The pumping technology of concrete has been widely used, especially commercial pumping concrete. Because the quality control of commercial concrete is much better than the quality control of mixed concrete on the construction site. At present, the domestic pumping level is also relatively high, and the vertical pumping can reach a pump height of 130m (Shanghai Oriental Pearl TV Tower).
Pumped concrete is different from ordinary concrete, it belongs to fluidized concrete. Fluidized concrete was first proposed by Germany to improve the construction performance of concrete. In 1974, the former Federal Republic of Germany formulated guidelines for the construction of fluidized concrete, and then the United States, Britain, Japan, etc. all put forward relevant reports, some of which are called superplastic concrete.
The characteristics of fluid concrete are:
For reference concrete with a small slump (3.5-9 cm slump), a fluidizer (composite of superplasticizer) is added before pouring, and mixed into concrete with a slump of more than 20cm and a fluidity. That is, without changing the original mix ratio and water consumption, use the method of adding additives to adjust the workability of the concrete to make it better fluidity. This kind of concrete has good viscosity, is easy to flow, does not segregate, and does not bleed.
Pumped concrete is a kind of fluidized concrete. Because it has pumping requirements, its admixture must also meet the special requirements of pumping. Pumping concrete occupies a large part of fluid concrete and commercial concrete, and pumping agent has become one of the important varieties of admixtures.
Composition and mechanism of pumping agent
Pumping agent is often not an admixture that can meet performance requirements, but is compounded with admixtures with different functions according to the characteristics of the pumping agent.
The specific compound ratio should be determined according to different purposes of use, different use temperatures, different concrete signs, and different pumping processes.
It is mainly composed of the following components:
1. Water reducing components
2. Retarding components
3. Air-entraining components
4. Water retention components
5. Superfine mineral admixture
6. Expansion component
Water reducing component
1) Ordinary water reducing agent
has a water-reducing effect, which can reduce the water-cement ratio under the condition of maintaining the required fluidity of the pumped concrete to improve the later strength.
Calcium lignosulfonate and sodium lignosulfonate are the most commonly used water reducers. In addition to the water reducing effect, it also has some retarding and air-entraining properties. Some pumped concretes with lower grades and low slump requirements can even meet the requirements by only adding lignosulfonate water-reducing agents.
The sugar-calcium water-reducing agent in the ordinary water-reducing agent is often introduced into the pumping agent as a retarding component
Water reducing component
(2) High-efficiency water-reducing agents must be used in pumping concrete with high concrete design strength and high slump value requirements, such as pumping agents for high-performance concrete, such as naphthalene-based water-reducing agents and melamine. Water reducing agent, fatty acid water reducing agent.
These water reducers have high water reduction rates and are suitable for the preparation of high-grade, large slump, self-leveling pumping concrete. The slump loss of these water reducers is relatively large, and a composite retarder is required. Amino sulfonate water reducer and polycarboxylate water reducer are low slump loss water reducers, and are more suitable for the preparation of high performance concrete with low water-cement ratio. When the water-cement ratio is 0.3, the water reduction rate of sulfamate can be as high as 30%, and when the water-cement ratio is large, they are prone to bleeding.
In the pumping agent, more than two kinds of water reducing agents are often used to compound. The common compounding methods are:
Naphthalene series + lignosulfonate series;
Melamine + lignosulfonate series;
Naphthalene series + sulfamate series, etc.
Compound use tends to have lower dosage and better effect than single use.
2. Retarding components
Commercial concrete is mostly used for pumping concrete. The slump loss is required to be small. Especially for large-volume concrete or high-temperature construction concrete in summer, retarding components must be added.
When ordinary water reducing agents cannot meet the retarding requirements, chemical retarders should be added, such as hydroxycarboxylates, sugars, polyols, etc. The slump loss is reduced, the hydration heat release of concrete can also be controlled, and temperature cracks can be avoided.
