Organo-silane chemistry: A water repellent
technology for coal ash and soils
John L. Daniels1, Mimi S. Hourani2 and Larry S. Harper3
1

AAAS Fellow and Program Director, Directorate for Engineering, Division of
Engineering Education and Centers, U.S. National Science Foundation, 4201 Wilson
Blvd, Arlington, VA 22230, USA (On leave from Department of Civil and Environmental
Engineering, University of North Carolina at Charlotte, Charlotte, NC 28223, USA)
2
Laboratory Services Manager, SUMMIT Engineering & Construction Services, Inc.
3575 Centre Circle, Fort Mill, SC 29715, USA
3
Partner, Hero Global Services, LLC, 207 Meacham Street Fort Mill, SC 29715
KEYWORDS: stabilization, infiltration, leachability, coupling agents
ABSTRACT
This manuscript provides insight into a new approach to chemically-based ash
improvement with organosilanes (OS). Specifically, OS serves as a coupling agent to
create a hydrophobic surface on virtually any silica-based material. As an illustration, a
laboratory and field testing program was conducted to evaluate the influence of OS
modification on the compaction, strength, swell and hydraulic properties of several
samples of Class F coal fly ash. OS modification resulted in notable changes to strength
and swell potential and a dramatic reduction in infiltration capacity. Concatenating with
previous results for reducing leachability, the data suggest that OS modification may
have wide application in the management of coal combustion products.
INTRODUCTION
Coal combustion fly ash has long been used as an ingredient to improve the
performance of materials such as concrete, flowable fill and stabilized soil. Likewise, a
number of standard and proprietary additives have been used to improve the properties
of fly ash in applications where it is the primary constituent, as in the case of a structural
fill or an embankment. Mostly, this has involved mixing fly ash with relatively small
proportions of lime1-3, or cement4, although flue gas desulfurization gypsum5, sodium
hydroxide6, bentonite clay7, enzymes8, among other additives have also been used.
The purpose of such modification has generally been to improve physical (e.g. strength,
compaction, permeability) or chemical (e.g., leachability) performance.
More recently, a form of organic modification that uses organosilanes (OS) to
irreversibly bond with fly ash or soil has been introduced as a water repellent
technology. The concepts and chemistry of this approach is provided by the fields of
applied clay science and catalysis. For example, the process of permanently grafting
organic molecules to various substrates has been explored to develop polymer-clay
nano composites10-12 and to strengthen calcium silicate hydrates in cement paste
1

 systems13. The purpose of this manuscript is to present example data related to ash
stabilization using OS as an alternative to conventional methods such as lime or
Portland Cement modification. OS modifies and reacts with individual particles without
binding them together, in contrast to a cementitious/pozzolanic approach.
Data have been obtained in the laboratory through compaction, strength, swell,
infiltration and hydraulic conductivity testing, as well as in the field through infiltration
measurements.
MATERIALS AND METHODS
The work presented herein comprises two separate experimental components. First,
the influence of OS on laboratory measurements of compaction, strength, swelling and
hydraulic conductivity was evaluated for a Class F fly ash from a coal-fired power plant
in the southeast U.S. Separately, a field trial was conducted to evaluate the potential
for OS use stabilizing ash monofills. OS was obtained from Zydex Industries
(Vadodora, India) through Hero Global Services (Fort Mill, SC, USA) and has the
commercial names Zycosil and Zycosoil. Details regarding the source product have
been presented in Daniels et al. (2009). Briefly, trialkoxy groups present in the OS
solution form siloxane (=Si-O-Si=) bonds with soil surfaces. In addition, there is an
organic group that contains a quaternary structure with a long alkyl chain (C18H37) which
imparts molecular level hydrophobicity on the treated surface.
Laboratory Measurements
Treated ash was prepared by mixing dry ash with a solution composed of 100 parts
water for every part of OS (100:1, by volume). This mixture was then allowed to fully airdry prior to subsequent testing. Treated and untreated samples were tested for
moisture-density relationships, California Bearing Ratio (CBR) and hydraulic
conductivity. Current ASTM standards were used to guide laboratory protocol, as
follows. Moisture-density relationships were evaluated in accordance with ASTM D698,
Method A with standard Proctor effort14. The CBR test was conducted as per ASTM
D188315. Hydraulic conductivity was evaluated in accordance with ASTM 508416 using
the constant head method, a back pressure of 345 kPa and a hydraulic gradient of 12.5.
Samples used for CBR or hydraulic conductivity were prepared at or above 95% of the
maximum dry density. In addition, approximately one month after treatment, infiltration
properties of previously treated and untreated compacted ash samples were evaluated
with a Mini Disk Infiltrometer from Decagon Devices. The device works by measuring
the rate of seepage into partially saturated soils using a slightly negative (suction)
pressure. Measurements were taken at a constant suction of -0.5 cm. Details on this
device and its application may be found in the manual17 with additional background
discussion provided separately18 as well as a comparison with other field techniques5.
Field Measurements
Depending on the field objectives, OS can be incorporated into the molding moisture
content during the construction of structural fills or it can be topically sprayed on the
2

