1. Introduction
Owing to its high strength, extra workability, excellent hydrothermal stability, and abundant raw material resources for easy mixing, normal concrete has become the building material with the most frequent use and widest application range [
1]. However, as the scope of concrete applications becomes wider, higher requirements for concrete properties (such as its durability and impact resistance) are being proposed, e.g., for high-rise and large-span buildings, building structures in severely cold areas, and other special purposes [
2,
3]. Therefore, the inclusion of different types of admixtures (such as various nanomaterials and fibers) in cement-based materials has become a common method for researchers to improve the properties of concrete [
4,
5,
6]. Nanomaterials are materials comprising of particles with particle sizes between 1–100 nm. Owing to the ultrafine sizes of the nanoparticles, the surface electronic structures and crystal structures of the nanoparticles undergo significant changes, resulting in unique nano-effects, for example, surface effects, small-size effects, quantum effects, and macro–quantum tunneling effects. Therefore, nanomaterials have attracted the attention of many researchers as a new high-tech material with a great potential for application. Owing to the unique physical and chemical properties of nanomaterials, many properties of concrete, such as its workability, strength, and durability, can be improved by introducing nanomaterials. Several researchers have conducted numerous investigations based on introducing low-dose nanomaterials for replacing the binding material in concrete mixtures to enhance the qualities of concrete, and have achieved important results [
7,
8,
9]. For instance, the incorporation of NS, nano-CaCO
3, or nano-TiO
2 particles has been shown to significantly improve the mechanical properties and endurance of concrete, chiefly owing to their nucleation and micro-aggregate filling effects [
10,
11,
12,
13]. In addition to nano-oxides, some nanocarbon materials, comprising of carbon nanotubes, carbon nanofibers, and graphene, have also been shown to dramatically increase the strength, fracture toughness, energy absorption capability, and ductility of ordinary concrete [
14,
15,
16].
Compared with other nanoparticles, NS particles have a higher activity and a particularly large specific surface area; therefore, NS can exhibit a higher level of pozzolanic activity in concrete. In addition, NS can provide nucleation sites for calcium silicate hydrate (C–S–H), and act as a catalyst for pozzolanic reactions to promote the dissolution of tricalcium silicate and the formation of C–S–H [
17,
18]. Therefore, among these nanomaterial particles, NS particles have received the most attention. Said and Zeidan et al. observed that the compressive strength of concrete blended with 6% of the cement weight of NS was 36% higher than the control concrete [
19]. Nazari and Riahi also got an analogous conclusion, but they suggested that NS particles as a fractional replacement of cement at, at most, 4 wt% could improve the formation of C–S–H gel and refine the pore structure of the concrete, and when NS content was in excess of 4 wt%, the strength of the concrete would decrease because of the uneven dispersion of nanoparticles [
20]. This is because Said and Zeidan et al. used colloidal NS in their experimental research, and colloidal NS can achieve better dispersion in a cement matrix than powdered NS. Zhang and Ling et al. found that adding 0–1.5% nanosilica particles was optimal for enhancing the compressive strength, flexural strength, and fracture energies of reinforced concrete with polyvinyl alcohol fibers. Moreover, 2.5% nanosilica particles was found to be the best for tensile strength; nevertheless, when the content was greater than 2%, the NS was prone to self-desiccation and flocking together, leading to micro cracks and strength losses in the composite [
21,
22]. According to the experimental results from most researchers on concrete with the addition of NS, it can be concluded that low doses of NS can significantly enhance the mechanical properties of concrete.
Reducing carbon emissions and promoting carbon neutrality are the main measures taken by authorities around the world to respond to environmental changes [
23]. Cement production is the main source of carbon dioxide; therefore, using supplementary cementitious materials (SCMs) such as granulated blast furnace slag and coal fly ash instead of the part of cement to make concrete is an effective way to reduce carbon emissions [
24,
25]. Coal fly ash is a type of ultrafine particle produced by coal carbon combustion in power plants. As an industrial byproduct, coal fly ash has been broadly utilized for concrete in recent decades, and its recovery has brought significant economic and environmental benefits [
26,
27,
28]. When coal fly ash is used as an SCM to prepare concrete, the loss on ignition is a problem that must be considered. Coal fly ash with a high unburned carbon content increases the conductivity of the concrete, changing the color of the mortar and concrete to black [
29]. In addition, coal fly ash with a high carbon content increases the corrosiveness of the metallic parts in concrete [
30]. Finally, it can lead to undesirable air entrainment behavior and mixture segregation [
31,
32]. The durability of high-performance road concrete incorporating coal fly ash was investigated by Li et al., who found that the use of coal fly ash as a cement replacement could significantly improve the permeability resistance and freezing–thawing resistance of concrete [
33]. Furthermore, Miguel et al. found that coal fly ash and silica fume improved the durability of mortar and concrete dramatically by effectively inhibiting the alkali–silica reaction [
34]. Zhang et al. investigated the effects of silica fume on the compressive strength and fracture properties of coal fly ash concrete. They detected that the compressive strength of concrete containing coal fly ash increased with the increase of silica fume content, but its ability to resist crack propagation gradually decreased [
35]. However, an increase in the amount of coal fly ash is not necessarily better. Miguel et al. found that excessive coal fly ash and granulated blast furnace slag will also reduce the carbonation resistance of concrete, especially in badly cured concrete [
36,
37].
