LIFAC ash – strategies for management

Waste Management 25 (2005) 265–279 www.elsevier.com/locate/wasman

LIFAC ash – strategies for management E.J. Anthony *, E.E. Berry, J. Blondin, E.M. Bulewicz, S. Burwell CANMET Energy Technology Centre-Ottawa, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ont., Canada K1A 1M1 Accepted 1 November 2004

Abstract LIFAC is a more recent addition to flue gas desulphurization methods for reducing sulphur emissions during coal combustion for the production of electricity. Ashes from the combustion of a low-sulphur lignite coal using LIFAC technology were used to evaluate different ash management strategies. The ashes, as produced and after treatment by the CERCHAR hydration process, were examined for their disposal characteristics and their utilization potential in concrete. They were also evaluated as underground disposal material using the AWDS process. Crown Copyright
2004 Published by Elsevier Ltd. All rights reserved.

1. Introduction The use of add-on technologies for the removal of flue gas components such as SO2 and NOx has altered the chemical and physical characteristics of the final ash and affected disposal and handling properties. In particular, the addition of limestone as a sorbent, produces ashes that behave quite differently from pulverized fuel ashes (PFAs), both in terms of disposal and during ash utilization. Ashes from the emissions control technology ‘‘limestone injection into the furnace and activation of unreacted calcium’’ (LIFAC), which is typically used for low- or medium-sulphur coals, were examined for characteristics and behaviour relevant to ash disposal and use. Both untreated and prehydrated ashes were used. The prehydration method used pressurized steam at about 165 C (the CERCHAR process – Centre ´ tudes et de Recherches du Charbon). d
E 1.1. LIFAC process LIFAC is a two-stage SO2 removal process developed by Tampella Power (Ryypoo¨ and Ekman, 2000). The *

Corresponding author. Tel.: +1 613 996 2868; fax: +1 613 992 9335. E-mail address: [email protected] (E.J. Anthony).

first stage consists of dry limestone injection into the upper portion of the pulverized coal-fired furnace, near the exit. The temperature in this region is typically 900– 1250 C and the limestone rapidly calcines to CaO. Approximately 25% of the SO2 is captured in the first stage. In the second stage, water is sprayed into a reaction chamber located between the air preheater and electrostatic precipitator. This activates the unused sorbent carried by the flue gas and the reinjected fly ash from the precipitator. Most of the SO2 capture (
50%) is achieved in this step (Ryypoo¨ and Ekman, 2000; Grace Dearborn Inc., 1993). The coal-derived portion of the fly ash is similar to conventional PFAs in physical and chemical composition. Sorbent injection and reactivation generate the CaO and CaSO4 portions of the ashes. The bottom ash collected from the furnace is entirely coal derived and, therefore, contains little or no calcium depending on the Ca content of the fuel ash. The LIFAC ash, therefore, consists only of fly ash, the calcinced sorbent and its reaction products. 1.2. CERCHAR process CERCHAR has developed a two-stage ash hydration process to quantitatively hydrate high-lime coal ashes using saturated steam at about 165 C. Initially, the

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2004 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2004.11.005

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Nomenclature LIFAC limestone injection into the furnace and activation of unreacted calcium FGD flue gas desulphurization ´ tudes et de Recherches du CERCHAR Centre d
E Charbon AWDS ash–water dense suspension PFA pulverized fuel ash FBC fluidized bed combustion

ashes came from conventional combustion of brown coal in Gardanne, France. Later the technology was shown to be applicable to ashes from circulating fluidized bed combustion (CFBC) of high-sulphur fuels using limestone or dolomitic sorbents (Blondin et al., 1993; Iribarne, 1991). 1.3. AWDS process The ash–water dense suspension (AWDS) method was developed in Poland for the disposal, on the surface or underground, of PFAs and other energy industry wastes. The suspension sets to a rock-like solid, strong enough to prevent subsidence in mines while maintaining extremely low permeabilities. Solid-to-water ratios of 1:1 to 3:1 were recommended, keeping water use and the water content of AWDS low (Anthony et al., 2003). 1.4. No-cement and roller-compacted concrete LIFAC ashes have received relatively little attention except in terms of disposal options. Other calcitic ashes (e.g., CFBC ashes) have been studied in much greater detail, albeit with mixed success in finding alternative uses. They have been used as binders, with PFA, in low performance concretes such as no-cement concrete (NCC) and roller-compacted concrete (RCC) (Rose et al., 1987; Bland et al., 1987a,b; Jones et al., 1987; Dearborn Environmental Consulting Group, 1988), the PFA acting as the pozzolan, reacting with the CaO in the FBC residues. Materials such as NCC and RCC appear to have considerable potential for non-structural applications, particularly mine support or backfill, where large quantities of cement are currently used. In such applications some level of controlled expansion may even be advantageous and the long setting times are not a limitation. RCC is a construction material with some of the characteristics of concrete, soil-cement and asphalt. Physically, RCC is a damp aggregate and can be produced in ready-mix cement trucks. Emplacement of RCC requires a spreader box, vibratory roller compac-

CFBC NCC RCC ICAP XRD TGA IC DSC

circulating fluidized bed combustion no-cement concrete roller-compacted concrete inductively coupled argon plasma X-ray diffraction thermogravimetric analysis ion chromatography differential scanning calorimetry

tor and a cover for curing. Mix designs of the concrete tested follow conventional guidelines, taking into account the interactions of: cement, fine and coarse aggregate and water, soil-cement design guidelines detailing flexure strength and workability, and asphalt design guidelines. No similar work has been done on the preparation of no-cement binder from LIFAC ashes. The presence of lime and calcium sulphate intimately mixed with PFA suggests that this material ought to be ideally suited to the production of no-cement concrete.

2. Experimental procedure 2.1. Residue feedstock Two types of residues were examined, both from SaskPower
s 300 MWe Shand station LIFAC two-stage SO2 capture process, with a Saskatchewan lignite coal. One portion of ash was used as is (LFC), while the other was hydrated by CERCHAR in France (LFC-C). 2.2. CERCHAR hydration The CERCHAR hydration process (Blondin and Baalbaki, 1991) was used to hydrate over 300 kg of LIFAC fly ash. Batches of the ash (30–50 kg batches) were placed in a 200-l spherical hydration reactor and wetted with an ash-specific amount of cold water. External heaters maintained the vessel wall temperatures at 170 C, while the pressure inside the reactor rose from 0.5 to 0.85 MPa. Mixing took place throughout the procedure to aid the conversion of lime to portlandite. The vessel was then opened and the mixing continued for several minutes to permit any excess water to evaporate, leaving the material air dry. After hydration, all ash lots were recombined and mixed to ensure the homogeneity of the product (LFC-C). 2.3. Ash characterization Chemical characterization by wet chemical methods, inductively coupled argon plasma (ICAP) and X-ray dif-

