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The influence of different retarding materials polycarboxylate superplasticizer on the performance of concrete during the compounding process

The influence of different retarding materials, polycarboxylate superplasticizer, on the performance of concrete during the compounding process.

Different manufacturers of polycarboxylate superplasticizers use different retarding materials and dosages during admixture compounding, which affect cement paste, concrete workability, slump retention, setting time, strength, etc. This article analyzes the effects of adding polycarboxylate superplasticizers with different retarding components on various properties of concrete.

Types of retarding materials

At present, there are many types of retarders on the market, but they can be roughly divided into two categories based on their chemical composition: organic retarders and inorganic retarders. Inorganic retarders mainly include phosphates, pyrophosphate salts, borax, sodium fluorosilicate, zinc chloride, zinc carbonate, and cadmium sulfate. Organic retarding materials are widely used, mainly including sugars (monosaccharides, oligosaccharides, polysaccharides, disaccharides, etc.), citric acid, potassium sodium tartrate, and alcohol derivatives (monoalcohols, diols, propylene glycol, etc.).

The role of retarding materials in concrete

Generally speaking, the polycarboxylate superplasticizer and its retarding components produce physical effects in concrete: the retarding material does not react chemically with the concrete or generate new substances, which can have a positive effect on various concrete properties.

The hydration heat release of Portland cement can be divided into several stages: pre-induction, induction, acceleration, deceleration, and stability. The action range of the retarder material is essentially only to prolong the hydration of cement during the induction period. Its mechanism of action usually includes the precipitation hypothesis, the complex salt hypothesis, the adsorption hypothesis, and the inhibition of calcium hydroxide crystal growth theory. However, the core is to delay the hydration of cement and delay the crystallization of hydration products to achieve the purpose of retarding. At the same time, it prevents temperature deformation from causing concrete cracks and improves the slump retention performance of concrete mixtures.

The mechanism of action is that the retarding material is adsorbed onto the surface of cement particles, forming hydrogen bonds that bind with water molecules within the cement concrete. The surface of the cement concrete particles will form a water film, slowing down the rate of water penetration into the cement. In addition, the retarding component solution slows the growth of hydration products, thus inhibiting changes in the properties of cement concrete.

Selection of retarding materials

The core issue of the rheological properties of various powders (including cement) is the fluidity of the slurry. Under the same conditions, if the cement slurry is dispersible and flowable, the rheological properties of the entire system will be good. Therefore, the flowability of the cement slurry reflects the loss of the system’s rheological properties over time.

Various retarding materials have their own advantages and disadvantages, ranging from the ease of material acquisition, price differences, a wide range of performance, to the effectiveness of retarding.

Conclusion

Under the same conditions, different retarding materials have their own characteristics, but they share a common feature in terms of concrete strength: the early strength of concrete mixed with retarding materials is not as good as that without retarding materials. This is because the addition of retarding materials effectively delays the early hydration of cement, hinders the formation and development of the initial structure of the slurry, and results in a relatively low 3-day compressive strength of concrete. With full cement hydration, the hydration products are fully interlaced, overlapped, and filled, resulting in a dense structure that makes the compressive strength of concrete in the later stages higher than that of benchmark concrete.

Due to the variety of retarder types and their different mechanisms of action, their impact on concrete performance will also vary. Improper use of different types, combinations, or dosages of retarding materials can not only affect the workability of concrete but also lead to abnormal setting time and ultimately result in strength issues. When selecting the type of retarding material, full consideration should be given to its compatibility with the concrete raw materials, as well as factors such as the construction season and economic cost. After determining the required retarding material, it should be used separately or mixed, according to the characteristics of various retarding materials, in a trial mix to find the optimal dosage range.