Technical Papers and Articles
Guidelines for Stabilization of Soils Containing Sulfates
1986, Jim Mitchell, professor of civil engineering at the University of
California at Berkeley, presented a paper in the Terzaghi Lecture
Series published by the American Society of Civil Engineers (ASCE).
This paper addressed several interesting and rather unique geotechnical
engineering problems. One of these problems was the Stewart Avenue
pavement failure in Las Vegas, Nevada.
A paper by Dal Hunter
followed Mitchell’s report and also addressed the Stewart Avenue
failure but with a more complete description of the chemical and
mineralogical aspects. Sulfate induced problems in soils stabilized
with calcium-based stabilizers such as lime, portland cement, and fly
ash have been documented since the late 1950's. The mechanism has been
studied by a number of highly qualified cement chemists in an effort to
understand and control sulfate attacks on portland cement concrete
Basic Mechanisms of Reactions
in-depth discussion of the complex reactions of sulfate-induced
distress in stabilized soils is not within the scope of this technical
memorandum. However, it is important to understand the fundamentals of
Basically four components are the
culprits in sulfate-induced distress in stabilized soils: calcium,
aluminum, water, and sulfates. Together in the right combination these
components will produce calcium-aluminate-sulfate-hydrate minerals with
very large expansion potential, in some cases as high as 250%. One of
these minerals is called ettringite. This mineral holds very large
quantities of water within its structure. During the formation of
ettringite, very high swell pressures can develop, and very large
volume increases can and do occur.
The formation of ettringite
and similar troublesome minerals can be prevented by interrupting the
supply of any one of the four components: calcium, aluminum, water, or
sulfate. When lime and water for construction are added to clay, the
calcium is supplied by the lime, and the aluminum is released from the
clay in the high pH system produced by lime and water. If the soil
contains a high sulfate concentration in the form of gypsum, for
example, all the ingredients with the exception of water are present
for the formation of the expansive minerals. Using a low aluminate
portland cement (such as type V, sulfate-resistant cement) does not
solve the problem because the source of the aluminum is not entirely
the portland cement but the soil.
There is no easy answer to the
problem. Calcium is present when either lime or portland cement are
used for soil stabilization. Soils containing clay are rich with
aluminum, a basic structural unit of clay. Water is necessary for
compaction and for stabilization reactions and is present within
pavement structures during their service life. Unfortunately, the
sulfates cannot be efficiently or economically removed from the soil.
Factors Affecting the Reactions
number of efforts have been made to control the reactions that result
in the formation of the problematic expansive minerals. Some of these
efforts have been successful, but others have not. Some are successful
but economically impractical.
Presently, the best approach when
dealing with lime stabilization of clay with a significant soluble
sulfate content is to force the formation of the deleterious minerals
prior to compaction. If these minerals form during the mellowing period
before placement and compaction, no damage will be done to the
pavement. Fortunately, the expansive minerals do form relatively
rapidly as long as the sulfates are soluble, the aluminum is released
from the clay and adequate water is available for the formation of the
minerals. The keys to success are to force the expansive mineral
ettringite to form prior to placement and compaction of the pavement
layer by providing adequate mellowing time (time delay between
application of the stabilizer and compaction of the stabilized soil)
and adequate water.
Adequate mellowing time may be as little as
24 hours or as much as 7-days, depending on the level of soluble
sulfates in the soil. An adequate amount of water is typically 3 to 5
percentage points above the optimum needed to achieve maximum density
during compaction. Excess water should be applied during the mellowing
period, and plentiful amounts of water should be applied to the surface
of the stabilized layer during curing.
Water is the most important component of the equation. Adequate water must be
supplied throughout the stabilization construction process to force
formation of the ettringite prior to compaction. The worst scenario
would be to compact a lime-treated sulfate-bearing clay with too little
water. This is especially a problem if quicklime is used, and too
little water is used to completely hydrate the quicklime. If this were
the case, water entering the soil subsequent to compaction would cause
development of expansive minerals in the compacted layer and produce
very high and very disruptive expansive pressures. For this reason use of lime slurry is always recommended in stabilization of sulfate-bearing clays. Lime slurry provides the abundance of water and uniformity of hydration
required to lower risk. Remember, quicklime was used at Stewart Avenue,
and forensic studies showed inadequate water and poor construction
techniques in many areas. The result was post-construction heave when
water ultimately reached the quicklime causing hydration of the
quicklime and the ensuing expansive chemical reactions.