E.J. Anthony et al. / Waste Management 25 (2005) 265–279 Table 1 Chemical characterization of the ashes

267

Table 3 Leachate characterization for the ashes (values in mg/l)

Component (%)

LFC

LFC-C

Parameters

LFC

LFC-C

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO TiO2 Cr2O3 B2O3 P2O5 SO3 CO2 LOI

33.8 21.3 2.61 25.9 3.10 6.25 0.46 0.02 <0.72 <0.01 – 0.43 3.88 – 1.70

33.2 18.8 3.52 21.5 2.51 6.67 0.74 0.02 0.66 <0.01 – 0.39 – – 5.4

Sum

99.4

93.4

PH (units) Conductivity (lmho) Phosphorus Zinc Lead Aluminum Iron Boron Chromium Magnesium Calcium Barium Sodium Potassium Silica Sulphate Nitrate

12.4 13,600 NA NA <0.039 0.051 0.012 0.8714 0.131 NA 2730 7.10 420 5.96 1.3 909 10

12.1 11,100 NA NA <0.043 <0.032 0.015 1.17 0.070 NA 2660 2.10 596 26.8 2.56 810 11.3

fraction (XRD) were used. Results for chemical analysis obtained by ICAP are presented in Table 1. Only small differences between the hydrated (CERCHAR) and unhydrated samples were seen, but in the hydrated material CaO (as quicklime) was no longer present, and by implication the hydration was complete. The particle size distribution of the ashes was determined by sieve analysis. Table 2 shows that the LFC-C ash has a somewhat different particle size distribution from untreated LFC, due presumably to mixing and the hydration process, which are capable of producing some size reduction and agglomeration, respectively. In leachate testing the method used was Canadian General Standards Board Leachate Extraction Procedure (CGSB 164-GP-IMP) (Canadian General Standards Board, 1987) in which 50 g (dry) of the ash is mixed in 800 ml of reagent water and the mixture is radially rotated in sealed nalgene bottles for 24 h. Acetic acid (0.5 M) may be added, to a maximum of 4 ml/g dry weight, if the pH of the leachate exceeds 5.2. After maximum acid addition, the leachate was filtered through a 0.45-lm filter paper. ICAP, ion chromatography (IC) and wet chemistry were used to analyze the leachates. In both cases the pH was above 12, conductivities ranged from 11,100 to 13,600 lX1, and sulphate

Table 2 Particle size distribution of the ashes Mesh

LFC

LFC-C

16 30 50 100 200 325 Pan

0.00 0.00 0.21 0.90 9.41 16.28 73.20

0.01 0.29 0.41 0.91 7.08 12.45 78.85

The following elements were all below detection limit (mg/l) for all samples: arsenic <0.069, selenium <0.08, mercury <0.03 ppb, cadmium <0.004, nickel <0.015, manganese <0.004, silver <0.008, titanium <0.003.

levels were about 900 mg/l. Calcium levels were 2700 mg/l. The results are presented in Table 3. 2.4. Geotechnical testing of the ashes Quick lime (CaO) albeit in low amounts, is present in the LFC ash, but there is no appreciable heat or steam generation on contact with water. Therefore, the ash can be used without prehydration. LIFAC ashes, due to the higher combustion temperature and the location of the limestone injection, contain spherical PFA particles. As the behaviour of such a mixture of PFA with calcined and sulphated sorbent has not been studied in detail, a geotechnical-test program was set up. The optimum moisture content of an ash is the quantity of water added to obtain the best compaction at a disposal site. For a soil, it can be defined as ‘‘the water content at which a soil can be compacted to a maximum dry unit weight (dry density) by a given compactive effort’’. The dry density is the mass of dry solid per unit volume of compacted sample. The point at which the maximum dry density is achieved is not necessarily the point of maximum sample density. The optimum moisture content was obtained from Proctor compaction tests following AASHTO T-134 (AASHTO, 1990). Both freeze/thaw (AASHTO T-136) (AASHTO, 1990) and wet/dry (AASHTO T-135) (AASHTO, 1990) stability tests require cylindrical specimens, compacted to their optimum densities and stored at 21 C and 100% relative humidity (RH) for 7 days. The freeze/thaw samples are then cycled at 23 C in air for 24 h, followed by 23 h at 21 C and 100% RH, for

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a total of 12 cycles. After each cycle the samples were weighed and their diameters and heights measured. The wet/dry samples were submerged in water at room temperature for 5 h, followed by drying for 42 h at 71 C, for a total of 12 cycles. After each period, the samples were weighed and their diameters and heights measured. After the 12th cycle samples were dried at 110 C. When accurate measurements could no longer be made due to sample deterioration, the sample was considered to have failed. The triaxial compressive strength (unconsolidated, undrained) of the ashes simulates isostatic pressures at various depths and was obtained at confinement pressures of 138, 275 and 415 kPa, by test method ASTM D2850-87 (ASTM, 1990). The direct, unconfined shear strength was tested according to ASTM D2166-91 (ASTM, 1990). For the ashes, hydraulic conductivity values were below 1 · 102 m/s, i.e., at the lower limit for permeability tests by ASTM D2434-68 (ASTM, 1990). The ASTM D-5084 method (ASTM, 1990) was therefore used. The tests were made in a triaxial constant head flexible wall apparatus, with saturation to a B-value (the change in sample pore water pressure divided by the change in confinement pressure) of at least 0.96. A hydraulic gradient of 20 and consolidation pressure of 37.7 kPa were used for all samples. 2.5. Physical testing of engineered materials No-cement and roller-compacted concrete test programs were set up following previous ash utilization work (Dearborn Chemical Company Ltd. and Ash Management Engineering Inc. Commercialization, 1991). The unconfined compressive strength of triplicate sets of 2-in. cubes (ASTM C-109) (ASTM, 1990) was used to determine the optimum ash/water blend. The cubes were cured for 7 days, demolded and their dimensions were measured. After curing for 7, 14, 28, 56 or 90 days the cubes were placed in an unconfined compressive strength machine for destructive testing. Unconfined compressive strength development was also used to evaluate the NCC and RCC. Modified ACI Portland cement proportioning guidelines were used to determine the concrete mix design (Table 4). Sample size for testing was 100 mm (width) · 200 mm

