Effects of Admixtures on the Properties of Cement Concrete
The incorporation of various admixtures is mainly to produce chemical or physical effects on cement. The physical effects are divided into participating in hydration reactions and not participating in hydration reactions. desired purpose.
1 Water reducing agent
Water reducer is the physical effect of participating in the hydration reaction. It refers to the ability to reduce the amount of mixing water and improve the strength of concrete under the condition of the workability of concrete and the amount of cement unchanged; or under the condition of constant workability and strength , an admixture that saves the amount of cement. According to its water reducing and strengthening ability, it is divided into ordinary water reducing agent and high efficiency water reducing agent. The dosage of ordinary water reducing agent is generally 0.1% ~ 0.3%, and the dosage of high performance water reducing agent is generally 0.3% ~ 1.5%. , the most widely used water reducer is high-efficiency polycarboxylate water reducer. The water reducer is the most used in the current project. Its main features: low dosage, high water reduction rate, when the dosage is about 1.0%, the water reduction rate exceeds 35%. Low slump loss, 1h slump retention rate is very good, it remains unchanged when the temperature is low, the temperature exceeds 20 degrees, the 1h slump slightly loses, but it also maintains above 95%, the temperature exceeds 30°C, 1h The slump retention value is still 93%. And concrete has good durability, and it is superior to ordinary water reducing agents in terms of filling, stability, pumpability, frost resistance, and shrinkage resistance.
The main working principle is: the water reducing agent has a good effect on the fluidity of the concrete. Normally, if the water reducing agent is not mixed with cement and water, the effect is not sufficient, there will be agglomeration, and the cement ion will not play a good role in wrapping part of the water. Hydration, after the water reducing agent is added, the chemical substances in the water reducing agent will be adsorbed to the surface of the cement, so that the similar cement particles will decompose due to mutual repulsion, and the wrapped water molecules will be released. This part of the water can participate The hydration reaction reduces the water-cement ratio, improves the strength and durability of concrete, and achieves good fluidity.
In the case of constant water-cement ratio, the addition of water reducing agent can increase the initial slump of concrete by more than 10cm, the fluidity is greatly increased, and it has replasticity. In the case of keeping the concrete slump basically unchanged, the addition of water reducing agent can save the amount of cement, and the early strength of concrete can still be improved compared with that without mixing, so as to achieve economic effects. The amount of superplasticizer needs to be properly adjusted in the same proportion in different seasons, because the activity of water molecules at different temperatures is different, and the effect of water molecules and cement will be affected by temperature, so when using a superplasticizer The configuration of water reducing agent in winter and summer will be different.
2 Air-entraining agent
Air-entraining agent refers to an admixture that can introduce a large number of tiny, closed and stable air cells into the concrete during the mixing process. It is mainly used in structures with high frost resistance requirements, such as concrete dams, roads, bridge decks, airport pavement and other large areas that are prone to frost.
The incorporation of air-entraining agent will form a large number of tiny closed spherical bubbles in the concrete mixture. These micro-bubbles, like balls, can reduce the frictional resistance between aggregate particles and increase the fluidity of the concrete mixture. If the fluidity remains the same, water consumption can be reduced. Water retention and cohesiveness are correspondingly improved. Due to the existence of a large number of air bubbles, the effective bearing area of the concrete is reduced, and the strength of the concrete is reduced. Therefore, the amount of air-entraining agent should be strictly controlled. In addition, due to the existence of a large number of air bubbles, the elastic deformation of concrete increases and the elastic modulus decreases, which is beneficial to improve the crack resistance of concrete. The main reason is that there is a large amount of water in the concrete mixture, and the bubbles of the air-entraining agent can block and fill the generated cracks, thereby improving the crack resistance of the concrete, reducing the influence of water on it, and enhancing the impermeability Improve frost resistance.
3 Retarder
Retarder is an additive that reduces the hydration speed and heat of hydration of cement or gypsum, and prolongs the setting time. Retarder is a common admixture used to prolong the setting time of concrete. The main function of the retarder is physical action, it does not participate in the hydration reaction of cement, and does not produce new substances. The chemical molecules in the retarder will adsorb on the surface of cement ions to form a thin film, thus slowing down the chemical reaction between cement and water. Response time delays setting time, reduces early hydration heat release, and reduces cracks. During summer construction and long-distance concrete transportation, adding an appropriate amount of retarder can make the fresh concrete maintain good plasticity for a long time, which is convenient for construction. Temperature cracks, adding retarder has become one of its important means.
4 Accelerator
Accelerator is a chemical admixture that can quickly set and harden cement concrete without excessively affecting its long-term strength. It has become one of the important components of shotcrete, especially with the increase in the number and scale of underground projects, the accelerator is not only more and more important as a component of concrete, but also cannot be used in some specific projects. Indispensable, especially in winter construction and some emergency repair projects.
Its working principle is: mixed with quick-setting agent, it can immediately react with gypsum, which acts as a retarding effect in cement, to form sodium sulfate, thereby eliminating the retarding effect of gypsum, making the cement hydrate rapidly, and its water is precipitated in the solution. The crystallization product crystallizes, which in turn leads to the rapid setting and hardening of cement.
Although the quick-setting agent can make the concrete set quickly and promote the rapid growth of the early strength, the later strength of the concrete is far less than that without the accelerator. This is mainly because the rapid setting and hardening of concrete will inevitably lead to the formation of large defects inside, and the crystal transformation of calcium aluminate hydrate is easy to occur, which also leads to an increase in the internal porosity of the slurry, which reduces the effective bearing area of the concrete and thus affect the later strength of concrete.
