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State of the Art Report Use of Waste Materials in Construction - Technological Development
Environmental Aspects of Construction with Waste Materials J.J.I.M. Goumans, H A . van der SImt and R G . Aalbers (Editors) @I994 Elsevier Science B.V. AN rights reserved.
STATE OF THE ART REPORT USE OF WASTE MATERIALS IN CONSTRUCTION DEVELOPMENT G.R.Woolley Civil Engineering Consultant.
963
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TECHNOLOGICAL
Abstract Massive quantities of waste materials are produced annually in the major industries of mineral extraction, electricity generation and steel production. Each in turn has vast stockpiles of their waste product. Interestingly the major chemical composition of these combined wastes are similar and each are used to a minor extent in the construction of roads, pavements cement production and asphalt. There are numerous other producers of waste materials and there is the important source of demolition waste and scalpings from road surfaces under repair. There is mounting concern for the environmental effect of depositing these wastes to spoil heaps or as in-till to large excavations. especially so in the event of leaching out of certain trace elements. In this paper, the major waste materials are introduced, considered and their utilization reviewed. Where information on development of the several wastes is available this is introduced. To mount any form of assault on the continuing use of natural aggregates or other materials in construction there has to be a fundamental change of emphasis in regard to using re-cycled waste materials. Selection of a suitable and effective stabilizing agent for these materials either in isolation or when combined is important but, although they have very similar chemical compositions, they are physically different. To reduce our dependence on natural aggregates and to increase our use of waste materials in construction there may be the opportunity to create manufactured items closer to the source of waste material production. Alternatively Legislation may be required to enforce such change. 1. INTRODUCTION lncreasingly environmental and commercial pressures are causing Government Agencies, Local Authorities and Business Ventures to question the practice of dumping waste material. The rising cost of disposal, reduction in industrial activity and clamour for preservation of the natural environment, corn bined with increasing commercial pressure, are promoting a renewed interest in ' using waste products. Some waste products have been in limited use for a number of years. Of these, perhaps the most internationally known is blast furnace slag, a by-product resulting from the production of iron. Determined by the way in which molten slag is cooled it may be a coarse mixture of vitreous agglomerates or granules of a shattered vitreous structure. These granules, granulated blast-furnace slag, are available to replace primary materials or for grinding to fotm a latent hydraulic binder. A not too dissimilar situation exists for fly ash, the solid material collected by mechanical o r electrostatic means from furnace exhaust giwes following the burning of coal in electricity generation. From the late 1930's , after its use in the United States of America, fly ash has been increasingly used as a partial replacement of cement in concrete. It has also been widely used to construct lightweight
964 fill embankments. Both slag and fly ash are used in the production of a number of sophisticated product\ like paint, powder etc. but, with tly ash there is a surplus which has to be disposed of somewhere and by someone. The two examples illustrate the dilemma of dumping to waste the balance of these useful materials, a situation which is increasingly becoming an imposition on the environment. Consideration of the current State of the Art presumes that some waste products are being used for construction but, begs the question ' are we concerning ourselves properly and fully in the on-going study and research into further applications '. Do we therefore prioritise our search for new or extended uses into
projects which provide the highest premium on our outlay, or seek to find outlets which will consume the greatest quantities of waste product. Both have advantages but, the environment is unable to distinguish between any or all of the wastes, we have that facility and only our due diligence and effort will incorporate more waste materials in construction, thereby contributing to the continued well being of our varied and several environments. We must also aim to conserve our primary material resource. By using waste products in a controlled way we should be able to reduce despoiling our environment by reducing extraction of primary materials and the deposition of unwanted waste products. Letters were sent to those named persons in ISCOWA who had recorded an interest in Technological Development associated with the use of waste materials in construction. Sadly the response was very limited, indeed most disappointing, even after a reminder to selected individuals but, some information was provided and I formally thank those respondents for their contributions. An analysis is given of data provided and of material found elsewhere in earlier papers and reports. The major individual waste materials are considered in turn, their properties given and current utilisation reported. Where new processes are under development or being newly practised, known details are given. It was not possible to mount a full literature review and in deference to the authors of papers published worldwide the references given are limited.
2. METALLURGICAL SLAGS 2.1 Blast-furnace[ iron ] slags - The Material The process of refining metallic ores involves raising the temperature of the ore in a turnace until the metal separates from the associated ore components and can be collected. The remaining matter, a mixture of ore residues and reactant used to promote separation is referred to as a metallurgical slag. The manner and rate of cooling of this molten slag will produce several different types of solid slag. If allowed to solidify naturally, a dense crystalline material referred to as air-cooled slag results. Slag may be foamed by introducing water jets into the stream of molten slag, causing it to expand and form a lightweight material. When cooled very quickly with large amounts of water it forms a glassy material called granulated slag. Pelletized slag is produced by a more sophisticated process involving water jets, revolving drums and air cascades resulting in small pellets of foamed or granulated slag ( I ).
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2.2 Composition All slags have similar composition. In crystalline blast-fumace slag the principle mineralogical phase lies in the series from akermanite [ 2CaO.Mg0.2Si0, ] to gehlenite [2CaO.A1 0 .Si 0 ] (2). When slags have a high lime content they may contain dicalcium silicate [2CaO.SiO 1. This inclusion can cause disruption of the slag through volume increase. If sulphur is present in a reactive, leachable form. mainly as calcium sulphide, problems may also occur. Typical chemical composition of a number of blast-furnace slags is reproduced in table I . Table I Typical Chemical Compositions of Blast-furnace Slag [weight%] Australia SiO, 33-37 Al;,O> 15-18 CaO 39-44 MgO 1-3 TiO, 0.6 Fe,O, 0.7 MnO 0.3-1.5 N a O 0.2 K 0 0.5 s 0.6-0.8
USA Germany 34-38 35 11-15 12.2 45-47 41 1-3 X 1.3-4.5
0.25 0.5 1.2 incl 0.6
Norway
34-38 7- 10 40-4x 6- 13 0.7- 1. I
0.1-0.4
UK 31-36 13.5 33-45 4-15 0.x
0.5 0.8
lridia 30-35
18-26 30-36 3-9
<0.5 <1.0
0.7 0.8 0.53
2.3 Utilisation Transport costs bear heavily on the use of slags worldwide but in general the percentage used is very high and in some countries approaching 100%. Road making uses the most amount of this material ahead of cement manufacture. Use of granulated slag to replace natural aggregate is well understood. Using the specifications for natural aggregates it may be used in blinding, base, sub-base and as coated macadam for the wearing course in roads. Granulated slag properties as a latent hydraulic binder suits it to act as the binder in gravel slag and sand slag, both used as base material in roads. When combined with water and an alkali activator it will produce cementing compounds, this hydraulic activity depending on the chemical compo on of the slag, physical properties and glass content (2). When combined with portland cement and water the alkalis and lime produced in the hydration of cement acts as the catalyst in producing the cementitious reactions of granulated slag (3). There is extensive use of this property in the manufacture of several slag cements which may contain granulated blast-furnace slag in proportions as high as 90%. Useful to reduce the heat of hydration in hardening concrete, these slag cements have lower early strength but may attain equivalent strength with time. The higher slag contents of these cements provide more resistance to attack by sulphate solution (4).
966 ple process and, subject to soundness, once Production of air cooled slag in roads, as dense aggregate in concrete or as cooled it may be crushed and graded fertilizer. It does need space for cooling pits a filter material. It is also crushed to but, the newer, larger furnaces are being equipped with granulating plant in order to maximise the advantages offered by blast-furnace slag cements. Rapid cooling by compressed air producers the slag wool product, a material used for insulating purposes. Pelletized slag offers a lightweight aggregate, the pellets being of the vitrified form and, like granulated slag, have the properties of a latent hydraulic binder. This attribute lends itself to the soil stabilisation process and production of lightweight building blocks (1).
3. STEELSLAG 3.1 The Material More variable than slag produced from iron ore smelting, steel slags result from the conversion of pig iron to steel. Chemical compositions of these slags introduce this variability but, as with blast-furnace slag the reactant used in smelting and impurities present in the pig iron produce a steel slag. Amongst these compounds is lime, which will quickly hydrate and magnesia with a slower hydration period. The basic oxygen process of steel making tends to produce a more vmable slag than the open hearth process (2).
