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Metallurgical slags are non-metallic secondaryproducts of the refining of metals from metallicores. Slags derived from the iron and steelindustries are by far the most common typesproduced and are used as aggregates inconstruction.Blastfurnace slag (BF slag) is an industrialby-product produced during the making ofiron. The UKs annual production of about fourmillion tonnes is processed into constructionaggregates, and ground granulated blastfurnaceslag (GGBS), a cementitious binder.Basic oxygen steel slag (BOS) is an industrialby-product of the steel making process, withapproximately one million tonnes producedannually. A different form of steel slag knownas Electric arc furnace (EAF) steel slag is alsoproduced but in lower quantities. Both types ofsteel slag are processed into constructionaggregates in a manner broadly similar to thatused for BF slag.

Supplies of quality controlled slags fromcurrent production should not be confusedwith Old bank slags, for example those foundin Cumbria which do not comply with the

quality requirements of BS 1047. These are stillin use but they may have an inherent variabilityand may contain volumetrically expansivematerials if not properly processed before use.

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Blastfurnace slagand steel slag:their use as aggregates

Andrew Dunster BSc, PhD, CChem, MRSC

BRE Centre for Concrete Construction

IP 18/01

Blastfurnace slag and steelslag are widely used asaggregates, in particularfor highways constructionand maintenance. Thevarious types of slagproducts have differentproperties andcharacteristics.

This Information Paperprovides specifiers andusers with information onthe types of slag productscurrently available andtheir recommended uses.This information is ofparticular interest toengineers involved in the

construction andmaintenance of highways,docks and similar pavedareas. It includes somefrequently asked questions,together with sources offurther help andinformation, and codes andstandards relevant to theproduction, testing and useof slag products.

constructing the future

Figure 1 Compacting BF slag as a sub-base in a trench reinstatement

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Modern slags are principally sourced from steel plantslocated in the north-east, east coast, south of Englandand South Wales. The slags derived from modernproduction of iron and steel are valuable raw materialsthat are processed for a range of construction uses[1][2][3]

including, but not limited to, roadstone, aggregates forconcrete as well as for bulk fills, and cement manufacture.Virtually all the modern production of blastfurnace slagand EAF steel slag from the major steel works in Englandand Wales is used but there are many opportunities tofurther develop uses for Basic Oxygen Steel Slag.

Descriptions of the products

Blastfurnace slagVarying the rate and manner of cooling the moltenmaterial produces several different forms of solidifiedblastfurnace slag [4]:l If allowed to solidify in open air pits, a semi-dense

porous crystalline material is formed; this is called air-cooled blastfurnace slag.

l Alternatively, the slag can be quenched rapidly by jetsof water to form granulated slag (GBS ) that is furtherground into GGBS for use as cementitious binder.

l Many years ago other forms of BF slag were producedusing different methods of cooling but the only oneproduced today is pelletised BF slag; this is usedmainly in block manufacture as lightweight aggregateand as feedstock for GGBS production.

Blastfurnace slag is a by-product of the manufacture ofiron by chemical reduction in a blast-furnace. The point

at which the slag is drawn off is shown in Figure 3. TheBF slag is formed in a continuous process by the fusion oflimestone (and/or dolomite) and other fluxes with theash from the carbon source (coke) and non-metalliccomponents of the iron ore. The slag floats on the surfaceof the molten iron in the blast-furnace and is drawn off(Figure 4). A high-level of quality control over the com-position of the slag indirectly ensures that the iron qualityrequirements are met and that the consistent chemicalcomposition and physical properties of the slag are inaccordance with the standards required for the end use.

Steel slagl Basic oxygen steel slag is produced by blowing oxygen

into molten iron mixed with additions of fluxes andrecycled steel scrap. During a vigorous but controlledprocedure, the slag temperature and composition ismonitored and, when judged to be correct, the oxygenlance is withdrawn and the slag drawn off from theconverter vessel. The process refines the iron by fusionwith a flux, such as limestone or dolomite, underoxidising conditions. Impurities from the iron (such ascarbon, silicon and other unwanted elements) areeither oxidised to gases or are chemically combinedinto the slag.

l Electric arc furnace slag is produced during theproduction of more specialist steels from the refining ofscrap iron in an electric induction furnace.

