United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR -93/099 August 1993
&EPA Project Summary
Parameters Affecting the
Measurement of Hydraulic
Conductivity for Solidified/
Stabilized Wastes
D. J. Conrad, S. A. Shumborski, L. Z. Florence, and A. J. Liem
A series of experiments conducted
at the Alberta Environmental Centre ex-
amined the variation in hydraulic con-
ductivity (K) within and among three
matrices formed by steel mill baghouse
dust treated with 8%, 9% and 10% Nor-
mal Portland Cement at a water/cement
ratio of 1:1 Within the 8% and 9% ma-
trices, test gradient (i) and back pres-
sure (P) were combined into 3x3 fac-
torial treatments. Commercially avail-
able equipment was modified to allow
sensitive and continuous monitoring of
hydraulic conductivity. A permeant-ma-
trix interaction was indicated by K de-
creasing with time at a rate that in-
creased with higher cement contents.
After hydraulic conductivity testing, the
samples were examined by scanning
electron microscopy and energy dis-
persive x-ray analysis. A cement hy-
dration product, identified as ettringite,
had formed in the solidified/stabilized
waste pores. This product reduced hy-
draulic conductivity by two orders of
magnitude by restricting conducting
pores. Four to seven weeks of testing
were required before an acceptable
equilibrium had been reached and sta-
tistical comparisons among the i x P
treatments were made. Within each ma-
trix, gradient was statistically the most
significant parameter accounting for
60% of the variation in results. The
response to gradient was different than
that observed with clay and soil-liners
in the literature. The overall mean hy-
draulic conductivity of the 8% matrix
(10 ± 5 x 10'6 cm.sec1) was significantly
greater (p<0.01) than that of the 9%
matrix (0.06 ± 0.03 x 106 cm.sec1). Tem-
poral effects, gradient and cement con-
tent were identified as important fac-
tors affecting hydraulic conductivity
measurements and must be considered
in regulatory tests. Bulk density was a
useful quality control criterion for mini-
mizing sample variance within each
matrix.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
Solidification/stabilization processes are
widely used to treat wastes before their
disposal, especially those containing heavy
metals. These processes reduce the po-
tential for release of such contaminants
by removing free water, i.e., forming a
monolith (thereby reducing the surface
area available for leaching) and by reduc-
ing contaminant solubility by alkaline pre-
cipitation or incorporation into cement hy-
d rat ion products.
One of the routes of contaminant re-
lease is through dissolution and flow
through the bulk of the treated waste.
Hydraulic conductivity, defined below by
Darcy's law, is thus important:
K =
Q
iA
where K is hydraulic conductivity (cm. sec'1),
Q is flow rate (cm3, sec1) through a cross-
sectional area of A (cm2), and i is gradient
Printed on Recycled Paper
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(dimensionless), defined as head loss or
pressure drop (cm H2O) per unit length of
the solid (cm).
Hydraulic conductivity and permeability,
which are often incorrectly interchanged,
are related by:
KU,
K = -
where K is permeability (cm2), u, and p are
absolute viscosity (g.cnr'.sec'1) and density
(g.crrr3) of the fluid, respectively, and g is
gravitational acceleration (980 cm.sec'2).
When there is no fluid-solid interaction,
permeability is an intrinsic and useful prop-
erty of a solid: the flow rates of different
fluids through it can be computed from its
permeability and the properties of the liq-
uids. As shown in the report as summa-
rized here, however, this is not necessar-
ily the case with solidified/stabilized waste.
Moreover, since it is not the solid property
per se, but the flow rate of aqueous
permeant through it which is of interest,
hydraulic conductivity will be used herein.
The available literature deals predomi-
nantly with clay and soil liners. Test and
instrument parameters, such as satura-
tion, gradient and back pressure, as well
as temporal effects and sample prepara-
tion, have been identified as being impor-
tant factors in hydraulic conductivity mea-
surements. The corresponding information
on solidified/stabilized waste is, however,
practically non-existent. Nor is there suffi-
cient information that addresses the dif-
ferences between clay or soil liners and
solidified/stabilized waste, such as those
in compressive strength and permeant-
matrix interactions.
