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Synthetic aggregates from combustion ashes using an innovative rotary kiln
Waste Management 21 (2001) 241±246
www.elsevier.nl/locate/wasman
Synthetic aggregates from combustion ashes using an innovative rotary kiln P.J. Wainwright *, D.J.F. Cresswell School of Civil Engineering, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Abstract This paper describes the use of a number of dierent combustion ashes to manufacture synthetic aggregates using an innovative rotary `Trefoil' kiln. Three types of combustion ash were used, namely: incinerated sewage sludge ash (ISSA); municipal solid waste incinerator bottom ash (MSWIBA - referred to here as BA); and pulverised fuel ash (Pfa). The ®ne waste ash fractions listed above were combined with a binder to create a plastic mix that was capable of being formed into `green pellets'. These pellets were then ®red in a Trefoil kiln to sinter the ashes into hard fused aggregates that were then tested for use as a replacement for the natural coarse aggregate in concrete. Results up to 28 days showed that these synthetic aggregates were capable of producing concretes with compressive strengths ranging from 33 to 51 MPa, equivalent to between 73 and 112% of that of the control concrete made with natural aggregates. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Rotary kiln; Synthetic aggregates; Concrete; Combustion ashes
1. Introduction
1.1. MSWIBA
This project is speci®cally aimed at using an innovative design of rotary kiln (known as the Trefoil kiln) to provide a solution to two modern-day dilemmas which confront both disposers of waste and users of natural aggregates for the production of concrete. The dilemmas are:
As the reliance on land®ll as the primary waste management option declines in the UK more municipal solid waste incinerators (MSWI) will be needed. Depending on the levels of minimisation, reuse, and recycling the number of new incinerators is estimated to be between 28 and 165 (200 000 t/a) [1] in the coming decades. This will produce a large and consistent supply of MSWI bottom ash. Such quantities will help to encourage the use of bottom ash (BA) as a construction material. This is a sensible method of utilisation as it avoids the problems and costs associated with disposal and provides an alternative aggregate source. Concern has been expressed over the use of BA in construction due to metal contamination and the associated expansion when incorporated in concrete [2]. Methods of sintering and vitri®cation of BA to decrease the availability of toxic elements are well documented [3±7] but energy costs are high and use is limited. The levels of metal contamination within BA mean that without vitri®cation utilisation requires some form of encapsulation or covering to prevent the leaching of contaminants into the groundwater. At present the main use of BA is in bound applications to reduce the potential for contaminant release [8±10], such applications include: asphaltic concrete, concrete blocks and cementitious subbases in roads. Washing and/or ageing [11] can bring
. how to overcome the con¯icting problems of dealing with the increasing amounts of domestic and industrial wastes and, at the same time, eect a reduction in the number of land®ll sites being used for disposal; and . how to limit the use of irreplaceable natural resources and still satisfy the growing demand for aggregate. The particular waste streams investigated in the project were: . incinerated sewage sludge ash (ISSA) . municipal solid waste incinerator bottom ash (MSWIBA- BA); and . pulverised fuel ash (Pfa) * Corresponding author. Tel.: +44-113-233-2264; fax.: +44-113233-2265. E-mail address: [email protected] (P.J. Wainwright).
0956-053X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00096-9
242
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
about improvements in BA quality, this decreases the mobility of metals by oxidation and carbonation reducing the pH and producing more stable mineral phases. Evidence from continental Europe (see [12], for example) suggests that BA is a potentially suitable material for some construction purposes. This has been borne out by recent developments in the UK where ash is being used from three incinerators and industry is drawing up guidelines on its use [13]. Satisfactory results have been obtained in previous work on producing a quality aggregate from sintering BA in a traditional rotary kiln [14±17]. The resultant aggregate was used to replace natural coarse aggregate in a number of concrete specimens, which were then tested over a 5-year period for structural performance and long term degradation. 1.2. ISSA A rotary kiln has been used to produce a lightweight aggregate (LWA) from a mixture of de-watered sewage sludge and clay [18]. This mix was extruded, chopped into pellets and ®red in a rotary kiln at up to 1090 C. Strengths of up to 35 MPa have been achieved from concretes in which this material was used to replace the natural coarse aggregate. A mixture of de-watered sludge and a binder has also been ®red in a brick kiln to produce a material which, after crushing, resulted in a hard angular rough textured aggregate with a loose bulk density of 600±650 kg/m3 and water absorption of approximately 7% [19,20]. Recent legislation preventing the dumping of sewage sludge at sea [21] from the end of December 1998 has meant that a number of alternatives are being sought by water companies to dispose of the sludge. One such method is incineration which produces a granular ash (ISSA) which itself needs to be disposed of. Currently the majority of the ash is disposed of to land®ll, but there has been some recent work undertaken to assess its potential for use as a construction material. A 3 tonne per day capacity plant has been constructed in Tokyo [22] producing a material called `sludgelight'. In this process the ISSA is mixed with an organic waste and pelletised before being ®red in a vertical shaft furnace at approximately 1050 C. The properties and performance of the aggregate produced were similar to other commercially available lightweight aggregates (LWA). Tay and Show [23] also demonstrated the use of ground ISSA as an ingredient in producing masonry cement.
