Improvement Of The Performance Of Concrete By Mineral Admixture

Adding mineral admixtures to concrete can generally achieve the following purposes: reduce the amount of cement, improve the performance of concrete, reduce the heat of hydration, increase the strength of the later stage, improve the internal structure of the concrete, improve the impermeability and corrosion resistance, and inhibit alkali— Aggregate reaction, etc. The national standard clearly stipulates that mineral admixtures are added to improve the durability of concrete, and are ground various mineral admixtures. The main feature is that they are ground mineral materials with a fineness smaller than cement particles. They are mainly used for Improve the durability and working performance of concrete.

1. Slag

The slag is the molten slag floating on the surface of the molten iron in the ironmaking furnace, and it is quenched with water when discharged to obtain the water-quenched slag. Finely ground slag is a kind of mineral admixture that is used to dry this granular blast furnace water-quenched slag, and then use a special grinding process to grind it to a specified fineness, and mix it with a mineral admixture during concrete preparation. (It is required that the specific surface area used for grinding the slag exceeds 400m2/kg, in order to fully exert its activity and reduce bleeding)

1. The chemical composition of slag

The main chemical composition of slag is CaO, SiO2, Al2O3, Fe2O3, etc. The quality coefficient K is generally used to evaluate the activity of granulated slag: the larger the quality coefficient K, the higher the activity of the slag. The activity of granulated blast furnace slag is also related to the material conditions (temperature of molten slag before quenching, quenching method and quenching speed, etc.)

2. The influence of ground slag on concrete performance

It is generally believed that the mixing of mineral powder will have the following effects on the cement paste. The first is dispersion. After the mineral powder replaces a part of the cement, the contact points of the cement particles are reduced. Therefore, when the entire system is added with water, the original ionization causes the cement particles to attract due to the different surface electrical properties and form a flocculated structure. The possibility of decline. Second, mineral powder has a filling effect; studies have shown that minimizing the porosity of particles can effectively reduce the viscosity of cement slurry. Generally, the overall fineness of mineral powder mixed with cement is finer than cement, and a certain amount of mineral powder is mixed After entering, the entire powder particle distribution can be widened, and the accumulation can be denser, thereby reducing the viscosity by reducing the accumulation of porosity. Thirdly, it is secondary hydration. The ground slag has reactivity under alkali excitation, sulfate excitation or composite excitation, and secondary hydration reaction with Ca(OH)2 produced by cement hydration to generate low calcium type The hydrated calcium silicate gel stimulates and induces the hydration degree of cement during the hydration process of cement, and accelerates the reaction process of cement hydration. In addition, slag powder can also improve the interface structure of concrete, thereby increasing the strength and durability of concrete.

(1) Impact on work performance

The water requirement of the cementitious material directly affects its use in concrete. Under the same water-binder ratio, the smaller the water requirement of the cementitious material, the greater the fluidity of the slurry, which will affect the flow of the concrete mixture. Sex is also more favorable. Tests show that the use of ground slag can reduce the water consumption of standard consistency of the system, and the water consumption of standard consistency increases with the increase of slag content. At the same water-to-binder ratio, the gelling system with ground slag will have better fluidity and pumpability. Moreover, the greater the specific surface area of the ground slag, the better the workability. The exothermic process of the hydration reaction of the cementitious material system is closely related to the hydration reaction performance of the material. At the same time, the heat of hydration of the cementitious material system is directly related to the volume stability of concrete. The influence of ground slag on the hydration characteristics of the cementing system is greatly related to the fineness and content of the slag. At the same amount of ground slag, as the fineness of the slag increases, the hydration rate of the cementitious material will be accelerated, resulting in an increase in the heat of hydration; at the same fineness, the increase in the amount of ground slag can delay the glue The hydration rate of the condensing material prolongs the setting time. In addition, experimental studies have shown that mixing proper amount of water reducing agent and retarder can give full play to the performance of ground slag, greatly reduce the heat of hydration, and delay the process of hydration heat release, which is very effective. After the ground slag is added to the concrete, the setting time of the concrete will be prolonged, and the smooth, dense, and low-adsorption ground slag will increase the bleeding of the concrete. The bleeding is related to the fineness of the slag. If the specific surface area of the ground slag is larger than that of the cement, the bleeding will decrease, otherwise, it will increase.

