1. Early strength agent
Early strength agent can promote setting, improve the early strength of concrete, and Admixtures that have no significant impact on later strength. Those that only play a role in promoting coagulation are called coagulants. At present, commonly used early strength agents include chloride salts, sulfates, and triethanolamine.
1. Chloride salt
Chloride salt early strength agent mainly includes calcium chloride ( CaCl2) and sodium chloride (NaCl), among which calcium chloride is the earliest and most widely used early strengthening agent. The early effect of chlorine salt is mainly through the formation of calcium chloroaluminate hydrate and calcium oxychloride to achieve early strength.
In addition to promoting coagulation and early strengthening effects, calcium chloride also has the effect of lowering the freezing point. Because it contains chloride ions, it will accelerate the corrosion of steel bars, so the dosage must be strictly controlled. The dosage is generally 1% to 2%, which can increase the ld strength by 70% to 140% and the 3d strength by 40% to 70%. It has little impact on the later strength and can improve the frost resistance, but it increases dry shrinkage and reduces Freeze resistance.
The dosage, function and application of sodium chloride are basically similar to calcium chloride, but the effect is slightly worse, and the strength will be reduced in the later period.
“Technical Specifications for the Application of Concrete Admixtures” (GB50119) and “Construction Quality Acceptance Specifications for Concrete Structure Engineering” (GB50204) stipulate that in reinforced concrete, calcium chloride The dosage is not more than 1%. In unreinforced concrete, the dosage is not more than 3%; concrete that is often in humid or water level changing areas, concrete that is affected by corrosive media, concrete with alkali active aggregates, thin-walled structural concrete, In mass concrete, prestressed concrete, decorative concrete, and concrete structures using cold-drawn or cold-drawn low-carbon steel wires, chlorine salt early-strength agents are not allowed to be mixed. In order to prevent the corrosion effect of calcium chloride on steel bars, it is often used with rust inhibitors. The agent is used in combination.
Chloride salt early-strength agent is mainly suitable for winter construction of concrete and early-strength concrete, but is not suitable for steam curing concrete.
2. Sodium sulfate
Sodium sulfate (Na2SO4), usually anhydrous sodium sulfate, also known as Yuanming powder , is one of the sulfate-based early strength agents and is a widely used early strength agent. The early strength effect of sodium sulfate is achieved by generating dihydrate gypsum and then generating hydrated calcium sulfoaluminate.
Sodium sulfate has retarding and early strength effects. The dosage is generally 0.5% to 2.0%, which can increase the 3d strength by 20% to 40%, and the strength after 28d is basically unchanged. Difference, no corrosion effect on steel bars. When the aggregate is alkali-active aggregate, sodium sulfate cannot be added to prevent alkali-aggregate reaction. Excessive dosage will cause sulfate corrosion. When the sodium sulfate dosage is large, It will significantly increase the viscosity of the concrete mixture, which is detrimental to construction (especially the surface is not easy to smooth and polish). The application range of sodium sulfate is wider than that of chloride salt early strength agent.
3. Triethanolamine
Triethanolamine is a colorless or light yellow oily liquid, non-toxic, alkaline and non-ionic Surfactant.
The early strength mechanism of triethanolamine is different from the first two early strength agents. It does not participate in the hydration reaction and does not change the hydration of cement. product. It can reduce the surface tension of the aqueous solution, make the cement particles easier to wet, and increase the dispersion of the cement, thus speeding up the hydration rate of the cement and playing a catalytic role in the hydration of the cement. The increase in hydration products will Improve the early strength of cement.
The dosage of triethanolamine is generally 0.02% ~ 0.05%, which can increase the 3d strength by 20% ~ 40%, and the later strength The impact is small, and the anti-freeze and anti-permeability properties are improved. It has no rust effect on steel bars, but it will increase dry shrinkage.
4. Other early strength agents
In addition to the above three early strength agents, engineering Gypsum, sodium thiosulfate (sodium soda), alunite (potassium aluminum sulfate), calcium nitrate, potassium nitrate, sodium nitrite, calcium nitrite, sodium formate, sodium acetate, sodium dichromate, etc. are also used. Early strengthening agents are more effective when used in combination. High-efficiency water-reducing admixtures can improve the early strength of concrete to varying degrees. If the early strength agent and water reducing agent are used in combination, the early strength can be further improved, the later strength can be increased, and the construction properties of concrete can be improved. Therefore, the combined use of early strength agent and water reducing agent, especially the composite early strength water reducing agent of chlorine-free salt early strength agent and water reducing agent, has developed rapidly. Compound early-strength water-reducing agents such as sodium sulfate, calcium lignin, calcium sugar and high-efficiency water-reducing agents have been widely used.
