With the exhaustion and limited exploitation of river sand resources, the contradiction between supply and demand of construction sand is becoming more and more fierce. China has a long coastline and vast shallow sea, and is rich in sea sand resources. Therefore, some areas begin to accelerate the exploitation of sea sand, such as Guangdong, but some areas still firmly resist sea sand, such as Shanghai.
The same coastal areas are also facing the imbalance between supply and demand of construction sand. Why do Guangdong and Shanghai have completely different attitudes?
[Definition] Sea sand, as its name implies, is sand in the sea. As the second largest marine mineral after petroleum and natural gas, sea sand has many uses, one of the most important uses is as raw materials for engineering construction, especially for large-scale construction of land reclamation. Because chloride ions in sea sand will erode steel bars and bring potential safety hazards to the project. Therefore, as early as August 2004, the Ministry of Construction issued "Opinions on Strict Management of Sea Sand for Construction".
[Distribution] Coastal sand mainly distributes in Shandong, Liaoning, Fujian, Guangdong, Guangxi, Hainan and Zhejiang coastal provinces, while shallow sea sand mainly distributes in Taiwan shoal, east mouth of Qiongzhou Strait and outside the Pearl River estuary.
[Advantages] On the one hand, the exploitation and utilization of sea sand effectively make up for the shortage of river sand resources, avoid the ecological environment problems caused by excessive exploitation of river sand, and alleviate the tension of using sand.
[Disadvantage] Undesalinated sea sand contains a high content of chloride ions, and improper use will cause serious corrosion of steel bars, resulting in a sharp decline in the durability of concrete structures, which may lead to serious engineering accidents such as "sea sand houses".
Sea sand contains a lot of harmful substances, such as mica, light substances, organic substances, sulfides, sulfates and chlorides. The harm of sea sand mainly comes from the chloride salt contained in sea sand. The chloride ion contained in chloride salt will make the steel bar in concrete lose passivation film and corrode. The volume of corrosion products will be 2.5 times larger than the original volume, which will cause concrete cracking, peeling, wall cracking, and eventually lead to structural damage, failure and even collapse of buildings.
Whether sea sand can be used depends on its durability and chlorine content in concrete. First of all, desalination of sea sand should be carried out to make the chloride salt contained in sea sand meet the requirements of national norms. In addition, the concrete and steel bars applied with sea sand should also be treated with anti-corrosion, so as to ensure the durability of concrete structures.
The methods of chlorine reduction include natural placement of sea sand, fresh water washing, mechanical method and mixed sand method. In coastal areas, sea sand is basically dechlorinated by fresh water washing method.
The application of sea sand is a systematic engineering, which involves many links such as concrete raw materials, mix ratio, supervision of construction process, late maintenance and so on. In order to avoid the improper application of sea sand in engineering, it is necessary to strengthen supervision on all aspects of the use of sea sand. At the same time, more effective chlorine removal methods and application safeguards should be developed in order to make more effective use of sea sand resources.