[A]Chemical and physical properties for FA to be utilised in blended cement and concrete are specified by two communitarian standards: EN 197-1 “Cement – composition, specifications and conformity criteria for common cement” and EN 450 “Fly ash for concrete – definitions,requirements and quality control”.

Table[SEE ATTACHMENT] ZOOM150.

It summarises requirements of standard EN 450 indicating chemical-physical properties and statistical control criteria for fly ashto be utilised in the production of structural concretes conforming to the standard EN 206.

According to EN 206, FA can be introduced as a pozzolanic addition in concrete mix

design with an equivalent factor k = 0.2 or 0.4, depending on the cement class, with respect to the minimum PC content required by the standard for each class of environmentalexposition of the final product.

[B]Before mentioning the various processes available in the industry to upgrade FA from a low-grade material to a high-grade material with pozzolanic properties, first a few points regarding its pozzolanic activity are given as:

[I]Typically the pozzolanic activity of a FA is proportional to the amount of particles under 10um whereas particles larger than 45um show a little or no pozzolanic property.

[II]Fly ash particles are spherical in shape, and this has a positive effect on the water requirement and workability of concrete mixtures.

[III] The pozzolanic properties of FA are not governed so much by the chemistry but by the mineralogy and particle sizes of the ash.

[IV]The high calcium FA exhibit both cementitious and pozzolanic properties, and some show a rapid set unless it is retarded by gypsum or other additions.

[C] The +45 micron (um) residue test is an excellent indicator for FA quality. Size smaller than 45 um resulting is a proven water reducer of ~/5 when used as 30 % replacement in concrete. This FA also shows a good strength development by stimulating cement hydration (fine Filler effect) and forming additional C-S-H 'needles'(Pozzolanic reaction). A number of density related durability aspects are also linked to controlled top particles size of FA. Unclassified FA (Pozz-Fill) taken from theash bunkers (before conditioning), with nominal particle size depending on prevailing burningconditions, coal milling and the condition of each of electrostatic precipitator fields, will be a bit variable and not as effective in reducing the water requirement of a mix. Pozzolanic reactivity will be somewhat lower than with classified ash (controlled by 45 um residue ).

[B] Morphology of FA ( presence of unburned carbon) can also affect concrete because during concrete compaction carbon[(~ LOI) to 5%: ] will float to the surface because of the low density, and larger amounts can cause discoloration. FA with very low carbon contents (LOI ~1%) are much more consistent in performance.

[C] Now a days, the superfine ash[['new generation' or “super- pozz”] from Lethabo, with a nominal particle size smaller than 15um(produced by a special separation process) has a high reactivity. But 45 um size, being easily available is still mostly used.

[D] Due to its lower density and higher volume per unit mass, FA is a more efficient HIGHER VOID FILLER than portland portland cement,i.e permeability decreases.

Having known FA role and benefits, we can improve its quality as:

[I] ReduceUnburned Carbon Content: UBC frequently represents a critical parameter for the modern,high productivity generation of boilers, characterised by smaller dimensions and consequent reduced fuel residence time.Introduction of low-NOx burner also produced an UBC increase, while continuous variations in the quality of the coal provisioned, depending on economics of international market, force power plant conductors to set mills, burners, and electrostatic precipitators on averaged operational configurations among those optimal for each coal type. A careful power plant operation allows to reduce LOI variations.

[II] PARTIAL RECIRCULATION into boilers is a simpler alternatives for FA quality improvement.

[III] MICRONIZATION of FA by mills or air-classification cyclones results, in fact, in a great enhancement of its pozzolanic and filler capacity in cementitious mixtures.

[IV] Integrated fly ash beneficiation* treatments:FA with typical LOI values up to 8 % can be beneficiated to less than 5% by means of its selection and partial recirculation. If higher carbon content could be expected,it will be necessary to adopt further solutions.

*FA post-treatment beneficiation processes for carbon removal have the disadvantage totreat the entire ash production, with consequent high costs of investment and exercise.

REFERENCE:

PRODUCTION OF HIGH VALUE

COAL FLY ASH

Giulio Belz, Pompilio Caramuscio

ENEL Produzione Ricerca – Italy.

Reduction of Carbon, Chlorine and Mercury can be accomplished using mechanical means through either static separation or desorption.  If the resulting material also has a high Calcium  / Lime content  with corresponding low Alkali content then they could be utilized in concrete as either a C-Ash or an F-Ash.  Other alternatives include use as a raw material in a cement kiln after a full review of the other chemical oxides to determine interactions within the kiln.

thanks Manohar for a comprehensive answer. Actually in India there are companies which have started manufacturing FA bricks and they are very good alternative both from cost and strength point of view.

May also be worth grinding the flyash to a coarser surface area and smaller particle size while injecting lime and/or a slag into the milling operation.  We are working on this type of a project now if you would like to discuss further please drop me a note.

It depends on the characteristics of these fly ashes. If it fails the standard because of the physical property, you might want to reduce the size by grinding technique. If it is because of chemical property of fly ash, it requires activation process.  For class F fly ash, you can use alkali activation process but it yields another geopolymer product. If you want to process it to be used as cement replacement, it requires another additive to activate fly ash.  We are using synthetic fly ash which has the homogeneous property to study its leaching process.

alternative use of low quality fly ash to produce brick is recommended. details are available in my paper entitled "Prefabricated building elements based on FGD gypsum and ashes….". See in my research gate publications.

Thank you very much for your answers. The main problem with this topic is that literature is actually scarce and scattered in journals/sources. I will have a deeper read on all of your answers and come back with you if I need further reference.

Thank you very much again and hopefully I'll be able to help you back. 

Julio

By converting into to activated processed fly ash by using start of the art production process