On completion of these processes, the liquid slag, whichfloats on top of the steel melt, is poured off into ladles,transported, tipped and allowed to air-cool undercontrolled conditions in pits prior to processing [4].

BOS slag is produced in a batch process as a by-product of the conversion of iron to steel. The point atwhich the slag is drawn off is shown in Figure 5.

EAF slags have properties and applications broadlysimilar to those of BOS slag although they generally havea lower content of free magnesium and calcium oxides.

Properties

Blastfurnace slagThis is consistent in its physical properties as would beexpected for a particular source of natural aggregates. It ismade up primarily of the silicates and alumino-silicates ofcalcium and magnesium together with other compoundsof sulfur, iron, manganese and other trace elements. Atypical mineralogy is shown in Table 1.

The controls within BS 1047 are intended to excludefrom use as aggregates those blastfurnace slags that mightbe prone to falling (dicalcium silicate unsoundness) oriron unsoundness. The Standard also sets a limit onsulfate levels (see FAQs on page 6). When air-cooledblastfurnace slag is crushed and screened, it produces anaggregate with a rough surface texture and relatively highporosity; this gives it good adhesion characteristics tocement and bituminous binders.

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Figure 2 top: BF slag base (roadbase)bottom: BF slag base course on a footpath

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3Iron ore

Limestone

Powdered coke Blastfurnace

Pre-blended and pre-heated

Hot-air blast

Iron to steel-making plant

Quenched to give gbs

Air-cooled in pits, crushed, screened, stockpiled and naturally weathered prior to use

BF slag

Hot metal mixer

Iron from blastfurnace

Scrap and fluxes (lime or dolomite)

Oxygen lance

BOS furnace vessel

Steel to plant for rolling and finishing

Slag-tipped out into ladles,transported, tipped in open-air pits and allowed to cool

De-metalled, crushed, screened, stockpiled and naturally weatheredprior to use

Figure 4 Molten BF slag flowing from the blast-furnace into open pits for air cooling

Figure 5 The process for BOS slags

Figure 3 The process for blastfurnace slags

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Basic oxygen steel slagThis consists primarily of calcium silicates together withoxides and compounds of iron, manganese, alumina andother trace elements. Table 1 shows a typical mineralogyof processed BOS aggregate. It is relatively non-porousand produces a high-density aggregate with highcrushing strength. The processed material is denser andstronger than blastfurnace slag and, when used in anasphalt surfacing, delivers a high resistance to abrasionand polishing under heavy traffic [4].

Table 2 compares key physical properties and how air-cooled BF slag and BOS steel slag differ from naturalaggregates. Table 3 shows typical chemical compositions.

Uses

Air-cooled blastfurnace slag and BOS steel slagProcessed blastfurnace slag (meeting therequirements of BS 1047) and basic oxygen steelslags provide quality controlled manufacturedaggregates for use in construction. Principalapplications include asphalts regulated byBS 4987 and BS 594, concrete [5], unbound andbound sub-bases, cappings, fills such asearthworks and backfill to structures (seeSpecification for Highway Works Series 500, 600and 800) and chippings for surface dressing.They can be blended to produce self-hardeningbases, free draining support layers.

BOS slag is particularly well suited to the newgeneration of quiet asphalt thin surfacingsbecause of its high abrasion resistance (seeTable 1) and aggregate shape that contributes tosurface texture, a key requirement for highspeed skidding resistance [6].

BOS slag is not generally used in concrete because ofconcerns about dimensional stability. BRE is aware oftwo current trials of four pre-cast concrete dollos: largeblocks used for sea defence essentially to replace naturalaggregates used for armourstone. These blocks, whichare made with BOS slag as coarse and fine aggregate togive a high density concrete, have been in service insevere wetting and drying environments for about 18months. Unpublished reports suggest that visualmonitoring shows no evidence of cracking or spalling ofthese units.

Further details of a variety of non-aggregateapplications for air-cooled and granulated blastfurnaceand BOS slags in cement, glass and thermal insulationmanufacture, block pavior bedding material, sewagetreatment media, as an agricultural liming agent, arereviewed by Robinson [3].