(height) cylinders. The samples remained in the disposable molds, sealed in plastic bags, until the curing period of 7, 14, 28, 56 or 90 days was completed. The compactive effort used was 3445 N/m3 by dropping a 4.5 kg rammer over a distance of 450 mm, with a total of 58 blows per cylinder. Expansion testing was carried out according to ASTM C-157 (ASTM, 1990). The samples were molded and cured in plastic bags prior to testing. At the end of each curing period the samples were measured for expansion and returned to the plastic bags until the end of the next curing period. Set times were determined in accordance with ASTM C-403 (ASTM, 1990). An initial set time occurs when the mortar can withstand a penetration of 3.45 MPa (500 psi). A penetration resistance of 27.58 MPa (4000 psi) is defined as the final set. The AWDS testing was carried out at Cracow University of Technology, Poland. The behaviour of AWDS mixtures with different solid-to-water ratios over time was examined. The containers used were screw-top glass jars of 750 ml capacity with an internal diameter of 85 mm. Samples were prepared by combining the required quantities of ash and water and mixing them in a small blender for 5–8 min. The solid-to-water mass ratios used ranged between 0.9 and 1.0, and about 2 and 3. Parallel samples were kept in closed and in open jars in the laboratory, at room temperature (12–30 C), without humidity control. Closed jars were considered to be equivalent to placement of the ash–water mixture in a confined space. Each jar was effectively a closed system, allowing the results to be interpreted quantitatively. Open jars, permitting evaporation of H2O and capture of atmospheric CO2, mimicked deposition of the ash in the open, under temperate conditions, in the absence of precipitation. The total volume of each sample, the volume of the sediment and of the supernatant water was measured 2–3 h after the samples were prepared. The frequency at which subsequent observations were made was dependent on the changes observed. The Az-NII cone spreading behaviour method (Bulewicz et al., 1992) was useful in making a quick judgment about the fluidity of various mixtures. The cone is filled with the mixture, covered with a glass plate, inverted and quickly lifted, allowing the mixture to spread on

Table 4 Mix design for no-cement and roller-compacted concrete Mix proportions (%)

NCC

NCC-H

NCC-C

RCC

RCC-H

RCC-C

Coarse aggregate 19 mm (3/4 in.) clear 9.5 mm (3/8 in.) clear Fine aggregate LIFAC ash Water

24.6 16.4 26.0 26.6 6.4

24.1 16.0 25.4 28.2 6.3

23.0 15.4 24.3 27.0 10.3

14.1 20.8 31.1 26.6 7.4

14.3 21.1 31.5 27.0 6.1

13.8 20.3 30.4 26.0 9.5

E.J. Anthony et al. / Waste Management 25 (2005) 265–279

the horizontal glass surface. The spread is the diameter of the circle occupied by the material. The optimum fluidity corresponds to a spread of 190–220 mm. The settling behaviour of the AWDS mixtures was tested following PN-80/B-04300 (Bulewicz et al., 1992) for cement mixtures, using the Vicat apparatus. Readings were taken of the depth to which the needle of the instrument sank into the mixture over 30 s. Setting began at a reading of 30 mm and was considered to be complete at less than 2 mm. The maximum amount of supernatant water was also recorded. The effect of freezing ash–water suspensions on the setting behaviour and the compressive strength of the final solidified and cured mixtures was examined. The method used has been described before (Bulewicz et al., 1992) and involves taking fresh samples frozen at 15 C for 24 h and allowing them to thaw at 20 C. Observations were made on the period of time, after thawing, required for the samples to set. The compressive strength of the samples was determined 28 days later, using a modified standard (PN-84B-04110 stone method) (Bulewicz et al., 1992). The ash–water suspensions were poured into molds to produce cylindrical solidified samples 80 mm · 80 mm. Tests were then carried out on samples kept for 28 days either at room temperature (dry air) or cured at 20 C at 100% relative humidity. The samples, with the top and bottom surface machined parallel to within 0.1 mm, were then placed in the test machine and subjected to compressive stress along the axis of symmetry. The stress was continuous, increasing at a rate of 10 Pa/s and the critical stress for structural failure, Rc, was determined. Samples were cured for 28 days, then immersed in water without stirring. Changes were monitored for periods of 4, 8, 24, and 48 h. In addition, the permeability of 28-day samples was determined using the Kamin´ski method (Mazurkiewicz, 1990). Rheological properties of ash–water suspensions were determined using a Fann-type rotary viscometer, model VG-35 S (USA) (Bulewicz et al., 1992). The viscometer consists of an inner test cylinder; a rotating cylinder and a reservoir in which the fluid tested is placed. The rotation speed can be varied in suitable steps from 3 to 600 Hz. 2.6. Chemical testing of engineered materials Chemical changes occurring in calcitic ashes during the curing process are still not fully understood. Ettringite formation is probably important, but for this type of ash reliable information on its rate of formation or for that matter the rate of hydration of the anhydrite itself, is not available in the open literature. After set curing periods, the LIFAC and CERCHARtreated LIFAC ash pastes were examined for geochemical changes. The materials were immediately finely crushed and then ground with pure acetone to extract any free

269

Table 5 Proctor compaction results Sample

Optimum moisture content (%)

Optimum density (g/cm3)

Optimum dry density (g/cm3)

LFC LFC-C

14.8 17.5

1.81 1.73

1.50 1.43

water and stop the hydration processes (Rokita and Tomaszewski, 1988). Grinding was first done by hand in an agate mortar then in a corundum ball mill, until particle size was <20 lm. The acetone/ash paste was then filtered and the solids washed with acetone and dried at <40 C under nitrogen. The dried samples were analyzed for free lime and total alkalinity and examined by XRD using a Siemens D500 instrument and Kevex detector. Thermogravimetric analysis (TGA) was also carried out in an O2-free current of Ar, using a Dupont 951 Thermobalance and Dupont Thermal Analyst 2100 control system, coupled to a VG Quadrupole Micromass PC 300D mass spectrometer. Differential scanning calorimetry (DSC) was also employed, using a Dupont differential scanning calorimeter, Model 910.

3. Results and discussion 3.1. Geotechnical testing of ashes 3.1.1. Proctor compaction Results of the Proctor compaction tests are presented in Table 5, giving the optimum moisture content, density and dry density. For the untreated LIFAC ash the optimum moisture content is quite low. It is hypothesized that this could be due to the spherical shape of most of the ash particles, facilitating efficient packing and hence promoting the tendency of the material to pack at its highest dry density, with a low water content. The higher moisture content of the LFC-C (17.5% vs. 14.8%) is presumably due to increased content of fines and irregularly shaped particles in the treated ash. 3.1.2. Freeze/thaw stability Both LFC and LFC-C survived 12 freeze/thaw cycles, with a falling tendency to take up water. LFC and LFCC showed some tendency towards shrinkage and expansion, respectively (Table 6). LFC-C was slightly less stable, but the observations indicate the ashes would be stable to freezing and thawing in use or in disposal sites. 3.1.3. Wet/dry stability Results of the wet/dry testing are presented in Table 7. The LFC did not show any significant deterioration under wet/dry exposure, although shrinkage of 2–3% was noted. The volume of LFC-C decreased by about

