Frequently Asked Questions

What is fly ash and Furnace Bottom Ash?

The ash carried out with the flue gases in a pulverised coal-fired power station is called ‘fly ash’.

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 traditionally dumped on the power station’s landfill site. However, over the last 30 years Ash Resources has pioneered the development of fly ash in a wide variety of construction and mining applications:

  • Sold dry for use in concrete or moistened (called conditioning) for less demanding applications such as fill, grouts or as a fine aggregate replacement.
  • Processed in equipment called classifiers which can separate the fly ash particles into specific size ranges. Classified fly ash is a more highly specified material that is marketed under Ash Resources’ brand names: DuraPozz®, SuperPozz®, DuraPozz®Pro™ and SuperPozz®Pro™.

Furnace Bottom Ash (FBA) is an agglomeration of larger particles that are too heavy to be removed in the flue gases and fall to the bottom of the furnace. FBA is normally extracted wet and graded. It is an excellent lightweight aggregate for making concrete bricks and blocks.

When fly ash is referred to as PFA, what does this mean?

PFA stands for Pulverised Fuel Ash and is a misnomer for fly ash since it refers to all the ash from the furnace. Pulverised fuel refers to coal that has been crushed and ground to a fine talcum-like powder in order to increase the efficiency of combustion in power station furnaces. The carbon in the pulverised coal burns very quickly (typically within 2-4 seconds) and is completely combusted at temperatures of over 1250oC.

To be correct, PFA is the collective expression for fly ash together with FBA.

What are some of the uses of fly ash?

Fly ash is referred to as a ‘cementitious’ material because it has properties that are similar to cement. This property has led to it becoming an important material in the cement and concrete industries where it is now used in the majority of site-mixed, ready-mixed and precast concretes. Incorporating fly ash in a concrete mix can be achieved either  by using factory pre-blended cements, in compliance with SANS 50450-1 common cements, or by the concrete producer blending it in the concrete mix.

Other uses for fly ash include:

  • As a raw material in the production of cement clinker because of its alumina content (Al2O3 > 30%)
  • In the precast market, conditioned fly ash is used successfully as a fine aggregate replacement (where sand of suitable quality is unavailable)
  • Road base materials, flowable fills (needs clarification?), waste stabilisation and manufactured coarse aggregate
  • Ultra-fine fraction fly ash is increasingly used as an inert mineral filler for composite plastics and rubber products

How much fly ash can be put into a concrete mix?

Large proportions of fly ash can be used in certain types of concrete. For example, having 60-70% of cementitious content as fly ash is commonplace for mass concrete in gravity dam walls.

The recognised limits to how much fly ash can be incorporated in the binder content are covered in SANS 50197-1 for factory made cements and for mixer blended concretes SANS 10100: Part 2 Materials and SANS 1200G, which provides guidelines on the use of fly ash in concrete.

What is the environmental impact of using fly ash in concrete?

Fly ash has three main beneficial effects:

  • Reduces landfill sites by transforming a by-product from coal-fired power stations into an important building material
  • It is a valuable source of alumina and silica in the cement manufacturing process. It is also an ingredient of factory blended cements.
  • Using fly ash to manufacture or blend cement burns less limestone in the cement kilns – the major source of harmful CO2 emissions. The total energy demand for producing cement is also reduced, resulting in less greenhouse gas emissions and consumption of hydrocarbon fuels.

What specifications do Ash Resources’ fly ash products conform to?

Ash Resources’ products conform to the relevant local standards and offer our customers ‘peace of mind’ in terms of consistent quality:

Our classified fly ash products (DuraPozz®, DuraPozz®Pro™ and SuperPozz®) comply with:

  • SANS 50450 -1. This covers the definitions, specifications and conformity criteria governing the use of fly ash in concrete. Under this new specification, Category “S” denotes a classified fly ash with a maximum 12% retained on a 45µm sieve.
  • ASTM C618: American Standard: DuraPozz® complies with most requirements in ASTM C618, Class F
  • SANS 50197-1 – Common Cement Specification: Composition, specifications and conformity criteria for constituents of common cements. When included as a constituent in common cements, the use of siliceous fly ash is denoted with a “V”.

Unclassified fly ash (PozzFilI®, PozzCem®) complies with:

  • SANS 50197-1 – Common Cement Specification: Composition, specifications and conformity criteria for constituents of common cements in terms of reactive silicates, free lime and loss on ignition (LOI) values.

Ultra-fine fly ash (UFFA) (SuperPozz®) complies with:

  • Category S in SANS 50450 – Part 1.

What is the difference in chemical composition between fly ash and Portland Cement?

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, as processed and sold by Ash Resources, is high in reactive silicate glass while PC has none.

This means that the two materials complement each other: the hydration of Portland cement releases calcium hydroxide, which reacts with the silicates in the fly ash to form strong, durable cementing compounds.

Does fly ash improve the workability of concrete?

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. The spherically shaped particles of fly ash make a concrete mix workable, enabling a typical water reduction of between 6% and 12%  (seems high? Should it say 2-12%? Or ‘up to 12%’?) when using classified fly ash. (is there nil benefit when using PozzFill?)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 with the same water content as a CEM I only concrete.
  • Fly ash in a mix allows coarse, clean sands to be used while still achieving good workability
  • Other benefits are that segregation and bleeding are reduced because of the increased cohesiveness of a fly ash mix. This enhances form finish and sharpness of detail.

How does fly ash reduce the heat of hydration in concrete?

The exothermic hydration reaction that rapidly raises the temperature of freshly placed concrete is not a problem in normal structures that allow the heat energy to dissipate easily.  However, in large structures the heat builds up, causing high temperatures and expansion of the concrete while it is hardening. The temperature differential between the interior of a mass pour and the cooling outer layers can generate sufficient tensile stresses to cause cracking, leading to reduced concrete integrity and durability.

Partially replacing Portland cement with fly ash reduces the rate of heat generation and resulting peak temperatures due to the pozzolanic reaction occurring over an extended period of time.

What is the impact of fly ash on the strength of concrete?

Fine fraction fly ash (i.e. classified fly ash with typically 90% of its particles passing a 45µm sieve) fills the voids among the cement and aggregate particles. Being spherical in shape, fly ash almost acts as ball bearings maintaining the plasticity of concrete and reduces the water requirement of the mix for a given workability.

The pozzolanic reaction of fly ash with calcium hydroxide produced during cement hydration forms stable calcium silicate hydrates (CSH), transforming the weak crystalline structure to a dense gel-like matrix. By replacing the weak calcium hydroxide, these hydrates increase the strength and reduce the permeability of hardened concrete.

The pozzolanic reaction takes place relatively slowly at normal temperatures, resulting in significant strength development at later ages when compared to Portland cement at 112 days and beyond. In this way, fly ash has a positive impact on the longer term strength of concrete, provided that good site and curing practices are followed.

How does fly ash decrease the permeability of concrete?

Fly ash increases the cementitious compounds in a mix, minimises water demand and the small 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.

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