Foam concrete is a kind of concrete with lightweight and has ideal strength. Because of the hollow structure in foam concrete, it has the function of absorbing heat and isolating sound.
The density of foamed concrete is 300-1200 kg/m3, and the thermal conductivity is between 0.08-0.3W/ (m ·K).
Foam concrete is widely used in CLC blocks, lightweight partition panels, roof insulation, floor cushion construction, floor heating backfilling, and other occasions.
At the same time, foam concrete is also a good sound absorption material, which can be used in highway sound insulation boards, sound absorption boards, and other fields.
Generally speaking, foam concrete is a kind of building material with multi-function, environmental protection, and economy, which has a wide application prospect.
Properties and functions of retarders
The substance that can delay the setting and hardening time of refractory castable is called retarder. The action mechanism of retarders varies with the properties of binders and retarders used. Generally speaking, it has the following two functions:
1. To form a complex. The retarder forms a complex with the positive ions dissociated from the binder, which inhibits the crystallization of hydrates or reaction products, or inhibits grain growth.
2. To form a thin film. The retarder is adsorbed on the surface of the cement particles, which prevents the cement particles from contacting with water and restrains the hydration reaction rate. Retarders are mainly used in calcium aluminate cement bonded castables, such as gluconic acid, ethylene glycol, glycerol, starch, phosphate, lignosulfonate, etc.
Properties and functions of delayed coagulant
The substance in which water is mixed for a certain period of time to promote the setting of the binder is called delayed coagulant. They are a kind of counterionic substances which are mixed with water and need to be hydrolyzed to provide counter-ionic substances which are different from those carried on the surface of the fine powder in the mixture. For example, the charge on the surface of ultrafine silica powder is negative, so the delayed coagulant used must be a substance that can provide positive ions. For example, if the castable with ultra-fine silica powder as the main binding phase, calcium aluminate cement can be used as delayed setting accelerator, because calcium aluminate cement can slowly release Al3+ and Ca2+ ions during hydrolysis, which can promote the coagulation of silica ultra-fine powder slurry.
Properties and functions of foaming agent (air entraining agent)
The substance that can reduce the surface tension of the liquid and produce a large number of uniform and stable foam is called foaming agent (air entraining agent). The principle of bubbles produced by foaming agent and air entraining agent is similar, that is, when it dissolves in water, it is easy to be adsorbed on the gas-liquid interface, which reduces the surface tension of barren liquid and increases the contact area between liquid and air. Because of the protective effect of the adsorbed foaming agent on the liquid film, the liquid film is relatively firm and the bubbles are not easy to burst. Foaming agents are rosin size, resin saponin grease, aluminum petroleum sulfonate, etc. Air entraining agents include rosin thermopolymers, Nonionic surfactants, alkylbenzene sulfonates, carboxylic acids and their salts, etc. In addition, there is a kind of aerating agent whose aerating effect is that the castable produces bubbles due to chemical and physical action during mixing and after pouring, which is uniformly distributed in the slurry. The aerating agent usually uses metal aluminum powder, which is highly active and reacts with water to replace hydrogen. The escaped H2 gas forms bubbles in the castable. However, the addition of aluminum powder should not be too large, otherwise the strength of castables will be greatly reduced. This kind of admixture is used to prepare lightweight porous castables.
Properties and functions of defoamers
The substance that can make the bubbles disappear quickly when the refractory castable is mixed and shaken is called defoamer. Defoamer is also a surface active material, its ability to reduce liquid surface tension is much stronger than foaming agent, so that the strength of the liquid film formed is very low, which can greatly reduce the stability of foam. And the spreading speed on the liquid surface is also faster, and the faster the spreading speed is, the stronger the defoaming effect is. The types of defoamers include alcohols, fatty acids and fatty acid salts, amides, organosilicon compounds, etc.
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1. Reduce the solubility of hemihydrate gypsum.
2. Slow down the dissolution rate of hemihydrate gypsum.
3. Retarding ions are adsorbed on the surface of dihydrate gypsum crystal, and these ions are combined into the lattice.
4. Form a complex to restrict the diffusion of ions near the dihydrate gypsum crystal.
Common retarders
1. Organic acids and their soluble salts: citric acid, tartaric acid, malic acid, etc.
2. Alkaline phosphate: sodium tripolyphosphate.
3. Proteins: bone glue, degraded proteins, polymerized amino acids.
4. Compounding of various retarders.
Retarding mechanism of citric acid
Citric acid is a ternary organic acid containing hydroxyl group, which ionizes H + and many kinds of acid ions under different conditions, which form different stable precipitates with calcium. Among them, the stability of the complex is the highest when the PH value is 8.5, so the setting time is the longest.
