What is Fly ash concrete, the use of fly ash, advantages and disadvantages
Fly ash concrete:- In view of global warming, efforts are being made to reduce carbon dioxide emissions into the environment. The cement industry is the main source of carbon dioxide emissions, as well as the main source of consuming large amounts of incoming resources in the cement production process.
By using materials with pozzolanic properties (such as fly ash) instead of cement, the cement and concrete industries can jointly meet the growing needs of the construction industry and help reduce environmental pollution. As we all know, the use of suitable fly ash can improve most of the properties of concrete.
In addition to economy and ecology, the use of fly ash in concrete improves the workability, bleeding, reduces separation, heat release and permeability of concrete, improves ultimate strength, enhances sulfate resistance, and inhibits alkali accumulation material.
Due to the high strength and low permeability of mature concrete containing fly ash, the ability to resist crack growth is increased. Studies have shown that the abrasion resistance of concrete containing fly ash has also been improved. Fly ash concrete reacts with silica and reduces the cracking tendency of concrete.
The improvement of various properties of concrete is due to the physical function of fly ash to improve the microstructure of hydrated cement paste. Fly ash does not have any beneficial effect on the alkali metal carbonate reaction.
Use of Fly ash concrete
The use of fly ash improves the workability of concrete. This phenomenon can not only use the unit water content of the mixture, but also reduce the amount of the admixture. The use of fly ash generally does not affect the density and air content of the concrete mixture. Generally, the use of fly ash slightly delays the setting time of concrete, but it is compensated by reducing the amount of admixtures to maintain the same workability. The bleeding of fly ash concrete is significantly reduced, and other properties such as cohesion, pumping characteristics and surface finish are improved.
As the content of fly ash increases, the strength of concrete will decrease. Compared with later ages, this reduction is greater at earlier ages. This is because the secondary hydration caused by pozzolanic action is slower in the initial state of fly ash concrete. The rate of strength development at different ages is related to W/Cm and the percentage of fly ash in the concrete mixture. For a given W/Cm, the elastic modulus of fly ash concrete also decreases as the percentage of fly ash increases. Compared with the compressive strength, the reduction of the E value is much lower.
The shrinkage of the fly ash concrete mixture is similar to that of the control concrete mixture. Compared with OPC concrete, fly ash concrete is more durable. The significant decrease in the 56-day and 90-day RCPT values indicates that the permeability of fly ash concrete is much lower than that of OPC concrete.
Advantages of fly ash concrete:
- The possibility of thermal cracking is reduced.
- Reduce the heat of hydration. Improve the permeability of water and other fluids.
- Improve workability under the same water-cement ratio.
- The drying shrinkage is reduced.
- Improve compressive strength and other long-term performance.
- The minimum value of pre-cooling. In large-scale construction, post-cooling is avoided.
- Increase resistance to sulfate attack.
- More resistant to alkali-silicon reaction.
- Concrete becomes more durable than ordinary concrete without fly ash.
Disadvantages of fly ash concrete:
- The fly ash produced by some power plants is usually compatible with concrete, while others require ore beneficiation, and there are few other types of fly ash that cannot actually be used for concrete.
- The quality of the fly ash to be used is very important.
- Poor quality usually negatively affects concrete.
- Poor quality will increase permeability and damage the building.
- Therefore, it is very important to use only high-quality fly ash to prevent negative effects on the building structure.
