Concrete admixture refers to a substance that can significantly improve the performance of concrete when it is mixed. Its dosage is generally not more than 5% of the cement quality. Due to the improvement of concrete performance by admixture, its application proportion in engineering is increasing. In many countries, the concrete mixed with admixture accounts for 60% ~ 90% of the total concrete. Therefore, the admixture gradually becomes the fifth component of concrete.
A, admixture classification
There are many kinds of concrete admixtures. According to the classification, naming and definition of concrete admixtures, concrete admixtures are divided into four categories according to their main functions:
1. Admixtures for improving workability: water reducing agent, pumping agent and air entraining agent.
2. Admixtures for adjusting setting and hardening time: retarder, early strength agent and accelerator.
3. Additives to improve durability: antirust agent, waterproof agent and air entraining agent.
4. Additives to improve other properties: air entraining agent, colorant, expansion agent and antifreeze.
Second, water reducing agent
Water reducer refers to the addition of additives that can significantly reduce the mixing water consumption under the condition that the slump of concrete is basically unchanged.
(A) the mechanism of water reducing agent
Water reducer is a kind of surface active substance, and its molecules are composed of hydrophilic groups and hydrophobic groups. When cement is mixed with water, the cement slurry forms a flocculation structure, which wraps a part of the mixed water and reduces the fluidity. The action mechanism of water reducing agent is shown in the following three aspects:
(1) Its hydrophobic groups are directionally adsorbed on the surface of cement particles, and its hydrophilic groups point to the aqueous solution, so that the surface of cement particles is charged with the same charge, and the repulsive force separates cement particles, releasing free water with flocculation structure and increasing fluidity;
(2) Hydrophilic groups adsorb a large number of polar water molecules, which increases the thickness of solvated water film on the surface of cement particles, plays a role in lubrication and improves workability;
(3) The water reducing agent reduces the surface tension, making the cement particles easier to wet and more fully hydrated, thus improving the strength of concrete.
Schematic diagram of flocculation structure of cement slurry and function of water reducer.
(B) the technical and economic effects of water reducing agent
1. Increase liquidity. When the water consumption and water-cement ratio are constant, the slump of concrete can be increased by 100 ~ 200 mm without affecting the strength of concrete.
2. Improve the strength of concrete. Under the condition of keeping the fluidity and cement dosage unchanged, the mixing water consumption can be reduced 10% ~ 15%, thus reducing the water cement ratio and improving the concrete strength 15% ~ 20%.
3. Save cement. Under the condition of keeping the fluidity and water-cement ratio unchanged, the mixing water consumption can be reduced, and the cement consumption can be reduced accordingly.
4. Improve the durability of concrete.
(3) Types of water reducing agents
According to chemical composition, there are lignin series, naphthalene series, water-soluble resin, molasses and compound water reducer.
1. Lignin water reducer
Include calcium lignosulfonate (lignosulfonate), sodium lignosulfonate (lignosulfonate), magnesium lignosulfonate (lignosulfonate), etc. Among them, wood calcium water reducer is widely used.
Wood-calcium water reducer is a brown powder made of sulfite pulp waste liquid left over from the production of pulp or fiber pulp, neutralized by lime milk, sugar removed by biological fermentation, evaporated and concentrated, and spray-dried.
The suitable dosage of wood calcium water reducer is generally 0.2% ~ 0.3% of cement quality. The water reduction rate is 10% ~ 15%, and the 28d compressive strength of concrete is increased 10% ~ 20%. If the water consumption is not reduced, the slump of concrete can be increased by 80 ~100 mm; If the compressive strength and slump of concrete remain unchanged, the cement consumption can be saved by about 10%. Wood-calcium water reducer has retarding effect on concrete, especially when it is added too much or at low temperature, and may also reduce the strength of concrete, so attention should be paid to it when using it.
Wood calcium water reducer can be used in general concrete engineering, especially in mass pouring, slip-form construction, pumping concrete and summer construction. Wood calcium water reducer should not be used alone in winter construction. When the lowest temperature of the day is lower than 5℃, it should be used with early strength agent or antifreeze. Steam-cured concrete and prestressed concrete should not use wood calcium water reducer alone, so as to avoid loose concrete surface after steam-curing.
2. Naphthalene sulfonate water reducer
Naphthalene water reducer is made from naphthalene or naphthalene homologues by sulfonation and methanol retraction. At present, China mainly produces NNO, NF, FDN, UNF, MF and Jianshi first-class water reducing agents, most of which are non-air-entraining water reducing agents.
The suitable dosage of naphthalene-based water reducer is 0.5% ~ 1.0% of cement quality, the water reducing rate is 10% ~ 25%, and the 28d strength of concrete is improved by more than 20%. Naphthalene superplasticizer has good water reducing and strengthening effects, and has strong adaptability to different kinds of cement. It is suitable for preparing early strength, high strength, fluid state and steam curing concrete.
3. Water-soluble resin water reducer
This kind of water reducer is made of some water-soluble resins, such as melamine resin and coumarone resin. This kind of water reducing agent has obvious strengthening effect and is a kind of high efficiency water reducing agent. Our products include SM resin water reducer.
The dosage of SM water reducer is 0.5% ~ 2.0% of the cement mass, the water reducing rate is 15% ~ 27%, the 3d strength of concrete is improved by 30% ~ 100%, and the 28d strength is improved by 20% ~ 30%. SM water reducer is suitable for preparing high strength concrete, early strength concrete, fluid concrete and steam curing concrete.
