# Hydraulic Conductivity of Compacted CementStabilized Fly Ash

Michael E. Kalinski 1 

### and Praveen K. Yerra 1

1 

### Universit y of Kent ucky, Department of Civil Engineering, 161 Raymond Building, Lexington, Kentucky 40506-0281

KEYWORDS: fly ash, portland cement, compaction, hydraulic c onductivity

BACKGROUND

Ash is produced from the combustion of c oal at a global rate of approximately 480 million metric tons per year 1 . This includes bottom ash and flue gas desulfurization (FGD) material, but about two thir ds of the ash is in the fo rm of airborne particles, also known as fly ash. Fly ash is a fine-grai ned, dusty material that consists of SiO 2 , Al 2 O 3 , CaO (quicklime), and other minor constituent s, and is primarily produced by electric power plants. Due to i t s abundance, it is advantageous for fly ash producers to identify practical uses for fly ash rather than dispose o f it in landfills at a significant cost. Numerous uses have been identified for fl y ash in cons truction, including soil stabiliz atio n and portland cemen t supplementation 2 . Fly ash usage is also gaining in popularity in the agriculture i ndustry for the construction of high-strength cattl e feedlot pads 3 . In the European Union, over 90% of coal combustion byproducts (CCBs), including fly ash, are recycled 1 . In other parts of the wo rld, a much smaller percentage of fly ash (for instance, about one quarter in the United States) is utilized, with the remaining fly ash disp osed in lan d fills 2 . Clearly, there i s significant room for improvement with respect to utiliz ing fly ash, and it is of benefit to identify uses in which fly ash is the primary constit uent, rather than an additive, to accelerate its consumption.

Class F fly ash is a product of the combustion of older, harder bituminous and anthracite coal. In contrast, class C fly ash is a pr oduct of the combustion of younger lignite and sub-bituminous. Class F fly ash contains le ss than 10% quicklim e, while class C fly ash contains more than 10% quicklim e. Since th is study was conducted using fly ash from coal mined in the Appalachian region of the U n ited States, class F fl y ash was used. With relatively little quicklime, class F fl y ash is not self-cementing, and a cementing agent, such as portland cement, qui cklime, or hydrated lime (CaOH 2 ), must be added. When combined with a cementing agent, hydr ated, and compacted, class F fl y ash forms a high-strength material with unconfined compressive strength as high as 5,500 kPa 3 . This material possesses numerous potent ial uses for construction, including embankments, highway base courses, and cattle feedlot pads.

1Coal combustion byproducts have also been us ed as low-permeability hydraulic barrier s in a manner similar to com pacted clay. Wolfe et al. 4 demonstrated the use of compacted FGD material for the constructi on of low-permeability liners for animal waste lagoons. In their research, FGD material was mixed wit h fly ash and compacted to form a hydraulic barrier at the bottom of the lagoon. Hydraulic conductivity ( k ) ranging from 3.6 x 10 -5 to 1.0 x 10 -8 cm/s was measured in labor atory specimens prepared using standard proctor compaction effort (ASTM D 698) , while large-scale field measurements yielded steady state k around 4.5 x 10 -7 cm/s. Ghosh and Subbar ao 5 also demonstrated that fly ash, when combined with hydrated lime an d compacted using standard proctor effort, possesses k on the order of 1.0 x 10 -5 cm/s. They furth e r demonstrated that k decreases with increasing hydr ated lime content and increasing water content ( w ), and that with the addition of around 1% gypsum (CaSO 4 ● 2 H 2 O)

, k can be reduced to around 1.0 x 10 -7 cm/s. With respect to the 1.0 x 10 -7 cm/s criterion commonly prescribed in regulations for co mpacted clay liners (CCLs) for waste containment, these results indicate that, in some instances, compacted CCBs, including compacted fly ash, may perform satisfactor ily. These studies als o demonstrated that the water quality of the effluent permeating through these materials is not significantly impacted by the CCB, and that acceptable water quality standards are achiev able.

Like compacted clay, the dry density of com pacted fly ash varies with water content. Dry density, ρ d , is defined as:

V M s d = ρ , ( 1 )

where M s is the mass of dry solids in a volume V . Water content, w , is defined as:

% x M M
w

s w 100 = , ( 2 )

where M w is the mass of water. For a given com paction effort, the relations h ip between

ρ

d

and w , referred to as a compaction curve, is a concave-downward plot of ρ d

versus

w , with a maximum dry density, ρ dm ax , at a corresponding optimum water content, w opt

. Ghosh and Subbarao 5 demonstrated that k of compacted fly ash is less when compacting wet of w opt , and recommend that w be wet of w opt to m i nimize k . Similar observations have been made for compacted clay by Mitchell et al. 6 . They demonstrated that k decreases by several orders of magnitude when going from dry of

w

opt to wet of w opt , and that k less than 1.0 x 10 -7 cm/s can be achieved when wet of

w

opt

.

