**2.4 Setting time**

The addition of FA or other raw material such as GGBFS generally delays the setting time of concrete. The initial and final setting time averages of class F and class C FA are 4:50, 4:40 and 6:45, and 6:15 (h:min), respectively. Setting time could be affected by different factors, for example, the amount of Portland cement, water demand, the reactivity of the pozzolan dosage or FA, and the temperature of concrete. Hot weather plays a positive effect on setting times and is considered as an advantage, by giving enough time for placing and finishing the handled work. On the other hand, if the weather is cold, setting time could be controlled by additives, which delay the finish operation. Some of accelerating admixtures and calcined shale or clay could be used to decrease setting time [27].

FA might have an influence on the rate of the hardening of cement [30, 36, 37] for the following reasons:


#### **2.5 Physical treatment**

A study carried out by Barry [38] shows that the CSA (Canadian Standards Association) standard A23.5-M1982 on plant scale gets an advantage by improving the quality of using FA with high finer size of particles. The results showed an improvement in terms of reactivity and activity of FA, reducing water requirement and resulting in an enhanced ability to control alkali-aggregate reaction. It is observed that the particle size (or the particle surface area) and the size distribution have a significant role in determining the activity of FA. Therefore, FA with finer particle size could replace a high proportion of cement without affecting the strength [7].

However, Ramezanianpour [30] and Adam [39] have performed more than 340 tests of 14 sources of FA. Their results showed that there is no correlation between fineness and compressive strength at the ages of 7 and 28 days for mortars, but a minor correlation was found at 90 days. Joshi [40] and Ravina [41] have exploited a new phenomenon which is called "particle size segregation phenomenon" electrostatic precipitators' method was used to obtain FA fractions of different fineness from a particular source. Another experiment was carried out by Joshi [40] by investigating the proportion of particles of four types of FA, which are up to 45 μm from a modern power plant. The retained percentages of 45 μm sieve for each type of FA are 5, 16, 32, and 38%. The results indicated that replacing 10 and 20% of finer FA in concrete leads to develop a significant strength. These results have been supported by those found by Ravina [41] when the pozzolanic activity index of low-calcium FA from the same precipitator was used for testing.

#### **2.6 Effect of FA on workability and water requirement**

In general, rheological properties of cement pastes could be impacted by the morphology and the small size of the spherical particles of FA. The amount of calcium in FA particles (low calcium) has a significant influence on the rheology of pastes by reducing the amount of water demand and increasing workability. According to Davis et al. [42], FA is considered as a particular material comparing with other pozzolans by leading to the increased water requirement of concrete mixtures. Owens [43] believed that the main characteristic of FA, which has a significant effect on workability of concrete, is the proportion of coarse material (up to 45 μm) which could exist in FA. The effect of coarse particles on the water requirement is shown in **Figure 4**.

Much research has been carried out by Lloyd and Rangan [44] on the use of FA in geopolymer concrete. It is investigated that not only compressive strength could be affected by the characteristics of initial materials but also workability of geopolymer concrete. However, other studies [45] have shown that workability might be related to the ratio of alkaline activator solution (AAS) to binder and composition and nature of the chemical admixture which has been used.

Sathia et al. [46] have reported that the ratio of H2O to Na2O of 10–14 is only used when FA content is about 408 kg/m3 in a designed concrete; this ratio could be changed depending on FA content. Thus, Siddique and Iqbal Khan [47] stated that for an equal w/c ratio and depending on the spherical shape and glassy phase on the FA particle surface, a greater workability could be achieved. Ramezanianpour [30] stated that, due to the necessity of mixing and placing concrete in a reinforced formwork, it is necessary to maintain its workability. This could be determined by the rheological properties of the system, which are in turn impacted by all the components. Thus, it is important to understand the rheological behavior and the main role of FA in the fresh concrete, which leads to exploit the potential role of FA for improving concrete.

**113**

**Figure 4.**

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

**2.7 The impact of FA on durability of concrete exposed to elevated temperatures**

Recently, the requirement of infrastructure and its development such as in nuclear reactor containment structures exaggerates the use of concrete, which could withstand high temperatures. Many researchers [48–50] have studied the effect of elevated temperature on FA concrete in the range of 230°C. Another study was carried out by Carette et al. [51] which showed the influence of a temperature 600°C on concrete with a mix of Portland cement, slag, and FA, as is illustrated in **Figure 5**. Under a high temperature, the addition of FA has no effect on the behavior of the concrete; however the changes in concrete properties or decreasing of

*Influence of coarse-particulate content of FA on water requirement for equal workability in concrete [43].*

