*2.2.3 Size and fineness of particle*

As mentioned before, FA particles have spherical solid forms with hollowing inside as cenospheres or plerospheres form. FA particle sizes vary from 1 μm to more than 100 μm. In general, 10–30% of particles are larger than 45 μm, with 300–500 m2 /kg of surface area. However, some types of FA have low or high surface area between 200 m<sup>2</sup> /kg and 700 m<sup>2</sup> /kg, respectively [27]. There are two ways to measure the particle size and fineness of FA:

• Specific surface area by Blaine apparatus: this method is based on the time passing through a bed of FA and correlated with its specific surface area in m<sup>2</sup> /kg.

ASTM does not exaggerate any specific requirement for the surface area of FA, which could be used in concrete, whereas the Indian Standard IS 3812 Part 1 [28] specifies 320 m2 /kg of FA as a minimum Blaine area for use in concrete.

• Residue on 45 μm sieve by wet-sieve analysis: this method is used to measure the percentage of particles in FA bigger than 45 μm as is referred to in ASTM430 [29]. Many countries follow this method for their national standards [7].

Some research showed that particles of raw FA mostly range from 1 to 100 μm in **Figure 2**. The particles less than 10 μm are the ones that react and contribute in the formation of early strength (7 and 28 days), whereas the particles between 10 and 45 μm react slowly and lead to the formation of a late strength (up to 1 year). The particles higher than 45 μm could be considered as inert and largely act as fine sand (filler) [7, 27].