Retarding mechanism of sodium polyphosphate.
Sodium polyphosphate is a straight-chain trimer, which combines with calcium ions in the liquid phase to form insoluble calcium phosphate covering the surface of hemihydrate gypsum, which hinders its dissolution, thus greatly reducing supersaturation and reducing the nucleation probability of dihydrate gypsum crystal.
At the same time, the adsorption of sodium tripolyphosphate on the crystal nucleus surface of dihydrate gypsum reduces the surface energy of the crystal nucleus, increases the nucleation barrier, and prolongs the time for the crystal nucleus to reach the critical nucleation size, which is macroscopically characterized by the prolongation of the induction period and setting time of gypsum.
Retarding mechanism of proteins
It is found that macromolecular retarders (proteins) are mixed into gypsum-based materials, combine with calcium ions on the surface of dihydrate gypsum, cover the crystal nucleus surface of dihydrate gypsum, and reduce the crystal nucleus surface can inhibit the growth of crystal nucleus.
At the same time, due to the adsorption and protection of protein colloid, protein colloid inhibits the crystal growth of gypsum, but does not change the crystal morphology, and has little effect on the strength of gypsum.
Summary
1. Under the same dosage, the retarding effect of citric acid is the strongest. When the content is more than 0.2%, the retarding time suddenly increases sharply, and when the dosage is 0.3%, the retarding time reaches more than 8 hours, which almost achieves the effect of preventing gypsum from setting.
2. The content of sodium polyphosphate is 0.1%, and the retarding effect is relatively not obvious, and then the time increases suddenly with the increase of the amount of polyphosphate, which is difficult to control when in use.
3. There is a linear relationship between the retarding time and the content of protein retarders, which is beneficial to the operation control.
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What is concrete superplasticizer?
Admixtures that can greatly reduce the amount of mixing water under the condition that the slump of concrete is basically the same are called superplasticizers.
Factors affecting the effect of concrete superplasticizer
Different cements have different clinker components. The higher the content of MgO in the clinker component, the greater the influence of the water reducer on the performance of concrete, and the easier it is to cause micro-cracks in the concrete.
Superplasticizer has the property of selective adsorption to mineral clinker. When C3A is in contact with water, it has enough adsorption layer immediately. less. This uneven adsorption will make the dispersion of superplasticizers not the same. The higher the C3A content, the worse the compatibility between cement and water reducing agent, which will cause the superplasticizer to have insufficient water reduction or no water reducing effect at all.
Experiments show that the setting time of concrete with water reducing agent is longer than that of concrete without water reducing agent.
In the concrete with superplasticizer added, the concrete system characterized by low water-cement ratio, its setting and rheology are very different from the cement mortar without superplasticizer.
1.Reduced water consumption. The amount of gypsum dissolved in the cement slurry is very small, and its concentration is not enough to control the hydration of C3A, resulting in rapid setting and rapid slump loss;
2. The addition of superplasticizer interferes with the power of cement hydration, reduces the solubility of gypsum, and makes the concentration of gypsum in the liquid phase lower. At the same time, due to the lack of sulfate ions, the sulfonate group on the superplasticizer molecule will combine with C3A, so that the dosage of superplasticizer in the liquid phase is reduced, the dispersing effect on cement is lost, and the slump loss is accelerated.
The specific surface area and particle gradation of cement affect the dispersion effect of superplasticizer. Generally speaking, cement with larger specific surface area, when mixed with the same superplasticizer, has obvious effect and greatly improves fluidity.
A lot of data show that the alkali content obviously affects the fluidity of the cement system of the superplasticizer. The higher the alkali content, the smaller the fluidity of the cement. Experiments show that the use of high-efficiency superplasticizer in high-alkali cement will reduce the water reduction rate.
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Concrete additive is a substance added in the process of mixing concrete to improve the performance of concrete, the amount of which is not more than 5% of the quality of cement (except in special cases).