In addition, degradation of concrete structures is strongly affected by the chemical attack. For example, the penetration of chloride ions into the concrete leads to chemical reactions which could help in the formation of corrosion around reinforcement. This could be the reason of an early end to a structure's life cycle. Other studies that have been carried out by Thomas et al. [52] and Uddin and Shaikh [53] have reported that resistance of concrete to the immigration of chloride ions is mainly controlled by porosity and inter-connectivity of pores system and

In order to achieve an efficient geopolymer synthesis, it is required that silica (SiO2), alumina (Al2O3), and iron (Fe2O3) should be in high proportions [54]. Further, the activity of FA or the formation of aluminosilicate gel is related to the nature of environment, which could be acidic or basic (Ferna and Deventer, 2007) [55], and also high concentration of calcium has an important effect on the reaction, by accelerating its rate. Nikolić et al. [56] reported that the reactivity of FA could be influenced by many factors which in turn affects the characteristics of FA-based geopolymer, such as glassy phase, particle size distribution, the presence of iron, calcium, and inert elements. However, the reactivity of FA is not dependent only on the glassy phase but on the whole FA; this means that the glassy phase has a limitation

strength could be observed at the same range of temperatures [37].

also depends to the chemical binding capacity of cement.

**2.8 FA requirements for geopolymer**

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

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A study carried out by Barry [38] shows that the CSA (Canadian Standards Association) standard A23.5-M1982 on plant scale gets an advantage by improving the quality of using FA with high finer size of particles. The results showed an improvement in terms of reactivity and activity of FA, reducing water requirement and resulting in an enhanced ability to control alkali-aggregate reaction. It is observed that the particle size (or the particle surface area) and the size distribution have a significant role in determining the activity of FA. Therefore, FA with finer particle size could replace a high proportion of cement without affecting the strength [7]. However, Ramezanianpour [30] and Adam [39] have performed more than 340 tests of 14 sources of FA. Their results showed that there is no correlation between fineness and compressive strength at the ages of 7 and 28 days for mortars, but a minor correlation was found at 90 days. Joshi [40] and Ravina [41] have exploited a new phenomenon which is called "particle size segregation phenomenon" electrostatic precipitators' method was used to obtain FA fractions of different fineness from a particular source. Another experiment was carried out by Joshi [40] by investigating the proportion of particles of four types of FA, which are up to 45 μm from a modern power plant. The retained percentages of 45 μm sieve for each type of FA are 5, 16, 32, and 38%. The results indicated that replacing 10 and 20% of finer FA in concrete leads to develop a significant strength. These results have been supported by those found by Ravina [41] when the pozzolanic activity index of

low-calcium FA from the same precipitator was used for testing.

tion and nature of the chemical admixture which has been used.

In general, rheological properties of cement pastes could be impacted by the morphology and the small size of the spherical particles of FA. The amount of calcium in FA particles (low calcium) has a significant influence on the rheology of pastes by reducing the amount of water demand and increasing workability. According to Davis et al. [42], FA is considered as a particular material comparing with other pozzolans by leading to the increased water requirement of concrete mixtures. Owens [43] believed that the main characteristic of FA, which has a significant effect on workability of concrete, is the proportion of coarse material (up to 45 μm) which could exist in FA. The effect of coarse particles on the water

Much research has been carried out by Lloyd and Rangan [44] on the use of FA in geopolymer concrete. It is investigated that not only compressive strength could be affected by the characteristics of initial materials but also workability of geopolymer concrete. However, other studies [45] have shown that workability might be related to the ratio of alkaline activator solution (AAS) to binder and composi-

Sathia et al. [46] have reported that the ratio of H2O to Na2O of 10–14 is only

changed depending on FA content. Thus, Siddique and Iqbal Khan [47] stated that for an equal w/c ratio and depending on the spherical shape and glassy phase on the FA particle surface, a greater workability could be achieved. Ramezanianpour [30] stated that, due to the necessity of mixing and placing concrete in a reinforced formwork, it is necessary to maintain its workability. This could be determined by the rheological properties of the system, which are in turn impacted by all the components. Thus, it is important to understand the rheological behavior and the main role of FA in the fresh concrete, which leads to exploit the potential role of FA

in a designed concrete; this ratio could be

**2.6 Effect of FA on workability and water requirement**

requirement is shown in **Figure 4**.

used when FA content is about 408 kg/m3

**2.5 Physical treatment**

**112**

for improving concrete.

