Classification of 1. concrete admixtures

According to the main function of the admixture in the national standard GB8075-87, the concrete admixture is divided into four categories

1. Admixtures to improve the rheological properties of concrete mixtures, including various water reducing agents, air entraining agents and pumping agents.

2. Admixtures to adjust the setting time and hardening performance of concrete, including retarder, early strength agent and accelerator, etc.

3. Admixtures to improve the durability of concrete, including air-entraining agents, waterproofing agents and rust inhibitors.

4. Admixtures to improve other properties of concrete, including air entraining agents, expansive agents, antifreeze agents, colorants, waterproofing agents and pumping agents.

(I) water reducing agent

Water reducing agent refers to the admixture that can keep the workability of concrete unchanged, and significantly reduce the mixing water. Since the water reducing agent is added to the mixing house, if the unit water consumption is not changed, the water reducing agent is also called plasticizer.

1. Mechanism of action of water reducing agent After mixing cement with water, the cement particles will attract each other and form many flocs in the water. In the flocculent structure, a lot of mixing water is wrapped, so that the water can not play the role of increasing the fluidity of the slurry. When the water reducing agent is added, the water reducing agent can disassemble these flocculent structures and liberate the free water in the package, thus improving the fluidity of the mixture. At this time, if you still need to keep the original concrete workability unchanged, you can significantly reduce the mixing water, play a role in reducing water, so called water reducing agent.

• If the strength remains unchanged, the amount of cement can be reduced while reducing water to achieve the purpose of saving cement.

• 2. The technical and economic effects of using water reducing agents have the following technical and economic effects.

•(1) When maintaining the same workability and not reducing the amount of cement, the amount of mixing water can be reduced by 5-25% or more. Due to the reduction of mixing water, the water cement ratio is reduced, and the strength can be increased by 15-20%, especially the early strength is improved more significantly.

•(2) In the case of keeping the original mix ratio unchanged, the slump of the mixture can be greatly improved (can be increased by 100~200mm), so that it is convenient for construction and can meet the requirements of pumping concrete construction.

(3) If the strength and workability are not maintained, the cement can be saved by 10 ~ 20%.

(4) due to the reduction of mixing water, the bleeding and segregation of the mixture can be improved, which can improve the frost resistance and impermeability of concrete. Therefore, the durability of the concrete can be improved.

• 3. Currently commonly used water reducing agent

Water reducing agent mainly includes lignin system, naphthalene system, resin system, molasses system and humus system, etc. All kinds can be divided into ordinary water reducing agent, high-efficiency water reducing agent, early strength water reducing agent, retarding water reducing agent, air-entraining water reducing agent and so on according to the main functions.

• The brief introduction is as follows:

• 1) lignin-based water reducing agent lignin-based water reducing agent is an important variety of calcium lignosulfonate, referred to as wood calcium powder or M agent, is a brown powder, the main component is calcium lignosulfonate, this water reducing agent has no corrosion hazard to steel bars, and can significantly improve the frost resistance, impermeability and durability of concrete. Due to the retarding effect, the early hydration heat of cement can be reduced, which is beneficial to the construction of hydraulic mass concrete engineering.

Lignin water reducing agent in addition to wood calcium powder, as well as MY water reducing agent, CH water reducing agent, CF-G,WN-1 type wood sodium water reducing agent and other products.

• 2) Naphthalene water reducing agent The main component of this type of water reducing agent is naphthalene and naphthalene congeners sulfonate and formaldehyde condensate, generally brown powder. Naphthalene series water reducing agent has a strong dispersion effect on cement, and its effect of water reduction, enhancement and durability improvement is better than lignin, which belongs to high-efficiency water reducing agent. There are many varieties of this kind of water reducing agent, including NNO,FDN,UNF,NF,MF,JN, Jian -1,AF, etc. Naphthalene water reducing agent is suitable for all concrete projects, and is more suitable for the preparation of high strength, early strength concrete and fluid concrete.

II) Early strength agent

The admixture that accelerates the early strength development of concrete becomes the early strength agent. This kind of concrete can accelerate the process of cement hydration, improve the early strength of concrete, and has no significant effect on the later strength. Commonly used are chloride, sulfate, triethanolamine three categories and their based on the compound early strength agent.

• 1. Chloride salt early strength agent commonly used chloride salt early strength agent mainly calcium chloride and sodium chloride. Chlorine salt admixture can obviously improve the early strength of concrete. Because the chloride salt has the effect of accelerating the corrosion of the steel bar, its dosage is often controlled, and the concrete without reinforcement is generally 1 ~ 3% of the cement quality.

• 2. Sulfate early strength agent The commonly used sulfate early strength agent mainly includes sodium sulfate, mirabilite, dihydrate gypsum and sea wave. They are all white powder, which can hydrate calcium sulphoaluminate crystals generated by cement hydration in concrete to accelerate the hardening of concrete. The appropriate dosage is 0.5 ~ 2% of the weight of cement.

• 3. Triethanolamine Triethanolamine is a kind of organic matter, colorless or light yellow oily liquid, soluble in water, alkaline, accelerate the role of cement hydration, excessive use will cause the strength decreased significantly.

• 4. Composite early strength agent The above three types of early strength agents can be used alone, but the composite use effect is better.

(III) air-entraining agent

In the process of mixing concrete, can introduce a large number of uniform distribution, stable and closed air bubble admixture called air-entraining agent. Air entraining agent can be introduced in the concrete mixture in the diameter of about 0.05~1.25mm bubbles, can improve the workability of concrete to improve the frost resistance of concrete, suitable for port workers, hydraulic, underground waterproof concrete and other projects, commonly used products are rosin thermopolymer, rosin soap, in addition to sodium alkyl sulfonate and sodium alkyl benzene sulfonate.

(IV) retarder

Admixtures that extend the setting time of concrete are called retarders. In concrete construction, in order to prevent the high temperature, transport distance is longer, the concrete mixture of premature condensation affect the quality of pouring, in order to extend the mass concrete heat release time or to prevent the construction of concrete layered pouring cracks in the project, often need to add retarder in the concrete.

(V) antifreeze

Can make the concrete at negative temperature hardening, and in the prescribed time to achieve sufficient antifreeze strength of the admixture called antifreeze. Concrete works under negative temperature conditions shall be mixed with antifreeze. In general, in addition to reducing the freezing point, antifreeze has the effects of promoting coagulation, early strength, and water reduction, so it is mostly a compound antifreeze. Commonly used are NC-3 type, MN-F type, FW2,FW3,AN-4 and so on.