5 Fly ash
Fly ash is a kind of industrial solid waste, generally refers to the fine ash collected from flue gas in coal-fired power plants, also known as fly ash and soot. The appearance of fly ash is similar to cement, in powder form, and the color is from milky white to gray black, generally silver and gray. Fly ash has morphology effect, filling effect and micro-aggregate effect. Because fly ash is sensitive to temperature and humidity, especially to water-binder ratio, the internal microstructure of fly ash concrete with too large water-binder ratio is not conducive to the development, and at the same time affects the durability of concrete. Therefore, it is necessary to control the water-binder ratio.
The main mechanism of action of fly ash is microbead effect, that is, the main shape of pulverized coal is the adhesion of tiny glass beads and sponges, including spherical particles and irregular debris particles. The spherical glass body is like a glass ball and has a dense texture. The surface is smooth, the particle size is fine, and the adsorption force to water is small, which improves the fluidity of concrete and plays a good role in lubrication during mixing and transportation. Moreover, they are often filled in the pores of the cement slurry, which greatly improves the compactness of the hardened concrete; the second is the secondary hydration reaction, the chemical molecules in the fly ash can react with the products of cement and water hydration Secondary hydration, thereby increasing the strength of concrete.
of concrete strength failure : one is aggregate failure, the other is cement stone failure, and the other is the destruction of hydration reactants between aggregate and cement stone. Therefore, the strength of the hydration reaction product is also one of the important factors of concrete strength. This is why the secondary hydration reaction will increase the strength of concrete. In practical application, there are the following three methods to mix fly ash in an appropriate amount. (1) The equivalent replacement method takes out the cement and replaces it with the same amount of fly ash to reduce the heat release of cement hydration and improve the crack resistance. (2) Excessive replacement method takes out excess cement and replaces it with fly ash, and the excess part is calculated as sand. (3) The amount of cement in the external mixing method remains unchanged, and a certain proportion of fly ash is directly mixed in, which has good workability.
6 Silica Fume
Silica fume is also called microsilica fume or superfine silica powder, and is generally referred to as Silica fume. Silica fume is an ultra-fine silicon powder material formed by rapid oxidation and condensation of SiO2 and Si gas produced during the smelting of ferrosilicon alloys and industrial silicon with oxygen in the air. It is a by-product of large-scale industrial smelting . The process needs to be recycled with dust removal and environmental protection equipment, because the density is small, it also needs to be encrypted with encryption equipment. Its particle size is generally 0.1-1.0 μm, which is finer than fly ash, and it is also a tiny bead particle. Due to its tiny characteristics, it can well improve the structure of the internal pores of the concrete (including the size of the pores, how many, Distribution), play a very good filling role, and can produce secondary hydration reaction, improve concrete strength, improve concrete cohesion and water retention, because silica fume particles are fine, specific surface area is large, and water demand is high, so in When concrete is mixed with silica fume, it must be used in combination with high-efficiency water reducer to achieve good results. After the concrete is mixed with silica fume, early maintenance must be strengthened to prevent plastic shrinkage cracks in the concrete. But because of its few sources and high cost, it is rarely used in the engineering field.
7 Slag Powder
Slag powder is granulated blast furnace slag that conforms to the standard and is dried and ground (or added with a small amount of gypsum to be ground together) to reach a certain fineness and meet the corresponding activity index.
The main chemical components of slag micropowder have ultra-high activity. When it is mixed into cement concrete as an admixture, these active chemical components can further react with the hydration reaction products in cement to form hydrated calcium silicate products, filling In the pores of cement concrete, the density of cement concrete is greatly increased, thereby improving the pore structure, reducing porosity and reducing the size of the largest pore diameter, so as to improve the impermeability of concrete. Concrete requires good workability and pumpability. In the early stage of cement hydration, slag powder is distributed and wrapped on the surface of cement particles, which plays a role in delaying and isolating, and can effectively inhibit the alkali-aggregate reaction of cement concrete, significantly The anti-alkali aggregate reaction performance of cement concrete is improved, and the durability of cement concrete is improved; the heat of hydration of cement concrete can be significantly reduced, the heat resistance is good, and the application prospect in the engineering field is broad. Slag powder improves the erosion resistance of seawater and fresh water of cement concrete, and resists sulfate erosion. Especially suitable for anti-seawater engineering, underground engineering and pavement engineering.
Epilogue: With the vigorous development of the world's construction industry, concrete plays an increasingly important role in various engineering constructions. Concrete with different property requirements is more and more widely used in applications, and admixtures and admixtures are becoming more and more indispensable in concrete engineering because of their unique workability. Due to the wide variety of admixtures and admixtures, the concrete properties required by the project are not the same. To use the admixtures and admixtures correctly and reasonably, it is necessary to be familiar with the performance of the product and the actual requirements of the project. Choose the appropriate type according to standard requirements, construction conditions and construction technology. In engineering practice, "double mixing" technology is often used, that is, adding water reducing agent while mixing different admixtures. The ordinary, high-strength and high-performance concrete prepared in this way can save cement, improve the workability, strength and durability of concrete, and can meet the construction technical requirements of different projects.
In short, different types of admixtures and admixtures have different effects on concrete, but they all have one thing in common: reasonable selection and use to improve concrete strength and fluidity, heat resistance, impermeability, freeze-thaw resistance, crack resistance and other properties, so as to better adapt to different requirements of engineering construction.
