The concrete industry places a heavy demand on primary resources. For example, it is estimated that 165 million tonnes of aggregate are used for concrete each year. This is considered unsustainable due to environmental impact and resource depletion. The UK Government has recognised this and introduced an aggregate levy to encourage the use of recycled materials and industrial by-products. For practical information on Standards, mix designs and specifications plus more detail on properties, a series of parallel technology application documents are available.

This research information digest series also provides technical information on:

* recycled concrete aggregate

* recycled glass

* conditioned fly ash

* fly ash

* granulated rubber.

These digests have been written by the Concrete Technology Unit, University of Dundee and funded by the Department of Trade and Industry ‘Partners in Innovation’ scheme, the RMC Environmental Fund and the following project partners:

* Aggregate Industries

* Amec Civil Engineering

* Ballast Phoenix

* British Cement Association

* Castle Cement Ltd

* Concrete Society

* Edmund Nuttall Ltd

* Halcrow Group

* Quarry Products Association

* RMC Materials Ltd

* RWE Innogy



This research information digest provides information based on worldwide research, development and practice that has shown that incinerator bottom ash aggregate (IBAA) can be used as a coarse and fine aggregate in low-strength concrete, and that incinerator fly ash (IFA) can be used as a pozzolanic activating addition.

Key messages

Incinerator ashes are the byproducts of energy-from-waste recovery. They are available in two forms:

* IBAA is a granular material that can be used as a partial replacement of natural aggregate (normally up to a maximum of 25% by mass) in new low-strength concrete.

* IFA is a powder-like material that can be used to promote the reaction between fly ash and Portland cement.

Environmental impact

600,000 tonnes of IBAA and 70,000 tonnes of IFA created from incineration of municipal solid waste are produced in the UK each year. The vast majority of this is currently sent to landfill.

While their use in concrete will have only a small impact on the 165 million tonnes of natural aggregate and 15 million tonnes of Portland cement used each year, they can be used to prevent highervalue materials being used inappropriately in low-value applications. Use of incinerator ashes in concrete will contribute towards sustainable development by:

* using a material that would otherwise be sent to landfill

* reducing overall greenhouse emissions

* reducing natural aggregate usage

* potential reduction in transport impacts.


Incinerator ashes, as defined in this Research Information Digest, are the byproduct of incineration of municipal solid waste during energy-from-waste recovery. Incinerator bottom ash aggregate (IBAA) is comprised of non-combustible solid waste constituents and can be graded for use as an alternative to natural aggregate and used in the same way. At present, only half of the 600,000 tonnes of IBAA produced each year are used as aggregates for roadbase, asphalt and concrete blocks.

Incinerator fly ash (IFA) is a fine powder captured in air pollution control apparatus. It has pozzolanic activating properties, and can be used as an addition to promote reactions between fly ash (to BS EN 450)(1) and Portland cement.

Due to high chloride contents, incinerator ashes are unsuitable for use in reinforced or prestressed concrete.


Origin of the material

IBAA comprises non-combustible clinker, stone, masonry and glass that has been screened and graded. Ferrous and non-ferrous metals are recovered from IBAA for recycling. Lightweight unburned material is removed. A period of stockpiling prior to use is beneficial to assist formation of stable compounds.

IFA is a grey (sometimes white) powder of similar particle size to Portland cement. Physical and chemical properties are dependent upon the incinerator type and the method of air pollution control. Its use as an addition in concrete is therefore plant dependent.

Properties and characteristics

Typical properties of IBAA are given in Table 1. It is a relatively lightweight aggregate when compared with natural sands and gravels, and has a higher porosity and water absorption.

Chloride content

The chloride content of incinerator ashes is high and the limit on chloride content of concrete of 0.4% by mass of cement for use in structural concrete is unlikely to be met. Therefore, incinerator ashes should not be used in reinforced or prestressed concrete(2).

Sulfate content

Sulfates within IBAA are present as relatively insoluble compounds e.g. gypsum and ettringite. IBAA with acidsoluble sulfate content less than 1.0% (the maximum limit for recycled aggregate in BS 8500-2)(3) has proved to be suitable for use in concrete.

Environmental considerations

Dioxin/furan levels in IBAA are typically less than 10ng/kg and well below the UK Government maximum permissible dioxin level of 50ng/kg(2). Levels of dioxin/furan in IFA vary from plant to plant and can be above the Government limit of 50ng/kg. Regular checks are required on batches of IFA if the material is to be used in concrete.

Handling, treatment and storage

Storage and exposure to natural elements generally improves the quality of IBAA since soluble elements may be washed away (subject to appropriate drainage), and processes such as swelling, hydration, carbonation and oxidation improve the chemical integrity and structural durability(4). Furthermore, IBAA is best added to the mixer in a saturated condition, due to its high water absorption. It is therefore favourable to allow absorption of water(5). IBAA can otherwise be handled in the same way as natural aggregates.

IFA may be used in the same way as other dry powders (e.g. fly ash to BS EN 450). However, environmental regulations may restrict the movement, handling, and storage of IFA as a consequence of the presence of heavy metals. Special handling equipment may be needed to produce concrete due to the aggressiveness of IFA towards steel when moist(6).

