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Property and effect of Polycarboxylate ether superplasticizer (PCE) as water reducer in concrete

In recent years, with the continuous development of the construction industry and the emergence of high-rise buildings, the demand for concrete is getting bigger and bigger, the strength grade is getting higher and higher, and the quality requirement is getting higher and higher.

Polycarboxylate high-efficiency water reducing agent is a kind of concrete admixture with high water reduction rate and low slump loss.

It can greatly increase the rheological properties of fresh concrete, and at the same time improve the compressive strength of hardened concrete and keep the water consumption unchanged.

It plays a vital role in the compatibility of fresh and hardened concrete. It also ensures the continuity of engineering construction, greatly shortens the construction period, promotes the development of fluid concrete technology and pumping and pouring technology, and accelerates the development of commercial concrete.

1、 The function of PCE (PolyCarboxylate Ester)

Polycarboxylate superplasticizer plays four main roles in concrete mixtures:

firstly, in order to improve the casting performance of concrete, and improve the ease of concrete without changing the composition of concrete;

Second: to reduce the water-cement ratio of mixing water and concrete under the given working conditions, and to improve the strength and durability of concrete;

Third: Reduce the amount of water and cement under the condition of ensuring the concrete casting performance and strength, and reduce the factors that cause the initial defects of concrete, such as creep, dry shrinkage, and heat of hydration of cement;

Fourth: the role of polycarboxylate superplasticizer in high-grade concrete is irreplaceable by other admixtures.. In the concrete of C50 standard or above, the ease of concrete is difficult to be achieved by other admixtures. It is expected that the improvement of concrete process performance and the breakthrough of concrete grouting technology in the future will mainly rely on the high-quality high-efficiency water reducing agent based on polycarboxylic acid polymer (PCE).

2、 The performance of PCEs :

Main technical characteristics of PCE polymer:

1. The content of polycarboxylic acid superplasticizer is low, and the water reduction rate is high. 
The content of polycarboxylic acid superplasticizer accounts for 0.80% – 1.25% of the cementitious material, and the water reduction rate can reach (20% – 35)%. When used in combination with fly ash, the water binder ratio is low, and it is suitable for the preparation of medium and high strength high-performance concrete.

2. The concrete has large fluidity and low slump loss. Due to the good dispersion stability of polycarboxylate superplasticizer, the large fluidity concrete (slump ≥ 18omm) prepared by polycarboxylate superplasticizer has low time loss, basically no slump loss in one hour, and the second hour loss is less than 15%, which makes up for the defects of concrete prepared by common naphthalene superplasticizer, such as high slump loss and easy bleeding, When used in combination with fly ash, excellent fluidity can be obtained with a small amount of water reducer, which can meet the process requirements of commercial concrete production, especially for pumped concrete, pipe plugging is not easy to occur.

3. It has good adaptability to cementitious materials. In engineering practice, the cement produced by different manufacturers is used to prepare pumping concrete, and a large amount of fly ash is mixed at the same time. After the polycarboxylic acid superplasticizer is mixed, it has good compatibility with different cements, and there is no obvious bleeding segregation, which hinders the growth of concrete strength. Because of its high water reduction rate, it is suitable to be used with fly ash, reducing the shrinkage of fly ash concrete, It also significantly improves the pumpability of concrete and improves the durability of concrete. When the strength of concrete is the same, the dosage of cement increases, the dosage of water reducing agent increases slightly, the water binder ratio decreases, and the strength of concrete increases; For concrete with different design strength grades, the dosage of water reducing agent increases slightly with the increase of the dosage of cementitious materials, the water binder ratio decreases, and the strength of concrete increases accordingly, but the workability of concrete remains stable on the whole, and the slump can reach (180-240) mm. 

