L E A C H A T E O F FLY ASH D E R I V E D F R O M R E F U S E I N C I N E R A T I O N TAY JOO HWA
School of Civil and Structural Engineering, Nayang Technological Institute, Nanyang Avenue, Republic of Singapore 2263
(Received November 1990) Abstract. For highly urbanized cities where there is a scarcity of land available for landfilling, incineration of refuse is a feasible solution for solid waste disposal. After incineration, about 20% by weight of fly ash and other residues are produced and disposed of by landfill. Leachate tests carried out on samples of fly ash show that the heavy metal concentrations of the leachate are in excess of the permissible limits set by WHO. Lime and cement are used to stabilize the fly ash. The concentrations of heavy metals in the leachates of lime and cement treated fly ash are non-detectable. In additional to pollution control, the lime and cement treated fly ash significantly improves the properties of soft clay.
Introduction
Singapore is an Island-City-State with a total land area of about 620 km 2. With rapid urbanization and industrialization in the last two decades, the amount of refuse generated has steadily increased. The average daily amount of refuse generated in 1969 was 1150 tonnes, and it increased to 2580 tonnes in 1979. In 1986, the daily refuse collected was about 4000 tonnes. The increase in the refuse output is not only attributed to population growth but also to the increase in per capit refuse load which has increased significantly from 0.56 kg per person per day in 1969 to 1.09 kg per person per day in 1979, and further increased to 1.69 kg per person per day in 1986. The increase in per capita load is mainly due to the growing affluence and improvement in the standard of living. Refuse collected daily from various parts of the Island is disposed of by landfilling and incineration. About 35% of the refuse is disposed of by landfilling and 65% by incineration. The Ulu Pandan Refuse Incineration Plant, with a capacity for handling 1600 tonnes of refuse per day, was built in 1979. The second refuse incineration plant, with a capacity to handle 2000 tonnes of refuse per day, was completed in 1987. A third incineration plant is under construction and will be completed by the end of 1990. The three incineration plants will be able to dispose of 80% of the refuse generated in the Island. After incineration, about 20% by weight of fly ash and other residues are produced. These incinerated residues are also commonly disposed of by landfilling. As fly ash contains heavy metals that might leach out and pollute the environment (Bagchi 1989, Francis 1987), leachate tests on the fly ash were conducted in the laboratory. Lime and cement were used to stabilize the fly ash. This paper presents the research results on the properties of fly ash and leachate characteristics of fly ash.
Environmental Monitoring and Assessment 19: 157-164, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
158
TAY
JOO
HWA
Properties of Fly Ash The samples of fly ash were obtained from the Tuas Refuse Incinerator plant. The fly ash is derived from the precipitation of flue gas following the combustion of municipal refuse. When examined under a microscope, the fly ash particles are generally spherical in shape. The typical range of size distribution is shown in Figure 1. The fly ash is predominantly a sand-size material, with between 12% to 18% greater than 2 mm in size, between 73% and 78% in the size range of 0.076 m m and 2 mm, between 3% and 5% in the size range of 0.002 m m and 0.075 mm, and between 0% and 6% less than 0.002 m m in size. Table I lists a summary of properties of the fly ash. The effective size for the fly ash is 0.12, the average uniformity coefficient is 10.9 and the average coefficient of curvature is 1.5. The specific gravity of the fly ash is about 1.71, which is 35% lower than sand, an indication that the carbon content is high or that a high proportion of the fly ash particles are hollow. The typical specific gravity of natural sand is 2.65. The loss on ignition for the fly ash was 15%, indicating that a significant portion of fine material was not completely burnt during the incineration process. As shown in Table I, the fly ash is weakly alkaline, with a p H value of 9.4. The chemical compositions are listed Gravel
Sand Coarse to medium
Silt
Clay
Fine
10o
80
60
o,.
40
20
0
lO
0.1
0.01
Grain diameter (mm)
Fig. 1. Grain size distribution of fly ash.
