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Introduction of early strength agent

Release time:

2022-04-14

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　early strength agentIt is an additive that can promote the development of early strength of concrete. It can make the concrete obtain higher strength in a short time after mixing concrete, so as to meet various requirements in engineering construction. In the 1990 s, due to the addition of sodium sulfate, the content of alkali in cement increased, so it was easy to produce the reaction between alkali and pipe. There are some restrictions on the use of sodium sulfate. High performance concrete, early strong with water reducing agent, of course, high cost. 1) Chlorine saltearly strength agent.

Calcium chloride, sodium chloride, potassium and aluminum chloride are commonly used as early strength agents for chloride salts in production. The mechanism of action is that the hydrated chloroaluminate generated by the reaction of C3A with chloride in cement is insoluble in water and can promote the hydration of C3A. Calcium hydroxide produced by cement hydration reacts with chloride, and the generated calcium oxychloride is insoluble in water, which can reduce the concentration of calcium hydroxide and accelerate the hydration reaction of C3S in the system. The resulting double salt product will increase the proportion of solid phase in the slurry, which is conducive to the formation of a strong cement paste structure. At the same time, chloride is usually soluble in water, and the salt effect will increase the solubility of minerals in cement, promote the hydration reaction rate of cement minerals, and shorten the hardening time of cement concrete. It is easy to cause corrosion of steel bars, which limits the application range of chlorine salt-based early strength agent to a great extent. GB8076-2008 "Concrete Admixtures" and China's concrete admixtures technical specifications do not

The chloride ion content in the same type of concrete is strictly regulated. Chlorine early strength agent can only be mixed in plain concrete without reinforcement. In the construction of reinforced concrete, prestressed reinforced concrete or concrete with metal embedded parts, the introduction of Cl-should be strictly limited, and the use of such early strength agents should be cautious or prohibited.

Sulphateearly strength agent

Sulphateearly strength agentThere are sodium sulfate, calcium sulfate, aluminum sulfate and aluminum potassium sulfate. The mechanism of action is that sulfate is usually a strong electrolyte, which can improve the ionic strength of cement paste. During the hydration of cement, the sulfate dissolved in water reacts with the calcium hydroxide produced by the hydration of cement to produce calcium sulfate and sodium hydroxide. Among them, calcium sulfate has a fine particle size and reacts with C3A to form hydrated calcium sulphoaluminate crystals, which is very fast. and hydrogen oxidation

Sodium, as an active agent, enhances the alkalinity of the system and can improve the solubility of C3A and gypsum, thereby increasing the amount of calcium sulphoaluminate in cement. In addition, calcium sulphoaluminate crystals cross and overlap each other during the growth process to form the initial skeleton of the cement, which is continuously filled and solidified by hydration products such as C- S-H gels, thereby significantly improving the early strength of the cement. However, since K and Na do not participate in the hydration process of cement, their soluble salts will remain in the concrete system. The sodium salt or potassium salt is easy to precipitate from the concrete surface after dehydration and drying, and crystallizes on the concrete surface, resulting in expansion and cracking of the concrete surface. Therefore, the amount of sulfate early strength agent in concrete should be determined through tests to avoid alkali-aggregate reaction or excessive sulfate corrosion hazards to the project.

Nitrate, silicateearly strength agent

The effective response of nitrate can promote the hydration of cement, improve the pore structure of hydration products, and make the mortar structure tend to be dense. In addition, adding nitrous acid to the concrete is beneficial to form a protective film on the surface of the steel bar, which can effectively avoid the corrosion of the steel bar in the concrete. Calcium nitrate was added to concrete as an early strength agent, and the effect of calcium nitrate on the early strength of concrete was discussed. Experiments show that according to the dosage standard of 1500mL calcium nitrate per 100kg cement, compared with concrete without early strength agent, Ca(NO3) is mixed.

The early strength of 2% concrete is increased by 66% at 1d and 29% at 7d.

Key words:

Early strength agent

The role of concrete early strength agent

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