replacement in concrete increases the workability of the plastic concrete, reduces the heat of hydration and is therefore useful in large structures such as dams. It also increases the long-term strength of the concrete and, by reducing the permeability, leads to an overall improvement in durability [24]. The use of pfa in the production of lightweight aggregate is also well-established (see [25]). Lytag is the trade name given to a LWA produced by sintering low carbon (circa 8%) ¯y ash. The ash is ®rst pelletised on an inclined disc pelletiser with 5±10% water and is then ®red on a `sinterstrand' at a temperature of about 1000 C. The resultant material is open textured with small voids that are interconnected and permeable to water. Lytag has a loose bulk density of approximately 825 kg/m3, and is capable of producing concretes with strengths in excess of 40 MPa. It is a well-established material and has been used in a number of large-scale projects such as, for example, the construction of North Sea oil production platforms [26]. 2. Experimental work 2.1. The Trefoil kiln The Trefoil kiln used in this project is much more thermally ecient than a traditional rotary kiln. Traditional rotary kilns consist of a thick steel drum lined with heavy manually laid ®re bricks which have a limited life and need regular maintenance entailing total shut down which can involve weeks of lost production. By contrast, the Trefoil kiln (see Fig. 1), which is potentially the only real advance in kiln design for the last 100 years [27], compresses a ceramic ®bre insulation between a thin steel alloy hot face and a tensioned thin mild steel cold face to form a cartridge with a triple-lobed cross section. The advantages of such a design include:
1.3. Pfa Pulverised fuel ash has a long history of use in construction. It can be used as a light base material when compacted, and as a pozzolan as it hardens when mixed with lime and water. The inclusion of pfa as cement
Fig. 1. Cross-section of the Trefoil kiln.
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
. excellent thermal eciency (an inner kiln temperature of 1200 C and an outer temperature of 60 C); . a much smaller and lighter structure which does not need costly maintenance or relining and which is easier to manufacture, transport, install, remove and replace; . much faster heat-up and cool-down periods (50 times quicker than conventional kilns); . vastly improved responsiveness (operating temperatures can be increased or decreased in minutes); and . higher load factors.
2.2. Materials The mineralogy of all three ashes used were similar consisting mainly of silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO) [19,24,28] with smaller amounts of Cu, Zn, K and other metal oxides. The pfa used had proportionally more silica and alumina than the other two ashes. The ISSA was derived from the incineration of sewage sludge from a plant in the UK. The ash was used in its conditioned state at dry solids content of about 75% with all the material passing 1mm. The MSWIBA (referred to here as BA) used was that passing 600 mm obtained from the 0±6 mm fraction of a sample taken from a bottom ash recycling plant in the UK. The processing undertaken at the plant consisted of a small amount of size reduction and removal of ferrous and non-ferrous metals. The pfa was obtained as a grab sample from the dry area of a settling lagoon of a UK power station. 2.3. Aggregate extrusion/pelletisation The ®rst stage in the aggregate production process was to blend the ashes with a binder to enable the mixture to be extruded and pelletised. In this study the binder used was clay, dewatered sewage sludge or a combination of both. The proportions of the dierent types of ashes and binders used are shown in Table 1. At the time of writing it is not possible, for commercial reasons, to disclose the proportions of pfa and sewage sludge that were used. Once blended the mixtures were extruded and then pelletised by means of a rotary drum pelletiser that also incorporated a burner drier. The combined action of the drier and the rolling motion of the drum produced a skin on the pellets that aided green pellet strength and created a thin denser outer rim on the ®red pellet.
