Lightweight Concrete Aggregate From Sintered Fly Ash

Transcription

1 Lightweight Conrete Aggregate From Sintered Fly Ash L. JOHN MINNICK, G. and W. H. Corson, In., Plymouth Meeting, Pennsylvania Lightweight onrete aggregate is being produed ommerially from pulverized oal fly ash by several plants. The produts are of good quality, although they vary somewhat with respet to shape and size. The type of fly ash used in the manufaturing proess is generally that produed from bituminous oal. The harateristis of fly ash that are required in the forming and sintering proess are desribed. A desription of the physial properties of the fly ash aggregate is presented. The evaluation of these aggregates by the use of ASTM methods gives a good indiation of the performane of the various materials in strutw al onrete. Comparisons are provided between other types of lightweight aggregate onrete and, in all ases, the fly ash onrete shows a favorable range of properties. Conrete strengths in exess of 6, psi (42 kg/m 2 ) an be produed with fly ash aggregate. Of signifiane in making omparisons is the high rate of absorption of the aggregate, the residual pozzolani ativity, and the relatively low ement fators required for lean onrete. Information is presented on typial mix designs for use in strutural onrete. THE PRODUCTION of lightweight aggregate from fly ash has reeived the attention of investigators throughout the world (1,2). Although a number of early attempts to produe a ommerial produt proved"unsuessful, reent developments give every indiation that the manufature of fly ash aggregate is now possible. Furthermore, the produt quality is quite aeptable for use in strutural onrete, for insulationtype onrete, and as an aggregate for masonry units. Fly ash serves as an exeptionally good raw material beause it is available in large quantities in geographi areas where lightweight aggregates are in demand. The ommerial methods in use today produe aggregate by first preparing an unfired pellet or granule of the fly ash followed by firing on some type of sinter strand. Beau.se fly ash usually ontains some unburned arbon, there is an opportunity to utilize the available heat generated by burning of residual ombustible material during proessing. Fly ash aggregates, therefore, are properly lassified as "sintered" produts rather than "bloated" materials, suh as those produed by heating lay or shale in rotary kilns. Historially, oal ash has often found appliation as an aggregate. Originally this was in the form of "inders" from traveling grates or stoke1 -fed oal burners. These systems have now been largely replaed by pulverized-oal burners and the resulting fly ash is olleted in a very finely divided ondition in various types of dust-olleting systems, suh as ylone olletors or eletrostati preipitators. It is the purpose of this paper to desribe the requirements for fly ash as used in the sintering proesses and the properties of the fly ash aggregates that are produed. Although it is not intended to present the tehniques involved in the prodution of aggregates, some information is inluded relating to the effet of the manufaturing proess on the harateristis of the aggregate. Paper sponsored by Committee on Mineral Aggregates and presented at the 49th Annual Meeting. 21

2 22 TABLE 1 TYPICAL FLY ASH ANALYSES FOR VARIOUS TYPES OF COAL Sample Soure LOI Si2 Al,O, Fe2:i Cao MgO so, Bituminous Illinois s 4 Kentuky <l 2 Ohio <l West Virginia <l <l Pennsylvania Soure <1 Soure I 1 Soure l 2 Soure <l <1 England TIl-...l UV l West Germany <l Ligniti Minnesota Montana <l North Dakota < North Dakota Canada Czehoslovakia Frane <l Greee East Germany <l Poland RAW MATERIALS The types of fly ash available over a broad spetrum of hemial and physial properties. Table 1 gives some typial analyses of fly ashes from various parts of the world. There are several fators that affet the quality of ash as used in the manufature of aggregate. Of prime importane is the type of oal used; thus, ash produed from bituminous oal is usually quite low in alium and magnesium oxides, whereas ash produed from lignite oal is usually high in these lwo onstituents. The presene of exessive amounts of free alium oxide in a fir ed aggrega te an h ave deleterious effets; among these is hydration of the oxide, resulting in the formation of "pits" or "pops" in the hardened onrete. A seond fator affeting the quality of the fly ash is the ash ontent of the oal. The majority of oals used in the United States is relatively low in ash, usually approximately 1 perent. The oals in Europe are frequently quite high in ash ontent (up to about 45 perent). European pratie, therefore, requires that the oal be burned more thoroughly to reover as muh fuel as possible. The resulting fly ash is low in residual arbon-in many instanes less than 1 perent. Carbon ontents in the United States are rarely this low and it is not unommon for the arbon ontent of the ash to be in exess of 1 perent. It is essential that fly ash ontain not less than about 4 perent and prefer ably not more than 8 per ent arb on to be an aeptable material for aggregate prodution. A third fator affeting the fly ash quality is the type of ash olletion systems in use. This an diretly influene the fineness of the ash. Fineness varies not only between different oal-burning operations but also within a given operation. For example, in a typial olletion system used in this ountry there may be as many as 2 hoppers removing ash from the furnae. The loation of these hoppers in relation to the gas flow results in substantial differenes in the partile size of the ash. The variation of properties of the ash influenes to a onsider able degree the quality ontrol of the sinter strand operations. Thus, if a fly ash hang es in its fuel ontent (residual arbon) from a low to a high value as it is being fed into th.e firing proess, serious diffiulties an be experiened in ontrolling the operation. Variation in iron oxide ontent is of onsiderable importane beause this ingredient serves as a primary

