Fly Ash as a Sustainable and Waste Material Rosli Hainin1, Hamed Niroumand1,a 1
Department of geotechnics and transportation engineering, Universiti Teknologi Malaysia a: Corresponding author:
[email protected]
Khairul Anuar Kassim, Ramli Nazir Department of geotechnical engineering, Faculty of civil engineering, Universiti Teknologi Malaysia
ABSTRACT The burning of coal in the process of generating electricity, leads to the production of solid waste such as the fly ash and bottom ash. Numerous studies had been carried out to determine the mechanical properties of fly ash and also bottom ash. An extension of these studies is necessary in order to explore further usage of these ashes, in particular when mixed together in optimum percentage. The first aim of this study is to provide a review of fly ash in construction. The second aim is to develop knowledge readers and researchers for advantages and disadvantages of fly ash to help focus future research in this area. In this paper, a brief review is presented of the theory and properties practice of fly ash as a waste and sustainable material. Mechanisms are discussed on fly ash in its applications, improves the workability, minimizes cracking due to thermal and drying shrinkage, and enhances durability to reinforcement corrosion, sulfate attack, and alkali-silica expansion.
KEYWORDS: Fly ash, Waste material, Sustainable material, Anti-corrosion
INTRODUCTION For the past several years, there have been limited studies to incorporate some of waste materials into HMA. Materials involved to date include ground rubber tires, ground glass, asphalt shingles, contaminated sand/soils, incinerator ash and various kinds of waste polymers (Waller, 1993). There are perhaps other waste materials that could be included in similar studies of hot mix asphalt in the future. One governing criteria would be to quantify material available for use. There must be a sufficient amount and a continuous supply in order for a specific material to be considered for use. There are two primary factors that must be taken into account when the matters of incorporating waste materials into hot mix asphalt are considered. One consideration is cost, there needs to be a balance between disposals of the waste material in the normal manner as compared to incorporation into the hot mix asphalt. A second consideration is the effect on quality and performance of the HMA. It would be poor economics indeed to incorporate a waste material that substantially increases the cost of the HMA and at the same time shortens the service life or increase maintenance costs. (Waller, 1993). Strategies need to evolve for sustainable development. Civil engineers are among the group of professionals who supervise use of large quantities of natural and processed materials in construction activities such as buildings, - 39 -
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highw way facilities,, water resouurces facilitiees, and envirronmental ap pplications. These T materiaals use naturaal resources and consum me large quantities of energy e to exxtract, proceess, and trannsport. Thereffore, civil en ngineers are in i a unique position p to apply a principles of sustainnable develoopment to connstruction maaterials procuurement. (Tu uncer B. Edil, 2006). Susstainable dev velopment reequires that enngineers emp ploy sustainaable engineeering practicees that meet additional constraints inn terms of envvironment beeing sustainaable. This cooncept of ennvironmentallly sustainabble project iss often referreed to in a sho ort hand as green g such as a “green buiildings” and “green high hways.” (Tunncer B. Edil, 2006). 2 Worldd industries aannually gennerate millionns of metric tons t of solid by-productss. Most of these materials have been landfilled l in the developped countriess at considerable cost sinnce the mental regulaations in the late 1970s and early 19880s. Recentlyy there incepttion of modeern environm has beeen a shift in i societal atttitudes resuulting in stroong interest in i developin ng beneficiall reuse markeets for indusstrial by prooducts. As a result, envvironmental regulations r h have changeed and Green benefiicial reuse off industrial by-products b is now perm missible in a variety of applications. a highw ways concept aims at encoouraging andd acceleratingg the wide sppread use of recycled r matterials. Fly assh and many other industtrial by-prodducts can be used beneficially as hig ghway constrruction materiials. (Miller and a Collins 1976)
Fly ash Flly ash is the finely f divideed residue thaat results from the combuustion of pullverized coall and is transpported from the combusttion chambeer by exhausst gases. Ovver 61 millio on metric tons (68 y ash were prroduced in 2001. millionn tons) of fly
Fly ash sou urce Flly ash is prooduced by coal-fired c eleectric and stteam generaating plants. Typically, coal c is pulverrized and blo own with air into the boiller's combustion chambeer where it im mmediately ignites, generaating heat annd producing a molten miineral residuue. Boiler tubbes extract heeat from the boiler, coolinng the flue gaas and causiing the molteen mineral reesidue to harrden and forrm ash. Coarrse ash particlles, referred to as bottom m ash or slagg, fall to the bottom of thhe combustion chamber,, while the ligghter fine ashh particles, teermed fly ash h, remain susspended in thhe flue gas. Prior P to exhaausting the fluue gas, fly ash a is removved by partiiculate emisssion control devices, suuch as electrrostatic precippitators or filtter fabric bagg houses.
