Fly ash is used in concrete as a substitute for Portland cement due to its pozzolanic qualities. In rare circumstances, fly ash can boost the ultimate strength of concrete as well as its chemical resistance and longevity. The addition of fly ash to concrete can considerably enhance its workability. This makes it possible to add more water to the mix while maintaining the desired consistency. Moreover, the increase in air content helps lighten the material and reduces heating during construction.
Concrete that contains 5% by volume or more of cement replacement (such as Class G fly ash) can be referred to as "pozzolan-based." Pozzolans are siliceous materials such as volcanic rocks, tuffs, and ash that contain relatively high levels of aluminum oxide (Al2O3). When water is added to these materials, an acid is formed which causes the hydration of the cementing particles. The resulting calcium aluminate compounds (CaO-Al2O3-XH2O) have properties similar to ordinary portland cement.
In order for pozzolan-based concrete to gain maximum benefits from the addition of fly ash, it must meet certain requirements. For example, it should be mixed with water before adding any other ingredients to prevent agglomeration during mixing. Also, the ratio of cement to pozzolan should be equal to or greater than 0.5.
The use of fly ash as a partial replacement for cement in mortar and concrete on construction sites is well recognized in the building industry all over the world, including India. The following are some of the key building materials that may be made from fly ash: The following are the benefits of fly ash concrete over regular cement concrete:
The global market for fly ash was valued at $1 billion in 2016 and is expected to reach $1.5 billion by 2021, at a CAGR of 5%. Fly ash has become widely used as a partial substitute for cement in concrete, owing to its low cost advantage over pure cement products.
The main application of fly ash is as a partial substitute for cement in concrete. This is because of its ability to react with water to form a hardened mass. As well as this, fly ash has other advantages over cement such as being cheaper, having better air quality when burned, and being able to be recycled. These properties make fly ash useful in concrete applications where these factors are important.
Fly ash can be used in various forms including powder, pellet, and liquid. It can be added directly to concrete or blended with other ingredients before mixing with water. The amount of fly ash used varies depending on the type of application; however, it usually represents between 10% and 50% of the total weight of the concrete.
Concrete that uses fly ash as a partial substitute for cement has many advantages over plain concrete.
The Negative Effects of Fly Ash The quality of fly ash can have an impact on the strength and durability of cement concrete. Poor grade fly ash can increase the permeability of concrete and cause structural damage. It is recommended that you use class-f or better fly ash when preparing your own mortar or concrete.
Class-F fly ash is clean, free of clinkers, and has a low iron content. Class-A fly ash is cleaner than class-F and has less plasticity than class-C fly ash. Class-B and class-C fly ashes have higher iron contents and lower calcium oxide ratios than class-F fly ash; thus they are more reactive with water and require careful monitoring during mixing and placement.
Iron in concrete reduces its strength and increases its porosity. Concrete containing 10% or more fly ash by volume should not contain more than 1.7% by weight of total iron content.
Concrete that contains excess iron may appear gray or brown instead of white. This color change is due to the presence of ferrous sulfate which forms when water reacts with the iron in the fly ash. Ferrous sulfate is very sensitive to light and will turn black if exposed to sunlight for several days. The concrete also may exhibit other undesirable properties such as high permeability or poor resistance to heat.
Fly ash is a byproduct of pulverized coal or coal dust in power plants and is increasingly being utilized as a cement replacement in the manufacture of bricks. However, fly ash has limitations since it can only replace up to 30% of the cement in concrete constructions. The rest of the cement content needs to be replaced with natural limestone which is more expensive than commercial cements.
There are two types of fly ash - bottom ash and top ash. Bottom ash is collected at the base of a boiler's furnace while top ash is removed from above the firebox. Both types of fly ash contain significant amounts of silicon, aluminum, magnesium, calcium, potassium, sulfur, oxygen, nitrogen, and carbon. The type and amount of elements present in fly ash depend on the type of coal that was burned in a power plant.
In India, coal is used to generate electricity mainly by three methods called "Rajasthan style firing", "Maghrib method", and "Wet combustion". Rajasthan style firing is the traditional method used in coal-based thermal power stations. In this method, coal is spread over the floor of a furnace and burns completely without any smoke. This type of firing is used by most of the older units still operating in India. Maghrib method is used in some units built before 1990.