Cui Jun 1. Xu Xun 1. You Panli 2, Ji Mengqi 1, Nie Yanni 1, Wang Zonghao 1, Yu Rui 1, Long Hao
(1) School of Civil Engineering and Architecture, Southwest University of Science and Technology. 621010 in Mianyang, Sichuan: 2. Such as price fluctuations, supply and demand imbalances, uneven quality of machine-made sand and gravel, etc. Put forward the methods to alleviate the shortage of construction sand and gravel resources and solve the existing problems, which can provide a reference for the healthy development of China's construction sand and gravel market.
[Key words] concrete; sand and gravel resources; machine-made sand and gravel; construction industry
0 Preface
Sand and gravel is an important building material. Sand and stone resources in China are abundant and widely distributed, and mines, rivers and lakes are common mining objects. According to statistics, the development and utilization of non-metallic minerals have increased by leaps and bounds for three times in human history, the first time around 1970, the second time around 1990, and the third time after 2000 [1]. Since the mid-1980s, in order to meet the growing demand for the development of the construction industry, China has carried out large-scale river sand and gravel mining [2], which has had a great impact on the geological environment of the Yangtze River and other basins. At the same time, with the restriction of the exploitation of natural sand and gravel resources. There is a shortage of sand and gravel for engineering construction. This has led to a series of engineering quality problems [3]. Therefore, the analysis of the current situation of sand and gravel resources in China and the problems faced in the development and utilization of sand and gravel resources is conducive to the rational utilization of sand and gravel resources and the promotion of the sustainable and healthy development of the construction industry.
This paper mainly analyzes the reasons for the shortage of sand and gravel resources for construction in China and the existing problems of sand and gravel for construction at the present stage, and puts forward relevant solutions, which can provide reference for the rational utilization of sand and gravel resources for construction in China. Sand and gravel resources are
widely distributed in nature, mainly used for road, building and other engineering construction materials, and are one of the necessary raw materials for social and economic development. Sandstone resources are easy to explore and exploit, and their distribution, quality and exploitation scale can be ascertained through investigation and evaluation or simple general survey. Sand and gravel mining is generally open-pit mining, with low mining technology requirements, less investment and low cost [4-5]. Sand and gravel is the largest, indispensable and irreplaceable raw material in China's infrastructure construction. According to statistics. The annual consumption of sand and gravel in China is about 20 billion tons [6], equivalent to the average annual consumption per person 13.
At present, there is a shortage of natural sand and gravel supply in China, mainly due to the unbalanced development and utilization of sand and gravel resources and the restricted exploitation of natural sand and gravel resources.
1.
The development and utilization of sand and gravel resources are related to local policies, economic development level and development stage. According to statistics, there are obvious differences in the development and utilization level of sand and gravel resources in different provinces of China. By the end of 2020, according to the distribution of sand and gravel mines in each province (see Table 1), the number of sand and gravel mines in Guizhou, Yunnan and Xinjiang is relatively large, with 1632, 1049 and 749 respectively, while the number of sand and gravel mines in Beijing, Tianjin and Shanghai is 0.
Table 1 Distribution
of sand and gravel mines in China 1. With the rapid development of the construction industry, the demand for sand and gravel is increasing, and the over-exploitation of natural sand and gravel is frequent, resulting in severe environmental problems. For example, the over-exploitation of river sand in the Mekong Delta region of southern Vietnam has led to more and more serious landslides. In addition, the exploitation of sand and gravel from the riverbed will inevitably cause longitudinal and transverse deformation of the riverbed, thus changing the river regime and affecting the river evolution [7-8]. Mud and sand in nature are mainly obtained through natural siltation, but relative to the amount of sand mining, its own supply speed is very slow, unable to meet the demand for sand mining [9-11]. With the continuous exploitation of natural sand resources, the ecological environment is gradually destroyed, and corresponding measures should be taken to protect the ecological environment [12]. For this reason, various regions have issued relevant policies to restrict the exploitation of natural sand and gravel resources in order to further standardize the exploitation of natural sand and gravel and protect the ecological environment.
With the restricted exploitation of natural sand and gravel resources and the rapid rise in the price of river sand, some areas in China have begun to use a large number of imported natural sand and gravel and machine-made sand and gravel. Chloride ions
in desalinated sea sand will lead to corrosion of steel bars, thus affecting the quality of the whole construction project, and should be used with caution. In addition, with the increasing demand for desalinated sea sand, a large number of unqualified desalinated sea sand flows into the market, seriously affecting the quality of the project and endangering the safety of people's lives and property. Since 2010, the mining work in Ningbo, Zhoushan, Pearl River Estuary and other sea sand mining areas has been suspended.
Imported natural sand and gravel (the main importing countries include Malaysia, Russia, North Korea, etc.) Is a part of the sand and gravel used in engineering construction in China, but the price of imported natural sand and gravel is expensive and not suitable for large-scale application.
With the limited exploitation of natural sand and gravel resources and the increasing demand for sand and gravel in construction projects, the application of machine-made sand and gravel in construction projects is increasing (see Figure 1). Compared with river sand, machine-made sand is more regular and can be processed into sand of different specifications and sizes to meet the needs of various projects. Divided by geographical regions, the production lines of machine-made sand and gravel in China are mainly concentrated in the western, central and eastern regions; divided by administrative regions, the production lines of machine-made sand and gravel are mainly distributed in Zhejiang, Guangdong, Guangxi, Guizhou and other places. It is worth noting that the preparation of machine-made sand and gravel requires higher equipment, which requires professional equipment to produce qualified and market-oriented machine-made sand and gravel.
