Geotechnical News • December 2017
29
THE GROUT LINE
the system has higher compressive
strength and improved durability , as
well as a reduced level of syneresis
(i.e. shrinking). A logical question is
if a 3.2 ratio is better than a 2.2 ratio,
why not use a 4.2 ratio or higher?
The answer is manufacturing limita-
tions; once the ratio of sand to soda
begins to exceed 3.2 there is a rapid
increase in temperature requirements
to make molten sodium silicate as well
as a corresponding increase in molten
glass viscosity. Assuming a high ratio
glass was produced in a furnace using
ordinary means, the material would
have limited solubility and stability in
water. Upon aging, it would be prone
to separation and formation of a gel.
To move to a lower alkali, i.e. more
siliceous product, the end-user needs
to jump from sodium silicate to col-
loidal silica. This takes the user from
using a solution of silicate molecules
to an aqueous suspension of discrete,
nonporous particles of amorphous
silicon dioxide. Colloidal silica or
sometimes under the banner of “nano
particles”, is used in grouting applica-
tions. This form of silica has several
desirable performance properties such
as longer, more controlled gelation
times and the produced gels tend to be
less prone to syneresis. The trade-offs
are the produced silica gel has less
strength, and a higher starting con-
centration than for sodium silicate is
required. Further, colloidal silica is a
considerably more expensive chemi-
cal.
A cost-effective procedure has been
developed to produce polysilicates
for subterranean applications. A 4.5
ratio was selected as the focus ratio for
comparative study with conventional
sodium silicate. Table 1 provides a
comparison of the 4.5 ratio vs. 3.2
ratio vs. colloidal silica.
Formulation development &
experimental procedures
Silicate-based grouts are generally
considered one of the safest types
of chemical grouts. Any concerns
related to environmental impact are
usually related to the choice of setting
agent and/or potential leaching of
sodium. The move to a lower alkalin-
ity sodium silicate addresses concerns
with sodium levels. Care was taken
in selection of setting agents so as to
maximize overall HS&E character-
istics of the grout system. Triacetin
was selected as a proven setting agent
and is environmentally safe. Citric
acid is not commonly used in grouting
applications but is a preferred setting
agent for oilfield applications. Similar
to triacetin, it is approved as a food
additive. Citric acid had the further
appeal of being a lower cost chemical
vs. triacetin. The other checkmark in
favor of citric acid is that it comes as
an easy to dissolve powder. Depending
on location and temperature, the use
of powder provides definitive handling
advantages over a liquid.
The 4.5 ratio sodium silicate was
benchmarked against traditional
sodium silicate using ASTM method-
ology to study the feasibility and per-
formance metrics for stabilizing soils.
The study at GRC was conducted
using liquid sodium silicates diluted
with tap water to concentrations rang-
ing from 30 to 60%. These concentra-
tions would be considered typical for
most geotechnical applications. Sands
used in this study were medium to fine
Barco silica sand. The use of just sand
represented a worst case scenario since
the presence of clays and metals are
known to improve strength and water
resistance. Formulations were devel-
oped to provide set times of approxi-
mately ~4hrs at room temperature. As
noted earlier, the presence of dissolved
metals such as calcium are known to
improve the performance of the sili-
cate grout. Testing later included the
performance of conventional and high
ratio silicate with a hardener.
Silica gel durability
As anticipated, the move to a less
alkaline form of sodium silicate pro-
duced silica gels with greater durabil-
ity vs. conventional sodium silicate.
This was quantified using ASTM
D559/D559M, “standard test methods
Table 1. Physical properties of colloidal silica vs. 3.2 ratio vs. 4.5 ratio
Colloidal
Silica*
3.2 ratio
4.5 ratio
SiO
2
:Na
2
O
52
3.22
4.5
% SiO
2
30
28.7
21.1
% Na
2
O
8.9
4.7
% Solids
30
37.6
25.8
pH
10.2
11.3
11.0
Density ( g/cm
3
)
1.22
1.38
1.22
Viscosity (centi-
poise)
5
180
50
Average size (nm)
7
1
3
*represents colloidal silica that has been used in the past for water blocking
Figure 1: Permeability test set up.
