Table of Contents
Introduction Of Concrete Retarder
Concrete retarder is an external additives that retard the hardening of cement pastes and also delay the hardening of cement-containing mixtures such as mortar and concrete.
Purpose Of Concrete Retarder
Adding a retarder to a concrete mix can allow it to set up to an hour later. The hot climate causes the curing process to take longer, giving workers more time to mix, transport and place the concrete. A diluent not only slows down the process, it also conserves water.
List Of Retardant
- Phosphonates
- Lignosulphonates
- Sugars
- Hydroxycarboxylic acids and their salts
- Salts of Pb, Zn, Cu, As, Sb
- Borates
Working Concept Of Retarders:
The hydration reaction shall be stopped momentarily and the rest period extended by dissolving the diluent in water or by spraying it on the surface of the concrete. The specifics of the retarder and cement used have an effect on the system that is formed. It’s also important to understand that the factors contributing to the gap are transitory. The mechanism will eventually stop working, although irrigation will still occur.
This process is completed in four steps.
Adsorption:
The diluent additive adheres to the surface of the cement particles. Diluent additives form a protective film around the cement particles. This diffusion barrier makes it difficult for water molecules to reach the surface of the dehydrated cement particles, slowing down the hydration process. As a result, cement pastes retain their plasticity for a long time because there are not enough hydration products for hardening. This retarding additive reacts with C3A in cement, comes out of solution and mixes with the hydrated material.
Nucleation:
The surface of cement particles releases calcium and hydroxyl ions in the presence of water. When the concentration of these ions exceeds a certain threshold, the hydration products C2S and CS crystallize. Until the cement reaches supersaturation, the calcium hydroxide nucleation delay adsorbs on the mixture and prevents further nucleation.
Complexation:
Within minutes, calcium ions released from the cement particles combine with other ions to form complexes. The formation of these compounds makes it easier for the cement to dissolve. When diluting mixtures are present, the concentration of Ca 2+ , OH- , Si, Al and Fe increases during the hydration process in the aqueous phase of the cement paste. The formation of calcium hydroxide is prevented when calcium ions and hydroxyl ions collect in solution, which slows down the hydration process.
Precipitation:
The diluent precipitates after reacting with extremely alkaline solutions to form an insoluble derivative. Adsorption and this process are very similar. The pH of the solution rises rapidly above 12 within the first few minutes of contact between water and cement. These insoluble chemicals precipitate around the cement particles and form a barrier that prevents hydration. The coating acts as a diffusion barrier by preventing the cement particles and water molecules from making good contact.
Uses Of Concrete Retarders
- Retarding admixtures are commonly added to concrete when it’s hot outside.
- If a diluent is sprayed on top of the formwork surface the matrix will not harden where it contacts the formwork. However, the rest of the concrete hardens.
- Retarders are used during the construction of large or very large structures.
- The grouting process for oil wells uses substances that slow down the reactions.
Advantages Of Concrete Retarders
Concrete retarder is a chemical that slows down the hardening process of concrete. This can be helpful in a few different ways:
- It allows concrete to remain workable for a longer period of time, which can be helpful in hot weather or when working on large projects.
- It can prevent the formation of cracks or seams between different pours of concrete.
- It can improve the overall quality of the concrete by reducing segregation and bleeding. However, it is important to use concrete retarder carefully, as too much can cause problems like bleeding or expansion. It may also have different effects on different types of cement.