**3.2 Fly ash**

Fly ash is a fine granular powder that exists in nature as a by-product of burning coal in power plants. Fly ash is used as a low-cost recycled material in concrete mixes to improve the concrete strength, reduce concrete viscosity and improve its pump ability, mitigate the ASR and its destructive impact on hardened concrete, and reduce the final cost of the produced concrete. The chemical composition of fly ash is shown in **Table 2**.

Two types of fly ash are used in the construction industry as SCMs to improve the fresh and hardened concrete properties. These are class C and class F fly ash. The main difference between the two types is that class F fly ash is a pozzolan; thus, it needs to react with the calcium hydroxide resulting from the cement hydration process to form the binding material. While class C fly ash is a cementitious material, it can produce binder directly once it reacts with water (direct hydration process).

The use of fly ash in concrete mixing provides fresh concrete with workability advantages. Fly ash particles are spherical in shape, which provides the concrete powder (cement, SCMs, and aggregates) with a higher tendency to flow. In addition to the lubricant effect provided, fly ash reduces the shear capacity of fresh concrete. Hence, the fly ash concrete mix has a higher tendency to flow, being pumped, and has a better hardened surface after formwork is removed.

Recent research shows that fly ash can partially replace up to 30% of the cement weight of conventional mixes. More than 30% fly ash could be used when mass concrete is poured [10]. The replacement of 20% of cement with an equivalent amount of fly ash results in improving the mix workability and/or maintaining similar workability while reducing 10% of mixing water.

In hardened concrete, class C fly ash provides improved mechanical properties due to its ability to form the additional binder. Class C and F fly ash results in reduced voids and lowered permeability in hardened concrete. Reduced concrete

#### **Figure 4.**

*Micro-silica concrete packing improved packing order [9].*


**Table 2.** *Chemical composition of fly ash.*

*Application of Supplementary Cementitious Materials in Precast Concrete Industry DOI: http://dx.doi.org/10.5772/intechopen.100249*

voids play a major role in improving durability and blocking moisture dissipation to the hardened concrete, which improves its alkali-silica resistivity.