Daniel and Benson 7 developed an acceptance window approach to specifying ρ d

and w for compacted clay to minimize k by combining two or more comp action curves generated at different compacti on efforts. By compacti ng within an acceptance window that includes the locus of points wet of w opt , k will be low. This approach can be expanded by measuring k of each compacted specimen, superimposing constant k

2contours over the compaction curves, and delineating an acc eptance window that satisfy a desired k crite r ion (e.g. k < 1.0 x 10 -7 cm/s). This approach is particularly attractive because it eliminates the require ment that the field compaction effort be equivalent to a specific l aboratory compaction effort.

As mentioned previously, fly ash compact ed with portland cement is currently being used as a construction material for livestock feedlot pads due to its strength and durability 3 . To provide information for the analysis of surface water and infiltration rates for this material, a ccurate estimate of k is required. Therefore, the study detailed herein was performed to measure k of this material using va rious cement contents, water contents, and compaction efforts. Another purpose of this study was to elaborate on existing research regarding the hydraulic char acteristics of compacted fly ash-cement mixtures to develop relationships between ρ d , w , and k in a manner similar to the approach used for compacted clay. By es tablis hing this relations hip, in situ k of compacted fly ash-cement mixtures can be predicted based on field measurement of ρ d and w using a number of in situ methods.

TESTING PROCEDURES

Specimens were prepared using fly ash obt ained from combustion of coal at the Kentucky Utilities electric power plant in Tyrone, Kentucky. The co mposition of a representative sample of the fl y ash, listed in Table 1, in dicates that the fly ash was class F, with less than 10% quicklime. The specific gravity of the fly ash solids, G s , was 2.24 as measured using ASTM D854.

Table 1 –Com pos ition of a r epresentative sample of fly ash used for this study
Constituent Percentage by Weight
SiO 2 54.4
Al 2 O 3 26.7
Fe 2 O 3 6.9 C a O 2

. 4
K 2 O 2

. 0 TiO 2 1.6
M g O 1

. 1 O t h e r s 4

. 9

The fly ash was combined with portland cem ent using cement contents of 0%, 5%, 10%, and 15%. Cement content, c , is defined as:

a c c

s c M M M

M M c + = = , ( 3 )

where M a is the mass of fly ash and M c is the mass of the added c e ment. Fly ash and cement were mixed, and water was added to achieve a desired w . After mixing and

3prior to compaction, the mixture was allowe d to “mello w” for one hour. Specimens were compacted using standard and modified proc tor compaction effort (ASTM D698 and D1557, respectively). Proctor compaction in volves dropping of a weight a fixed number of times from a fixed heigh t. For standard proctor testin g, a 24-N weight is dropped 75 times from a height of 30 cm. For modified proctor testi ng, a 45-N weig ht is dropped 125 times from a height of 45 cm. Thus, the standard test corresponds to low compaction effort, while the modified test corresponds to higher compaction effort.

Several specimens were compacted over a range in w that included w opt to fully define each compaction curve. Based on previous research with the same material 3 , it was expected that w opt would be around 35% and 25% for specimens compacted using standard and modified proctor effort, respecti vely. After compacti on, the specimens were placed in sealed plastic bags and allow ed to cure for seven days. The specimens were compacted in a cylindric al mold, wit h a length and diameter of 11.6 cm and 10.2 cm, respectively.

After curing, the specimens were plac ed in a permeameter, and flexible wall permeability testing was performed in accordance with ASTM D5084 to measure k as illustrated in Fig. 1. To perform the test, a cell pressur e of 59 kPa was used, along with an influent and effluent pressure of 48 kPa and 0 kPa, respectively . The net pressure gradient of 48 kPa was equivalent to a stat ic head of 4.9 m. Deaired tap water was used as a permeant, and permeation was in an upwar d direction to help dislodge air bubbles trapped in the specimen. To asse ss the effect of long-term curing on k

, specimens were stored in plastic bags afte r permeation, and re-tested after 60 days.

