Fly ash
Fly ash (crushed fuel ash) is a byproduct of a power-generating plant that uses coal as fuel. During the combustion of crushed coal in modern power plants, volatile matter, and carbon burn as they pass through the high-temperature zone in the coal furnace, but most of the mineral impurities such as soil, quartz, and feldspar melt at high temperatures.
The fused material is quickly transported to low-temperature zones where it hardens into glass spherical particles. Some of the mineral substances accumulate and form bottom ash, but most of it flows with the flue gas stream and is called fly ash.
The ash is then removed from the gas by the electrostatic precipitators. It can be classified into two categories: charcoal (ASTM C618):
Class F ash is obtained by burning anthracite or bituminous coal and Class C lignite or sub-bituminous coal. The chemical compositions of Class F and Class C are listed in Table 2-13.
It can be seen from the table that class F contains less than 10% CaO and class C 15 to 30% of Cao. Thus, sometimes class F is called low-calcium fly ash and class C is called high-calcium fly ash.
Usually, high-calcium ash is more reactive because it contains most of the calcium in the form of reactive crystalline compounds, such as C3A and CS. The morphology of this is compared with that of silica fume in Figure 2-41.
This shows that most of the particles in it, which occur as solid spheres of glass, but sometimes a small number of hollow spheres, called nanospheres (completely empty) and planisphere (filled with several small spheres), see Figure 2-42.
Fly ash size distributions are slightly smaller than Portland cement, more than 50% above 20 µm. It can be used as a mixing cement. In this case, the final method of cement production is milling this with clinker and gypsum.
This can also be used to produce modified concrete by adding it in a blending process. Research over the last 40 years shows that adding this to concrete has some advantages and some disadvantages.
Adding this to concrete can improve the spherical shape and viability of fly ash particles on the glass surface. By replacing cement with this, concrete costs can be reduced because fly ash is less expensive than cement.
This is an industrial by-product that reduces the energy demand in concrete production. In addition, adding it to concrete can reduce the hydration heat of fresh concrete and is good for mass concrete structures.
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The disadvantages of this concrete are the early opening time due to its low strength and low reactivity it. Much research has been done to improve reactivity by chemically activating it (Xi and Xi, 2000). One method is to add an alkali activator 1 or 2% NaOH or KOH to the concrete mixture.
Another is to use lime to mix with fly ash for a few days before adding fly ash to the concrete mix. Recently, some studies have been conducted that use a mechanism to wipe or mill ash to “activate” its functionality, and some promising results have been obtained (Blanco et al., 2005).
The normal replacement of cement by fly ash is around 25 to 30% by weight. However, in high-volume fly ash concrete, it content up to 60% is reached (Langley et al., 1989).
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