 finished grade surface. For this work, the goal was to investigate the ability to create a
thin barrier layer against infiltration. As such, OS was topically applied to two sections of
an ash monofill, a relatively flat (< 5% slope) section and a slope (1.5H:2V) section, as
shown in Figure 1. A Finn T60 Hydroseeder was used which incorporated a cannon
sprayer, hose and built in agitator, with a 2271 liter tank. Water from the ash basin was
used by way of a pump. A 30kg drum of OS concentrate solution was mixed with a full
tank to produce a water:OS ratio of 75:1. The flat section was essentially square, with a
total treatment area of approximately 900 m2. The slope section was approximately 7 m
in width and 12 m in length. Mini Disk Infiltrometer measurements were made
approximately one month after treatment in both the slope and flat sections.

Treated Slope Section

Treated Flat Section

Figure 1. Treated and untreated sections of field test site (ash monofill).
RESULTS AND DISCUSSION
The results for the laboratory-derived moisture density relationships, hydraulic
conductivity and CBR data are provided in Table 1, while Figure 2 provides a plot of the
cumulative infiltration versus the square root of time18 for both laboratory samples and
field test sections.

3

 Table 1. Laboratory data
Sample
Molded
Description
w*
%
Ash
(Untreated)

Soaked

MDD** w*
kN/m3 %

K20***
(cm/s)
MDD**
kN/m3

CBR****
%

Swell
(%)

2.5 5.0 4.5
mm mm kg

25.6

11.6

33.5

11.4

1.40 x 10-4 3.3

4.3

2.7

Ash
(OS treated) 24.8

12.4

24.4

12.4

5.50 x 10-5 5.1

6.5

0.0

*Optimum Moisture Content, **Maximum Dry Density, ***Hydraulic Conductivity,
corrected for 20°C, ****California Bearing Ratio, calculated at penetration shown
3.5

Untreated - Flat Section (Field)

y = 0.0013x2 + 0.1796x
R2 = 0.9989

Untreated - Slope Section (Field)

3.0

Cumulative Infiltration (cm)

Untreated - Laboratory Sample
Treated - Laboratory and Field

2.5

Trendline - Flat Section
Trendline - Slope Section

2.0

Trendline - Laboratory Sample

y = 0.0106x2 + 0.0739x
R2 = 0.9995

1.5

y = 0.0104x2 + 0.0608x
R2 = 0.9992

1.0

Zero infiltration for treated laboratory sample as
well as treated flat and slope sections in field

0.5
0.0
0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

Square Root of Time
Figure 2. Infiltrometer results for laboratory and field samples
As indicated by Table 1, the influence of OS treatment was to reduce the optimum
moisture content and increase maximum dry density. A similar trend was also reported9
whereby a modest but consistent trend of increasing OS dosage correlated with
increasing dry density and decreasing optimum moisture content for another Class F
coal combustion fly ash. In terms of CBR results, the strength of the treated ash
increases, with a dramatic difference observed in terms of the change in water content

4

 after soaking. Specifically, the water content of the untreated ash sample increased
from 25.6% to 33.5% while the water content of the treated sample actually decreased
slightly, from 24.8% to 24.4%. These differences also manifest as a commensurate
reduction in swell, i.e., from 2.7% to 0%. As such, OS-treatment appears to mitigate
volume changes in ash that could otherwise be susceptible to swelling. Naturally, this
has implications according to ash mineralogy, e.g., neomineral formation and ettringite
control.
Moisture content changes in untreated ash have practical implications relative to on site
workability and slope stability, both of which can be exacerbated with increases in
moisture. Figure 3 illustrates the typical hydrophobicity that developed on the OStreated flat section of the field site. While untreated ash would immediately imbibe
moisture, OS-treated surfaces serve to pond water according to the grade. Likewise,
Figure 4 shows a picture of the slope section of the field site after treatment and
subsequent precipitation events. The left side of the slope shown in Figure 4 is
untreated while the right side is OS-treated. The untreated section is a visibly darker
hue, owing to previous saturation as was confirmed on-site.

