Mechanism of Action and Evaluation Methods of Retarders

A retarder is an additive that delays the hydration reaction of cement, thereby extending the setting time of concrete. The use of retarders allows freshly mixed concrete to maintain plasticity for a longer period, facilitating pouring, improving construction efficiency, and not adversely affecting the later performance of the concrete.

For ready-mixed concrete or concrete poured in high-temperature environments during summer, using a retarder can also reduce slump loss, ensuring normal transportation and pumping of concrete, improving work efficiency, and avoiding material waste. For large-volume concrete, adding a retarder can lower the absolute temperature rise of the concrete, delay the peak temperature, and effectively prevent temperature stress cracks in the concrete.

Retarders come in various types and can be classified into two main categories based on their chemical composition: inorganic retarders and organic retarders.

Inorganic retarders primarily include phosphates, pyrophosphates, zinc salts, ferrous sulfate, copper sulfate, fluorosilicates, and borates. Recently, phosphates and pyrophosphates have been widely used as inorganic retarders.

Organic retarders mainly include the following types:
(1) Hydroxycarboxylic acids, amino acids, and their salts, common examples being citric acid, gluconic acid, and salicylic acid, typically used in dosages of 0.005% to 0.02% of the cement mass;
(2) Polyols and their derivatives. These retarders have stable retarding effects and are less influenced by temperature, generally used in dosages of 0.005% to 0.02% of the cement mass;
(3) Sugars, such as glucose, sucrose, molasses, and their derivatives. Due to their wide availability, low cost, and stable retarding effects, they are widely applied, typically in dosages of 0.001% to 0.03% of the cement mass.

The mechanism of action of retarders is complex and difficult to summarize with a single theory. Currently, several theories are recognized: adsorption theory, chelation theory, precipitation theory, and the theory of controlling calcium hydroxide crystal growth.

3.1 Adsorption Theory
Retarders adsorb onto the surface of cement particles, forming a dense adsorption film that alters the double electric layer structure of the cement particle surface, inhibiting the process of moisture adsorption and the hydration reaction. Additionally, some ions in retarders can adsorb onto the surface of hydrated cement product crystals, inhibiting crystal growth and further delaying hydration. This theory is particularly applicable to sugars and polyols and their derivatives.

3.2 Chelation Theory
The saturation of Ca²⁺ leads to the formation of Ca(OH)₂ crystals, which is a significant reason for the end of the induction period of cement hydration. Functional groups such as -OH and -COO⁻ in retarder molecules can form chelates with Ca²⁺ in the solution, inhibiting the crystallization of Ca(OH)₂, thereby effectively extending the induction period of cement hydration. This theory is more applicable to hydroxycarboxylic acids, amino acids, and their salts.

3.3 Precipitation Theory
The precipitation theory posits that retarders can form a poorly soluble precipitate layer on the surface of cement particles, preventing moisture from contacting the cement particles and inhibiting the dissolution of surface components, thus delaying the hydration reaction of cement. This theory is particularly applicable to molasses retarders.

3.4 Control of Calcium Hydroxide Crystal Growth Theory
This theory suggests that retarders hinder the crystallization of Ca(OH)₂, preventing C₃S from generating hydrated calcium silicate gel properly, thus inhibiting the hydration reaction of cement. This theory is more applicable to inorganic retarders.

Currently, the evaluation methods for the retarding effect of retarders are still somewhat limited, primarily focusing on the measurement of setting time and setting time differences.

For neat cement paste, the standard method is described in the "Testing Method for Water Requirement, Setting Time, and Stability of Cement Standard Consistency" (GBT 1346-2011). The method uses a standard Vicat apparatus to determine setting times, requiring prior measurement of the standard consistency water requirement, and then preparing the cement paste according to that water requirement to measure initial and final setting times. The advantage of this method is its convenience and speed, while the drawback is the significant potential for human error, particularly in the operational process and reading stages, especially when determining the final setting time.

For concrete, the standard method is specified in "Concrete Admixtures" (GB 8076-2008). This method uses a penetration resistance apparatus, requiring the concrete mixture to be screened through a 5mm sieve to separate the mortar, which is then placed in a standard metal cylinder to obtain a curve of penetration resistance versus time. The time corresponding to a penetration resistance of 3.5 MPa is then taken as the initial setting time, and 28 MPa as the final setting time. The advantage of this method is its accuracy and small error margin, while the disadvantages include a more complex and time-consuming operational process.

With advancements in technology, researchers both domestically and internationally have attempted to use new methods to evaluate the retarding effects of retarders, such as hydration temperature rise, X-ray diffraction, low-field nuclear magnetic resonance, and resistance methods. While these methods are simple and effective, their accuracy still requires extensive practical engineering validation, and no related standards have been established. Therefore, traditional methods remain the primary approach in engineering.

The difference in setting time is the most intuitive reflection of the effectiveness of a retarder, but the quality of a retarder is not solely determined by these two factors. An ideal retarder should exhibit significant retarding effects at low dosages, have strong adjustability in setting time within a certain dosage range, and not cause abnormal setting phenomena. Additionally, it is particularly important that the retarder significantly delays the initial setting time of the cement paste while minimizing the interval between initial and final setting times.

In today's rapidly developing ready-mixed concrete industry, retarders play a crucial role. As understanding of retarders deepens, their applications will become increasingly widespread, especially in the production of large-volume concrete where it is necessary to reduce the instantaneous hydration heat of cement and delay the peak of cement hydration.