Difference in the use of water reducing agent in various cement

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New Earth Group


Difference in the use of water reducing agent in various cement

China's cement production is large, many production enterprises, material sources are wide. The mineralization composition of different cement is complex, and there are many varieties of mixed materials, so the technical effect of admixture used in different varieties of cement is very different. Therefore, there is a problem of adaptability of admixtures to different cements. This is the skill of construction technicians for a long time.technical problems. The following is a brief introduction to solve the problem of the adaptability of admixtures to several special cements.
The soluble alkali in high-alkali cement is usually expressed as Na2O equivalent, which mainly comes from the clay and admixture of cement production, and the appropriate amount of soluble alkali is conducive to promoting cement hydration, which is more conducive to the development of early strength of concrete. The fluidity of cement concrete increases with the increase of alkali content. But arrivingIn a certain amount, the cement will hydrate sharply and the fluidity of the cement slurry will decrease significantly. The plasticizing effect was also significantly reduced after adding water reducing agent. The water reducing agent is used to increase the loss rate of the slump of commercial concrete and pumped concrete construction. When using high alkali cement, water reducing agent with high sulfate content should be used (sodium sulfate contentMore than 20%) the use effect will be significantly improved. This is mainly because CaSO4 contained in low-concentration water reducing agent is produced during synthesis and neutralization, and has excellent water solubility. Gypsum in cement is dissolved in water in large quantities before dissolution. When higher alkali accelerates the dissolution of C3A, a large amount of SO3 already exists in water and reacts with C3A to form AFt,The decrease in fluidity due to the formation of calcium aluminate is thereby prevented and the slump loss is reduced. It is not difficult to see that the water reducing agent with high sodium sulfate content is more suitable for high alkali cement.
Low alkali and sulfur-deficient cement: The optimal content of soluble alkali in cement is generally believed to be 0.4-0.6. Cement with an alkali content of less than 0.4 percent is usually referred to as low-alkali cement. The water-soluble alkali is mostly in the presence of alkali sulfate, so low-alkali cement is also called sulfur-deficient or sulfur-deficient cement. Sulfur-deficient cement mixed with water reducing agent usually flowsPoor, and increase the amount of water reduction agent although there is a certain effect, but more will increase the concrete bleeding, the preparation of concrete homogeneity is poor, slump loss fast, so the commonly used water reduction agent is difficult to adapt, even if the amount of retarder doubled also has no effect. It is not difficult to see that sulfur-deficient cement produces the above-mentioned non-adaptation phenomenon.The fundamental reason is that SO3 in cement is not enough, which reduces the hydration effect of C3A in cement, and the rapid adsorption of C3A on admixtures also reduces the plasticizing function of water reducing agent. Therefore, only the supplement of soluble alkali (sulfate) is effective to solve the problem of low alkali and sulfur deficiency cement adaptability. And the commonly used increase the amount of retarder.The effect of the method is not obvious.
Cement with high C3A content: the main components of cement are C3S, C2S, C3A and C4AF. the adsorption activity order of these mineralized components is generally considered to be C3A>C4AF>C3S>C2S, of which C3A has the largest adsorption amount of water reducing agent. therefore, when the water reducing agent content is constant, the fluidity of concrete increases with the content of C3A.And lower. The time loss rate of slump also increases. This is mainly due to the incorporation of water reducing agent will be C3A adsorption, and accounted for the main mineralization component C3S but not enough water reducing agent to adsorption dispersion, so that the fluidity of cement slurry decreased. The content of C3A in cement exceeds 8%, which will adversely affect the fluidity of concrete.ring. The test proves that the use of water reducing agent with high sulfate content to supplement SO3 in cement slurry has a certain effect. The adsorption and hydration of C3A can also be inhibited by mixing a certain amount of hydroxycarboxylate retarder, while the effect of using retarder such as polyol is not obvious. You can also use low-cost water-reducing agent and appropriately increase the amount of mixing., meet the C3A adsorption and have more residual water reducing agent to improve the fluidity of C3S and other mineralized components. Due to the low price of this kind of water reducing agent, it will not increase the cost of use.
