Sand and gravel, also known as aggregates, is both ancient and modern material. It is called ancient because as early as the 10th century BC, the ancient Greek Olympians used gelatin and natural sandstone in the Aegean Sea to build house; it is called modern because it is possible to mine and produced with modern advanced technology today.
Because the early aggregates industry use traditional and decentralized production method, people did not know enough about aggregates products and industry. In terms of science and technology, many people believe the aggregates industry is low in technology, and research and development of its mining, technology, quality, and its performance and role in various concrete constituent materials are insufficient; from the industry level, the aggregates industry is often regarded as the simple basic materials industry, so the government did not formulate enough policies and standards for its development. Therefore, it is very important to raise awareness and publicize the importance of aggregates mining, processing technology, product quality and the importance of the industry.
1. Classification and source
(1) Classification of aggregates (sand and stone)
Sand generally refers to particles with a size of less than 4.75mm, including river sand, mountain sand, lake sand, desalinated sea sand, etc. The thickness of the sand is divided into 4 grades according to the fineness modulus, the coarse sand’s fineness modulus is from 3.7 to 3.1, and the average particle size of coarse sand is more than 0.5mm; the medium sand’s fineness modulus is from 3.0 to 2.3, and the average particle size is from 0.5mm to 0.35mm; fine sand’s fineness modulus is from 2.2-1.6, the average particle size is 0.35mm-0.25mm; extra fine sand’s fineness modulus is 1.5-0.7, the average particle size is less than 0.25mm. The larger the fineness modulus, the coarser the sand. The fineness modulus of ordinary concrete sand is in the range of 3.7 to 1.6, and medium sand is suitable for concrete, or coarse sand plus a small amount of fine sand, with the ratio 4:1.
Stone refers to particles with a particle size greater than 4.75mm. There are two kinds, which are gravel and pebble. Gravel is a particle with a particle size greater than 4.75mm made from natural rock or produced through mechanical crushing and screening; Pebble is a particle with a particle size greater than 4.75mm formed by natural weathering, water flow handling and sorting. If the length of the particles in the pebbles and gravels is 2.4 times greater than the average particle size, they are needle-shaped particles, and if the thickness is less than 0.4 times the average particle size, they are flaky particles. The average particle size refers to the average value of the upper and lower limits of the particle size.
Natural sand and gravel refers to a loose mixture of sand and gravel, consisting of rock debris and cement. The composition of the rock is mainly quartz and feldspar, and it is related to the surrounding rock or the type of rock exposed in the upper stream of the river; the cement in the rock is commonly calcareous, siliceous, iron, and muddy. Different rocks differ in cement.
The structure of different sands and stones is quite different, and some particles have uniform thickness and some have different sizes; some sands and stones have good roundness and some have sharp edges and corners. Sand and stone belongs to loose materials, but its particles are generally hard and have stable chemical properties in the surface environment.
Natural sand and gravel are generally obtained by screening directly. The manufactured sand is obtained by mining the mountain and processing the ore through crushing, screening and shaping. The characterization method of natural sandstone is adopted for manufactured aggregates.
In actual use, the fineness modulus of aggregates can only be used as a reference, and the true gradation of aggregates should be paid attention to.
(2) The source of aggregates
People usually call the sand collected from rivers, mountains, seas, etc. river sand, mountain sand and sea sand, or collectively called natural sand, which is a natural mineral resource. With the increase in the use of buildings, roads, bridges and infrastructure, as well as the reduction of natural sand and gravel resources and the protection of the environment, mining in recent years has been the main source of manufactured aggregates. There are three types of minerals commonly used in manufactured aggregates: magmatic rocks, sedimentary rocks, and metamorphic rocks. Magmatic rocks are divided into basalt, andesite, gritstone, rhyolite, granite, syenite, diorite, gabbro, and peridotite; sedimentary rocks are divided into conglomerate, sandstone, siltstone, limestone, dolomite; metamorphic rock is divided into marble, quartzite, gneiss, corner shale, serpentine. Different mine ores have different physical and chemical properties.
2. The role of aggregates in construction and infrastructure
Aggregates are the basic materials in national economic construction, mainly used for basic engineering, concrete, mortar and corresponding products, and is the foundation for construction, highway, railway, bridge, municipal engineering, water conservancy engineering, hydropower engineering, nuclear power plant engineering, airport, wharf, etc. Aggregate materials are indispensable and irreplaceable in facility construction. At present, cement concrete is the largest and most widely used in engineering construction, followed by asphalt concrete, and the amount of polymer concrete is relatively small.