0.001
LEACHATE
OF
FLY
ASH
159
TABLE I Summary of properties of fly ash. Properties
Average
Specific gravity Effective size, mm Uniformity coefficient Coefficient of curvature Loss on ignition, % pH
1.71 0.12 10.9 1.50 15 9.40
in Table II. The percentages o f various inorganic chemical c o m p o n e n t s for all the samples collected are quite consistent. The p r e d o m i n a n t chemical constituents in the fly ash are silicate and calcium. Other m a j o r chemical c o m p o u n d s are ferrous, potassium a n d sodium. The a m o u n t s of heavy metals such as zinc, lead, nickel, c h r o m i u m and c a d m i u m are in trace quantities. Detailed analysis o f the c o m p o u n d s o f the fly ash is b e y o n d the scope o f this study.
Leaehate Tests A m a j o r concern in disposing the fly ash on landfill is the characteristics o f the leachate generated. Leachate tests on the fly ash were carried out in the l a b o r a t o r y using the setup shown in Figure 2. The 0.1 m thick samples were sandwiched between two layers of sand in a perspex c o l u m n o f 0.15 m internal d i a m e t e r a n d 0.5 m height. The sand layer was 0.1 m thick a b o v e a n d 0.05 m thick below the samples. T a p water with p H value o f 7.2 was used. A constant head o f water level was m a i n t a i n e d to allow a continuous d o w n w a r d flow at an average rate o f 50 L / d a y . The tests were carried out for a period of 90 days. TABLE II Chemical constituents of fly ash. Elements
Concentration (% by weight)
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Maganese Nickel Chromium Cadmium
42.00 15.14 3.30 1.24 1.19 0.83 0.78 0.36 0.17 0.11 0.04 0.03 0.01
160
TAY
I_
i
0.15.8
JOO
HWA
~
Inflow.~ 0.05 V ---
~
Outflow to maintain constant head
-~
Perspex c o l u m n
0.2
. . -.
0.1
.-
9 .-..
. . .
~
0.1
/
~
.-
~
Soil sample
0.05
t
Sand layers
Outflow for chemical composition test All d i m e n s i o n s in M Fig. 2.
Leachate test appar~tatus.
The variations of the pH values are shown in Figure 3. The pH values generally decrease with time and becomes less alkaline with continue leaching. The initial pH value for the leachate is 9.4, and decreases to 7.2 after 28 days. The pH value after 28 days is 7.2, which is identical to the potable water used for leaching. The initially higher pH value is to be expected as the fly ash contains high concentrations of alkali metals such as calcium, potassium and sodium as discussed earlier. Table IIl shows the metal concentrations in the leachate at various days. The chemical components that are readily leach out by the water are calcium, potassium and sodium. The initial concentrations of calcium, potassium and sodium in the leachate are 1500 rag/L, 245 m g / L and 180 m g / L respectively. However, the quantities of these chemicals that are leach out decrease significantly with time. After 28 days, the concentrations of these chemicals in the leachate fell to very low levels. The concentration of other chemical components, such as silicate, ferrous, magnesium, zinc and copper, are insignificant in the leachate. The leaching of these chemicals are
L E A C H A T E O F FLY ASH
161
]
14 13 12
7
11
~
FI
~
]
A8
F ~ A e h + Cement t
10
q
!
9
/ + Fly , ~ h
"r
.2
t
6
4
I
1
I
7
28
60
90
Time (daye)
Fig. 3.
pH values ofleachates.
practically diminished after 28 days. However, the trace quantities of chromium and cadmium are found initially to exceed the World Health Organization (WHO) International Standards for Drinking Water. The initial concentrations of chromium, cadmium and lead in the leachate are 0.20 mg/L, 0.03 mg/L and 0.18 mg/L respectively. TABLE III Leachate chemical composition from fly ash. Leaching Element (mg/L)
Time (Days) 1 7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
3.0 1500 0.4 245 180 0.9 0.5 0.05 7.4 0.0 0.18 0.20 0.03
2.0 560 0.3 70 40 0,7 0.1 0.0 0.2 0.0 0.06 0.0 0.0
2.0 32 0.0 6.0 31 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 22 0.0 5.0 30 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 987 0.3 77 115 0.8 0.4 0.0 0.3 0.0 0.14 0.01 0.03
162
TAY JOO
HWA
The W H O limits for c h r o m i u m , c a d m i u m and lead are 0.05 m g / L , 0.005 m g / L and 0.05 m g / L respectively. However, after 28 days o f leaching, the concentrations o f toxic chemical fall below the W H O limits.
