Avoidance of waste: Beneficial use of industrial by-products as constituents of concrete
Waste and industrial by-products should be considered as a potentially valuable resource merely awaiting appropriate treatment and application. Concrete plays an important role in the beneficial use of these materials in construction. Although some of these materials can be beneficially incorporated in concrete, both as part of the cementitious binder phase or as aggregates, it is important to realise that not all waste materials are suitable for such use.
This information sheet examines how these alternative materials become accepted materials/products and have their suitability for use in concrete established. It also explores the range of materials that have been established and identifies the limitations placed on their use. Incorporating wastes or other alternative materials should not undermine the long-term durability of concrete or its fresh or hardened engineering properties. There should be no increased environmental risk resulting from the use of these materials.
Establishing suitability for use in concrete
In common with other ‘new’ materials, waste or industrial by-products cannot be used as constituents of concrete conforming to British/European Standards (currently BS 5328: 1997 Concrete(1) and BS 8500: 2002 Concrete(2), the complementary BS to BS EN 206-1: 2000 Concrete – Part 1: Specification, performance, production and conformity(3). until their suitability for use has been established by the appropriate British Standards Institution committee.
‘Establishing suitability for use’ goes beyond the necessary development of a technical specification for the material such as a British British/European Standard, British Board of Agrement Certificate, or equivalent UK Technical Approval, and European Technical Approvals (ETA). It also involves a semi-formalised assessment by the BSI committee (e.g.B517/1 Concrete production and testing) of the technical specification, the evidence that underpins it and any related research findings, whether laboratory, exposure site trial or prior use elsewhere. However, the latter rarely proves to be definitive, given the historical concern that practice overseas may differ significantly from that in the UK.
Suitability for use in specific exposure classes is generally established by comparing the performance of concrete containing the ‘new’ material with performance obtained using those already established. Such evidence of durability, if not available, would have to be acquired. Only after the committee has accepted the evidence as credible and demonstrating the required performance will a new material ‘gain entry’ to these national standards. It is noted that innovation is always ahead of standardisation and so some non-conforming products may well be used with discretion.
Use of waste in concrete
There is considerable potential for use of specific wastes and by-products in each concrete component, including admixtures and mixing water. However, the major use, by volume, has been in cementitious binders or aggregates after the ‘beneficiation’, improving properties and reducing variability, of various inorganic waste streams.
There is a long history of using by-products that have undergone physical or chemical beneficiation as part of the cementitious phase of concrete. The material can be interground/blended with Portland cement (or clinker) in a factory or at a purpose-built blending facility to produce a factory-made cement. Alternatively, it can be regarded with the other components of the fresh concrete as an ‘admixture’ during mixing to produce a ‘mixer combination’. The latter approach is usually adopted in the UK, although factory-made cements are available. Commonly used alternative materials are described below.
Pulverised-fuel ash (pfa)
Pfa is a by-product of coal-fired power stations. The properties required for its use in concrete are specified in BS EN 450: 1995 Fly ash for concrete. Definitions, requirements and quality control(4), where it is known as ‘fly ash’ and also in BS 3892, where it is called pfa. BS 8500(2) defines the exposure classes in which cements and combinations containing BS EN 450 fly ash can be used.
Pfa to BS 3892-1: 1997 Pulverised-fuel ash – Specification for pulverised-fuel ash for use with Portland cement(5) is normally classified to be finer than BS EN 450 fly ash. Pfa has been used for over 20 years as a component of cement and for even longer as part of a mixer combination(6). The proportion of pfa is usually 25-35% by mass of the combination. Pfa undergoes a pozzolanic reaction with the lime released during Portland cement hydration to produce additional hydrates. Compared to Portland cement concrete at the same free water/cement ratio(6), concrete containing pfa can have a lower heat of hydration and enhanced resistance to sulfate attack or chloride ingress by diffusion.
Ground granulated blastfurnace slag (ggbs)
Blastfurnace slag, a by-product of iron manufacture, can be granulated by water quenching and subsequently finely ground to produce a material that is cementitious when activated by the alkalis produced during Portland cement hydration. Ggbs is usually used at levels of 40-70% by mass of the the total cement/combination, and is covered by BS 6699: 1986 Specification for ground granulated blastfurnace slag for use with Portland cement(7). Compared to Portland cement concrete at the same free water/cement ratio(6), concrete containing high levels of ggbs in the cement or combination is characterised by low heat of hydration and enhanced resistance to sulfates and chloride ingress.
Silica fume (microsilica) is a very fine, silica-rich powder, produced during the manufacture of silicon metal and ferrosilicon. It is pozzolanic, like pfa, reacting with the lime produced during Portland cement hydration. Due to its high silica content and extreme fineness, it is much more reactive. Typically, it is used at around 5-10% by mass of total cement or combination.
A European standard defining the properties required for its use in concrete is currently under preparation. In the UK, this material has a more limited history of use in concrete than either pfa or ggbs, but in Scandinavia, Canada, France and USA the material has proved particularly effective for the production of very high strength or very impermeable and durable concretes(8). British Board of Agrement Certificate 85/1568 covers the properties of silica fume available from the major UK supplier.
In the UK, limestone fines (a by-product of quarrying) are interground/blended with Portland cement clinker at levels up to 20% by mass of the total cement to produce Portland-limestone cement. Higher proportions are used elsewhere in Europe. Portland-limestone cement is covered by the British/European Standard for common cements, BS EN 197-1: 2000 Cement composition, specifications and conformity criteria for common cements(9). Limestone fines can also be added to the mixer alongside Portland cement to form a combination. These are covered by BS 7979: 2001 Specification for limestone fines for use with Portland cement(10). Evidence indicates that limestone fines make a positive contribution to the strength of the binder, although the physical/chemical mechanism is uncertain.
