Hydraulic Conductivity of Compacted CementStabilized Fly Ash 

Michael E. Kalinski1

 and Praveen K. Yerra1 

1

University of Kentucky, Department of Civil Engineering, 161 Raymond 

Building, Lexington, Kentucky 40506-0281 

KEYWORDS: fly ash, portland cement, compaction, hydraulic conductivity

BACKGROUND 

Ash is produced from the combustion of coal at a global rate of approximately 480 

million metric tons per year1

. This includes bottom ash and flue gas desulfurization 

(FGD) material, but about two thirds of the ash is in the form of airborne particles, also 

known as fly ash. Fly ash is a fine-grained, dusty material that consists of SiO2, Al2O3, 

CaO (quicklime), and other minor constituents, and is primarily produced by electric 

power plants. Due to its abundance, it is advantageous for fly ash producers to identify 

practical uses for fly ash rather than dispose of it in landfills at a significant cost. 

Numerous uses have been identified for fly ash in construction, including soil 

stabilization and portland cement supplementation2

. Fly ash usage is also gaining in 

popularity in the agriculture industry for the construction of high-strength cattle feedlot 

pads3

. In the European Union, over 90% of coal combustion byproducts (CCBs), 

including fly ash, are recycled1

. In other parts of the world, a much smaller percentage 

of fly ash (for instance, about one quarter in the United States) is utilized, with the 

remaining fly ash disposed in landfills2

. Clearly, there is significant room for 

improvement with respect to utilizing fly ash, and it is of benefit to identify uses in which 

fly ash is the primary constituent, 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 product of the combustion of younger lignite and 

sub-bituminous. Class F fly ash contains less than 10% quicklime, while class C fly ash 

contains more than 10% quicklime. Since this study was conducted using fly ash from 

coal mined in the Appalachian region of the United States, class F fly ash was used. 

With relatively little quicklime, class F fly ash is not self-cementing, and a cementing 

agent, such as portland cement, quicklime, or hydrated lime (CaOH2), must be added. 

When combined with a cementing agent, hydrated, and compacted, class F fly ash 

forms a high-strength material with unconfined compressive strength as high as 5,500 

kPa3

. This material possesses numerous potential uses for construction, including 

embankments, highway base courses, and cattle feedlot pads. 

1
Coal combustion byproducts have also been used as low-permeability hydraulic barriers 

in a manner similar to compacted clay. Wolfe et al.4

 demonstrated the use of 

compacted FGD material for the construction of low-permeability liners for animal waste 

lagoons. In their research, FGD material was mixed with 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 laboratory specimens prepared using 

standard proctor compaction effort (ASTM D698), while large-scale field measurements 

yielded steady state k around 4.5 x 10-7 cm/s. Ghosh and Subbarao5

 also 

demonstrated that fly ash, when combined with hydrated lime and compacted using 

standard proctor effort, possesses k on the order of 1.0 x 10-5 cm/s. They further 

demonstrated that k decreases with increasing hydrated lime content and increasing 

water content (w), and that with the addition of around 1% gypsum (CaSO4 ● 2 H2O), 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 compacted clay liners (CCLs) for waste 

containment, these results indicate that, in some instances, compacted CCBs, including 

compacted fly ash, may perform satisfactorily. These studies also 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 achievable. 

Like compacted clay, the dry density of compacted fly ash varies with water content. 

Dry density, ρd, is defined as: 

V

Ms ρ d = , (1)

where Ms 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 Mw is the mass of water. For a given compaction effort, the relationship between 

ρd and w, referred to as a compaction curve, is a concave-downward plot of ρd versus 

w, with a maximum dry density, ρdmax, at a corresponding optimum water content, wopt. 

Ghosh and Subbarao5

 demonstrated that k of compacted fly ash is less when 

compacting wet of wopt, and recommend that w be wet of wopt to minimize 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 

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

wopt. 

Daniel and Benson7

 developed an acceptance window approach to specifying ρd and w

for compacted clay to minimize k by combining two or more compaction curves 

generated at different compaction efforts. By compacting within an acceptance window 

that includes the locus of points wet of wopt, k will be low. This approach can be 

expanded by measuring k of each compacted specimen, superimposing constant k

2
contours over the compaction curves, and delineating an acceptance window that 

satisfy a desired k criterion (e.g. k < 1.0 x 10-7 cm/s). This approach is particularly 

attractive because it eliminates the requirement that the field compaction effort be 

equivalent to a specific laboratory compaction effort. 

As mentioned previously, fly ash compacted with portland cement is currently being 

used as a construction material for livestock feedlot pads due to its strength and 

durability3

. To provide information for the analysis of surface water and infiltration rates 

for this material, accurate estimate of k is required. Therefore, the study detailed herein 

was performed to measure k of this material using various cement contents, water 

contents, and compaction efforts. Another purpose of this study was to elaborate on 

existing research regarding the hydraulic characteristics of compacted fly ash-cement 

mixtures to develop relationships between ρd, w, and k in a manner similar to the 

approach used for compacted clay. By establishing this relationship, in situ k of 

compacted fly ash-cement mixtures can be predicted based on field measurement of ρd

and w using a number of in situ methods. 