3.2 Composition Not dissimilar in appearance to an igneous rock, steel slag chemical compositions vary greatly but, generally they have a higher CaO I SiO proportion than blast-furnace slags. Typical chemical constituents are shown in table 2. Of concern are the free calcium, magnesium oxides and metallic iron phases found. These compounds will hydrate and expand in the presence of moisture. Table 2 Typical Chemical Constituents of Steel Slags ( weight % ) Basic Oxygen Steel Open Hearth Electric Arc Germany USA UK USA UK Low C High C CaO 47.2 59.9 40.3 33-5 I 36.17 3 1-50 SiO, 14.X 13.8 21.7 9-19 18.02 11-24 Fe,O, 1.53 10.5 16.3 24-45 17.46 5-30 (total iron) x.54 5-18 3.8 0.5-3 Al,o, 0.6 2.10 9.96 2-8 1.5 0.9 4.4 0.5-4 MgO Mn 0 5.4 3.0 4 3-10 4 6-22 3.3 Utilisation Iron and iron compounds are contained in steel slag and much is recycled as a minor component in blast-furnace charges. When the metallic iron is removed the remainder is principillly used for land reclamation and unrestrained fill to allow for any expansion that may occur. Presence of dicalcium silicate restricts the material from use as concrete aggregate but it is used for road making when unbound or bound with bitumen as a bound macadam. After controlled pre-weathering, steel slags provide a good quality
967 artificial stone for road making (5).The material may also be ground to provide a high quality fertilizer and depending on slag composition will provide phosphate or lime as a soil conditioner. Granulated electric arc furnace steel slag is used for cement manufacture in some puts of the world with evidence of hydrate formation similar to portland cement. 3.4 Developments Work is reported o n the use of alkali activated slag cement to immobilize heavy metal ions in waste ( 6 ) .Inclusion of slag in the production of lime - sand bricks has shown to be promising whereby slag may replace up to 15% of the lime constituent in the process (7 )
4. FLY ASH ( referred to in the United Kingdom as Pulverised Fuel Ash ) 4.1 The Material
Coal used for steam raising in modern power stations is finely ground prior to combustion. This will typically reduce it in size from an incoming average passing a 12.7 mm sieve of 94% to 80% passing a 75 micron sieve before entering the furnace. The constituents, which may be separated to some degree by grinding, consist of carbonaceous matter and various minerals in the form of shales, clays, sulphides and carbonates. This pulverized coal is injected into the furnace at high speed in a stream of hot air and it burns instantaneously in the range of 1500 +/- 2000 deg.C . This is above the melting point of most minerals present and as a result they undergo various chemical and physical changes. For example pyrite is converted into oxides of iron, including spherical particles of magnetite ; clay forms glass spheres of complex sulphates. The exact nature of changes depends on the type of coal, fineness of grinding, temperature and retention period within the hot zone of the furnace. Between 75 - X5% of the resulting ash is carried out of the furnace with the flue gases and is known as fly ash. The remainder falls to the bottom of the furnace where it sinters to form a c o m e r material known as furnace bottom ash. Fly ash is extracted from the flue gases by electrostatic precipitators and / or mechanical methods and is collected in a series of hoppers for disposal ( 8 ) .
4.2 Composition Fly ash consists principally of glass spheres together with some crystalline matter and a varying amount of carbon. Overall colour will depend to some degree on the proportions of carbon, iron and moisture. It will have a specific gravity in the range 1.98 to 2.38 and average specific surface in the range of 260 to 595 sq.m / kg (10). Collected ash will be graded. Some suggest that the finer ash is more reactive but it dws not follow that all fine ashes have high reactivities ( I I ) . The grading from any given source under steady load is normally very consistent. There are three predominant elemene in fly ash, silicon, aluminium and iron, the oxides of which together account for approximately 75% of the material. Silicon is present partly in the crystalline form of quartz (SiO,) and in association with the aluminium as mullite (3A120, ,2Si02 ), the rest in the glassy phase. The iron appears partly as the oxides magnetite (Fe203),and haematite (Fe,O,), the rest in the glassy phase. Carbon, determined as loss-on-ignition, is present in fly ash in amounts
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which vary with the efficiency of combustion. A typical chemical analysis of fly ash collected from a modern furnace burning sub - bituminous coal is given in table 3 (12). The operating regime differs when lignite is used as fuel and this will produce an ash which differs markedly in respect of calcium oxide. It is known as a high lime fly ash (cli~ss C) and has a typical chemical composition as shown in table 3 (13). Examination of fly ash taken from the flue gases of low NOx boilers which carry a limited amount of ammonia have been shown to have no affect on the production and properties of concrete (14). Table 3 Typical Chemical Composition of Fly Ashes From sub - bituminous coal Si 0, 49 - 53 ,4203 25 -29 Fe20, x- II Ca 0 1.x - 5.3 Mg 0 1.3 - 2.2 K2 0 2.2 - 3.7 Na, 0 0.7 - 1.5 I .o - 2.0 Ti 0, 0.3 - 1.7 so3 Loss on Ignition 1.5 - X.8 Blaine specific surface 220 - 5 10 (sq. m / k g ) Fineness >45 micron 6.5 - 25.0
( weight % ) From lignite coal 2 I .o - 35.0
10.0 - 14.0
4.5 - 6.5 30.0 - 45.0 (10% free) 1.5 - 3.0 0.4 - 0.9 0.5 - 1.0 --_4.0 - x.0 3.0 - 7.0 250 - 2x0
----
Utilisation Fly ash has been used quite extensively as a fill material in road construction. When placed and compacted at optimum moisture content ( generally in the range 17 - 23% ) it has a lower density than most other fill materials, an advantage when placed on highly compressive soils. Many fly ashes have self - cementing properties when compacted which reduce settlement potential within the fill mass, a distinct advantage when placed to bridge abutments (15). Mostly however, they are trost susceptible and d o require a minimal protection. Recent developments in the technique of reinforced soil embankments have shown how suitable tly ash can be for this application (16). Stabilization of tly ash with lime or cement has allowed the material to be used for sub - base construction and as a binder it may be used to improve the physical properties of soil either alone or with lime or cement. Lime - fly ash mixes have been successfully used to stabilise a range of materials but, when clay is present the lime appears to react primarily with the clay ( I 7). Numerous applications of the use of cement / tly ash grout mixes are reported for the filling of mine workings, rock fissures and the like ( 18). As a pozzolanic material fly ash will react with lime in the presence of water. This cementitious property is used in concrete where fly ash will react with the hydration products of portland cement. It may be included by intergrinding with the cement clinker to form blended cement or included in the site mixer with portland cement. The physical properties of fly aqh produce a rheological change in fly ash concrete which allows a small
4.3
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reduction in water content when compared to an all cement concrete of similar workability ; generally thought to give a 3 8 water reduction for each 10% cement replaced with tly has a lower temperature rise and i ash (19). Hardening mes experienced in large sections. affected by high tern mass concrete in dams and there has been considerable use of the material in these structures. Recently the traditional method of including and placing concrete in dams has received competition from the new practice of roller compacted concrete, a formation whereby layers of low water / cementitious concrete are built - up and compacted insitu by roller. In this way it is claimed that construction costs are reduced through a shorter construction programme and inclusion of high volumes of tly ash often in the range 40 60% (20). In Spain similar and even larger volumes of fly ash have been used in dam structures placed and compacted conventionally (2 1). Early records of fly ash structural concrete report inclusion of the material in small proportions by volume. Introduction of weigh batching at the mixer, perhaps more sophisticated mix designs and increasing awareness of the potential engineering and commercial benefits has raised the general replacement level of cement to 30%. Inclusion by weight of 30 - 40% is not uncommon and there are now a variety and increasing number of structures built with fly ash concrete. Fly ash is widely used for the production of lightweight aggregate. Using a sintering process pellets are formed in a variety of ways (22) (23). Sinter cake may be formed at I 150 - 1200 deg.C , a product used in the manufacture of building blocks. The lightweight aggregate produced is of good regular shape of high quality with moderate water absorption, used for structural lightweight concrete. Aerated concrete blocks are made in an autoclave process using a mixture of fly ash. cement and sand with an air entraining agent. This produces a high quality thermally efficient building block.
4.4 Development The proportions of fly ash being included in concrete as cement replacement continues to increase. In real terms however, the total amount of the material used in concrete is small compared to total production. Development work is in progress to design high volume fly ash boulders (24) and other structural elements (25), the latter including FGD gypsum with fly ash, for use offshore as coastal protection. Evaluation continues on a trial section of road surfacing which included fly ash as replacement for filler in the asphalt ( 2 6 ) .This gave a significant reduction in energy demand in the production process. The residues of tluidised bed combustion have been successfully tried in the making of artificial gravel (27). There are products in research which combined with fly ash produce construction materials suitable to replace natural aggregates in use today. This is early work, shrouded in commercial secrecy but it does illustrate a willingness to use waste materials. 5.0 COLLIERY SPOIL ( Minestone Waste )
5.1 The Material Colliery spoil is produced during the extraction of coal by deep mining. It may be pure rock removed in formation of underground roadways, the coarse material separated from the coal in the washery, tine material resulting from degradation during the mining
and transportation or tailings collected in the final separation. The latter which is almost total coal is used elsewhere for combustion. Rock types associated with coal seams are mudstones, siltstones, sandstones and shale. Quartz and clay minerals, kaolinite and illite, are the principle minerals included. Naturally grey in colour this shaley material weathers rapidly. When placed in spoil heaps and compacted in layers to exclude oxygen it will remain in the excavated form but, the older heaps often suffered combustion and the resulting " burnt shale " is a reddish-coloured material with higher strength and better resistance to weathering ( 5 ) .
