Concrete is a mixture of cement, coarse aggregates, fine aggregates or sand and water. Among them, natural river sand is the main source of fine aggregate or sand. However, with the exhaustion of river sand resources and restricted mining, the contradiction between the supply and demand of construction sand has become more and more intense. Although artificial manufactured sand has become a perfect substitute for natural river sand, but it still cannot meet the needs of the market.

Some coastal countries and regions have begun to consider using sea sand instead of natural river sand, such as the Netherlands, France and Denmark. China, with its long coastline and vast shallow seas, is rich in sea sand resources. Therefore, some areas such as Guangdong Province have begun to accelerate the mining of sea sand, but some areas still resolutely resist sea sand, such as Shanghai. The same is the coastal area, why is the attitude towards sea sand completely different? Can sea sand be used for concrete aggregate? If so, how to use sea sand for construction?

Read also: What is manufactured sand?

Advantages and disadvantages of sea sand

Advantage

  • Sea sand is more like river sand, rounder or more cubic.
  • The resources are sufficient and can be mined at low cost, which can effectively make up for the shortage of river sand resources, avoid the ecological environmental problems caused by excessive mining of river sand, and alleviate the tension of sand use.

Disadvantage

Undiluted sea sand contains a high content of chloride ions, and improper use can cause severe corrosion of steel bars, leading to a sharp decline in the durability of concrete structures.

Sea sand contains a lot of harmful substances, such as mica, light substances, organic matter, sulfide, sulfate and chloride salt, etc. The harm of sea sand mainly comes from the chloride salt contained in sea sand. The chloride ions contained in the chloride salt will cause the steel bars in the concrete to lose the passivation film, which will cause the steel bars to corrode. The volume of the corrosion products will be more than 2.5 times larger than the original volume, which will cause the concrete to crack and peel off, and the wall will crack, Will eventually lead to structural damage, failure, and even building collapse.

Can sea sand be used for concrete aggregate

Can be used for concrete aggregate mainly depends on its durability and chlorine content for concrete. If the durability and chlorine content problems can be effectively solved, sea sand can be used as concrete aggregate. First, the sea sand must be desalinated to make the chloride salt contained in it meet the requirements of the national standard. In addition, anti-corrosion treatments should be applied to the concrete and steel bars using sea sand to ensure the durability of the concrete structure.

Sea sand dechlorination method

The chlorine reduction methods of sea sand include: sea sand natural placement method, fresh water washing method, mechanical method and mixed sand method. In coastal areas, sea sand is basically reduced by fresh water washing method.

1. Natural placement

Naturally accumulate sea sand to a certain thickness. Under the effects of natural wind, rain and sun, after several months or longer, it can be used after sampling and testing the salt content to meet the requirements of the specification.

Disadvantages: It takes a long time and cannot solve emergency needs.
Advantages: low cost, no need to occupy too much area.

2. Fresh water washing method

It is to use fresh water to wash the sea sand to make the salt content meet the standard requirements.

Disadvantages: waste of fresh water resources and high cost.
Advantages: simple operation, short processing time, meeting emergency needs.

3. Mechanical law

Disadvantages: The water consumption is large, and each cubic meter of sand needs to consume more than 1.5t of fresh water. The cost is too high, requiring classification equipment, centrifugal equipment, water supply and drainage equipment, etc.
Advantages: It can be operated on a narrow site, takes a short time, and can be industrially mechanized. 
 

4. Mixed method

The mixing method is to blend sea sand and river sand in an appropriate ratio, and the basic principle is to reduce the salt content.

Disadvantages: There is no specific specification.
Advantages: easy operation and simple principle.

Tips

  • For the sea sand applied by the mixed sand method, the chloride ion content before and after the sea sand is mixed should be accurately measured, and a separate sea sand metering hopper should be added to the mixing station to ensure the accurate amount of sea sand mixed.
  • The mechanical mixing method of sea sand and river sand in the storage bin should be avoided to reduce the influence caused by human factors.
  • At the mixing station, a separate closed silo should be built for the storage of sea sand, and a certain distance should be kept from other raw materials such as river sand.
  • Drainage facilities must be built in front of the sea sand silo to prevent the water discharged from the sea sand silo from polluting other raw materials.

Treatment measures for existing sea sand structures

For buildings (structures) that have used unqualified sea sand, or where the chloride ion content exceeds national regulations due to other factors, the following measures can usually be taken to remove chlorine.

  • Electrochemical dechlorination method
  • Repair directly   
  • Dechlorination of concrete salt discharge agent

Summary

The application of sea sand is a systematic project, involving multiple links such as concrete raw materials, mix ratio, construction process supervision, and post-maintenance. In order to prevent sea sand from being improperly used in the project, it is necessary to strengthen the supervision of all aspects of the use of sea sand. At the same time, it is necessary to further develop more effective sea sand dechlorination methods and application safeguard measures in order to make more effective use of sea sand resources.

Read also: Different types of aggregates

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