- Mar 30, 2018 -
The performance and strength of concrete have always been closely related to the water-cement ratio. For example, the preparation of high-strength concrete usually requires the use of a low water-cement ratio, and a low water-cement ratio will inevitably lead to a decrease in the fluidity of the concrete, which is not conducive to construction. The success of the water-reducing agent has successfully mitigated the conflict between the work performance and strength of the concrete caused by the water-cement ratio, making it possible for the formulated concrete to have both good workability and high strength. Polycarboxylate superplasticizer has the advantages of low content, high water reduction, good water repellency, adjustable molecular structure, and environmental protection. It makes the application range more and more widely. High-grade pumping concrete, self-compacting concrete, high-strength concrete, and other high-performance concrete preferred water reducer.
Although the polycarboxylic acid water reducing agent has been more and more widely used, but in the actual application process, there have been polycarboxylate water reducing agent water reducing and dispersing effect is not good, the concrete is abnormal condensation, slump loss over time, even The decrease in the concrete strength and the like, among which the compatibility problem between the polycarboxylic acid water reducing agent and the concrete raw materials is one of the main reasons. According to JC/T1083-2008 "Compatibility test of cement and water-reducing agent," the definition of compatibility of cement and water-reducing agent, if the added water-reducing agent saturation point is clear, the initial fluidity of cement paste is better. Loss of small time, and no obvious bleeding, segregation, etc., then shows that the superplasticizer and cement compatibility. There are compatibility issues between polycarboxylate water reducer and concrete raw materials, mainly in the following aspects:
When using the same kind of polycarboxylic acid water-reducing agent and different cement, it may happen that the polycarboxylic acid water-reducing agent has good compatibility with one kind of cement but has poor compatibility with another cement, when the same kind of cement is used. When different polycarboxylate water reducers are used, similar conditions may occur. On the surface, it is due to the difference in the types of cement and polycarboxylate water reducers. The substance is related to their physical and chemical properties. For example, the mineral composition of cement clinker is the most important factor affecting its compatibility with polycarboxylate water reducer, mainly related to the adsorption of polycarboxylic acid water reducer by clinker minerals, and the positive hydration of aluminate phase hydration. Compared with the silicate phase hydration, the negatively charged polycarboxylate water-reducing agent is more easily adsorbed. It is generally believed that the cement with a high C3A content and a high degree of crystallization has poor compatibility with the polycarboxylic acid water reducer. In addition, the cement specific surface area also affects the compatibility of the cement with the polycarboxylate water reducer. When the specific surface area of the cement increases, the contact area between the cement particles and the water and polycarboxylic acid water reducer molecules increases, resulting in a surface It can increase, accelerate the hydration rate of cement, and increase the adsorption effect, which will cause the loss of cement slurry to increase over time, resulting in poor compatibility.
(2) mineral admixtures
Industrial solid wastes such as fly ash, slag, and silica fume not only play a role in environmental protection as a concrete admixture, but they are also an indispensable raw material for formulating high-performance concrete. The compatibility between the mineral admixture and the polycarboxylic acid water reducing agent is mainly caused by the adsorption of the admixture on the polycarboxylic acid water reducing agent, granulated blast furnace slag and high quality fly ash containing spherical glass bodies. Since the polycarboxylic acid water-reducing agent has less adsorption, the compatibility between them is good. Carbon content is the main reason affecting the compatibility of fly ash and polycarboxylic acid water reducing agent. Fly ash with high carbon content can adsorb a large amount of polycarboxylic acid water reducer and water, making it less than polycarboxylic acid. The water agent has poor compatibility. Due to its high specific surface area, silica fume can adsorb a large amount of polycarboxylic acid water reducer, which is poorly compatible with polycarboxylic acid water reducer.
(3) Other chemical admixtures
The compounding of admixtures can play a super-additive effect and is increasingly applied to the concrete field as a technical route. Sometimes only one water-reducing agent can not meet the construction requirements, so different water-reducing agents need to be compounded. The adaptability of polycarboxylic acid water-reducing agent and other water-reducing agents varies depending on the types of other water-reducing agents. Polycarboxylic acid The water-reducing agent has good adaptability to the lignosulfonate-based water-reducing agent, and the use of a naphthalene-based water-reducing agent will have a negative effect. In addition, there are also compatibility issues between polycarboxylic acid water reducers and other additives such as early strength agents, retarders and bulking agents. In particular, the sulfate content and alkali content in the additive have a great influence on the compatibility. .
(4) Sand aggregate
Gravel aggregates have a direct impact on the performance of concrete, and mud content is one of the main indicators to measure aggregate quality. Before the appearance of polycarboxylate water-reducing agent, due to the relatively small influence on the performance of concrete in the scope of the specification, the mud powder in the sandstone did not attract much attention. In recent years, with the increase in the use of polycarboxylic acid water reducer, natural sand and sand resources with better quality are gradually in short supply. When using concrete with high powder content and natural sand with high mud content, the performance The polycarboxylic acid water reducing agent has poor compatibility with it. The content of stone powder in the mechanized sand has little influence on the polycarboxylic acid water reducer, and it is generally considered that the stone powder content is within 20% of the range, which is even advantageous for the performance of the concrete. Polycarboxylate water-reducing agent is particularly sensitive to the clay content of sand and gravel. When the content of clay in gravel is high, the water-reducing and dispersing properties of the polycarboxylic acid water-reducing agent are seriously affected, and the work performance and strength of the concrete are reduced.
(5) Sulfate and alkali content
Sulfate in concrete is mainly derived from cement clinker, admixtures, admixtures, and externally added gypsums that act as a retarder on cement. A large number of studies have shown that sulfate on the one hand competes with polycarboxylate superplasticizers to reduce the polycarboxylate water-reducing agent that acts as a dispersed cement particle. On the other hand, the presence of sulfate reduces the space of polycarboxylic acid water-reducing agent. Resistant, thus affecting the water-reducing dispersion properties of polycarboxylate water reducers. Kazuo Yamada et al. found that the steric bulk of polycarboxylic acid-based water-reducing agent molecules decreases with the increase of sulfate ion concentration in the solution, indicating that sulfate can be influenced by reducing the steric hindrance of polycarboxylic acid water-reducing agent molecules. Its water reducing and dispersing properties. Wang Zhi et al. found that different types of sulphate have different effects on the water-reducing and dispersing properties of polycarboxylic acid water-reducing agents. The effects of potassium sulphate and sodium sulphate are more significant than those of calcium sulphate, but they are easily soluble in potassium sulphate and sodium sulphate etc. When the sulfate content is low (within 0.2% of the cement mass), it has no negative effect on the water-reducing dispersion performance of the polycarboxylate water-reducing agent. At the same time, the type of gypsum has a great influence on the polycarboxylate water reducer, which mainly depends on the dissolution speed and solubility of different types of gypsum.
Alkali (Na2O and K2O) in concrete is generally derived from the clay raw materials and additives used in calcining cement clinker. As the alkali content increases, the amount of water required for the concrete to reach the same fluidity increases, the setting time decreases, and the initial fluidity of the water-reduced concrete blended with carboxylic acid reduces and the loss increases with time.