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Table 3 Typical chemical composition % and how it differs betweenair-cooled BF slag and BOS steel slag after Robinson [3]

CaO SiO2 Al2O3 MgO S Fe2O3 MnO P2O5 TiO2BF slag 41 36 13 7 0.8 - - - -BOS 42 44 10 12 1 2 5 6 - 27 31 3 6 2 3 0.5

Table 2 Typical properties of BF slag and BOS slag aggregates compared with natural aggregates after Robinson [3]

BF slag BOS Quartz Carboniferousdolerite limestone

Apparent density 2.55 3.55 2.95 2.72Water absorption 4 1.3 1 0.7Impact value dry 34 7 7 19Crushing value 34 9 10 2110% fines soaked kN 85 360 360 160Polished stone value 53 57 53 62 55 64 38 48Abrasion value 5 7 1.5 3.8 8

Air-cooled blastfurnace slagl Melilite (solid solution series of calcium aluminium

silicates and calcium magnesium silicates)l Small amount of calcium sulfide 3.0

Table 1 Mineralogy of air-cooled BF slag and BOS slag after Robinson [3]

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Environmental considerations

Adverse environmental impacts associated with the useof these materials are rare and can be attributed to badpractice or inappropriate use. The potentially leachablelime and sulfur compounds present in BF slag, and thelime present in BOS slag, can be contained byappropriate material selection and pavement design.Blastfurnace slag typically contains less than 1% totalsulfur; only a very small part of this is potentiallyleachable when in contact with groundwater becausemost of it is bound up within the aggregate. In the UK, atleast 100 million tonnes of unbound blastfurnace slaghave been used in construction projects with very fewcases of environmental pollution. Most BF and BOS slaghas been used within a thirty-mile radius of the steelworks, providing ample opportunity for majorenvironmental incidences to occur. That this has nothappened is largely due to industry self-regulation andcompliance with the quality requirements in BS 1047.

Joint guidelines issued by the Environment Agencyand British Aggregate Construction Materials Industry(BACMI, now the Quarry Products Association, QPA) [7]

give specific guidance to avoid leaching of harmful sulfurspecies and other compounds into the waterenvironment. These guidelines state that unboundblastfurnace slag should not be used below the watertable or in waterlogged or poorly drained areas. There isno migration of sulfur compounds from the interior of BFslag particles to replace the dissolved material on thesurface. Therefore, after initial dissolution, the processrapidly slows down, only restarting if the slag is crushedto expose fresh surfaces.

A CIRIA Report [8] reviews the leaching behaviourunder rigorous laboratory conditions of a range ofsecondary materials. Each material is ranked in terms ofits potential to contaminate groundwater and classifiedwith respect to drinking water standards. Blastfurnaceslag is placed into Category 1 for unrestricted use, ieequivalent to limestone.

Conclusions

As a sustainable by-product of iron and steel making, it isclearly in the national interest to fully utilise slag, whereappropriate, to reduce the need to extract finite primaryminerals. This is in accord with government policy aimedat developing a sustainable construction industry.l As a road aggregate, BF slag has excellent resistance to

binder stripping in asphalt mixtures caused by thecombined action of water and traffic. Also, BF slag hasrelatively low density, providing good surface coveragewith the associated economic benefits.

l Steel slag is particularly suitable as an aggregate forsurface dressings, in asphalts and macadams, or forasphalt chippings on heavily-trafficked roads.

l Supplies of slags from current production must not beconfused with Old Bank slags. These are still in usetoday but they have an inherent variability and maycontain expansive particles although they can beconsidered satisfactory with controlled processing.

l The high level of performance of these productsemanates from well-controlled production processeswith good quality control procedures.

l BF slag and steel slags can be substituted for naturalaggregates in most applications.

l There is very little evidence of slags causinggroundwater pollution when correctly specified andinstalled; this is due largely to industry self-regulationand compliance with appropriate product standards.

