Although many scientific studies have been done on ionic chemical stabilization, one of the most thorough studies is one done at Texas A&M University (Ionic Stabilization of Expansive Clays) and it’s conclusions mirror those in other studies. It fully explores and explains the mechanism of ionic stabilization and the evidence and conclusions help to explain the effects on the clay soils that are routinely seen in the field and in the long term proven results for roads, paving and structures.
The mechanism is explained in detail here and more information about the study and it’s findings can be found below.
Theory of Expansive Clays
Expansive clay soils can experience significant changes in volume with changes in moisture content. The cause of this expansion potential is due to the electrical charge characteristics of the clay lattice structure. Clay particles are made up of a series of clay lattice structures.
The clay lattice structure of the montomorillonite clay is shown above. The montomorillonite clay is a form of smectite clay and is one of the more common high swell clays. As illustrated in the figure above the montomorillonite clay lattice structure is made up of (2) tetrahedral sheets and (1) octahedral sheet. The tetrahedral sheet is made up of a sheet of tetrahedrals with a silica cation in the center surrounded by (4) oxygen’s. The octahedral sheet is made up of a sheet of octahedrals with either aluminum, iron, or magnesium at the center surrounded by (6) hydroxyls. Between the clay lattice sheets is a diffuse double layer of polarized water molecules and exchangeable cations.
As a result of missing cations in the tetrahedral and octahedral sheets or the substitution of aluminum for silica in the tetrahedral sheet and iron or magnesium for aluminum in the octahedral sheets produces a negative internal charge. Adding to the internal negative charge of the clay lattice is a negative exterior charge resulting from the geometry of the exterior tetrahedral sheets. The outer exterior of the clay lattice consists of tetrahedral sheets with negatively charged oxygen anions on the outside of the tetrahedral sheets which are on the outside of the clay lattice sheets. The swell potential of a clay is determined by the magnitude of the electrical charge of the clay lattice and the surface area of the clay particle. To offset or neutralize the electrical charge of the clay particles, exchangeable cations and polarized water molecules are attracted around the surfaces of the negatively charged clay particles. The combination of exchangeable cations and polarized water molecules form a layer around the clay particle called the diffuse double layer. The expansion capabilities of the diffuse double layer will determine whether the clay soil has a higher or lower potential for swell.
How AGSS-ICS Works to Stabilize Clay Soils
Treating expansive clay soils with AGSS-ICS significantly reduces the clay soils ability to expand and also reduces their soil suction capabilities. Treatment with AGSS-ICS utilizes physio-chemical soil principles to change the swell/shrink characteristics of the clay soils. The AGSS-ICS chemical processes that produce these results are referred to as isomorphic substitution and cation exchange which results in a reduction of the internal and external negative charges in the clay lattice. Isomorphic substitution of missing cations in the tetrahedral and octahedral clay lattice sheets reduces the internal negative charges of the clay lattice sheets. Aluminum substitutes for silica in the tetrahedral sheets and iron and magnesium substitutes for aluminum in the octahedral sheets. The external negative charges of the clay lattice are reduced by a chemical process called cation exchange. The AGSS-ICS chemical solution neutralizes the negative charges on the surface of the clay lattice by providing very large amounts of the hydrogen cation in the diffuse double layer that is present between the clay lattices. These hydrogen cations are then attracted to the outer negatively charged surface of the clay lattice, thus neutralizing their negative charge. The diffuse double layer between the clay lattices normally contains polarized water molecules and positive and negative ions. The cation exchange process that occurs is the exchange of the existing large cations that are in the diffuse double layer (primarily sodium and calcium) with the hydrogen cation that is the smallest cation that exists. Due to the very high charge density of very small hydrogen cations (H+1), they can easily exchange with the much larger sodium (Na+1) and calcium (Ca+2) cations and more effectively neutralize the negative charges on the surface of the clay lattices and collapse the diffuse double layer.