5.2 Composition Spoil will contain varying amounts of carbonaceous matter but, the principle minerals will comprise quartz and the clay minerals kaolinite and illite. Chemically colliery spoil will typically include Si 0, , range 38 - 60%, A1,0, ,range 14 - 30% and Fe,O,, range 3 - 1 I %. It will include a number ot other minor elements. Chemical changes occur when spontaneous combustion has taken place which include il reduction in the proportwn of combustible matter, a breakdown of clay minerals, decomposition of carbonates and the oxidisation of sulphides to soluble sulphates. The latter change producing an undesirable increase in sulphate content affecting possible use of the material. Precautions may be necessary to avoid close contact with other engineering materials if the shale has a high sulphate content (2).
5.3 Utilisation Construction of embankments using colliery spoil is perhaps the only situation where large scale quantities are used. It is used as fill beneath dwellings and roads, the burnt shale being the preferred material because of its good grading and granular nature. Tests may be necessary to ensure against frost damage and limits may be placed on sulphate content. To minimise the potential of frost heave shale has been stabilised with ;L nominal amount of cement. Precautions may be necessary to prevent sulphate and acid attack of concrete which may have direct contact with colliery burnt shale and the use ot appropriate cements and / or pozzolanas are recommended (27). Colliery spoil has been widely used for brick making where carbonaceous matter in the spoil can contribute to the energy requirements in the kiln. A number of practical points associated with the use of colliery spoil in brick making contribute to its relatively variable appearance but, it makes a brick with good durability. Colliely spoil is also used in the production of a lightweight aggregate. The two processes used, deliver a rough, angular aggregate suitable for use in the production of lightweight concrete building blocks. 5.4 Development Studies are progressing into the use of colliery spoil in combination with tly ash, cement and bentonite for liners to waste disposal areas (29). There is continuing interest in using large quantities of the material off - shore in the formation of protective islands. In this situation it will be necessary to overcome the risks of limited pollution from surface washings and the plate like structure of shales. Perhaps in cornbination with cement and another tiller new formulations can be found.
97 I 6. MINING and QUARRYING WASTE
6.1 The Material Considerable quantities and notably different wastes are produced by the mining and quarrying industries. Those wastes produced in large quantity generally have a chemical analysis in which Si 0, predominates with varying amounts of Fe203and A203 and lesser percentages of the minor elements. Quiurying wastes will include overburden. reject rock and the fine material arising from transportation and washing. Mineral mining produces waste rock removed to expose the ore and tailings obtained through the separation of minerals from the ore. Geological formations will determine the composition of wastes which will vary by site and operation. Rock type, excavation method , transportation and preparation process are the determinants in particle size distribution. The coarser fraction of wastes are usually deposited in spoil heaps local to the source 01 used to provide landscape screening. The finer material is generally transported as water slurry to settling lagoons. 6.2 Utilisation Amongst the most common uses are for road construction or the manufacture of calcium silicate products. The coarser fraction from a number of mining and quarrying wastes are used in road construction or to produce concrete aggregate. These include the wastes from the winning of copper ore, iron and gold ores in addition to the rejected rock removed in overburden to other quarrying activities (2). The p produces a coarse waste of size > 20mm to 75 micron and a fi - 10 micron. The coarser material is used in block making and the fine sand for concrete ). China clay and laterite are used in brick making or burnt to form umber of fine wastes are used as fillers in bitumen or in the manufacture of products. Manganese mud is used as a pigment to colour bricks and ground slate as a filler in paints and plastics. Slate granules provide the decoration to bitumen felts and it has also been used to produce a lightweight aggregate. Phosphate sand can be used in concrete or for ground modelling but, the colloidal clay sized particles of phosphate slime remains unstable without mixing with sand (2).
6.3 Developnient Geographical remoteness of mining ilnd quarrying activities presents a major vansport problem for the considerable quantities of waste being produced. There appears to be little work being done on the utilisation of these many wastes in the production of a manufactured item. Investigations have been noted into the suitability of using certain shales and slates for the production of lightweight aggregates however, the energy demanded to cause them to expand makes the unit production costs too expensive, 7. MUNICIPAL SOLID WASTE INCINERATOR ASH 7.1 The Material Municipal Solid Waste is the ash resulting from the incineration of municipal
912 waste. It is quite variable, the product being determined by the process conditions, furnace type and temperature and composition of the feedstock which is largely domestic and trade refuse. Typically the majority of the ash has particles of less than 3mm with mainly glass, ceramics and slag-like matenal. The more variable nature of this ash suggestc a noticeably greater environmental impact, one demanding strict control of use or placement. 7.2 Composition In primary selection the rnedlic refuse is recovered for re-cycling. Determined by the original composition of the waste, these ashes show great variations in chemical composition. They are highly siliceous with minor components of iron calcium and aluminium. There are also numerous trace elements. Typically the Dutch AVI- Slag composition produced from MSW is given in table 4.
Table 4 General Composition AVI - slag 4% organic material glass 18% rubble, ceramics I1% 13% slag - like material 2% iron 2% nun - ferro metals fraction < 3mm 50% ( not recognizable
)
7.3 Utilization In Holland, recognising the potential for environmental impact, there are strict rules for its use. These include limits on leachability, soil-ground water protection, impermeable encasement and the subsequent facility for removal. The coarse fraction > 2mm is used as a concrete aggregate where generally only 15 - 20% of natural ,coarse aggregate may be replaced in non - reinforced items. However high chloride contents may be present, zinc and aluminium can lead to expansion and the other engineering parameters of concrete, setting time, shrinkage and high cement I water contents may be affected (3 I). This aggregate has also been included in the composition of asphalt. The pulverised refuse has been used to replace a small proportion of the pulverised coal feed for cement production without affecting the cement quality. 7.4 Development It is the practice to store this ash in the open air after production. This is to allow natural stabilization of the product and reduce the potential for leaching of certain metals. Research is indicating that there is secvndary mineral formation during this storage period,as tound in the natural pozzolana. which influence the leaching of contaminant\ (32). Post - treatment systems which include washing, sintering and smelting are being considered to ctabilize the material, thereby widening the scope for inclusion in construction. These in-plant measures may provide a vehicle for improving the ash quality and assist with fixation of heavy metals (33). Exploratory work is reported of the use of
973 ash derived from refuse incineration as a partial replacement of cement (34). Each secondary treatment, including storage before use, carries a clear cost implication. 8. DEMOLITION WASTE 8.1 The Material Largely demolition waste consists of concrete and brick rubble with minor quantities of secondary materials including steel, tiles, plaster boards, wood etc. The majority of the metal will be re-cycled and individual contractors will sepwate the minor componena from the bulk concrete and brick waste. There is also waste macadam and asphalt scalpings recovered from worn road surfaces in the process of repair or resurfacing.
8.2 Composition Concrete waste may be infinitely variable in particle size at recovery but, brick or concrete building block will be of a more uniform size. Subject to controlled c r u s h n g both will be available for grading to demand. Asphalt and macadam planings from roads under replcir will provide a coated aggregate material > 40mm m size. Reinforced concrete is clearly more difficult to handle but, it may be recovered with the reinforcing steel going for re-cycling.
8.3 Utilization Re-cycled concrete and brick rubble crushed to produce graded aggregates are used in foundations to properties and as sub-base to roads. The graded aggregate has been successfully used in new concrete but, it h been shown to be more water demanding and requires an increased cement content to fu 1 strength demands (35). The greater proportion of road planings are re-used in footpaths, car parks and the like and when crushed and graded used a s road sub-base material. Using the Minesotta process up to 25% of old asphalt is added to the other constituents in the asphalt batch. This is a practice favoured in several countries. Graded re-cycled concretes and asphalt may be stabilized with cement to provide hard standings and secondary road structures. Crushed brick and tile waste has been successfully used as backfill to trenches (36).