243
Table 1 Ratio of ash to binder in the aggregates Aggregate
ISSA 1 ISSA 2 BA 1 BA 2 Pfa
% Ash
63 64 82 (< 600 mm) 85 (< 300 mm) 5 (>300 < 600 mm) @
% Binder % Clay
% Sewage sludge (dry weight)
10 20 18 10
27 16 ± ±
@
@
the inert material in the pellet begins to sinter. From previous experience it was known that this stage can take between 5 and 40 min and occurs between 700 and 800 C, depending on the amount and type of carbon (organic material) present and the internal structure of the pellet. The carbon content of a lightweight aggregate produced from pfa needs to be below 4% to conform to BS 3797 [29] and it was considered appropriate to adopt this ®gure in the study. To ensure that this 4% level was reached before the sintering stage begins, samples of the aggregate were taken at various times during burnout and analysed for the carbon content. It was assumed that the remaining carbon would be burnt during further ®ring and could provide a volatile material for the production of voids during sintering, thus helping to give a lightweight structure to the pellet. All aggregates produced were tested for relative density and water absorption in accordance with BS 812 [30]. 2.5. Casting A number of dierent concrete mixes were made in which the natural coarse aggregate was replaced with the synthetic aggregates. The mix ratios used together with the free water/cement ratios and the workabilities are shown in Table 2. All mixes were designed to have nominally the same volume proportions with adjustments being made on the basis of loose bulk density. All the sintered ash aggregates were soaked in water prior to casting to ensure that they were in a saturated surface dried condition. Cubes were cast from all mixes for measurement of compressive strength in accordance with BS 1881 [31]. 3. Results
2.4. Aggregate ®ring
3.1. Aggregate properties
Firing of the synthetic aggregate consisted of two stages; the ®rst is known as the `burnout' stage where the carbon content of the ash/binder mix is allowed to combust (burnout) at a temperature below that where
The densities and water absorption ®gures for the aggregates produced from the combustion ashes are shown in Table 3, where they are compared with natural aggregate and with Lytag. The relative densities range
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P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
Table 2 Mix ratios (by weight of cement) for all concrete specimens
Coarse aggregate Sand OPC Free water:cement Slump (mm)
ISSA 1
ISSA 2
Pfa
BA 1
BA 2
Control
Lytag
2.21 2.23 1.0 0.55 20
2.34 2.23 1.0 0.55 50
2.20 2.23 1.0 0.53 50
2.52 2.23 1.0 0.53 95
2.37 2.23 1.0 0.53 135
3.64 2.23 1.0 0.53 20
2.47 2.23 1.0 0.53 10
Table 3 Aggregate properties Aggregate
Dry loose bulk density (kg/m3)
Loose bulk density (SSD) (kg/m3)
Relative density (SSD)
24 h water absorption (% dry mass)
BA 1 BA 2 Pfa ISSA 1 ISSA 2 10 mm Nat. Agg. Lytag
920 985 816 630 830 1510 780
1036 1034 960 730 900 1520 875
1.9 1.9 1.7 1.6 1.7 2.6 1.8
12.7 4.5 17.7 15.5 8.1 0.8 12.3
from 1.6 to 1.9, which are signi®cantly lower than the natural aggregate at 2.6 and similar to Lytag at 1.7. All have dry bulk densities below 1000 kg/m3, which classify them as lightweight aggregates. The water absorption ®gures do not all follow the expected trend of high absorption corresponding to low density. Two of the aggregates (BA2, ISSA2) have low densities 985 and 830 kg/m3, respectively, and relatively low absorption ®gures (when compared to Lytag) of 4.5 and 8.1%, respectively. This would suggest that the pore structure is more segmented in these two aggregate types than in the others. A closed pore structure is considered more bene®cial because it is likely to result in higher strength and lower shrinkage than with a more open structured aggregate of similar density. Further work is on going to study in more detail the microstructure of the aggregates and to develop a model to be able to predict this structure from knowledge of the constituent materials and kiln ®ring conditions. 3.2. Concrete workability The workabilities (slump) of all seven mixes made to nominally the same water/cement ratios of 0.53 are shown in Table 2. All mixes, with the exception of the control mix, had similar densities and it is reasonable therefore to make comparisons between the Lytag mix and the remainder of those mixes made with the synthetic aggregates manufactured in this study. Using any of the synthetic aggregates as a coarse aggregate replacement leads to an increase in workability when compared with Lytag. In the case of the two mixes made with the bottom ash aggregates (BA1, 2) the increases
