**2. What is FA?**

#### **2.1 Classification**

American Society for Testing and Materials (ASTM C618) [6] classified FA into two main classes based on the source of mineral coal; these categories are appropriately considered as important classes in the uses of concrete. The named class F and class C of FA have many similarities in terms of physical characteristics. However, a chemical composition analysis is required to distinguish between both classes. The total amount of silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) as the constituents of FA will determine the type of class. Fly ash is therefore classified as class F if the silica, alumina, and iron oxide content is at least 70% of the total mass and has a limited percentage of calcium oxide (CaO) (content no more than 10%). Class C FA constitutes at least 50% of silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) of the total mass and the calcium oxide (CaO) content is high (from 10 to 30%), with a high reactivity of almost all constituents [7].

Recently, many studies have been attempted in the analysis and synthesis of geopolymer. Some challenges have been faced in researching geopolymer process conditions and trying to identify the main aspects that limit and determine the reactivity of FA and geopolymers structure and its characteristics. FA-based geopolymer could be affected by many parameters [8, 9]; these parameters are significantly related to the primary materials and their characteristics, such as size and distribution of particles, the glassy phase in the content, the reactivity of both silicon and aluminum, constituent of iron, calcium and inert particles, and also the type of activator solution and its concentration.

Diaz et al. [10] supposed that the mechanical strength of geopolymer could be affected by many parameters of the mix design, for example, the ratio of NaOH to Na2SiO3 and activator solution to FA ratio. In addition, other factors could have significant impact on the behavior of fresh and hardened geopolymer, such as the physical and chemical properties and also the crystallographic of FA.

Particle size of FA could have a significant impact on the strength development in two ways. Firstly, when the particles are up of 45 μm, this has an influence on the water requirement in an adverse way. Particles size has an important effect on the reaction rate of FA at early stages. Secondly, once diffusion and dissolution of materials occur in concentrated pastes, surface area of the particles might play a considerable role in determining the kinetics of different processes [7]. Salloum [11] concluded that, from a study of 36 different concrete mixtures, there was a relationship linked to the fineness of FA and strength development in concrete.

**107**

**Table 2.**

*a*

*b*

*c*

*d*

*Fly Ash as a Cementitious Material for Concrete DOI: http://dx.doi.org/10.5772/intechopen.90466*

**2.2 Physical characteristics**

reducing agents.

i Particle density (kg/m3

ii Blaine fineness (m2

iii Retention on 45 μm (325 mesh)

*The individual standards may be referred for more details.*

*Comparison of some standards on PFA for use in concretea*

*control Portland cement (PC) mortar. The ASTM standard for the purpose: ASTM C311,*

*The equivalent alkali content, expressed as Na2O, is obtained as: Na2O + 0.658 K2O.*

*"Standard test methods for sampling and testing fly ash or natural pozzolans for use in Portland-cement concrete."*

 *[7].*

*Not specified but generally below 10% when FA is produced from burning of anthracite or bituminous coal.*

*The 28-day compressive strength (N/mm2*

sieve (%, max)

**Sl no Particulars ASTM C618** 

Due to the fineness of ash particles, the reactivity level increases, this could appear in the case of low-calcium ashes compared to those of higher in calcium content.

The performance of concrete is significantly impacted by the physical characteristics of FA; these characteristics could be the volume, rheology, and water content in the slurry, pore distribution, and also the reactivity of constituents. **Table 2** presents different standards of pulverized FA (PFA) and its uses in concrete [7]. Brahammaji and Muthyalu [12] claimed that, the production of an optimal properties of a geopolymer binder, class F fly ash should contain less than 5% of unburned material, no high than 10% of Fe2O3 and lower in CaO content. Also the reactive silica amount should be between 40 and 50%, and 80 and 90% of particles should be smaller or in the range of 45 μm. A high amount of CaO leads to produce higher compressive strength, due to the formation of calcium-aluminate-hydrate (C-A-H) at the early age. The other characteristics which could influence the suitability of FA as a source material for geopolymers are, amorphous content, this means the amount of SiO2, Al2O3 and Fe2O3 and also the morphology of FA. Other researchers [13] have reported that the amount of CaO + MgO could controls the characteristics of surface and the degree of progress of mortar and concrete carbonation. This occurs by providing anions and controls dosage requirements of water-

**Type F**

iv Loss on ignition (%, max) 6.0 7.0 5.0 v Water requirement (% of PC, max) 105 95 Not specified vi Moisture content (%, max) 3.0 0.5 2.0 vii Soundness (autoclave, max) 0.8% 10 mm 0.8% viii Strength activity index (%)b 75 80 80 ix SiO2 + Al2O3 + Fe2O3 (%, min) 70 Not specified 70 x SiO2 (%, min) Not specified Not specified 35.0 xi Reactive silica (%, min) Not specified Not specified 20.0 xii CaO (%, max) Not specifiedc 10.0 Not specified xiii MgO (%, max) Not specified Not specified 5.0 xiv SO3 (%, max) 5.0 2.0 3.0 xv Alkalis as Na2O (%, max)d 1.5 Not specified 1.5 xvi Total chlorides (%, max) Not specified 0.10 0.05

, min) Not specified 2000 Not specified

34.0 12.0 34.0

/kg) Not specified Not specified 320

*) of blended cement mortar is expressed as the percent of that of the* 

**BS 3892 Part 1 IS 3812**

Due to the fineness of ash particles, the reactivity level increases, this could appear in the case of low-calcium ashes compared to those of higher in calcium content.
