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Sulfates in Soil, Part IV

Feb 24

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HOW TO PREVENT FAILURE

Known Sulfate Counties

The Oklahoma Department of Transportation (ODOT) and the Texas Department of Transportation (TexDOT) keep a record of known sulfate counties in their respective states. 

These are counties where sulfates have been verified through laboratory testing and/or sulfate heave has been a problem in the past.  These maps of known sulfate occurrences are helpful for the government agencies and consultants who may have projects in the areas.  If sulfates have been found in or near their project area, sulfate testing and/or preventative measures in design can be taken.

The sulfates counties for Oklahoma and Texas are shown below, courtesy of ODOT and TexDOT (Harris, Sebesta, Scullion, 2004).

 

 Sulfate Counties in Oklahoma

Oklahoma Sulfate Counties

  

            

Sulfate Counties in Texas

Texas Sulfate Counties

Sulfates in Soil, Part III

Feb 5

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In Part II I talked about the soil stabilization chemicals which react with sulfate-rich soils.  This section covers what happens when the sulfate soil, chemicals and water react.

SULFATE INDUCED HEAVE

Effects on Soil

There is more than one potential heave mechanism by which sulfate-induced heave may occur.  The two most discussed mechanisms are through the oxidation of sulfide minerals to form gypsum and the formation of the mineral ettringite (Harris, Sebesta, Scullion, 2004), and possibly of the mineral thaumasite.

The first mechanism is the oxidation of sulfide minerals to form gypsum.  Sulfides such as pyrite and marcasite are created in oxygen deficient environments.  When these sulfides are exposed to oxygen, such as when the soil they are in is excavated, they become unstable.  When these sulfides are exposed, the oxygen acts as an oxidizing agent and surface water oxidizes them.  This reaction causes the soil to become very acidic, which helps to dissolve any limestone that may be nearby.  The dissolution of limestone supplies calcium which can combine with sulfates and water to form gypsum.  The oxidation of sulfides and formation of gypsum can produce significant amounts of distress, but when the gypsum rich soils are treated with calcium based stabilizers, as discussed next, the amount of heave is greatly multiplied (Harris, Sebesta, Scullion, 2004).   

The formation of ettringite is a bit more complex, in that more precise conditions have to be in place for the mineral to form.  In the formation of ettringite, sulfate ions combine with calcium from lime or cement stabilizers, aluminum from stabilizers and/or the clay minerals in the soil and water to generate sulfate heave.  Ettringite precipitates in environments with high pH and high sulfate activity.  This is a common scenario when a calcium-based stabilizer is added to sulfate rich soils.  For example, at standard temperature, the pH has to be above 10 and a water source has to be present.   When sulfate rich clay soils are treated with lime or cement based stabilizers, the pH rises to above 12 and water is supplied during the stabilization process (Harris, Sebesta, Scullion, 2004).  When all of the ingredients are present, they form highly expansive sulfate minerals such as ettringite and thaumasite. 

Ettringite is also known as hydrated calcium aluminum sulfate hydroxide and is known to swell up to 250 percent of its original size when water is present.  Four out of every five atoms in both ettringite and thaumasite minerals are either a part of a water molecule or a hydroxide, which is what gives them the ability to swell to such a large extent (www.Galleries.com, 2007).   Ettringite damages the soil structure by expanding as it precipitates, resulting in expansion of the soil (Little, Herbert, Kungalli, 2005).  The amount of sulfates present in the soil determines the amount of ettringite that can be formed.  The more sulfates that are present, the more ettringite that can be formed (Ferris, Eades, Graves, McClellan, 1991).

Thaumasite is a mineral that forms in addition to ettringite and is also known as hydrated calcium silicon carbonate sulfate hydroxide.  Thaumasite has an extremely high swelling potential as well, but only forms at low temperatures.  If soluble silica and carbonate are present during the isostructural transformation of ettringite at temperatures below approximately 59° F (15° C), thaumasite can form.  While ettringite formation results in expansion of the soil, thaumasite formation results in a loss of strength of the soil (Little, Herbert, Kunagalli, 2005).

In addition to the amount of sulfates in a soil, the amount and type of clay in a soil also contributes to the extent of sulfate mineral formation and associated heave (Ferris, Eades, Graves, McClellan, 1991).  The addition of lime to sulfate rich soils provides the calcium that reacts with aluminum from stabilizers and/or the clay minerals in the soil and water to form ettringite and to generate sulfate heave.  If more calcium from stabilization chemicals is available to the sulfate rich soil for reaction, then more ettringite can be formed.  Therefore, soils with a high clay content, or higher plasticity clays, which require a higher percentage of lime, cement, flyash or CKD for stabilization, can result in greater amounts of heave (Ferris, Eades, Graves, McLellan, 1991).