3 N!i ' e n 41--.i i..-<--- BO a & JOOt--f-r+-----:,...,'--+--t-6 l -August ---September 2L ' IOO 2 JOO per ent Carbon per ent thru 325 Mesh Sieve Figure 1. Effet of arbon ontent and sieve fineness on pellet strength and perentage of high-strength pellets. "' 23 flux and substantially influenes the operation of the plant proess as well as the produt quality. As a result of an intensive study made in the author's laboratory, it has been found that the most important requirements for suitable fly ash are the arbon ontent (or loss on ignition), the fineness (as measured with a No. 325 mesh sieve), and the iron oxide ontent. It has also been found from statistial studies that these individual harateristis do not reliably predit the resultant aggregate properties, but that when these harateristis are used as multiple fators they give signifiant relationships between the fly ash and the properties of the final produt. Figure 1 shows the effet of arbon and No. 325 mesh sieve fineness on both the perentage of high-strength pellets and the fired pellet strength on a large number of samples (at a relatively onstant Fe2 Os ontent) taken at different periods of time from one power plant. Although arbon ontent and sieve fineness do not give a high degree of orrelation individually, the produt of the perent of arbon times the perent through the No. 325 mesh sieve does, as shown in Figure 1. Based on studies suh as these, limits an be established to provide adequate quality ontrol of the raw fly ash for use in the sintering proesses. The interrelationship between fineness and arbon (also Fe2a ontent) an be useful in arriving at the seletion of fly ash material at the power station. Although fly ash from both mehanial and eletrostati olletors an be used, there are situations that require the rejetion of a portion of the fly ash that does not onform to the general requirements of the lightweight aggregate proess. In addition to the ontrol of properties of the raw material, an important fator in the preparation of the aggregate is the water ontent used in the formation of the green pellets prior to firing. The water is neessary not only to develop adequate physial strength of the pellet but also to ontrol the porosity or absorption of the fired aggregate. SHAPE AND SIZE OF FLY ASH AGGREGATE Although there are a few exeptions, most fly ash aggregates develop the ultimate shape and volume of the individual partiles at the time the material is formed in the unfired (green) state. Several devies are urrently used to form the unfired pellets or granules. Pelletizing drums or pans are1 used in a few plants. These partiular devies form spherially shaped partiles usually of a very uniform diameter. In the proess developed by the Corson Company, an extrusion priniple is used that results in partiles of varying shapes and sizes, depending on the seletion of the forming equipment. Figures 2, 3, and 4 show typial shapes produed by the different forming proesses. One signifiant fator resulting fro.m the hoie of the forming method is the final density that is ahieved in the aggregate. Furthermore, this density also ontributes, to a substantial degree, to the absorption oj the aggregate. The ommerial operations are all engineered to produe a produt that retains the disrete shape of the unfired pellet without formation of large linkers, whih are often produed from other types of raw materials. From the standpoint of produing aggregate for sh utural onrete, the range of sizes of the pellets are set to meet the basi requirements of the ASTM speifiation for onrete aggregates. In order to get the proper gradation, usually several methods

4 24 Figure 2. Extruded fly ash aggregate (rushed and graded). Figure 3. Unfired and fired fly ash aggregate made by pelletizing proess. Figure 4. Unfired and fired fly ash aggregate made by extrusion proess.