are signi®cant from 10 mm to between 95 and 135 mm respectively. All the aggregates produced in this study tended to be slightly more rounded and smoother than Lytag which would account for some of the increase, but no reason can be given at this stage for the signi®cant increases observed with the two bottom ash aggregate mixes. 3.3. Compressive strength Unfortunately at the time of writing strength results are only available up to the age of 28 days and these are shown for all mixes in Fig. 2. In addition the ratio of the 28-day strengths of the mixes compared to both the Lytag and the natural aggregate control concretes are shown in Table 4. The compressive strength results for all the dierent aggregate types are encouraging, in most cases they are similar in performance to both the control mixes and dier little from each other. Only two mixes show any signi®cant dierences in trend, that made from the incinerated sewage sludge ash (ISSA 2) and that made with the bottom ash (BA 1). In the case of the former the 28 day strength is 20% higher than with Lytag (51 cf. 42 MPa) and 12% stronger than even the natural aggregate control (51 cf. 45 MPa). Comparing ISSA 2 with ISSA 1 (Table 1) would indicate that the addition of 10% extra clay as the binder at the expense of the sewage sludge is bene®cial in terms of compressive strength (Fig. 2) and water absorption (Table 3). Interestingly the reverse appears to be true in the case of those aggregates made with bottom ash where the aggregate with the lower clay content (BA 2) performs
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
245
Fig. 2. Compressive strengths of synthetic aggregate concrete. Table 4 Strength of sintered ash aggregate concrete compared to control and Lytag concrete Aggregate
% of Lytag concrete 28 day strength
% of Control concrete 28 day strength
BA 1 BA 2 PFA 1 ISSA 1 ISSA 2
79 95 98 90 120
73 88 90 84 112
term data is needed to determine whether or not there are any potentially deleterious reactions taking place between the aggregates and the cement paste. The work programme currently underway is designed to address these questions as well as provide important information on the leaching characteristics of the synthetic aggregates. Future planned work will also look at other properties of concretes made with these aggregates such as: permeability, elasticity, shrinkage and creep. 4. Conclusions
better than BA1 containing 8% more clay. The 28-day strength of the mix made with the BA 1 aggregate is the lowest of all at 28 MPa which is 21 and 27% lower than the Lytag and natural aggregate control mixes, respectively. 3.4. Future work programme These results show that concretes with reasonable strengths can be made with synthetic aggregates produced from any of the waste streams investigated, but more work is needed to ®nd the mix proportions (in terms of ash and binder contents) to give the optimum performance from the aggregates. Although the strength results obtained to date are satisfactory, much longer-
The results of the tests reported here have shown that it is possible to successfully manufacture synthetic lightweight aggregates from the combustion ashes derived from the incineration of: sewage sludge, municipal solid waste and pulverised coal (pfa). The ashes were combined with a binder, extruded to form pellets and then ®red in a Trefoil rotary kiln to produce synthetic lightweight aggregate. Concretes made with the synthetic aggregate achieved 28 day strengths of between 33 and 51 MPa compared with 45 and 42 MPa for the natural aggregate and Lytag aggregate control mixes, respectively. Further work is being undertaken to establish the leaching characteristics of the aggregates and the optimum mixture proportions to give the most desirable aggregate properties.
246
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
References [1] DETR A way with waste. A draft strategy for England and Wales, 1999. p. 24±5. [2] Alkemade MMC. How to prevent expansion of MSWI bottom ash in road construction. In: Goumanns J.J.J., et al., editors. Environmental aspects of construction with waste materials. Amsterdam: Elsevier, 1994. p. 863±77. [3] Abe S. Construction materials manufacturing by the technology of melting. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 25±30. [4] Nishigaki M. Producing permeable bricks and paving slabs from molten slag. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 31±40. [5] Osada M. Direct melting process for MSW recycling. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 73±8. [6] Selinger A, Schmid TV. The ABB dry ash concept: InRECTM. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 79± 84. [7] Selinger A, et al. Investigation of sintering processes in bottom ash to promote their reuse in civil construction. Parts 1 & 2. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 41± 58. [8] Pfrang-Stotz G, Reichelt J. Municipal solid waste incinerator bottom ashes as granular base materials in road construction. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 85± 90. [9] Jansegers E. The use of MSWI bottom ash in hollow construction materials. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 431±6. [10] Phil KA. The application of incinerator bottom ash in road construction. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 541±50. [11] Steketee JJ. Quality improvements of MSW bottom ash by enhanced ageing washing and combination processes. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 13±24. [12] Born JGP, Veelenturf RAL. MSWI residues in the Netherlands, putting policy into practice. In: Goumanns J.J.J., et al., editors. Waste materials in construction: putting theory into practice. Amsterdam: Elsevier, 1997. p. 841±50.