5

Table 4 Uses of air-cooled BF slag and steel slag as aggregates and fillAir-cooled BF slag Steel slag

Bound and Concrete coarse aggregates Road sub-base [2]

unbound aggregates Capping layers Base (roadbase) [2], binder (base) courseRoad sub-base Surface (wearing) course/chippings [2]

Base (roadbase), binder (base) course Armourstone [2]

Surface (wearing) course/chippings Bituminous bound material in generalBituminous bound material in general Aggregates in dense concretes for specific uses [3]

Other construction and Fill and backfill [1] Earthworks, fill and backfill (not under foundations)non-construction applications Pipe bedding Cement manufacture

Biological filter media Agricultural liming agentFill beneath groundbearing slabs under houses [4] Track ballast

Notes[1] Environment Agency/QPA guidelines set limits on circumstances for use based on potential groundwater effects[2] Must be adequately weathered/quality controlled to minimise risk of expansion or particle disintegration[3] See Uses on page 4. Adequate weathering and quality control is especially important[4] See NHBC Technical Circular 5.1/02

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6Frequently asked questionsWhat are slags ?Non-metallic by-products of the refining of metals from metallic ores. Some have valuable properties thatcan be used in construction.

What is meant by falling of air-cooled BF slag ?One of the minerals (beta dicalcium silicate) present in some crystalline blastfurnace slags can undergo atransformation in its crystalline structure on cooling that can cause disintegration of the slag in aphenomenon known as falling, dicalcium silicate unsoundness or lime unsoundness. BS 1047 gives testmethods for identifying beta dicalcium silicate. Chemical equations and microscopic tests included inBS 1047 are intended to exclude slags that might contain this compound from use as aggregate. When itdoes occasionally occur, falling happens only during cooling and will not affect slag aggregates in service.

What about expansion of slags ?Steel slag and old bank slags may undergo volumetric expansion in the presence of water. The risk ofdamaging expansion occurring when using steel slag produced in modern processes is minimised by acombination of careful production control and processing. Air-cooled blastfurnace slag is non-expansive.

What advantages do slag aggregates offer over other aggregates ?Steel slags have a high abrasion and crushing resistance for road applications; air-cooled blastfurnace slagbonds particularly well to cement and bituminous binders. They also offer significant environmental benefitssuch as the re-use of secondary materials and avoidance of the need to quarry natural aggregates.

How will slags be affected by the aggregates tax ?The introduction in April 2002 of taxation on primary aggregates sold for construction purposes should notaffect secondary or recycled aggregates such as BF slag and BOS. However, this is still under discussion.

Is air-cooled BF slag from modern production suitable as a backfill, eg for pipe bedding orhardcore against concrete slabs or foundations ?Yes. Modern BF slags meet the requirements of BS 1047 and have a sulfate content that corresponds tothe lowest design sulfate class (DS-1) given in BRE Special Digest 1. However, old BF slags can contain highlevels of sulfates with an associated risk of sulfate attack to concrete under wet conditions (BS 5328).

Can BF slag be placed up against steel structures ?Yes. Corrosion occurs mainly in acidic conditions pH

7Codes and Standards

Concrete, aggregates and civil engineering work ingeneralBS 5328: 1997 Concrete

BS882: 1992 Aggregates from natural sources for concrete

BS812 Testing aggregates

BS4987 Part 1: 1993 Coated macadam for roads and otherpaved areas

BS594 Part 1: 1992 Hot rolled asphalt for roads and otherpaved areas

BSEN 12620 Aggregate for concrete

BSEN 13043 and UK Annex. Aggregates for bituminousmixtures and surface treatments for roads, and othertrafficked areas

BSEN 13242 and UK Annex. Aggregates for unbound andhydraulically bound materials for use in civil engineering workand road construction

BSEN 13383-1 Armourstone Part 1: Specification

BSEN 13450 Aggregates for railway track ballast

BSEN 1744-1 1998 Tests for chemical properties ofaggregates Part 1: Chemical analysis

Blastfurnace slagBS1047: 1983 Specification for air-cooled blastfurnace slagaggregate for use in construction

BSEN 1744-1 1998 Tests for chemical properties ofaggregates. Part 1: Chemical analysis

