Materials other than water, aggregate and hydraulic cement that are used as an ingredient of concrete or mortar are termed admixtures. However, they are added to the batch immediately before or during mixing for uniformity. In addition, admixtures are materials added to the concrete mix to improve its workability, acceleration, retarding settling time, controlling the development of concrete and enhancing durability to deterioration process. More also, admixtures used in concrete are divided into many categories based on the source as well as ASTM has its own classification of admixtures.
Properties of Concrete by Admixtures.
Admixtures affect among others the following properties of concrete.
- Concrete strength.
- Air content
- Water content
- Others include corrosion control, shrinkage resistance etc
Classification of admixtures.
The are many categories based on the source as well as ASTM has its own classification of admixtures. Based on the ASTM classification, there are two main classes of admixtures. These are;
Chemical admixtures which comprise of;
- Air-entraining admixtures
- Water reducing admixtures
- Retarding admixtures
- Accelerating admixtures
- Miscellaneous admixture
Mineral admixtures which comprise of;
- Fly ash
- Rice husk ash
- Silica Fume
- Ground granulated blast finance slag.
The finely divided mineral materials which are added to a concrete mix to obtain a specific engineering property of concrete are referred to as mineral admixture. The use of the mineral material as admixture gives economic benefits. It also brings about environmental safety. For instance, recycling of by-products and other industrial waste.
Types of mineral admixtures.
There are two types of mineral material. These are;
- Pozzolanic materials
- Inert filler materials
These are mineral admixture that contains reactive silica which when added to cement, they react with calcium hydroxide to form C-S-H. Examples of Pozzolanic materials include; volcanic ash, burnt clay, fly ash, and rice husk ash.
Note: C-S-H as used above simply means Calcium silicate hydrate. The reaction of the mineral materials (admixtures) with Portland cement form gel called C-S-H. It is not the only most abundant reaction product. And it occupying about 50% of the paste volume. In other words, it is responsible for most of the engineering properties of cement paste. Calcium silicate hydrate is the main product of the hydration of Portland cement. It is primarily responsible for the strength in cement-based materials.
Importance of Pozzolanic material – mineral admixtures.
- Lowering the heat of hydration.
- Increase in the durability of concrete
- Concrete strength is increased
Inert filler materials.
These are mineral admixtures, which do not affect the strength of the concrete been produced. Although, there is a greatly achievement of workability through them. This is to increase the workability of such concrete. Examples of inert filler materials include; hydrated lime, a dust of normal weight aggregate and the coloring pigment