Fig. 1—Fle x ible wall p e rm eab ility test configuration

RES U LTS AND DIS C USSION

Results are summarized in Tables 2-5 for each c , indicating w
, ρ d , and k for e a ch specimen. As illustrated in Fig. 2, the fly ash-cement mixtures exhibited compaction behavior similar to that of compacted clay, with ρ dm ax and corresponding w opt . As compaction effort increased from standard to modified proctor effort, ρ dm ax incr eased

4and w opt decreased. The zero air voids (ZAV) curve on each plot corresponds to a degree of saturation, S , of 100%. Degree of saturation is defined as:

v w V V S = , ( 4 )

where V w

and V v are the volume of water and volu me of voids, res pectively. As expected, the compaction curves are shifted to the left r e lative to the ZAV curve, indic a ting that S was initially less than 100%. At the conclusion of hydraulic conductivity testing, specimens were weighed, and S was recalculated. The average degree of saturation after permeation was 94%.

Hydraulic conductivity is also plotted as a function of w in Fig. 2. A s indicated in these plots, k decreased by about an order of magnit ude when going from dry to w e t of w opt

. When compacting wet of w opt , k on the order of 10 -6 cm/s was measured, and k was independent of compaction effo rt. When increasing from st andard to modified proctor compaction effort, the k versus w curve shifts to the left i n a manner similar to the compaction curves. Thus, com paction effort plays a role in k , and k is a functi on of both

ρ

d

and w . With respect to the k criterion of 1.0 x 10 -7 cm/s commonly used for waste containment liners, this material is not acceptable, although Ghosh and Subba r ao demonstrated that the addition of 1% gypsum can reduce k to this level 5 . Nev e rtheless, relatively lo w values of k that may be suitable for other applic ations are attainable.

By superimposing cont ours of constant k over the compaction curves, k can be predicted based on in situ measurement of ρ d

and w . A s seen in Figs. 2 and 3, c has little effect on k , but there is a clear relationship between ρ d , w , and k . Thus, for a given acceptance criterion (e.g. k < 1.0 x 10 -5 cm/s ), ρ d

and w can be measured in situ, and a

ρ

d

- w - k relationship s u ch as those shown in Fig. 3 can be used to determine whether or not the material satisfies the criterion. Ther e are a number of me thods for measuring

ρ

d

and w of field compacted materials, incl uding the nuclear gauge (ASTM D3017 and D2922), balloon test (ASTM D2167), and sand cone test (ASTM D1556). By using one or more of these methods on field compacted fly ash-cement mixtures, k can be accurately estimated.

To assess the effect of curing time on k , hydraulic cond uctivity of the fly ash-cement specimens was re-measured after a curing period of 60 days. Measured values for k are also inc l uded in Tables 3-5, and illustrated graphically in Fig. 4. These data indicate that with additional curing time , only minor decreases in k are observed, with k decreasing by an average of approximately 13%.

5Table 2. Data for specimens compacted with c = 0% Specimen ID w (% ) ρ d (kg/m 3 ) k (cm/s) Standard Proctor (ASTM D698)
0s1 26.3 990 5.04 x 10 -5

0s2 33.1 988 4.01 x 10 -5

0s3 36.6 999 3.67 x 10 -5

0s4 41.0 1067 1.64 x 10 -5

0 s 5 4 4

. 0 1 0 2 6

- Modified P r octor (ASTM D1557)
0m1 16.6 1127 3.22 x 10 -5

0m2 21.5 1132 3.50 x 10 -5

0m3 24.0 1175 1.96 x 10 -5

0m4 28.6 1200 1.38 x 10 -5

0m5 32.6 1149 1.26 x 10 -5

0m6 35.9 1131 0.79 x 10 -5

Table 3. Data for specimens compacted with c = 5% Specimen ID w (% ) ρ d (kg/m 3 ) 7-day k (cm/s) 60-day k (cm/s) Standard Proctor (ASTM D698)
5s1 21.2 1091 2.99 x 10 -5 2.40 x 10 -5

5s2 24.6 1132 2.31 x 10 -5 1.88 x 10 -5

5s3 29.1 1144 1.20 x 10 -5 0.99 x 10 -5

5s4 32.5 1179 0.35 x 10 -5 0.27 x 10 -5

5s5 36.7 1143 0.28 x 10 -5 0.25 x 10 -5

5s6 41.0 1093 0.20 x 10 -5 0.17 x 10 -5

Modified P r octor (ASTM D1557)
5m1 17.3 1213 1.75 x 10 -5 1.42 x 10 -5

5m2 21.2 1223 1.02 x 10 -5 0.91 x 10 -5

5m3 25.3 1246 0.34 x 10 -5 0.29 x 10 -5

5m4 28.1 1256 0.24 x 10 -5 0.21 x 10 -5

5m5 32.6 1196 0.34 x 10 -5 0.32 x 10 -5

6Table 4. Data for specimens compacted with c = 10% Specimen ID w (% ) ρ d (kg/m 3 ) 7-day k (cm/s) 60-day k (cm/s) Standard Proctor (ASTM D698)
10s1 19.3 1030 5.06 x 10 -5 4.06 x 10 -5