Burnt oil shale
Although uncommon in the UK, burnt oil shale is used elsewhere in Europe, particularly in Germany, as a constituent of Portland-burnt shale cement, as standardised in BS EN 197-1(9). Oil shale is ignited in a special kiln at about 800[degrees]C to produce a material that, when ground, has both hydraulic (dicalcium silicate and monocalcium aluminate) and pozzolanic (silica) components and properties.
While the by-products most commonly used in the cementitious phase of concrete have been described above, mention should be made of other materials that may, in time, be used as part of the cement in concrete. Ash from burning of rice husks (although unlikely to be of importance in the UK) has been found to be pozzolanic and can be used with benefit in a cement or combination and incinerator bottom ash may also be beneficial.
Research underway at a number of UK universities is indicating that glass cullet, when finely ground, may have pozzolanic potential. This may prove to be a way of disposing of the waste coloured glass not used for making new glass.
Wastes can also be incorporated in concrete in the form of aggregates. At its simplest, this involves the use of suitably sized recycled construction and demolition debris as coarse aggregate, plus natural fines. Such aggregate can be divided into two main groups(3):
* recycled concrete aggregate derived from crushed concrete and generally containing less than 5% of other materials, such as brick and asphalt, with restrictions on other material
* recycled aggregate derived from mixed-source demolition debris, often containing a high proportion of masonry.
BS 8500(2) permits the use of these materials to replace some of the natural coarse aggregate in concrete, although restricting the level of replacement and the exposure conditions for the concrete. Recycled concrete aggregate can produce concrete with properties very similar to concrete containing all natural aggregate, although cement contents are generally slightly higher.
Many secondary materials can also be used to manufacture aggregates for use in concrete. Pfa has been used for many years to produce a sintered pfa lightweight aggregate, and coarse-grain furnace-bottom ash has also been used as a concrete aggregate. Blastfurnace slag is used to produce aggregates(11), either as crushed air cooled slag (BS 1047: 1983 Specification for air-cooled blastfurnace slag aggregate for use in construction(12)) or as foamed or pelletised lightweight aggregates. In addition, recent research has demonstrated the potential for the use of incinerator bottom ash (mixed with a proportion of clay) as raw material for making lightweight aggregates for concrete(13). Likewise, rubber crumb from recycling of tyres has been successfully incorporated in the fine aggregate fraction of concrete.
There is a long history of using processed by-products from the timberand paper-making industries as admixtures. For example, lignosulfonates are active components of some water-reducing admixtures and are processed by paper-making products from wood pulp. Vinsol resins, neutralised wood resins, are the active components of many air-entraining admixtures, and admixture formulators are constantly alert to the possibilities that industrial by-products can bring. Most concrete admixtures are produced in conformity to BS EN 934-2: 2001 Admixtures for concrete, mortar and grout. Concrete admixtures. Definitions, requirements, conformity, marking and labelling(14).
Minimising waste in concrete production
Producers of ready-mixed concrete have almost eliminated waste at their plants. Aggregates can be reclaimed from returned concrete and used in further concrete production. Specialised admixtures may also be used to delay cement hydration, enabling the returned concrete to be used the following day.
Wash water and run-off water from aggregate stockpiles is also collected in settlement tanks for use as concrete mixing water, thus reducing the demand on the mains water supply and the risk of watercourse contamination. The solids may even be reused as fine aggregates for non-critical concrete.
Efforts to seek beneficial uses for waste materials continue throughout the construction industry in general, and concrete construction in particular. However, the technical and environmental justification for their incorporation must always be examined stringently and pragmatically before suitability is established via the recognised procedures.
1 BRITISH STANDARDS INSTITUTION. BS 5328: 1997 Concrete
2 BRITISH STANDARDS INSTITUTION. BS 8500: 2002 Concrete
3 BRITISH STANDARDS INSTITUTION. BS EN 206-1: 2000 Concrete – Part 1: Specification, performance, production and conformity, 74pp.
4 BRITISH STANDARDS INSTITUTION. BS EN 450: 1995 Fly ash for concrete. Definitions, requirements and quality control, 20pp.
5 BRITISH STANDARDS INSTITUTION. BS 3892-1: 1997 Pulverised-fuel ash – Specification for pulverised-fuel ash for use with Portland cement, 22pp.
6 THE CONCRETE SOCIETY. Technical Report 40: The use of ggbs and pfa in concrete, The Society, Slough, 1991.
7 BRITISH STANDARDS INSTITUTION. BS 6699: 1986 Specification for ground granulated blastfurnace slag for use with Portland cement, 20pp.
8 THE CONCRETE SOCIETY. Technical Report 41: Microsilica in concrete, The Society, Slough, 1993.
9 BRITISH STANDARDS INSTITUTION. BS EN 197-1: 2000 Cement composition, specifications and conformity criteria for common cements, 50pp.
10 BRITISH STANDARDS INSTITUTION. BS 7979: 2001 Specification for limestone fines for use with Portland cement, 20pp.
11 CLARKE, J. (ed.) Structural lightweight aggregate concrete, Blackie, 1993.
12 BRITISH STANDARDS INSTITUTION. BS 1047: 1983 Specification for air-cooled blast furnace slag aggregate for use in construction, 16pp.
13 NEWMAN, J. and OWENS, P. Recycling London’s waste – a blueprint for manufacturing construction products in Institute of Concrete Technology Yearbook 2001-02, ICT, Crowthorne, pp.95-106.
14 BRITISH STANDARDS INSTITUTION. BS EN 934-2: 2001 Admixtures for concrete, mortar and grout. Concrete admixtures. Definitions, requirements, conformity, marking and labelling. 28pp.
Copyright The Concrete Society May 2003
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