This report deals with the effects of
parameters affecting the measurement of
hydraulic conductivity of solidified/stabilized
waste. The study was undertaken to form
bases for the development of a regulatory
test method - to improve intra- and inter-
laboratory precision - and for correlating
accelerated laboratory test results to those
occurring under field conditions.
The scope of the investigation was lim-
ited to:
• one waste, steel mill baghouse dust,
treated with Normal Portland Cement
at different formulations to produce a
range of hydraulic conductivities
typical of those achieved by other
commercial solidification/stabilization
processes, and
• the following test and instrument
parameters: sample preparation,
temporal effects, gradient and back
pressure.
New equipment was acquired and modi-
fied to allow for sensitive, accurate, and
continuous flow measurements. Particular
attention was given to minimize variance
due to sample preparation, statistical meth-
ods, and experimental designs. Electron
microscopy, chemical analyses, and mea-
surements of physical properties were con-
.ducted to assist in the interpretation of the
observed phenomena.
Materials, Methods and
Procedures
Sample Preparation
Equal portions of steel mill baghouse
dust and silica sand (42, 41, and 40 wt.%)
were treated with equal portions of Nor-
mal Portland Cement and water (8, 9 and
10 wt.%). The products are referred to as
8%, 9% and 10% matrices, respectively.
The waste and the treatment were cho-
sen to be those commonly encountered in
practice. The formulations, including the
use of sand, were selected to produce
samples with a range of hydraulic conduc-
tivities typical of solidified/stabilized wastes
(10~6 to 10~8 cm.sec'1). To obtain samples,
the dry ingredients were mixed, water was
added and mixed, and the product was
compacted into cylindrical plastic molds
(7.6 cm diameter by 15.2 cm long) with
the use of a standard method. The
samples were cured for at least 28 days
at 23°C and a minimum relative humidity
of 95%.
After curing, the samples were removed
from the molds and then trimmed to en-
sure parallel end faces. The bulk density
of each sample was computed from mea-
surements of its mass and dimensions.
For each matrix, the mean bulk density of
all the samples prepared was computed.
Only those samples with bulk densities
within 0.5% of the mean were used.
Sample Characterization
In addition to bulk density and water
content, the specific gravity of the dried
samples (true density) was measured, from
which porosity and degree of saturation
could be computed. To determine the ef-
fect of hydraulic conductivity testing, these
destructive measurements were made on
samples after testing as well as on com-
panion samples that had not undergone
testing. The unconfined compressive
strengths of these companion samples
were also measured.
Equipment
Three flexible-wall permeameters were
used. The equipment featured a novel
method for measuring permeant flow rate,
using the movement of a piston inside a
permeant interface. This piston was con-
nected to a linear variable displacement
transducer, the output of which was digi-
tized and logged in a data logger. The
inlet and outlet flow rates were simulta-
neously measured.
Two major modifications were made.
The first was the replacement of the trans-
ducers to achieve 0.001-mm accuracy over
a 30-mm range. The second was the use
of a bladder interface to isolate the pneu-
matic pressurization system from the
permeameter cell water.
Hydraulic Conductivity
Measurements
After vacuum saturation was applied,
the inlet and outlet flow rates were com-
puted using volume measurements, from
the calibrated piston movement, and the
corresponding elapsed times. Hydraulic
conductivity was computed when the inlet
and the outlet flows were within 5%.
Other Measurements
Electron microscopic analyses were per-
formed on a Hitachi S510* scanning elec-
tron microscope (SEM), a Hitachi X-650
(SEM) with energy dispersive X-ray spec-
trometer, and a Hitachi H-600 scanning
transmission electron microscope (STEM)
equipped with a Kevex Be window X-ray
detector. Chemical analyses of the waste
and the exuded permeant were performed
with the use of ion coupled spectroscopy,
atomic absorption spectroscopy, colorim-
etry and potentiometric titration.
Experimental Design
The effects of sample formulation, time
and instrument parameters (gradient, i,
and back pressure, P) were studied as
follows. A 3 x 3 full factorial design was
used to determine the effects of i and P.
The values for i were 8, 116, and 227,
and for P were 14, 69, and 124 kPa.
