Before stating the types of concrete admixtures, let us start first by the definition of “Admixtures”. According to ACI Committee 116, an admixture is ‘a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that is added to the batch before or during mixing’.
Now in the market, a wide variety of admixtures exists and if properly used, admixtures are very beneficial to the properties of fresh or hardened concrete. They can improve the quality of concrete, control the setting time, enhance the workability of concrete, etc…
In general, concrete admixtures can be divided into two major categories:
- Chemical admixtures and;
- Mineral admixtures.
Chemical admixtures are those admixtures that are added to concrete in very small amounts to improve or add a specific function to concrete. They are used to improve the quality of concrete during mixing, transporting, placement and curing. It is worth noting that if chemical admixtures are added with a higher dosage than specified, they might cause negative effects on the properties of fresh and/or hardened concrete.
Chemical admixtures include:
- Air entrainers
- Water reducers
- Setting retarders
- Setting accelerators
- Special admixtures such as corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring.
Chemical admixtures can be used to achieve one or more functions at the same time. For instance, ASTM C494 specifies seven chemical admixture types:
- Type A: Water-reducing admixtures
- Type B: Retarding admixtures
- Type C: Accelerating admixtures
- Type D: Water-reducing and retarding admixtures
- Type E: Water-reducing and accelerating admixtures
- Type F: Water-reducing, high range admixtures
- Type G: Water-reducing, high range, and retarding admixtures
Mineral admixtures, also called cement replacement materials (CRM) or supplementary cementitious materials (SCM), are fine ground insoluble solid materials such as fly ash, slag and silica fume. They are added to the concrete in larger amount than any other type. Mineral admixtures have the ability to enhance workability as well as finish-ability of fresh concrete. They can also be utilized as a cement replacement material which reduce the concrete cost because these admixtures are far less expensive than the cement.
Besides the economic advantage, mineral admixtures can enhance the strength, durability and serviceability of concrete. These benefits are attributed to their pozzolanic activity. Moreover, mineral admixtures are waste products of industries. Thus by using them in concrete, maximum sustainability can be achieved.
Mineral admixtures can be used with portland cement either individually or in combinations.