As we know that, usually we do not make any construction without any concrete additives in a construction site for getting better improvement in properties. The use of pozzolanic or mineral admixtures is old as that of the beginning of concrete construction.
The use of mineral admixtures modify the fresh and hardened concrete properties. The mineral admixtures are also called supplementary cementing materials or concrete additives.
According to the definition of ASTM, Pozzolans are siliceous or alumina materials, they behave no or less cementitious properties. They are present in fines which react with calcium hydroxide in the presence of water at ordinary temperature results in the formation of compounds possessing cementitious properties.
The reaction can be shown as
Pozzolan(Mineral Admixtures)+Calcium Hydroxide+Water=C-S-H (Gel)
This reaction is called pozzolanic reaction. The consumption of calcium hydroxide improves the durability of concrete by making more denser and impervious paste.
Advantages of Mineral Admixtures
- Less costs
- Results in Energy Savings
- Improve Workability
- Improve extensibility
- Reduce the alkali-aggregate reaction
- Increase water tightness
- Increase strength
- Less water demand
- Less Heat of Hydration
- Less thermal shrinkage
- Ease of Compaction
- How can concrete be compacted?Every civil engineer should know this.
- Do you know what are the types of cement used for concrete construction?.
Types of Mineral Admixtures used in concrete
The concrete additives or pozzolans are classified into Natural pozzolans and Artificial Pozzolans. Let see the essential minerals classification.
- Clay and shales
- Volcanic tuffs
- Opaline Cherts
- Fly ash
- Rice husk ash
- Silica Fume
- Blast furnace slag
The natural pozzolans are naturally available on earth. Now let me describe more about the artificial pozzolans and properties of mineral types of admixtures.
The fly ash is the first type of admixtures that we are going to discuss. Fly ash is finely divided residue from the combustion of coal. The fly ash is collected by an electrostatic precipitator.
The fly ash is also known as pulverized fuel ash(PFA).In recent year fly ash becomes common ingredients in cement additives.
Fly ash contribute to high strength and high performance.The fly ash is an industrial waste, its use in concrete significantly improves strength in long-term
ASTM classified fly ash into Class F and Class C
- Class F-Fly Ash normally produced by burning anthracite and bituminous coal
- Class C-Fly ash produced from lignite or sub-bituminous coal.
The fly ash used in cement additives have high fineness, low carbon content and highly reactive forms
The effect of fly ash is high strength at later stages. The reaction proceeds slowly than ordinary, the strength at initial stages is low compared to ordinary concrete. The fly ash results in high durability and low water content
Silica fume is another mineral types of admixtures used for concrete construction. The silica fume is manufactured during the production of silicon or Ferrosilicon by electric arc furnaces at a temperature of 2000 degree Celsius. Silica fume is a spherical particle with an average diameter of 150nm.
Due to its high fineness, the addition of silica fume causes an increase in water demand used for making concrete.In addition, concrete becomes more sticky and cohesive.
3.Ground Granulated Blast furnace slag.
Ground granulated blast furnace slag is another types of admixtures used for concrete making. They are produced by heating of limestone, iron ore and coke at very high temperature. These processes are usually done in a blast furnace.
The below video shows the effect and usage of mineral admixtures
Hope you can understand the various types of admixtures used in concrete. If you like this post, don’t forget to share it with your friends.