High mixed material dosage cement: According to China's cement standards, cement can be mixed with a large number of mixed materials. At present, fly ash, volcanic ash, slag and ground limestone are widely used. The activity, water requirement, mineralization composition and adsorption properties of these mixtures are quite different, which affects the admixture pair.Adaptability of cement. Higher carbon content is more likely to deteriorate concrete performance. Unburned carbon is mostly porous particles, easy to absorb water, high water demand in concrete, overflow will increase concrete bleeding, and will increase the shrinkage deformation of concrete, but also affect the bonding performance of cement slurry and aggregate interface. After carbon meets water, it can also beIt can form a layer of hydrophobic film on the surface of the particles, which hinders the further penetration of water and affects the activity of fly ash. The study also found that the carbon in the fly ash has a strong adsorption capacity, and the water-reducing agent will compete with the cement for adsorption, which affects the fluidity of the cement slurry. Solve high loss on ignition fly ash, pozzolan cementAt present, the commonly used method of compatibility with admixtures is to increase the mixing amount of admixtures and mix a certain number of high-quality air-entraining agents.
Gypsum cement: anhydrite and waste gypsum have poor water solubility and are slowly dissolved in water. In the admixture, retarding and water reducing agents such as wood calcium or sugar calcium with high cost performance are usually added, and the incorporation of these water reducing agents will affect the solubility of gypsum. Since gypsum does not dissolve quickly, C3A in cement will hydrate quickly, producingA large number of calcium aluminate crystals are generated, resulting in pseudo-coagulation of concrete (I. e., a small amount of cement has been coagulated and a large number of cement particles have not been hydrated and coagulated, and the cement slurry loses fluidity). In order to prevent false coagulation of cement mixed with anhydrite or cement mixed with other poor water-soluble gypsum, it is best not to use wood calcium, wood sodium, sugar calcium, etc. to affect the dissolution of gypsum.Water reducing agent. It can also be mixed with a large amount of admixture that can supplement SO3 in cement to control pseudocoagulation.
Cement kiln storage time and specific surface area will also affect the adaptability of admixture. Usually, we call cement with a short storage time after being made "fresh cement" because of the short storage time of the above cement, the high temperature of the cement, the extremely fast hydration speed of the cement, and the electric charge generated by the cement in the grinding processThe mutual adsorption between particles affects the dispersion of water reducing agent and increases the loss rate of concrete slump. Prolonging the cement storage time and waiting for the temperature to drop below 50 ℃ is beneficial to improve the compatibility with the admixture. If the cement storage time cannot be prolonged, the dosage of the retarder can be increased to a certain extent. of cementThe specific surface area has a certain effect on the adaptability of the admixture. Cement with larger specific surface area needs more water, and more admixtures are required to achieve certain fluidity. It is generally believed that the more suitable specific surface area of cement is about 5000CM2/g. The early strength of cement with larger specific surface area develops faster, but it has great influence on the later strength of concrete.and slump retention will adversely affect the performance. When using cement with large specific surface area, the dosage of admixture should be increased. Considering that it does not increase the cost of use, low-cost water reducing agent can be used and the dosage of water reducing agent and retarder can be appropriately increased. Still can achieve better technical and economic benefits.
Due to the cement clinker and mixed material mineralization composition and form of complex, the compatibility of water reducing agent influence factors too much, it is difficult to use a simple way to solve all the water reducing agent on the problem of cement adaptability, research shows that the current is vigorously promoting the application of polycarboxylate high-performance water reducing agent although the adaptability of cementRelatively better than the commonly used a variety of high-efficiency water-reducing agent, can also be used to change the water-reducing agent mixing time and mixing method is conducive to improving adaptability.

Key words:

Polycarboxylate superplasticizer