*2.2.2 Particle-specific gravity*

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used one in the large thermal plants [20].

Particle distribution and their size are considered the main physical factor for the geopolymerization process [14, 15]. Komljenovic et al. [16] stated that, the reactivity of FA increases with increasing its fineness, which leads to an improvement of geopolymer properties. Basically, the formation of ash particles occur during the condensation and liquefaction process of incombustible inorganic matter, which is remained after coal combustion [17–19]. The shape of FA particles depend on the combustion conditions and condensation process. In general, there are two major combustion processes. The first process occurs when the temperature ranges from 1204 to 1727°C, this process is called the pulverized coal firing system. The second process is known as fluidized bed combustion which could be peaked at temperature ranged between 827 and 927°C. Typically, the first process is the most common

Surface tension of the melt plays a significant role in the formation of spheroidization of pulverized FA particles. Two types of particles could be formed, cenospheres, which are ash particles hollow from the inside, and plerospheres which are hollow ash particles but including smaller particles inside as is shown in **Figure 1**. Brouwers and Van Eijk [21] suggested that the formation of plerospheres is as a result of the cracking or puncturing of the primarily hollow particles during handling work, but not related to the melting process. Jayant reported that the shape and surface characterization of FA particles have an impact on concrete in terms of water demand, in particular at the desired slump stage [7]. The spherical forms of FA particles minimize interparticle friction and leads to the creation of a dynamic system between particles in a concrete. This process improves the flow properties of the concrete. An experimental study was carried out by Atiş et al. [22] on the properties of different types of FA. Their results showed that there are many similarities between the chemical and mineralogical composition of all types of FA and also the physical properties such as specific surface area, particle shape, and their distribution. To explain the performance of concrete from the strength and workability point of view, some authors proposed a new parameter called "shape factor" which is mainly based on the specific surface area of FA particles [7].

Another study shows that around 90% of tested FA could reduce water requirement of mortar mixtures. A correlation has been proposed to show the relationship between water demand and fineness and also water requirement and loss on ignition. Further, the addition of FA has a significant effect on the rheological properties of cement paste and workability of concrete, due to the small spherical particles of FA. Givi et al. [23] believed that the proportion of coarse material in the

*Scanning electron microscope of FA: (a) cenosphere and (b) plerosphere particles [7].*

*2.2.1 Particle shape and form*

**108**

**Figure 1.**

According to ASTM C188 [25], the specific gravity of FA particles can be determined by the same method that is used for hydraulic cement. If there is a water-soluble molecule in FA, it is recommended to use nonaqueous solvent as a replacement for water. ASTM C188 classified the specific gravity of various and common mineral admixtures such as FA, PC, and GGBFS as follows: 2.0–2.7, 3.0–3.20, and 2.9–3.0, respectively [7]. Sabat [26] assumed that FA could be the most suitable geotechnical material, due to its resistivity in terms of high shear strength, low specific gravity, less compressibility, and good physicochemical properties. FA mainly contains silica, alumina, iron, and calcium, with less quantities of magnesium, sulfur, sodium, potassium, and carbon. The density or specific gravity of FA depends on its chemical compounds and typically ranges between 1.9 and 2.8 [27].
