Use of concrete admixtures

Hydraulic concrete must be mixed with an appropriate amount of admixtures. Commonly used admixtures include water-reducing agents, retarders, air-entraining agents, etc. There are also admixtures that use two functions in combination. The admixtures used need to be tested and optimized for adaptability. During use, the quality and dosage of admixtures must be strictly controlled.

In the preparation process of concrete, mortar and paste, a small amount (not exceeding 5% of the cement dosage) is added into a product that can change the properties of concrete, mortar or paste, which is called concrete Admixtures.

Adding an appropriate amount of admixtures to concrete can improve concrete quality, improve concrete properties, reduce concrete water consumption, save cement, reduce costs, and speed up construction progress. With the advancement of technology, admixtures have become the fifth essential material besides cement, coarse and fine aggregates, admixtures and water. Adding admixtures is an important measure to optimize the design of concrete mix proportions and improve the durability of concrete. . Therefore, the newly revised "Hydraulic Concrete Construction Specifications" (DL/T5144-2001) emphasizes that appropriate amounts of admixtures must be added to hydraulic concrete.

1 Classification of admixtures

Concrete admixtures can be divided into the following four categories according to their main functions:

(1) Improve the flow deformation properties of concrete mixtures Admixtures. Including ordinary water-reducing agents and high-efficiency water-reducing agents, air-entraining agents and pumping agents, etc.

(2) Admixtures that adjust the setting time and hardening properties of concrete. Including retarder, early strength agent and quick-setting agent.

(3) Admixtures that improve the durability of concrete. Including air-entraining agents, waterproofing agents and rust inhibitors.

(4) Admixtures that improve other properties of concrete. Including expansion agents, antifreeze, colorants, etc.

The types of admixtures commonly used in hydraulic concrete mainly include water-reducing agents, retarders, air-entraining agents and various composite admixtures, such as retarding water-reducing agents or retarding high-efficiency water-reducing agents. , early-strength water-reducing agent, air-entraining water-reducing agent, and other types of admixtures, such as pumping agents, waterproofing agents, antifreezes, etc., are also mixed according to special needs. National and industry standards such as GB8076-1997 "Concrete Admixtures" and DL/T5100-1999 "Technical Specifications for Hydraulic Concrete Admixtures" have strict requirements on the performance indicators and technical specifications of these admixtures, which can be carried out according to the different needs of concrete. Select.

2 Water-reducing agent

Water-reducing agent is also called plasticizer or dispersant. Adding an appropriate amount of water-reducing agent when mixing concrete can make the cement particles disperse evenly and release the moisture wrapped in the cement particles, thereby significantly reducing the water consumption of concrete.

The function of the water-reducing agent is to improve the workability of the concrete while maintaining the same mix ratio; or to reduce the water consumption and improve the strength of the concrete while maintaining the same workability; or to maintain the workability of the concrete. When the strength remains unchanged, the cement consumption is reduced, cement is saved, and costs are reduced. At the same time, adding water-reducing agent to the concrete will make it more uniform and dense, improve a series of physical and chemical properties, such as impermeability, frost resistance, erosion resistance, etc., and improve the durability of the concrete.

In the past, the water-reducing agents used in hydraulic concrete were generally ordinary water-reducing agents such as pulp waste liquid, calcium lignin, and molasses. The water-reducing rate was not high, generally 5% to 10%. As the quality requirements for hydraulic concrete increase, the quality requirements for water-reducing agents are also getting higher and higher. Naphthalene-based high-efficiency water-reducing agents are widely used in large hydropower projects such as Ertan and Three Gorges. The water-reducing rate is as high as 20% to 30%. They are mainly used to prepare high-strength and high-flow concrete or concrete that requires a significant reduction in water consumption. High-efficiency water-reducing admixture has a significant effect on improving cement usage efficiency and can save about 20% of cement usage. When ordinary water-reducing agents are used in the granite artificial aggregate concrete of the Three Gorges Project, the water consumption is as high as about 110 kg/m3. The preferred ZB-1A high-efficiency water-reducing agent is used and mixed with DH9 air-entraining agent and Grade I fly ash. , the water consumption is reduced to about 85 kg/m3, reaching the advanced level at home and abroad. The Three Gorges Project also used acrylic high-efficiency water-reducing agent X404 with a higher water-reducing rate of more than 30%. However, this type of water-reducing agent is expensive and is mainly used in high-strength concrete parts.