BS1377: 1990 Parts 1 to 9 Methods of test for soils for civilengineering purposes

BS6699: 1992 Ground granulated blastfurnace slag for usewith Portland cement

Steel slagBSEN 1744-1 1998 Tests for chemical properties ofaggregates Part 1: Chemical analysis

BSEN 13383-2 Armourstone Part 2: Test methods

What is covered in relation to slags

Properties and applications

Quality, grading and testing requirements for aggregates for concrete.Requirements can be applied both to natural and secondary aggregates such asslags

Various test methods including those for sulfate content, TFV and AIV*

Specification for constituent materials and for mixtures

Specification for constituent materials and for mixtures

Refers to constituents of air cooled BF slag

Physical and chemical requirements. Specifically mentions air cooled BF slag andsteel slag aggregates and sets maximum expansion limits for steel slag. Notnecessary to test volume stability of steel slag aggregates having a satisfactoryperformance record

Physical and chemical requirements. Specifically mentions air cooled BF slag andsteel slag aggregates and sets maximum expansion limits for steel slag.Recommends that specifier obtains advice of supplier in respect of expansion ofsteel slag

Physical and chemical requirements. Specifically mentions air cooled BF slag andsteel slag aggregates and sets maximum expansion limits for steel slag.Disintegration of steel slag is assessed using a method given in Part 2 of thisStandard

Manufactured aggregates are covered in the scope, which includes steel slags

Method of test for unsoundness of blastfurnace and steel slags, and fordetermination of expansion of steel slag, also free lime in steel slag

Materials and properties and testing of air-cooled BF slag including requirementsfor coarse aggregate for concrete, bituminous bound and unbound applications

Assessment method for falling will supersede the BS 1047 method after 1 December 2003

Test methods for soils in civil engineering purposes

Specifies requirements for comparison and manufacture and for chemical andphysical properties of GGBS

Measurements of the rate of expansion for steel slag

Boiling test for visually assessing stability of steel slags

*TFV = 10% Fines ValueAIV = Aggregate Impact Value

Notes:The Highways Agency Manual of Contract Documents governs the use of slags in highway works.The British Standards covering the requirements for aggregates will be superseded after 1 December 2003 by a series of new CEN standards for aggregates in concrete (prEN 12620), road and civil engineering work (prEN13242 and prEN13043).

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References and further reading[1] Gutt, W and Nixon, P J. Use of waste material in the constructionindustry; analysis of RILEM symposium by correspondence (Chapter 3,Slags). Materials and Structures, 12, 70, 255-306.[2] Lee, AR. Slag for roads - Its production, properties and uses.Journal of Institution of Highway Engineers, 1968, 16, 2, 2-14. [3] Robinson, H L. The utilisation of blastfurnace and steel makingslags as aggregates for construction. 36th Forum on the Geology ofIndustrial Minerals/11th Extractive Industry Geology Conference, Bath,UK 7-12 May 2000.[4] Lee, A.R. Blastfurnace and steel slag - Production, properties anduses. The British Quarrying & Slag Federation Ltd. Edward Arnold[publishers] Ltd. 1974.[5] Gutt, W, Teychenne, DC and Harrison, WH. The use of lighter-weight blastfurnace slag as dense coarse aggregate. BRE CP93/74.[6] Manual of contract documents for highway works: Volume 1:Specification for highway works : Series 900, Clause 942 Thin asphaltsurfacings[7] Environment Agency. The use of air-cooled blastfurnace slag asan unbound aggregate in the construction industry. QPA/Environment Agency Joint Guidelines.[8] CIRIA. Use of industrial by-products in road construction - waterquality effects. Construction Industry Research and InformationAssociation Report 167.[9] Garvin, S et al. Risks of contaminated land to buildings, buildingmaterials and services. A literature review. R & D Technical Report 331(for Environment Agency).

