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1.   What are flyash and furnace bottom ash?
The ash carried out with the flue gases in a pulverised coal fired power station is called ‘Flyash’(FA). To prevent the ash polluting the atmosphere, the flue gases are passed through electrostatic precipitators or bag filters to separate the Flyash from the flue gases. The Flyash is collected in silos from where it is either solid dry for use in concrete or moistened (called conditioning) for less demanding applications such as fill, grouts or as a fine aggregate replacement. Conditioned ash (PozzSand®) can also be easily transported in open top trucks which are covered to suppress dust. Flyash of a high quality is processed a step further by Ash Resources. The ash is passed through equipment called Classifiers from which particles of a specific size range are extracted. This more highly specified material is marketed under Ash Resources brand names: DuraPozz® and SuperPozz. Furnace Bottom Ash (FBA) is an agglomeration of larger particles that were not removed in the flue gases and thus fall to the bottom of the furnace. It is normally extracted wet, graded and sold to make concrete bricks and blocks. Internally FBA has gained an excellent reputation as a lightweight aggregate in these applications
2.   When flyash is referred to as PFA, what does this mean?
PFA stands for Pulverised Fuel Ash and is a misnomer for Flyash. Pulverised fuel refers to coal that has been crushed and ground to a fine talcum-like powder. This increases the efficiency of combustion in power station furnaces. The carbon in the pulverized coal burns very quickly (typical within 2-4 seconds) and completely combusted at temperature of over 1250 C. The sum total of all the ash is PFA. Thus to be correct PFA = Flyash + bottom ashdration in conc
3.   What are some of the uses of flyash?
Flyash can be used in all site mixed, readymixed or precast concretes. Incorporating Flyash in a concrete mix can be achieved by using factory pre-blended cements, or by blending by the concrete producer in the concrete mixer. Cement producers often use Flyash to replace natural materials such as aluminates and silicates in the production of Portland Cement, and in the precast market conditioned Flyash is successfully used as a fine aggregate replacement. Other applications for Flyash include road base materials, flowable fills, waste stabilization and manufactured coarse aggregate. In recent years, ultra fine fraction ash has been used as an inert mineral filler for composite plastic and rubber products
4.   How much flyash can be put into a concrete mix?
There are no technical reasons why large proportions of Flyash cannot be used in certain types of concrete. Having 60-70% of the cementitious content as Flyash is, for example, commonplace for mass concrete in gravity dam walls. The recognized limits to how much Flyash can be utilized as part of the binder content are covered in SANS 50197-1 (formerly SABS EN 197-1) for factory made cements and for mixer blended concretes SANS 1200G which provides guidelines on the use of Flyash in concrete. More Flyash may be added as Type 1 addition which treats Flyash as an inert material
5.   Does Flyash improve the workability of concrete?
Yes, it is one of the major reasons why concrete producers and contractors like working with Flyash mixes. ‘Workability’ expresses the ease of handling, placing and finishing fresh or ‘plastic’ concrete mix workable enabling a typical water reduction of between 6% and 12% when using classified Flyash.
6.   How does Flyash reduce the heat of hyrete?
During the hydration of cement, heat is generated, causing the concrete temperature to rise which further accelerates the setting time and strength gain of the concrete. In the case of mass concrete, the rapid temperature rise increases the chances of thermal cracking, leading to reduced concrete integrity and durability, which can significantly increase the potential for thermal cracking. Partially replacing Portland Cement with Flyash reduces the rate of heat generation due to the pozzolanic reaction with lime occurring over extended period of time.
