhu et al. [
13] pre-coated recycled aggregate and attempted to improve the drying shrinkage and durability of the corresponding recycled concrete. Several researchers have also doped some fibers or nanoparticles, such as PVA fiber [
14], basalt fiber [
15], nano-silica [
16], nano-SiO
2, and/or nano-TiO
2 [
17,
18] to effectively improve the mechanical properties, pore structure, and chloride diffusivity of recycled concrete. Xiao et al. [
18] documented that nano-TiO
2 was a little better than nano-SiO
2 for refining recycled concrete, and the corresponding recycled concrete doping with 2 wt% nano-TiO
2 had the best resistance to chloride diffusion.
As a graphene derivative, a graphene oxide (GO) sheet comprises a hexagonal carbon network bearing hydroxyl, epoxide, carboxyl, and carbonyl functional groups [
19]. These oxygen-containing groups contribute to making GO sheets hydrophilic and highly dispersible in water, in contrast to carbon nanotubes [
20,
21,
22]. The various functional groups of a GO sheet and its large surface area make it the perfect nano-reinforcement to aqueous systems due to the improvement in solubility, ease of production, and positive interaction with an aqueous cement matrix [
23,
24,
25,
26]. Hou et al. [
27] studied the intrinsic interactions between a GO nanosheet and cement hydration products through the reactive molecular dynamics simulation method and found that GO dosage endowed the cementitious matrix with a high cohesive force and enhanced plasticity owing to the H-bonds and covalent–ionic bonds. Quite a few related researchers reported that GO sheet dosage could significantly improve the mechanical properties, fracture toughness, as well as the microstructures of hardened cement concrete [
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40]. The transport properties, corrosion resistance, freeze-thawing durability, electrical resistivity, multifunctionality/intelligence, and electromagnetic shielding functional capacities of GO-reinforced concrete were also investigated [
41,
42,
43,
44,
45]. As Mohammed et al. argued, GO could effectively improve the transport property, reshape the microstructure, and enhance the frost resistance of GO-reinforced concrete [
41,
42]. As Chen et al. found, with cement with 0.4 wt% GO––carbon fiber (CF) and a shield thickness of 5 mm, a shielding effectiveness of 34 dB was attained at the X-band region, a 31% increase over that of CF/cement (26 dB) in the same fraction [
43]. In addition, Lu et al. investigated the influence of GO on the workability, hydration degree, mechanical behavior, and microstructures of magnesium potassium phosphate cement (MKPC) paste and concluded that the addition of GO shortened the final setting time and decreased the workability of the MKPC paste, but the mechanical strength of the MKPC paste with the proper addition of GO was improved [
44].
However, as Ghazizadeh et al. documented, the direct incorporation of GO nano-reinforcement into a cementitious matrix hardly guaranteed a good dispersion of GO in an alkaline cementitious matrix, and even some agglomeration occurred due to the temporarily retardant interaction of GO with the surface of cement hydrating grains [
24]. Li et al. also argued that GO agglomerates, after direct incorporation of GO into the cement matrix, were more electrically insulative than cement paste, and the resultant composite had no piezoresisitive effect [
46]. A few scholars endeavored to achieve sufficient dispersion and stabilizing of GO in cement matrix employing ultrasonication [
47], polycarboxylate superplasticizer [
37,
48], and/or silica fume aiding [
48]. Here, we use the dispersion techniques of ultrasonication and polycarboxylate superplasticizer pre-stabilization to achieve sufficient dispersion of GO.
To the best of the authors’ knowledge, there are no reports addressing the effect of GO on the fracture toughness and microhardness of recycled concrete. In this study, diluted GO dispersion was firstly treated with the dispersion techniques of ultrasonication and polycarboxylate superplasticizer pre-stabilization; diluted GO dispersions with varied contents of GO were then cast mixed into cement mortars along with natural (NS) or recycled sand (RS) to fabricate six groups of GO-modified NS- or RS-based mortars (GONMs or GORMs). The corresponding mechanical strength, fracture toughness, and microhardness of the 28 d-cured GONMs or GORMs were comprehensively investigated. The reinforcing mechanisms of GO on the physical properties of the RS- or NS-based mortars were also investigated, as well as their crystal phases and microstructures.
4. Conclusions
In this study, six groups of GO-modified cement mortars associated with natural or recycled sand (GONMs or GORMs) are prepared, and their mechanical strength, fracture toughness, and microhardness in tandem with microstructures are accordingly characterized.
1) The f28t and f28c, KIC and δc, and Hv of GORM with 0.05 wt‰ wGO are 6.15 MPa and 55.0 MPa; 0.7139 MPa.m1/2 and 0.2076; and 104.6 kgf, respectively, whereas those of GONM with same wGO are just 6.35 MPa and 59.5 MPa; 0.7671 MPa.m1/2 and 0.1276; and 104.7 kgf, respectively. This is attributed to RS with zigzag surfaces, irregular interface slips, and relatively inferior strengths bringing the superiority of the template and the reshaping effects of GO into full play in GORM.
2) RS-based mortars with only 0.025 wt‰ and 0.05 wt‰ GO dosage achieve significant improvements in mechanical strengths (4.50% and 10.61% in f28t, respectively, and 4.76% and 13.87% in f28c, respectively), fracture toughness (16.49% and 38.17% in KIC, respectively, 160.14% and 286.59% in δc, respectively), and microhardness (21.02% and 52.70% in Hv, respectively) with respect to the control without GO.
3) XRD and SEM images demonstrate that GO incorporation can only improve the overlapping pattern and size-regular morphology of hydration products, rather than introduce new crystal phases into cementitious matrix.
These findings effectively show a promising future for the use of small amounts of GO to reinforce recycled concrete, and we will focus on the influence of GO incorporation on the transport properties and durability of recycled concrete in the future.