TESTING PROCEDURES 

Specimens were prepared using fly ash obtained from combustion of coal at the 

Kentucky Utilities electric power plant in Tyrone, Kentucky. The composition of a 

representative sample of the fly ash, listed in Table 1, indicates that the fly ash was 

class F, with less than 10% quicklime. The specific gravity of the fly ash solids, Gs, was 

2.24 as measured using ASTM D854. 

Table 1 –Composition of a representative sample of fly ash used for this study 

Constituent Percentage by Weight 

SiO2 54.4 

Al2O3 26.7 

Fe2O3 6.9 

CaO 2.4

K2O 2.0

TiO2 1.6 

MgO 1.1

Others 4.9

The fly ash was combined with portland cement using cement contents of 0%, 5%, 

10%, and 15%. Cement content, c, is defined as: 

c a

c

s

c

M M

M

M

M

c

+ = = , (3)

where Ma is the mass of fly ash and Mc is the mass of the added cement. Fly ash and 

cement were mixed, and water was added to achieve a desired w. After mixing and 

3
prior to compaction, the mixture was allowed to “mellow” for one hour. Specimens were

compacted using standard and modified proctor compaction effort (ASTM D698 and 

D1557, respectively). Proctor compaction involves dropping of a weight a fixed number 

of times from a fixed height. For standard proctor testing, a 24-N weight is dropped 75 

times from a height of 30 cm. For modified proctor testing, a 45-N weight 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 wopt to fully define 

each compaction curve. Based on previous research with the same material3

, it was 

expected that wopt would be around 35% and 25% for specimens compacted using 

standard and modified proctor effort, respectively. After compaction, the specimens 

were placed in sealed plastic bags and allowed to cure for seven days. The specimens 

were compacted in a cylindrical mold, with a length and diameter of 11.6 cm and 10.2 

cm, respectively. 

After curing, the specimens were placed 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 pressure 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 static head of 4.9 m. Deaired tap water was 

used as a permeant, and permeation was in an upward direction to help dislodge air 

bubbles trapped in the specimen. To assess the effect of long-term curing on k, 

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

Fig. 1—Flexible wall permeability test configuration 

RESULTS AND DISCUSSION 

Results are summarized in Tables 2-5 for each c, indicating w, ρd, and k for each 

specimen. As illustrated in Fig. 2, the fly ash-cement mixtures exhibited compaction 

behavior similar to that of compacted clay, with ρdmax and corresponding wopt. As 

compaction effort increased from standard to modified proctor effort, ρdmax increased 

4
and wopt 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 Vw and Vv are the volume of water and volume of voids, respectively. As 

expected, the compaction curves are shifted to the left relative to the ZAV curve, 

indicating 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. As indicated in these 

plots, k decreased by about an order of magnitude when going from dry to wet of wopt. 

When compacting wet of wopt, k on the order of 10-6 cm/s was measured, and k was 

independent of compaction effort. When increasing from standard to modified proctor 

compaction effort, the k versus w curve shifts to the left in a manner similar to the 

compaction curves. Thus, compaction effort plays a role in k, and k is a function of both 

ρd 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 Subbarao 

demonstrated that the addition of 1% gypsum can reduce k to this level5

. Nevertheless, 

relatively low values of k that may be suitable for other applications are attainable. 

By superimposing contours of constant k over the compaction curves, k can be 

predicted based on in situ measurement of ρd and w. As seen in Figs. 2 and 3, c has 

little effect on k, but there is a clear relationship between ρd, w, and k. Thus, for a given 

acceptance criterion (e.g. k < 1.0 x 10-5 cm/s), ρd and w can be measured in situ, and a 

ρd - w - k relationship such as those shown in Fig. 3 can be used to determine whether 

or not the material satisfies the criterion. There are a number of methods for measuring 

ρd and w of field compacted materials, including 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 conductivity of the fly ash-cement 

specimens was re-measured after a curing period of 60 days. Measured values for k

are also included 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%. 

5
Table 2. Data for specimens compacted with c = 0% 

Specimen 

ID 

w

(%) 

ρd

(kg/m3

)

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

0s5 44.0 1026 -

Modified Proctor (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

(%) 

ρd

(kg/m3

)

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 Proctor (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

6
Table 4. Data for specimens compacted with c = 10% 

Specimen 

ID 

w

(%) 

ρd

(kg/m3

)

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 Proctor (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

(%) 

ρd

(kg/m3

)

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

15s5 38.3 1169 - -

Modified Proctor (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

7
1400

1300

1200

1100

1000

D

900 ry Density,

ρd, (kg/m3

)

10 20 30 40 50

Water Content, w (%)

