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.


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 (, 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).