The hydraulic conductivity of each of
the 8%, 9%, and 10% matrices was con-
currently measured using the three
permeameters. To study the initial tempo-
ral effects, median levels of i and P were
used. After suitable equilibrium was
reached, i and P were then varied in a
random manner according to that design.
Equilibrium was defined as when the varia-
tion of hydraulic conductivity with time, as
computed from the slope of a linear re-
gression, could not be proven to be differ-
ent from zero. Because of time constraints,
the full factorial design was applied only
to the 8% and 9% matrices. In addition,
three 8% matrix samples were tested con-
currently at median levels of i and P over
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
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a 29-day period to obtain precision val-
ues.
Results and Discussion
Bulk Density
Although a standard compaction proce-
dure was carefully applied, there were
overlaps in bulk density among the se-
lected matrices, as shown in Table 1.
The criterion of selecting only those
samples within ±0.5% of the sample mean
of each matrix was thus applied to ensure
distinct populations.
Temporal Effect
The hydraulic conductivity of each ma-
trix decreased with time in a manner that
could be described by the following equa-
tion:
Kx ^06=A(T+^)B
where T is elapsed time, in days, from the
first day of testing at T=0. The least-
squares values of A and B (referred to as
the power function intercept [initial value]
and slope, respectively, and computed
over 80 days of testing at median levels
of i and P) are given in Table 2.
Two interesting observations can be
made: there was a marked difference in
initial value between the 8% and 9% ma-
trices but not between the 9% and 10%
matrices; and the negative slope increased
in magnitude with increasing cement con-
tent. The first observation can be readily
explained in terms of granular versus
paste-like behavior resulting from differ-
ences in cement and water contents. The
second, which suggests some form of ce-
ment hydration reaction, will be discussed
in the following section.
The decrease in hydraulic conductivity
with time, up to two orders of magnitude
for the 10% matrix, was the opposite of
what was anticipated at the onset of the
project. Due to matrix dissolution, it was
expected that the connecting pores in the
matrix, and hence hydraulic conductivity,
would increase. Matrix dissolution did oc-
cur quite significantly. In some cases, more
than 1% of the sample weight was lost
over 80 days of testing. Yet, the hydraulic
conductivity decreased with time. The re-
sults of the investigation into this phenom-
enon are described below.
Mechanisms of the Decrease In
Hydraulic Conductivity
Visual inspections of the samples after
testing showed the presence of white ma-
terials in the dark-colored matrix. SEM
examinations revealed profuse fibrous
growth, the morphology of which was simi-
lar to that of ettringite (3CaO.AI2O3.
CaSO4.31H O), rather than to that of a
cement hydration product, such as cal-
cium silica-hydrate or calcium hydroxide.
X-ray analyses of the individual fibres re-
vealed the presence of Ca, Al, S and
traces of Fe, and a Ca/S ratio of 2.62±0.52.
Table 1. Bulk Density of Stabilized Waste Samples
Matrices, cement percent weights
8%
9%
10%
Mean Bulk
Density, (SD) *
(gms per cm3)
Relative Standard
Deviation (n)+
2.398 (±0.024)
1.00 (16)
2.480 (±0.043)
1.73(18)
2.527 (±0.363)
0.36 (16)
* SD = Standard Deviations.
* = Number of Samples.
Table 2. Temporal Effect: Regression Analysis Results *
Matrix
8%
9%
10%
Initial Value, ±se *
Slope, ±se
82.4 ± 4.4
-0.413 ±025
1.571 ± 044
-0.743 ±0.032
1.356 ±0.148
-1.476 ±0.070
* The models account for more than 98% of observed variation in hydraulic conductivity.
* Standard error; significance levels p<0.001.
These results suggest the presence of
mainly aluminoferrite trisulphate (Ca/S=2.1 )
and some aluminoferrite monosulphate
(Ca/S=4.1).
The formation of fibrous materials was
a result of permeant-matrix reactions. No
such formation was observed in compan-
ion samples that were kept in a humidity
chamber.