Guidelines for Using Lime in Sulfate Bearing Soils
an effort to assist you in recommending lime stabilization in
sulfate-bearing clays, the following general recommendations are made:
Sulfate Levels Too Low to be of Concern
the total level of soluble sulfates is below 0.3%, or 3,000 parts per
million (ppm), by weight of soil, then lime stabilization should not be
of significant concern. The potential for a harmful reaction is low.
However, good mix design and construction practices should be followed
as usual. If soluble sulfates are detectable at all, lime slurry should
be used in lieu of dry lime and adequate water (optimum for compaction
plus at least 3%) should be used for mixing.
Sulfate Levels of Moderate Risk
soluble sulfate levels of between 0.3% (3,000 ppm) and 0.5% (5,000 ppm)
are of moderate concern. Generally, these sulfate levels do not result
in harmful disruption, but on occasion have caused localized distress.
Localized distress is often due to seams of higher sulfate
concentration not detected in testing. The potential for some localized
distress is a “fact of life” with sulfate levels in this range.
encountering sulfate levels in the range of 0.3% to 0.5%, it is
imperative to follow good mix design and good construction techniques
explicitly. Special attention must be given to using excess water
during mixing, mellowing, and curing. Mixing water should be at least
3% to 5% above optimum for compaction. Lime slurry must be used in lieu
of dry quicklime or hydrated lime.
The mellowing period should typically be at least 72-hour, but may need to be longer depending on experience.
Sulfate Levels of Moderate to High Risk
soluble sulfate levels between 0.5% (5,000 ppm) and 0.8% (8,000 ppm)
represent moderate to high risk. These soils can and have been
successfully treated but require very close attention to construction
technique. Generally, the same mix design and construction guidelines
as described for soils containing sulfate levels between 0.3% and 0.5%
should be followed. However, before treating these soils with lime,
laboratory testing to determine swell potential is recommended. This
testing will not only establish the approximate amount of swell but
also will help establish the required period of mellowing between
mixing and compaction.
Sulfate Levels of High and Unacceptable Risk
soluble sulfate levels of greater than 0.8% (8,000 ppm) are generally
of high risk to stabilize with lime. In certain situations, such soils
have been successfully treated. However, the risk is generally too high
for routine work. If such soils are to be treated, it should only be
done following laboratory testing and by an experienced contractor,
well-schooled in lime stabilization of high sulfate soils.
of such high sulfate soils requires lime slurry, mixing, mellowing,
curing water contents of 3% to 5% above optimum for compaction, and may
require an extended mellowing period of longer than 72-hours. The
required mellowing period may be as long as 7-days during which
monitoring of density is recommended. Double application techniques
(discussed below) may be effective in successfully treating high
Soils with total soluble sulfate contents greater
than 1.0% (10,000 ppm) generally are not suitable for lime
stabilization because of the high risk of sulfate-induced disruption
Double Application of Lime
situations, a double application of lime is effective in reducing heave
potential and in providing successful long-term stabilization. Double
mixing is obviously more expensive, and, therefore, must be
cost-effective. Double mixing uses one-half the required lime
initially. The soil, excess water, and lime are then mixed followed by
a mellowing period of from 72-hours to about 7-days. The purpose of the
long mellowing period is to allow time for expansive reactions prior to
compaction. Then the second lime treatment is applied (the other half
of the required lime is used). The lime-soil mixture is then compacted.
Double treatment does not mean twice the amount of lime. It means that
the same amount of lime is added in two increments. This technique
should be thoroughly evaluated through laboratory testing of
site-specific soils to establish appropriate lime application amounts,
mellowing times, etc. before proceeding with field construction.
Determining Whether or Not Sulfates May Be of Concern
only “fool proof” way to know whether or not sulfates will be a problem
is to test the soil for sulfates. This is done by sampling the soil at
enough locations and at the appropriate depths to reasonably assess the
level and extent of sulfates.
Quantitative sulfate testing
requires the extraction of sulfates from the soil. This is done by
solubilizing the sulfates in water, followed by quantitative
measurement. Since sulfate salts, such as gypsum (calcium sulfate),
have a specific level of solubility, the amount of sulfate extracted
from the soil is determined by the type of sulfates present and amount
of water added. Therefore, ten parts water to one part soil will result
in more solubilized sulfates than three parts water to one part soil,
especially at higher sulfate contents. Experience has shown that an
extraction protocol using ten parts water to one part soil is the best
for evaluating potential problems resulting from sulfate reactions.
This also allows better comparison with most of the test data developed
in related research efforts to date. Note that the sulfate levels
and associated treatment guidelines provided in this document are based
on the ten parts water to one part soil testing ratio and may not be
applicable to other water:soil ratios.
