RESEARCH INFORMATION DIGEST 3: Fly ash

RESEARCH INFORMATION DIGEST 3: Fly ash

This technology digest provides information based on worldwide research, development and practice that has shown that fly ash is an important cement constituent and pozzolanic addition for use in concrete.

Key messages

Fly ash, or pulverised-fuel ash (pfa), is used as a constituent of blended cement or in combination with Portland cement. As a blend or a combination the normal proportion of fly ash is 20-35% of the total cement. Compared with Portland cement concrete at equal water/cement ratio, the use of fly ash in concrete will give:

* lower heat of hydration

* enhanced resistance to sulfate attack

* enhanced resistance to chloride ingress

* more effective mitigation of damaging alkali-silica reaction

* lower compressive strength

* less resistance to carbonation.

The mix design of concrete containing fly ash needs to be modified to take account of changes in the concrete performance that results. Fly ash has a long history of successful use in concrete and requirements for properties for its use in concrete are given in BS EN 450(1).

Environmental impact

Fly ash is a by-product of coalfired power stations. Approximately 5.5 million tonnes are produced per annum, of which only 50% currently finds use. This compares with the 15 million tonnes of Portland cement produced each year. Increased use of fly ash as a cement component in concrete will contribute towards policies of sustainable development by:

* reducing overall greenhouse gas emissions

* reducing natural aggregate usage

* reducing Portland cement consumption

* improving some aspects of concrete durability and prolonging service life of structures.

1. INTRODUCTION AND USES IN CONCRETE

Fly ash, also called pulverised-fuel ash, is used by: (i) blending with Portland cement in the cement factory or (ii) adding as a dry constituent (Type II addition) at the concrete mixer. It is normally added in proportions of between 20% and 35% by mass of the combination (fly ash + Portland cement), although up to 55% is allowed by BS EN 197-1(2). Fly ash has a long history of use in concrete in the UK, particularly relatively fine fly ash (as specified in BS 3892-1 )(3). The requirements for fly ash are given in Table 1.

2. MANUFACTURE AND PROCESSING

Origin of the material

Fly ash is a by-product of coal-fired power stations.

Properties and characteristics

Fly ash is grey in colour and the particular shade is dependent upon the quantity of unbumed carbon as well as the iron content. Fly ash particles are spherical and are finer than those of Portland cement. Fly ash has pozzolanic properties, i.e. it reacts with the lime released during Portland cement hydration to produce calcium-silicate-hydrates and calcium-aluminate-hydrates. The reactivity of fly ash is dependent upon its particle size, i.e. finer fly ash as specified in BS 3892-1 is more reactive than coarser fly ash allowed within BS EN 450(1).

Handling and storage

Fly ash is commonly supplied in a dry form that can be handled and added to concrete at the mixer in the same way as Portland cement.

3. STANDARDS AND SPECIFICATIONS

The European Standard BS EN 450 describes the requirements for fly ash for use as a constituent of concrete. BS EN 450 covers only siliceous fly ash: fly ashes with high calcium levels are excluded. Fly ashes produced from burning materials other than coal are also not included. It should be noted that fly ashes conforming to BS 3892-1(3) fall within the scope of BS EN450(1).

The use of fly ash as a cement constituent in BS EN 197-1(2) will normally fall within the designation of Portland fly ash cement (CEM II A-V and CEM II-V) or pozzolanic cement (CEM IV/B) (see Table 2).

4. PROPERTIES OF CONCRETE

The use of fly ash may favourably affect the properties of concrete. To obtain the maximum benefit from the use of fly ash, the mix design should be suitably modified to account for any changes. There are a number of mix design methods that acknowledge this(4,5).

The effects of fly ash on the properties of concrete are well documented. In general, the use of fly ash in concrete: (i) improves consistence (workability), (ii) reduces heat of hydration, (iii) produces a less permeable and more durable matrix and (iv) prolongs strength gain beyond 28 days(4).

Fresh properties

Fly ash inclusion in concrete reduces water demand, improves consistence and reduces bleeding and segregation. The benefits associated with these effects enable concrete to be designed with reduced water contents, compared with Portland cement (CEM I) concrete of the same consistence. The beneficial effects of improved consistence may reduce as the coarseness of the fly ash increases.

Engineering properties

When designed for a given consistence and 28-day strength, fly ash concrete and CEMI concrete have similar rates of strength gain up to 28 days. However, thereafter, fly ash concrete gradually develops higher strength.

For a given 28-day strength, fly ash concrete has similar elastic modulus, creep coefficient and drying shrinkage (see Table 3) to CEMI concrete, regardless of fly ash fineness.