A growing number of concrete structures are of the types frequently affected by impact loads under service conditions. For example, airport runways are subject to impacts from aircraft landing, and offshore structures are subject to the impacts of waves. Therefore, under these impact loads, the structural safety of traditional Portland cement concrete with brittle fracture characteristics poses a significant challenge. Researchers have conducted numerous studies on enhancing the impact resistance of ordinary concrete. For example, Siddique et al. evaluated a concrete that replaced 40% natural sand with fine ceramic aggregate. They observed that the impact energy absorbed by the specimens increased from 0.94 J with plain concrete to 0.99 J [
38]. Li et al. used a Hopkinson pressure bar to study the impact resistance of self-compacting concrete with asphalt-coated coarse aggregate. The results showed that applying asphalt onto the surface of coarse aggregate distinctly increased the impact toughness index of the self-compacting concrete, and that when the asphalt layer thickness was 120 µm, the impact resistance was optimal [
39]. Gonen found that when the usage rate of waste crumb rubbers in ordinary concrete was 4%, the impact resistance of the specimen was 200% higher than that of a control concrete [
40]. Carmichael et al. experimentally studied the influence of 10–50% nanomaterial particle replacement on the impact resistance of concrete relative to a control concrete (without nanomaterials), and found that the nanomaterial concrete had better impact resistance [
41]. In addition, incorporating various fiber materials into concrete is considered to be an ideal optimization method for improving the impact resistance of concrete. Numerous research results on fibers have shown that the impact resistance of fiber-containing concrete is significantly improved compared with ordinary concrete, since fiber-containing concrete can consume more impact energy [
42,
43,
44]. Therefore, most of the current methods used to improve the impact resistance of concrete incorporate the addition of energy-absorbing components into the concrete.
It is well known that durability is a critical factor in evaluating the lifespan of concrete. Resistance to chloride permeability and freezing–thawing are two vital elements of this durability. In an environment with a high concentration of chloride ions, for instance, coastal areas and cold areas where deicing salt is used, chloride ions can access the concrete through diffusion, adsorption, and capillarization, causing damage to the internal reinforcement and corrosion of the concrete [
45]. Freezing–thawing cycles also pose a severe threat to the durability of concrete structures. In concrete structures, repeated freezing–thawing can cause surface erosion and crack propagation, accelerate the oxidation and corrosion of steel bars, and significantly reduce the service life of the reinforced concrete structure [
46,
47]. The durability of concrete mixed with NS particles was studied by Du et al.; even at a dosage of 0.3%, the water resistance and chloride ion permeability were greatly improved [
8]. They observed that the internal structure of the concrete was denser, specifically in the interfacial transition zone (ITZ), owing to the pozzolanic reaction and nano-filling effect of the NS. Behfarnia and Salemi found that the freezing–thawing resistance of concrete mixes could be noticeably improved by adding nano-Al
2O
3 and NS, and that the performance of a concrete containing nano-Al
2O
3 was better than that of a concrete containing the same dose of NS [
48]. They also found that the appearances of the concrete specimens containing NS after freezing–thawing cycles were distinctly improved, and that the quality loss was significantly reduced. According to Zhang et al., the amount of NS added to a mixture significantly improves the freezing and thawing resistance of a high-performance concrete [
49]. Gonzalez et al. observed that adding NS to a concrete mixture can reduce the external damage caused by freezing–thawing cycles, owing to the production of denser and lower-permeability concrete [
50].
Although there have been a large number of studies on the impact resistance of ordinary concrete, and the mechanical properties and durability of concretes containing NS, results correlated with the impacts of NS particles on the impact resistance and durability of coal fly ash concretes are relatively rare. Therefore, in this study, the effects of different NS dosages on the impact resistance, chloride penetration resistance, and freezing–thawing resistance of concretes containing coal fly ash were studied using a drop-weight impact test, single-sided freezing–thawing test, and rapid chloride migration (RCM) test, respectively; then, the optimal NS content was determined.