8.4
Development Work is reported on the use of pozzolanic additions to concrete mix designs using recovered concrete waste in order to counter the effect of higher water demand and increased cementitious content (37). Quality Control schemes are offered to ensure that re-cycled aggregates comply with given specifications and a Model is proposed for the prediction ot demolition waste in the future (3X). Work is reported on the recovery of raw materials from reclaimed asphalt (39). 9. DISCUSSION There are massive quantities of waste being produced by the major industries of mineral extraction, electricity generation and steel production. There are also innumerable smaller industries which produce waste often of a more complex nature. The latter by their
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nature and quantity are unlikely to offer the opportunity of re-cycling or integration with other materials to form a product available for use in construction. It seems therefore entirely appropriate to consider the main contributors to an increasingly large stockpile of waste. Mounting demands for natural aggregates and a reducing availability of environmentally acceptable placements for waste materials conspire to strengthen the need to bring these waste materials back into construction. There is a predominance in the major sources of waste being considered of silica, alumina and iron. These wastes have a not dissimilar chemical composition which would suggest a similarity of outlet, even though physically there is great variation in particle size and shape. Crushing and grading of individual wastes could reduce this disparity and offer a complimentary span of product with uniformity of outlet. This may not he practical but, in developing the iugument, it should be stressed that similar end uses of these major wastes are perceived. Indeed, it is interesting to note how all are currently accepted for use in road construction to a greater or lesser degree, each opportunity and location of the waste being determined by geographical location, physical capacity of the site and ground soil conditions. All these wastes include a small, aid in some cases significant, proportion of potentially harmful mineral element. Consideration must therefore be given to reducing the potential for leaching out of these elements. Stabilization of waste products with cement offers a solution which is perceived by some to be effective. G are currently investigating the role of cement minerals. MSWl ash when stored is thought to produce secondary minerals and it could be nce of an activating agent that in combination with other wastes or alone with the similar secondary agencies would suffice. It is useful to note the success of combining fly ash, cement and bentonite in cut-off walls (42). Whatever the choice or complexity of combining and I or stabilizing wastes to contain undesirable leaching there is perhaps a more fundamental problem inhibiting the increased use of waste materials in construction. Geographically it appears that the source of major wastes is remote from the main construction activities. There is therefore likely to be a significant transport problem to overcome when trying to relate a waste stockpile to a construction site. Intrinsic production costs of natural aggregate for concrete and road construction iue low. Hence geographically remote waste stockpiles are disadvantaged even though the material may be free for collection. Attention therefore must turn to the practicability of manufactuting construction items with waste materials at the source of those waste materials. It may be that waste heat from the main production process is available to supplement the unit cost of production but avenues must be sought to overcome the transport costs of removing waste materials in bulk. In the event manufactured items may command a premium as well as reducing the stockpile at source. A draconian alternative to initiatives being sought for the inclusion of more waste materials in construction may lie in Legislation. This could take the form of a levy on unit quantities of natural aggregates used, o n transportation of those aggregates, or a simple Directive that all construction above a minimum cost shall include a given proportion of re-cycled waste product. In the more densely populated countries of the developed world this may have an appeal but, it could offer significant logistical, cost and control problems elsewhere.
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10. CONCLUSION The major sources of waste suitable for re-use in construction have been described. reviewed and considered. There is a certain chemical similarity in these major waste materials but, real difference in mineralogyand physical properties. Limited use is being made of these materials largely as a result of geographical location in regard to construction activities. All the major wastes are used in varying quantities for road construction, general fill, to construct embankments and to a limited extent in the production of cement and asphalt. However. the replacement of natural aggregates with re- cycled w u t e materials is minimal. It is an attractive thought that because of similarity of main chemical composition in these wastes it would be relatively easy to suggest that some may combine to offer a real alternative in construction to natural materials. To reduce the opportunity for leaching of certain trace elements cement is commonly used but, given the plethora of chemical agents available it may be possible to promote secondary mineral formations within these combined wastes which would stabilize the offending elements and deny leaching activity. There are real problems associated with the geographical location of wastes in regard to construction activities. It is suggested that thought be given to manufacturing construction items at the source cif waste production, perhaps utilizing waste heat to assist with production procedures and unit costs. Ultimately it may be that the problems of geography, leaching and natural reluctance to use other than natural materials in construction will not be resolved by innovations for the use of waste materials. Reluctantly it is suggested that Legislation may have to be considered to enforce the controlled use of these wastes. This would reduce the risk of environmental pollution and lessen the burden of recovering natural aggregate.
Acknowledgement The Author gratefully acknowledges contributions received from a number of respondents interested in this important subject.
11. REFERENCES I. 2. 3. 4.
5.
Reeves C M. The Production, Properties and Application of Blast-furnace Slag etc. ICT Dip. Cem & Conc Assn, Wexham, 1980 Gutt W and Nixon P J. Use of waste materials in the construction industry, Materials and Structures vol. 12 No.70,1979, 255 - 306 Neville A M. Properties of Concrete, 2nd edition, Pitman, London, 1977, 69 - 70 Lea F M. The Chemistry of Cement and Concrete. 3rd Edition, Edward Arnold 1970 Whitbread M. et al, Occurrence and Utilization of Mineral and Construction Wastes, Dept. Environment. HMSO, London, IW I , 25
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6. 7. X. 9. 10.
I I.
12.
Malolepszy J and Deja J. Immobilization of heavy metal ions by the alkali activated slag cementitious materials, Wascon '94, Maastricht, 1994 Maloleopszy J et al., The granulated foundry slag a%a valuable raw material in the concrete and sand-lime brick production, Wascon '9 1, Maastricht, I99 I , 475 - 47X The Concrete Society. The use of GGBS and PFA in concrete. Technical Report No.40. The Concrete Society, Wexham, London, I99 I Barber E G. et al., PFA Utilization, C E G B, London, 197 I Mathews J D and Gutt W H. Studies of fly ash as a cementitious material, Proc. of Conference. Ash Technology and Marketing, London, 1978 Cabreri J G, Woolley G R et al., Proc. Ash Tech X4, London, 19x4, 303 -312 Cabrera J G and Woolley G R., Fly Ash Utilisation in Civil Engineering, Wascon '94,
13. 14. 15.
I 6. 17.
I x. 19.
20. 21. 22. 23. 24. 25. 26. 27. 2x. 29.
Maastricht, 1994 Papayiaini J. Use of a high - calcium fly ash in blended type cement production, Int. Conf. Blended Cements, Shetfield, 199 I Backes H P and Koch H J., The properties of concrete made with NH - bearing coal fly ash, Betonwerk, vo1.3, I9XX. 71 - 76 Fox H. Pulverised fuel ash as structural fill, Proc. Ash Tech, London, I9X4,495 500 Jones C J F P et al., Reinforced earth trial structure for Dewsbury ring road, Proc. hst. Civ Eng, vol. XX, Part I , 1990,321 -345 Manz 0 E. Lime - Fly Ash stabilization for road building, Proc, Ash Tech., London, 19x4, 505 -5 I 2
Hughes D C., Performance of hardened grouts, Nat. Seminar, Dundee, 1992, X9 I03 Hobbs D W.. Portland - pulverised fuel ash concretes etc.. Proc.Inst. Civ Engrs, Part 2, VOI X5, 19x5, 317 - 331 Dunstan M R H., Development of high fly ash content concrete, Proc.Inst.Civ.Engrs, Part I , vol. 74, 19X3,495 -513 Cabrera J G. et al., Concrete International 1994, to be publi Boral Lytag, Lytag structural guide-lines and properties, 19x9 Boas A and Spanjer JJ., The manufacture and the use of artificial aggregates from fly ash to Aardelite process, Proc. Ash Tech., London, 19x4, 577 - 5x2 Woolley G R and Cabrera J G., Coastal Protection Project (1994), University of Leeds ( in progress) Gera F et al., Utilization of ash and gypsum produced by coal burning power plants, Wascon '9 I , Maastricht, I99 I , 433 -440 Cabrera J G and Zoorob S., Final Report ETSU Contract No. EICS/3580/2669, 199 I University of Leeds. Mulder E and Houtepen S M., Artificial gravel as a substitute in asphaltic concrete Lnt. Coal ash Util. Symp. Orlando, 1991 BS 6543 : 19x5, Use of industrial by-products and waste materials in building and civil engineering, BSI, London 19x5 Lattey S E and Trew J C., State of the art report un the use of low cost materials
911 30. 31. 32. 33. 34.
35. 36.
37. 38.
39. 40. 41.
42.
( Colliery spoil and pfa ) as landfill liners, Internal report. Univ. of Leeds. 1990 Walters G V., Economic treatment of quarrykhina clay waste materials Schuur H M L and Broadbent C P., Slag utilization in the Netherlands Zevenbegen C., Geochemical factors controlling the mobilization of the major elements during weathering of MSW bottom ash. Wascon '94, Maastricht. 1094 Schneider J et al.. Improving the MSWI bottom ash quality by simple in-plant measures. Wascon '94. Maastricht, I994 Hwa T J and Kiat C H., User of ash derived from refuse incineration as a partial replacement of cement, lnt. Conf. Blended Cements, Sheffield. I991 Hendriks Ch F., Ecological use of construction and demolition waste. Private communication, 1994 Folkenberg J., Application of crushed tile and concrete as filling material in pipe trenches, Danish Tech. Inst., 1994 Wainwright P J and Cabrera J G., The performance of concretes made with combinations of pfa and re-cycled concrete, Wascon '94, Maastricht, 1994 Walker I and Trankler J 0 V.. Model for the prediction of demolition waste composition, Wascon '94, Maastricht, 1994 Mulder E et al.. Recovery of raw materials from reclaimed asphalt pavement, Wascon '94. Maastricht, 1994 Glasser F P., Immobilisation potential of cementitious matrices, Wascon '94. Maastricht, 1994 Sprung S et al., Environmental compatibility of cement. Wascon '94 Maastricht, 1994 National Ash, Case Study No. 5. National Power. UK, 1990.
