Concrete admixtures are substances added in the process of mixing concrete to improve the performance of concrete.
1. Classification of concrete admixtures
Chinese standard GB8075-87 divides concrete admixtures into four categories according to their main functions
1. Admixtures for improving the rheological properties of concrete mixtures, including various water reducers, air-entraining agents and pumping agents.
2. Admixtures for adjusting the setting time and hardening properties of concrete, including retarders, early-strength agents and accelerators.
3. Admixtures for improving the durability of concrete, including air-entraining agents, waterproofing agents and rust inhibitors, etc.
4. Admixtures to improve other properties of concrete, including air-entraining agents, expansion agents, antifreeze agents, colorants, waterproofing agents and pumping agents, etc. (1) Water reducing agent
Water reducing agent refers to an admixture that can keep the workability of concrete unchanged and significantly reduce its mixing water consumption. Since the water reducing agent is added to the mixing house, if the unit water consumption is not changed, its workability can be significantly improved, so the water reducing agent is also called a plasticizer.
1. The mechanism of action of the water reducing agent After the cement is mixed with water, the cement particles will attract each other and form many flocs in the water. In the floc structure, a lot of mixing water is wrapped, so that these water cannot play the role of increasing the fluidity of the slurry. When the water-reducing agent is added, the water-reducing agent can disintegrate these flocculent structures and liberate the encapsulated free water, thereby improving the fluidity of the mixture. At this time, if the workability of the original concrete still needs to be kept unchanged, the mixing water can be significantly reduced and the water reducing effect can be achieved, so it is called a water reducing agent.
The first generation, ordinary water reducer. The second generation, superplasticizer. The third generation, polycarboxylate water reducer.
Today we mainly explain the third-generation polycarboxylate (PCE) water reducing agent. Its advantages are: the concrete is dense, so the concrete mixed with PCE has good durability and high water reduction rate, so the strength of the concrete increases rapidly and the final strength is high. . The surface quality of concrete is smooth and dense, the slump loss of fresh concrete is small, and the adaptability to cement is easy to adjust, especially suitable for the preparation of concrete with special technology and construction requirements.
The polyether PCE commonly used at home and abroad is the full name of allyl polyether-acrylic acid copolymer. The structural formula is as follows:
The molecular composition of PCE high-performance superplasticizer, the length of main and side chains, the type of functional groups, and the density of branches all have obvious effects on the performance of the superplasticizer.
The type and length of functional groups in the molecular chain are crucial. The sulfo group (-SO3H) in the molecular structure is beneficial to produce a high-efficiency dispersion effect and improve the water reduction rate. The increase of the amount of carboxylic acid group (-COOM) is beneficial to retard setting and slump retention, delay the slump loss of concrete, and also help to increase the water reduction rate. The increase in the amount of ester groups (-COO-) is beneficial to retard setting and slump loss but has no water-reducing effect. Excessive levels of these functional groups can cause negative effects.
The high content of ester groups makes PCE high in oxygen content, and the volume of oxygen bubbles increases, and the concrete water reduction rate of products with too high carboxyl group content will decrease. The sulfonic acid group will reach the saturation value faster, and the water reduction rate will also reach the maximum value. Therefore, each functional group should be connected with the main chain of the PCE polymer in an appropriate ratio to achieve the effect of high water reduction and better encapsulation performance.
PCE high performance water reducing agent quality standard 2) Early strength agent
Admixtures that accelerate the development of early strength of concrete are called early strength agents. This type of concrete can accelerate the process of cement hydration, improve the early strength of concrete, and have no significant effect on the later strength. Commonly used are chloride salts, sulfate salts, triethanolamine three categories and composite early strength agents based on them.
1. Chlorine salt early strength agent Commonly used chloride salt early strength agent is mainly calcium chloride and sodium chloride. Chloride salt admixture can significantly improve the early strength of concrete. Because chloride salts have the effect of accelerating the corrosion of steel bars, their dosage is often controlled. For unreinforced plain concrete, it is generally 1~3% of the cement quality.
2. Sulfate early strength agent Commonly used sulfate early strength agents mainly include Yuanmingfen, Glauber's salt, dihydrate gypsum and Haibo. They are all white powders, which can form hydrated calcium sulfoaluminate crystals that can be hydrated with cement in concrete to accelerate the hardening of concrete.
3. Triethanolamine Triethanolamine is an organic substance, which is a colorless or light yellow oily liquid. It dissolves in water and is alkaline. It has the effect of accelerating the hydration of cement. Excessive use will cause a significant decrease in strength.
4. Compound early strength agent The above three types of early strength agent can be used alone, but the combined effect is better. (3) Air-entraining agent
In the process of mixing concrete, the admixture that can introduce a large number of uniformly distributed, stable and closed micro-bubbles is called air-entraining agent. Air-entraining agents can be introduced into concrete mixtures with a diameter of approximately
The bubbles of 0.05-1.25mm can improve the workability of concrete and improve the frost resistance of concrete. It is suitable for port engineering, hydraulic engineering, underground waterproof concrete and other projects. Commonly used products include rosin thermopolymer, rosin soap, etc. There are sodium alkyl sulfonate and sodium alkyl benzene sulfonate. (4) Retarder
Admixtures that prolong the setting time of concrete are called retarders. In concrete construction, in order to prevent the premature setting of the concrete mixture affecting the pouring quality in the case of high temperature and long transportation distance, in order to prolong the heat release time of mass concrete or prevent construction cracks in the concrete poured in layers For projects, it is often necessary to add retarders to the concrete. (5) Antifreeze
The admixture that can harden concrete at negative temperature and achieve sufficient antifreeze strength within a specified time is called antifreeze. Concrete works constructed under negative temperature conditions must be mixed with antifreeze. In general, in addition to reducing the freezing point, antifreeze also has the functions of accelerating coagulation, early strength, and water reduction, so it is mostly composite antifreeze. Commonly used are NC-3 type, MN-F type, FW2, FW3, AN-4 and so on.