[13] ENDS. Progress on recycling incinerator ash. ENDS Report 290 March 1999. p. 16. [14] Wainwright PJ, Boni SPK. Arti®cial aggregates from domestic refuse. Concrete 1981;5(5):25±9. [15] Wainwright PJ, Boni SPK. Some properties of concrete containing sintered domestic refuse as a coarse aggregate. Magazine of Concrete Research 1983;35(123):75±85. [16] Wainwright P.J., Hadzinakos I., Robery P. A review of the methods of utilisation of incinerator residues as a construction material. In: Proceedings of the International Conference on Low Cost and Energy Saving Construction Materials. Rio de Janeiro, Brazil, 1984. [17] Wainwright P.J., Robery P. Production and properties of sintered incinerator residues as aggregates for concrete. In: Goumanns J.J.J. et al., editors. Waste materials in construction. Proc. Int. Conf. on Environmental Implications of Construction with Waste. Amsterdam: Elsevier, 1991. p. 425-32. [18] Elkins BV et al. Complete reclamation of wastewater and sludge. Water Science and Technology 1985;17:1453±4. [19] Yip W-K, Tay J-H. Aggregate made from incinerated sludge residue. Journal of Materials in Civil Engineering 1990;2(2):84±93. [20] Yip W-K et al. Clay-blended sludge as lightweight aggregate concrete material. Journal of Environmental Engineering 1991;117(6):834±44. [21] EEC. Directive 91/271/EEC concerning urban wastewater treatment, 1991. [22] Yasuda Y. Sewage sludge utilisation technology in Tokyo. Water Science and Technology 1991;23:1743±52. [23] Tay J-H, Show K-Y. Properties of cement made from sludge. Journal Environmental Eng 1991;117(2):236±45. [24] Woolley GR. State of the Art Report: use of waste materials in construction- technological development. In: Goumanns J.J.J. et al., editor. Environmental aspects of construction with waste materials. Amsterdam: Elsevier, 1994. p. 963±75. [25] Clarke J.L., editor. Structural lightweight aggregate concrete. London/New York: Blackie Academic & Professional, 1993. [26] Price W. Lightweight concrete for the North Sea: the BP Harding (Forth) Field gravity base tank. Quarterly Journal of the Federation de la Precontrainte 1995;2:3±7. [27] Parks L.F., Sherwen R.G. The use of new materials in an improved design of rotary kiln. Materials and Design 1986; VII(5). [28] Tay J-H. Engineering properties of incinerator residue. Journal of Environmental Eng 1991;117(2):233. [29] BS 3797. Lightweight aggregate for use in masonry units and structural concrete. [30] BS 812. Methods for the testing of aggregates for use in concrete. [31] BS 1881. Part 116. 1983 methods for determination of compressive strength of concrete cubes.
www.elsevier.nl/locate/wasman
Synthetic aggregates from combustion ashes using an innovative rotary kiln P.J. Wainwright *, D.J.F. Cresswell School of Civil Engineering, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Abstract This paper describes the use of a number of dierent combustion ashes to manufacture synthetic aggregates using an innovative rotary `Trefoil' kiln. Three types of combustion ash were used, namely: incinerated sewage sludge ash (ISSA); municipal solid waste incinerator bottom ash (MSWIBA - referred to here as BA); and pulverised fuel ash (Pfa). The ®ne waste ash fractions listed above were combined with a binder to create a plastic mix that was capable of being formed into `green pellets'. These pellets were then ®red in a Trefoil kiln to sinter the ashes into hard fused aggregates that were then tested for use as a replacement for the natural coarse aggregate in concrete. Results up to 28 days showed that these synthetic aggregates were capable of producing concretes with compressive strengths ranging from 33 to 51 MPa, equivalent to between 73 and 112% of that of the control concrete made with natural aggregates. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Rotary kiln; Synthetic aggregates; Concrete; Combustion ashes
1. Introduction
1.1. MSWIBA
This project is speci®cally aimed at using an innovative design of rotary kiln (known as the Trefoil kiln) to provide a solution to two modern-day dilemmas which confront both disposers of waste and users of natural aggregates for the production of concrete. The dilemmas are:
As the reliance on land®ll as the primary waste management option declines in the UK more municipal solid waste incinerators (MSWI) will be needed. Depending on the levels of minimisation, reuse, and recycling the number of new incinerators is estimated to be between 28 and 165 (200 000 t/a) [1] in the coming decades. This will produce a large and consistent supply of MSWI bottom ash. Such quantities will help to encourage the use of bottom ash (BA) as a construction material. This is a sensible method of utilisation as it avoids the problems and costs associated with disposal and provides an alternative aggregate source. Concern has been expressed over the use of BA in construction due to metal contamination and the associated expansion when incorporated in concrete [2]. Methods of sintering and vitri®cation of BA to decrease the availability of toxic elements are well documented [3±7] but energy costs are high and use is limited. The levels of metal contamination within BA mean that without vitri®cation utilisation requires some form of encapsulation or covering to prevent the leaching of contaminants into the groundwater. At present the main use of BA is in bound applications to reduce the potential for contaminant release [8±10], such applications include: asphaltic concrete, concrete blocks and cementitious subbases in roads. Washing and/or ageing [11] can bring
. how to overcome the con¯icting problems of dealing with the increasing amounts of domestic and industrial wastes and, at the same time, eect a reduction in the number of land®ll sites being used for disposal; and . how to limit the use of irreplaceable natural resources and still satisfy the growing demand for aggregate. The particular waste streams investigated in the project were: . incinerated sewage sludge ash (ISSA) . municipal solid waste incinerator bottom ash (MSWIBA- BA); and . pulverised fuel ash (Pfa) * Corresponding author. Tel.: +44-113-233-2264; fax.: +44-113233-2265. E-mail address: [email protected] (P.J. Wainwright).