7.   What is the impact of Flyash on the strength of concrete?
Fine fraction Flyash, i.e. classified ash (Dura Pozz®) with typically 90% of its particles passing a 45um sieve, acts as a plasticiser in the concrete. Being spherical in shape, they almost act as ball bearings reducing the water requirement of the concrete for a given workability. This water reduction and resultant lower water: cement ration increases the compressive strength Flyash, also enhances the strength and overall performance of concrete by the pozzolanic reaction with lime (calcium hydration). This reaction forms stable Calcium Silicate Hydrate (CSH) transforming the weak crystalline structure to a dense gel like matrix. These hydrates fill the voids within the concrete removing the lime thereby increasing the strength and reducing the permeability. The pozzolanic reaction takes place over time at normal temperatures, resulting in significant strength development at later ages when compared to PC at 112 days and beyond. Flyash therefore has a positive impact on the longer term strength of concrete, provided that good site and curing practices are followed.
8.   How Does Flyash decrease the permeability of concrete?
Flyash increases the cementitious compounds in a mix, minimizes water demands and the spherical particles fill in potential bleed channels. These factors increase the density of the concrete and give less internal voids with a corresponding reduction in permeability.
9.   How does Flyash reduce chloride ingress to concrete?
Not only does the lower permeability of a Flyash concrete impede penetration by chlorides – normally in the forms of seawater or seahorse mist-but chlorides chemically bind to the Flyash due to the high aluminate content (3 to 5 times higher than cement ). These effects can increase the protection for steel reinforcing with sufficient cover by as much as 98%.
10. How does Flyash decrease the alkali/silica reaction?
Some aggregates contain reactive silica that can react with soluble alkalis, such as the free lime in cement. The reaction products are voluminous and can create destructive expansion forces. When Flyash is incorporated in a mix, it reacts chemically with the alkalis and lime liberated by cement hydration so that they are unavailable for subsequent formation of expansive products by reaction with a potentially reactive aggregate. The potential damage from the alkali/silica reaction can be serious. In addition to causing the concrete surface to disintegrate, interior stresses may occur which cause sufficient cracking to critically impair the structural integrity of the concrete and enable the reinforcement to corrode. Including Flyash in the mix will usually reduce the reaction sufficiently to protect the concrete and steel reinforcement from these forms of deterioration.
11. Can Flyash be used in mortars and plaster?
Yes! Plasters and mortars made with Flyash as part of the binder will however develop strength slowly, and will have to be cured correctly in line with good site practices. In cold weather strength development at early ages will be protracted as is the case with most sand: cement mixes containing blended cements or other extenders. It is important to have a clean sharp sand in the mix, as often fine clayey sands are used for economic reasons, these sands lead to plaster / mortars of poor strength and that are prone to crack and craze. (SACAA report – South African Flyash: A Cement Extender by J. Kruger).
12. What is the difference in chemical composition between Flyash and Portland Cement ?
A major difference between Flyash and Portland Cement (PC) is that PC is rich in calcium silicates while its level in Flyash is low. High quality Flyash is high in reactive silicate glass while PC has none. Lime is released during the hydration of Portland Cement. This free lime is a key ingredient in the reaction with the silicates in the Flyash to form strong, durable cementing compounds.
13. What is difference between fly ash and pulverised coal ash?
Fly ash and pulverized coal ash are the two names of the same thing. It is also known as pulverized fuel ash.
14. Can ESP ash/chimney ash/dry fly ash be used for agricultural application?
Such a fine ash in dry condition is not advisable to be used in agriculture application as its transportation, application (spreading and ploughing) would be costlier, difficult and may cause air pollution.
Pond ash is best suited for this application as it contains some moisture and does not become air borne easily.
15. Is fly ash radio active? Is it harmful ?
Each material, including fly ash is radio active. It is the degree of radio activity that determines its impact. The radio activity level of fly ash is 1/10th to 1/20th that of the level that cause any harm. This has been tested and certified by Department of Atomic Energy, Government of India.
Radio activity levels of fly ash bricks, agriculture soils treated with fly ash as well as the agriculture produce grown on fly ash treated soils have been tested for radio activity and are certified as very safe.