5 are used; either the material is made in different sizes and then blended, or, as in the ase of extruded produts, the distribution of the size may be established in the green bodies prior to the firing yle. To produe finer aggregate, it is ustomary to rush the material, sreen it, and proportion it so that it meets the requirements of the appliable speifiation for size gradation. For this reason, the fly ash aggregate plants usually inorporate rushing, sreening, and blending operations in the manufaturing proess. PROPERTIES OF FLY ASH AGGREGATE There is as yet no standard method to measure the strength of the individual granules, although several methods are used in individual laboratories. A study reported by the Central Eletriity Generating Board (3) provides some results of tests of pellet strength made on aggregate utilizing differentbu1 ni11g tehniques. This report shows that the strengths are quite good for those pellets produed by the sintering grate, as well as those produed experimentally in rotary or shaft kiln equipment. In general, the aggregate produed from fly ash is evaluated on the basis of ASTM Speifiation C 33 and the referene test methods as alled for in this speifiation. The tests inlude measurements of onrete-making properties, that is, strength and unit weights, drying shrinkage, popouts, and durability. Also of importane is the test for stai.ning. The firing proess results in some sintering of individual partiles of fly ash on the outer surfae of the aggregate oupled with a more general fusion ation in the interior. The sintered exterior normally represents less than 1 perent of the total volume. Figure 5 shows a lose-up view of the typial struture of a ross setion of fly ash aggregate. The struture of the exterior sintered portion of the aggregate results from the individual partiles of the fly ash adhering to adjaent partiles by partial melting or softening. The struture of the interior of the aggregate represents a 25 Figure 5. Close-up of a sawed partile of fly ash aggregate enlarged 1 times.

6 26 TABLE 2 FLY ASH LIGHTWEIGHT AGGREGATE PROPERTJES Test Unit weight, lb/ft' (kg/m') At F. M. = 6. 5 (nnminol 3 /, in. to No. 4) At F. M. = 5.8 (nominal 'le in. to No. 8) At F. M. = 3.5 (nominal 'le in, to O) At F. M. = 3. (nominal No. 4 to ) 24 -hour absorotion, perent Staining Loss on ignition, perent NOTE: F.M means Ummosi modulus. Normal Range for Fly Ash Aggregate 37 to 48 (593 to 769) 41 to 51 (657 to 817 ) 53 to 61 (849 to 977) 56 to 63 (897 to 1,9) 14 to 24 None to very light to 3.4 ASTM Speifiation maximum 55 maximum 65 maximum 7 maximum 5.maximum multiellular struture of a more ompletely melted produt. In both ases the voids in the aggregate are aused by evaporation of the mixing water and elimination of arbon during the sintering proess. Table 2 gives the normal ranges of properties on a number of fly ash aggregates. It is ta be noted that the values for absorption are quite high. The rate of absorption is also high and the effet that this has on ompositions suh as onrete is believed to be more benefiial than delete1 ious. It has been reported (!) that the water absorbed by this aggregate when preparing a bath of onrete is ontinuously available during the uring proess. The aggregate an thereby provide uring water r ather than absorbing water out of the ement matrix of the onrete mix. This may explain in part why these materials ofteo provide somewhat superior performane when used in onrete. Field experiene has indiated that plasti shrinkage f the onrete is also substantially redued (presumably as a result of the highly saturated ondition of the aggregate). The relative absorption of the different portions of the fly ash aggregate, as well as the unr ushed and rushed aggr egate mater ials, is shown in Figure 6. This graph is representative of materials produed ommel'ially at the Corson Plant at Plymouth Meeting, Pennsylvania v A A =: _-_--_ --t ; -: t ! l -r_!!sd-xtruded Porlites_(:3!!! ::.:'!! l 5 to B- Extruded Whole Pellets :: :: I---_-_ Minutes 5 Hours 24 Hours Time as indiated Figure 6. Moisture absorption of different fly ash aggregate produts made from a single soure of fly ash.

7 TABLE 3 POZZOLANIC STRENGTH OF FINELY GROUND FLY ASH AGGREGATE 3 Material Under Test Fly ash from soure A Ground lightweight aggregate made from soure A fly ash Fly ash from soure B Ground lightweight aggregate made from soure B fly ash am odified test aording to ASTM Method C T. With respet to the bulk density of aggregate produed from fly ash, some of the variation depends on the raw material from Blaine Pozzolani whih the aggregate was produed. For Fineness, Index, instane, fly ash with very high fineness Perent of m'/gm Control and low Carbon COntent Will USUally TeSUlt in a produt having the higher densities. 2, The data given in Table 2 illustrate the 2, effet of aggregate size on bulk density. 5,42 13 Any residual fine fly ash partiles that 5, are either arried through the burning proess or are formed by abrasive ation of the aggregate during handling are still highly pozzolani and will reat in muh the same manner when mixed with portland ement as the unfired fly ash. This results in the usual long-term improvements in strength and dimensional stability of the onrete. When the lightweight aggregate produed from fly ash is pulverized to pass a No. 2 mesh sieve, virtually all of the OJ:iginal pozzolani ativity is reovered. Table 3 gives data on this effet as measured with two lightweight aggregate materials from different soures of fly ash. Figure 7 is a lose-up of a fratured speimen of onrete ontaining fly ash aggregate. The exellent bond established at the interfae between the aggregate and the mortar matrix is typial of results with fly ash material. 27 Figure 7. Fratured speimen of lightweight onrete ontaining fly ash aggregate.

