Geoscience Reference
In-Depth Information
7.9.1 Cement-sodium silicate stabilized columns
As stated earlier, the organic materials in organic soil and peat contain substances such
as humus and humic acids which act as retarding materials during hydration and other
chemical reactions with cement. During stabilization with cement, the humic acids
react with Ca(OH)
2
(from burnt cement) to form insoluble reaction products which
precipitate out on the clay particles, thus inhibiting the strength gain via pozzolanic
reactions. Secondly, the acids also cause the soil pH to drop and hinder cementation.
Thirdly, since organic soils contain fewer solid particles to stabilize, and it is the solid
particles that provide a definite structure, a greater quantity of stabilizer needs to be
added. Furthermore, organic soils have a considerably higher water:soil ratio than clay,
and hence a large amount of water in the soil implies larger voids, thus requiring more
stabilizers, which is not cost-effective (Carlsten and Ekstrom, 1995; Moayedi
et al
.,
2011). As Müller-Vonmoos (1983) emphasized, in soils with high organic content, the
quantity of binder needs to exceed a 'threshold'. As long as the quantity of binder is
below the threshold the soil will remain unstabilized. These factors affect the reaction
rate of the binders, resulting in a slower strength gain or higher cost of stabilizing
organic soil than clay.
Sodium silicates have been developed into a variety of different grout systems and
are widely using as a chemical grout. The sodium silicate systems consist of sodium
silicate and a reactor/accelerator (i.e. calcium chloride) which can be compatible with
cement to get strong bonding properties in a two-compound system. Two-compound
systems have been used in grouting in the soil, below a water table or with high
moisture content, and produce a high-strength permanent grout if not allowed to dry
out (Clarke, 1984; Shroff and Shah, 1999). Sodium silicate is the common name for the
compound sodium metasilicate, Na
2
SiO
3
, also known as water glass or liquid glass.
Sodium silicates function as rapid-setting additives for cement or cement clay grouts
and can alleviate the problems of cement grout application, such as separation of settle
out due to excessive water bleed, equipment and manpower being idle while waiting
for curing to take place, and surrounding water diluting the grout and preventing it
from setting in flooded environments (Houlsby, 1990).
To overcome the difficulties in using cement alone for the stabilization of organic
soils Kazemian
et al.
(2011c, 2012a) have used a cement-sodium silicate system (i.e.
calcium chloride, cement and sodium silicate) to stabilize organic soils. It was observed
by the authors that the trend of gain in shear strength of the organic soils changed
with an increase in calcium chloride content. It increased until the net charge of the
sample became zero. Thereafter, it decreased with an increase in calcium chloride
as deflocculation of the larger size particles took place. The shear strength of the
samples was observed to increase by increasing the concentration of sodium silicate.
This behaviour is due to the hydration and pozzolanic reactions of cement with water
and the rapid reaction between cement, sodium silicate and calcium chloride. However,
upon increasing the concentration of sodium silicate, the strength of the mixture was
observed to decrease due to re-stabilization, which prohibits an increase in the shear
strength of the mixture.
Huat
et al.
(2011) and Kazemian
et al
. (2012b) carried out a series of tests on peat
collected fromKampung Jawa, Selangor (Peninsular Malaysia), using a block sampling
method. Various types of binder were used to stabilize peat and the shear strength of