F Figure 1: M Method of flly ash transffer can be dry,wet d or both.
Fly ash applic cations Cuurrently, oveer 20 millionn metric tonns (22 millioon tons) of fly f ash are used u annuallly in a varietyy of engineeering applicaations. Typiccal highway engineering applicationss include: poortland cemennt concrete (P PCC), soil and a road basee stabilizatioon, flowable fills, grouts,, structural fill f and asphallt filler.
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Fly ash a pote ential Flly ash is mosst commonlyy used as a po ozzolan in PC CC applicatiions. Pozzolaans are siliceeous or siliceoous and alum minous materrials, which in a finely ddivided form m and in the presence of water, react with w calcium m hydroxide at ordinary temperatures t s to produce cementitiou us compoundds. The uniquee spherical sh hape and parrticle size disstribution off fly ash makke it a good mineral m fillerr in hot mix asphalt a (HM MA) applicatiions and im mproves the fluidity of flowable fiill and grouut. The consisstency and abundance a o fly ash inn many areaas present unnique opporttunities for use in of structuural fills and other highw way applicatioons.
Fly Ash A Enviironmen ntal bene efits. Flly ash utilizaation has siggnificant envvironmental benefits b inclluding: (1) Producing1 ton of cemennt will produ uce 1 ton of CO C 2,so replaccing 1 ton off cement by fly ash mean ns preventingg 1 ton of CO O2 going intoo atmospheree; (2) Net reeduction in energy use and greenho ouse gas andd other adversse air emissions when flyy ash is used d to replace manufacture m d cement or hydrated lim me; (3) Reducction in amouunt of coal coombustion byproducts thhat must be disposed d off in i landfills, and a (4) Conseervation of otther natural resources r andd materials.
Fly As sh Produ uction Flly ash is prodduced from the t combustiion of coal inn electric utiility or indusstrial boilers. There or traveling grate, are foour basic typpes of coal-ffired boilers:: pulverized coal (PC), stoker-fired s cyclonne, and fluid dized-bed coombustion (F FBC) boilerss. The PC bo oiler is the most m widelyy used, especiially for largge electric geenerating uniits. The otherr boilers are more comm mon at industtrial or cogeneration facillities. Fly ash is captureed from the flue gases using u electrostatic precippitators mmonly refeerred to as baghouses. The physicaal and (ESP) or in filterr fabric colllectors, com a combbustion meth hods, coal soource, and particle p chemiical characteeristics of flyy ash vary among shape..
Taable 1: 20011 Fly ash prooduction annd use.
Ass shown in Table T 1, of the t 62 millio on metric tonns (68 millio on tons) of fly fl ash produuced in 2001, only 20 million metric ttons (22 milllion tons), orr 32 percentt of total production, wass used. following is a breakdow wn of fly ashh uses, mucch of whichh is used in the transportation The fo industtry.
Tab ble 2: Fly assh uses
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Fly Ash A Hand dling Thhe collected fly ash is typically coonveyed pneeumatically from the ES SP or filter fabric hoppeers to storagee silos wherre it is kept dry pendingg utilization or further processing, p o to a or system m where the dry ash is mixed m with water w and connveyed (sluicced) to an on n-site storagee pond. The drry collected ash is normaally stored and a handled using u equipm ment and proocedures sim milar to those used for haandling portlland cement:: a) Fly ashh is stored inn silos, dom mes and otheer bulk c be transsferred usingg air slides, bucket connveyors and screw storagge facilities, b)Fly ash can conveyyors, or it can c be pneuumatically co onveyed throough pipelin nes under po ositive or neegative pressuure condition ns, c)Fly ashh is transpo orted to marrkets in bulkk tanker tru ucks, rail cars and bargess/ships, d)Flyy ash can be packaged inn super sackss or smaller bags b for speccialty applications. Dry collected fly ash can also be moisteened with waater and wetting agents,, when applicable, using specialized equipment (conditioneed) and hauuled in coveered dump trucks for sspecial applications such as structuraal fills. Watter conditionned fly ash can be stocckpiled at joobsites. Expossed stockpileed material must m be kept moist or covvered with taarpaulins, plaastic, or equiivalent materiials to preven nt dust emisssion.