0956-053X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00096-9
242
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
about improvements in BA quality, this decreases the mobility of metals by oxidation and carbonation reducing the pH and producing more stable mineral phases. Evidence from continental Europe (see [12], for example) suggests that BA is a potentially suitable material for some construction purposes. This has been borne out by recent developments in the UK where ash is being used from three incinerators and industry is drawing up guidelines on its use [13]. Satisfactory results have been obtained in previous work on producing a quality aggregate from sintering BA in a traditional rotary kiln [14±17]. The resultant aggregate was used to replace natural coarse aggregate in a number of concrete specimens, which were then tested over a 5-year period for structural performance and long term degradation. 1.2. ISSA A rotary kiln has been used to produce a lightweight aggregate (LWA) from a mixture of de-watered sewage sludge and clay [18]. This mix was extruded, chopped into pellets and ®red in a rotary kiln at up to 1090 C. Strengths of up to 35 MPa have been achieved from concretes in which this material was used to replace the natural coarse aggregate. A mixture of de-watered sludge and a binder has also been ®red in a brick kiln to produce a material which, after crushing, resulted in a hard angular rough textured aggregate with a loose bulk density of 600±650 kg/m3 and water absorption of approximately 7% [19,20]. Recent legislation preventing the dumping of sewage sludge at sea [21] from the end of December 1998 has meant that a number of alternatives are being sought by water companies to dispose of the sludge. One such method is incineration which produces a granular ash (ISSA) which itself needs to be disposed of. Currently the majority of the ash is disposed of to land®ll, but there has been some recent work undertaken to assess its potential for use as a construction material. A 3 tonne per day capacity plant has been constructed in Tokyo [22] producing a material called `sludgelight'. In this process the ISSA is mixed with an organic waste and pelletised before being ®red in a vertical shaft furnace at approximately 1050 C. The properties and performance of the aggregate produced were similar to other commercially available lightweight aggregates (LWA). Tay and Show [23] also demonstrated the use of ground ISSA as an ingredient in producing masonry cement.
replacement in concrete increases the workability of the plastic concrete, reduces the heat of hydration and is therefore useful in large structures such as dams. It also increases the long-term strength of the concrete and, by reducing the permeability, leads to an overall improvement in durability [24]. The use of pfa in the production of lightweight aggregate is also well-established (see [25]). Lytag is the trade name given to a LWA produced by sintering low carbon (circa 8%) ¯y ash. The ash is ®rst pelletised on an inclined disc pelletiser with 5±10% water and is then ®red on a `sinterstrand' at a temperature of about 1000 C. The resultant material is open textured with small voids that are interconnected and permeable to water. Lytag has a loose bulk density of approximately 825 kg/m3, and is capable of producing concretes with strengths in excess of 40 MPa. It is a well-established material and has been used in a number of large-scale projects such as, for example, the construction of North Sea oil production platforms [26]. 2. Experimental work 2.1. The Trefoil kiln The Trefoil kiln used in this project is much more thermally ecient than a traditional rotary kiln. Traditional rotary kilns consist of a thick steel drum lined with heavy manually laid ®re bricks which have a limited life and need regular maintenance entailing total shut down which can involve weeks of lost production. By contrast, the Trefoil kiln (see Fig. 1), which is potentially the only real advance in kiln design for the last 100 years [27], compresses a ceramic ®bre insulation between a thin steel alloy hot face and a tensioned thin mild steel cold face to form a cartridge with a triple-lobed cross section. The advantages of such a design include:
1.3. Pfa Pulverised fuel ash has a long history of use in construction. It can be used as a light base material when compacted, and as a pozzolan as it hardens when mixed with lime and water. The inclusion of pfa as cement
Fig. 1. Cross-section of the Trefoil kiln.
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
. excellent thermal eciency (an inner kiln temperature of 1200 C and an outer temperature of 60 C); . a much smaller and lighter structure which does not need costly maintenance or relining and which is easier to manufacture, transport, install, remove and replace; . much faster heat-up and cool-down periods (50 times quicker than conventional kilns); . vastly improved responsiveness (operating temperatures can be increased or decreased in minutes); and . higher load factors.