Excessive dosage of early-strength agent or early-strength water-reducing agent will cause frost on the concrete surface, reduce the later strength and durability, and also affect the protection of steel bars. adverse effects. Sometimes it can also cause premature setting of concrete or false setting.
2. Antifreeze
1. Commonly used antifreeze
Antifreeze is an admixture that can harden concrete at negative temperatures and achieve expected performance under specified curing conditions. In northern my country, in order to prevent early freezing of concrete, antifreeze is often added during winter construction (when the daily average temperature is below 5°C). Antifreeze can lower the freezing point of water so that cement can continue to hydrate at negative temperatures and improve the early strength of concrete to resist water freezing.The expansion pressure generated acts as an anti-freeze function.
Commonly used antifreezes include glycerol, diethylene glycol, dimethyl sulfoxide, calcium chloride, sodium chloride, ammonium chloride, and potassium carbonate , ethylene glycol, calcium formate, calcium acetate, sodium nitrite, calcium nitrite, calcium nitrate, etc.
2. Application scope and dosage of antifreeze
Sodium nitrite and nitrous acid The appropriate dosage of calcium is 0.3% to 1.0%, which has the effects of lowering the freezing point, preventing rust, and early strength. The appropriate dosage of calcium chloride and sodium chloride is 0.5% to 1.0%, which has the effect of early strength and lowering the freezing point, but it has a rusting effect on steel bars. Antifreezes of glycerol, diethylene glycol and dimethyl sulfoxide can be used in various concrete projects. Antifreezes containing nitrites and carbonates are strictly prohibited for prestressed concrete projects. Ammonium salts and urea are strictly prohibited for use in prestressed concrete projects. Office, residential and other indoor construction projects.
In order to improve the antifreeze effect of antifreeze, antifreeze is often compounded with water reducing agent, early strength agent and air entraining agent to make it have better antifreeze effect. sex.
3. Waterproofing agent
1. Commonly used waterproofing agents
Waterproof agent refers to an admixture that can reduce the water permeability of mortar or concrete under hydrostatic pressure. Concrete is a heterogeneous material with pores of different sizes (gel pores, capillary pores and macropores) distributed in the body. The main function of the waterproofing agent is to reduce the pores inside the concrete, improve the density or change the pore characteristics and block the water seepage paths, so as to improve the impermeability of the concrete.
Air-entraining agents, air-entraining water reducing agents, swelling agents, ferric chloride, aluminum chloride, triethanolamine, sodium stearate, methyl Admixtures such as sodium silanolate and sodium ethyl silanolate are used as waterproofing agents. Composite waterproofing agents are mostly used in projects. In addition to the above ingredients, sometimes a small amount of highly active mineral materials, such as silica fume, are also added.
2. Penetrating waterproof material
(1) Cement-based penetrating crystalline waterproof material The function of It is a penetrating waterproof material composed of base materials, mixed with active chemicals (catalysts) and other auxiliary materials. Its waterproofing mechanism is that through the capillary pores or micro-cracks in the concrete, it gradually penetrates into the interior of the concrete in the presence of water, and reacts with the cement hydration products to form crystals to make the concrete dense. The product is divided into waterproofing agent (Type A) and waterproof coating (Type C). When used, directly mix Type A) into the cement concrete or add water to make a slurry and apply it (Type C) on the surface of the cement concrete or dry sprinkle ( Type C) Apply pressure on the newly formed cement concrete surface (appropriate amount of water can be sprinkled to wet the waterproof material). The dosage of type A is 5% to 10%, and the amount of type C (dry spreading amount) is 1kg/m2 to 1.5kg/m2. Cement-based permeable crystallized waterproofing materials have good waterproofing effects and can increase the strength of surface concrete by 20% to 30%. Cement-based penetrating crystallized waterproofing materials must be sprayed and cured after initial setting so that they can fully penetrate into the concrete.
(2) Raw materials for cement-based penetrating crystallized waterproofing materials
Cement-based penetrating The raw materials of crystallized waterproofing materials mainly include unsaturated polyester resin, methyl methacrylate, and water-absorbing resin. These ingredients have good penetration ability in concrete and quickly enter the pores of the concrete, blocking the pores to achieve waterproofing and impermeability. role.
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