Durability characteristics

A comparison of durability characteristics of CEMI concrete and fly ash concrete is given in Table 4.

Chloride ingress

Ten-year tests show that at equal w/c ratio and strength class, concrete containing fly ash provides significantly better resistance than CEMI concrete. There is no significant effect of fly ash fineness or loss on ignition (LOI) on the resistance to chloride ingress(4).

Carbonation resistance

Long-term tests (up to 10 years)(4,8) have shown that the depth of carbonation of concrete containing fly ash is higher than that of CEMI concrete of equal consistence and strength (see Table 4). For practical purposes, fly ash fineness and LOI have no influence on carbonation rates and fly ash concrete can be used on all carbonation exposure at up to 35% by mass.

Sulfate resistance

Long-term tests (up to 10 years) show that concrete (at equal strength) containing fly ash has excellent resistance to the ettringite form of sulfate attack(4,9). BS 8500-1(10) recommends that a minimum of 25% fly ash by mass should be used where fly ash is used as a cement constituent or in combination with a CEMI cement for sulfate resistance. Guidance on the use of fly ash to resist the thaumasite form of sulfate attack is given in BRE Special Digest 1(11) and BS 8500-1.

Freeze/thaw resistance

Fly ash has no effect on the performance of concrete specimens exposed to freeze/thaw conditions. As for all concretes, a minimum air content of 3.5% should be used to obtain satisfactory performance in aggressive freeze/thaw environments. The dosage of air-entraining admixture required to attain the minimum air content may vary with the LOI of fly ash(12).

Abrasion

Research indicates that concrete (at equal strength) containing fly ash has equivalent resistance to abrasion as CEM I concrete(4).

Leaching

Research has shown that concrete containing fly ash poses no environmental concern with respect to leaching.

5. AVAILABILITY

Coal-powered thermal power stations are expected to continue for the foreseeable future. Therefore, fly ash will continue to be produced at a rate of approximately 7.5 million tonnes per annum in the UK. The geographical distribution of power stations enables fly ash to be supplied to all major cities and industrial centres.

There is an increasing emphasis placed by Government and the market on sustainable development. These objectives are met by the utilisation of fly ash, which has a proven track record of use in the construction industry. Further information regarding availability can be obtained from the United Kingdom Quality Ash Association (UKQAA), Regent House, Bath Avenue, Wolverhampton, WVl 4EG.Tel: +44(0)1902576586;Fax: 444(0)1902 576 596; http://www.ukqaa.org.uk

References:

1. BRITISH STANDARDS INSTITUTION. BS EN 450: Fly ash for concrete – Definitions, requirements and quality control. 1995.

2. BRITISH STANDARDS INSTITUTION. BS EN197-1: Cement – Part 1: Composition, specifications and conformity criteria for common cements. 2000.

3. BRITISH STANDARDS INSTITUTION. BS 3892: Part 1: Pulverised-fuel ash – Specification for pulverised-fuel ash for use with Portland cement. 1997.

4. DHIR, R., MCCARTHY, M. and MAGEE, B. Impact of BS EN 450 pfa on concrete construction in the UK. Construction and Building Materials, Vol.12, No. 1, 1998, pp.59-74.

5. BUILDING RESEARCH ESTABLISHMENT. Design of normal concrete mixes. Watford, 1997.

6. BRITISH STANDARDS INSTITUTION. BS1881-121: Testing concrete – Part 121: Method for determination of static modulus of elasticity in compression. 1983.

7. AMERICAN SOCIETY FOR TESTING AND MATERIALS. C666/C666M: Standard test method for resistance of concrete to rapid freezing and thawing. 2003.

8. MATHEWS, J. Performance of pfa concrete in aggressive conditions, 4: Carbonation. Building Research Establishment, Watford, 1995,28pp.

9. MATHEWS, J. Performance of pfa concrete in aggressive conditions, 1: Sulfate resistance. Building Research Establishment, Watford, 1995,38pp.

10. BRITISH STANDARDS INSTITUTION. BS 8500-1 : Concrete – Complementary British Standard to BS EN 206-1 – Part 1: Method of specifying and guidance for the specifier. 2002.

11. BUILDING RESEARCH ESTABLISHMENT. Special Digest 1: Concrete in aggressive ground, Part 3: Design guides for common applications. Watford, 2003,28pp.

12. DHIR, R., MCCARTHY, M., LIMBACHIYA, M.etai. Pulverised-fuel ash concrete: air entrapment and freeze/thaw durability, Magazine of Concrete Research, Vol. 51, No.1, 1999, pp.53-64.

Copyright The Concrete Society Jul 2005

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