Stabilization of Fly Ash As the initial leachate o f fly ash contains heavy metal concentrations exceeding the limits o f d r i n k i n g water standards, an experimental stabilization process was conducted in the laboratory. Lime and cement are two c o m m o n l y used agents for the stabilization o f h a z a r d o u s wastes. Mixtures o f equal a m o u n t s o f lime a n d fly ash a n d cement a n d fly ash were tested. Leachate tests on the mixtures were conducted in the l a b o r a t o r y using the setup shown in Figure 2. Figure 3 also shows the variations o f the p H values for the samples of two mixtures. The significantly higher contents o f calcium present in lime a n d cement contributed to the higher p H values of the samples treated with lime a n d cement. F o r the lime treated fly ash sample, the p H values o f the leachate are higher than 11 for the leaching p e r i o d o f 90 days a n d below. The p H o f the leachate for the cement treated fly ash samples are well above 10. Tables IV a n d V show the concentrations of various chemical components in the leachate at different days. All the chemical concentrations in the leachate for the fly ash samples treated with lime a n d cement are lower t h a n the values stated in Table III. The concentrations of toxic chemicals in the leachate are non-detectable. The results show that lime a n d cement are the two effective agents for the stabilization of fly ash derived from refuse incineration. In view o f the relatively high calcium a n d silicate contents o f the fly ash derived from refuse incineration, investigations were carried out in the l a b o r a t o r y to study the effects o f lime a n d cement treated fly ash on the geotechnical properties o f soft marine clay. The TABLE IV Leachate from lime treated fly ash. Leaching Element
Time (days)
(mg/L)
1
7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
2.2 604 0.05 24 24 0.1 0.3 0.0 0.1 0.0 0.06 0.0 0.0
2.1 34 0.03 0.J 1.7 0.0 0.2 0.0 0.02 0.0 0.0 0.0 0.0
2.1 14 0.0 0.3 0.8 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0
2.0 14 0.0 0.3 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 13 0.0 0.3 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
LEACHATE
OF
FLY
163
ASH
TABLE V Leachate from cement treated fly ash. Leaching element
Time (days)
(mg/L)
1
7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
10.7 172 0.1 59 76 0.1 0.05 0.0 0.1 0.0 0.0 0.0 0.0
10.7 40 0.1 5.0 1.0 0.1 0.03 0.0 0.04 0.0 0.0 0.0 0.0
8.7 37 0.1 4.1 1.0 0.1 0.01 0.0 0.0 0.0 0.0 0.0 0.0
8.7 22.5 0.0 2.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
6.4 19.5 0.0 1.2 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
marine clay is f o u n d in m a n y parts of the Island a n d construction difficulties are encountered where this soil type prevails. The samples of lime and cement treated fly ash were mixed with marine clay for the studies. 10% by weight of the lime a n d cement treated fly ash were used. Table VI presents the results of the effects of the mixtures o n the properties of marine clay. The properties were measured after 28 days of curing. The plasticity index of the marine clay decreases from 56% to 39% when lime treated fly ash is used. The cement treated fly ash further decreases the index to 28%. The reduction in the plasticity is of significance as it is related to the lower compressibility properties and higher shear strength properties. The compression index also decreases from 0.53 to 0.28 and 0.15 for the samples of marine clay, lime treated fly ash with clay and cement treated fly ash with clay respectively. The lower compression indices are advantgeous as they imply smaller a m o u n t s of settlements. The permeabilities of samples increase with the additions of admixtures. It increases from 5.8X 1O-~ m / s to 2.5X 10-9 m / s a n d 1.3X 10-8 m / s for the samples of marine clay, lime
TABLE VI Geotechnical properties of samples. Properties
Marine Clay
Fly Ash+ Lime+Clay
Fly Ash+ Cement+Clay
Plasticity index (%) Compression index Permeability(m/s) Unconfined compressive strength (kPa)
56 0.53 5.8X10-u 11.3
39 0.28 2.5X10-9 57.6
28 0.15 1.3X10-~ 73.5
164
TAY
JOO
HWA