As the negative charges of the clay lattice structure are neutralized the swell potential of the clay lattices are reduced significantly. The reduction of negative charges of the clay lattice will allow for attractive forces to become more dominant and repulsive forces less dominant between the clay lattice structures. These increased attractive forces between the clay lattices results in a collapse of the diffuse double layer between the clay lattices with a subsequent tendency for the clay lattices to become more flocculent (edge to face) and less dispersed (face to face). Clay lattices with greater attractive forces will have more flocculent tendencies and will result in significantly lower swell potential and a reduction in soil suction (approximately a ten times reduction in soil suction). The reduction of soil suction will act like a moisture barrier in clay soils that have been treated since the ability for moisture to migrate into clay soils that have been treated with AGSS-ICS is significantly reduced.
Scientific Studies on Ionic Soil Stabilization
One of the most thorough studies on ionic soil stabilization was performed by Dr. Shondeep L. Sarkar and Dr. Bruce E. Herbert with Texas A&M University. A summary of the conclusions from their research is provided below:
The ASCE summary of the Texas A&M study states: Soil samples treated in the field (Crowley, Texas) with a hydrogen ion exchange chemical (for the study referred to as HIExC) solution were analyzed in the laboratory using different techniques, such as X-ray diffraction, zeta potential, transmission electron microscopy/ energy dispersive X-ray analysis and environmental SEM to better understand the mechanism of soil stabilization by the HIExC solution. A limited number of engineering properties such as swell-shrink, strength, and permeability were also tested. Untreated soil samples from the same location were included as a “control,” to help delineate the intricate changes in the soil structure and engineering properties that occur as a result of the amendment with HIExC solution. The principal conclusion of this study is that a change in the shrink-swell behavior of smectite in soils treated with HIExC occurs, which is distinctly beneficial from the standpoint of its stabilization.
- Identifiable smectite peaks were noted to be absent in the Hydrogen Ion Exchange Chemical – treated coarse and fine factions.
- The treatment of clay samples reduced the Zeta potential of the treated sample by 32 % as compared with the untreated clay sample. The Zeta potential is a measure of the negative charge in the clay and on the clay surface. As the Zeta potential decreases it is considered that the double diffuse layer is also reducing.
- Soluble ions analysis of Al and Fe supports the hypothesis that the Hydrogen Ion Exchange Chemical treatment changes the surface chemistry of clays through a complex precipitation-dissolution process; the low pH may be a contributory factor. The treatment of the clay with the Hydrogen Ion Exchange Chemical reduced the soluble Al ions by 43 % and increased the soluble Fe ions by 173 %.
- The Environmental Scanning Electron Microscope examination showed that no morphological changes occur when clay sample is treated with the Hydrogen Ion Exchange Chemical solution, but swelling becomes evident in the counterpart water-treated sample.
- The Transmission Electron Microscope examination revealed that the crystalline electron diffraction pattern of smectite in the treated sample is replaced by diffuse halos, implying a change in the clay structure or a smearing of the smectite particles by an amorphous or non-crystalline material.
- During sample handling and processing, the treated sample was continually recognized as having different behavior compared to the untreated sample. The untreated sample was more “stickier”, as opposed to the treated sample that was much more difficult to disperse.
- The dominant conclusion of this study is that soil samples amended with the Hydrogen Ion Exchange Chemical solution exhibit a change in the crystallographic characteristics of the soil, which can be associated with a change in the shrink-swell behavior of smectite.
The question is asked how effective and how permanent is the treatment of the clay soils with AGSS-ICS? We have accumulated a significant amount of field and laboratory testing and monitoring data that has shown conclusively the successful performance of the AGSS-ICS treated clays. The results of the testing and monitoring is presented in the “Data – Proven Results” section of this website. With regard to the question of how permanent is the AGSS-ICS treatment on clay soils can best be answered by considering the results of the previously described research and our own experiences presented in the “Data – Proven Results” section of this website, we can say that the AGSS-ICS treatment of clay soils is permanent. If the crystallographic characteristics of the soil are changed, then we can conclude that the AGSS-ICS treatment of the clay soils is permanent.
If you want to dig into every detail of the Texas A&M study, click the link below.