How to Process Sand-gravel Aggregate?

What is Sand-gravel Aggregate?

Sand-gravel aggregate is a general term for sand, gravel, stone and other materials in construction projects, which is the main building material of concrete and masonry stone in hydraulic engineering. Aggregate, a type of granular loose material, acts as the skeleton or filling in concrete. As the main raw material in concrete, aggregate plays the role of skeleton and support in buildings.

Natural sand is rock particles with particle size less than 4.75mm formed by natural weathering, water transport, sorting and accumulation, but excluding soft rock and weathered rock particles. After the materials are mixed, cement is stirred by water and becomes the thin paste. If there is no aggregate, it will not be able to take shape and thus cannot be used.

Aggregate's Type ^[1]

Classification of aggregates (by size)

Aggregate with particle size larger than 4.75 mm is called coarse aggregate, commonly known as the stone. Gravel and pebble are most common. Gravel is naturally formed or made by mechanical crushing and screening system and its particle size is greater than 4.75 mm, which are formed by natural weathering, water transport, sorting and accumulation.

Acicular particles are those with the length of pebble and gravel particles larger than 2.4 times of the average particle size of the corresponding grade. Flaky particles are those whose thickness is less than 0.4 times of the average particle size (mean grain size refers to the average particle size of the upper and lower limit of the particle size).

Aggregate with particle size less than 4.75 mm is called fine aggregate, commonly known as sand. Sand is divided into natural sand and artificial sand. Natural sand, a type of rock particles whose particle size is less than 4.75 mm, is formed by natural weathering, water transport, sorting and accumulation, but excluding soft rock and weathered rock particles.

Natural sand includes river sand, lake sand, mountain sand and desalinated sea sand. Artificial sand is the general name of mechanism sand and mixed sand after soil removal treatment. Let's give an insight into the details.

Fine aggregate (sand)

Rock particles with particle size between 0.15 mm and 4.75 mm are fine aggregate, commonly known as sand. They are divided into natural sand and artificial sand according to their sources.

Sand particles are smooth and clean but often contain broken shells, soluble salt and other harmful substances. Mountain sand granule has many edges and corners, rough surface, mud content and organic impurities. Mechanical sand is natural rock with mechanical crushing, being made after screening. And mixed sand is a mixture of natural sand and artificial sand with no fixed mixing ratio.

Coarse aggregate (stone)

The aggregate with particle size between 4.75 mm and 9.0mm is coarse aggregate, commonly known as the stone. It can be divided into two types, that is, gravel and pebble.

Gravel is natural rock or rock by mechanical crushing, screening system, and the particle size is larger than 4.75 mm of rock particles. Pebble is a kind of rock particle with a diameter of more than 4.75 mm, which is formed by natural weathering, water current transportation, sorting and accumulation. According to the different sources, it can be divided into river pebble, sea pebble and mountain pebble.

Aggregate in concrete

Aggregate, a kind of granular loose material, acts as the frame or filling in concrete. Compared with cement stone, it can make concrete more stable and durable. Also, it is essential in reducing shrinkage and hydration heat, inhibiting crack expansion and force transmission and providing wear resistance. The cost of concrete can be reduced by adding as much aggregate as possible without affecting the performance of concrete.

As the main raw material in concrete, aggregate plays the role of skeleton and support in buildings. When mixing materials, cement is mixed into a thin paste by water. If it is not added with aggregate, it will not be made and can not be used eventually. Therefore, aggregate is a very important raw material in construction. The requirements for aggregate in concrete are as follows:

1. With good particle size distribution, the bulk porosity is smaller than the surface to reduce the amount of cement slurry.

2. The surface of aggregate particles is clean to ensure good adhesion with cement slurry.

3. Aggregate with unharmful impurities is better, or it will affect the cement condensation hardening and later mixed soil durability components.

4. It has enough strength and robustness to play the role of skeleton force transmission.

Market demand for aggregate

Global demand for sand and aggregate could reach 51.7 billion tonnes in 2019 as the property market in developed countries recovers and rising per capita wages in developing countries drive demand for modern housing.

In the aspect of infrastructure, state investment and business investment will also play a part. In 2019, the Asia-pacific region will see the fastest growth in aggregate demand worldwide, with India, Vietnam, Malaysia, Indonesia and other regions among the fastest growing.

However, due to the economic transformation, the growth rate of aggregate demand in China will slow down. By 2019, China will still be the country with the largest aggregate demand in the world, accounting for nearly half of the total aggregate consumption in the world. The growth rate of sand aggregate demand in Africa and the Middle East will be higher than the global average.

In Latin America, many government-supported affordable housing projects will boost construction and aggregate demand in the region. The promotion of the construction industry in North America will benefit the local sand aggregate sales. The sand aggregate industry in Western Europe is about to usher in a new round of growth after a long period of decline, which is a piece of good news for sand aggregate suppliers in the region. In addition, the construction industry in eastern Europe will also warm up.