2.2. Materials The mineralogy of all three ashes used were similar consisting mainly of silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO) [19,24,28] with smaller amounts of Cu, Zn, K and other metal oxides. The pfa used had proportionally more silica and alumina than the other two ashes. The ISSA was derived from the incineration of sewage sludge from a plant in the UK. The ash was used in its conditioned state at dry solids content of about 75% with all the material passing 1mm. The MSWIBA (referred to here as BA) used was that passing 600 mm obtained from the 0±6 mm fraction of a sample taken from a bottom ash recycling plant in the UK. The processing undertaken at the plant consisted of a small amount of size reduction and removal of ferrous and non-ferrous metals. The pfa was obtained as a grab sample from the dry area of a settling lagoon of a UK power station. 2.3. Aggregate extrusion/pelletisation The ®rst stage in the aggregate production process was to blend the ashes with a binder to enable the mixture to be extruded and pelletised. In this study the binder used was clay, dewatered sewage sludge or a combination of both. The proportions of the dierent types of ashes and binders used are shown in Table 1. At the time of writing it is not possible, for commercial reasons, to disclose the proportions of pfa and sewage sludge that were used. Once blended the mixtures were extruded and then pelletised by means of a rotary drum pelletiser that also incorporated a burner drier. The combined action of the drier and the rolling motion of the drum produced a skin on the pellets that aided green pellet strength and created a thin denser outer rim on the ®red pellet.
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Table 1 Ratio of ash to binder in the aggregates Aggregate
ISSA 1 ISSA 2 BA 1 BA 2 Pfa
% Ash
63 64 82 (< 600 mm) 85 (< 300 mm) 5 (>300 < 600 mm) @
% Binder % Clay
% Sewage sludge (dry weight)
10 20 18 10
27 16 ± ±
@
@
the inert material in the pellet begins to sinter. From previous experience it was known that this stage can take between 5 and 40 min and occurs between 700 and 800 C, depending on the amount and type of carbon (organic material) present and the internal structure of the pellet. The carbon content of a lightweight aggregate produced from pfa needs to be below 4% to conform to BS 3797 [29] and it was considered appropriate to adopt this ®gure in the study. To ensure that this 4% level was reached before the sintering stage begins, samples of the aggregate were taken at various times during burnout and analysed for the carbon content. It was assumed that the remaining carbon would be burnt during further ®ring and could provide a volatile material for the production of voids during sintering, thus helping to give a lightweight structure to the pellet. All aggregates produced were tested for relative density and water absorption in accordance with BS 812 [30]. 2.5. Casting A number of dierent concrete mixes were made in which the natural coarse aggregate was replaced with the synthetic aggregates. The mix ratios used together with the free water/cement ratios and the workabilities are shown in Table 2. All mixes were designed to have nominally the same volume proportions with adjustments being made on the basis of loose bulk density. All the sintered ash aggregates were soaked in water prior to casting to ensure that they were in a saturated surface dried condition. Cubes were cast from all mixes for measurement of compressive strength in accordance with BS 1881 [31]. 3. Results
2.4. Aggregate ®ring
3.1. Aggregate properties
Firing of the synthetic aggregate consisted of two stages; the ®rst is known as the `burnout' stage where the carbon content of the ash/binder mix is allowed to combust (burnout) at a temperature below that where
The densities and water absorption ®gures for the aggregates produced from the combustion ashes are shown in Table 3, where they are compared with natural aggregate and with Lytag. The relative densities range
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Table 2 Mix ratios (by weight of cement) for all concrete specimens
Coarse aggregate Sand OPC Free water:cement Slump (mm)
ISSA 1
ISSA 2
Pfa
BA 1
BA 2
Control
Lytag
2.21 2.23 1.0 0.55 20
2.34 2.23 1.0 0.55 50
2.20 2.23 1.0 0.53 50
2.52 2.23 1.0 0.53 95
2.37 2.23 1.0 0.53 135
3.64 2.23 1.0 0.53 20
2.47 2.23 1.0 0.53 10
Table 3 Aggregate properties Aggregate
Dry loose bulk density (kg/m3)
Loose bulk density (SSD) (kg/m3)
Relative density (SSD)
24 h water absorption (% dry mass)
BA 1 BA 2 Pfa ISSA 1 ISSA 2 10 mm Nat. Agg. Lytag
920 985 816 630 830 1510 780
1036 1034 960 730 900 1520 875
1.9 1.9 1.7 1.6 1.7 2.6 1.8
12.7 4.5 17.7 15.5 8.1 0.8 12.3