10s2 24.1 1012 5.04 x 10 -5 4.10 x 10 -5

10s3 28.1 1020 4.16 x 10 -5 3.60 x 10 -5

10s4 34.0 1030 2.86 x 10 -5 2.19 x 10 -5

10s5 36.7 1049 1.39 x 10 -5 1.22 x 10 -5

10s6 41.5 1056 0.44 x 10 -5 0.40 x 10 -5

10s7 43.7 1035 0.42 x 10 -5 0.37 x 10 -5

Modified P r octor (ASTM D1557)
10m1 16.8 1129 2.42 x 10 -5 2.16 x 10 -5

10m2 19.4 1134 2.43 x 10 -5 2.12 x 10 -5

10m3 24.5 1151 1.39 x 10 -5 1.29 x 10 -5

10m4 29.0 1197 0.32 x 10 -5 0.29 x 10 -5

10m5 32.9 1197 0.34 x 10 -5 0.29 x 10 -5

10m6 36.8 1142 0.32 x 10 -5 0.27 x 10 -5

Table 5. Data for specimens compacted with c = 15% Specimen ID w (% ) ρ d (kg/m 3 ) 7-day k (cm/s) 60-day k (cm/s) Standard Proctor (ASTM D698)
15s1 20.7 1151 2.88 x 10 -5 2.46 x 10 -5

15s2 25.4 1181 1.68 x 10 -5 1.52 x 10 -5

15s3 29.9 1208 0.62 x 10 -5 0.57 x 10 -5

15s4 33.4 1213 0.41 x 10 -5 0.38 x 10 -5

1 5 s 5 3 8

. 3 1 1 6 9

- - Modified P r octor (ASTM D1557)
15m1 17.3 1279 1.52 x 10 -5 1.24 x 10 -5

15m2 20.9 1274 0.92 x 10 -5 0.82 x 10 -5

15m3 25.0 1318 0.33 x 10 -5 0.28 x 10 -5

15m4 28.9 1289 0.18 x 10 -5 0.16 x 10 -5

15m5 32.4 1217 0.33 x 10 -5 0.31 x 10 -5

15m6 35.9 1176 0.20 x 10 -5 0.19 x 10 -5

714 00

13 00

12 00

11 00

10 00

90 0Dry Density, ρd,(kg/m3)

50 40 30 20 10

W a te r C o n t e n t, w (% ) St a n d a r

d

Mo d i f i e d
ZA V 1400

1300

1200

1100

1000

900Dry Density, ρd, (kg/m3)

50 40 30 20 10

W a ter Co nten t, w (% )

10 -6 2 4 10 -5 2 4 10 -4HydraulicCond.,k(cm/s)

50 40 30 20 10

W a te r C o n t e n t, w (% ) 10 -6 2 4 10 -5 2 4 10 -4Hydraulic Cond.,k(cm/s)

50 40 30 20 10

W a ter Co nten t, w (% ) a) c = 0% b ) c = 5%

1400

1300

1200

1100

1000

900Dry Density, ρd, (kg/m3)

50 40 30 20 10

W a ter Co nten t, w (% ) 1400

1300

1200

1100

1000

900Dry Density, ρd, (kg/m3)

50 40 30 20 10

W a ter Co nten t, w (% )

10 -6 2 4 10 -5 2 4 10 -4Hydraulic Cond.,k(cm/s)

50 40 30 20 10

W a ter Co nten t, w (% ) 10 -6 2 4 10 -5 2 4 10 -4Hydraulic Cond.,k(cm/s)

50 40 30 20 10

W a ter Co nten t, w (% )

c ) c = 10% d ) c = 15%

Fig. 2 – Compaction c u rves and hydraulic c onductivity data for fly ash-cem e nt mixtures (7-day curing tim e )

8a) c = 0 % b ) c = 5 %

c) c = 10% d) c = 15%

Fig. 3 – Density-moist u re content-hyd raulic c onductivity relationships for

com pacted fly ash-portland cem e nt m i xtures (7-day curi ng tim e )

910 -6 2 4 6 10 -5 2 4 6 10 -4HydraulicCond.,k(cm/s)

50 40 30 20 10

W a te r C o n t e n t, w (% )

7- d a y c u r i n

g
st a n d a r d

mo d i f i e d
60

- d a y c u

r i ng st a n d a r

d

mo d i f i e d

10 -6 2 4 6 10 -5 2 4 6 10 -4HydraulicCond.,k(cm/s)

50 40 30 20 10

W a te r C o n t e n t, w (% ) a) c = 5 % b ) c = 10 %

10 -6 2 4 6 10 -5 2 4 6 10 -4HydraulicCond.,k(cm/s)

50 40 30 20 10

W a te r C o n t e n t, w (% ) c) c = 15%

Fig. 4 – Hydraulic conductivity dat a for 7- and 60-day curing periods.

