1 The classification of concrete admixtures is based on my country's current national standard "Classification, Naming and Definition of Concrete Admixtures" CB8075-87. Concrete admixtures are added during the process of mixing concrete. Added, substances used to improve the properties of concrete, the dosage should not exceed 5% of the mass of cement (except for special circumstances). From the perspective of usage functions, concrete admixtures are divided into five categories: ① Improve the rheological properties of fresh concrete - water reducing agents, air entraining agents, water retaining agents, etc.; ② Adjust the setting and hardening properties of concrete - retarder , early strength agent, accelerating setting agent, etc.; ③ Adjust the gas content of concrete - air-entraining agent, air-generating agent, foam agent, defoaming agent, etc.; ④ Improve the durability of concrete - air-entraining agent, rust inhibitor, Antifreeze agents, impermeability agents, anti-corrosion agents, etc.; ⑤ Provide special properties for concrete - gas-generating agents, foaming agents, colorants, expansion agents, alkali-aggregate reaction inhibitors, etc.
2 Purposes of concrete admixtures 2.1 Water-reducing agent Water-reducing agent is a surface-active material that is added to concrete to diffuse cement particles and reduce the amount of cement particles. The free water in the aggregate is released, thereby keeping the working performance of the concrete unchanged and significantly reducing water consumption, lowering the water-cement ratio, improving workability, increasing fluidity, and saving cement. Types of water-reducing agents include lignin-based, sulfonate-based, water-soluble melamine resins, sugar-based melamines, etc. 2.2 Air-entraining agent Air-entraining agent can produce a large number of fine, evenly distributed closed bubbles in mortar and concrete, which can improve the workability, impermeability, frost resistance and durability of concrete. Types of air-entraining agents include sodium alkyl sulfonate, rosin soap, OP emulsifier, 801-2, etc. 2.3 Setting regulating agent Setting regulating agent is an admixture that adjusts the setting time of cement. It has a significant impact on the setting time and strength development of concrete. The setting-adjusting agents include retarder, early-strengthening agent and accelerating setting agent. A retarder is an admixture that delays the reaction between cement and water, thereby prolonging the setting time of concrete. Retarder varieties include boric acid, phosphoric acid, NNO, calcium lignosulfonate, sodium, etc. Early strength agents can improve the early strength of concrete, and have obvious effects on accelerating formwork turnover, accelerating project progress, and saving winter construction costs. Early strengthening agent varieties include chloride series, sulfate series, triethanolamine, calcium formate, etc. Accelerated setting agents are generally used in spraying or emergency repair projects. Accelerated setting agents can set and harden concrete within a few minutes and have the characteristics of high adhesion. Generally, the initial setting is 1min to 5min, and the final setting is 2min to 10min. Types of accelerating setting agents include Hongxing Type 1, Type 711, Type 782, etc. 2.4 Expansion agent Expansion agent is an admixture that causes expansion of concrete or mortar due to chemical reaction. Its main function is to compensate for the shrinkage of concrete. Expanding agent products include calcium sulfoaluminate, gypsum, iron powder, lime, alunite, etc. 2.5 Air-entrained agent Adding air-entrained agent to concrete can cause the air-entrained agent to react with the alkali in the cement to generate gas to form countless micron independent bubbles to generate air-entrained concrete. Air-entraining agent products include scaly aluminum powder, sodium rosin acid, rosin thermopolymer, etc. 2.6 Foaming agent Foaming agent can reduce the surface tension of the solution, produce uniform and stable foam, and generate foam concrete. Foaming agents also have the effect of strongly slowing down agglomeration. Foaming agent products include rosin soap, animal hair, resin saponin grease, aluminum petroleum sulfonate, etc. 2.7 Rust inhibitors Rust inhibitors are admixtures that prevent or reduce the corrosion of steel bars or metal embedded parts in concrete. Rust inhibitors include benzoate, phosphate, fluorosilicate, chloroaluminate, etc. 2.8 Antifreeze Antifreeze is an admixture that lowers the freezing point of water in concrete. Antifreeze types include calcium chloride, sodium chloride, sodium sales, ammonia, urea, etc.