Concrete is a composite material, and aggregates is one of the main component materials with the largest proportion. In concrete structures, aggregates mainly serve as a skeleton, stabilizing the volume of concrete, transmitting stress, while suppressing shrinkage and preventing cracking. It plays an extremely important role in construction and infrastructure. It can be said that without aggregates, there will be no concrete, and there will be no concrete buildings and basic engineering.
3. Reflections on the development of manufactured aggregates industry
(1) The aggregates industry should transform into industrialized, large-scale and intensive mode
Aggregates resources are rich, which can form industrialized and large-scale production, and product quality is stable. The development manufactured aggregates not only saves natural sand and gravel resources and protects the environment, but also provides important conditions for the aggregates industry to move towards a standardized and industrialized development path, and at the same time can accelerate the adjustment and technical upgrade of the traditional aggregates industry structure .
The development manufactured aggregates is important for rational use of resources, standardization of mining order, and protection of the environment, the development of high-performance concrete technology, and the needs of the social development process.
(2) At present, it is the best time for aggregates production enterprises to integrate regional markets and drive regional industrial structure adjustment and technological upgrading. In recent years, the central and local governments have strengthened the planning of aggregates mining and the management and rectification of enterprises. They have adopted “withdrawal, merger, and closing down” for small enterprises. Some enterprises in some areas have transformed and upgraded mining equipment to form a larger scale. Some powerful enterprises have invested a lot to build manufactured sand production lines and adopted advanced environmental protection measures. They have become a national demonstration enterprise, leading the development of aggregates enterprises towards large-scale, intensive and environmental protection.
At present, there are dozens of aggregates production lines with an annual output of several million tons to 10 million tons or even more than 10,000 tons in China, and they are increasing at a rate of more than a dozen production lines per year. All newly-built aggregates production lines have adopted a full range of environmental protection and safety measures.
In recent years, China's aggregates industry has accelerated the pace of industrial restructuring and technological upgrading, from the traditional manual mining to the large sets of technical equipment mining and production, from the traditional decentralized production method to a relatively concentrated large-scale production mode. These companies generally attach great importance to large-scale development, pay attention to quality, energy saving, production safety, and recycle wastewater and dust in the production process, pay attention to environmental protection, and the industrial structure is constantly upgraded.
At present, the number of aggregates companies in the country has been rapidly reduced from more than 20,000 to around 10,000. The industry is rapidly changing to the direction of industrialization, large in scale and intensiveness, which has made great progress, and the image has been greatly improved.
(3) To formulate reasonable mining plans and corresponding policies to guide the healthy development of the industry
At present, China’s aggregates industry develop rapidly, especially the development of manufactured aggregates that has entered an important period of transformation, and requires re-planning and layout, and reasonable policy to guide and support.
Aggregates are local resources. Local governments should re-plan and lay out aggregates mining according to local market demand and mine resources. Relevant policies need to be sorted out, and revised or formulated according to the new situation and requirements. Policies encourage enterprises to reorganize and unite, and mine according to the state’s requirements for resource development and environmental protection.
Of course, aggregates companies also need self-discipline and environmental protection awareness. Companies should recycle stone powder and waste water produced in the production process to meet the requirements of "zero discharge". The government should formulate policies and measures that encourage restoration of mining areas.
(4) Like other materials, aggregates need to be studied
Traditional aggregates products are mainly obtained by mining natural resources. Sand quarrying is mainly excavation, screening, transportation, etc. The collected aggregates are hardly processed, or processed in a small amount. Due to the simple production of traditional aggregates, it is generally believed that the technology involved is not complicated, and few people have studied it as a specialized technology. Aggregates belong to local resources. For a long time, due to the lack of reasonable planning and effective management of aggregates mining by local governments, some enterprises do not produce according to the regulations, resulting in people generally thinking that the aggregates industry is a traditional extensive industry. Once the aggregates industry is talked about, people considered it dirty and messy, destroying the environment and affecting the lives of the mining area and surrounding residents.