STATE OF THE ART REPORT USE OF WASTE MATERIALS IN CONSTRUCTION DEVELOPMENT G.R.Woolley Civil Engineering Consultant.
963
-
TECHNOLOGICAL
Abstract Massive quantities of waste materials are produced annually in the major industries of mineral extraction, electricity generation and steel production. Each in turn has vast stockpiles of their waste product. Interestingly the major chemical composition of these combined wastes are similar and each are used to a minor extent in the construction of roads, pavements cement production and asphalt. There are numerous other producers of waste materials and there is the important source of demolition waste and scalpings from road surfaces under repair. There is mounting concern for the environmental effect of depositing these wastes to spoil heaps or as in-till to large excavations. especially so in the event of leaching out of certain trace elements. In this paper, the major waste materials are introduced, considered and their utilization reviewed. Where information on development of the several wastes is available this is introduced. To mount any form of assault on the continuing use of natural aggregates or other materials in construction there has to be a fundamental change of emphasis in regard to using re-cycled waste materials. Selection of a suitable and effective stabilizing agent for these materials either in isolation or when combined is important but, although they have very similar chemical compositions, they are physically different. To reduce our dependence on natural aggregates and to increase our use of waste materials in construction there may be the opportunity to create manufactured items closer to the source of waste material production. Alternatively Legislation may be required to enforce such change. 1. INTRODUCTION lncreasingly environmental and commercial pressures are causing Government Agencies, Local Authorities and Business Ventures to question the practice of dumping waste material. The rising cost of disposal, reduction in industrial activity and clamour for preservation of the natural environment, corn bined with increasing commercial pressure, are promoting a renewed interest in ' using waste products. Some waste products have been in limited use for a number of years. Of these, perhaps the most internationally known is blast furnace slag, a by-product resulting from the production of iron. Determined by the way in which molten slag is cooled it may be a coarse mixture of vitreous agglomerates or granules of a shattered vitreous structure. These granules, granulated blast-furnace slag, are available to replace primary materials or for grinding to fotm a latent hydraulic binder. A not too dissimilar situation exists for fly ash, the solid material collected by mechanical o r electrostatic means from furnace exhaust giwes following the burning of coal in electricity generation. From the late 1930's , after its use in the United States of America, fly ash has been increasingly used as a partial replacement of cement in concrete. It has also been widely used to construct lightweight
964 fill embankments. Both slag and fly ash are used in the production of a number of sophisticated product\ like paint, powder etc. but, with tly ash there is a surplus which has to be disposed of somewhere and by someone. The two examples illustrate the dilemma of dumping to waste the balance of these useful materials, a situation which is increasingly becoming an imposition on the environment. Consideration of the current State of the Art presumes that some waste products are being used for construction but, begs the question ' are we concerning ourselves properly and fully in the on-going study and research into further applications '. Do we therefore prioritise our search for new or extended uses into
projects which provide the highest premium on our outlay, or seek to find outlets which will consume the greatest quantities of waste product. Both have advantages but, the environment is unable to distinguish between any or all of the wastes, we have that facility and only our due diligence and effort will incorporate more waste materials in construction, thereby contributing to the continued well being of our varied and several environments. We must also aim to conserve our primary material resource. By using waste products in a controlled way we should be able to reduce despoiling our environment by reducing extraction of primary materials and the deposition of unwanted waste products. Letters were sent to those named persons in ISCOWA who had recorded an interest in Technological Development associated with the use of waste materials in construction. Sadly the response was very limited, indeed most disappointing, even after a reminder to selected individuals but, some information was provided and I formally thank those respondents for their contributions. An analysis is given of data provided and of material found elsewhere in earlier papers and reports. The major individual waste materials are considered in turn, their properties given and current utilisation reported. Where new processes are under development or being newly practised, known details are given. It was not possible to mount a full literature review and in deference to the authors of papers published worldwide the references given are limited.
2. METALLURGICAL SLAGS 2.1 Blast-furnace[ iron ] slags - The Material The process of refining metallic ores involves raising the temperature of the ore in a turnace until the metal separates from the associated ore components and can be collected. The remaining matter, a mixture of ore residues and reactant used to promote separation is referred to as a metallurgical slag. The manner and rate of cooling of this molten slag will produce several different types of solid slag. If allowed to solidify naturally, a dense crystalline material referred to as air-cooled slag results. Slag may be foamed by introducing water jets into the stream of molten slag, causing it to expand and form a lightweight material. When cooled very quickly with large amounts of water it forms a glassy material called granulated slag. Pelletized slag is produced by a more sophisticated process involving water jets, revolving drums and air cascades resulting in small pellets of foamed or granulated slag ( I ).
965
2.2 Composition All slags have similar composition. In crystalline blast-fumace slag the principle mineralogical phase lies in the series from akermanite [ 2CaO.Mg0.2Si0, ] to gehlenite [2CaO.A1 0 .Si 0 ] (2). When slags have a high lime content they may contain dicalcium silicate [2CaO.SiO 1. This inclusion can cause disruption of the slag through volume increase. If sulphur is present in a reactive, leachable form. mainly as calcium sulphide, problems may also occur. Typical chemical composition of a number of blast-furnace slags is reproduced in table I . Table I Typical Chemical Compositions of Blast-furnace Slag [weight%] Australia SiO, 33-37 Al;,O> 15-18 CaO 39-44 MgO 1-3 TiO, 0.6 Fe,O, 0.7 MnO 0.3-1.5 N a O 0.2 K 0 0.5 s 0.6-0.8
USA Germany 34-38 35 11-15 12.2 45-47 41 1-3 X 1.3-4.5
0.25 0.5 1.2 incl 0.6
Norway
34-38 7- 10 40-4x 6- 13 0.7- 1. I
0.1-0.4
UK 31-36 13.5 33-45 4-15 0.x
0.5 0.8
lridia 30-35
18-26 30-36 3-9
<0.5 <1.0
0.7 0.8 0.53
2.3 Utilisation Transport costs bear heavily on the use of slags worldwide but in general the percentage used is very high and in some countries approaching 100%. Road making uses the most amount of this material ahead of cement manufacture. Use of granulated slag to replace natural aggregate is well understood. Using the specifications for natural aggregates it may be used in blinding, base, sub-base and as coated macadam for the wearing course in roads. Granulated slag properties as a latent hydraulic binder suits it to act as the binder in gravel slag and sand slag, both used as base material in roads. When combined with water and an alkali activator it will produce cementing compounds, this hydraulic activity depending on the chemical compo on of the slag, physical properties and glass content (2). When combined with portland cement and water the alkalis and lime produced in the hydration of cement acts as the catalyst in producing the cementitious reactions of granulated slag (3). There is extensive use of this property in the manufacture of several slag cements which may contain granulated blast-furnace slag in proportions as high as 90%. Useful to reduce the heat of hydration in hardening concrete, these slag cements have lower early strength but may attain equivalent strength with time. The higher slag contents of these cements provide more resistance to attack by sulphate solution (4).
966 ple process and, subject to soundness, once Production of air cooled slag in roads, as dense aggregate in concrete or as cooled it may be crushed and graded fertilizer. It does need space for cooling pits a filter material. It is also crushed to but, the newer, larger furnaces are being equipped with granulating plant in order to maximise the advantages offered by blast-furnace slag cements. Rapid cooling by compressed air producers the slag wool product, a material used for insulating purposes. Pelletized slag offers a lightweight aggregate, the pellets being of the vitrified form and, like granulated slag, have the properties of a latent hydraulic binder. This attribute lends itself to the soil stabilisation process and production of lightweight building blocks (1).
3. STEELSLAG 3.1 The Material More variable than slag produced from iron ore smelting, steel slags result from the conversion of pig iron to steel. Chemical compositions of these slags introduce this variability but, as with blast-furnace slag the reactant used in smelting and impurities present in the pig iron produce a steel slag. Amongst these compounds is lime, which will quickly hydrate and magnesia with a slower hydration period. The basic oxygen process of steel making tends to produce a more vmable slag than the open hearth process (2).