3. Air-entraining components
Appropriate concrete air content can reduce pumping resistance, prevent concrete bleeding and segregation, and can also improve anti-permeability and freeze-thaw resistance.
Almost all foreign concrete maintains a certain amount of air.
The use of air-entraining agents with good bubble properties will not affect the strength of concrete. For example, air-entraining agents are almost always mixed in Japanese concrete.
Air-entraining components can choose air-entraining agent and air-entraining water reducer.
4. Water retention components
Water retention agent is also called thickener. Its function is to increase the viscosity of the concrete mixture, so that the concrete will not bleed and segregate under the conditions of large water-cement ratio and large slump.
Some water-retaining agents also have the properties of reducing water and maintaining slump.
These materials include the following:
(1) The content of polyvinyl alcohol is below 0.3%, which has the effect of retarding and thickening. Commonly used polyvinyl alcohols are 1799, 0588 and so on.
(2) The content of methyl cellulose and carboxymethyl cellulose is very small, accounting for only 0.01 to 0.05% of the cement consumption.
(3) Hydroxypropyl fiber reduces the slump loss and increases the consistency. The content is 0.01%.Others include dextrin, xylitol mother liquor, animal glue and so on.
5. Superfine mineral admixture
These materials all have certain pozzolanic activity, or have hydration activity under alkaline excitation conditions. Such as silica fume, fly ash, slag powder, zeolite powder, shale powder, bentonite, stone powder, diatomaceous earth, etc., they can improve the gradation, prevent bleeding segregation, increase volume stability, and increase concrete durability. , Prevent alkali and aggregate reaction.
The surface area of these materials is larger than that of cement, and the mixing amount is larger. Generally, the internal mixing method is adopted, which can replace the same amount of cement as the cementing material. Among them, fly ash, slag powder, silica fume, and zeolite powder are most commonly used. The content of silicon powder is 5-10%, and the content of other types is 15-30%.
6. Expansion component
In large-scale foundations and large-volume concrete, expansive agents are often added to compensate for concrete shrinkage.
The pumping agent has a relatively large water ash and a large slump. In order to maintain volume stability, especially for the expansion joints of large-volume concrete, expansion agents must be used in the post-casting belt.
It is best not to compound other additives in the selected expansion agent, but to use together with the pumping agent and admixture.
Pumping agents are generally compounded into products in admixture factories, but do not include admixtures and expansion agents.
Early strength agent
Divided into inorganic salt early-strength agent, organic salt early-strength agent, composite early-strength agent, etc.
1. Inorganic salt early strength agent:
1. Calcium chloride "Calcium chloride has obvious early strength effects, especially low temperature early strength and freezing point reduction. Adding calcium chloride to concrete can speed up the early hydration of cement, and the heat of hydration in the first few hours has a significant increase. This is mainly due to the fact that calcium chloride can react with the tricalcium aluminate in the cement and is on the surface of the cement particles. On the formation of hydrated calcium chloroaluminate. It can promote the hydration reaction of tricalcium silicate and dicalcium silicate to improve early strength. When it is mixed with less than 1%, it has no obvious effect on the setting time of cement. When it is mixed with 2%, the setting time is advanced by about 2 hours. When it is mixed with more than 4%, the cement will set quickly.
Inorganic salt early strength agent
Calcium chloride significantly increases the shrinkage value of concrete. When it is mixed with 0.5%, the shrinkage increases by about 50%, when it is mixed with 2.5%, it reaches 115%, and when it is mixed with 3%, it increases to 165%. At the same time, the introduction of chloride ions can promote the corrosion of steel bars, so it is best to use it together with rust inhibitors (such as sodium nitrate). Based on the adverse effects of calcium chloride on reinforced concrete, when using calcium chloride early-strength agent, it should be used in accordance with the relevant construction acceptance specifications.
