How Does Fly Ash in Concrete Work?

How Does Fly Ash in Concrete Work?

Fly ash, a byproduct from burning coal in power plants, is one of the most commonly used supplementary cementitious materials in concrete. It can improve the concrete's workability, strength, durability, and long-term performance, while reducing its carbon footprint and cost. In this article, we will explore how fly ash in concrete works.

1. Chemical Composition.

Fly ash is a finely divided powder that consists primarily of silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3), with minor amounts of calcium, magnesium, sulfur, and other minerals. Its chemical composition and physical properties vary depending on the type of coal, the combustion conditions, and the collection and storage methods.

2. Pozzolanic Reaction.

Fly ash is a pozzolan, which means it can react with water and calcium hydroxide (Ca(OH)2) in the presence of Portland cement to form a cementitious material called calcium silicate hydrate (C-S-H) gel, similar to the one formed by the hydration of cement particles. This reaction, known as pozzolanic reaction, is a slow and continuous process that can continue for decades.

3. Particle Size Distribution.

Fly ash has a wide range of particle sizes, from submicron to micron to millimeter scale. The finer particles can fill the voids between the cement particles and improve the packing density and workability of the concrete, while the coarser particles can provide a ball-bearing effect and reduce the internal friction and shrinkage of the concrete.

4. Water Demand.

Fly ash can absorb water and increase the water demand of the concrete, especially if it is hygroscopic or has a high calcium oxide (CaO) content. This can affect the slump, setting time, and strength of the concrete, and require adjustments in the mix design and curing methods.

5. Strength Development.

Fly ash can improve the early and long-term strength of the concrete, depending on the amount, type, and fineness of the fly ash, and the curing conditions. The pozzolanic reaction can provide additional cementitious material and reduce the porosity and permeability of the concrete, which can enhance its durability and resistance to chemical attack, sulfate attack, and alkali-silica reaction. The ball-bearing effect can reduce the stress concentration and increase the flexural and compressive strength of the concrete.

6. Durability.

Fly ash can improve the durability of the concrete by reducing the amount of free lime, which can react with the carbon dioxide in the air and form calcium carbonate (CaCO3), causing cracking and scaling. Fly ash can also reduce the permeability and absorption of the concrete, which can prevent the ingress of water, chloride ions, and other harmful substances that can cause corrosion, spalling, and deterioration.

7. Sustainability.

Fly ash can contribute to the sustainability of concrete by reducing its carbon footprint, as it is a waste product that would otherwise be disposed of in landfills. The use of fly ash can reduce the cement content of the concrete, which can decrease its CO2 emissions, energy consumption, and cost. The use of fly ash can also conserve natural resources and reduce the environmental impact of coal mining and transportation.

Conclusion.

Fly ash is a versatile and beneficial material that can enhance the properties and performance of concrete. Its use requires careful consideration of its chemical composition, particle size distribution, water demand, strength development, durability, and sustainability. The proper selection, proportioning, mixing, curing, and testing of fly ash concrete can ensure its successful application and long-term benefits.

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