Polycarboxylate high-performance water reducer refers to the copolymerization of unsaturated monomers containing carboxyl groups and other monomers. It is used in cement concrete with high water reduction, plasticization, slump retention and low shrinkage performance. Due to the excellent comprehensive performance of this product, its mechanism of action is also different from that of commonly used high-efficiency water reducers, so it is called the third-generation water reducer. Although Polycarboxylate Superplasticizer has excellent water-reducing performance, good slump-preserving performance and low shrinkage rate, good technical and economic benefits cannot be achieved just by mixing this kind of product. Its application technology is more complicated than that of commonly used high-efficiency superplasticizer. Improper use will not only fail to achieve the expected technical effect, but may even cause engineering accidents. Some of the problems we have collected in the application of polycarboxylate superplasticizers are summarized as follows.
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1 Adaptability of polycarboxylate superplasticizer to cement
Since the Polycarboxylate Superplasticizer is not sensitive to the type and concentration of ions in the cement slurry solution, the longer side chain is not easy to be covered by cement hydrate, so it can exert its dispersibility for a longer period of time. It has better adaptability to various cements than naphthalene-based and other high-efficiency water reducers, but it also has the problem of cement adaptability, and it cannot be adapted to any cement.
The cement mineral composition and content ratio, the specific surface area of cement, the alkali content in cement, the type, dosage and solubility of cement coagulant will affect the cement adaptability of superplasticizer. These factors also exist when using polycarboxylate high-performance water reducer. The more important problem is that the content of sulfate ions in cement has a great influence on the application effect of polycarboxylate high-performance water reducer. This is mainly because a large number of sulfate ions and polycarboxylate high-performance superplasticizer molecules compete to adsorb cement particles, reducing the adsorption capacity of polycarboxylate high-performance superplasticizers. Therefore, when the sulfate content in cement is high, the water-reducing function of polycarboxylate high-performance superplasticizers will decline sharply, and a large amount of bleeding will occur after a certain period of time.
There are also studies that the presence of a large amount of sulfate ions in water will shrink the EO chain in the polycarboxylate high-performance superplasticizer, thereby weakening the volume exclusion effect of the EO chain. The above-mentioned cement with high sulfate content is mixed with naphthalene-based high-efficiency superplasticizer, but because sulfur trioxide inhibits the hydration of C3A, the slump retention of concrete will be improved.
2 Influence of admixture and aggregate on the application effect of PCE
Tests have proved thatPolycarboxylate Superplasticizer is very sensitive to the selection of fly ash. The effect of using grade I ash is better, and the fluidity of concrete is poor when using grade II or grade III ash. Sometimes it even loses fluidity. This is mainly due to the high carbon content of grade Ⅱ and grade Ⅲ fly ash with high loss on ignition, and the adsorption of carbon particles to the admixture is large, thereby reducing the adsorption of the admixture to the cement and affecting the fluidity of the concrete.
The test also found that when using pozzolan or pozzolan cement, the water-reducing effect of polycarboxylate high-performance water-reducing agent is poor. The influence of aggregate on the use effect of polycarboxylate high-performance superplasticizer is mainly the mud content in sand and gravel in addition to the gradation and dosage of aggregate. When the mud content increases from 0.5% to 4%, the water-reducing rate of the concrete mixed with polycarboxylate high-performance water-reducing agent decreases, or even completely loses fluidity, and the slump-retaining performance is also poor.
3 Effect of water-cement ratio and concrete mix ratio on the effect of PCE
Polycarboxylate Superplasticizer is very sensitive to the water consumption of concrete. When it is too small, the fluidity is not good. If it is slightly increased, bleeding may occur, which affects the overall homogeneity of concrete.
Concrete sand rate will also affect the application effect of polycarboxylate superplasticizer. When using commonly used high-efficiency superplasticizers, the fluidity of concrete is improved by appropriately increasing the sand rate, but when the Polycarboxylate Superplasticizer is mixed, the greater the sand rate, the worse the concrete fluidity.