With the increasing shortage of natural aggregates resources and the continuous improvement of national requirements for environmental protection, most areas have restricted the exploitation of natural sand and gravel. In order to meet demand, manufactured aggregates has become an inevitable choice. Manufactured aggregates are very different from traditional sandstone production because the former involves multiple technologies. To obtain high-quality aggregates, the mine needs to be explored, and the mining and processing production lines should be reasonably laid out, especially for the large-scale production lines, they need factory design, including construction, operation management, and safety of the system , environmental protection and other aspects. Comprehensive consideration is made from mining exploitation, resource utilization, energy consumption, dust, noise, waste water treatment and recycling, and the concept of intelligent energy saving is implemented through all-round green manufacturing and the entire system. The core technology of mining technology, ore processing technology and mechanical equipment needs to be researched; the particle gradation, particle type, surface characteristics, needle flake particle content, stone powder content, void content and aggregate-slurry need to be researched. To make high-performance and high-strength concrete and basic engineering, more in-depth and meticulous research is required on the composition, particle shape, particle size, particle grading, water absorption rate, linear expansion coefficient, etc. to effectively solve the problems of fineness modulus, stone powder content and water content of aggregates.
(5) Establishing aggregates R&D and quality inspection institutions, and increasing R&D and testing efforts are urgent tasks
With the continuous development of China's society and economy, the requirements for the quality of buildings and infrastructure are getting higher and higher, and the requirements for comprehensive utilization of resources and environmental protection are becoming stricter. In particular, the usage of high-performance concrete are constantly increasing which requires high quality aggregates. Improving the overall level of technology, quality standards, testing and management in the aggregates industry has become a top priority. In the case where manufactured sand has become the main development direction and production method, to realize industrialization and large-scale production, we must pay attention to the research and development of mining, crushing, and screening technologies. We must have theoretical guidance, technical support, and we need to set up R&D institutions, intensify R&D efforts, and conduct in-depth research on mining and equipment technology, product standards, quality, and testing. It is necessary to strengthen the construction of the quality inspection system, formulate more standards, strengthen the formulation and promotion of standards, and enhance quality awareness.
(6) The aggregates industry has a promising future in the construction waste recycling
With the rapid development of construction and infrastructure construction, China consumes about 11 billion tons of aggregates annually. At the same time, the reconstruction of old buildings and infrastructure in cities in China will generate a large amount of construction waste. Among all construction waste, the amount of waste concrete is the largest, which brings great pressure to resources and the environment. If these wastes are not used, it will increase the environmental load and waste resources. Many countries in the world treat waste concrete and use it as recycled aggregate in reconstruction projects, which not only saves natural aggregate resources, but also benefits the environment.
The aggregates industry has significant advantages in the recovery and reuse of waste concrete, and it has an important impact on the implementation of a resource-saving and environmentally-friendly sustainable development national policy, and is of great significance for the implementation of resource recycling.
At present, the development of the aggregates industry is in a critical period, where challenges and opportunities coexist. The challenge is to change the traditional mining and production methods; the opportunity is to accelerate the process of industrialization, intensive and large-scale development. Colleagues in the industry need to be firm in their beliefs, use their wisdom and courage to speed up industrial restructuring and technological upgrading, and create a new situation in China’s aggregates industry.
Editor's postscript:
On November 4, 2019, 10 departments including the Ministry of Industry and Information Technology, the National Development and Reform Commission, the Ministry of Natural Resources, the Ministry of Ecology and Environment, the Ministry of Housing and Urban-Rural Development, the Ministry of Transport, the Ministry of Water Resources, the Ministry of Emergency Management, the General Administration of Market Supervision, and the National Railway Group jointly issued “Several Opinions on High-Quality Development of Aggregates Industry”. This is the first special guidance document issued by the central government for the aggregates industry.
On March 25, 2020, 15 departments including the National Development and Reform Commission, the Ministry of Industry and Information Technology, the Ministry of Public Security, the Ministry of Finance, the Ministry of Natural Resources, the Ministry of Ecology and Environment, and the Ministry of Housing and Construction jointly issued the "Guiding Opinions on Promoting the Healthy and Orderly Development of the Aggregates Industry". The National Development and Reform Commission made a special statement when issuing this opinion: aggregates are the most basic and indispensable building material in engineering construction. The Communist Party Central Committee and the State Council attach great importance to the healthy and orderly development of the aggregates industry. In order to stabilize the supply of the market, maintain the overall price stability, and promote the healthy and orderly development of the industry, 15 departments jointly issued the "Guiding Opinions" with the approval of the State Council.
In recent years, more than ten departments of the country have jointly issued a document to vigorously promote the healthy development of the aggregates industry, which is unprecedented and means a great opportunity for development the aggregates industry. The Chinese Aggregates industry must cherish this precious opportunity and comprehensively promote the healthy and orderly development and high-quality development of the industry.