3 Retarder

Retarder can delay the setting and hardening time of concrete and facilitate construction; it can slow down the hydration rate of concrete slurry and prolong the hydration heat release process, which has the effect of Conducive to temperature control of large-volume concrete. Retarder will reduce the early strength of concrete in 1 to 3 days, but has no effect on the normal development of later strength.

General retarders can extend the initial setting time of concrete by 1 to 4 hours, but this is not enough for concrete construction in large warehouses under high temperatures. In order to meet the needs of large-volume concrete construction in high-temperature areas and high-temperature seasons, the national "Eighth Five-Year Plan" scientific and technological research project has developed a high-temperature retarder. This retarder can be used when the temperature is (35+2) ℃ and the relative humidity is (60+5)%. Under the conditions, the initial setting time of concrete is 6 to 8 hours. The retarding high-efficiency water-reducing admixture currently used in large quantities in the Three Gorges and other projects can make the initial setting time of concrete reach more than 6 to 8 hours by appropriately increasing the dosage, which meets the needs of concrete construction in large warehouses during high-temperature seasons around 35°C.

4 Air-entraining agent

Air-entraining agent is a surface-active substance and one of the commonly used admixtures for concrete. It can introduce a large amount of air from the atmosphere during the mixing process of concrete. Small air bubbles that are evenly closed allow the concrete to contain a certain amount of air. Good leadThe agent can introduce as many as 1 billion bubbles into concrete, with pore diameters ranging from 0.05 to 0.2 mm. They are generally discontinuous closed spheres with even distribution and good stability. This can significantly improve the frost resistance and durability of concrete. (The internal concrete of the Three Gorges Project can withstand more than 150 freeze-thaw cycles, and the external concrete can resist more than 300 cycles); at the same time, it can also improve the workability of concrete, especially when the artificial aggregate or natural sand particles are coarse and the gradation is poor. It has better effect when used in cement-poor concrete; improves the bleeding and segregation of concrete; reduces the permeability of concrete and improves the corrosion resistance of concrete.

The dosage of air-entraining agent is generally in the range of 0.3 to 2 ten thousandths of the weight of cement. Since the dosage is small, it must be weighed accurately and mixed evenly. In addition, there are many factors that affect the amount of air entrainment, such as water-cement ratio, cement dosage, sand rate, aggregates, vibration methods, mixing time, slump, molding temperature, etc., all of which need to be operated strictly, otherwise the desired effect will not be achieved. .

5 Composite admixtures

Composite admixtures are admixtures with more than two main functions. For example, retarding water-reducing admixture has both retarding and water-reducing functions. Aqueous agents have both air-entraining and water-reducing functions.

Many hydropower projects, especially the Three Gorges Project, use two kinds of admixtures in combination, such as retarding high-efficiency water-reducing agent and air-entraining agent, which simultaneously have the functions of efficient water reduction, air entrainment, and retarding. Very good results have been achieved, which not only meets the requirements for retarding the concrete poured on the large warehouse surface, but also achieves the goals of reducing water and improving durability.

6 Principles for Selection of Admixtures

What kind of admixtures should be used in a project should be carefully selected according to the engineering design and construction technical requirements before the project starts, and strictly based on the raw materials. Adaptability test demonstration and confirmation. In the early stages of construction of the Three Gorges Project, more than 30 varieties produced by nearly 30 formal manufacturers of admixtures across the country were initially tested in accordance with national standards. On this basis, several high-quality products were selected from the preliminary trials and were selected by 3 Qualified test units conducted a comprehensive concrete adaptability test based on the raw materials of the Three Gorges Project. After full demonstration and strict review, 2 to 3 admixture varieties with excellent quality and suitable for the actual conditions of the Three Gorges Project were finally selected to meet the requirements. It meets the construction needs of large warehouse area and high-strength concrete, improves various properties of concrete, and achieves good economic benefits.

In order to facilitate management, it is appropriate to select 1 to 2 types of similar admixtures for a large or medium-sized project (including backup). Under normal circumstances, do not change admixture types casually during project construction.

Compared with other raw materials, although the dosage of admixtures is small, it is crucial to the quality of concrete. Therefore, after its dosage is determined through experimental verification, it should be strictly controlled to ensure that the quality of admixtures is extremely stable. The properties should be strictly inspected in accordance with relevant standards during delivery and use, and the transportation and storage of admixtures should also be strictly implemented in accordance with relevant standards.

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