(2) Impact on strength

When it is mixed with ground slag, its early compressive strength decreases with the increase of the content, but it can reach the same level in the later stage. Some experimental studies [2] showed that the compressive strength of cement mixed with ground slag for 3d~7d was lower than that of pure cement, and reached the same level at 28d, and the compressive strength of cement mixed with slag was higher than pure cement at 56d. The cement is even higher. That is to say, the cement mixed with ground slag has a more obvious increase in later strength. As the fineness of slag increases, its early compressive strength increases, and its later strength is also higher. In order not to reduce the strength of the concrete structure, the fineness of the slag should be increased as much as possible when economic and various conditions permit. The grinding method of slag also affects the development of concrete strength. Some experimental studies [3] found that the slag powder processed by ball milling has a wider particle size distribution and circularity quotient than the slag powder processed by tuning; when the specific surface area of the slag powder is similar (about 430m2/kg), the strength ratio of the ball milled slag cement Vertical mill slag cement has high strength. In terms of flexural strength, the flexural strength in the early stage is similar to that of pure cement, and the flexural strength in the later stage is significantly higher than that of pure cement, and the flexural strength increases with the increase of slag content.

(3) Impact on long-term durability

The incorporation of slag improves the pore structure of concrete, increases the number of pores with pore size≦50nm [5], reduces the number of macropores and connected pores in the slurry, increases the density of the slurry, and improves the durability. Replacing part of Portland cement with ground slag can effectively improve the sulfate corrosion resistance of concrete. The reason is: the addition of ground slag reduces the content of C3A in the concrete mixture; due to the formation of calcium silicate hydrate, the soluble calcium hydroxide is reduced, which reduces the conditions for the formation of calcium sulfate; and improves the penetration resistance of concrete When calcium silicate hydrate is formed in the micropores, there are generally alkali and calcium hydroxides, which reduce the permeability of concrete, thereby preventing the intrusion of corrosive sulfate and improving the sulfate resistance of concrete. Slag is beneficial to inhibit alkali-aggregate reaction in concrete [1]. The reason is: the addition of slag reduces the alkali content per unit of concrete; the filling effect of slag improves the compactness and impermeability of concrete, reduces the permeability, and greatly reduces the activity of alkali ions, which effectively prevents alkali- An important factor in the occurrence of aggregates, and its inhibitory effect is related to the fineness of the slag. The incorporation of slag can also improve the freeze-thaw resistance of concrete. Because it can reduce the number of connected pores in the pores, it must be mixed with air-entraining agents. Appropriate air content and spacing coefficient are necessary for the freeze-thaw resistance of concrete.

2. Fly ash

Fly ash is dust collected by the chimneys of coal-fired power plants. It contains a large number of spherical glass beads, as well as mullite, quartz, and a small amount of mineral crystals.

1. The chemical composition of fly ash

The main chemical components of fly ash are: SiO2, Al2O3, Fe2O3, CaO, and a small amount of loss on ignition. The chemical composition of fly ash discharged and produced by most thermal power plants in China is: SiO240%~50%, Al2O320%~30%, Fe2O35%~10%, CaO2%~5%, and loss on ignition is 3%~ 8%. SiO2 and Al2O3 are the main active components in fly ash. According to the content of CaO, fly ash is divided into high-calcium fly ash (CaO content ≥ 10%) and low-calcium fly ash (CaO content <10%).