Classification of concrete additives
Concrete additives are divided into four categories according to their main functions.
1. Additives to improve the rheological properties of concrete mixing, including various water reducers, air entraining agents and pumping agents, etc.
2. Additives that regulate the setting time and hardening properties of concrete, including retarders, early strength agents, and accelerators, etc.
3. Additives to improve the durability of concrete, including air entraining agent, waterproof agent and rust inhibitor, etc.
4. Additives to improve other properties of concrete, including aerator, expansive agent, antifreeze, colorant, waterproofing agent and pumping agent, etc.
Water reducing agent:
Water reducing agent or superplasticizer refers to the additive which can keep the workability of concrete unchanged and significantly reduce the amount of water used for mixing. Because the water reducer is added to the mixing house, if the unit water consumption is not changed, it can obviously improve its workability, so the superplasticizer is also called plasticizer.
Action mechanism of superplasticizer
After cement is mixed with water, the cement particles will attract each other and form many flocs in the water. In the flocculent structure, a lot of mixed water is wrapped, so that the water cannot increase the fluidity of the slurry. When the superplasticizer is added, the superplasticizer can disassemble these flocculent structures and liberate the wrapped free water, thus improving the fluidity of the mixture. At this time, if the workability of the original concrete still needs to be kept unchanged, the mixing water can be significantly reduced and play a role in reducing water, so it is called superplasticizer.
If the strength remains unchanged, the amount of cement can be reduced while reducing water, so as to achieve the purpose of saving cement.
Commonly used water reducing agent
The main types of superplasticizers are lignin series, naphthalene series, resin series, molasses series and humic series, which can be divided into common superplasticizer, superplasticizer, early strength superplasticizer, retarding water reducer, air-entraining superplasticizer and so on according to their main functions.
Early strength agent:
The additive which accelerates the development of early strength of concrete is the early strength agent. This kind of concrete can accelerate the hydration process of cement, improve the early strength of concrete, and has no significant effect on the later strength. There are three kinds of commonly used chloride, sulfate, triethanolamine and compound early strength agents based on them.
1. The commonly used chloride early strength agents are mainly calcium chloride and sodium chloride. Chloride additive can obviously improve the early strength of concrete. Because chloride salt can accelerate the corrosion of steel bar, the content of chloride salt is often controlled, and the content of unreinforced plain concrete is generally 1% of the cement quality.
2. The commonly used sulfate early strength agents are mirabilite, dihydrate gypsum and Haibo. They are all white powders, which can be hydrated calcium sulphoaluminate crystals formed by hydration with cement in concrete, which can accelerate the hardening of concrete, and the suitable content is 0.5% of the weight of cement.
3. Triethanolamine. Triethanolamine is a kind of organic matter, which is colorless or yellowish oily liquid, soluble in water, alkaline, and has the effect of accelerating cement hydration. Overuse will cause a significant decrease in strength.
4. Compound early strength agents. The above three kinds of early strength agents can be used alone, but the effect of compound use is better.
Air entraining agent:
In the process of mixing concrete, the additive which can introduce a large number of evenly distributed, stable and closed micro-bubbles is called air entraining agent. The air entraining agent can be introduced into the concrete mixture with a diameter of about.
0.05~1.25mm air bubbles can improve the workability and workability of concrete and improve the frost resistance of concrete. it is suitable for port engineering, hydraulic engineering, underground waterproof concrete and other projects. The commonly used products are rosin heat polymer, rosin soap, sodium alkyl sulfonate and sodium alkylbenzene sulfonate.
Retarder:
The additive that prolongs the setting time of concrete is called retarder. In concrete construction, in order to prevent the premature setting of concrete mixture from affecting the pouring quality under the conditions of high air temperature and long transportation distance, in order to prolong the heat release time of mass concrete or to prevent construction cracks in layered concrete, it is often necessary to add retarder to concrete.
Antifreeze:
The additive that can harden concrete at negative temperature and achieve sufficient antifreeze strength within a specified period of time is called antifreeze. Concrete projects constructed under negative temperature conditions should be mixed with antifreeze. In general, antifreeze can not only reduce the freezing point, but also promote coagulation, early strength, water reduction and so on, so it is mostly compound antifreeze. The common ones are NC- 3, MN- F, FW2, FW3, AN-4, etc.
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If you are looking for high-quality concrete additives, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com)
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