3.2 Composition Not dissimilar in appearance to an igneous rock, steel slag chemical compositions vary greatly but, generally they have a higher CaO I SiO proportion than blast-furnace slags. Typical chemical constituents are shown in table 2. Of concern are the free calcium, magnesium oxides and metallic iron phases found. These compounds will hydrate and expand in the presence of moisture. Table 2 Typical Chemical Constituents of Steel Slags ( weight % ) Basic Oxygen Steel Open Hearth Electric Arc Germany USA UK USA UK Low C High C CaO 47.2 59.9 40.3 33-5 I 36.17 3 1-50 SiO, 14.X 13.8 21.7 9-19 18.02 11-24 Fe,O, 1.53 10.5 16.3 24-45 17.46 5-30 (total iron) x.54 5-18 3.8 0.5-3 Al,o, 0.6 2.10 9.96 2-8 1.5 0.9 4.4 0.5-4 MgO Mn 0 5.4 3.0 4 3-10 4 6-22 3.3 Utilisation Iron and iron compounds are contained in steel slag and much is recycled as a minor component in blast-furnace charges. When the metallic iron is removed the remainder is principillly used for land reclamation and unrestrained fill to allow for any expansion that may occur. Presence of dicalcium silicate restricts the material from use as concrete aggregate but it is used for road making when unbound or bound with bitumen as a bound macadam. After controlled pre-weathering, steel slags provide a good quality
967 artificial stone for road making (5).The material may also be ground to provide a high quality fertilizer and depending on slag composition will provide phosphate or lime as a soil conditioner. Granulated electric arc furnace steel slag is used for cement manufacture in some puts of the world with evidence of hydrate formation similar to portland cement. 3.4 Developments Work is reported o n the use of alkali activated slag cement to immobilize heavy metal ions in waste ( 6 ) .Inclusion of slag in the production of lime - sand bricks has shown to be promising whereby slag may replace up to 15% of the lime constituent in the process (7 )
4. FLY ASH ( referred to in the United Kingdom as Pulverised Fuel Ash ) 4.1 The Material
Coal used for steam raising in modern power stations is finely ground prior to combustion. This will typically reduce it in size from an incoming average passing a 12.7 mm sieve of 94% to 80% passing a 75 micron sieve before entering the furnace. The constituents, which may be separated to some degree by grinding, consist of carbonaceous matter and various minerals in the form of shales, clays, sulphides and carbonates. This pulverized coal is injected into the furnace at high speed in a stream of hot air and it burns instantaneously in the range of 1500 +/- 2000 deg.C . This is above the melting point of most minerals present and as a result they undergo various chemical and physical changes. For example pyrite is converted into oxides of iron, including spherical particles of magnetite ; clay forms glass spheres of complex sulphates. The exact nature of changes depends on the type of coal, fineness of grinding, temperature and retention period within the hot zone of the furnace. Between 75 - X5% of the resulting ash is carried out of the furnace with the flue gases and is known as fly ash. The remainder falls to the bottom of the furnace where it sinters to form a c o m e r material known as furnace bottom ash. Fly ash is extracted from the flue gases by electrostatic precipitators and / or mechanical methods and is collected in a series of hoppers for disposal ( 8 ) .
4.2 Composition Fly ash consists principally of glass spheres together with some crystalline matter and a varying amount of carbon. Overall colour will depend to some degree on the proportions of carbon, iron and moisture. It will have a specific gravity in the range 1.98 to 2.38 and average specific surface in the range of 260 to 595 sq.m / kg (10). Collected ash will be graded. Some suggest that the finer ash is more reactive but it dws not follow that all fine ashes have high reactivities ( I I ) . The grading from any given source under steady load is normally very consistent. There are three predominant elemene in fly ash, silicon, aluminium and iron, the oxides of which together account for approximately 75% of the material. Silicon is present partly in the crystalline form of quartz (SiO,) and in association with the aluminium as mullite (3A120, ,2Si02 ), the rest in the glassy phase. The iron appears partly as the oxides magnetite (Fe203),and haematite (Fe,O,), the rest in the glassy phase. Carbon, determined as loss-on-ignition, is present in fly ash in amounts
968
which vary with the efficiency of combustion. A typical chemical analysis of fly ash collected from a modern furnace burning sub - bituminous coal is given in table 3 (12). The operating regime differs when lignite is used as fuel and this will produce an ash which differs markedly in respect of calcium oxide. It is known as a high lime fly ash (cli~ss C) and has a typical chemical composition as shown in table 3 (13). Examination of fly ash taken from the flue gases of low NOx boilers which carry a limited amount of ammonia have been shown to have no affect on the production and properties of concrete (14). Table 3 Typical Chemical Composition of Fly Ashes From sub - bituminous coal Si 0, 49 - 53 ,4203 25 -29 Fe20, x- II Ca 0 1.x - 5.3 Mg 0 1.3 - 2.2 K2 0 2.2 - 3.7 Na, 0 0.7 - 1.5 I .o - 2.0 Ti 0, 0.3 - 1.7 so3 Loss on Ignition 1.5 - X.8 Blaine specific surface 220 - 5 10 (sq. m / k g ) Fineness >45 micron 6.5 - 25.0
( weight % ) From lignite coal 2 I .o - 35.0
10.0 - 14.0
4.5 - 6.5 30.0 - 45.0 (10% free) 1.5 - 3.0 0.4 - 0.9 0.5 - 1.0 --_4.0 - x.0 3.0 - 7.0 250 - 2x0
----
Utilisation Fly ash has been used quite extensively as a fill material in road construction. When placed and compacted at optimum moisture content ( generally in the range 17 - 23% ) it has a lower density than most other fill materials, an advantage when placed on highly compressive soils. Many fly ashes have self - cementing properties when compacted which reduce settlement potential within the fill mass, a distinct advantage when placed to bridge abutments (15). Mostly however, they are trost susceptible and d o require a minimal protection. Recent developments in the technique of reinforced soil embankments have shown how suitable tly ash can be for this application (16). Stabilization of tly ash with lime or cement has allowed the material to be used for sub - base construction and as a binder it may be used to improve the physical properties of soil either alone or with lime or cement. Lime - fly ash mixes have been successfully used to stabilise a range of materials but, when clay is present the lime appears to react primarily with the clay ( I 7). Numerous applications of the use of cement / tly ash grout mixes are reported for the filling of mine workings, rock fissures and the like ( 18). As a pozzolanic material fly ash will react with lime in the presence of water. This cementitious property is used in concrete where fly ash will react with the hydration products of portland cement. It may be included by intergrinding with the cement clinker to form blended cement or included in the site mixer with portland cement. The physical properties of fly aqh produce a rheological change in fly ash concrete which allows a small
4.3
969
reduction in water content when compared to an all cement concrete of similar workability ; generally thought to give a 3 8 water reduction for each 10% cement replaced with tly has a lower temperature rise and i ash (19). Hardening mes experienced in large sections. affected by high tern mass concrete in dams and there has been considerable use of the material in these structures. Recently the traditional method of including and placing concrete in dams has received competition from the new practice of roller compacted concrete, a formation whereby layers of low water / cementitious concrete are built - up and compacted insitu by roller. In this way it is claimed that construction costs are reduced through a shorter construction programme and inclusion of high volumes of tly ash often in the range 40 60% (20). In Spain similar and even larger volumes of fly ash have been used in dam structures placed and compacted conventionally (2 1). Early records of fly ash structural concrete report inclusion of the material in small proportions by volume. Introduction of weigh batching at the mixer, perhaps more sophisticated mix designs and increasing awareness of the potential engineering and commercial benefits has raised the general replacement level of cement to 30%. Inclusion by weight of 30 - 40% is not uncommon and there are now a variety and increasing number of structures built with fly ash concrete. Fly ash is widely used for the production of lightweight aggregate. Using a sintering process pellets are formed in a variety of ways (22) (23). Sinter cake may be formed at I 150 - 1200 deg.C , a product used in the manufacture of building blocks. The lightweight aggregate produced is of good regular shape of high quality with moderate water absorption, used for structural lightweight concrete. Aerated concrete blocks are made in an autoclave process using a mixture of fly ash. cement and sand with an air entraining agent. This produces a high quality thermally efficient building block.
4.4 Development The proportions of fly ash being included in concrete as cement replacement continues to increase. In real terms however, the total amount of the material used in concrete is small compared to total production. Development work is in progress to design high volume fly ash boulders (24) and other structural elements (25), the latter including FGD gypsum with fly ash, for use offshore as coastal protection. Evaluation continues on a trial section of road surfacing which included fly ash as replacement for filler in the asphalt ( 2 6 ) .This gave a significant reduction in energy demand in the production process. The residues of tluidised bed combustion have been successfully tried in the making of artificial gravel (27). There are products in research which combined with fly ash produce construction materials suitable to replace natural aggregates in use today. This is early work, shrouded in commercial secrecy but it does illustrate a willingness to use waste materials. 5.0 COLLIERY SPOIL ( Minestone Waste )
5.1 The Material Colliery spoil is produced during the extraction of coal by deep mining. It may be pure rock removed in formation of underground roadways, the coarse material separated from the coal in the washery, tine material resulting from degradation during the mining
and transportation or tailings collected in the final separation. The latter which is almost total coal is used elsewhere for combustion. Rock types associated with coal seams are mudstones, siltstones, sandstones and shale. Quartz and clay minerals, kaolinite and illite, are the principle minerals included. Naturally grey in colour this shaley material weathers rapidly. When placed in spoil heaps and compacted in layers to exclude oxygen it will remain in the excavated form but, the older heaps often suffered combustion and the resulting " burnt shale " is a reddish-coloured material with higher strength and better resistance to weathering ( 5 ) .