The type of clay is believed to play a role in the amount of heave in sulfate rich soils.  Smectites are very expansive clays consisting of three layers.  Clays containing high levels of smectite will require increased amounts of stabilization chemical to become stabilized (Ferris, Eades, Graves, McLellan, 1991), much as described above with high plasticity clays.    Kaolinite is another type of clay mineral.  Kaolinite has only two layers, but the lesser amount of layers may help it to provide more aluminum from its structure to help form ettringite and produce greater amounts of sulfate induced heave (Ferris, Eades, Graves, McLellan, 1991).

Sulfates in Soil, Part II

Oct 22

Posted by kkb in Sulfates, Uncategorized | Comments off

Last week was the first post in a series about sulfates in soils.  This week I will talk about soil stabilization chemicals and their relationship to sulfate soils.

SOIL STABILIZATION CHEMICALS

Chemical modification of soil involves mixing soil with a chemical additive such as lime, Portland cement, flyash, cement kiln dust, or a combination of any of the four.  Soils treated with chemicals may expand to an even greater extent if chemical modification is used in soils which contain high levels of sulfates, organic matter, or salts. 

 

Lime

Lime is the most common chemical additive for treating highly plastic clay soils.    When treating soils, lime can be used in two different forms: quicklime or hydrated lime. 

Quicklime, or calcium oxide, is the most reactive form of lime.  In the presence of moisture, it is corrosive to equipment and to human skin.  When mixed with water it produces heat and swells.  Lime mixed with water is hydrated lime, also known as slaked lime.  Slaked lime comes as a fine powder or mixed in a slurry, where quicklime is coarse powder (Hausmann, 1990).  Quicklime is more cost-effective because it takes less to react with the soil, but slaked lime is often used instead.  More and more communities in Oklahoma have regulations against using quicklime because of its corrosive nature and ability to travel in windy conditions. 

Quicklime reacts with moisture in the clay soils and generates heat and expands, consolidating the soil.  This is especially effective when the lime is injected or placed in layers, as opposed to simple soil mixing (Hausmann, 1990).

Slaked lime, used in a slurry, can be more costly to transport, but is safer for the contractors, equipment, and persons down-wind of the project.

In either the quicklime or slaked form, when lime is mixed with clay, two pozzolanic chemical reactions occur.  These reactions are cation exchange and flocculation-agglomeration.  In the cation exchange reaction, calcium ions are exchanged with the adsorbed cations attached to the montmorillonite surfaces.  The flocculation-agglomeration reaction causes the clay particles to flocculate and agglomerate into large clumps.  This decreases the liquid limit and increases the plastic limit, which decreases the plasticity index (Das, 2004).  The strength, shrinkage limit and workability of the soil are also increased.   In addition to decreasing the plasticity index, increasing the shrinkage limit, increasing the workability and improving the strength of high plasticity index soils, lime treatment also increases a soil’s permeability and optimum water content and decreases its dry density (Hausmann, 1990). 

Cation exchange and flocculation-agglomeration are immediate reactions when soil is mixed with lime.  A secondary reaction causes cementation of the soil by removing the silica and alumina from the clay’s crystal lattice structure.  This increases the strength of the clay soil, both immediately and long-term (Hausmann, 1990). 

Lime stabilization is the most common form of chemical stabilization for high plasticity index soils.  Lime can be used when sulfates are present in low levels and at higher levels if the conditions are tested and monitored correctly.

Portland Cement

Portland cement is the most commonly used additive for general soil stabilization.  Cement stabilization is effective for a wide variety of soil types, including expansive clays, but is usually used to increase strength in soils.  Cement has the same effects on sulfate rich soils as lime does.

With cement stabilization of soils, cementation occurs between the soil and the calcium silicate and aluminate hydration products (Gromko, 1974).  Cement treatment of soils generally increases the maximum dry density and reduces the plasticity index of an expansive soil.  A small addition of cement to an expansive subgrade material can often reduce the shrink/swell potential below 1 percent (Hausmann, 1990). 

Cement stabilization is a viable option for treating expansive soils.  As with lime, it can be used when sulfates are present in low levels and at higher levels if the conditions are tested and monitored correctly.