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E.J. Anthony et al. / Waste Management 25 (2005) 265–279

Table 6 Freeze/thaw results for the ashes

Table 9 Direct shear strength results

#

Sample ID

Sample specific gravity

Compressive stress (kPa)

Shear strength (kPa)

Strain at failure (%)

LFC LFC-C

2.30 2.70

4250 8620

2130 4310

1.30 0.96

Sample mass and volume changes (M% and V%) LFC

1 2 3 4 5 6 7 8 9 10 11 12

LFC-C

M%

V%

M%

V%

14.6 15.3 15.6 15.9 16.0 16.1 16.2 16.2 16.3 16.3 16.5 16.5

0.35 0.13 0.18 0.22 0.33 0.08 0.20 0.07 0.12 0.27 0.40 0.49

10.5 11.1 11.3 11.5

0.24 0.41 0.31 0.36

11.7 11.8 11.9 12.0 12.1 12.1 12.2

0.24 0.38 0.31 0.36 0.02 0.04 0.19

Table 7 Wet/dry results for the ashes #

Sample (Mass gain %/volume increase %) LFC

1 2 3 4 5 6 7 8 9 10 11 12

LFC-C

M%

V%

M%

V%

0.9 0.1 4.6 0.1 2.5 0.5 1.8 2.2 0.5 2.5 0.3 2.0

0.45 1.2 0.12 1.83 2.19 3.30 2.39 2.82 2.82 3.38 2.23 3.18

3.1 2.7 4.3 5.2 5.3 6.9 5.3 5.9 8.2 6.6 6.5 9.1

0.17 0.18 0.10 0.17 0.11 0.26 0.47 0.54 0.95 0.93 0.93 1.03

3.1.5. Shear strength The shear strengths are low, as expected for brittle samples, without any cementing agents (Table 9). The LFC and LFC-C samples show some evidence of strength development, indicating significant self-cementing. Shear strengths, although low, were still significantly higher than those seen previously with FBC fly ash (Anthony et al., 2003). 3.1.6. Permeability Permeability results are given in Table 10. The compacted LFC had a very low permeability (4.6 · 107 mm/s) while the LFC-C was more permeable by an order of magnitude (2.6 · 106 mm/s). This could be due to the differences in optimum water content for maximum compaction. The lower water demand and porosity of the LFC samples produces a less permeable, compacted solid. These results also tend to explain the lack of water uptake by the LFC and LFC-C samples when subjected to wet/dry conditions and are consistent with the high compressive strengths observed for these materials. 3.2. Physical testing of engineered materials

1% over the duration of the test. After the 12 wet/dry cycles the sample also showed noticeable surface cracks and some spalling of the top edge. 3.1.4. Triaxial compressive strength Both the LFC and LFC-C show relatively high strengths at low and high confinement pressures (Table 8). These samples are sufficiently strong for the confinement pressures to have virtually no effect on their strength properties. The LFC-C is significantly stronger than the untreated LFC (12.3 MPa compared with 9.9 MPa at 415 kPa confinement pressure), indicating that the CERCHAR process has some beneficial effect on the self-cementing ability of the LIFAC ash. Table 8 Triaxial compressive strength results Sample ID

LFC LFC-C

Mixes without PFA or lime added were prepared to determine the best water-to-solids ratio. The best LFC paste design was 81% LIFAC ash and 19% water (water-to-solids ratio of 0.228). The results are illustrated in Fig. 1. The strength development of the LFC mixes was substantial, bearing out the initial evaluation of the characteristics of the ash. The formulation for the LFC-C was 77.4% LIFAC ash and 22.6% water (CERCHAR, 1994). The results for the unconfined compressive strength development of the two optimized LIFAC mortars (LFC and LFC-C) are shown in Fig. 2. The figure shows that the strength development curves for the two materials Table 10 Permeability results

Compressive stress (kPa), confinement pressure 138 kPa

415 kPa

6670 12,370

9900 12,300

Sample

Time to steady state/duration (min)

Hydraulic head (m)

Hydraulic conductivity (mm/s)

LFC LFC-C

168/4143 180/360

4.36 5.17

4.61 · 107 2.6 · 106

Unconfined Compressive Strength (MPa)

E.J. Anthony et al. / Waste Management 25 (2005) 265–279

271

45 40 35 30 25 20

7Days Curing

15

42Days Curing

10

Poly. (7DaysCuring)

5

Poly. (42DaysCuring)

0 0.225

0.23

0.235

0.24

0.245

0.25

0.255

0.26

0.265

0.27

Water/Solids Ratio

Fig. 1. Unconfined compressive strength development of cured samples as a function of the water/solids mass ratio.

are similar. Considerable compressive strength is developed, greater for the LFC mortar than the LFCC one.

Unconfined Compressive Strength (MPa)

3.2.1. Effect of different curing conditions Triplicate sets of the LFC-C ash were cured under two types of simulated environmental conditions. In the first case the samples were cured in sealed plastic bags at 23 C, so there was no exchange of water between the sample and the environment. In the second case the samples were cycled through an immersion in water and a sealed drainage cycle, to represent an unprotected environment (noted-E). Before the tests started, the samples were all cured for 7 days in the sealed condition. All samples were tested for unconfined compressive strength and dimensional changes (Fig. 3). The LFC materials all showed very little expansion and good strength development. The environmental cycling seemed to have had only a minor effect on the strength development and expansion.

3.2.2. No-cement concretes Unconfined compressive strength (UCS) development of the mixes is shown in Fig. 4. The conventionally hydrated LIFAC (NCC-H) exhibited least strength development, with only 15.4 MPa achieved at 90 days. Greatest strength development was obtained with the untreated LIFAC (NCC), with 24.3 MPa attained at 90 days. The CERCHAR treated LIFAC (NCC-C) outperformed the NCC-H. The NCC-C has a higher water requirement and this may have affected the development of compressive strength. As shown previously, high water content (without the water required for hydration) results in lower unconfined compressive strength development. 3.2.3. Set times Previously, no-cement concretes have exhibited extended set times (CERCHAR, 1994). LIFAC mortars (both LFC and LFC-C) had much shorter set times, as indicated in tests with the LFC materials. For LFC

45 40 35 30 25 20 15

LFC

10

LFC-C 5 0 0

10

20

30

40

50

60

70

80

90

100

Curing Time (Days) Fig. 2. Unconfined compressive strength of samples with an optimum water/solids ratio as a function of the curing time.