Fly Ash Charactteristics Size aand Shape Flly ash is typically finer thhan portland cement and lime. Fly ashh consists off silt-sized paarticles whichh are generally spherical,, typically raanging in sizze between 10 1 and 100 micron m (Figuure 2). These small glass spheres impprove the fluiidity and woorkability of the t mix. Finneness is one of the importtant properties contributiing to its wid despread application in hiighways.
Figure 2: Fly ash particles p at 2,000x maggnification. Chem mistry Flly ash consists primarilyy of oxides of silicon, aluminum irron and calccium. Magnnesium, potasssium, sodium m, titanium, and sulfur are also preesent to a leesser degree. When used as a mineraal admixturee, fly ash is classified ass either Classs C or Classs F ash baseed on its chemical compoosition. Ameerican Assocciation of Sttate Highwaay Transporttation Officiaals (AASHT TO) M 295 [A American Society for Testing T and Materials (A ASTM) Speecification C 618] definnes the chemiical composition of Class C and Class F fly ash.. Class C ashhes are geneerally derivedd from sub-biituminous cooals and connsist primarilly of calcium m alumino-su ulfate glass, as well as quartz, q tricalccium aluminaate, and free lime (CaO).. Class C ashh is also refeerred to as hiigh calcium fly f ash becausse it typically contains more m than 20 percent CaO O. Class F asshes are typiically derivedd from bitumiinous and annthracite coaals and consist primarilyy of an alum mino-silicate glass, with quartz, q mullite, and magneetite also preesent. Class F, F or low calcium fly ashh has less than n 10 percentt CaO.
Table 3 Sam mple oxide analyses off ash and poortland cem ment
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Colorr Flly ash can bee tan to dark gray, depennding on its chemical c andd mineral con nstituents. Tan and light colors c are typically t asssociated with high limee content. A brownish color is typpically associiated with the iron contennt. A dark gray g to blackk color is typ pically attribuuted to an ellevated unburnned carbon content. c Fly ash color is usually veryy consistent for each pow wer plant annd coal sourcee.
Figure 3: Typical ash colors.
Fly Ash Qua ality Quuality requirrements for fly ash varyy dependingg on the inttended use. Fly ash quaality is affecteed by fuel characteristics (coal), coffiring of fuels (bitumino ous and sub bituminous coals), and vaarious aspectts of the com mbustion and d flue gas cleeaning/collecction processes. The fouur most relevaant characteriistics of fly ash a are loss on ignition ((LOI), finenness, chemicaal compositioon and uniforrmity. LO OI: is a meaasurement off unburned carbon c (coal)) remaining in the ash and a is not a ccritical characcteristic of flly ash when used as mineeral filler in asphalt. Som me fly ash usses are not afffected by thee LOI, likee, filler in asphalt, a flow wable fill, annd structural fills can acccept fly ashh with elevated carbon co ontents. Fiineness of fly ash is mosst closely related to the ooperating conndition of thee coal crusheers and the grrindability off the coal itsself. For fly ash use in applications a such as asph halt filler, finneness shouldd be enough for fly ash to pass 0.07 75 mm sievee. A coarserr gradation can c result in a less reactivve ash and could contaain higher caarbon contennts. Limits on fineness are addresssed by ASTM M and state trransportationn departmentt specificatioons. Fly ash can be proceessed by screening or air classification to improvee its fineness and reactivvity. Some non-concrete n applicationss, such as struuctural fills are a not affectted by fly ash h fineness. However, H othher applicatioons such as asphalt a filler, are greatly dependent d onn the fly ash fineness f and its particle size s distributtion.
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Chemical composition of fly ash relates directly to the mineral chemistry of the parent coal and any additional fuels or additives used in the combustion or post-combustion processes. The pollution control technology that is used can also affect the chemical composition of the fly ash. Electric generating stations burn large volumes of coal from multiple sources. Coals may be blended to maximize generation efficiency or to improve the station environmental performance. The chemistry of the fly ash is constantly tested and evaluated for specific use applications. Some stations selectively burn specific coals or modify their additives formulation to avoid degrading the ash quality or to impart a desired fly ash chemistry and characteristics. Uniformity of fly ash characteristics from shipment to shipment is imperative in order to supply a consistent product. Fly ash chemistry and characteristics are typically known in advance so asphalt mixes are designed and tested for performance.