2. Sodium chloride
Sodium chloride is an early strength agent and a good antifreeze material for reducing freezing point. Moreover, the price is cheap and the source of raw materials is wide. When the dosage is the same, the effect of sodium chloride in reducing the freezing point is better than that of calcium chloride, and it is almost the best effect among all materials for reducing the freezing point. However, as an early-strength agent, its concrete strength will be reduced in the later stage, and it will also corrode the steel bars. The compound rust inhibitor must be used in reinforced concrete according to regulations. Sodium chloride is generally not used as an early strength agent alone, but is mostly used as an antifreeze component in an antifreeze. It has a better effect when used in combination with triethanolamine, and the general usage amount is ≤1%. Alkali metal chloride salts of the same family as sodium also have a good early-strength effect, such as potassium chloride and lithium chloride. According to the metal activity sequence table, chlorides increase the hydration promotion effect of cement with the increase of the cation radius, in the following order: potassium chloride>sodium chloride>lithium chloride. However, potassium chloride and lithium chloride are more expensive, and there are many lithium salt residues and potassium salt by-products in Northwest my country that can be used.
3. Ferric chloride
When the content is not more than 2%, ferric chloride has an early strength effect, and when the content is more than 3%, it is mostly used as a waterproofing agent. The advantages of ferric chloride as an early-strength agent are early-strength, good compactness, and the strength of the latter 28 days is higher than that without the early-strength agent. The disadvantage is that the chloride-containing salt has a corrosion effect on the steel bar, but there is no obvious corrosion effect when the amount is small.
It is rarely used as an early-strength agent alone, and other additives are often used in concrete that requires early-strength, waterproof, and anti-freezing requirements. Some chlorides such as aluminum chloride, stannous chloride, and ammonium chloride also have good early strength effects, but they are rarely used due to cost and source issues.
It is the most widely used early strength agent, especially sodium sulfate and calcium sulfate.
Sodium sulfate is also known as sodium sulfate and anhydrous Glauber's salt. Its natural mineral is called Glauber's salt. It is white crystal, which is easily weathered and loses water and turns into white powder, namely sodium sulfate. Sodium sulfate is rich in resources and the price is relatively low. Sodium sulfate is easy to dissolve in water. When the cement hardens, it reacts with the calcium hydroxide produced during cement hydration: Na2SO4+Ca(OH)2+2H2O→Ca2SO4.2H2O+2NaOH
The generated dihydrate gypsum has fine particles, and it participates in the hydration reaction faster than the original dihydrate gypsum in the cement clinker: Ca2SO4.2H2O+C3A+12H2O→3Ca.Al2O3.CaSO4.12H2O to make sulfoaluminate, the product of hydration Calcium is generated faster, which speeds up the hydration and hardening of cement. Its 1-day strength increase is especially obvious. Since the early hydrate structure is formed faster and the structure is less dense, the strength will decrease slightly in the later 28 days. The faster the early strength increases, the more the later strength will be affected. Therefore, the sodium sulfate content should have a maximum The best control amount is generally 1-3%, and the early strength effect is not obvious when the amount is less than 1%, and the late strength loss is also large when the amount is too large, generally 1.5% is appropriate.
In the hydration reaction, the sodium sulfate early strength agent increases the alkalinity due to the generation of sodium hydroxide. This has an early strength effect on the cement mixed with pozzolan and slag, and the concrete mixed with active superfine admixtures. More obvious. But at the same time, it is easy to cause alkali-aggregate reaction for active aggregate.
When using sodium sulfate early-strength agent in steam-cured concrete, attention should be paid to the dosage. When the dosage is too high, high-sulfur hydrated calcium sulfoaluminate (ettringite) will be formed quickly and the concrete will expand and cause cracks.
Sodium sulfate is used in concrete. When the dosage is too large or the curing conditions are not good, it is easy to produce "return alkali" phenomenon on the surface of the concrete, that is, a layer of hairy calcium hydroxide fine crystals will precipitate on the surface of the concrete, which will affect the concrete. The smoothness of the surface is also not conducive to the further decoration of the surface. Construction in winter or dry weather is particularly prone to occur.
