These are chemicals, mostly sold in a liquid form to influence the behavior of the wet mortar, the drying time, and ultimately influence the MPa of the cured concrete. They are added to the mortar mix during mixing.
The five main functions
There are five distinct classes of chemical admixtures:
- Accelerating
- Retarding,
- Water-reducing,
- Plasticizes (super-plasticizers).
- Air-en-training
All other varieties of admixtures fall into specialty category’s whose functions include corrosion inhibition, shrinkage reduction, alkali-silica reactivity reduction, work-ability enhancement, bonding, damp proofing, reducing permeability and coloring.
We are not taking here about foaming agents as admixtures, because it is a given that we use these to make FC
Using admixtures in FC is mostly done to increase the “strength” after curing; one of the most used types is the super-plasticizer.
Another new group are the nano-carbon materials. These are very successfully used to enhance the MPa. Go to the page Making FC stronger for more details.
When selecting an admixture, one of the first things you need to look at is the compatibility with the foaming agent you are going to use. If you use products from the same manufacturer, they may be able to give you some information on this. Otherwise I suggest you do some tests before you start your “real” project.
The effect of some super-plasticizers in the FC is that the air bubbles are more uniform. This is beneficial.
I suggest keeping the use of admixtures to the minimum requirement for what you want to achieve. Changing one thing in the FC mix seems to have an influence on most of all the other properties, as FC is already much more complex than “normal” concrete. Change what you do already rather than adding another product in the mix to achieve a desired result. There is usually more than one way to skin a cat!
Here is an overview of what they are used for in “normal” concrete
Accelerating admixture
These are added to concrete to reduce curing time of the concrete and to speed up early strength. The amount of reduction in curing time varies depending on the amount of accelerator used. Calcium chloride is a low cost accelerator, but specifications often call for a non-chloride accelerator to prevent corrosion of reinforcing steel.
Retarding admixtures
These are often used in hot weather conditions to delay setting time. They are also used to delay set of more difficult jobs or for special finishing operations like exposing aggregate. Many retarders also act as a water reducer.
Water reducing admixtures
These reduce the amount of water needed in the concrete mix to create a workable mortar and increase the strength. Most low range water reducers reduce the water needed in the mix by 5%-10%. High range water reducers reduce the mix water needed by 12% to 30% but are expensive.
Shrinkage Reducing
This type of admixture could reduce early and long-term drying shrinkage. They can be used in situations where shrinkage cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable. Shrinkage reducing admixtures can, in some cases, reduce strength development both at early and later ages.
Air-entraining
The name says it all, this is done in an attempt to reduce the mass and to improve the work-ability. Air-en-training admixtures, which are used to purposely place microscopic air bubbles into the concrete. There are also admixtures that combine functions.
Other additives
Mainly used to be able to make concrete in unusual conditions, or for very high performance
There are untold other additives doing all sorts of things to the concrete. I found one company that has 60 on their list! http://www.muhuchem.com