8 28 1 l 8 u kg I m2 6 6 l iii '"'.:. '"'e.. N e 6 u,s '... u ) 2 Vf "' '... "' Normal 3.;l.. a. - N u,., a: '"'e " 25 e u ' ' '.. i 2... "' :!... "' " ::E JOO 2 Moist :E "i "' 24 ;r: Normal Cure " 1 1 ;: 22 "' => :E ' o "'g J: -... u a. 12 ' "' " 8 Ii.!: "' " <... ' '... 1 Ii " m N u... U) "' 'g 'el 'o N " "- - iii Ii.?- " 'O... ' :5 2 a "' Dry Aggregate 6 Air Enlrained ::E Doy Compressive Slrenglh, ksi S and Fly Ash Lighlwelghl Sand Lighlweighl Rtfereno Conrelo Conrelo ' Cone role Figure 8. Comparison of lightweight onretes using sand as the fine aggregate.

9 Another important differene between aggregates produed from fly ash is in the shape of the individual granule. Unrushed materials, suh as those produed by the Corson proess (whih makes an extruded shape), have been demonstrated to have some advantage over the round pellets in improved workability and pumpability of the onrete. Crushed materials have been blended with sand and have been found to be effetive in pakaged onrete. The overall balane of properties, i.e., density, strength, and workability, in these latter mixtures is quite good. PROPERTIES OF HARDENED CONCRETE Considerable strutu ral onrete has been produed with lightweight aggregate made from fly ash. In general, the onrete has demonstrated satisfatory performane and ompares very favorably with other forms of lightweight aggregate onrete. A reent report of Committee 213 of the Amerian Conrete Institute (5) provides a good desription of lightweight aggregate onrete and inludes fly ash aggi egate in the total evaluation made. Figure 8 shows graphs desribing onrete properties using fly ash agg1 egates produed in various plants throughout the world. The Wlshaded bands shown in the graphs are taken diretly from the ACI repol't, as are the dotted lines representing a typial normal weight onrete. The shaded bands, as superimposed on this bakgrowld, represent the fly ash lightweight aggregate onrete. The graphs in Figure 8 are based on the use of normal weight natural sand as the fine aggregate. Most of the ompanies produing onrete ommerially use this type of omposition beause of diffiulties in workability experiened when lightweight aggregate is used in both the oarse and fine frations. Figure 9 shows data on the latte1 type mixtures. Beause the soure of this information is limited to fly ash aggregate onrete produed at the Corson Company plant, the pertinent data a.j:e given in the form of a single line rather U1an in a band, as is the ase in Figure 8. For omparative purposes, Figure 9 also inludes the bakgi ound information as presented by ACI; t11e shaded band l'epresents all lightweight aggregates and t11e dotted line again represents normal onrete. A study reently reported by the Portland Cement Assoiation Laboratories (6) provides additional information on fly ash aggregate onrete. The Portland Cement Assoiation program has utilized material that has been supplied from a number of proessing plants in this ountry. An interesting report of studies arried out in Yugoslavia (7) gives information on the feasibility of using fly ash from lignite oal to produe aeptable lightweight materials. A ooperative investigation is urrently Wlder way (8) to determine the fire-resistane of onretes produed with fly ash lightweight aggregate. An early report by Pearson and Ase (9) states that fire tests on fly ash aggregate onrete onduted for New York City, Using ASTM Method E , shows that a nominal-size speimen of 4 by 12 by 6 in. has a fire-resistane of 3 hours. Figure 1 provides some data on the thermal values for fly ash lightweight aggregate onrete. Inluded in this figu1 e are data supplied tlwougb the ourtesy of the Centre Sientifique et Tehnique du Batiment (1). The spread in the K-values as plotted reflets the onditions present at t11e time the measurements are made. The higher value of K at eah density results primarily from tests of onrete ma.de in a moist ondition. There is onsiderable interest, in Frane as well as in other parts of the world, in the use o! this type of onrete in building prefabriated strutures. The very favorable K-values that are obtained, together with the high strength and other properties of the fly ash aggregate onrete, enable U1e fabriator to produe exterior walls and other units of minimum thikness and weight but of adequate strep.gth. Table 4 gives design riteria that a.re u1tently in use for ommerial mixtures in this ountry. Comparison of t11ese designs with the reommended pratie, as developed by AC! 613A-59, indiate that t11e fly ash aggregate falls well within the ranges of that study. These design riteria provide adequate fators of safety when ompared with the strength results as shown in Figures 8 and 9. 29

