1. Introduction
The traditional cement concrete industry is unprepossessing due to its high consumption of natural resources and energy, and major environmental damage [
1]. In 2020, China produced 2.5 billion m
3 of concrete, which consumed nearly 1.65 billion tons of sand and gravel, including a large number of natural river sand (RS). As a non–renewable resource, RS is overconsumed. Long–term mining of RS has caused serious threats to the safety and stability of bridges and river banks, the ecological system of origin, and the health of local residents [
2,
3,
4]. In addition, China has promulgated a policy of restriction and prohibition of mining RS. This further leads to less and less high–quality RS being available for the concrete industry, and the rising cost of RS. Therefore, it is imperative to find alternative materials for RS in order to meet the goal of sustainable development in the concrete industry.
Manufactured sand (MS) is a kind of fine aggregate with particle size less than 4.75 mm that is obtained by crushing and screening rocks. It is regarded as a potential alternative material for RS [
5]. However, due to its preparation process, MS has defects such as irregular particle shape, unreasonable gradation, high stone powder content, large methylene blue value (MBV), etc. These defects also lead to the performance differences (workability, mechanical properties, and durability) between MS concrete and RS concrete [
6,
7,
8,
9]. Many scholars have studied the influence of MS characteristics on the performance of concrete. Shen et al. [
10,
11] studied the effects of MS of different crushing values, stone powder content and the MBV on performance of C50 MS concrete. It was found that the stone powder content and MBV had the greatest impact on the workability of the concrete. When the stone powder content and MBV remained unchanged, the compressive strength was negatively correlated with the crushing value. Liu et al. [
12] studied the influence of MMBV on the durability of medium– and low–strength MS concrete. MMBV is the multiplication of stone powder content and MBV. MMBV has shown an effect on the workability of medium– and low–strength concrete. With increasing MMBV, concrete workability decreases. The compressive strength and chloride penetration resistance of medium–strength concrete were more affected than those of low–strength concrete. Chen et al. [
13] studied the coupling effect of different stone powder content and MBV in the performance of C40 concrete, and they calculated the coupling coefficient of stone powder content and MBV on concrete performance. Li et al. [
14] studied the effect of stone powder content on the durability of MS fully replaced medium–strength marine concrete. MS with low crushing value and MBV was selected to prepare marine concrete. The optimum content of stone powder was about 7.5%. Li et al. [
15] used MS of different crushing value and stone powder content to prepare pavement concrete. The results of research showed that with increasing stone powder content, the compressive strength of pavement concrete improved. The lower the crushing value, the higher the compressive strength. To sum up, the MS characteristics have significant effects on the mechanical properties and durability of low– and medium–strength concrete. However, there are few studies on the effects of MS characteristics on performance of high–strength concrete.
In recent years, the market share of MS has been increasing [
16]. Hence, it is generally believed that MS is a low–quality substitute for RS and is only suitable for the preparation of medium– and low–strength concrete [
17]. This can also be found from the above studies. However, Shen et al. [
18] proved that MS with low crushing value, stone powder content, and MBV can be used to completely replace RS in the preparation of ultra–high–strength concrete, and the compressive strength at 28 d exceeded 130 MPa. This indicates that MS has the potential to be used to prepare high–strength concrete. In fact, on the basis of the main quality characteristic parameters of MS, including stone powder content, MBV, crushing value and soundness, the quality of MS can be divided into categories I, II and III, corresponding to high, medium and low quality, respectively, according to the Chinese standard [
19].
Table 1 presents the quality of the MS used in some studies according to the Chinese standard. It can be found that in the past, most studies changed one or two characteristic quality parameters to study their influence on concrete performance. However, they did not reckon with the influence of overall MS quality on concrete performance. The category of a certain quality characteristic parameter cannot be used to determine the overall quality of MS. Hence, the overall quality of MS needs to be comprehensively determined on the basis of multiple quality characteristic parameters. As shown in
Table 1, the stone powder content, crushing value, and MBV of the MS used in reference [
18] are determined to be class I. The MS can be regarded as being of high quality, and is able to completely replace RS in the preparation of high–strength concrete. On the other hand, for medium– and low–quality MS, the optimum rate of replacement for the preparation of high–strength concrete remains unclear. In addition, few studies about manufactured sand recycled aggregate concrete (MSRAC) have been reported, especially with respect to the effect of different levels of MS quality on the durability of high–strength RAC.
The research and use of RAC in China started later than that in developed countries. At present, the use rate of waste concrete in China is low, at only about 10%. A large amount of waste concrete has piled up, resulting in serious land pollution [
20,
21,
22]. The application of RAC contributes to solving this problem. Many studies have investigated the durability of RAC [
23,
24,
25]. Chloride penetration resistance is one of the most important durability properties of RAC. Mata et al. [
26] found that RAC had higher chloride permeability than NAC. The lower resistance of RAC to chloride ion penetration compared with NAC can be attributed to the old interfacial transition zone and cement paste attached to the recycled aggregate, which make RAC more penetrable than NAC. Due to the presence of old mortar, RAC has multiple interfaces, such as new mortar–old mortar, new mortar–aggregate, and old mortar–aggregate, leading to RAC having more complex and heterogeneous material properties. The old mortar layer is the fundamental reason for the poor durability of RAC [
27]. Loose and porous old mortar and the weak interfacial transition zone provide a large number of channels for chloride ions to enter the concrete [
28,
29]. Chloride ions are present and migrate in the concrete in the free and chemically or physically bound states, deteriorating the interface structure of the concrete and causing the corrosion of steel bars. This leads to the cracking and destruction of reinforced concrete structures, and continuously reduces the bearing capacity of concrete structures. Especially in concrete structures exposed to marine environments, deicing salt environments and coastal underground environments, the chloride penetration resistance of the concrete protective layer is particularly important.
Therefore, in order to improve the use rate of MS and promote the application of medium– and low–quality MS in chloride ion environments, it is necessary to clarify the influence of different levels of MS quality on the chloride penetration resistance of high–strength RAC at different rates of replacement. In this experiment, high–, medium– and low–quality MS are used to prepare MSRAC. The effects of MS of different quality on the compressive strength and chloride penetration resistance of high–strength RAC with different rates of replacement are explored in order to define the optimum rate of replacement of MS of different quality. The microstructure of MSRAC was observed using Scanning Electron Microscope (SEM) technology to clarify the influence of different levels of MS quality on MSRAC.