treated fly ash with clay and cement treated fly ash with clay respectively. The higher permeabilities are advantageous as they imply faster rates of foundation settlement. The unconfined compressive strength of marine clay increase significantly with the additions of lime and cement treated fly ash. The strength increase about 5.1 times and 6.5 times with the additions of lime treated fly ash and cement treated fly ash respectively. Conclusion The fly ash derived from refuse incineration is essentially a granular and spherical particle with an effective size of 0.12 mm. The average coefficient of uniformity is 10.9 and the coefficient of curvature is 1.5. The specific gravity of the fly ash particles is 1.71 with ignition loss of 15%, indicating that substantial quantities of organic matters are found in the fly ash. The fly ash is alkaline with a pH value of 9.4. The predominant chemical constituents are silicate and calcium. The amounts of heavy metals are in trace quantities. Leachate tests were carried out on samples of the fly ash. The initial p H value for the leachate is 9.4 and decreases to 7.2 after 28 days. The concentrations of chromium, cadmium and lead in the leachate initially exceeding the limits of W H O Drinking Water Standards. However, after 28 days of leaching, the concentrations are well below the limits. Lime and cement are two effective agents for the stabilization of fly ash. All the chemical concentrations in the leachate for the fly ash samples treated with lime and cement are lower than that of the fly ash samples. The toxic chemical concentrations in the leachate for the lime and cement treated fly ash samples are non-detectable. In addition to pollution control for the leachate, the lime and cement treated fly ash improve the properties of soft marine clay significantly. References Bagchi, A. and Sopcich,C.:1989,'Characterization ofMSWlncinerator Ash',Z Environ. Eng, ASCE, 115(2), 447-452. Francis, C.W. and White, G. H.:1987,'Leaching ofToxicMetals From lncineratorAshes',WaterPo~Cont. Fed, 59 (11), 979-986.
School of Civil and Structural Engineering, Nayang Technological Institute, Nanyang Avenue, Republic of Singapore 2263
(Received November 1990) Abstract. For highly urbanized cities where there is a scarcity of land available for landfilling, incineration of refuse is a feasible solution for solid waste disposal. After incineration, about 20% by weight of fly ash and other residues are produced and disposed of by landfill. Leachate tests carried out on samples of fly ash show that the heavy metal concentrations of the leachate are in excess of the permissible limits set by WHO. Lime and cement are used to stabilize the fly ash. The concentrations of heavy metals in the leachates of lime and cement treated fly ash are non-detectable. In additional to pollution control, the lime and cement treated fly ash significantly improves the properties of soft clay.
Introduction
Singapore is an Island-City-State with a total land area of about 620 km 2. With rapid urbanization and industrialization in the last two decades, the amount of refuse generated has steadily increased. The average daily amount of refuse generated in 1969 was 1150 tonnes, and it increased to 2580 tonnes in 1979. In 1986, the daily refuse collected was about 4000 tonnes. The increase in the refuse output is not only attributed to population growth but also to the increase in per capit refuse load which has increased significantly from 0.56 kg per person per day in 1969 to 1.09 kg per person per day in 1979, and further increased to 1.69 kg per person per day in 1986. The increase in per capita load is mainly due to the growing affluence and improvement in the standard of living. Refuse collected daily from various parts of the Island is disposed of by landfilling and incineration. About 35% of the refuse is disposed of by landfilling and 65% by incineration. The Ulu Pandan Refuse Incineration Plant, with a capacity for handling 1600 tonnes of refuse per day, was built in 1979. The second refuse incineration plant, with a capacity to handle 2000 tonnes of refuse per day, was completed in 1987. A third incineration plant is under construction and will be completed by the end of 1990. The three incineration plants will be able to dispose of 80% of the refuse generated in the Island. After incineration, about 20% by weight of fly ash and other residues are produced. These incinerated residues are also commonly disposed of by landfilling. As fly ash contains heavy metals that might leach out and pollute the environment (Bagchi 1989, Francis 1987), leachate tests on the fly ash were conducted in the laboratory. Lime and cement were used to stabilize the fly ash. This paper presents the research results on the properties of fly ash and leachate characteristics of fly ash.