5.2 Composition Spoil will contain varying amounts of carbonaceous matter but, the principle minerals will comprise quartz and the clay minerals kaolinite and illite. Chemically colliery spoil will typically include Si 0, , range 38 - 60%, A1,0, ,range 14 - 30% and Fe,O,, range 3 - 1 I %. It will include a number ot other minor elements. Chemical changes occur when spontaneous combustion has taken place which include il reduction in the proportwn of combustible matter, a breakdown of clay minerals, decomposition of carbonates and the oxidisation of sulphides to soluble sulphates. The latter change producing an undesirable increase in sulphate content affecting possible use of the material. Precautions may be necessary to avoid close contact with other engineering materials if the shale has a high sulphate content (2).
5.3 Utilisation Construction of embankments using colliery spoil is perhaps the only situation where large scale quantities are used. It is used as fill beneath dwellings and roads, the burnt shale being the preferred material because of its good grading and granular nature. Tests may be necessary to ensure against frost damage and limits may be placed on sulphate content. To minimise the potential of frost heave shale has been stabilised with ;L nominal amount of cement. Precautions may be necessary to prevent sulphate and acid attack of concrete which may have direct contact with colliery burnt shale and the use ot appropriate cements and / or pozzolanas are recommended (27). Colliery spoil has been widely used for brick making where carbonaceous matter in the spoil can contribute to the energy requirements in the kiln. A number of practical points associated with the use of colliery spoil in brick making contribute to its relatively variable appearance but, it makes a brick with good durability. Colliely spoil is also used in the production of a lightweight aggregate. The two processes used, deliver a rough, angular aggregate suitable for use in the production of lightweight concrete building blocks. 5.4 Development Studies are progressing into the use of colliery spoil in combination with tly ash, cement and bentonite for liners to waste disposal areas (29). There is continuing interest in using large quantities of the material off - shore in the formation of protective islands. In this situation it will be necessary to overcome the risks of limited pollution from surface washings and the plate like structure of shales. Perhaps in cornbination with cement and another tiller new formulations can be found.
97 I 6. MINING and QUARRYING WASTE
6.1 The Material Considerable quantities and notably different wastes are produced by the mining and quarrying industries. Those wastes produced in large quantity generally have a chemical analysis in which Si 0, predominates with varying amounts of Fe203and A203 and lesser percentages of the minor elements. Quiurying wastes will include overburden. reject rock and the fine material arising from transportation and washing. Mineral mining produces waste rock removed to expose the ore and tailings obtained through the separation of minerals from the ore. Geological formations will determine the composition of wastes which will vary by site and operation. Rock type, excavation method , transportation and preparation process are the determinants in particle size distribution. The coarser fraction of wastes are usually deposited in spoil heaps local to the source 01 used to provide landscape screening. The finer material is generally transported as water slurry to settling lagoons. 6.2 Utilisation Amongst the most common uses are for road construction or the manufacture of calcium silicate products. The coarser fraction from a number of mining and quarrying wastes are used in road construction or to produce concrete aggregate. These include the wastes from the winning of copper ore, iron and gold ores in addition to the rejected rock removed in overburden to other quarrying activities (2). The p produces a coarse waste of size > 20mm to 75 micron and a fi - 10 micron. The coarser material is used in block making and the fine sand for concrete ). China clay and laterite are used in brick making or burnt to form umber of fine wastes are used as fillers in bitumen or in the manufacture of products. Manganese mud is used as a pigment to colour bricks and ground slate as a filler in paints and plastics. Slate granules provide the decoration to bitumen felts and it has also been used to produce a lightweight aggregate. Phosphate sand can be used in concrete or for ground modelling but, the colloidal clay sized particles of phosphate slime remains unstable without mixing with sand (2).
6.3 Developnient Geographical remoteness of mining ilnd quarrying activities presents a major vansport problem for the considerable quantities of waste being produced. There appears to be little work being done on the utilisation of these many wastes in the production of a manufactured item. Investigations have been noted into the suitability of using certain shales and slates for the production of lightweight aggregates however, the energy demanded to cause them to expand makes the unit production costs too expensive, 7. MUNICIPAL SOLID WASTE INCINERATOR ASH 7.1 The Material Municipal Solid Waste is the ash resulting from the incineration of municipal
912 waste. It is quite variable, the product being determined by the process conditions, furnace type and temperature and composition of the feedstock which is largely domestic and trade refuse. Typically the majority of the ash has particles of less than 3mm with mainly glass, ceramics and slag-like matenal. The more variable nature of this ash suggestc a noticeably greater environmental impact, one demanding strict control of use or placement. 7.2 Composition In primary selection the rnedlic refuse is recovered for re-cycling. Determined by the original composition of the waste, these ashes show great variations in chemical composition. They are highly siliceous with minor components of iron calcium and aluminium. There are also numerous trace elements. Typically the Dutch AVI- Slag composition produced from MSW is given in table 4.
Table 4 General Composition AVI - slag 4% organic material glass 18% rubble, ceramics I1% 13% slag - like material 2% iron 2% nun - ferro metals fraction < 3mm 50% ( not recognizable
)
7.3 Utilization In Holland, recognising the potential for environmental impact, there are strict rules for its use. These include limits on leachability, soil-ground water protection, impermeable encasement and the subsequent facility for removal. The coarse fraction > 2mm is used as a concrete aggregate where generally only 15 - 20% of natural ,coarse aggregate may be replaced in non - reinforced items. However high chloride contents may be present, zinc and aluminium can lead to expansion and the other engineering parameters of concrete, setting time, shrinkage and high cement I water contents may be affected (3 I). This aggregate has also been included in the composition of asphalt. The pulverised refuse has been used to replace a small proportion of the pulverised coal feed for cement production without affecting the cement quality. 7.4 Development It is the practice to store this ash in the open air after production. This is to allow natural stabilization of the product and reduce the potential for leaching of certain metals. Research is indicating that there is secvndary mineral formation during this storage period,as tound in the natural pozzolana. which influence the leaching of contaminant\ (32). Post - treatment systems which include washing, sintering and smelting are being considered to ctabilize the material, thereby widening the scope for inclusion in construction. These in-plant measures may provide a vehicle for improving the ash quality and assist with fixation of heavy metals (33). Exploratory work is reported of the use of
973 ash derived from refuse incineration as a partial replacement of cement (34). Each secondary treatment, including storage before use, carries a clear cost implication. 8. DEMOLITION WASTE 8.1 The Material Largely demolition waste consists of concrete and brick rubble with minor quantities of secondary materials including steel, tiles, plaster boards, wood etc. The majority of the metal will be re-cycled and individual contractors will sepwate the minor componena from the bulk concrete and brick waste. There is also waste macadam and asphalt scalpings recovered from worn road surfaces in the process of repair or resurfacing.
8.2 Composition Concrete waste may be infinitely variable in particle size at recovery but, brick or concrete building block will be of a more uniform size. Subject to controlled c r u s h n g both will be available for grading to demand. Asphalt and macadam planings from roads under replcir will provide a coated aggregate material > 40mm m size. Reinforced concrete is clearly more difficult to handle but, it may be recovered with the reinforcing steel going for re-cycling.
8.3 Utilization Re-cycled concrete and brick rubble crushed to produce graded aggregates are used in foundations to properties and as sub-base to roads. The graded aggregate has been successfully used in new concrete but, it h been shown to be more water demanding and requires an increased cement content to fu 1 strength demands (35). The greater proportion of road planings are re-used in footpaths, car parks and the like and when crushed and graded used a s road sub-base material. Using the Minesotta process up to 25% of old asphalt is added to the other constituents in the asphalt batch. This is a practice favoured in several countries. Graded re-cycled concretes and asphalt may be stabilized with cement to provide hard standings and secondary road structures. Crushed brick and tile waste has been successfully used as backfill to trenches (36).