The above phenomenon has been pre-
viously reported. The hydraulic conductiv-
ity of cement pastes could be reduced by
six orders of magnitude as a result of
curing under water. The explanation was
given in terms of expansion in volume of
hydrated paste and hydration products fill-
ing the pores and cavities, thereby reduc-
ing and blocking flow channels.
Effect of Matrix
As previously mentioned, there was a
marked difference in hydraulic conductiv-
ity between the 8% and 9% matrices. Even
when the effects of time, i and P are not
considered, a statistically significant differ-
ence (p<0.01) in hydraulic conductivity ex-
ists between them: 10±5 x 10~6 cm.sec'1
versus 0.06±0.03 x 10'6 cm. sec'1. Hydrau-
lic conductivity is thus very sensitive to
matrix composition.
Effect of Instrument Parameters
The effects of i and P were investigated
after equilibrium conditions were reached.
By the criterion used, these were reached
after 27, 34, and 59 days of testing for the
8%, 9% and 10% matrices, respectively.
Note however that the criterion is based
on failure to reject the null hypothesis of
zero slope. The power of the test and the
Type II error were not considered. Tem-
poral effect could still be present, and this
would be considered as random errors.
The variation of hydraulic conductivity
with gradient and back pressure was mod-
elled according to a second order polyno-
mial of the form:
, x2+bt1 x,2+b22 x22+ bl2 x,x2
where x, and Xj, are the transformed (range
from -1 to +1) i and P, respectively. The
regression results are shown in Table 3.
The results show statistically significant
effects of gradient for the 8% matrix, with
a positive linear term and a negative qua-
dratic term; and gradient and back pres-
sure for the 9% matrix, with only positive
linear terms. The equations show that K is
less sensitive to i and P at high and me-
dian levels, and thus is the region where
hydraulic conductivity should be measured
to minimize variability.
The gradient was varied by a factor of
25, and the back pressure by 8. Yet the
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Table 3. Regression Analysis Results
Coefficient
Matrix
8%
9%
13.01*
(0.8) S
0.06"
(.005)
2.33 +
(0.7)
0.023 *
(.005)
0.18
(0.7)
0.015"
(.0005)
-4.3 +
(1.1)
-0.013
(.008)
"& bi2 n *
-1.2
(1.1)
0.003
(.008)
-0.3
(0.9)
-0.001
(.006)
22
26
The models account for 68% and 58% of the total variance for the 8% and 9% matrices
respectively.
n+ Number of data points.
s Standard error of the coefficient.
" Significant effect at p<0.01.
variation in hydraulic conductivity was less
than 4-fold for the 9% matrix, and less
than 3-fold for the 8% matrix. Therefore,
as a first approximation and only with re-
spect to the effects of i and P, accelerated
laboratory conditions produced results
close to what might be found in the field.
Distribution and Precision
Log-transformation has been suggested
as a way to normalize the data and obtain
constant variance. The triplicate measure-
ment results for the 8% matrix were log-
transformed. The null hypothesis of nor-
mal distribution could not be rejected for
the transformed data. The precision val-
ues are shown in Table 4.
For comparison, a precision of x/+ 7.3
for four replicates was reported in a previ-
ous study on solidified/stabilized waste.
The improvement could be attributed to
sample preparation, sample acceptance
criterion and measurements at higher lev-
els of i and P.
Correlation with Porosity
For the 8%, 9%, and 10% matrices, the
hydraulic conductivity and porosity E could
be correlated by:
K=2.83x 10-29 10725E
with p<0.0001 and ^=0.89. It should be
emphasized, however, that only one waste/
treatment combination was used.
Saturation
Saturation is defined as the ratio of free
water to pore volume. Changes in satura-
tion as a result of testing are shown in
Table 5.
There was a marked increase in satura-
tion, which in part was effected in the
beginning of the testing when vacuum
saturation was applied. In the field, how-
ever, the treated waste would not be satu-
rated. The effect of not saturating the
sample, coupled with applying low levels
of gradient and back pressure, still needs
to be investigated.