Environmental Monitoring and Assessment 19: 157-164, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
158
TAY
JOO
HWA
Properties of Fly Ash The samples of fly ash were obtained from the Tuas Refuse Incinerator plant. The fly ash is derived from the precipitation of flue gas following the combustion of municipal refuse. When examined under a microscope, the fly ash particles are generally spherical in shape. The typical range of size distribution is shown in Figure 1. The fly ash is predominantly a sand-size material, with between 12% to 18% greater than 2 mm in size, between 73% and 78% in the size range of 0.076 m m and 2 mm, between 3% and 5% in the size range of 0.002 m m and 0.075 mm, and between 0% and 6% less than 0.002 m m in size. Table I lists a summary of properties of the fly ash. The effective size for the fly ash is 0.12, the average uniformity coefficient is 10.9 and the average coefficient of curvature is 1.5. The specific gravity of the fly ash is about 1.71, which is 35% lower than sand, an indication that the carbon content is high or that a high proportion of the fly ash particles are hollow. The typical specific gravity of natural sand is 2.65. The loss on ignition for the fly ash was 15%, indicating that a significant portion of fine material was not completely burnt during the incineration process. As shown in Table I, the fly ash is weakly alkaline, with a p H value of 9.4. The chemical compositions are listed Gravel
Sand Coarse to medium
Silt
Clay
Fine
10o
80
60
o,.
40
20
0
lO
0.1
0.01
Grain diameter (mm)
Fig. 1. Grain size distribution of fly ash.
0.001
LEACHATE
OF
FLY
ASH
159
TABLE I Summary of properties of fly ash. Properties
Average
Specific gravity Effective size, mm Uniformity coefficient Coefficient of curvature Loss on ignition, % pH
1.71 0.12 10.9 1.50 15 9.40
in Table II. The percentages o f various inorganic chemical c o m p o n e n t s for all the samples collected are quite consistent. The p r e d o m i n a n t chemical constituents in the fly ash are silicate and calcium. Other m a j o r chemical c o m p o u n d s are ferrous, potassium a n d sodium. The a m o u n t s of heavy metals such as zinc, lead, nickel, c h r o m i u m and c a d m i u m are in trace quantities. Detailed analysis o f the c o m p o u n d s o f the fly ash is b e y o n d the scope o f this study.
Leaehate Tests A m a j o r concern in disposing the fly ash on landfill is the characteristics o f the leachate generated. Leachate tests on the fly ash were carried out in the l a b o r a t o r y using the setup shown in Figure 2. The 0.1 m thick samples were sandwiched between two layers of sand in a perspex c o l u m n o f 0.15 m internal d i a m e t e r a n d 0.5 m height. The sand layer was 0.1 m thick a b o v e a n d 0.05 m thick below the samples. T a p water with p H value o f 7.2 was used. A constant head o f water level was m a i n t a i n e d to allow a continuous d o w n w a r d flow at an average rate o f 50 L / d a y . The tests were carried out for a period of 90 days. TABLE II Chemical constituents of fly ash. Elements
Concentration (% by weight)
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Maganese Nickel Chromium Cadmium
42.00 15.14 3.30 1.24 1.19 0.83 0.78 0.36 0.17 0.11 0.04 0.03 0.01
160
TAY
I_
i
0.15.8
JOO
HWA
~
Inflow.~ 0.05 V ---
~
Outflow to maintain constant head
-~
Perspex c o l u m n
0.2
. . -.
0.1
.-
9 .-..
. . .
~
0.1
/
~
.-
~
Soil sample
0.05
t
Sand layers
Outflow for chemical composition test All d i m e n s i o n s in M Fig. 2.
Leachate test appar~tatus.
