*2.2.5 Unburned carbon*

Unburned carbon (UBC) is mostly the significant affecting particles on the loss on ignition (LOI). During hydration process the carbon particles do not have any part in the chemical reactions. However, they have an impact on the water requirement in concrete. Carbon particles have a very strong affinity and attraction to

**111**

**2.4 Setting time**

for the following reasons:

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

the organic chemical admixtures. For example, air-entraining agents (AEA) are a chemical agent that has absorbed on the carbon and negatively affects the hardened concrete. In general, the absorption degree depends on many factors, such as surface area and type of carbon in terms of its polarity and particle size. An experimental study showed that FA with less than 3–4% of carbon does not have a greater effect on the performance of organic chemical admixtures [7]. On the other hand, Ha et al. [32] reported that the use of FA as raw material, which contains around 8%

FA has varieties of chemical compositions; the averages of the main elements of FA in some European countries (France, UK, Germany), USA, and far Asian countries (Japan, China, India) are given as follows: 53.05%, 27.24, and 5.50% for SiO2, Al2O3, and Fe2O3, respectively [7]. However, another study [33] reported that FA consisted of a heterogeneous mixture of complex aluminosilicate glasses and other crystalline elements. The structure of aluminosilicate glass is an amorphous form, but it could be modified due to the addition of alkaline and metal oxides such as Na2O, K2O, MgO, CaO, and FeO. A study carried out by Das and Yudhbir [34] showed that a strong correlation exists between the glass content and the ratio of potassium to aluminum oxides (K2O/Al2O3). ASTM classification shows that the composition of glass in class F fly ash is different from that in class C. A high polymerized glass network is observed in class F FA, but the glass matrix depolymerizes

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

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

• FA could contain sulfates which lead to a reaction with cement in the same way

• The fly ash cement mortar might contain less water, and this has a significant

• The surface-active agent which could be added to modify the rheology (water reducers) of concrete could be absorbed by FA, and this leads to an influence on

• FA particles could act as nuclei for crystallization of cement hydration products.

• FA is considered as cementitious and contains high calcium (class C FA).

of UBC, could accelerate the corrosion of reinforcement steel.

when the CaO increases in comparison with Al2O3 content [7, 21, 35].

shale or clay could be used to decrease setting time [27].

as when gypsum is added to Portland cement.

effect on the rate of stiffening.

the stiffness of mortar.

**2.3 Chemical and mineralogical composition of FA**

**Figure 3.** *Munsell color circle [31].*

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

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iron compounds are (+2) [7].

**Figure 2.**

*2.2.5 Unburned carbon*

percentage of carbon is low or absent in ash, then the color might be brown, due to the presence of iron (+3) compounds. The color changes to bluish gray to gray if the

Tanosaki et al. [13] have reported the use of colorimetric methods or as is known as the Munsell system to identify colors by following the next three dimensions, hue, value (lightness), and chroma (color purity). In 1905, Professor Albert H. Munsell created the Munsell system. According to Malacara [31], **Figure 3**, describes the color circle system. The system is divided into five principle hues: red, yellow, green, blue, and purple, along with five intermediate hues halfway between adjacent principle hues. Each of these ten categories is used to divide into other ten

Unburned carbon (UBC) is mostly the significant affecting particles on the loss on ignition (LOI). During hydration process the carbon particles do not have any part in the chemical reactions. However, they have an impact on the water requirement in concrete. Carbon particles have a very strong affinity and attraction to

sub-categories, so that 100 hues are given integer values.

*Scanning electron microscope (SEM) micrographs of fly ash particles [30].*

**110**

**Figure 3.**

*Munsell color circle [31].*

the organic chemical admixtures. For example, air-entraining agents (AEA) are a chemical agent that has absorbed on the carbon and negatively affects the hardened concrete. In general, the absorption degree depends on many factors, such as surface area and type of carbon in terms of its polarity and particle size. An experimental study showed that FA with less than 3–4% of carbon does not have a greater effect on the performance of organic chemical admixtures [7]. On the other hand, Ha et al. [32] reported that the use of FA as raw material, which contains around 8% of UBC, could accelerate the corrosion of reinforcement steel.