Sulfates soluble in
water are measured in parts per million (ppm) and often expressed
either in ppm or percent. 10,000 ppm are equivalent to 1.0%. Therefore,
3,000 ppm are equivalent to 0.3% and 5,000 ppm to 0.5%, etc. The
soluble sulfate content should be reported on a dry soil basis to insure consistency of test results. Soluble sulfates should be
extracted from the soil using ten parts distilled water to one part
soil. Test method Tex-620-J (Appendix A) prepared by the Texas
Department of Transportation is recommended. Any of several
quantitative methods (barium precipitation, ion chromatography, etc.)
may be used effectively to measure the water solubilized sulfates.
Again, the important thing to remember is that the water:soil ratio
used in preparation of the solution will control the amount of sulfates
solubilized and measured by any of these methods, and that the
guidelines presented here are based on 10:1 extractions.
testing for sulfates, it is important to remember that sulfates often
are present in concentrated areas and may not be distributed uniformly.
Seams or veins of sulfates are common. It is also important to realize
that sulfates tend to concentrate at a certain depth below the surface
of the soil. This depth of concentration is dependent on the climatic
conditions of the area or region. In Texas, this depth is often three
to six feet (about one to two meters) below the surface.
typically are concentrated nearer the surface in drier, western
regions. As we move eastward into wetter and more humid climates, the
general rule is that sulfates, if present, tend to concentrate at lower
Probably the most beneficial and reliable tool for
assessing the presence and significance of sulfates within an area is
the United States Department of Agriculture’s County Soils Report. A
report is available for every county in the United States and can be
obtained from the Soil Conservation Service, a County Agent or the
State land grant university. The soils report provides an abundance of
engineering information conveniently tabulated. There is also a
discussion of each soil series within the county and a discussion of
the soil profile. This discussion will generally identify the presence
of gypsum and other sulfate salts and the depth of significant
concentrations, if any. This is an extremely valuable investigative
tool. Keep in mind that it is very important not only to identify the
presence of sulfates but also the depth of occurrence. For example, a
soil may be essentially sulfate free in the upper two or three feet
(0.67 to 1.0 meters) but have sulfate concentrations at a depth of 6
feet (approximately 2 meters). In this case, sulfates would not be of
concern during normal surface stabilization operations but could be of
concern in cut and fill areas.
Required Testing and Frequency of Testing
best approach in checking for sulfates is to ask the county agent where
sulfates typically occur and at what depth to expect significant
concentrations. It is also wise to buy or check out a County Soil
Report. You can locate the construction job of interest to you on the
aerial photographs of the county in the back of the report. From these
photos the soil series in the area can be identified. Pertinent
information on each soil series in presented in the discussion section
and in the tabulated agricultural and engineering data for each soil.
sulfates are present and identified in the County Soils Report, a field
testing plan should be established with the geotechnical engineer. The
frequency of testing depends on the level of sulfates present and the
geological information for the region.
If total soluble sulfate
levels are above 0.5%, tests to determine the degree of expansion that
may occur should be performed. These tests require monitoring the
vertical and circumferential swell on compacted lime-soil cylinders
(see Appendix B). The cylinders are subjected to water by placing them
on porous stones, surrounding them with absorptive towels, and allowing
the sample to take on water for at least 30 days or until swell levels
off. The measured circumferential and vertical swells are then compared
to criteria established by the engineer. If total soluble sulfate
levels exceed 0.8%, this type of testing should be mandatory.
Addressing and Countering Inaccurate and Misleading Assertions
Probably the most common misconception is that lime is the only stabilizer that causes sulfate-induced heave. The fact is that any calcium-based stabilizer has the potential to
cause heave in sulfate-bearing soils. Not only lime but also portland
cement and type C fly ash are sources of calcium. In fact the Portland
Cement Association (PCA) promotes the concept that lime results from
the hydration of portland cement and is available for soil
stabilization. Many cases have been documented of sulfate-induced heave
or damage in cement- and fly ash-stabilized soils. Indeed some fly
ashes high in sulfates have been the source of the distress.
Another common assertion is that sulfate
resistant portand cement can be used to effectively stabilize
sulfate-bearing clays without the fear of deleterious reactions. This claim is not true. Sulfate resistant portland cement was developed
to resist the attack of sulfate-bearing water on concrete.