The use of recycled aggregate from waste concrete and slag will grow twice compared with the growing rate of natural sand. The reserves of natural sand are gradually decreasing worldwide and illegal sand mining is still banned in those developing countries which have a huge demand for sand and gravel aggregate.

By 2019, natural sand reserves in some countries will be rapidly depleted. The depletion of natural sand resources will lead to the rapid rise in the price of sand aggregate, especially in the areas where construction projects with huge aggregate demand are concentrated. This will prompt big resource groups to switch to more affordable alternatives, such as construction waste. A large amount of construction waste generated in modern society will cause a huge waste of resources if it is not utilized but directly buried.

How to process the aggregate (taking granite as an example)

Granite is one of the most common aggregates and is considered the best building materials. It is formed by the slow crystallization of magma beneath the earth's surface. It is a kind of high-grade concrete; It is smooth and polished, hence it is also a beautiful decorative hue for houses and offices. Granite is composed of feldspar, quartz and mica crystals, available in gray, red, pink and many other colors.

Typically, granite is crushed to less than 70 mm of dust to form an easily compacted aggregate.

• 0-2 mm and 0-5 mm granite sand

  It can be used for road construction and decorative paving and playgrounds. In addition, these aggregate parts are best used for the construction of concrete structures and landscaping of garden areas in any home.

• 3-5 mm granite sand

  It can be used to build roads and railways. It sometimes combines with 5-10 mm granite sand because they form the best foundation.

• 5-10 mm granite sand

  It is one of the most commonly used aggregate ranges. It is widely used in the construction of concrete structures, roads, buildings, bridges, drainage foundation and septic tank drainage field construction.

• 5-20 mm granite sand

  For road concrete mixtures, 5-20 mm granite sand is preferred. The reason for using this fraction polymerization is simple. They are the glue that holds the solid concrete together, making the surface smooth and even. They are best added with composite materials for building walls and floors.

• 10-20 mm granite sand

  For laying and filling concrete roads, 10-20 mm granite sand is perfect. They are also used for base preparation of road surfaces.

• 20-40 mm granite sand

  Starting from the class of medium-grade aggregate, 20-40 mm is used to construct the substratum (below the asphalt surface) of the pavement. Large-scale structures such as highways and railways, as well as manufacturers of heavy construction machinery, need to produce reinforced concrete with the help of 20-40 mm granite sand.

• 20-70mm and 40-70mm granite sand

  This bulk of aggregate can be used when construction workers use a large amount of concrete. They are the first choice for industrial spaces and housing construction.

Then, how to process granite aggregate?

In practical applications, granite needs to be crushed into stones of different sizes, so it is necessary to use machines to process and crush them. The selection of the aggregate production line is an important link in the production of modern aggregate mines. The complete production line consists of four systems: feeding, crushing, screening and grinding.

1. Preprocessing

   In this stage, it is mainly used to pre-crush the bulk materials in the raw ore of sand and stone aggregate by means of hammering, sort the sundries which are not suitable for crushing, and remove the sundries such as waste wood and plastic, so as to facilitate the next processing procedure. The primary magnetic separation is carried out to remove the waste iron from the aggregate.

2. Crushing and sand-making

   Firstly, use a vibrating feeder to feed the raw materials evenly. Then use a jaw crusher to crush them. The jaw crusher uses the movement of the jaw plate to crush the large aggregate of sand and gravel into smaller particles. The raw material will be transported to the transit heap by the conveying equipment. Then use the sand making machine to turn the materials into fine plastic, sand and stone aggregate.

3. First-stage screening

   In this stage, the circular vibrating screen is used to divide the coarse cracked concrete aggregate into blocks of 0-40 mm and crushed materials of less than 5mm. The coarse aggregate of 5-40 mm can be directly used for the concrete making of general C15-C20 after being collected. Fine aggregate can be transported to the transit reactor for the next process.

4. Secondary crushing and screening

   The ball mill can be used to grind the fine aggregate again. The crushed aggregate is divided into high-quality recycled aggregate of 5-20 mm by secondary screening, which can be used in the concrete manufacturing of C25-C30. The high-quality recycled fine aggregate of 0.15 mm-5 mm and the fine powder of 0.15 mm or less are selected by tertiary screening.

Common aggregate production line configuration

Generally speaking, the configuration of the aggregate production line should be combined with the user's working condition, capacity, finished product granularity, energy consumption and other requirements.

Common equipment combinations of the aggregate production line are as follows:

1. Jaw crusher + Cone crusher + Sand making machine + auxiliary equipment (feeder, conveyor, etc.).

2. Jaw crusher + Impact crusher + Sand making machine + auxiliary equipment.

Since there is little information related to aggregate in the news and policies, gravel aggregate is generally misunderstood as stone. But stones can be found everywhere, how can sand making be called an industry? This may be the ideas of a considerable number of people of non-industry on the sand aggregate industry.

However, sand aggregate really exists in every aspect of our production and life. People build houses, roads and buildings based on it. Without the supply of sand-gravel aggregate, most of our construction activities will be forced to stop. Therefore, the sand-gravel aggregate industry in people's life and modern industry is particularly important, which can not be ignored.