A retarder of concrete is a chemical admixture that slows down the rate of cement hydration and extends the setting time of concrete. It is widely used in construction scenarios where controlling the setting speed is crucial, such as in high-temperature environments, large-volume concrete works, and long-distance transportation. The addition of a retarder of concrete makes construction operations easier and helps reduce the occurrence of cracks during the curing process, thus significantly improving the quality and durability of concrete.
Classification Based on Retarding Duration
Retarders used in concrete can be classified into normal and super retarders. Normal retarders extend the setting time of concrete by over 10 hours, while super retarders can delay the setting time of cement for over 70 hours.
Classification Based on Chemical Composition
From the perspective of chemical composition, retarders used in concrete are divided into two major categories: inorganic and organic. Organic retarders include compounds such as hydroxycarboxylic acids and their salts, lignosulfonates and their derivatives, sugars and carbohydrates, and polyols and their derivatives. Inorganic retarders cover phosphates, polyphosphates, borates, fluorosilicates, as well as salts of zinc, iron, and copper.
Delaying Setting Time
The primary function of a retarder used in concrete is to delay both the initial and final setting times, preventing cold joints during construction. It is especially useful in high-temperature seasons or for long-distance pumping.
Improving Workability
It maintains the plasticity of the concrete for a longer period, making it easier to pour and consolidate.
Reducing Heat of Hydration
It reduces the temperature-induced cracking in large-volume concrete caused by early heat of hydration.
Regulating Construction Schedule
It adapts to the needs of complex structures or continuous pouring, providing flexibility in the construction process.
Effects on Fresh Concrete Properties
A retarder used in concrete significantly extends both the initial and final setting times. It also improves the workability of the concrete, reducing slump loss and decreasing water consumption, making the mixing and construction processes smoother. The retarder forms a layer on the surface of the cement particles, acting as a barrier and slowing down the hydration process of cement. It inhibits the nucleation and formation processes during cement hydration, further delaying the hydration reaction.
Effects on Hardened Concrete Properties
In terms of strength, concrete with a retarder used in concrete or a retarder-reducing agent will have slightly lower early strength compared to untreated concrete. This reduction is more significant when a retarder is used. However, over time, the strength gradually recovers.
In terms of shrinkage, concrete with a retarder used in concrete or retarder-reducing agent will experience slightly increased shrinkage, which grows with the amount of retarder used. However, when a retarder-reducing agent is used to reduce water content and the cement ratio, the shrinkage value is reduced.
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