The variations of the pH values are shown in Figure 3. The pH values generally decrease with time and becomes less alkaline with continue leaching. The initial pH value for the leachate is 9.4, and decreases to 7.2 after 28 days. The pH value after 28 days is 7.2, which is identical to the potable water used for leaching. The initially higher pH value is to be expected as the fly ash contains high concentrations of alkali metals such as calcium, potassium and sodium as discussed earlier. Table IIl shows the metal concentrations in the leachate at various days. The chemical components that are readily leach out by the water are calcium, potassium and sodium. The initial concentrations of calcium, potassium and sodium in the leachate are 1500 rag/L, 245 m g / L and 180 m g / L respectively. However, the quantities of these chemicals that are leach out decrease significantly with time. After 28 days, the concentrations of these chemicals in the leachate fell to very low levels. The concentration of other chemical components, such as silicate, ferrous, magnesium, zinc and copper, are insignificant in the leachate. The leaching of these chemicals are
L E A C H A T E O F FLY ASH
161
]
14 13 12
7
11
~
FI
~
]
A8
F ~ A e h + Cement t
10
q
!
9
/ + Fly , ~ h
"r
.2
t
6
4
I
1
I
7
28
60
90
Time (daye)
Fig. 3.
pH values ofleachates.
practically diminished after 28 days. However, the trace quantities of chromium and cadmium are found initially to exceed the World Health Organization (WHO) International Standards for Drinking Water. The initial concentrations of chromium, cadmium and lead in the leachate are 0.20 mg/L, 0.03 mg/L and 0.18 mg/L respectively. TABLE III Leachate chemical composition from fly ash. Leaching Element (mg/L)
Time (Days) 1 7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
3.0 1500 0.4 245 180 0.9 0.5 0.05 7.4 0.0 0.18 0.20 0.03
2.0 560 0.3 70 40 0,7 0.1 0.0 0.2 0.0 0.06 0.0 0.0
2.0 32 0.0 6.0 31 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 22 0.0 5.0 30 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 987 0.3 77 115 0.8 0.4 0.0 0.3 0.0 0.14 0.01 0.03
162
TAY JOO
HWA
The W H O limits for c h r o m i u m , c a d m i u m and lead are 0.05 m g / L , 0.005 m g / L and 0.05 m g / L respectively. However, after 28 days o f leaching, the concentrations o f toxic chemical fall below the W H O limits.
Stabilization of Fly Ash As the initial leachate o f fly ash contains heavy metal concentrations exceeding the limits o f d r i n k i n g water standards, an experimental stabilization process was conducted in the laboratory. Lime and cement are two c o m m o n l y used agents for the stabilization o f h a z a r d o u s wastes. Mixtures o f equal a m o u n t s o f lime a n d fly ash a n d cement a n d fly ash were tested. Leachate tests on the mixtures were conducted in the l a b o r a t o r y using the setup shown in Figure 2. Figure 3 also shows the variations o f the p H values for the samples of two mixtures. The significantly higher contents o f calcium present in lime a n d cement contributed to the higher p H values of the samples treated with lime a n d cement. F o r the lime treated fly ash sample, the p H values o f the leachate are higher than 11 for the leaching p e r i o d o f 90 days a n d below. The p H o f the leachate for the cement treated fly ash samples are well above 10. Tables IV a n d V show the concentrations of various chemical components in the leachate at different days. All the chemical concentrations in the leachate for the fly ash samples treated with lime a n d cement are lower t h a n the values stated in Table III. The concentrations of toxic chemicals in the leachate are non-detectable. The results show that lime a n d cement are the two effective agents for the stabilization of fly ash derived from refuse incineration. In view o f the relatively high calcium a n d silicate contents o f the fly ash derived from refuse incineration, investigations were carried out in the l a b o r a t o r y to study the effects o f lime a n d cement treated fly ash on the geotechnical properties o f soft marine clay. The TABLE IV Leachate from lime treated fly ash. Leaching Element
Time (days)
(mg/L)
1
7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
2.2 604 0.05 24 24 0.1 0.3 0.0 0.1 0.0 0.06 0.0 0.0