Ponded water on OS-treated compacted fly ash

Figure 3. Development of hydrophobicity on OS-treated flat section of field site (typical)
As noted17 and similarly presented19, the polynomial used to fit the infiltration data
collected for the untreated sections is of the form:

5

 Untreated Section

Treated Section

Figure 4. Example of slope after precipitation event – right side remained dry, as
evidenced in part by a lighter color (from top to bottom) and verified on-site.
I = C 1t + C 2 t

(1)

In Eq. 1, I is the cumulative infiltration, C1 is related to the hydraulic conductivity, C2 is
related to the soil sorptivity at t is the elapsed time. Hydraulic conductivity, k, is then
given by:
k=

C1
A

(2)

In Eq. 2, A is computed for a given level of suction pressure and soil properties17. In
particular, it is related to van Genuchten parameters, as tabulated separately20.
Accordingly, a value of A equal to 8.1 was selected, which approximates a fly ash
(analogous to a silt loam) at the -0.5 cm suction applied by the infiltrometer to measure
hydraulic conductivity. Note that the value computed by Eq. (2) is the corresponding in
situ hydraulic conductivity at the prevailing moisture content and level of suction. No
infiltration was recorded for the treated material, which translates to a hydraulic
conductivity of zero at this level of suction. Table 2 provides the values of C1, C2 (from
trendlines plotted in Fig. 2), A and the computed value for hydraulic conductivity, k, for
the laboratory samples and field test sections.

6

 Table 2. Infiltrometer data and computed values of unsaturated hydraulic conductivity

Description
Lab Sample
(Untreated)
Lab Sample
(OS-treated)
Flat Section-Field
(Untreated)
Flat Section-Field
(Treated)
Slope Section-Field
(Untreated)
Slope Section-Field
(Treated)

C1

C2

A

k (cm/s)

0.0013 0.1796 8.1 1.60x10-4
0

0

8.1

0

0.0104 0.0608 8.1 1.28x10-3
0

0

8.1

0

0.0106 0.0739 8.1 1.31x10-3
0

0

8.1

0

Note that the observed values of saturated and unsaturated hydraulic conductivity are
about the same (1.40x10-4 cm/s vs. 1.60x10-4 cm/s) for the untreated ash. As such, it's
worth ruminating as to why OS treatment resulted in slight reductions in terms of
saturated hydraulic conductivity (1.40x10-4 cm/s to 5.50 x 10-5 cm/s) while the influence
was far more dramatic in the case of infiltration-derived values of unsaturated hydraulic
conductivity (1.60x10-4 cm/s to 0 cm/s). These results illustrate a vital point as it relates
to determining the value of hydraulic conductivity in the laboratory, versus what may
realistically develop in the field. In particular, laboratory tests commonly ensure
saturation through back pressurization, as was likewise reported herein (345 kPa).
Indeed the hydraulic conductivity of a soil is at a maximum in the saturated condition,
which with time, can develop in natural field conditions (e.g., near or below the
groundwater table, landfill liners with sufficient head, etc.). However, it does not appear
that ash treated with OS can be saturated in the absence of a considerable amount of
artificial pressure, as similarly suggested19 and reiterated herein. Soil-water or ashwater interaction is more often characterized by a contact angle of approximately zero,
implying near perfect wetting21. However, the contact angle is likely greater than 90°
after OS-modification9. As such, it may be instructive to compare water/OS-modified ash
matrix with a Mercury/unmodified ash matrix. The latter scenario occurs when using the
mercury intrusion method to force mercury into porous media22. The pressure required
for a given level of penetration varies according to pore size and so one can determine
the prevailing pore size distribution. By analogy, water only penetrates OS-treated ash
provided sufficient pressure is available. This characteristic emphasizes the need to
carefully match laboratory testing against anticipated field conditions.
CONCLUSIONS
OS modification represents another approach to ash improvement, with possible
increases in strength and reductions in swell potential and hydraulic conductivity. More
dramatic results were observed for in situ measurements of infiltration. In particular, the
hydraulic conductivity, as measured with a negative pressure of -0.5 cm, was reduced
7