Table 5. Permeant Saturation Before and
After Testing
Matrix
Testing
8%
9%
10%
Before
After
35%
96%
42%
93%
52%
94%
Table 4. Results of Log-Transformation for the 8% Matrix
Test Day
Log Transformation 1 8
15
22
29
Mean
(SD)*
Precision *: xA
-3.94
(.18)
3.80
-4.31
(.25)
4.4
-4.48
(.23)
4.25
-4.61
(.26)
4.48
-4.78
(.24)
4.36
* Standard Deviation.
+ 95% confidence limits for mean K for three samples.
Effect of Gradient: Comparison
with Soil/Clay Liners
High gradients applied to soil/clay
samples have been reported to cause con-
solidation and the consequent reduction
in hydraulic conductivity. This seems rea-
sonable considering that clay, with a typi-
cal unconfined compressive strength
(UCS) of 100 kPa, would be subjected to
a pressure of up to 450 kPa (for 15 cm
sample length at a gradient of 300). For
comparison, the samples used in this study
had a range of UCS from 3000 to 6000
kPa, and were subjected only to a maxi-
mum pressure of 340 kPa. This may ex-
plain the difference in the effect of gradi-
ent between soil/clay and solidified/stabi-
lized waste.
Conclusions and
Recommendations
Conclusions
1. Hydraulic conductivity was sensitive to
matrix composition. Significantly differ-
ent hydraulic conductivities were mea-
sured between samples differing only
by 1% in cement content. The ability to
distinguish such samples was attrib-
uted to the institution of a strict quality
control criterion on the basis of bulk
density. The corollary is that variance
resulting from sample preparation can
be minimized by using that criterion.
2. Hydraulic conductivity decreased with
elapsed time during testing. A power
function in the form of y=axb describes
the relationship. The decrease could
be explained by long-term cement hy-
dration reactions forming ettringite in
the permeant-conducting pores. Al-
though matrix dissolution occurred, no
effect was observed over the testing
period of up to 80 days.
3. The effects of gradient and back pres-
sure on hydraulic conductivity were the
following:
• Gradient was the most significant
parameter, and its correlation with
hydraulic conductivity was positive,
the opposite to that for soil and clay
liners. This was attributed to the
higher unconfined compressive
strength and the corresponding lesser
degree of sample consolidation.
• Medium and high levels of gradient
and back pressure were the less
sensitive region for hydraulic
conductivity measurements. Falling-
head permeameters, in which low
levels of these parameters are used,
are thus operated in the more
sensitive region.
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• Over the entire region of the chosen
experimental levels, the measured
hydraulic conductivities varied by a
factor of four or less. Values obtained
in the laboratory are thus reasonable
estimates of those in the field
conditions, provided that compaction
and curing conditions are similar.
4. An exponential relationship between hy-
draulic conductivity and sample poros-
ity was shown to be statistically signifi-
cant. How such a relationship varies
with different matrices was not investi-
gated.
Recommendations for
Regulatory Test Development
1. Bulk density should be used as a qual-
ity control criterion to reduce variance
resulting from sample preparation.
2. To improve precision, temporal effects
should be taken into account and mea-
surements carried out at high levels of
gradient and back pressure.
3. To estimate maximum hydraulic con-
ductivity, measurements should be
made as soon as the sample is cured.
Recommendations for Future
Work
1. Other common waste/treatment sys-
tems should be studied to investigate
permeant-matrix interactions and the ef-
fect of instrument parameters.
2. Saturation effects should be studied to
predict field hydraulic conductivity.
The full report was submitted in fulfill-
ment of CR#814860-01-1 by Alberta Envi-
ronmental Centre under the sponsorship
of the U.S. Environmental Protection
Agency.
6ll.S. GOVERNMENT PUNTING OFFICE: 1993 • 75WJ7I/W026
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D. J. Conrad, S. A. Shumborski, L. Z. Florence, and A. J. Liem are with Alberta
Environmental Centre, Vegreville, Alberta, Canada JOB 4LO.
C. I. Mashni is the EPA Project Officer (see below).
The complete report, entitled "Parameters Affecting the Measurement of
Hydraulic Conductivity for Solidified/Stabilized Wastes," (Order No. PB93-
199 396/AS; Cost: $19.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA/600/SR-93/099
BULK RATE
POSTAGES FEES PAID
EPA
PERMIT No. G-35
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