4 Air-entraining properties of polycarboxylate superplasticizer
The main chain of polycarboxylate high-performance water reducer contains carboxyl adsorption genes, and a large number of polyether side chains are connected to the branch chains to provide steric hindrance, and polyethers are mostly air-entraining. Proper air-entrainment can improve concrete fluidity, pumpability, water retention and durability. However, excessive air-entraining will not only fail to achieve the above-mentioned improvement goals, but the strength of concrete will decrease due to excessive air content. The main reason is that the diameter of these air bubbles is too large. At the same time, because the concrete mixed with Polycarboxylate Superplasticizer is generally relatively viscous, these air bubbles are difficult to get rid of by vibration during construction, resulting in more honeycomb pockmarks on the surface of the concrete, which affects the appearance of the concrete. In response to this situation, the best solution at present is still to mix defoamers. Considering the actual application, it is the best effect to reduce the amount of defoaming agent and to suppress the generation of large bubbles and leave an appropriate amount of fine bubbles, especially for concrete with certain air-entraining requirements.
5 Compatibility of polycarboxylate superplasticizer with other additives
In order to further improve the performance of the Polycarboxylate Superplasticizerand reduce the cost of use, it is best to use the polycarboxylate high-performance water-reducer in combination with other admixtures. The current problem is that most admixtures are incompatible with the polycarboxylate high-performance water-reducer. The performance is that the products cannot be miscible or there is no superposition effect after mutual solubility. Sometimes the fluidity or slump retention of the polycarboxylate high-performance water-reducer will be reduced, and some will increase the bleeding and segregation of concrete.
Polycarboxylate Superplasticizer are completely incompatible with naphthalene-based superplasticizers, anthracene-based superplasticizers, and sulfamate superplasticizers, but they are compatible with melamine-based superplasticizers in a certain proportion. It is only compatible with aliphatic high-efficiency water reducers.
Polycarboxylate high-performance superplasticizer is compatible with Sodium Gluconate and sugar retarders, but incompatible with various phosphate and citric acid retarders. When preparing concrete with certain early strength requirements, it is very difficult to use Polycarboxylate Superplasticizer, and only the early strength agent (such as sodium thiocyanate) mixed with a certain coagulation accelerator will be effective.
When the concrete needs a certain amount of air, a small amount of defoaming (foam suppressing) agent can be added to the polycarboxylate high-performance water reducer, and then an air-entraining agent can be added. Rosin thermopolymer air-entraining agents and triterpene saponins are incompatible with polycarboxylate high-performance water-reducers, and the application effect of fatty alcohol sulfate or polyether composite air-entraining agents is better.
6 Application range of polycarboxylate high-performance water reducer
Polycarboxylate Superplasticizer is characterized by low dosage, high water reduction and high reinforcement. Therefore, it is more suitable for the preparation of medium and high strength concrete and high fluidity concrete with higher requirements for water reduction. In addition, the product has good slump retention performance, so it has a better application effect in high-strength fluidity commercial concrete and pumped concrete.
Because the low-strength grade concrete uses less cementitious materials, it is difficult for polycarboxylate high-performance water reducers to exert their advantages. Moreover, due to the high water consumption of low-strength concrete, a small amount of polycarboxylate high-performance superplasticizer cannot inhibit the bleeding of concrete, and the quality of concrete is difficult to guarantee. Its use effect is not even as good as that of commonly used naphthalene-based superplasticizers. Especially the early strength, the concrete mixed with polycarboxylate high-performance superplasticizer is particularly low, so currently polycarboxylate high-performance superplasticizer is rarely used in concrete below C30.
7 Shrinkage reduction and anti-crack performance of high-performance superplasticizer mixed with polycarboxylate
Due to the reduction of the surface tension of the liquid phase, the concrete mixed with Polycarboxylate Superplasticizer has a much lower shrinkage rate than the concrete mixed with commonly used water reducers, and sometimes even lower than blank concrete without admixtures. This is very beneficial to the improvement of concrete crack resistance. A large number of engineering reports have also confirmed that the addition of polycarboxylate high-performance water reducer can indeed reduce the generation of concrete shrinkage cracks.
We believe that with the continuous deepening of research on Polycarboxylate Superplasticizerproducts, the application technology of polycarboxylate high-performance superplasticizer will be mastered more professionally and skillfully, and better technical and economic effects will be achieved, so as to facilitate the higher and faster development of concrete technology.
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