272

E.J. Anthony et al. / Waste Management 25 (2005) 265–279 45

Unconfined Compressive Strength Development (MPa)

LFC 40

LFC-C

35

LFC-C-E

30 25 20 15 10 5 0 0

10

20

30

40

50

60

70

80

90

100

Curing Time (days)

Unconfined Compressive Strength Development (MPa)

Fig. 3. Unconfined compressive strength of mortars containing LIFAC ashes as a function of the curing time.

30

25

20

15

10 NCC

5

NCC-C NCC-H

0 0

10

20

30

40

50

60

70

80

90

100

Curing Time (days)

Fig. 4. Unconfined compressive strength of no-cement concretes containing LIFAC ashes as a function of the curing time.

and LFC-C setting started after 8 and 26 h, respectively and the process was complete after 26 and 52 h. The set times followed the general pattern seen in this work. The untreated LFC residue consistently showed a tendency to develop higher compressive strength than the CERCHAR-treated material and to achieve it faster. 3.2.4. Roller-compacted concretes The mix design for roller compacted concretes is given in Table 4. The RCC-H contained ash prehydrated in the laboratory and, therefore, the amount of mix water required was lower than for the RCC, with untreated ash. The RCC-C (CERCHAR-treated LIFAC ash) had a higher water requirement than the RCC-H. Strength development (UCS) of the RCC series concretes is illustrated in Table 11. Most strength development takes place over the first 14 days. Strength does continue to increase past this time, but only slowly. Again, the greatest initial strength development is seen with RCC, the lowest with RCC-H. At 90 days the

strength in RCC appeared to be lower than that for RCC-C. The result for RCC at 90 days appeared odd, but the quantity of the residues available for this work was limited, and some test cylinders were prepared in duplicate or even singly and this particular test could not be repeated. However, the LIFAC materials do seem to expand at lower water contents, possibly causing the loss of strength at 90 days.

Table 11 Unconfined compressive strength development for roller-compacted concrete Mix

Unconfined compressive strength (MPa) 7 days

14 days

28 days

56 days

90 days

RCC RCC-H RCC-C

15.1 9.7 13.5

17.2 11.5 16.2

20.1 12.1 16.8

23.1 13.4 17.8

17.2 14.2 19.2

E.J. Anthony et al. / Waste Management 25 (2005) 265–279

273

Maximum Supernatant Water (%)

30 LF C

25

LF C- C

20

15

10

5

0 0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

Solid/Water Ratio Fig. 5. Supernatant water over LIFAC ashes, as a function of the solids/water ratio.

3.2.5. Ash–water dense suspension Both untreated LIFAC and CERCHAR-treated LIFAC (AWDS-LFC and AWDS-LFC-C) were used in the AWDS work. When mixtures were made with LFC, there was a temperature rise of several degrees Celsius. The heat of reaction estimated from a plot of temperature rise vs. the W/S ratio was about 20 kJ/kg. With LIFAC-C there was no temperature rise. Two types of experiments were made. In one the containers were left open to ambient air and in the other the jars containing the mixture were sealed. In the closed jar tests, for the AWDS-LFC, and AWDS-LFC-C the initial densities of the settled material (i.e., solids with water filling the inter-particle spaces) were 1540 and 1590 kg/m3, respectively, with the amount of supernatant water decreasing as the solids-to-water ratio rose. This is shown in Fig. 5. In the closed containers LFC slowly took up water for over about 30 days, while with LFCC there was considerable initial sedimentation but then little further change. In the open vessels, the water level

steadily fell, mainly due to evaporation. After solidification, the AWDS-LFC was a smooth, hard, rock-like solid while the AWDS-LFC-C set to a much softer material and crumbled easily. There was a tendency for Na2SO4 crystals to form on the surface of the samples. 3.2.6. Spreading behaviour of AWDS mixtures The AWDS-LFC was more ‘‘runny’’ with water. Since there is evidence that the degree of spread is inversely related to the mean size of the particles, this may simply reflect particle agglomeration in the CERCHAR hydration process. Following the original criteria (Bulewicz et al., 1992) for optimum mixtures in terms of fluidity, the AWDS-LFC and AWDS-LFC-C should have S/W ratios of 2.0–2.25 and 1.7–1.85, respectively, Fig. 6. Both the time to the onset of hardening and the duration of the process decreased rapidly with increasing solids/water ratio. The results are given in Table 12. At low solids-to-water ratios (S/Wm) the AWDSLFC-C mixtures were relatively slow to set. It is evident

400

AWDS-LFC 350

AWDS-LFC-C

Spread (mm)

300

250

200

150

100 0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

Solid/Water Ratio

Fig. 6. Spread of AWDS samples based on LIFAC ashes, as a function of the solids/water ratio.

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Table 12 Spreading behaviour, supernatant water and setting of AWDS materials

Table 13 Effect of freezing on setting times and compressive strength of cured samples

Mix

Sample

S/W

Thawed

No freezing

AWDS-LFC

1.50 2.00

90 72

2.51 3.43

2.67 3.91

AWDS-LFC-C

1.50 1.72

168 70

0.41 0.41

0.45 0.59

AWDS-LFC

AWDS-LFC-C

S/W

Spread (mm)

Maximum water (%)

Setting time (h)

1.00 1.25 1.50 1.75 2.00 2.35

350 330 300 270 220 185

12.9 7.2 4.0 1.5 2.0 0.5

98 62 20 18 14 12

170 123 95 76 50 36

1.00 1.25 1.50 1.75 1.90

340 305 240 205 175

26.7 17.0 9.2 4.5 2.4

168 96 48 36 24

372 288 168 72 52

Initial

Final

that the setting of AWDS mixtures based on LIFAC does not depend on the crystallization of gypsum or significant ettringite formation. XRD phase analysis showed that in both the solids were predominantly amorphous, with some quartz, calcite and traces of hematite. Since AWDS-LFC sets to a hard smooth mass and AWDS-LFC-C to a crumbly solid, the differences between them must be significant. Comparison of the chemical composition (Table 1) and XRD data (Table 19) suggest that both in the original material and in the solidified mass the silica is present mostly in the form of amorphous silicates or aluminosilicates, perhaps in association with Na+ ions, since the Na content of these ashes is elevated. It can be supposed that when sufficient water is added the Ca2+ ions are precipitated with amorphous silicates, aluminates or aluminosilicates and the þ SO2 4 and Na ions are left free to migrate to the surface with the residual water (Nixon et al., 1979). With AWDSLFC, these amorphous phases probably form networks of colloidal dimensions, leading to solidification. This could explain why AWDS-LFC-C samples set more slowly to a mechanically weak solid, since if amorphous Ca silicates are precipitated at the CERCHAR treatment stage, extensive cross-linking during the seasoning of the AWDS mixtures must of necessity be reduced. 3.2.7. Effect of freezing on AWDS mixtures The effect of freezing ash–water suspensions on the setting behaviour and on the compressive strength of the final solidified, cured mixtures was also examined. The compressive strength of comparable samples after 28 days is given in Table 13 and results for samples not subjected to freezing are compared with those for the frozen samples. A slight reduction in strength was seen in the AWDS-LFC following freezing, but there were no systematic differences between the setting times for mixtures that had and had not been frozen. However, the difference in strength was much less than be-