Compositional variability

The composition of IBAA is dependant upon the composition of the material incinerated. While this may vary on a day-to-day basis, it has been observed that the variability is relatively small. Given that most IBAA is stockpiled prior to use, there is unlikely to be significant variations in composition.

While the composition of IFA depends on the incinerator and the method of air pollution control, there is relatively little variation in composition from a single plant.


The European Standard for aggregates in concrete, BS EN 12620


The use of incinerator ashes will affect the properties of concrete. To obtain the maximum benefit from use of incinerator ashes the mix design should be suitably modified to account for any changes. There are mix designs that acknowledge this(2).

Engineering properties

Influence on cube strength

Because of the low density and friable nature of some IBAA particles, 28-day cube strength of IBAA concrete will be approximately 25^5% less than that of an equivalent natural aggregate (NA) concrete for the same w/c ratio, with the greatest differences in strength being found for higher strength concretes(2). IBAA concretes can be proportioned for a given design strength by using a lower w/c ratio than that of an otherwise comparable NA concrete. A comparison of engineering and durability properties of IBAA and NA concrete at equal strength is given in Table 2.

A mixture comprising IFA at 10% by mass in combination with fly ash (10-25% by mass) and Portland cement (65-80% by mass) will have equivalent 28-day cube strength to Portland cement (CEM I) at the same w/c ratio(2).

Tensile strength

IBAA and natural aggregate concrete designed for the same design strength through a reduction in w/c ratio will have equivalent tensile strength. IFA has no effect on tensile strength at a given cube strength.

Modulus of elasticity

IBAA concrete has a lower modulus of elasticity than natural aggregate at equal cube strength. However, concretes with up to 25% IBAA will tend to fall within the typical range of values expected for the given strength. IFA in equal strength concrete has no effect on elastic modulus.

Drying shrinkage

IBAA is less effective than natural aggregate in restraining drying shrinkage. On average, for every 1 % replacement by mass of natural aggregate, there will be a 1% increase in shrinkage. For recommended IBAA contents (

Durability characteristics

Initial surface absorption

Results indicate that use of IBAA will increase surface absorption. However, with up to 25% IBAA values will tend to fall within the range expected of normal concrete(2).

Sulfate resistance

Overall, results suggest little effect of IBAA content on sulfate resistance(2).

Freeze/thaw resistance

Results indicate that there is negligible effect of IBAA on freeze/thaw resistance of concrete(8).

Alkali-silica reaction

Despite the potentially high glass content in IBAA, tests have shown that IBAA is benign in terms of alkali-silica reaction(8).


Incinerator ashes can contain significant proportions of heavy metals. Tests have shown that when bound in cement-based products leaching of these elements is insignificant(9).

Table 3 gives the service concentration of species detected leaching from cement-bound IBAA (cube strength


There are 14 municipal solid waste incinerators in operation in the UK, producing 600,000 tonnes of IBAA and 70,000 tonnes of IFA per year. Incinerator ashes are generally managed by the waste-to-energy facilitator who should be contacted for acquisition of IFA. IBAA in a processed form, suitable for use in concrete, is usually made available from specialist aggregate suppliers.


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

2. DHIR, R., DYER, T., HALLIDAY, J. and PAINE, K. Valueadded recycling of incinerator ashes. Concrete Technology Unit, CTU/1802, 2002,267pp.

3. BRITISH STANDARDS INSTITUTION. BS 8500-2, Concrete – Complementary British Standard to BS EN 206-1-Part 2: Specification for constituent materials and concrete. 2002.

4. COVENTRY, S., WOOLVERIDGE, C. and HILLIER, S. The reclaimed and recycled construction materials handbook. CIRIA C513,1999.

5. HALLIDAY, J. and DHIR, R. Full-scale trials using incinerator bottom ash in cement based products. Sustainable Concrete Construction, (Eds. DHIR, R. ef al), Thomas Telford, 2002, pp.429-438.

6. HEMMINGS, R. and CORNELIUS, B. Evaluation of G VRD municipal incinerator ash as a supplementary cementing material in concrete. AMEC Report No. VA06294, March 2004.

7. BRITISH STANDARDS INSTITUTION. BS EN 12620: Aggregates for concrete. 2002.

8. BERG, E. and NEAL, J. Municipal solid waste bottom ash as Portland cement concrete ingredient. Journal of Materials in Civil Engineering, ASCE, Vol. 10, No. 3, 1998. pp.168-173.

9. PAINE, K., DHIR, R. and DORAN, V. Incinerator bottom ash: Engineering and environmental properties as a cement bound material. Internationa/Journal of Pavement Engineering, Vol. 3, No.1, 2002. pp.43-52.

10. SCHREURS, J., van der SLOOT, H. and HENDRICKS, Ch. Verification of laboratory-field leaching behaviour of coal fly ash and MSWI bottom ash as a road base material. Waste Management, 20,2000. pp.193-201.

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