Third, the early strength agent
Early strength agent is an additive that accelerates the early strength development of concrete and has no obvious influence on the later strength. Early strength agent can accelerate the hardening process of concrete at normal temperature, low temperature and negative temperature (not lower than -5℃), and is mostly used in winter construction and emergency repair projects. Early strength agents mainly include inorganic salts (chloride salts and phosphates), organic amines and organic-inorganic compounds.
1. chlorine salt early strength agent
Chlorine salt early strength agents mainly include calcium chloride, potassium chloride, aluminum chloride and ferric chloride, among which calcium chloride is the most widely used. Calcium chloride is a white powder, and its suitable dosage is 0.5% ~ 1.0% of cement quality, which can improve the 3d strength of concrete by 50% ~ 10% and the 7d strength by 20% ~ 40%. At the same time, it can reduce the freezing point of water in concrete and prevent early freezing of concrete.
The biggest disadvantage of using calcium chloride as early strength agent is that it contains Cl- ions, which will corrode steel bars and lead to concrete cracking. In order to inhibit the corrosion of calcium chloride on steel bars, calcium chloride is often used in combination with sodium nitrite, a rust inhibitor.
2. Sulfate early strength agent
Sulfate early strength agents mainly include sodium sulfate, calcium thiosulfate, aluminum sulfate and potassium aluminum sulfate, among which sodium sulfate is widely used. Sodium sulfate is white powder, and the general dosage is 0.5% ~ 2.0%. When the dosage is 1% ~ 1.5%, the time for concrete to reach 70% design strength can be shortened by about half.
Sodium sulfate has no corrosive effect on steel bars and is suitable for concrete where chloride salt is not allowed. However, due to the reaction with calcium hydroxide to generate strong alkali NaOH, in order to prevent alkali-aggregate reaction, it is forbidden to use sodium sulfate in concrete containing active aggregate, so be careful not to add too much, so as to avoid the later expansion, cracking and damage of concrete and prevent the surface of concrete from "frosting".
3. Organic amine early strength agent
The organic amine early strength agents mainly include triethanolamine and triisopropanolamine, among which triethanolamine is the best early strength agent.
Triethanolamine is a colorless or yellowish oily liquid, alkaline and soluble in water. The dosage is 0.02% ~ 0.05% of the cement quality, which can improve the early strength of concrete. Triethanolamine has a slight retarding effect on concrete, and excessive dosage will cause serious retarding of concrete and reduce the strength of concrete, so the dosage should be strictly controlled.
Fourth, the retarder
Retarder is an additive that can delay the setting time of concrete and has no adverse effect on the later strength development of concrete. There are four main types of retarders: sugars, such as molasses; Lignosulfonates, such as calcium lignosulfonate and sodium lignosulfonate. Lignosulfonate and molasses are commonly used as retarders, among which molasses has the best retarding effect.
Retarder has the functions of retarding setting, reducing water, reducing hydration heat and strengthening, and has no corrosive effect on steel bars. It is mainly suitable for mass concrete and concrete constructed in hot climate, as well as concrete that needs long-term parking or long-distance transportation.
Verb (abbreviation of verb) air entraining agent
Air entraining agent refers to an additive that can introduce a large number of evenly distributed tiny bubbles in the process of concrete mixing, thus reducing bleeding and segregation of concrete mixture, improving workability and significantly improving frost resistance and durability of hardened concrete. At present, the commonly used air entraining agents are rosin hot polymer, rosin soap, alkylbenzene sulfonate, etc. Due to the existence of a large number of tiny, closed and evenly distributed bubbles, some properties of concrete have been obviously improved or changed.
1. Improve the workability of concrete mixture
2. Significantly improve the impermeability and frost resistance of concrete.
3. Reduce the strength of concrete. When the air content of ordinary concrete increases by 65438 0%, the compressive strength decreases by 4% ~ 6% and the flexural strength decreases by 2% ~ 3%.
Air entraining agent can be used for impervious concrete, frost-resistant concrete, sulfate-resistant concrete, concrete with serious bleeding, lean concrete, light concrete and concrete with decorative requirements, but it is not suitable for steam curing concrete and prestressed concrete.
Six, antifreeze
Antifreeze is an additive that can harden concrete at negative temperature and achieve the expected performance under specified curing conditions. Commonly used antifreeze is chloride, chloride antirust agent and chloride.
Antifreeze is used for concrete constructed at negative temperature. At present, the domestic antifreeze is suitable for the temperature of 0 ~- 15℃, and other concrete winter construction measures should be added when constructing at lower temperature.
Seven. accelerator
Accelerator refers to the admixture that can make concrete set and harden quickly. Accelerators mainly include inorganic salts and organic compounds. Inorganic salts are commonly used accelerators in China, mainly including Hongxing I, 7 1 1, 728 and 8604.
After the accelerator is mixed into concrete, the initial setting of concrete can be achieved within 5 minutes, and the strength of 1h can be increased by 2-3 times, but the strength will decrease in the later period, and the strength at 28d is about 80%-90% of that without admixture. The mechanism of quick setting and early strength of accelerator is to make gypsum in cement become Na2SO4 and lose its retarding effect, thus promoting the quick hydration of C3A, and precipitating the crystals of its hydration products in the solution, leading to the quick solidification of cement paste. Accelerator is mainly used in mine tunnel, railway tunnel, drinking water culvert, underground engineering and shotcrete or mortar engineering.