3 Application of admixtures in engineering 3.1 Select appropriate admixtures according to project characteristics Almost all kinds of concrete can be mixed with admixtures, but they must be based on the engineering concrete Choose the appropriate type of admixture according to different standard requirements, construction conditions and construction techniques. 3.2 Pay attention to the quality of admixtures. Currently, there are many brands of admixtures in my country. Many manufacturers do not clearly indicate their product varieties and the quality is different. Therefore, before application in the project, the selected admixtures should be dosed and mixed in accordance with the quality standards. Performance testing. 3.3 Pay attention to the selection of cement varieties. Among admixture materials, cement has the greatest impact on water-reducing admixtures. Different types of cement will affect the water-reducing and enhancing effects of the water-reducing agent, especially the water-reducing effect. High-efficiency water-reducing agents are more selective for cement. The water-reducing rates of different cements vary greatly. Cement mineral composition, admixtures, setting agents, alkali content, fineness, etc. will all affect the effectiveness of the water-reducing agent. Therefore, the same water-reducing agent at the same dosage often has significantly different effects due to different cements, or the same water-reducing agent can be used in different cements to achieve the same water-reducing and enhancing effect. The dosage of the water-reducing agent must be Also obviously different. When a water-reducing agent is available for selection, a water-reducing agent that is more suitable for the cement used in construction should be selected to make the water-reducing agent perform better. Before use, a suitability test of cement and admixtures should be conducted. 3.4 Pay attention to the dosage. Each admixture has a suitable dosage. Even the same type of admixture has different suitable dosages for different uses. Excessive dosage is not only economically unreasonable, but may also cause quality accidents. If the dosage of high-efficiency water-reducing agent is too small, the high-efficiency effect will be lost, while if the dosage is too large, the quality will be affected due to bleeding. 3.5 Pay attention to adjusting the mix ratio of concrete. Generally speaking,, admixtures have no special requirements for concrete mix proportions and can be designed according to ordinary methods. However, when reducing water or saving cement, appropriate adjustments should be made to the sand rate, cement dosage, water-cement ratio, etc. 3.5.1 Sand rate Sand rate has a great influence on the workability of concrete. Excessive sand rate not only affects the strength of concrete, but also increases the plastic shrinkage of concrete. The specific ratio should be determined by the trial matching results. 3.5.2 Amount of cement The use of water-reducing admixtures in concrete can save cement to varying degrees. The use of ordinary water-reducing admixtures can save 5% to 10%, and the use of high-efficiency water-reducing admixtures can save 10% to 20%. 3.5.3 Water-cement ratio The water-cement ratio of concrete mixed with water-reducing agent should be determined according to the water-reducing rate of the mixed type. 3.6 Pay attention to the construction characteristics 3.6.1 During the mixing process, the amount of water-reducing agent and water must be strictly controlled, and the appropriate mixing method and mixing time should be selected to ensure that the water-reducing agent is fully effective. There should be different process requirements and precautions for different mixing methods. 3.6.2 Note that the slump loss of concrete mixed with admixtures (especially early-strength admixtures or early-strength water-reducing admixtures) is generally faster, and the transportation and parking time should be shortened, generally not more than 30 minutes, otherwise the post-mixing method must be used. 3.6.3 Pay attention to concrete pouring and forming operations. When pouring concrete, attention should be paid to maintaining its homogeneity to avoid segregation. 3.7 Pay attention to economic benefits (pay attention to the relationship between the price and dosage of admixtures). When selecting admixtures, admixtures should be reasonably selected based on the quality requirements of the concrete for construction, construction process conditions, construction methods used, ambient temperature conditions and the measures taken. At the same time, on the premise of ensuring the quality of the admixtures, admixtures of the same type, for the same purpose and to achieve the effect of concrete use must be compared with three companies, and the dosage should be as low as possible and the price is reasonable. </p
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