CONCLUS ION

The hydraulic conductivity of compacted portland cement-fly ash mixtures is affected by compaction effort and w . When compacting relatively dry mixtures ( w < 20% )

, k appears to be independent of compaction effort, and is on the order of 10 -5 cm/s. When compacting between w of 20% and w opt , compaction effort affects k , and, at a given w
, k decreases by about an order of magnitude when increasing from standard to modified proctor effort. When w e t of w opt , k is on the order of 10 -6 cm/s regardless of compaction effort or water content. With respect to curing time, ext ended curing time has relatively little effect on k within a 60-day time fr ame. Since most str ength gain and pozzolanic reaction occurs within the first 28 days of portl and cement hydration, it is unlikely that k would change signific a nt ly beyond 60 days.

10Based on the results of this study, an appr oach to construction quality assurance (CQA) testing of compacted portland cement-fly ash mixtures can be applied to estimate k based on in situ measurement of ρ d

and w using a number of different methods. Hydraulic conductivities of compacted portland cement-fly ash mixtur es on the order of 10 -5 to 10 -6 cm/s can be readily ac hieved. Th is is not adequate with respect to the criterion of 1.0 x 10 -7 c m /s often used for waste contai nment layers, but this material may be suitable for other geotechnical applic ations such as earth dams and highway base courses. Herein, portland cement was us ed as a cementing agent due to its rapid hydration, high strength, and availability. However, it is expected that similar measurements and behavior would be obs erved if either hydrat ed lime or quicklime were used.

ACK N OWLEDGMENT S

The authors would lik e to thank Dr. Jose Bi cudo of CH2M Hill (formerly of the Universit y of Kentucky Department of Biosystems and Agri cultural Engineering) for his support and assistance in performing research in compact ed fly ash-cement materials. We would also lik e to thank Mr. Thomas Moore of the Kentucky Utilities Tyrone Power Plant for providing us with fly ash for testing, as we ll as Mr. Gen e Yates of the University of Kentucky for his assistance in the laboratory.

REFE RE NCES

[1] Vom Berg. W, and Feuerborn, H. J., 2001, “CCPs in Europe,” Proceedings of Clean Coal Day in Japan 2001 , September 3-6, Tokyo, ECO BA European Coal Combustion Products Association, http://www.energiasko r.se /rapporter/ECOBA_paper.pdf

.
[2] FHWA, 1999, “Fly Ash Fact s for Highway Engineers,” Federal Highway Administration Docu ment FHWA-SA-94-081

, Third Printing, American Coal As h Association.

[3] Kalinski, M. E., Bicudo, J. R., H i ppley, B ., and Nanduri, S. R., 20 05, “Development of Construction Specifications and Quality A ssurance Criteria for Compacted Fly AshCement Feedlot Pads, ” Applied Engineeri ng in Agriculture , American Society of Agricultural Engineers (accepted for publication).

[4] Wolfe, W. E., Buta lia, T. S., Walker, H., and Whitlach, E. E., 2003, “Use of Stabilize d FGD Materials in the Construc tion of Low Permeability Liners, Proceedings of the 2003 International Ash Utilization Sy mposium , University of Kentu cky Center for Applied Energy Research, October 19-22, Lexington, Kentucky.

[5] Ghosh, A., and Subbarao, C., 1998, “Hydraulic Conductivity and Leachate Characteris t ics of Stabiliz ed Fly A s h,” Journal of Environmental Engineering , American Society of Civil Engineers, Vol. 124, No. 9, pp. 812-820.

11[6] Mitchell, J. K., D. R. Hooper, and R. G. Campanella, R. G., 1965, “Permeability of Compacted Clays,” Journal of Soil Mechanic s and Foundation Engineering , Vol. 91, No. 4, pp. 41-65.

[7] Daniel, D. E., and C. H. Benson, 1990, “Water Content-Density Criteria for Compacted Soil Lin e rs,” Journal of Geotec hnical Engineering , Vol. 116, No. 12, pp. 1811-1830.

12