 Standard

 Modified

 ZAV

1400

1300

1200

1100

1000

Dr

900

y Density, 

ρd, (kg/m3

)

10 20 30 40 50

Water Content, w (%)

10-6

2

4

10-5

2

4

10-4

Hydraulic Cond., k (c

m/s)

10 20 30 40 50

Water Content, w (%)

10-6

2

4

10-5

2

4

10-4

Hydraulic Cond., k (cm/s)

10 20 30 40 50

Water Content, w (%)

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

1400

1300

1200

1100

1000

Dr

900

y Density, 

ρd, (kg/m3

)

10 20 30 40 50

Water Content, w (%)

1400

1300

1200

1100

1000

Dr

900

y Density, 

ρd, (kg/m3

)

10 20 30 40 50

Water Content, w (%)

10-6

2

4

10-5

2

4

10-4

Hydraulic Cond., k (cm/s)

10 20 30 40 50

Water Content, w (%)

10-6

2

4

10-5

2

4

10-4

Hydraulic Cond., k (cm/s)

10 20 30 40 50

Water Content, w (%)

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

Fig. 2 – Compaction curves and hydraulic conductivity data for fly ash-cement 

mixtures (7-day curing time) 

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

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

Fig. 3 – Density-moisture content-hydraulic conductivity relationships for 

compacted fly ash-portland cement mixtures (7-day curing time) 

9
10-6

2

4

6

10-5

2

4

6

10-4

Hy

draulic Cond., k (c

m/s)

10 20 30 40 50

Water Content, w (%)

7-day curing

 standard

 modified

60-day curing

standard

 modified

10-6

2

4

6

10-5

2

4

6

10-4

Hy

draulic Cond., k (c

m/s)

10 20 30 40 50

Water Content, w (%)

 a) c = 5% b) c = 10% 

10-6

2

4

6

10-5

2

4

6

10-4

Hy

draulic Cond., k (c

m/s)

10 20 30 40 50

Water Content, w (%) c) c = 15% 

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

CONCLUSION 

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 wopt, 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 wet of wopt, k is on the order of 10-6 cm/s regardless of compaction 

effort or water content. With respect to curing time, extended curing time has relatively 

little effect on k within a 60-day time frame. Since most strength gain and pozzolanic 

reaction occurs within the first 28 days of portland cement hydration, it is unlikely that k

would change significantly beyond 60 days. 

10
Based on the results of this study, an approach 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 ρd and w using a number of different methods. 

Hydraulic conductivities of compacted portland cement-fly ash mixtures on the order of 

10-5 to 10-6 cm/s can be readily achieved. This is not adequate with respect to the 

criterion of 1.0 x 10-7 cm/s often used for waste containment layers, but this material 

may be suitable for other geotechnical applications such as earth dams and highway 

base courses. Herein, portland cement was used as a cementing agent due to its rapid 

hydration, high strength, and availability. However, it is expected that similar 

measurements and behavior would be observed if either hydrated lime or quicklime 

were used. 

ACKNOWLEDGMENTS 

The authors would like to thank Dr. Jose Bicudo of CH2M Hill (formerly of the University

of Kentucky Department of Biosystems and Agricultural Engineering) for his support and 

assistance in performing research in compacted fly ash-cement materials. We would 

also like to thank Mr. Thomas Moore of the Kentucky Utilities Tyrone Power Plant for 

providing us with fly ash for testing, as well as Mr. Gene Yates of the University of 

Kentucky for his assistance in the laboratory. 

REFERENCES 

[1] Vom Berg. W, and Feuerborn, H. J., 2001, “CCPs in Europe,” Proceedings of Clean 

Coal Day in Japan 2001, September 3-6, Tokyo, ECOBA European Coal Combustion 

Products Association, http://www.energiaskor.se/rapporter/ECOBA_paper.pdf. 

[2] FHWA, 1999, “Fly Ash Facts for Highway Engineers,” Federal Highway 

Administration Document FHWA-SA-94-081, Third Printing, American Coal Ash 

Association. 

[3] Kalinski, M. E., Bicudo, J. R., Hippley, B., and Nanduri, S. R., 2005, “Development of 

Construction Specifications and Quality Assurance Criteria for Compacted Fly AshCement Feedlot Pads,” Applied Engineering in Agriculture, American Society of 

Agricultural Engineers (accepted for publication). 

[4] Wolfe, W. E., Butalia, T. S., Walker, H., and Whitlach, E. E., 2003, “Use of Stabilized

FGD Materials in the Construction of Low Permeability Liners, Proceedings of the 2003 

International Ash Utilization Symposium, University of Kentucky Center for Applied 

Energy Research, October 19-22, Lexington, Kentucky. 

[5] Ghosh, A., and Subbarao, C., 1998, “Hydraulic Conductivity and Leachate 

Characteristics of Stabilized Fly Ash,” 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 Mechanics 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 Liners,” Journal of Geotechnical Engineering, Vol. 116, No. 12, pp. 

1811-1830.

12