Sulfate-bearing water will react with calcium and aluminum in the
concrete to form the expansive ettringite mineral in the hardened
concrete causing cracking and degradation of the concrete. Cement
chemistry researchers found low-aluminum cement to be effective in
reducing the expansive reaction. This is logical as one of the
components of ettringite—aluminum—has been reduced.
this approach does not work in soil stabilization because clay is a
source of abundant quantities of aluminum. Therefore, using a low
aluminum cement is a mute point.
An assertion of some credibility is that low calcium fly ashes will minimize heave potential. The problem with this statement is that low calcium ashes are low in
the component that is the key to stabilization of clay soils—available
calcium. Low calcium fly ash is primarily a pozzolan—a finely divided
source of silicates and aluminates that has the potential to develop
cementitious properties in the presence of water and lime. Clay is also
a pozzolan. Therefore, adding pozzolans to pozzolans without the key
ingredient, calcium, is poor engineering judgement. In other words,
adding low calcium ash to a clay may not induce heave, but neither is
it an effective stabilizer of the clay.
Appendix A: Texas Department of Transportation
Determining Chloride and Sulfate Content in Soils
Test Method TEX-620-J
This method describes how to determine the chloride and sulfate content in soil.
- Balance, calibrated to weigh to nearest 0.1 g (0.004 oz.)
- Balance, calibrated to weigh to nearest 0.0005 g (0.00002 oz.)
- Sieves, U.S. Standard 4.75 mm (No. 4) and 425 mm (No. 40)
- Pulverizer and Crusher
- Oven, capable of maintaining a temperature of 60 + 5°C (140 + 9°F)
- Beakers - 400 mL (13.5 oz.)
- Stirring rod
- Hot Plate
- Whatman #42 filter paper, 185 mm (7.4 in.) (round)
- Wash bottle
- Volumetric Flask - 500 mL (15 oz.)
- Dilute Silver Nitrate Solution
1) Obtain 300 g (10.5 oz.) representative sample when material top size is smaller than 4.75 mm (No. 4).
2) Pulverize the 300 g (10.5 oz.) to pass the 425 mm (No. 40) sieve.
3) Weigh to the nearest 0.1 g (0.004 oz.)
If material top size is larger than 4.75 mm (No. 4), obtain
approximately 3000 g (105 oz.) representative sample and crush/grind to
pass the 4.75 mm (No. 4) sieve.
5) Obtain 300 g (10.5 oz.) representative sample of the minus 4.75 mm (No. 4) material.
6) Pulverize the 300 g (10.5 oz.) to pass the 425mm (No. 40) sieve.
7) Weigh to the nearest 0.1 g (0.004 oz.).
8) Dry the sample in a 60 + 5°C (140 + 9°F) oven and cool to 25 + 3°C (77 + 5°F) in a desiccator to constant weight.
Weigh 30 g (1.05 oz.) of the sample material into a 400 mL (14 oz.)
tall form beaker and add 300 mL (10.5 oz.) of deionized water.
2) Place the beaker on a hot plate and heat to near boiling for 24 hours.
3) Stir the sample into solution occasionally throughout the 24 hours and keep the beaker covered with a watch glass.
At the end of the 24 hour digestion period filter the sample through a
No. 42 Whatman filter and wash with hot water until filtrate is free of
NOTE: Test the filtrate for chloride by adding one to
two drops of filtrate from the funnel to a dilute silver nitrate
solution. Any turbidity indicates chlorides present.
5) Pipette an Aliquot from the filtrate and determine the sulfate and chloride content according to Tex-619-J.
6) Calculate the sulfate and chloride contents:
Analysis of Water for Chloride and Sulfate
Test Method TEX-619-J
method covers the calculation of chloride and sulfate ions in water to
determine its suitability for concrete, sprinkling or similar uses.
Interferences and methods of treating them may be found in ASTM D512
(Method B) for chloride and ASTM D516 (Method A) for sulfate.
- Muffle furnace, 427 to 593°C (800 to 1,100°F)
- Oven, 100°C (212°F)
- Balance, analytical
- Magnetic stirrer
- Hot Plate
- Meeker burner
- Filter papers, No. 2 and No. 42 Whatman or equals
- pH paper, range 8 to 9
- Platinum crucible
- Volumetric flask 500 mL (15 oz.)
- Beaker, 200 mL (6 oz.) tall form
- Beaker, 250 mL (7.5 oz)
- Buret, 50 mL (1.5 oz.)
- Graduated cylinder, 25 mL (0.75 oz.)
- Filter Funnel
- Wash Bottle
All reagents must be ACS reagent grade. Use deionized or distilled water to prepare solutions.
