The ash carried out with the flue gases in a pulverised coal fired power station is called ‘Fly Ash’(FA). To prevent the ash polluting the atmosphere, the flue gases are passed through electrostatic precipitators or bag filters to separate the Fly Ash from the flue gases.
The Fly Ash 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.
Fly Ash 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.
PFA stands for Pulverised Fuel Ash and is a misnomer for Fly Ash. 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 = Fly Ash + bottom ash.
Fly Ash can be used in all site mixed, readymixed or precast concretes. Incorporating Fly Ash 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 Fly Ash to replace natural materials such as aluminates and silicates in the production of Portland Cement, and in the precast market conditioned Fly Ash is successfully used as a fine aggregate replacement. Other applications for Fly Ash 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.
There are no technical reasons why large proportions of Fly Ash cannot be used in certain types of concrete. Having 60-70% of the cementitious content as Fly Ash is, for example, commonplace for mass concrete in gravity dam walls. The recognized limits to how much Fly Ash 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 Fly Ash in concrete. More Fly Ash may be added as Type 1 addition which treats Fly Ash as an inert material.
Fly Ash has three main beneficial effects:
- Using Fly Ash in concrete transforms a waste product from power stations into a valuable by-product.
- Cement producers, as mentioned, can use Fly Ash as a valuable source of alumina and silica in the cement making process as well as its use in the manufacture of factory blended cements, Using Fly Ash in these ways reduces the need for the extraction of raw materials in the case of blended cement. Fly Ash reduces the total energy demand of producing cement. Cement process kilns require high temperature normally generated using hydrocarbon fuels. Fly Ash blended cements enable the cement producer to burn less fuel in the manufacturing process and reduce the quantity of harmful CO2 emissions.
- When Fly Ash is incorporated in structural concrete there is a reduction in the potential for leaching. The pozzolanic reaction of the Fly Ash reduces the permeability of the concrete effectively preventing any significant leaching from Fly Ash, the Portland Cement or the aggregates.
Ash Resources believes that by conforming to the respective standards and specification for our products we offer our customers ‘peace of mind ‘ in terms of products consistency and quality.
Classified Fly Ash –i.e. Dura –Pozz ® and DuraPozz®ProTM complies to:
SANS 1491-Part 2 (formerly SABS 1491): Fly Ash – Portland cement extenders.
BS EN 450- Part 1 2005 European Standard Fly Ash for concrete – definition specifications and conformity criteria. This specification is soon to replace BS 3892 – Part 1. Under the new specifications, Category “S” denotes a classified Fly Ash with a maximum 12% retained on a 45um sieve.
ASTM C618: American Standard
DuraPozz® is a Classified Fly Ash in line with this standard.
SANS 50197-1 – Common Cement Specification:
Composition specification conformity criteria for constituent in common cements the use of Fly Ash is denoted with a “V”
Unclassified Fly Ash – i.e. PozzFill complies to:
SANS 50197-1 – Common Cement Specification:
Composition, specifications and conformity criteria for constituent in common cements. In terms of reactive silicate, free lime and loss on ignition (LOI) values, PozzFill® is fully compliant to the standard.
EN 450 – European Standard
Whilst carefully selected by Ash Resources fro the Power Station bunkers, PozzFill® fineness can fll outside the maximum EN 450 value (40%) passing the 45um sieve.
Ultra-fine Fly Ash – Portland cement extenders (new standard Incorporates UFFA)
BS EN 450 – Part 1: 20058 – European Standard
A major difference between Fly Ash and Portland Cement (PC) is that PC is rich in calcium silicates while its level in Fly Ash is low. High quality Fly Ash 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 Fly Ash to form strong, durable cementing compounds.
Yes, it is one of the major reasons why concrete producers and contractors like working with Fly Ash 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 Fly Ash.
Some benefits of this are:
- Lower slump concrete can be placed more easily because of the better plasticity or higher slump concrete can be produced at the same water content as a CEM I only concrete.
- Segregation and bleeding are reduced because of the increased cohesiveness of a Fly Ash mix. This enhances form finish and sharpness of detail.
- The presence of Fly Ash in a mix allows coarse, clean sands to be used while still having good workability.
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 Fly Ash reduces the rate of heat generation due to the pozzolanic reaction with lime occurring over extended period of time.
Fine fraction Fly Ash, 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.
Fly Ash, 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.
Fly Ash therefore has a positive impact on the longer term strength of concrete, provided that good site and curing practices are followed.
Fly Ash 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.
Not only does the lower permeability of a Fly Ash concrete impede penetration by chlorides – normally in the forms of seawater or seahorse mist-but chlorides chemically bind to the Fly Ash 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%.
Fly Ash improves the resistance of concrete to sulphate attack through three main mechanisms:
- The reduced permeability of a Fly Ash concrete minimises penetration of harmful sulphates.
- Fly Ash chemically binds the free lime in cementitious compounds rendering it unavailable.
- Replacing a portion of Portland Cement with Fly Ash reduces the amount of reactive aluminates available for sulphate reaction (expansion).
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 Fly Ash 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 Fly Ash in the mix will usually reduce the reaction sufficiently to protect the concrete and steel reinforcement from these forms of deterioration.
Plasters and mortars made with Fly Ash 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 Fly Ash: A Cement Extender by J. Kruger).
Some benefits of using Fly Ash in mortar and plaster:
- Water reducing properties at a given workability
- Better water retention that Portland cement (PC) mortar.
- Better resistance to the elements that PC mortar
- Lower drying shrinkage
Fly Ash when used in mortars and plasters will therefore have no detrimental effects on the subsequent plastic or hardened properties.