What are the varieties of retarders?

Retarders are chemical additives that slow down the setting or hardening of cement. They are used in concrete applications where a delayed setting is desirable, such as in hot weather, or when extended mixing or placement times are needed. There are several types of retarders available, each with its own set of properties and benefits. Here are some of the varieties of retarders:

1. Organic Acids: Organic acids such as citric, tartaric, and gluconic acid are commonly used as retarders in cement-based materials. They work by reacting with the free lime in the cement, which slows down the hydration process. Organic acid retarders are generally non-toxic and biodegradable, making them an environmentally friendly choice.
2. Sugars: Sugars such as glucose, sucrose, and fructose can also be used as retarders in cement-based materials. They work by binding to the surface of the cement particles, which slows down the hydration process. Sugar retarders are often used in combination with other retarders to provide a more controlled setting time.
3. Inorganic Salts: Inorganic salts such as borax, zinc sulfate, and sodium silicate are commonly used as retarders in cement-based materials. They work by forming a thin film on the surface of the cement particles, which slows down the hydration process. Inorganic salt retarders are often used in combination with organic acid or sugar retarders to provide a more consistent and predictable setting time.
4. Lignosulfonates: Lignosulfonates are natural polymers that are derived from wood pulp. They are commonly used as retarders in cement-based materials, as they work by binding to the surface of the cement particles and slowing down the hydration process. Lignosulfonate retarders are generally more effective in high-alumina cement than in ordinary Portland cement.
5. Hydroxycarboxylic Acids: Hydroxycarboxylic acids such as gluconic and citric acid are commonly used as retarders in cement-based materials. They work by chelating the free calcium ions in the cement, which slows down the hydration process. Hydroxycarboxylic acid retarders are often used in combination with other retarders to provide a more consistent and predictable setting time.
6. Polycarboxylate Ether (PCE) Superplasticizers: PCE superplasticizers are commonly used as retarders in concrete applications where a delayed setting time is desirable. They work by dispersing the cement particles and reducing the surface tension of the water, which slows down the hydration process. PCE retarders are often used in combination with other superplasticizers to provide a more consistent and predictable setting time.

In conclusion, retarders are an important component of cement-based materials, as they provide a more controlled setting time and can help to improve the workability of the material. The type of retarder used will depend on the specific application and desired setting time, as well as the properties of the cement and other additives used. By choosing the right type of retarder, contractors and engineers can ensure that their cement-based materials are strong, durable, and perform well over time.