1. Definition and Characteristics of Retarders

A retarder is an additive that can delay the hydration reaction of cement, thereby extending the setting time of concrete. The use of retarders can maintain the plasticity of fresh concrete for a long period of time, facilitate pouring, improve construction efficiency, and do not have adverse effects on the later performance of the concrete.

For commercial pumped concrete or concrete constructed in high temperature environments in summer, using retarders can also reduce slump loss, ensure normal transportation and pumping construction of concrete, improve work efficiency, and avoid material waste. For large volume concrete, adding retarders can also reduce the absolute temperature rise of the concrete, delay the occurrence of temperature peaks, and effectively avoid the occurrence of temperature stress cracks in the concrete.

2. Types of Retarders

There are many types of retarders, which can be divided into two categories based on their chemical composition: inorganic retarders and organic retarders. 

Inorganic retarders mainly include phosphate, metaphosphate, zinc salt, iron sulfate, copper sulfate, fluorosilicate, and borax. In recent years, the widely used inorganic retarders are phosphate and metaphosphate type retarders.

Organic retarders mainly include the following types:

(1) . Hydroxycarboxylic acids, amino carboxylic acids and their salts, commonly including citric acid, gluconic acid, salicylic acid, and their salts. The dosage is generally 0.005%~0.02% of the cement mass;

(2) . Polyols and their derivatives. This type of retarder has a relatively stable retarding effect and is less affected by temperature. The dosage is generally 0.005%~0.02% of the cement mass;

(3) . Carbohydrates, such as glucose, sucrose, molasses, and their derivatives. Due to its wide range of raw materials, low price, and stable retarding effect, it is widely used, with a dosage generally ranging from 0.001% to 0.03% of the cement quality.

3. Retarders’ Mechanism of Action

The mechanism of action of retarders is quite complex and difficult to generalize with a single theory. The currently recognized theories include adsorption theory, formation complex salt theory, precipitation theory, and control calcium hydroxide crystal growth theory.

3.1 Adsorption Theory

The retarder adsorbs on the surface of cement particles to form a dense adsorption film layer, changing the double layer structure of the cement particle surface, and inhibiting the process of water adsorption and hydration reaction of cement particles. In addition, some retarder ions can adsorb onto the surface of cement hydration product crystals, inhibit crystal growth, and also play a role in delaying hydration. This theory is more applicable to sugars, polyols, and their derivatives.

3.2 Formation Complex Salt Theory

The saturation of Ca2+and the formation of Ca (OH) 2 crystals are important reasons for the end of the cement hydration induction period. Functional groups such as - OH and - COO - in the retarder molecule can form complex salts with Ca2+in the solution, inhibiting the crystallization of Ca (OH) 2 and effectively prolonging the induction period of cement hydration. This theory is more applicable to hydroxyl carboxylic acids, amino carboxylic acids, and their salts.

3.3 Precipitation Theory

The sedimentation theory suggests that retarders can form a insoluble precipitation layer on the surface of cement particles, prevent the contact between water and cement particles, inhibit the dissolution of surface components of cement particles, and thus delay the hydration reaction of cement. This theory is more applicable to molasses retarders.

3.4 Control Calcium Hydroxide Crystal Growth Theory

According to the theory of controlling the growth of calcium hydroxide crystal, the retarder blocks the crystallization of Ca (OH) 2, so that C3S cannot normally generate hydrated calcium silicate gel, thus inhibiting the hydration reaction of cement. This theory is more applicable to inorganic retarders. 

4. Evaluation Method for the Retarding Effect of Retarders

At present, the evaluation methods for the retarding effect of retarders are still relatively limited, mainly focusing on the determination of setting time and the difference in setting time. 

For the clean slurry, the standard method is the testing method specified in the "Test Method for Water Consumption, Setting Time, and Soundness of Cement Standard Consistency" (GBT 1346-2011). The instrument used in this method is a standard Vicat apparatus. Before measuring the setting time, the water consumption for standard consistency needs to be determined in advance, and then the cement slurry is prepared according to the water consumption for standard consistency. The initial and final setting times of the cement slurry are measured. The advantages of this method are convenience and speed, while the disadvantage is that there is a significant human error, mainly reflected in the randomness of the operation process and readings, especially when determining the final setting time, the human error is more obvious.

For concrete, the standard method is the testing method specified in "Concrete Admixtures" (GB8076-2008). The instrument used in this method is a penetration resistance meter. Before measuring the setting time, the concrete mixture needs to be sieved out of mortar using a 5mm (circular hole sieve) vibrating screen and loaded into a standard metal cylinder to obtain the relationship curve between the penetration resistance value and time. Then, when the penetration resistance value reaches 3.5MPa, the corresponding time is calculated as the initial setting time; When the penetration resistance value reaches 28MPa, the corresponding time is used as the final setting time. The advantages of this method are accurate results and small errors, while the disadvantages are that the operation process is relatively complex and time-consuming.

With the advancement of technology, domestic and foreign scholars have also attempted to use some new methods to evaluate the retarding effect of retarders, such as hydration temperature rise, X-ray diffraction, low-field nuclear magnetic resonance, resistance method, etc. Although simple and effective, the accuracy still needs to be verified by a large amount of engineering practice, and no relevant standards have been formed. Therefore, traditional methods are still the main method in engineering at present.

The difference in setting time is the most intuitive manifestation of the effectiveness of a retarder, and the quality of a retarder is not solely determined by these two factors. An ideal retarder should have a significant retarding effect in a small dosage range, with strong adjustability of setting time within a certain dosage range, and no abnormal setting phenomenon. Furthermore, it is particularly important that the retarder can significantly delay the initial setting time of the cement slurry, while minimizing the interval between initial and final setting.

5. Conclusion

In the rapid development of commercial concrete today, retarders play a very important role. With the continuous deepening of people's understanding of retarders, their application will also become more and more widespread, especially in the production of large volume concrete that needs to reduce the instantaneous hydration heat of cement and delay the peak hydration of cement, retarders are indispensable.

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