16. Can fly ash be used for back filling of open cast mines?
Yes fly is a good material for back filling of open cast mines. It can be used as a structural fill material that is by laying in layers and with appropriate compaction. It can also be used alongwith overburden with intermittent mixing i.e. if 10 dumpers are bringing over burden, two dumpers can be employed bring in fly ash and all these materials can be backfilled in a regular manner as per the existing practice of back filling. The fly ash would get automatically intermixed and fill up the voids in the over burden material, providing it stability and nutrition as well as water holding capacity for aforestation.
17. Can fly ash be used, in place of sand, for stowing of underground mines?
Yes, fly ash is a good material to substitute sand for back filling of open cast mines. Coarser portions of fly ash that is bottom ash and coarse ash from pond ash are even better materials than sand for this purpose. Fine ash particles that take relatively longer time for settlement, can also be settled faster with application of appropriate technology.
18. Can fly ash be ground to improve its fineness?
Yes, the fineness of the fly ash can be improved by grinding.
19. Will cementatious property of fly ash will create hard lumps of soil when used for agriculture applications?
No it will not.
Fly ash is a pozzolonic material which behaves like a cement in presence of lime/cement and water. In common soils fly ash will not behave like cementatious material. It should not be used in the soils where lime treatment is being given.
20. What is the difference between fly ash (pulverised coal ash, pulverised fuel ash) ESP ash, bottom ash, pondash ?
When pulverized coal (bituminous/lignite is burnt in the boiler of a thermal power station, a part of ash falls down at the bottom of the boiler and is known as bottom ash. Whereas, the major portion of the ash comes out alongwith the flue gases and is collected through electro static precipitator or filter bags or other means before allowing the exhaust gases through escape the chimney, this part of ash is generally known as ESP ash. The un-utilised ESP ash and bottom ash are taken to lagoons known as ash ponds for deposition. The ash deposited in the ash pond is known as pond ash. In some cases this ash is deposited in the form of a mount or hillock and is known mound ash. Fly ash or pulverised coal ash or pulverised fuel ash in general term represents all types of ashes produced in the thermal power station unless otherwise specifically referred to as ESP ash(chimney ash, dry fly ash), bottom ash, pond ash and mound ash.
Small part of ash is also collected in the economizer & pre-heater. This ash should generally be discharged alongwith bottom ash, being a coarse ash. However, at number of power stations it is discharged with ESP ash.
21. Can bottom ash and pond ash be used for fly ash lime gypsum bricks or fly ash sand cement bricks?
ESP ash/chimney ash/dry fly ash is recommended for this use. However, pond ash or bottom ash can also be used but the results (strength of the bricks) would not be as good as in case of ESP ash.
22. Is the quality of pond ash same at all locations in the ash pond?
No. The quality of ash in different zones of pond ash is different.
The ash deposited within about 100 meters of ash slurry discharge point in the pond is coarser ash as compared to the ash deposited within about 100 mtrs. of water overflow discharge point in the ash pond. The ash deposited in between these two areas is of medium particle size. Further, during the process of settling of ash in the pond, natural segregation process takes places. Coarser ash settles down first and the final particle in the top layer. With the general practice intermittent filling of ash ponds, repetitive layers of about 1 meter thickness get developed with segregated particles.
23. Can same pond ash be used for all types of soils for agriculture applications?
For optimum results the fly ash needs to be selected based on physical and chemical properties of soil and the cropping pattern. However, all bituminous coal fly ashes(Class F fly ash) give good results in all soils and all cropping patterns.
Application of acidic fly ashes (which is rarely available in India) should be avoided. For best results the selection of fly ash and dosage be done under the advice of agriculture fly ash experts. The selected fly ash and the dozes should maximise the supply of micro nutrient required for the cropping pattern deficient in the soil and should improve the soil texture/physical properties of the soil.
24. What is the chemical composition of flyash?
SourceSiO2(%)Al2O3(%)Fe2O3(%)CaO(%)MgO(%)SO3(%)Na2O(%)K2O(%)LOI(%)
Bituminous20-605-3510-401-120-50-40-40-30-15
Subbituminous40-6020-304-105-301-60-20-20-40-3
Lignite15-4510-254-1515-403-100-100-60-40-5