8.4
Development Work is reported on the use of pozzolanic additions to concrete mix designs using recovered concrete waste in order to counter the effect of higher water demand and increased cementitious content (37). Quality Control schemes are offered to ensure that re-cycled aggregates comply with given specifications and a Model is proposed for the prediction ot demolition waste in the future (3X). Work is reported on the recovery of raw materials from reclaimed asphalt (39). 9. DISCUSSION There are massive quantities of waste being produced by the major industries of mineral extraction, electricity generation and steel production. There are also innumerable smaller industries which produce waste often of a more complex nature. The latter by their
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nature and quantity are unlikely to offer the opportunity of re-cycling or integration with other materials to form a product available for use in construction. It seems therefore entirely appropriate to consider the main contributors to an increasingly large stockpile of waste. Mounting demands for natural aggregates and a reducing availability of environmentally acceptable placements for waste materials conspire to strengthen the need to bring these waste materials back into construction. There is a predominance in the major sources of waste being considered of silica, alumina and iron. These wastes have a not dissimilar chemical composition which would suggest a similarity of outlet, even though physically there is great variation in particle size and shape. Crushing and grading of individual wastes could reduce this disparity and offer a complimentary span of product with uniformity of outlet. This may not he practical but, in developing the iugument, it should be stressed that similar end uses of these major wastes are perceived. Indeed, it is interesting to note how all are currently accepted for use in road construction to a greater or lesser degree, each opportunity and location of the waste being determined by geographical location, physical capacity of the site and ground soil conditions. All these wastes include a small, aid in some cases significant, proportion of potentially harmful mineral element. Consideration must therefore be given to reducing the potential for leaching out of these elements. Stabilization of waste products with cement offers a solution which is perceived by some to be effective. G are currently investigating the role of cement minerals. MSWl ash when stored is thought to produce secondary minerals and it could be nce of an activating agent that in combination with other wastes or alone with the similar secondary agencies would suffice. It is useful to note the success of combining fly ash, cement and bentonite in cut-off walls (42). Whatever the choice or complexity of combining and I or stabilizing wastes to contain undesirable leaching there is perhaps a more fundamental problem inhibiting the increased use of waste materials in construction. Geographically it appears that the source of major wastes is remote from the main construction activities. There is therefore likely to be a significant transport problem to overcome when trying to relate a waste stockpile to a construction site. Intrinsic production costs of natural aggregate for concrete and road construction iue low. Hence geographically remote waste stockpiles are disadvantaged even though the material may be free for collection. Attention therefore must turn to the practicability of manufactuting construction items with waste materials at the source of those waste materials. It may be that waste heat from the main production process is available to supplement the unit cost of production but avenues must be sought to overcome the transport costs of removing waste materials in bulk. In the event manufactured items may command a premium as well as reducing the stockpile at source. A draconian alternative to initiatives being sought for the inclusion of more waste materials in construction may lie in Legislation. This could take the form of a levy on unit quantities of natural aggregates used, o n transportation of those aggregates, or a simple Directive that all construction above a minimum cost shall include a given proportion of re-cycled waste product. In the more densely populated countries of the developed world this may have an appeal but, it could offer significant logistical, cost and control problems elsewhere.
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10. CONCLUSION The major sources of waste suitable for re-use in construction have been described. reviewed and considered. There is a certain chemical similarity in these major waste materials but, real difference in mineralogyand physical properties. Limited use is being made of these materials largely as a result of geographical location in regard to construction activities. All the major wastes are used in varying quantities for road construction, general fill, to construct embankments and to a limited extent in the production of cement and asphalt. However. the replacement of natural aggregates with re- cycled w u t e materials is minimal. It is an attractive thought that because of similarity of main chemical composition in these wastes it would be relatively easy to suggest that some may combine to offer a real alternative in construction to natural materials. To reduce the opportunity for leaching of certain trace elements cement is commonly used but, given the plethora of chemical agents available it may be possible to promote secondary mineral formations within these combined wastes which would stabilize the offending elements and deny leaching activity. There are real problems associated with the geographical location of wastes in regard to construction activities. It is suggested that thought be given to manufacturing construction items at the source cif waste production, perhaps utilizing waste heat to assist with production procedures and unit costs. Ultimately it may be that the problems of geography, leaching and natural reluctance to use other than natural materials in construction will not be resolved by innovations for the use of waste materials. Reluctantly it is suggested that Legislation may have to be considered to enforce the controlled use of these wastes. This would reduce the risk of environmental pollution and lessen the burden of recovering natural aggregate.
Acknowledgement The Author gratefully acknowledges contributions received from a number of respondents interested in this important subject.
11. REFERENCES I. 2. 3. 4.
5.
Reeves C M. The Production, Properties and Application of Blast-furnace Slag etc. ICT Dip. Cem & Conc Assn, Wexham, 1980 Gutt W and Nixon P J. Use of waste materials in the construction industry, Materials and Structures vol. 12 No.70,1979, 255 - 306 Neville A M. Properties of Concrete, 2nd edition, Pitman, London, 1977, 69 - 70 Lea F M. The Chemistry of Cement and Concrete. 3rd Edition, Edward Arnold 1970 Whitbread M. et al, Occurrence and Utilization of Mineral and Construction Wastes, Dept. Environment. HMSO, London, IW I , 25
976
6. 7. X. 9. 10.
I I.
12.
Malolepszy J and Deja J. Immobilization of heavy metal ions by the alkali activated slag cementitious materials, Wascon '94, Maastricht, 1994 Maloleopszy J et al., The granulated foundry slag a%a valuable raw material in the concrete and sand-lime brick production, Wascon '9 1, Maastricht, I99 I , 475 - 47X The Concrete Society. The use of GGBS and PFA in concrete. Technical Report No.40. The Concrete Society, Wexham, London, I99 I Barber E G. et al., PFA Utilization, C E G B, London, 197 I Mathews J D and Gutt W H. Studies of fly ash as a cementitious material, Proc. of Conference. Ash Technology and Marketing, London, 1978 Cabreri J G, Woolley G R et al., Proc. Ash Tech X4, London, 19x4, 303 -312 Cabrera J G and Woolley G R., Fly Ash Utilisation in Civil Engineering, Wascon '94,
13. 14. 15.
I 6. 17.
I x. 19.
20. 21. 22. 23. 24. 25. 26. 27. 2x. 29.
Maastricht, 1994 Papayiaini J. Use of a high - calcium fly ash in blended type cement production, Int. Conf. Blended Cements, Shetfield, 199 I Backes H P and Koch H J., The properties of concrete made with NH - bearing coal fly ash, Betonwerk, vo1.3, I9XX. 71 - 76 Fox H. Pulverised fuel ash as structural fill, Proc. Ash Tech, London, I9X4,495 500 Jones C J F P et al., Reinforced earth trial structure for Dewsbury ring road, Proc. hst. Civ Eng, vol. XX, Part I , 1990,321 -345 Manz 0 E. Lime - Fly Ash stabilization for road building, Proc, Ash Tech., London, 19x4, 505 -5 I 2
Hughes D C., Performance of hardened grouts, Nat. Seminar, Dundee, 1992, X9 I03 Hobbs D W.. Portland - pulverised fuel ash concretes etc.. Proc.Inst. Civ Engrs, Part 2, VOI X5, 19x5, 317 - 331 Dunstan M R H., Development of high fly ash content concrete, Proc.Inst.Civ.Engrs, Part I , vol. 74, 19X3,495 -513 Cabrera J G. et al., Concrete International 1994, to be publi Boral Lytag, Lytag structural guide-lines and properties, 19x9 Boas A and Spanjer JJ., The manufacture and the use of artificial aggregates from fly ash to Aardelite process, Proc. Ash Tech., London, 19x4, 577 - 5x2 Woolley G R and Cabrera J G., Coastal Protection Project (1994), University of Leeds ( in progress) Gera F et al., Utilization of ash and gypsum produced by coal burning power plants, Wascon '9 I , Maastricht, I99 I , 433 -440 Cabrera J G and Zoorob S., Final Report ETSU Contract No. EICS/3580/2669, 199 I University of Leeds. Mulder E and Houtepen S M., Artificial gravel as a substitute in asphaltic concrete Lnt. Coal ash Util. Symp. Orlando, 1991 BS 6543 : 19x5, Use of industrial by-products and waste materials in building and civil engineering, BSI, London 19x5 Lattey S E and Trew J C., State of the art report un the use of low cost materials
911 30. 31. 32. 33. 34.
35. 36.
37. 38.
39. 40. 41.
42.
( Colliery spoil and pfa ) as landfill liners, Internal report. Univ. of Leeds. 1990 Walters G V., Economic treatment of quarrykhina clay waste materials Schuur H M L and Broadbent C P., Slag utilization in the Netherlands Zevenbegen C., Geochemical factors controlling the mobilization of the major elements during weathering of MSW bottom ash. Wascon '94, Maastricht. 1094 Schneider J et al.. Improving the MSWI bottom ash quality by simple in-plant measures. Wascon '94. Maastricht, I994 Hwa T J and Kiat C H., User of ash derived from refuse incineration as a partial replacement of cement, lnt. Conf. Blended Cements, Sheffield. I991 Hendriks Ch F., Ecological use of construction and demolition waste. Private communication, 1994 Folkenberg J., Application of crushed tile and concrete as filling material in pipe trenches, Danish Tech. Inst., 1994 Wainwright P J and Cabrera J G., The performance of concretes made with combinations of pfa and re-cycled concrete, Wascon '94, Maastricht, 1994 Walker I and Trankler J 0 V.. Model for the prediction of demolition waste composition, Wascon '94, Maastricht, 1994 Mulder E et al.. Recovery of raw materials from reclaimed asphalt pavement, Wascon '94. Maastricht, 1994 Glasser F P., Immobilisation potential of cementitious matrices, Wascon '94. Maastricht, 1994 Sprung S et al., Environmental compatibility of cement. Wascon '94 Maastricht, 1994 National Ash, Case Study No. 5. National Power. UK, 1990.