Fly Ash

Fly ash is a waste product created by coal combustion.  It is a fine-grained dust primarily composed of silica, alumina, and various oxides and alkalies (Das, 2004). Fly ash is usually combined with lime to stabilize soils.  Therefore, it poses the same risk to sulfate rich soils as does lime and cement.

Class F fly ash is produced when anthracite or bituminous coal is burned.  It has pozzolanic properties and can react with hydrated lime to produce cementitious products (Hausmann, 1990).  Class C fly ash is produced when subbituminous or lignite coal is burned.  It contains up to 25 percent free lime, which means it can produce cementitious reactions without the addition of manufactured lime (Das, 2004).  Additive mixtures of fly ash and lime contain 10 to 35percent fly ash and 2 to 10 percent lime (Hausmann, 1990). 

As with lime and cement, flyash can be used when sulfates are present in low levels and at higher levels if the conditions are tested and monitored correctly.

Cement Kiln Dust

Cement kiln dust (CKD) is a byproduct of Portland cement rotary kiln operations.  It is a fine powder that is chemically similar to Portland cement.  It is used in the same fashion as Portland cement.

Because CKD has many of the same properties as Portland cement it too has negative effects on sulfate rich soil.  But as the other products, it can be used when sulfates are present in low levels and at higher levels if the conditions are tested and monitored correctly.

Sulfates in Soil

Oct 12

Posted by kkb in Sulfates | Comments off

INTRODUCTION 

Naturally sulfate rich soils can be found in the southern, western and southwestern regions of the United States.  Sulfate rich soils that are stabilized with calcium based chemicals can result in what is called sulfate-induced heave.  Calcium based stabilization chemicals include lime, cement, and their derivatives.

Lime, cement, flyash, or cement kiln dust can react with clay soil and the sulfates in the clay to form expandable minerals, such as ettringite, which can expand up to 250 percent of its original size when exposed to moisture.  This can create large hills of soil, destroying roadways and foundations.  Sulfate-induced heave causes millions of dollars in damage each year to roads, highways, runways, parking lots, buildings, and other earth structures created by treating sulfate rich soils with cement or lime products (Puppala, Griffin, Hoyos, Chomtid, 2004). 

Simple laboratory testing may be performed on soil samples to determine on-site sulfate levels.  With proper awareness and testing, sulfate-induced heave can be prevented.

Sulfates, stabilization chemicals, sulfate-induced heave, heave prevention, and alternative treatments will be discussed through a series of posts with an emphasis on Oklahoma and Texas.

 

SULFATES

What is a Sulfate?

Sulfates are salts of sulfuric acid.  The sulfate ion is an anion, SO4 2-.  It consists of one central sulfur atom surrounded by four equivalent oxygen atoms.  The sulfate ion carries a negative two charge and is the conjugate base of the hydrogen sulfate ion, HSO4–, which is the conjugate base of sulfuric acid, H2SO4.

 

3-D Rendering of a Sulfate Ion

3-D Rendering of a Sulfate Ion

 

Sulfate compounds are created when cations, such as calcium, combine with the sulfate anion.  Sulfate compounds include the sulfate minerals gypsum, ettringite and thaumasite, which are the culprits in sulfate induced heave.

 

Sulfates in Soil
Vertical heave during construction of US 67 near Midlothian, TX (Harris, Sebesta, Scullion, 2004)

Vertical heave during construction of US 67 near Midlothian, TX (Harris, Sebesta, Scullion, 2004)

 

Sulfate problems resulting from lime and cement stabilization in soils have been reported since 1962, but were not paid much attention to until the mid-1980s(Harris, Sebesta, Scullion, 2004).    Sulfates occur naturally in soils and have been reported in Oklahoma, Texas, Nevada, Louisiana, and Kansas (Puppala, Griffin, Hoyos, Chomtid, 2004). 

 

Sulfates in soils alone do not pose a problem for roadways and structures.  When sulfate rich soils are treated with calcium-based stabilizers and subjected to moisture, however, results can be devastating.  Sulfate ions in the soil combine with calcium from lime or cement stabilizers, aluminum from stabilizers and/or the clay minerals in the soil and water to generate sulfate heave.

 

Sulfate levels in soil can vary greatly in an area.  Seams of high concentrations of sulfates can be found on a project site.  It is important to determine where these seams are and to either remove them or properly mix them with non or low sulfate soils.  The amount of sulfates in soil can be tested by collecting a soil sample and testing it in a laboratory.  These laboratory tests will be discussed further in an upcoming post.