Fly Ash Quality Assurance and Quality Control Criteria vary for each use of fly ash from state to state and source to source. Some states require certified samples from the silo on a specified basis for testing and approval before use. Others maintain lists of approved sources and accept project suppliers' certifications of fly ash quality. The degree of quality control requirements depends on the intended use, the particular fly ash, and its variability. Testing requirements are typically established by the individual specifying agencies.
Fly Ash Uses in Highways Fly-ash can be used as: Fly Ash in Portland Cement Concrete (rigid highways), Fly Ash in Stabilized Base Course, Fly Ash in Flowable Fill, Fly Ash in Structural Fills/Embankments, Fly Ash in Soil Improvements, Fly Ash in Asphalt Pavements, Fly Ash in Grouts for Pavement Subsealing. The unique spherical shape and particle size distribution of fly ash can make it good mineral filler in asphalt pavement applications. The consistency and abundance of fly ash in many areas present unique opportunities for use in structural fills and other highway applications. (Tuncer B. Edil, 2006)
Fly ash in asphalt pavements (Flexible Highways) Fly ash can be used as mineral filler in HMA paving applications. Mineral fillers increase the stiffness of the asphalt mortar matrix, improving the rutting resistance of pavements, and the durability of the mix. Some of the benefits of fly ash in asphalt pavements can be as under depending upon the quality and proportioning of fly ash available which needs to be properly researched.(Miller and Collins 1976)
Fly Ash Potential Benefits. Fly ash can have properties of mineral filler for gradation. Due to hydrophobic properties of fly ash, reduced asphalt stripping is expected, Lime in some fly ashes may also reduce stripping, Where available locally, fly ash may cost less than other mineral fillers, Also, due to the lower specific gravity of fly ash, similar performance is expected using less material by weight, further expected to reduce the material cost of HMA, Fly ash is normally expected to meet mineral filler specification requirements for gradation, organic impurities and plasticity.
Utilization of Mineral-Fillers Mineral fillers increase the stiffness of the asphalt mortar matrix, improving the rutting resistance of pavements. Mineral fillers also help reduce the amount of asphalt drain down in the mix during construction, which improves durability of the mix by maintaining the amount of asphalt initially used in the mix. Mineral fillers have become more necessary as mixture gradations have become coarser (eg Stone Mastic Asphalt SMA). Asphalt pavements with coarse
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gradattions are in ncreasingly being b designned becausee they perfoorm well un nder heavy traffic condittions. Some mixtures m reqquire higher dust d to asphaalt ratios.
Mix des sign and specificati s ion requirrements Flly ash must be b in a dry form f when used u as minerral filler. Ty ypically, fly ash a is handleed in a similaar manner to hydrated lim me - it is trannsported to thhe HMA plannt in pneumaatic tankers; stored in wattertight silos at the plant; and meteredd into the HM MA using an auger. En ngineering Properties. P The physicaal requiremennts for mineeral filler in bituminous b p paving are defined in AAS SHTO M 17.
Tablee 4: AASH HTO M 17 specificatio on requirem ments for mineral m filller using asphalt a pavinng mixtures.
Organic impu urities. Althoough no stan ndard for carrbon content or LOI is sppecified for fly f ash used aas mineral filller, laboratoory asphalt mortar m evaluaations incorpporating fly ashes a with LO OIs up to 10 percent p perfo orm satisfacttorily. Pllasticity. Fly y ash is a nonn-plastic material. Gradation. Most M fly ashees typically fall f within a size range of o 60 to 90 percent p passiing the 75 μm m (No. 200 sieve). Fiineness. Theere is no fiineness standard for miineral filler beyond the AASHTO M 17 gradattion requirem ments; howevver, often a requirement r mum percent passing the 20 μm for a maxim (No. 6635) sieve waas specified. Typically, fly fl ash has 400 to 70 perceent passing th he 20 μm sieve and perforrms well in mortar m testingg and field peerformance. Sp pecific graviity. The speccific gravity of fly ash varies v from source s to souurce; it is typpically 2.0 to 2.6. Most "non-fly " ashh" mineral fiillers have a specific graavity rangingg from 2.6 to t 2.8; fore, HMA deesigned withh fly ash willl usually requuire a lower percentage by b weight to obtain therefo the sam me performaance (e.g., vooids in minerral aggregatee, stiffness, drrain down, etc.). e Riigden voidss. Research indicates i thaat mineral fiillers with more m than 500 percent vooids as determ mined using the t modifiedd Rigden's vooids test tendd to overly stiffen s the assphalt binderr. Most fly ashhes have a Rigden void oof less than 50 percent.