5. Calcium sulfate
Calcium sulfate, also known as gypsum, has been used as a setting adjuster in cement production. The general content is about 3%. It is used to adjust the setting time and mixed in the cement. When the concrete is mixed with calcium sulfate, it has obvious early strength effect. As calcium sulfate reacts with the tricalcium aluminate in the cement, a large amount of calcium sulfoaluminate is quickly formed, which quickly crystallizes and forms a crystal nucleus, which promotes the crystallization and growth of other components of the cement, thereby improving the early strength of the concrete.
The optimum dosage of calcium sulfate in concrete varies with the content of tricalcium aluminate and tetracalcium aluminate ferrite in the cement. The dosage should not be too large, otherwise it will reduce the later strength and even cause expansion cracks. Other sulfates such as potassium aluminum sulfate (alunite), potassium sulfate, sodium thiosulfate, aluminum sulfate, iron sulfate, zinc sulfate, etc. have early-strength effects, but the amount used is not large.
6. Nitrate early strength agent
Sodium nitrate, sodium nitrite, calcium nitrate, calcium nitrite all have early strength effects, especially as early strength and antifreeze at low temperature and negative temperature.
Sodium nitrite and sodium nitrate can promote the hydration of cement, and can improve the pore structure of concrete and make the structure of concrete more compact. Sodium nitrite is also a good rust inhibitor, especially suitable for use in reinforced concrete.
Calcium nitrite and calcium nitrate are often used in combination. The former Soviet Union has this product. They can promote cement hydration at low and negative temperatures. It has a good effect on accelerating the hardening of concrete and improving the compactness and impermeability of concrete. In the microstructure of cement stone, it strengthens the hydration process of cement minerals, increases the volume of gelatinous substances, closes pores and capillaries, and plays a good role in improving the durability of concrete.
Ferric nitrate can also be used as an early-strength agent. It reacts with calcium hydroxide formed by hydrolysis and hydration of clinker components to form ferric hydroxide and calcium nitrate. It not only has early-strength effect, but also uses ferric hydroxide colloid to close the pores. Achieve the effect of anti-seepage.
7. Carbonate early strength agent
Both sodium carbonate and potassium carbonate can be used as early-strength agents and accelerators for concrete.
Use in winter construction can significantly accelerate the concrete setting time and increase the growth rate of concrete negative temperature strength. And because carbonate can change the distribution of the internal pore structure of concrete,
Reduce the total porosity of concrete, and improve the impermeability of concrete after adding carbonate. Carbonate is also a raw material with a wide source of raw materials and a relatively low price.
Organic salt early strength agent
triethanolamine molecule has a nitrogen atom (N), it has a pair of unshared electrons, it is easy to form covalent bonds with metal ions, complex, and form a more stable complex with metal ions.
These complexes form many soluble areas in the solution, thereby increasing the diffusion rate of hydrated products. Can shorten the incubation period in the process of cement hydration; improve the early strength.
When the triethanolamine content is too large, C3A and gypsum in the cement mineral will quickly generate ettringite under its catalysis, which speeds up the setting time. Triethanolamine has a certain inhibitory effect on the hydration process of C3S and C2S, which in turn allows the later hydration products to fully grow and densify, ensuring the improvement of the later strength of concrete.
Organic salt early strength agent
Others such as diethanolamine and triisopropanolamine also have similar effects. Therefore, in use, the cheaper triethanolamine residue is often selected, which is actually a mixture of triethanolamine, triisopropanolamine, diethanol gum, etc., due to the super stacking effect, its effect is sometimes better than pure triethanolamine.
When triethanolamine is used as an early-strength agent, the dosage is 0.02-0.05%, and if the dosage is more than 0.1%, it has the effect of accelerating coagulation.