2. Improvement of concrete performance

(1) Improvement of work performance

The incorporation of fly ash can significantly improve the performance of concrete, which is mainly reflected in the following aspects. Fly ash can reduce the water demand of concrete. A large number of experimental studies have shown that the same amount of fly ash replaces cement to ensure that the slump is basically the same. As the amount of fly ash increases, the amount of concrete mixing water decreases significantly. If the same water consumption is ensured, the slump of the mixture will increase significantly after adding fly ash to the concrete, and the mixing of fly ash will significantly improve the pumpability of the concrete. In terms of improving bleeding and segregation, it is generally believed that the incorporation of fly ash can reduce the bleeding of concrete, but the test results of many researchers show that the bleeding rate of fly ash concrete is greater than that of ordinary concrete with the same slump. The incorporation of fly ash can make the concrete slump have a relatively large retention value over time, so that the concrete mixture still has better working performance in a certain period of time, that is, reduces the slump loss of concrete.

(2) Improvement of mechanical properties

The incorporation of fly ash reduces the early strength of concrete. As the amount of fly ash increases, the early strength decreases more. Generally, the strength decrease is very obvious after 7 days. After 90 days, the strength of fly ash concrete is close to that of ordinary concrete with a relatively small amount, and even exceeds the strength of ordinary concrete after 1 year. The flexural strength of fly ash is proportional to the compressive strength. The elastic modulus of fly ash concrete is also proportional to the compressive strength. Compared with ordinary concrete, the elastic modulus of fly ash concrete is not lower than or even higher than ordinary concrete with the same compressive strength after 28 days. The elastic modulus also increases with age. Since the incorporation of fly ash reduces the water consumption of concrete, its shrinkage is significantly reduced with the increase of fly ash content; but if high-calcium fly ash is used, the incorporation of fly ash will make its shrinkage value Higher than ordinary concrete. Maintaining moisture for a long time is beneficial to reduce the shrinkage of fly ash concrete.

(3) Improvement of long-term performance

The incorporation of fly ash can improve the pore structure of cement paste and mortar. Specifically, the incorporation of fly ash reduces the number of small pores in the cement paste, and these pores are closed. The voids of pure cement paste have relatively high connectivity. And the finer the fly ash, the more beneficial it is to reduce the porosity, especially the volume of macropores. Therefore, the compactness of concrete is improved, and the long-term durability is also improved. Existing research results show that fly ash concrete has good resistance to sulfate attack compared with ordinary concrete. Due to the pozzolanic chemical reaction of fly ash, the amount of Ca(OH)2 and free calcium oxide in the concrete is reduced. The secondary hydration product of fly ash fills the coarse pores in the concrete and improves the impermeability of the concrete. However, it should be pointed out that the high calcium fly ash has poor resistance to sulfate attack, so use it with caution. Concrete mixed with fly ash can also improve the corrosion resistance of chloride ions. Some experimental studies have found that the chloride ion diffusion coefficient of concrete mixed with fly ash is reduced to 1/3 of that of pure cement concrete. But the carbonization resistance of fly ash concrete is reduced. Because fly ash concrete consumes a large amount of Ca(OH)2 due to the pozzolanic reaction of fly ash, its content of Ca(OH)2 is relatively low compared with ordinary concrete, which is unfavorable to the anti-carbonization performance of concrete. With the increase of fly ash content, the carbonization rate also increases. To improve the anti-carbonization performance of fly ash concrete and appropriately increase the alkali reserve, for example, a more economical way is to add lime. The frost resistance of concrete mixed with fly ash is related to the quality of fly ash itself. If the quality of fly ash itself is good, because it reduces the water-cement ratio, it can also improve the frost resistance, but if the quality of fly ash is compared Poor, it will seriously reduce the frost resistance of concrete. Domestic practice has shown that fly ash is also beneficial to inhibit alkali-aggregate reaction, but it has a great relationship with the quality of fly ash.

Three, silica fume

Silica fume is industrial smoke recovered from the flue gas purification device when ferroalloy plants smelt ferrosilicon or metallic silicon, and is collected in a bag filter.