from 1.6 to 1.9, which are signi®cantly lower than the natural aggregate at 2.6 and similar to Lytag at 1.7. All have dry bulk densities below 1000 kg/m3, which classify them as lightweight aggregates. The water absorption ®gures do not all follow the expected trend of high absorption corresponding to low density. Two of the aggregates (BA2, ISSA2) have low densities 985 and 830 kg/m3, respectively, and relatively low absorption ®gures (when compared to Lytag) of 4.5 and 8.1%, respectively. This would suggest that the pore structure is more segmented in these two aggregate types than in the others. A closed pore structure is considered more bene®cial because it is likely to result in higher strength and lower shrinkage than with a more open structured aggregate of similar density. Further work is on going to study in more detail the microstructure of the aggregates and to develop a model to be able to predict this structure from knowledge of the constituent materials and kiln ®ring conditions. 3.2. Concrete workability The workabilities (slump) of all seven mixes made to nominally the same water/cement ratios of 0.53 are shown in Table 2. All mixes, with the exception of the control mix, had similar densities and it is reasonable therefore to make comparisons between the Lytag mix and the remainder of those mixes made with the synthetic aggregates manufactured in this study. Using any of the synthetic aggregates as a coarse aggregate replacement leads to an increase in workability when compared with Lytag. In the case of the two mixes made with the bottom ash aggregates (BA1, 2) the increases
are signi®cant from 10 mm to between 95 and 135 mm respectively. All the aggregates produced in this study tended to be slightly more rounded and smoother than Lytag which would account for some of the increase, but no reason can be given at this stage for the signi®cant increases observed with the two bottom ash aggregate mixes. 3.3. Compressive strength Unfortunately at the time of writing strength results are only available up to the age of 28 days and these are shown for all mixes in Fig. 2. In addition the ratio of the 28-day strengths of the mixes compared to both the Lytag and the natural aggregate control concretes are shown in Table 4. The compressive strength results for all the dierent aggregate types are encouraging, in most cases they are similar in performance to both the control mixes and dier little from each other. Only two mixes show any signi®cant dierences in trend, that made from the incinerated sewage sludge ash (ISSA 2) and that made with the bottom ash (BA 1). In the case of the former the 28 day strength is 20% higher than with Lytag (51 cf. 42 MPa) and 12% stronger than even the natural aggregate control (51 cf. 45 MPa). Comparing ISSA 2 with ISSA 1 (Table 1) would indicate that the addition of 10% extra clay as the binder at the expense of the sewage sludge is bene®cial in terms of compressive strength (Fig. 2) and water absorption (Table 3). Interestingly the reverse appears to be true in the case of those aggregates made with bottom ash where the aggregate with the lower clay content (BA 2) performs
P.J. Wainwright, D.J.F. Cresswell / Waste Management 21 (2001) 241±246
245
Fig. 2. Compressive strengths of synthetic aggregate concrete. Table 4 Strength of sintered ash aggregate concrete compared to control and Lytag concrete Aggregate
% of Lytag concrete 28 day strength
% of Control concrete 28 day strength
BA 1 BA 2 PFA 1 ISSA 1 ISSA 2
79 95 98 90 120
73 88 90 84 112
term data is needed to determine whether or not there are any potentially deleterious reactions taking place between the aggregates and the cement paste. The work programme currently underway is designed to address these questions as well as provide important information on the leaching characteristics of the synthetic aggregates. Future planned work will also look at other properties of concretes made with these aggregates such as: permeability, elasticity, shrinkage and creep. 4. Conclusions
better than BA1 containing 8% more clay. The 28-day strength of the mix made with the BA 1 aggregate is the lowest of all at 28 MPa which is 21 and 27% lower than the Lytag and natural aggregate control mixes, respectively. 3.4. Future work programme These results show that concretes with reasonable strengths can be made with synthetic aggregates produced from any of the waste streams investigated, but more work is needed to ®nd the mix proportions (in terms of ash and binder contents) to give the optimum performance from the aggregates. Although the strength results obtained to date are satisfactory, much longer-
The results of the tests reported here have shown that it is possible to successfully manufacture synthetic lightweight aggregates from the combustion ashes derived from the incineration of: sewage sludge, municipal solid waste and pulverised coal (pfa). The ashes were combined with a binder, extruded to form pellets and then ®red in a Trefoil rotary kiln to produce synthetic lightweight aggregate. Concretes made with the synthetic aggregate achieved 28 day strengths of between 33 and 51 MPa compared with 45 and 42 MPa for the natural aggregate and Lytag aggregate control mixes, respectively. Further work is being undertaken to establish the leaching characteristics of the aggregates and the optimum mixture proportions to give the most desirable aggregate properties.
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