Setting, final (h)

Compressive strength (MPa)

tween the AWDS-LFC and AWDS-LFC-C materials. It is also evident that the compressive strength depends on the S/W ratio. 3.2.8. Chemistry of seasoned LIFAC ash LIFAC ashes are quite different from other calcitic ashes (e.g., FBC ashes) in terms of both chemical and phase composition. The differences are evidently the key to understanding their behaviour in the AWDS process and the set samples were phase-analyzed by XRD. When the material from the inside (i.e., not in contact with air) of set and seasoned AWDS-LFC samples was examined, it was found to be mostly amorphous and the only sharp line in the diffraction pattern corresponded to a crystal lattice spacing of
0.795 nm. In a system consisting essentially of Ca(OH)2, Al(OH)2 and H2O, strong lines corresponding to lattice spacing of 0.76–0.82 nm can be tentatively attributed to AFm species such as, in cement notation (with C–CaO, A–Al2O3 and H–H2O), C3AHx and C4AHx. AFm species are mentioned by Taylor (1997), who gives the layer thickness for C4AH13 and C4AC0.5H12 as 0.794 and 0.819 nm, respectively, and states that ‘‘strong peaks in the XRD powder patterns of AFm phases normally include the first and second orders of the layer thickness’’. Material taken from the top layer of the deposit, however, gives a clear multi-line diffraction pattern, which indicates the presence of calcite and anhydrous Na2SO4, as well as of some calcium orthosilicate, Ca2SiO4, and quartz. For the AWDS-LFC-C material, the dried-out ‘‘froth’’ from the sample surface also appeared to consist mainly of anhydrous Na2SO4, with some calcite. No gypsum or portlandite could be detected. On exposure to the air, a sample of the AWDS-LFC-C crumbled, suggesting reaction with CO2, although in the absence of portlandite it was clear what chemical component was carbonating. These results were also confirmed by TGA. For seasoned AWDS-LFC-C samples, there was no indication of the presence of either gypsum or portlandite. Apart from the initial moisture loss, the subsequent mass loss was gradual, without any clear endothermic effects, except for a weak one
100 C below that associated with calcite decomposition. Given the relative

E.J. Anthony et al. / Waste Management 25 (2005) 265–279 Table 14 Compressive strength of solidified AWDS samples Sample

S/W

Rc values (MPa) Air dried

100% R.H.

AWDS-LFC

1.50 1.75 2.00 2.25

2.59 3.35 3.80 4.65

2.67 3.49 3.91 4.74

AWDS-LFC-C

1.00 1.25 1.50 1.75 1.90

0.39 0.39 0.47 0.58 0.62

0.36 0.40 0.45 0.59 0.60

weakness of the XRD lines for calcite, this is consistent with most of the Ca being chemically combined in the form of silicates, aluminates or aluminosilicates. 3.2.9. Compressive strength of set ‘‘optimum’’ LIFAC AWDS mixtures The compressive strength data are given in Table 14. The compressive strength increases slowly with increasing S/W ratio, and samples conditioned at 100% relative humidity showed a slightly improved mechanical strength. This suggests that early removal of water prevented the formation of some of the strength-forming phases. It is also evident and consistent with the results from Cracow University of Technology that the AWDSLFC-C gave a solid of considerably lower mechanical strength.

275

3.2.10. Rheological properties of LIFAC ash suspensions The fine particle size of the mixtures allows tests to be made over a relatively wide range of S/W ratios with both LIFAC ashes (1.5–2.0 for AWDS-LFC and 1.5– 1.75 for AWDS-LFC-C). Samples tested had similar rheological parameters, which qualitatively showed similar variation when the shear rate was changed, Tables 15 and 16, but LFC-C mixtures were systematically less viscous. 3.2.11. Effects of water on solidified LIFAC AWDS samples For these tests, the samples were cured for 28 days, immersed in water without stirring and changes were monitored for 4, 8, 24, and 48 h. The results are presented in Table 17. In addition, the permeability of the 28 d samples was determined using the Kamin´ski method (Mazurkiewicz, 1990). The permeability values for the AWDS mixtures fell within the range of 106–105 m/s. The LIFAC based materials, including even the weaker AWDS-LFC-C ones, behaved well in the test and did not fall apart, which is surprising given the apparently mechanically weak structures and the poor compressive strength. It is also clear that the mixtures become more permeable with increasing S/W ratios. However, the permeabilities were typical of conventional AWDS materials, which supports the contention that LIFAC-based ashes perform acceptably well in the AWDS process.

Table 15 Rheological parameters of AWDS-LFC Shear rate (s1)

5.11 85.17 170.34 340.68 511.02 1022.04

S/W = 1.00

S/W = 1.25

S/W = 2.00

S/W = 2.25

Tang. stress, pi (Pa)

App. visc. **(Pa, s)

Tang. stress, pi (Pa)

App. visc. **(Pa, s)

Tang. stresss, pi (Pa)

App. visc. **(Pa, s)

Tang. stress, pi (Pa)

App. visc. **(Pa, s)

1.533 3.577 5.110 8.176 11.242 10.951

0.3000 0.0420 0.0300 0.0240 0.0220 0.0205

2.044 5.621 8.687 13.797 19.929 36.792

0.4000 0.0660 0.0510 0.0405 0.0390 0.0360

3.066 10.731 16.352 26.061 35.259 66.941

0.6000 0.1260 0.0960 0.0765 0.0690 0.0655

4.599 20.951 33.215 54.166 76.650 137.970

0.9000 0.2460 0.1950 0.1590 0.1500 0.1350

Table 16 Rheological parameters of AWDS-LFC-C Shear rate (s1)

5.11 85.17 170.34 340.68 511.02 1022.04

S/W = 1.00

S/W = 1.25

S/W = 2.00

S/W = 2.25

Tang. stress, pi (Pa)

App. visc. **(Pa, s)

Tang. stress, pi (Pa)

App. visc. **(Pa,s)

Tang. stresss, pi (Pa)

App. visc. **(Pa, s)

Tang. stress, pi (Pa)

App. visc. **(Pa, s)

0.511 1.022 1.533 3.066 4.499 10.220

0.1000 0.0120 0.0090 0.0090 0.0090 0.0100

0.511 2.044 3.066 6.132 10.731 22.995

0.1000 0.0240 0.0180 0.0180 0.0210 0.0225

2.044 4.088 6.132 13.286 23.506 41.902

0.4000 0.0480 0.0360 0.0390 0.0460 0.0410

3.066 11.242 19.929 35.259 55.699 94.535

0.6000 0.1320 0.1170 0.1035 0.1090 0.0925

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Table 17 Effect of water on solidified AWDS samples Sample

S/W

State of sample after test period (h)

Permeability coefficient (m/s)

4

8

24

48

AWDS-LFC

1.50 1.75 2.00 2.25

NC NC NC NC

NC NC NC NC

NC NC NC NC

Cracked NC NC NC

8.4 · 106 9.1 · 106 9.5 · 106 1.2 · 105

AWDS-LFC-C

1.00 1.25 1.50 1.70

NC NC NC NC

NC NC NC NC

NC NC NC NC

NC NC NC NC

3.2 · 106 6.4 · 106 7.1 · 106 7.2 · 106

1.90

NC

NC

NC

NC

8.9 · 106

NC, no change.