 from the range of 10-4 to 10-3 cm/s, for a laboratory sample and field sections to 0 cm/s,
upon treatment with OS. Concatenating with previous results for reducing leachability,
the data suggests that OS modification may have wide application in the management
of coal combustion products, along with geotechnical applications for soils.
REFERENCES
[1] Davidson, D.T., Sheeler, J.B. and Delbridge, N.G. Jr. (1958) “Reactivity of four types
of fly ash with Lime” HRB Bulletin 193, pp. 24-31.
[2] Usmen, M.A. and Bowders, J.J. (1990). “Stabilization characteristics of Class F fly
ash” Transportation Research Record No. 1288, National Research Council.
Washington, D.C., pp. 59-69.
[3] Daniels, J.L. and Das, G.P. (2006) “Leaching Behavior of Lime-Fly Ash Mixtures”
Journal of Environmental Engineering Science Vol. 23, No. 1, pp. 42-52.
[4] MacMurdo, F.D. and Barenberg, E.J. (1973) “Determination of realistic cutoff dates
for late-season construction with lime-fly ash and lime-cement-fly ash mixtures”
Highway Research Record No. 442, pp. 92-101.
[5] Daniels, J.L. and Das, G.P. (2008) “Field scale characterization of coal combustion
fly ash stabilized with lime and FGD gypsum” GeoCongress 2008, Geotechnical Special
Publication Number 177, pp. 684-691. ASCE, Reston, VA.
[6] Sanusi, O., Tempest, B., Ogunro, V.O., Gergely, J., Daniels, J.L. (2009) “Effect of
Unreacted Hydroxyl Ion on Release of Trace Metals from Geopolymer Concrete” World
of Coal Ash, Lexington, KY.
[7] Bowders, J.J., Gidley, J.S. and Usmen, M.A. (1990). “Permeability and leachate
characteristics of stabilized Class F fly ash” Transportation Research Record No. 1288,
National Research Council. Washington, D.C., pp. 70-77.
[8] Khan, L.I. and Sarker, M. (1993). “Enzyme enhanced stabilization of soil and fly ash”
Fly Ash for Soil Improvement, ASCE Geotechnical Special Publication No. 36, K. Sharp,
Editor, New York, pp. 43-57.
[9] Daniels, J.L., Mehta, P., Vaden, M., Sweem, D., Mason, M.D., Zavareh, M. and
Ogunro, V.O. (2009) “Nano-scale organo-silane applications in geotechnical and
geoenvironmental engineering” Journal of Terraspace Science and Engineering 1(1),
21-30.
[10] Dean, K.M., Bateman, S.A. and Simons, R. (2007). “A comparative study of UV
active silane-grafted and ion-exchanged organo-clay for application in photocurable
urethane acrylate nano- and micro-composites.” Polymer, 48, 2231-2240.

8

 [11] Tartaglione, G., Tabuani, D. and Camino, G. (2008) “Thermal and morphological
characterization of organically modified sepiolite” MicroPorous and MesoPorous
Materials, 107, 161-168.
[12] Xue, S. and Pinnavaia, T.J. (2008) “Porous synthetic smectic clay for the
reinforcement of epoxy polymers” MicroPorous and MesoPorous Materials, 107, 134140.
[13] Pellenq, R.J.-M., Lequeux, N. and van Damme H. (2007). “Engineering the bonding
scheme in C-S-H: the iono-covalent framework” Cement and Concrete Research, 38,
159-174.
[14] ASTM D698 Standard Test Methods for Laboratory Compaction Characteristics of
Soil Using Standard Effort (12 400 ft-lbf/ft3 (600 kN-m/m3)), ASTM International, West
Conshohocken, PA
[15] ASTM D1883 Standard Test Method for CBR (California Bearing Ratio) of
Laboratory-Compacted Soils, ASTM International, West Conshohocken, PA
[16] ASTM D5084 Standard Test Methods for Measurement of Hydraulic Conductivity of
Saturated Porous Materials Using a Flexible Wall Permeameter, ASTM International,
West Conshohocken, PA
[17] Decagon (2007). Mini Disk Infiltrometer, User's Manual for Version 5, Decagon
Devices, Inc. Pullman, WA.
[18] Zhang, R. (1997). “Determination of soil sorptivity and hydraulic conductivity from
the disk infiltrometer” Soil Science Society of America Journal, 61, 1024-1030.
[19] Daniels, J.L. and Hourani, M.S. (2009). “Soil improvement with organo-silane” U.S.China Workshop held in conjunction with the International Foundations Congress and
Equipment Expo ‘09 (IFCEE09), ASCE, Reston/VA.
[20] Carsel, R. F. and Parrish, R.S. (1988). “Developing joint probability distributions of
soil water retention characteristics” Water Resources Research 24: 755-769.
[21] Fang, H.Y. and Daniels, J.L. (2006) Introductory Geotechnical Engineering: An
Environmental Perspective, Taylor and Francis, London, 545 p.
[22] Mitchell, J.K. and Soga, K. (2005) Fundamentals of Soil Behavior, Third edition,
John Wiley & Sons, New York, 577 p.

9