**Figure 4.** *Influence of coarse-particulate content of FA on water requirement for equal workability in concrete [43].*

#### **2.7 The impact of FA on durability of concrete exposed to elevated temperatures**

Recently, the requirement of infrastructure and its development such as in nuclear reactor containment structures exaggerates the use of concrete, which could withstand high temperatures. Many researchers [48–50] have studied the effect of elevated temperature on FA concrete in the range of 230°C. Another study was carried out by Carette et al. [51] which showed the influence of a temperature 600°C on concrete with a mix of Portland cement, slag, and FA, as is illustrated in **Figure 5**. Under a high temperature, the addition of FA has no effect on the behavior of the concrete; however the changes in concrete properties or decreasing of strength could be observed at the same range of temperatures [37].

In addition, degradation of concrete structures is strongly affected by the chemical attack. For example, the penetration of chloride ions into the concrete leads to chemical reactions which could help in the formation of corrosion around reinforcement. This could be the reason of an early end to a structure's life cycle. Other studies that have been carried out by Thomas et al. [52] and Uddin and Shaikh [53] have reported that resistance of concrete to the immigration of chloride ions is mainly controlled by porosity and inter-connectivity of pores system and also depends to the chemical binding capacity of cement.

#### **2.8 FA requirements for geopolymer**

In order to achieve an efficient geopolymer synthesis, it is required that silica (SiO2), alumina (Al2O3), and iron (Fe2O3) should be in high proportions [54]. Further, the activity of FA or the formation of aluminosilicate gel is related to the nature of environment, which could be acidic or basic (Ferna and Deventer, 2007) [55], and also high concentration of calcium has an important effect on the reaction, by accelerating its rate. Nikolić et al. [56] reported that the reactivity of FA could be influenced by many factors which in turn affects the characteristics of FA-based geopolymer, such as glassy phase, particle size distribution, the presence of iron, calcium, and inert elements. However, the reactivity of FA is not dependent only on the glassy phase but on the whole FA; this means that the glassy phase has a limitation

**Figure 5.** *Compressive strength of concretes after 1 month of exposure to various elevated temperatures [51].*

degree. Therefore, the reactivity of FA usually depends on the dissolution level of FA in the alkaline activator. As aforementioned, loss on ignition (LOI) is defined as the unburned carbon present in FA and how that affects the quality of paste or concrete by increasing the water requirement and reducing the reactivity of pozzolanic constituents. ASTM C618 (2008) [39] has reported that the required percentage of LOI is limited to 6% maximum. Another study showed that a high proportion of SO3 in concrete could lead to instability in volume, which in turn has an impact on durability. However, it is reported that about 5% of SO3 of FA could be used as a concrete binder.

#### **2.9 Chemical activation**

Blanco et al. [57] have proposed a procedure of using of wet milling and leaching with sulfuric acid to activate FA. One of the main applications to use the activated FA is to substitute silica fume in concrete, which could lead to achieve a high strength, due to the decrease of pore size in the hardened concrete. The addition of a limited amount of sodium sulfate or potassium sulfate (Na2SO4 or K2SO4) mixed with calcium hydroxide (Ca(OH)2) has a substantial effect on acceleration of hydration and compressive strength. A study carried out by Görhan and Kürklü [58] investigated that the activation of mortar samples by using NaOH of 6 M leads to increase in compressive strength values by 21.3 MPa and 22 MPa, compared to those samples which are activated by 9 M NaOH. Therefore, it has been reported by other authors [59, 60] that to achieve a great reactivity of FA particles within the activator solution, the liquid phase plays a significant role as a transport medium and a less smoothly gel is formed, due to the faster reaction of NaOH.

#### **2.10 Addition of FA to cement and concrete**

FA is classified into two classes by ASTM C618. Class F FA is pozzolanic, with minimum or no cementing value, whereas class C FA is cementitious as the same as

**115**

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

**Characteristics ASTM C618 FA** Type ASTM class F and C

Specific gravity 2.0–2.7 Fineness 1–100 μm Color Darker gray buff to tan LOI Depends to the source

Shape and particles Cenospheres, plerospheres, spherical particles

tics of FA that can be referred to in any uses.

ization as normalized by ASTM C618.

ability as a mineral admixture for cement and concrete.