1. The chemical composition of silica fume

The main component of silica fume is SiO2. In addition, it contains a small amount of Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, SO3, P2O5, etc. Silica fume used in concrete generally requires SiO2 to be greater than 85%, and most of it is amorphous. The more amorphous SiO2, the greater the pozzolanic activity of silica fume, and the stronger the reaction ability in alkaline solutions. The specific surface area of silica fume can reach 15m2/g or more, the particle shape is spherical, and the average particle size is about 0.1~0.2μm, which is two orders of magnitude finer than cement particles.

2. The improvement of silica fume to concrete performance

Silica fume has small particles, large specific surface area, and high purity SiO2. Adding silica fume as a mineral admixture to concrete can improve the performance of concrete. This beneficial effect is mainly manifested in: the role of ultra-fine filler of silica fume; it acts as a nucleus in the early hydration process and has a high pozzolanic activity.

(1) Impact on work performance

Silica fume particles are very small and have a very large specific surface area, which can fill the pores between cement particles. The particles are tightly packed, so the cohesiveness of the silica fume concrete mixture is increased, bleeding and segregation are reduced, and the average pore diameter of the pores is reduced. However, due to the large specific surface area of silica fume, the water consumption of concrete increases after the addition of silica fume. In order to achieve the required slump, it is necessary to add a superplasticizer to adjust the water consumption of concrete. However, the hydration heat of cement added with silica fume will greatly increase. It can be seen that silica fume greatly promotes the hydration of cement. The large increase in the early heat of hydration should be carefully considered.

(2) Impact on strength

The early strength and late strength of concrete mixed with silica fume will increase. Due to the good filling effect of silica fume, when it is mixed with silica fume, it can effectively fill the gaps between particles. When silica fume and superplasticizer are used in combination, the water-binder ratio of concrete drops to 0.13~0.18. Its compressive strength is several times that without silicon powder. However, studies have shown that if the content of silica fume is too high, the later strength of the concrete will decrease, and the content should not exceed 35%. According to information, silica fume concrete can increase the erosion resistance by 3 times and the cavitation erosion resistance by 14 times, which has outstanding advantages in underwater projects.

(3) Impact on long-term durability

Silica fume particles are very small and can fill the pores between cement particles. The particles are tightly packed, reducing bleeding, reducing the average pore size of the pores, and reducing water demand. The incorporation of silica fume improves the compactness of concrete, greatly reduces cement porosity, and improves concrete's impermeability and chemical resistance. A large number of domestic and foreign studies have shown that the frost resistance of silica fume concrete with a content of less than 15% is basically the same, and sometimes it will be improved, but the addition of more than 20% will significantly reduce the frost resistance of silica fume concrete [1]. When silica fume is added to concrete, the silica fume reacts with the alkali in the cement, which can inhibit the expansion caused by this alkali-aggregate reaction. However, the incorporation of silica fume will cause excessive early shrinkage of concrete, so the amount of silica fume should be controlled. High-performance concrete should be controlled at 5%~10%, and attention should be paid to moisture curing during curing. When silica fume is mixed into concrete, it can improve its sulfate corrosion resistance. Because silica fume can interact with the Ca(OH)2 generated by cement hydration after mixing, the content of free Ca(OH)2 is significantly reduced, resulting in post-corrosion The formation of ettringite is reduced, reducing the swelling effect. However, the test found that in the dry and wet cycle environment, the corrosion resistance of concrete mixed with silica fume decreased, so careful consideration should be given to the use. Because silica fume can improve the pore structure of concrete and improve its compactness, it also has good performance in anti-chloride ion corrosion.

These mineral admixtures have a good effect on improving the performance of concrete when mixed alone, and better results can be obtained when mixed. For example, when slag and fly ash are mixed together, it can reduce the heat of hydration and effectively reduce the number of temperature shrinkage cracks in large-volume structures. When the silica fume, grinding