Fig. 1 Market demand
for sand and gravel application in China from 2012 to 2020 2. However, in recent years, the price of machine-made sand and gravel has also remained high, resulting in a shortage of supply and demand. Figure 2 shows the price trend of machine-made sand and natural sand in China in 2021.
2. However, with the restricted exploitation of natural sand resources, the output of natural sand has been declining, resulting in an imbalance between supply and demand.
In recent years, the sand content of major rivers in China has decreased sharply. At the same time, some local governments do not have a good understanding of the importance of sand and gravel resources, simply and broadly carry out sand and gravel remediation work, the relationship between "blocking the back door" and "opening the front door" has not been well coordinated, the number of enterprises and sand and gravel production has dropped dramatically, resulting in short-term imbalance between supply and demand of sand and gravel in the region, and the price has risen sharply. This may lead to the following problems: a large number of low-quality sand and gravel enter the market, which not only indirectly leads to the increase of construction costs, but also affects the progress of construction and brings safety problems.
2. At present, many researchers have done a lot of research on the application of machine-made sand instead of natural sand in construction engineering. Zhong Meiqin et al. [18] found that the production method (equipment) of machine-made sand has an impact on its particle morphology, needle-like particle content and other properties. Compared with the machine-made sand produced by the frontal crusher, the machine-made sand produced by the CXL fine crusher has a less sharp angle, a smoother surface with flaky particles and no needle-like particles. Moreover, the content of flaky particles is low, and the amount of concrete mixing water is low. However, at present, more jaw crushers are used in small and medium-sized enterprises, and the production process is simple, resulting in uneven quality of machine-made sand [19]. Machine-made sand for concrete has high requirements for sand particle morphology and gradation, so it is necessary to select appropriate production methods (equipment) of machine-made sand according to different engineering needs.
At present, the shapes of machine-made sand particles on the market are different, and the fineness modulus is more than 3.0 [20-21], which is quite different from the particle gradation of natural sand. Generally, the particle size of machine-made sand is more than 2.50mm and less than 0. In addition, machine-made sand usually contains a certain amount of stone powder and mud. The particle size of stone powder is generally less than 0.075 mm, mostly between 0.016 and 0.075 mm. The fineness of slurry is relatively small, and the particle size is generally between 0. Stone powder and slurry are easily adsorbed on the surface of machine-made sand, which reduces the bonding performance between machine-made sand and cement matrix. Thereby affecting the performance of the manufactured sand concrete.
3. Solution
3. Due to the need of ecological environment protection, the exploitation of natural sand and gravel is restricted, which makes the price of sand and gravel unable to be regulated by means of market competition to balance supply and demand. In this case, if the right to exploit and sell sand and gravel is completely handed over to an individual group or legal entity, it will inevitably lead to price monopoly and lead to the rise of sand and gravel prices. Therefore, if sand and gravel resources can be used as public goods such as natural gas and gasoline, the monopoly of sand and gravel prices can be effectively avoided.
Whether in the structural reform of the supply side or in the exploitation and utilization of the mechanism, government intervention is needed. In addition to formulating policies related to sand mining management, special policies based on market mechanism are also needed. As a kind of public goods, sand and gravel are provided, purchased, pre-homogenized and sold to the demanders by the government departments, so as to realize the transformation from the industrial structure of "exploiter → enterprise mixing station" to the industrial structure of "government → exploiter → government → enterprise/mixing station", so as to effectively control the price of sand and gravel and ensure the best quality of sand and gravel. Considering that the public goods in the current market are between the quasi-public goods with strong marketization and the pure public goods with high natural monopoly. Therefore, if sand and gravel are classified into public goods, the government should cooperate with producers through entrusted management, so as to avoid some economic risks to a certain extent.
3. However, the slurry and stone powder in the machine-made sandstone will have a negative impact on the workability and mechanical properties of concrete. At present, small and medium-sized enterprises mostly use collar crusher to prepare machine-made sandstone, which is prone to excessive crushing, resulting in excessive content of stone powder in machine-made sandstone. Therefore, in order to produce the required Youtuan machine-made sand and gravel, the machine-made sand and gravel are usually washed, but this will increase the cost of labor, machinery and water and electricity. It is only applicable to the project with small amount of machine-made sand and gravel. In addition, the washing process may destroy the particle distribution of the machine-made sand and gravel, thus affecting the workability of the mixed soil [27-28].
In view of the above problems, the content of mud and stone powder in machine-made sand can be reduced by changing the preparation method of machine-made sand. For example, the use of fly ash geopolymer to prepare artificial sand can not only realize the recycling of industrial solid waste fly ash, but also prepare machine-made sand with good quality [29]. Gao Yuxin et al. [30] used silica-based waterproofing agent and clay swelling agent as surface modifiers of machine-made sand, which effectively reduced the negative impact of slurry and stone powder in machine-made sand on the workability of mixed sand.
Conclusion
Due to the needs of relevant policies and ecological environment protection, the shortage of natural sand and gravel supply will exist for a long time, and the resulting problems such as price mechanism and different quality of sand and gravel need to be solved urgently. On the one hand, the government can establish a unified public goods service platform for the purchase and sale of sand and gravel to macroscopically control the price and quality of sand and gravel; on the other hand, the sand and gravel industry can optimize the production mode of machine-made sand and gravel, promote the standardized development of sand and stone production equipment, and formulate relevant standardized production and technical standards to promote the healthy development of China's construction sand and gravel market.
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