CON NCLUSIION Flly ash use im mproves soil and concrette performannce, making it stronger, more durablle, and more resistant to o chemical attack. Fly ash use also a creates significant benefits foor our enviroonment. All fly ashes exxhibit cementtitious propeerties to varyying degreess depending on the chemiical and physsical propertiies of both thhe fly ash andd basic materials.
REF FERENC CES 1. American Society for Testing and Materials. (1992). ( Stand dard Test Meethod for Buulk G and Density of Compacted C Bituminous Mixtures Using U Saturatted Specific Gravity Surface-D Dry Specimenns. Philadelphia, ASTM D 2726.
Vol. 17 [2012], Bund. HN 2. American Society for Testing and Materials. (1992). Standard Test Method for Percent Air Voids in Compacted Dense and Open Bituminous Paving Mixtures. Philadelphia, ASTM D 3203. 3. American Society for Testing and Materials. (1992). Standard Test Method for Resilient Modulus of Bituminous Mixtures. Philadelphia, ASTM D 4123 - 82. 4. American Society for Testing and Materials. (1992). Standard Test Method for Coating and Stripping of Bitumen Aggregate Mixtures. Philadelphia, ASTM D 1664 – 80. 5.
(FHWA-IF-03-019; 2003), Fly Ash Facts For Highway Engineers, American Coal Ash Association.
6. L. Allen Cooley, Jr. and Michael H. Huner. Evaluation of Fly Ash Sources for Use as Mineral Filler in Hot Mix Asphalt, Proceedings: 14th International Symposium on Management and Use of Coal Combustion Products, Volume 2, Palo Alto, California, January 2001. 7. Miller R. H. and Collins R. J. (1976) “Waste Materials as Potential Replacement forHighway Aggregates,” National Cooperative Highway Research Program Report No.166, Transportation Research Board, 1976, 1-24 8. Niroumand, H. (2008) Investigation and comparison of the earthquakes of Silakhor desert and Manjil. Proceedings of the 4th International Structural Engineering and Construction Conference, ISEC-4 - Innovations in Structural Engineering and Construction 2 , pp. 1011-1015 9. Niroumand, H., Kassim, K.A. (2012) Experimental performance of soil hook system as an innovative soil anchors in sand. Advanced Science Letters 13 , pp. 417-419 10. Adhami, B., Niroumand, H., Khanlari, K. (2012) A new algorithm for the system identification of shear structures. Advanced Materials Research 457-458 , pp. 495499 11. Niroumand, H., Kassim, K.A. (2011) Uplift response of square anchor plates in dense sand. International Journal of Physical Sciences 6 (16) , pp. 3938-3942 12. Niroumand, H., Kassim, K.A. (2011) Simulation comparison of the dispersion behaviour of dry sand subjected to explosion. International Journal of Physical Sciences 6 (7) , pp. 1583-1590 13. Niroumand, H., Millona, K. (2010) Mud bricks and shred geogrids as sustainable material. Geotechnical News 28 (4) , pp. 59-61 14. Niroumand, H., Kassim, K.A., Nazir, R. (2010) Experimental studies of horizontal square anchor plates in cohesionless soil. Electronic Journal of Geotechnical Engineering 15 O , pp. 1703-1711 15. Niroumand, H., Kassim, K.A. (2010) Analytical and numerical study of horizontal anchor plates in cohesionless soils. Electronic Journal of Geotechnical Engineering 15 C , pp. 1-12 16. Niroumand, H., Kassim, K.A. (2010) Uplift response of horizontal square anchor plates in cohesive soil based on laboratory studies. Electronic Journal of Geotechnical Engineering 15 Q , pp. 1879-1886 17. Niroumand, H. (2010) Performance of shred tires and wood particles in earth bricks. 2nd International Conference on Sustainable Construction Materials and Technologies , pp. 1083-1091 18. Niroumand, H., Kassim, K.A., Nazir, R. (2010) Anchor plates in two-layered cohesion less soils. American Journal of Applied Sciences 7 (10) , pp. 1396-1399
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19. Tuncer B. Edil (2006) “Green Highways: Strategy for Recycling Materials for Sustainable Construction Practices.”, University of Wisconsin-Madison, Geological Engineering Program and Department of Civil & Environmental Engineering, Madison, WI, USA.
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