3.2.12. Curing behaviour Two different materials were used to study ash properties during curing: LIFAC and CERCHAR-treated LIFAC. Before curing, 29% water was added to the samples, which were identified as: LFC0, LFC-C0 – cured for 4–5 h; LFC7, LFC-C8 – cured for 7 or 8 days; LFC28, LFC-C28 – cured for 28 days; LFC90, LFC-C100 – cured for 90 or 100 days. Samples were cured in sealed plastic bags, except for sample LFC-C100, which was cured in a bag for 56 days and then stored on an open shelf until 100 days had passed. The free lime (FL) results (i.e., the sum of CaO and Ca(OH)2, expressed as CaO), obtained by two different wet chemical methods (sucrose and Franke) determined by DSC and by TGA are collected in Table 18. All values have been corrected so as to refer to the final sample mass, after ‘‘ignition’’ at 950 C and are thus independent of the amount of water taken up during the curing (Rokita and Tomaszewski, 1988). As expected TA 0 (total alkalinity) was found to be constant within reasonable experimental Table 18 Free lime and total alkalinity (CaO%) from geochemical examination of mortar pastes Sample

LOI

TA 0

FL 0 sucrose

FL 0 Franke

CaðOHÞ02 from TGA

LFC LFC0 LFC7 LFC28 LFC90 LFC-C0 LFC-C8 LFC-C28 LFC-C100

1.7 5.3 14.9 14.2 15.0 7.6 17.8 20.1 18.8

28.5 29.1 28.4 28.5 29.1 27.1 26.1 27.9 28.1

8.9 8.4 5.2 5.0 5.2 7.0 4.6 4.8 4.1

6.6 4.3 5.4 3.8 4.9 5.55 1.75 1.7 4.5

– – 2.3 2.1 1.9 – 0 0 0

Note: FL sucrose is determined by modified ASTM C-25. FL Franke is determined by the modified Franke test according to ASTM C-114. Ca(OH)2 is the result of TGA and DSC recordings.

error (28.7 for LFC and 27.3 for LFC-C). The values of LOI and FL 0 from the sucrose method gave similar but more consistent results than the Franke method. X-ray diffraction was used to confirm the chemical speciation during the curing process. The results obtained for LIFAC materials are summarized in Table 19. The phases identified were classified as ‘‘major, minor or trace’’ according to their importance in the diffractograms. Somewhat surprisingly, gypsum was absent from these samples. LFC-C28 and LFC-C100 gave XRD line patterns very similar to those for the untreated LIFAC ash. It is important to note the development of the hydrated tetracalcium aluminate, which implies that complicated chemical processes were occurring, involving the fuel ash components. TGA, with the identification of the gases evolved during successive stages of mass loss, was carried out to confirm and amplify the findings on chemical speciation of the cured samples. The following mass loss steps were observed in the TGA and DTG (differential TGA) curves: A (100–120 C): ettringite and 4CaO Æ Al2O3 Æ 13H2O; B (170–180 C): 4CaO Æ Al2O3 Æ 13H2O; C (300–350 C): 4CaO Æ Al2O3 Æ 13H2O; D (420–430 C): Ca(OH)2; and E (680–700 C): CaCO3. There was very little difference between samples LFC7 to LFC90. Since ettringite decomposition and the first step in the loss of water from the aluminate are superimposed, quantification is difficult, but the ettringite effect may be roughly estimated at about 10% for LFC7 to between 20% and 25% for LFC90. Ca(OH)2 decreases from 2.3% for LFC7 to 1.9% for LFC90. Since the concentrations are small, these estimates have an error margin of about ±0.5%. For LFC-C100, two TGA runs were carried out: one at a heating rate of 25 K/min and the other at 10 K/min. With the higher heating rate, dehydration steps A, B and C appeared as before, at similar temperatures (about 10 C higher) and were interpreted in the same way. No Ca(OH)2 was detected. Sample LFC-C100, when heated at 25 K/min, behaved similarly to LFCC8. However,

E.J. Anthony et al. / Waste Management 25 (2005) 265–279

277

Table 19 Summary of X-ray analyses for mortar pastes Sample

Major components

Minor components

Trace components

LFC LFC28 and LFC90 LFC-C LFC-C8

Lime Ettringite, 4CaO Æ Al2O3 Æ 13H2O, portlandite Halite (NaCl), portlandite, amorphous Ettringite, 4CaO Æ Al2O3 Æ 13H2O, amorphous

12CaO Æ 7Al2O3 Unidentified Anhydrite Fe oxides

LFC-C28 and LFC-C100

Ettringite, 4CaO Æ Al2O3 Æ 13H2O, amorphous

Quartz, anhydrite, calcite Quartz, calcite Calcite, quartz, 4CaO Æ Al2O3 Æ 13H2O Calcite, dolomite, quartz, Ca–Fe hydrated salt, (CO3, Cl) Calcite, dolomite, quartz