Three, composite early strength agent
The composite early-strength agent can be a composite of inorganic materials and inorganic materials, or a composite of organic materials and inorganic materials, or a composite of organic materials and organic materials.
Compound early-strength agents often have better early-strength effects than single-component early-strength agents. The dosage can also be lower than the single-component early strength agent.
Among the many composite early-strength agents, triethanolamine and inorganic salt-based composite early-strength agents have the best effect and have the widest application range.
This kind of early strength agent mainly includes: triethanolamine composite early strength agent, inorganic salt early strength agent and so on.
1. Triethanolamine composite early strength agent
A. Triethanolamine-sodium sulfate composite early-strength agent is the most commonly used composite early-strength agent. The composite early-strength agent is more effective at low temperature. When the curing temperature is lowered when the temperature is lower than 20 ℃, the early and late strength of the composite early-strength agent increases significantly.
The early-strength effect of triethanolamine-sodium sulfate composite early-strength agent is often greater than the arithmetic superposition of the enhancing effects of triethanolamine and sodium sulfate alone. Moreover, the 28-day strength of the composite early-strength agent is significantly higher than that without the addition of sodium sulfate, which is completely different from the decrease in the 28-day strength of the single-doped sodium sulfate early-strength agent.
When triethanolamine is compounded with sodium sulfate, the content of triethanolamine is 0.02-0.05%, and sodium sulfate is 1-3%, which is mainly determined according to the use temperature and different types of cement.
It is also possible to use triisopropanolamine, diethanolamine, etc. instead of triethanolamine for compounding, or use several mixtures of triethanolamine residues instead.
B. Triethanolamine-chloride compound early strength agent
Through experiments, it is found that the enhancement effect of triethanolamine-chloride compound early-strength agent, whether it is triethanolamine-sodium chloride or triethanolamine-calcium chloride or triethanolamine residue-sodium chloride compound early-strength agent, its The growth value of the early strength exceeds the arithmetic superposition value of the enhancement value of each single component. However, the 28-day intensity was slightly lower than the arithmetic stack value or remained the same.
Triethanolamine (including triisopropanolamine, diethanolamine and triethanolamine residues) can also be used in combination with nitrate, nitrite, carbonate, etc., but these are mostly used as antifreeze agents and used at negative temperatures.
2. Inorganic salt compound early strength agent
Sulfate composite early-strength agents mainly include sodium sulfate-sodium chloride early-strength agents, sodium sulfate and calcium sulfate or combined with triethanolamine at the same time.
Inorganic salt early-strength agents usually have the best effect at low temperatures, but their early-strength effect is not obvious as the temperature rises. Therefore, the early-strength effect is lower at temperatures above 20°C (normal temperature) and the temperature difference is much lower. The main reason is that the hydration and hardening speed of cement is greatly affected by temperature, and the hydration and hardening speed at room temperature is much faster than that at low temperatures.
The early-strength agent mainly increases the hydration reaction rate of cement in the early stage (1-7 days). The hydration rate of cement is fast enough at room temperature, and the promotion effect of the early-strength agent is not obvious. At low temperatures, the promotion of the early strength agent can significantly affect the hydration rate, so that the early strength can reach or exceed the level at room temperature.
Inorganic salt compound early strength agent also has some shortcomings:
Generally speaking, the early strength, especially when the strength increases rapidly in 1 day and 3 days, because the internal structure of the cement stone quickly laps and grows to produce strength, this fast-growing structure is often looser and not dense enough, which gives the later Strength brings some loss.
In addition, the sodium sulfate early-strength agent will introduce a large amount of Na+ ions due to its relatively large content (1-3%), which increases the alkali content of concrete. If it encounters active aggregates, it is prone to alkali-aggregate reaction.
Chloride-based early-strength agents will introduce chloride ions to cause corrosion of steel bars, which have an adverse effect on the durability of concrete. These influencing factors must be considered during use.