Fine-grained mineral additives are mixed together with the basic materials used in concrete production (cement, aggregate and water) and included in the concrete mixture. The proportion in which they are used is usually 10%-50% of the amount of cement contained in the concrete mixture. Fine-grained mineral additives affect many properties of fresh and hardened concrete, depending on the type and proportion of the material used. Fly ash, which we will talk about in this article, is one of the most widely used of these fine-grained mineral additives.
What is the Use of Fly Ash in Cement?
Pulverized coal is used as fuel in most thermal power plants for the production of electrical energy. With the combustion of pulverized coal, ash grains appear, a large amount of which are very fine, and some of which have relatively slightly larger sizes. Relatively heavy coarse ash grains fall to the bottom of the pit as bottom ash. The ashes that have very fine grains “fly” with the fuel gases and move out of the chimney. These ashes are called “fly ash”.
In case the ash goes out with the gases, the environment of the thermal power plant will be covered with ashes in a short time, so the ashes that will come out of the chimney are kept by electrostatic or electromechanical methods and channeled to the ash collector silos.
Since they have a siliceous and aluminous amorphous structure and are obtained as very fine-grained, fly ashes have pozzolanic properties. When combined with calcium hydroxide, which is the hydration product of cement, in an aqueous environment, it has hydraulic binding.
Therefore, it is directly used both in the production of portland-pozzolan type cement and as a concrete additive. Usually, a very large amount can be used as a concrete additive. The amount of fly ash contained in the concrete mixture varies around 15%-50% of the cement weight. With the use of fly ash in cement production,
- Less use of clinker,
- Reduction of greenhouse gas emissions,
- Reduction of energy consumption,
- By using waste, benefits such as less pollution of nature are provided.
What are the Fly Ash Classes? What is the Chemical Composition of Fly Ash?
The chemical compositions and physical properties of fly ashes vary according to the type of coal burned in a thermal power plant, the combustion temperature and the collection method.
The amount of “SiO2+Al2O3+Fe2O3” in many fly ashes is over 85%. In order for fly ashes to show sufficient pozzolanic properties, the total amount of these oxides should not be less than 70% and 50% in F-type and C-type ashes, respectively.
| Class | Definition |
| F | SiO2+Al2O3+Fe2O3≥%70 CaO ratio is lower than 10%. They are obtained from anthracite or bituminous coals and have pozzolanic properties. |
| C | SiO2+Al2O3+Fe2O3≥%50 The CaO ratio is usually more than 10%. They are obtained from lignite or low bituminous coals, and they have pozzolanic properties. Because they contain high amounts of CaO, they have some inherent binding. |
Table 1. The ASTM C 618 Standard evaluates fly ashes in two groups as class F and C.
| Chemical content | Class F Ash | Class C Ash |
| SiO2 | 43.6-64.4 | 23.1-50.5 |
| Al2O3 | 19.6-30.1 | 13.3-21.3 |
| Fe2O3 | 3.8-23.9 | 3.7-22.5 |
| CaO | 0.7-6.7 | 11.5-29.0 |
| MgO | 0.9-1.7 | 1.5-7.5 |
| Na2O | 0-2.8 | 0.4-1.9 |
| Loss of Ignition | 0.4-7.2 | 0.3-1.9 |
Table 2. Chemical Compositions of Different Thermal Power Plant Fly Ashes
What Should Be Considered in the Use of Fly Ash?
MgO, SO3 and alkali are present in fly ash, albeit in small amounts. It is desirable that the proportion of them is not high.
MgO leads to expansion in concrete as a result of hydration. SO3 causes the formation of ettringite in hardened concrete and thus causes very large expansions. High alkali content can increase the possibility of reaction with aggregates containing reactive silica, leading to expansion in concrete.
Physical and chemical analyzes of the fly ash to be used in concrete should be carried out and its suitability for use should be checked according to the limit values specified in the TS EN 450-1 standard.
| Chemical Properties | TS EN 450-1 Limits |
| Loss of ignition | Category A≤ % 5.0; Category B≤% 7.0; Category C≤% 9.0 |
| Chloride | ≤% 0.10 |
| Sulfate (SO3) | ≤% 3.0 |
| Free calcium oxide | If ≥ 1.5%, it shall be tested according to the expansion requirements. |
| Reactive calcium oxide | ≤% 10.0 |
| Reactive silicon dioxide | ≥% 25 |
| SiO2+Al2O3+Fe2O3 | ≥% 70 |
| Total alkali | ≤% 5.0 |
| Magnesium oxide (MgO) | ≤% 4.0 |
| Phosphate (P2O5) | ≤% 5.0 |
Table 3. TS EN 450-1 Chemical Properties Limits
| Physical Features | TS EN 450-1 Limits |
| Fineness (the ratio of the mass of ash remaining on the sieve when sifted through a 0.045 mm mesh sieve) | Category N : ≤% 40 Category S : ≤% 12 |
| 28-day activity index | ≥% 75 |
| 90-day activity index | ≥% 85 |
| Expansion (on the mixture with 30% fly ash and 70% cement) | ≤10 mm |
| Setting start time | It should be determined on a mixture containing 25% fly ash and 75% test cement and should not be more than twice the initial setting time of the cement paste prepared with 100% test cement. |
Table 4. TS EN 450-1 Physical Properties Limits
What is the Effect of Fly Ash on Concrete Properties?
We can list the effects of fly ash used as an additive in concrete on concrete properties as follows:
Positive Effects:
- Increases the ability to be processed in unset concrete.
- Reduces perspiration in fresh concrete.
- Reduces concrete hydration heat and thermal crack formation.
- It greatly increases the ultimate strength.
- Reduces the water permeability of hardened concrete.
- Increases the resistance of concrete to adverse effects.
- Prevents efflorescence by reacting with the cement hydration product calcium hydroxide.
- Increases the resistance of hardened concrete to sulfates.
- Reduces the potential for the formation of an alkali-silica reaction.
- Provides affordability.
Since the spherical shape of the ash grains leads to less friction, it can ensure that there is less need for water. This situation varies depending on the fineness of the fly ash and the amount of use.
Potential Harmful Effects:
- It slightly delays the outlet of concrete, which can be a problem in cold weather.
- It reduces the speed of gaining strength of concrete in the first couple of days.
- It requires a longer curing of concrete.
Why Should Fly Ash Be Used?
The use of fly ash provides high durability, low hydration temperature and high ultimate strength values in concrete. It also has environmental benefits as well as improving concrete performance. It ensures less use of cement clinker and therefore it provides less emission of greenhouse gas emissions and less energy consumption. It also provides less pollution of nature as the use of thermal power plant waste.
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