decreasing the heating rate to 10 K/min resolved the first peak into two components, (i) and (ii). It may be assumed (Taylor, 1997; Anthony et al., 1997) that (i) corresponds to the first dehydration step of the tetracalcium aluminate, with loss of two water molecules, (ii) to a further loss of four molecules and the well-separated 3rd peak to a loss of another four molecules of water. If this interpretation is correct, the concentration of the hydrated aluminate may be estimated to lie between 9% in LFC7 and about 11% in both LFC28 and LFC90. To complement the TGA work, and reveal the endotherms corresponding to different dehydration reactions, between room temperature and 600 C, DSC was also carried out on the samples. The assumptions used to make the quantitative estimates were the same as those outlined in previous work (Anthony et al., 1997). Four endotherms were immediately distinguished. Endotherm I appeared centred at about 135 C but on closer inspection a shoulder at about 145 C could be seen, indicating a probable superposition of two peaks, Ia and Ib. This was attributed to the effect due to hydrated tetracalcium aluminate (Anthony et al., 2003) followed by the endotherm for ettringite. Endotherms II and III appeared at 220 and 325 C, respectively, and were also attributed to the hydrated aluminate. Endotherm IV appeared at 520 C and was due to Ca(OH)2. There is very little difference in the appearance of this endotherm between the diagrams obtained for LFC7, LFC28 and LFC90. For LFC-C pastes the initial endotherms I, II and III were identified as above, but for LFC-C8, endotherm I appeared to be distinctly resolved into two peaks, Ia (125 C) and Ib (155 C). The first peak was attributed to the hydrated calcium aluminate and the second to ettringite. The two components were still distinguishable in LFC-C28, but for LFC-C100 the two effects appeared to be combined into a single peak. Peaks II (at 230 C) and III (at 340 C) were assumed to be associated with the later steps in aluminate dehydration. It should be noted that no Ca(OH)2 was detected in any of the three samples. The LIFAC ashes contain more than twice as much analytical Al2O3 as high-calcitic ashes, such as FBC ashes from burning high-sulphur coals, much less sulphate sulphur and Fe2O3, but an appreciable proportion

of sodium compounds. Hydrated tetracalcium aluminate, a compound known to occur in the hydration of Portland cement, is one of the few crystalline phases present, and gypsum is totally absent. Based on the XRD spectrum, the hydrated 4CaO Æ Al2O3 Æ 13H2O (C4AH13 in cement notation) is suggested, but cannot be distinguished from two similar layer structures (other AFm phases) containing sulphate or carbonate ions, such as: 4CaO Æ Al2O3 Æ CO3 Æ 10H2O and 4CaO Æ Al2O3 Æ SO3 Æ 8H2O. However, the additional information from TGA and DSC and the fact that the ash is relatively poor in sulphate suggest that C4AH13 is present in the samples studied. It is noted that the total alkalinity measurements (ignited samples
TA 0 ) give, within analytical error, a constant value: between 28.4 and 29.1 for LFC pastes and between 26.1 and 28.1 for LFC-C pastes. The results on free lime and Ca(OH)2 content, however, show striking discrepancies, which are discussed below. With LFC-C, Ca(OH)2 is not detectable in samples LFC-C8, LFC-C28 and LFC-C100, by TGA, DSC or XRD. However, the sucrose method (ASTM C-25 modified) gives FL 0 concentrations of 4.1–4.8%, expressed as CaO. This discrepancy may be explained by supposing that the calcium aluminate, 12CaO Æ 7Al2O3, detected by XRD in LFC-C is reactive and decomposes immediately in water (Taylor, 1997). The hydrate found in the other samples, 4CaO Æ Al2O3 Æ 13H2O begins to lose water at relatively low temperatures. It is probable that its lime content is extracted by the sucrose. In fact, the contents of the hydrate estimated from TGA, are 9– 11%, equivalent to 3.6–4.4% CaO; which is reasonably consistent with the FL 0 values. It is concluded that in the sucrose method any free lime as well as the hydrated aluminate are reacted. The method of Franke (ASTM C-114), utilizing ethyl acetoacetate for the extraction, seems to extract some, but not all of the CaO contained in the aluminate. Therefore, although the values obtained by his method are included in Table 18 for the sake of completeness, they are probably overestimates (with respect to the true FL 0 values). The LFC samples did contain some Ca(OH)2, as indicated by TGA, DSC and XRD. The estimates from TGA are probably the most reliable, although there is some risk of overprediction, if the decomposition of

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Ca(OH)2 is superimposed on the slow dehydroxylation of silicates. DSC gives lower values, but these may depend on other reactions occurring simultaneously with the portlandite dehydration. The values of FL 0 from the sucrose method should include both the calcium hydroxide (about 2% for LFC7, LFC28 and LFC90) and the hydrated calcium aluminate (a difference of about 3% for these samples). For the non-treated LIFAC samples, after the first 7 days of curing there appears to be very little change. In both LFC and LFC-C, tetracalcium aluminate hydrate develops and for the LFC-C samples, it appears to be present in higher concentrations at shorter times than for the LFC samples.

4. Conclusions Both untreated and CERCHAR-treated LIFAC ashes were subjected to a range of tests to determine their chemical, physical, geotechnical, geochemical and utilization characteristics. Morphologically, the ash particles were spherical, with some irregular particles formed in the CERCHAR ash. In this material there was also a slight increase in the proportion of both 45 lm and coarse particle size fractions. The geotechnical tests showed both ashes to be somewhat selfcementing. Both ashes performed well in the freeze/ thaw, wet/dry and strength development areas. The untreated LIFAC ash outperformed the CERCHAR-treated ash in terms of strength development and setting times with regard to use in no-cement and roller compacted concrete. The geochemical examination indicated the presence of hydrated tetracalcium aluminate but not of any gypsum. In samples cured over long periods, the aluminate formed faster than ettringite. Based on these tests, the LIFAC ash can either be used in concrete or disposed of in a landfill with minimal water addition. No prehydration is necessary as a safety measure before disposal, as is the case with FBC ashes. LIFAC ash can also be used with the AWDS technology, with no need for pretreatment to hydrate the lime present.

Acknowledgements The Canadian Electrical Association (CEA) financially supported this work. It was also co-funded by Natural Resources Canada through CANMET and Environment Canada through the Panel on Energy Research and Development (PERD) of the Federal Government. Additional co-funding was provided by Centre d
E´tudes et de Recherches du Charbon (CER-

CHAR). The supply of LIFAC ashes is gratefully acknowledged from SaskPower.

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E.J. Anthony et al. / Waste Management 25 (2005) 265–279 Nixon, P.J., Collins, R.J., Raiment, P.E., 1979. The concentration of alkalies by moisture migration in concrete – a factor influencing alkali aggregate reaction. Cement and Concrete Research 9, 417– 423. Rokita, J., Tomaszewski, S., 1988. A new technology of hydraulic transport and storage of fly-ash and energistic slag in the form of a mixture with high percentage of the solid phase. In: 11th International Conference on the Hydraulic Transport of Solids in Pipes, Stratford-upon-Avon, UK, Paper 7.3, pp. 481–493.

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Rose, J.G., Bland, A.E., Jones, C.E., 1987. Production of concrete using fluidized bed combustion waste and power plant fly ash. Contract Report No. TV-60443A for Tennessee Valley Authority, Kentucky Energy Cabinet, Lexington, KY, p. 79. Ryypoo¨, M., Ekman, I., 2000. Improving the performance of LIFAC FGD in Chinese boilers. Modern Power Systems, 31–32. Taylor, H.F., 1997. Cement Chemistry, second ed. Academic Press, New York.