**3. Conclusion**

**Table 3.** *FA characteristics.*

pozzolanic properties [6]. The main parameter to formulate a concrete mix design with the addition of FA is the proportion of the mix under consideration of the variation of water-cementitious ratio. This could lead to achieve the requirements for compressive strength at different ages, air content, and workability. ACI 211.1 or 211.2 has determined the procedures for the mix design in details, in terms of the proportioning of water, cement (or cement plus FA), and aggregate materials [61]. However, the specific gravity of FA is lower than PC, which needs to be taken into consideration in the mix proportioning process. Other standards such as the European standard BS EN206 provide some requirements for the use FA in concrete [7, 27, 62]. Further study has been carried out by Horpibulsuk et al. [63] which reported that FA could be considered as a dispersing material, when it is mixed with cement. This in contrast when FA is used in concrete as a pozzolanic material, where a pozzolanic reaction can be occurred by consuming of Ca(OH)2 during the hydration process. **Table 3** summarizes the main physical and chemical characteris-

Mineral compositions Rich in aluminosilicate, iron oxide, and calcium oxide Thermal resistance High resistance to elevated temperatures Uses in concrete production Improves concrete's strength and durability

This chapter mainly reviews FA as basic raw materials, which can be introduced

The chemical composition analysis shows that FA consists of a complex oxide mixture of aluminosilicate glasses and other crystalline elements with the presence of the amorphous phase. This latter explains the high reactivity of FA and its suit-

Numerous studies show that including FA in the production of geopolymers concrete provides a greater mechanical and microstructure properties, due to its physical characteristics compared to that given by OPC. These findings make the

in the production of concretes so-called geopolymer concrete. As per several reports, a huge quantity of FA is disposed and landfilled. Besides, it has been estimated that the cement industry contributes approximately 7% of global warming, due to the substantial increase in carbon dioxide. The raw materials and the manufacturing process of the conventional cement are found to be the main reason behind this increase. As a solution for this serious environmental issue, much research has been conducted to investigate other alternative materials to Portland cement. FA is the most investigated material, due to its suitability and its physicochemical properties including microstructure, reaction mechanism, and character-


#### **Table 3.**

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degree. Therefore, the reactivity of FA usually depends on the dissolution level of FA in the alkaline activator. As aforementioned, loss on ignition (LOI) is defined as the unburned carbon present in FA and how that affects the quality of paste or concrete by increasing the water requirement and reducing the reactivity of pozzolanic constituents. ASTM C618 (2008) [39] has reported that the required percentage of LOI is limited to 6% maximum. Another study showed that a high proportion of SO3 in concrete could lead to instability in volume, which in turn has an impact on durability. However, it is reported that about 5% of SO3 of FA could be used as a concrete binder.

*Compressive strength of concretes after 1 month of exposure to various elevated temperatures [51].*

Blanco et al. [57] have proposed a procedure of using of wet milling and leaching with sulfuric acid to activate FA. One of the main applications to use the activated FA is to substitute silica fume in concrete, which could lead to achieve a high strength, due to the decrease of pore size in the hardened concrete. The addition of a limited amount of sodium sulfate or potassium sulfate (Na2SO4 or K2SO4) mixed with calcium hydroxide (Ca(OH)2) has a substantial effect on acceleration of hydration and compressive strength. A study carried out by Görhan and Kürklü [58] investigated that the activation of mortar samples by using NaOH of 6 M leads to increase in compressive strength values by 21.3 MPa and 22 MPa, compared to those samples which are activated by 9 M NaOH. Therefore, it has been reported by other authors [59, 60] that to achieve a great reactivity of FA particles within the activator solution, the liquid phase plays a significant role as a transport medium and a less

FA is classified into two classes by ASTM C618. Class F FA is pozzolanic, with minimum or no cementing value, whereas class C FA is cementitious as the same as

smoothly gel is formed, due to the faster reaction of NaOH.

**2.10 Addition of FA to cement and concrete**

**114**

**2.9 Chemical activation**

**Figure 5.**

*FA characteristics.*

pozzolanic properties [6]. The main parameter to formulate a concrete mix design with the addition of FA is the proportion of the mix under consideration of the variation of water-cementitious ratio. This could lead to achieve the requirements for compressive strength at different ages, air content, and workability. ACI 211.1 or 211.2 has determined the procedures for the mix design in details, in terms of the proportioning of water, cement (or cement plus FA), and aggregate materials [61].

However, the specific gravity of FA is lower than PC, which needs to be taken into consideration in the mix proportioning process. Other standards such as the European standard BS EN206 provide some requirements for the use FA in concrete [7, 27, 62]. Further study has been carried out by Horpibulsuk et al. [63] which reported that FA could be considered as a dispersing material, when it is mixed with cement. This in contrast when FA is used in concrete as a pozzolanic material, where a pozzolanic reaction can be occurred by consuming of Ca(OH)2 during the hydration process. **Table 3** summarizes the main physical and chemical characteristics of FA that can be referred to in any uses.