3、 Some influencing factors of polycarboxylic acid water reducer in concrete

1.cement.

a.Influence of mineral composition of cement

The shrinkage rate of C3A, C3S, C2S and C4AF in Portland cement is C3A>C3S=C2S>C4F. 
The increase of C3A and C3S content in cement clinker will increase the self shrinkage and dry shrinkage of concrete and increase the chance of cracking. C3A has the highest calorific value, and high hydration heat leads to accelerated slump loss of concrete.

b.Effect of cement fineness

Increasing the specific surface area of cement is intended to increase the rate of hydration and hence the early strength of concrete. However, most of the cement under grinding conditions, the particles increase and the exothermic heat of early hydration increases, which has no effect on improving the late strength. And the larger the cement fineness, the worse the frost resistance of concrete and the lower the tensile strength.

What’s more, the increase of cement fineness makes the compatibility between cement and high efficient water reducing agent worse. With the increase of cement fineness, the saturation point of high efficient water reducing agent dosage is greatly increased, and the dosage of high efficient water reducing agent required to reduce the loss of concrete slump is also greatly increased.

c.Influence of gypsum form on compatibility between water reducer and cement

Different forms of gypsum on the production of cement according to the cement standard product testing is not much difference, but only add water reducing agent will be different, this is due to the reducing sugar and polyol on the dihydrate gypsum, gypsum hard solubility of different (hard gypsum) and gypsum fluoride.

d.Effect of soluble alkali

Many studies have proved that the sulphate of alkali in cement, i.e. soluble alkali, has a very important influence on the adaptability of additives to cement. Studies have shown that when the C3A mass fraction is not higher than 8 per cent and the cement soluble alkali content is high, the concrete has good compatibility and low slump loss. When the soluble alkali is very low, the compatibility of concrete will be lost quickly. It is suitable for cement with low soluble alkali content. Increasing the content of high efficiency water reducers gives a larger initial slump, but the loss is still rapid. When a little is added above the saturation point, sedimentation and water secretion increase significantly.

e.Influence of other factors of cement

Factors such as cement temperature, cement ageing time, dosage, type and amount of filler will affect the workability of concrete. The higher the cement temperature, the worse the plasticising effect of the water reducing agent.

2.Fly ash

Fly ash mainly changes the ease of concrete. Due to the fine particles, it mainly fills the voids in the cement, and some of the crystal beads act as balls to increase the fluidity. With the increase of fly ash dosage, the quality of fly ash in the market varies. Generally, concrete below C50 in the mixing plant is class II, and concrete above C50 is class I. Due to the shortage of fly ash, many commercial concretes use Class III fly ash or even raw ash. Due to the large fineness, high burning loss and high water retention of Class III fly ash and raw ash, it leads to poor concrete compatibility and reduced concrete strength.

3. Mineral powder

Granulated blast furnace slag powder improves the initial fluidity of the slurry and reduces the fluidity loss. This is mainly due to the different chemical composition and fineness of different manufacturers, resulting in different water requirements of mineral powders and different contributions to concrete strength.

4. Sand and stone

The main effects on PCE-based superplasticizers water reducer are:

(1) high sand content, the mud component in concrete adsorption of water reducer, resulting in high loss of concrete. The high fineness modulus of the water reducer mainly affects the ease of concrete, and the water absorption of sand leads to the water consumption of concrete;

(2) Mud content, clay content, particle size and stone dust content of stones

The particle size in the stone determines the water consumption. The content of stone powder has a great influence on the compatibility of concrete, mainly changing the composition of stone powder below 0.315mm in concrete and affecting the compatibility of concrete.

4. Conclusion

In conclusion, cement, fly ash, mineral powder, coarse and fine aggregates in concrete will affect the adaptability of polycarboxylic acid water reducing agent, which in turn will affect the performance and quality of the final concrete product. Therefore, it is necessary to design the compatibility of the admixture with the various components it serves according to the site conditions, so that qualified materials can finally produce qualified concrete. Polycarboxylic acid high-efficiency water reducing agent plays a very important role as a kind of concrete admixture with high water reduction rate and small slump loss. It can improve the ease of concrete, increase the strength and rheological properties of concrete, thus reducing the difficulty of concrete construction in many building structures and ensuring the safety of the structure. Therefore, it will play a great role in the construction industry and produce good benefits.

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