BREGutt, W, Nixon, PJ, Smith, MA, Harrison, WS, and Russell, AD. A survey of the locations, disposal and prospective uses of the majorindustrial products and waste materials. BRE Current Paper CP19/74.Lawson, EM and Nixon PJ. A survey of the locations, disposal andprospective uses of the major industrial products and waste materialsin Scotland. BRE Current Paper CP50/78.BRE Digests276 Hardcore427 Low-rise buildings on fillBRE Special DigestConcrete in aggressive groundBRE Information Paper5/97 Building on fill: collapse compression on inundation

Highway Works documentsManual of contract documents for highway worksVolume 1: Specification for highway worksSeries 500: Drainage and service ductsSeries 600: EarthworksSeries 700: PavementsSeries 800: UnboundSeries 900: Bituminous bound

Useful contactsBRE www.bre.co.ukEuroslag www.euroslag.orgQPA www.qpa.orgTarmac www.tarmac.co.ukCementitious Slag Makers Association (ggbs only)

www.ukcsma.co.uk

8

BRE is the UKs leading centre of expertise on building andconstruction, and the prevention and control of fire. Contact BREfor information about its services, or for technical advice, at:BRE, Garston, Watford WD25 9XXTel: 01923 664000Fax: 01923 664098email: enquiries@bre.co.ukWebsite: www.bre.co.uk

Details of BRE publications are availablefrom CRC Ltd or from the BRE website.Published by CRC Ltd, 151 Rosebery Avenue, London EC1R 4GBTel: 020 7505 6622Fax: 020 7505 6606email: crc@construct.emap.co.uk

BRE is committed to providing impartial andauthoritative information on all aspects of the builtenvironment for clients, designers, contractors,engineers, manufacturers, occupants, etc. Wemake every effort to ensure the accuracy andquality of information and guidance when it is firstpublished. However, we can take no responsibilityfor the subsequent use of this information, nor forany errors or omissions it may contain.

Requests to copy any part of thispublication should be made to:

CRC Ltd, PO Box 202,Watford, Herts WD25 9ZW

Copyright BRE 2001September 2001

ISBN 1 86081 510 3

www .bre.co.uk

Definition of termsAggregate PrEN13242: 2001A granular material used in construction. Aggregate may be natural,manufactured or recycled.Polished stone value BS112 Part 114: 1989A measure of the resistance of roadstone to the polishing action ofvehicle tyres under conditions similar to those occurring on thesurface of a road. Where the surface of a road consists largely ofroadstone, the state of polish of the sample will be one of the mainfactors affecting the resistance of the surface to skidding.Impact value BS812 Part 112: 1990A relative measure of the resistance of an aggregate to sudden shockor impact. The test specimen is subjected to a number of standardimpacts from a dropping weight. This breaks the aggregate by anamount dependent on the impact resistance of the material,assessed by a sieving test on the impacted specimen.Crushing value BS812 Part 110: 1990A relative measure of the resistance of an aggregate to crushingunder a gradually applied compressive load. The test specimen iscompacted in a standardised manner into a steel cylinder fitted with afreely moving plunger. The specimen is then subjected to a standardloading regime applied through the plunger, which crushes theaggregate by an amount dependent on the crushing resistance of thematerial, assessed by a sieving test on the crushed specimen.

Ten percent fines value BS812 Part 111: 1990A relative measure of the resistance of an aggregate to crushingunder a gradually applied compressive load. The test specimen iscompacted in a standardised manner into a steel cylinder fitted with afreely moving plunger, which crushes the aggregate by an amountdependent on the crushing resistance of the material, assessed by asieving test on the crushed specimen. Repeated with various loads todetermine the maximum force which generates a given sieveanalysis, this force is taken as the ten per cent fines value (TFV).Sub-base prEN13383-1: 1998A layer of material, usually granular, below the base (roadbase) to:p insulate the pavement from the subgrade (the rock or soil horizonimmediately beneath a pavement);p increase the pavement thickness to protect against frost;p provide an adequate surface for construction plant.Base (roadbase) prEN13383-1The main structural element in a road pavement. It spreads theconcentrated loads from traffic over an area of subgrade largeenough to sustain them.Wearing course prEN13383-1The uppermost layer in a road pavement. It contains the highestquality material in the pavement and its function is to provide thedesired riding and non-skid properties to the road.Armourstone prEN13383-1Unbound large coarse aggregates used in hydraulic structures.

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