Chemical Stabilization

As the name implies, it is dependent on the chemical reaction that occurs between the chemical/stabilizer
used and the soil particles. Cement, lime, bitumen emulsion, fly ash, Rice husk ash, Ground granulated
blast furnace slag and magnesium chloride are a few examples that are used.
Lime & Cement Soil Stabilization
One of the most common methods of soil stabilization is lime or cement soil stabilization. This method
of soil stabilization involves mixing lime or cement into the soil to boost its strength and resistance. The
percentage of lime or cement mixed into the soil varies depending on the qualities of the native soil. The
higher the plasticity, the more lime or cement is usually added in. Because lime and cement are both
employed as binders, they are frequently paired together.
Although soil stabilization with cement or lime is a common method, it is most typically used on paved
roadways. Typically, treating unpaved roads with cement is cost-prohibitive. The usage of lime or cement
to stabilize soil is frequently determined by geographical area. Some areas have easy access to lime,
whereas others do not, making cement more cost-effective. Soil stabilization with lime or cement works
by binding all of the soil’s particles together, improving the strength of the soil. Because this approach
necessitates the addition of cement or lime to the soil, practically all soil types are compatible with this
method of soil stabilization. Soil testing is essential to ensure that the appropriate amount of additives is
applied. If only a small amount of additive is employed, the soil will not achieve the appropriate strength.
If a greater amount is used, the soil may shrink or crack.
Chemical composition of Lime - Chemical composition of Cement -

Soil Pozzolans -
Pozzolans are a broad class of siliceous and aluminous materials which, in themselves, possess little or
no cementitious value but which will, in finely divided form and in the presence of water, react
chemically with calcium hydroxide (Ca(OH)2) at ordinary temperature to form compounds possessing
cementitious properties. The quantification of the capacity of a pozzolan to react with calcium hydroxide
and water is given by measuring its pozzolanic activity. Pozzolana are naturally occurring pozzolans of
volcanic origin.
Principle of Soil–Cement Stabilization
Cement has widely been used as a binding agent for the stabilization of soil which improves its generic
properties such as strength and deformation characteristics. Generally, portland cement particles are
largely heterogeneous in nature and the theory of soil–cement stabilization is fundamentally governed
by the four major strength-producing compounds that are present in the cement (Bergado et al. 1996),
i.e., dicalcium silicate [2CaO · SiO2 (C2S)], tricalcium silicate [3CaO · SiO2 (C3S)], tricalcium aluminate
[3CaO · Al2O3 (C3A)], and tetracalcium alumino-ferrate [4CaO · Al2O3 · Fe2O3 (C4AF)]. Bergado et al.
(1996) mentioned the two main chemical reactions that occur in the cement stabilization process: first is
the primary hydration reaction between cement and water, and the second is the secondary pozzolanic
reaction between cement and soil minerals. Basically, the two phases of calcium silicate (i.e., C 2S and
C3S) upon hydration produce calcium hydroxide [Ca(OH)2] which releases available calcium for cation
exchange, flocculation and agglomeration, and stabilizes the soil.
Cation exchange: sodium, magnesium, and other cations are replaced by the calcium cations from the
available calcium hydroxide.
Flocculation and agglomeration: flocculation of the clay particles increases the effective grain size and
reduces plasticity, thus increasing the strength of the matrix.
Pozzolanic reaction: the high pH environment created by the available calcium hydroxide solubilizes
silicates and aluminates at the clay surface, which in turn react with calcium ions to form cementitious
products that are composed primarily of calcium silicate hydrates or calcium aluminate hydrates, or both.
Carbonate cementation: calcium oxide reacts with carbon dioxide from the atmosphere to form calcium
carbonate precipitates, which cement the soil particles.
The other products generated are hydrated calcium silicate (C2SHX, C3S2HX) and hydrated calcium
aluminates (C3AHX, C4AHX) during a high-rate hydration process of these two calcium silicate phases.
These two hydrated products are known as primary cementitious products that each bear a strong
adhesive property. These adhesive properties of the hydrated products glued the neighbouring soil–
cement particles together while hardening, ultimately forming a strong soil skeletal structure. Further,
this primary hydration reaction is also held responsible for early strength development because the
product of cementation is formed as a result of the water drying up. On the other hand, the other product
that deposited separately as a crystalline solid phase is the hydrated lime Ca(OH)2. The cement hydration
process leads to the dissociation of the hydrated lime, which causes the release of hydroxide and calcium
ions into the solution, and makes the pH rise beyond 12 within a minute. The strong bases that generate
from the dissociation of calcium hydroxide dissolve the silica and alumina from both the soil minerals
and from the binder itself, which is similar to the reaction between a weak acid and strong base. This
hydrous silica and alumina will then go into the reaction with the calcium ions released from the
hydrolysis of cement to form insoluble compounds (secondary cementitious products), which are
hardened under curing to stabilize the soil. This reaction is known as a secondary pozzolanic reaction,
which is also understood as a process of solidification that hardens the soil structure with increasing
strength over time.

Advantages
• Long-lasting and permanent.
• Tested and proven.
• Most soil types are compatible with it.
• The soil turns dense.
• Reduces soil moisture content.
Disadvantages
• Expensive.
• Potential health risks.
• Before application, comprehensive soil testing is required.
Other mostly used materials are fly ash (FA), Rice husk ash (RHA), Ground granulated blast furnace slag
(GGBS) either as direct or partial replacement of the primary binder.
Fly Ash (FA) is the fine particulate residual outcome of pulverized coal burning obtained primarily from
the coal-based electricity generation plants. It is a pozzolanic material. It is a finely-divided amorphous
alumino-silicate with varying amounts of calcium, which when mixed with portland cement and water,
will react with the calcium hydroxide released by the hydration of portland cement to produce various
calcium-silicate hydrates (C-S-H) and calcium-aluminate hydrates.
Rice husk ash (RHA) is an agricultural by-product. During milling of paddy from the feld, nearly 78%
of the weight is collected as rice, bran and broken rice, while the remaining 22% is collected as a husk.
This husk carries nearly 75% of volatile organic matter and the rest 25% is turned into ash by controlled
exposure of pyro processing which is termed as rice husk ash. This rice husk ash (RHA) contain around
(85–90)% silica which is highly pozzolanic and suitable for replacement of cement.
Ground granulated blast furnace slag (GGBS) is the by-product of iron and steel-making processes.
It is obtained by reducing smelted iron slag from a blast furnace in water or steam, to produce a smooth
and grainy material that is then cooled rapidly and milled into a fine powder. This finely grounded
material behaves near similar to that of cement to facilitate its practice as a cementitious material.
Silica Fume (SF) Silica fumes, also known as microsilica, are a highly reactive pozzolanic material that
is obtained as a byproduct during the production of silicon or ferrosilicon alloys. This amorphous and
ultra-fine material is composed of very fine particles of silicon dioxide, typically with an average particle
size of less than 1 micron. Silica fumes possess unique properties that make them a valuable additive in
a variety of construction applications.
Chemical composition of GGBS and RHA - Chemical composition of Fly Ash -

Chemical composition of Silica Fume -