10 3 kg I m , E "' JOO Moist 2 Vf "..., g. u..., E a: 4 - L ;; " ' ' "' -'< 3: "O 9 Normal Cure Moist N E u -'< "' ::;:- ; "' 24 Normal Cure 3: ;: 22 "' " :::>,., _ "' 2...,E Q L U 12 Vl Cl Q. " BOO L ' < ::! 1 " ' "" Cl 6 CD N BO 4 " r--.t2 u UJ,., 4. L u;> 1 Q u ' Q N "... w " 3. - ;; Q. ao "'.a -'< "' " "3 " 2. "O 1. 6 Air Entrained " Doy Compressive Strength, ksi o:s::s::s::sl All lighlweight Conrete All Fly Ash Aggregofe Referene Conrete Conrete Figure 9. Comparison of onretes using all lightweight aggregate.

11 M ;::: ',a CJ., Normal Woighl Conrete <E-Fly Ash lightweighl Aggregale Conrete Thermal Condulivily (K Fator) Figure 1. Range of thermal ondutivities for fly ash aggregate onrete. TABLE 4 CONCRETE MIX DATA Quantities Per Cubi Yard Design Mix Saks Cement P ounds Aggregate (lb) Slump (in.) Watera Corson- Lite, Conrete Sand, Pounds Gallons %-in.size, Bone Dry S. S. D. Air- Entraining Agent (oz/sak) Perent Air Weight of Plasti Conrete (lb/u ft) Weight of Conrete, 28 Days (lb/u ft) , , , , , l. 5 l. 75 G 6 l l. 75 G 2. 6 atotal water, inluding water of absorption, ho as.od on Corson-Lite, 44 lb/u ft density, s.s.d. = saturated su rfa e dry SUMMARY Fly ash aggregate is being suessfully used in strutural onrete. Beause of the light weight of the aggregate, it finds appliation for strutural onrete in the range of 95 to 116 pf (1,52 to 1,86 kg/ m 2 ) and may also be used for insulating on1 ete at lower densities. With proper quality fly ash, ommerial sintering proesses produe various shapes of aggregate similar in properties to other stn1tural lightweight aggregates. However, the high rate of absorption and residual pozzolani properties provide some advantage over other materials. Tests of onrete using fly ash aggregate give results falling well within the range of values typial of lightweight onrete in general use. REFERENCES 1. Short, A., and Kinneburgh, W. Lightweight Conrete. John Wiley and Sons, New York, 1963.

12 32 2. Minnik, L. J. Corson Company Researhes Fly Ash Lightweight Aggregate. Rok Produts, Sept. 1964, pp Sintered Lightweight Aggregate. Central Eletriity Generating Board, England, Sept Corson, B. L., and Pfau, J. H. Lightweight Aggregate. U.S. Patent No. 3,341,341, Sept. 12, Gujde for Strutural Lightweight Aggregate Conr ete. Amerian Conrete Institute Committee Rept, Jour. Amerian Conrete Institute, Pro., Vol. 64, No. 39, Aug. 1967, pp Pfeifer, D. W. F ly Ash Aggregate Ligh tweight Conrete. P o1 tland Cement As sn. Res. and Dev. Bull., Brzakovie; Prerlrag. Properties of Aggregate of Fly Ash From Yugoslav Power Stations and Some Types of Lightweight Conretes. Seond Fly Ash Utilization Symposium, Pittsburgh, Penn., Marh National Ash Assoia tion. Private ommuniation. 9. P earson, A. S., and Ase, F. Lightweight Aggregate From Fly Ash. Civil Engineer ing, Vol. 43, No. 9, Sept. 1964, pp Croiset, M. Centre Sienlif.ique et Tehnique du Batiment. Private ommuniation.