- Silver Nitrate Solution, 0.1 Normal. Standardize against a sodium chloride solution of known concentration
- Potassium Chromate Indicator Solution. Dissolve 50 g (1.75 oz.) of potassium chromate (KCrO) in 100 mL (3 oz.) of water
- Nitric Acid (1 + 19). Mix 1 volume of concentrated nitric acid (70% by wt) with 19 volumes of water
Sodium Hydroxide Solution (1 g/l). Dissolve 1 g (0.03 oz.) of sodium
hydroxide (NaOH, pellet form ) in water and dilute to 100 mL (3 oz.)
- Barium Chloride Solution, 10%. Dissolve 10 g (0.03 oz.) of barium chloride (BaCl22H2O) in 90 mL (2.7 oz.) of water
- Hydrochloric Acid. Concentrated hydrochloric acid (HCL) (37% by wt.)
Procedure - Sample Preparation
Filter 500 mL (15 oz.) of the as-received water sample into a 500 mL
(15 oz.) volumetric flask using a No. 2 Whatman filter paper.
2) Weigh 50 g (1.75 oz.), to the nearest milligram, of the filtered sample into a 200 mL
(6 oz.) tall bottom beaker.
3) Adjust the sample pH to between 8 and 9 using nitric acid or sodium hydroxide solution.
4) Add 11 drops of the potassium chromate indicator and titrate using the silver nitrate solution in a 25 mL (0.75 oz.) buret.
5) The end-point is reached when a brick red color persists throughout the sample.
6) Determine the chloride ion concentration (weight percent) as follows: % Chloride = 3.545 VN/S
V = mL of silver nitrate solution
N = normality of silver nitrate solution
S = sample weight, grams (ounces)
Sulfate Ion Determination
1) Weigh 80 g (2.8 oz.), to the nearest milligram, of the filtered sample into a 250 mL (7.5 oz.) beaker.
2) Add 10 mL (0.34 oz.) of concentrated hydrochloric acid to the sample.
3) Heat to near boiling.
4) Add 25 mL (0.75 oz.) of barium chloride solution and heat again for 10 minutes.
5) Remove from the hot plate and let cool for 15 minutes.
Filter through a No. 42 filter paper and wash the precipitate with hot
water until the washings are free of chlorides, as indicated by testing
the washings with silver nitrate.
7) Place the filter paper and precipitate in a weighed platinum crucible and dry in a 100°C (212°F) oven for 1 hour.
8) Remove crucible from oven and slowly char the paper to a white ash using a Meeker burner.
9) Place the crucible and residue into the muffle furnace for 1 hour.
10) Cool in a desiccator and weigh.
11) Determine the concentration of sulfate: % Sulfate = 41.15 R/S
R = residue weight, grams (ounces)
S = sample weight, grams (ounces)
Appendix B Simplified Swell Test
1. Compaction mold and compaction equipment to meet ASTM D 698 or D 1557.
2. Calipers to measure vertical height and diametral width of the compacted sample.
3. Porous stone, water reservoir, and absorbent fabric to transmit water to soil.
Compact two replicate samples at 100% and 95% of the required
compaction energies (i.e., ASTM D 698 or ASTM D 1557 or Tex-113a).
Immediately after compaction, place the sample on a porous stone in a
water reservoir with the water level even with the top of the porous
3. Place an absorbent fabric around the circumference of
the samples with the bottom 25-mm of the fabric below the top of the
water reservoir so that the fabric can “wick” water to the
circumference of the sample. The samples are placed in a 25°C
temperature environment for 7-days. Vertical height and diametral
dimensions are recorded at the end of each day. Six diametral
measurements are made: two at 25-mm below the surface, two at the
mid-height and two at 25-mm above the base. The diametral dimensions
are recorded at approximately 90° from one another. Measure the
vertical height at two random points.
4. After 7-days the
samples are carefully placed in a 12°C (plus or minus one degree)
temperature room for an additional 21-days or until swell stops.
Calculations and Presentation of Data
Calculate the average of the diametral dimension measurements (average
of six) after sample fabrication and the average of the vertical height
after sample fabrication. Use these values as datum values.
Calculate average diametral and vertical measurements at the end of
each day. Calculate the percentage of change in diametral measurement
as the ratio of the average diametral measurement divided by the
diametral datum multiplied by 100%. Calculate the percentage change in
vertical measurement as the ratio of the average vertical height
divided to the vertical height datum multiplied by 100%.
the data as % diametral and vertical dimension (ordinate) change versus
time (abscissa) for a 28-day period or until swell ceases to occur.