2.1 34 0.03 0.J 1.7 0.0 0.2 0.0 0.02 0.0 0.0 0.0 0.0
2.1 14 0.0 0.3 0.8 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0
2.0 14 0.0 0.3 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.0 13 0.0 0.3 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
LEACHATE
OF
FLY
163
ASH
TABLE V Leachate from cement treated fly ash. Leaching element
Time (days)
(mg/L)
1
7
28
60
90
Silicate Calcium Ferrous Potassium Sodium Zinc Magnesium Lead Copper Manganese Nickel Chromium Cadmium
10.7 172 0.1 59 76 0.1 0.05 0.0 0.1 0.0 0.0 0.0 0.0
10.7 40 0.1 5.0 1.0 0.1 0.03 0.0 0.04 0.0 0.0 0.0 0.0
8.7 37 0.1 4.1 1.0 0.1 0.01 0.0 0.0 0.0 0.0 0.0 0.0
8.7 22.5 0.0 2.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
6.4 19.5 0.0 1.2 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
marine clay is f o u n d in m a n y parts of the Island a n d construction difficulties are encountered where this soil type prevails. The samples of lime and cement treated fly ash were mixed with marine clay for the studies. 10% by weight of the lime a n d cement treated fly ash were used. Table VI presents the results of the effects of the mixtures o n the properties of marine clay. The properties were measured after 28 days of curing. The plasticity index of the marine clay decreases from 56% to 39% when lime treated fly ash is used. The cement treated fly ash further decreases the index to 28%. The reduction in the plasticity is of significance as it is related to the lower compressibility properties and higher shear strength properties. The compression index also decreases from 0.53 to 0.28 and 0.15 for the samples of marine clay, lime treated fly ash with clay and cement treated fly ash with clay respectively. The lower compression indices are advantgeous as they imply smaller a m o u n t s of settlements. The permeabilities of samples increase with the additions of admixtures. It increases from 5.8X 1O-~ m / s to 2.5X 10-9 m / s a n d 1.3X 10-8 m / s for the samples of marine clay, lime
TABLE VI Geotechnical properties of samples. Properties
Marine Clay
Fly Ash+ Lime+Clay
Fly Ash+ Cement+Clay
Plasticity index (%) Compression index Permeability(m/s) Unconfined compressive strength (kPa)
56 0.53 5.8X10-u 11.3
39 0.28 2.5X10-9 57.6
28 0.15 1.3X10-~ 73.5
164
TAY
JOO
HWA
treated fly ash with clay and cement treated fly ash with clay respectively. The higher permeabilities are advantageous as they imply faster rates of foundation settlement. The unconfined compressive strength of marine clay increase significantly with the additions of lime and cement treated fly ash. The strength increase about 5.1 times and 6.5 times with the additions of lime treated fly ash and cement treated fly ash respectively. Conclusion The fly ash derived from refuse incineration is essentially a granular and spherical particle with an effective size of 0.12 mm. The average coefficient of uniformity is 10.9 and the coefficient of curvature is 1.5. The specific gravity of the fly ash particles is 1.71 with ignition loss of 15%, indicating that substantial quantities of organic matters are found in the fly ash. The fly ash is alkaline with a pH value of 9.4. The predominant chemical constituents are silicate and calcium. The amounts of heavy metals are in trace quantities. Leachate tests were carried out on samples of the fly ash. The initial p H value for the leachate is 9.4 and decreases to 7.2 after 28 days. The concentrations of chromium, cadmium and lead in the leachate initially exceeding the limits of W H O Drinking Water Standards. However, after 28 days of leaching, the concentrations are well below the limits. Lime and cement are two effective agents for the stabilization of fly ash. All the chemical concentrations in the leachate for the fly ash samples treated with lime and cement are lower than that of the fly ash samples. The toxic chemical concentrations in the leachate for the lime and cement treated fly ash samples are non-detectable. In addition to pollution control for the leachate, the lime and cement treated fly ash improve the properties of soft marine clay significantly. References Bagchi, A. and Sopcich,C.:1989,'Characterization ofMSWlncinerator Ash',Z Environ. Eng, ASCE, 115(2), 447-452. Francis, C.W. and White, G. H.:1987,'Leaching ofToxicMetals From lncineratorAshes',WaterPo~Cont. Fed, 59 (11), 979-986.
