2642
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 MATERIALS AND METHODS
2.1
Binders, soils and sample preparations
A model soil was used, namely a slightly clayey silty sand,
consisting of 90% sharp sand, 5% kaolin clay and 5% silica
flour. The sharp sand (obtained from Ridgeons, Cambridge,
UK), had a D50 of 0.8 mm and coefficient of uniformity of 4.3,
and the kaolin clay (obtained from Richard Baker Harrison,
Ilford, UK) had a liquid limit of 51% and plastic limit of 30%.
The silica flour was obtained from David Ball Group,
Cambridge. The water content of the soil was 10%. Reactive
MgO (obtained from Richard Baker Harrison, Ilford, UK) and
PC (obtained from Castle Cement, UK) were applied at 13%
dry content to the soil. The reactive MgO had the following
oxide composition: MgO: 97.2%, CaO: 1.2%, SiO
2
: 1.2%,
Al
2
O
3
: 0.2% and Fe
2
O
3
: 0.2%, while the PC had: CaO: 63.6%,
SiO
2
: 13.6%, Al
2
O
3
: 10.2%, Fe
2
O
3
: 2.7%, SO
3
: 6.9%, MgO:
0.6% and K
2
O: 0.9%.
The sharp sand, kaolin clay and silica flour were initially
mixed and homogenised for 5 minutes in a bench-top food
mixer after which water was added and the mixing continued
for further 5 minutes. The MgO was then applied to the model
soil and the entire system was then mixed twice for 5 minutes
with an inspection inbetween the two intervals. The
homogenised mix was then placed in cylindrical moulds, with
50 mm diameter and 100 mm height, applying consistent
moderate compaction in three layers by rodding and eliminating
any trapped air pockets. The samples were demoulded ~1 hour
later, then placed in the triaxial apparatus and subjected to the
carbonation procedures detailed below. A subset of MgO and
PC stabilised samples was also cured in their moulds under
“ambient” conditions, of 20±2
℃
and 95±3% relative humidity,
for 1, 7, 28 and 90 days.
2.2
Carbonation procedure and testing
A triaxial apparatus was used to permeate pressurised gaseous
CO
2
through the MgO-treated soil as shown in Figure 1. The
samples were subjected to 400 kPa confining pressure and then
followed by upward permeation of the gaseous CO
2
. First, the
CO
2
outflow tap was open during the carbonation process, and
the inflow CO
2
pressure was maintained at a low value of 20
kPa to reduce leakage. However, the CO
2
leakage was still
serious, and hence only four carbonation periods were
conducted using this method, which were 0.75, 1.5, 3 and 6
hours. Thereafter, the carbonation process was modified: the
CO
2
outflow tap was closed two minutes after turning on the
CO
2
inlet while keeping the inlet open therefore maintaining the
CO
2
pressure at the 200 kPa level for the designated carbonation
periods: 0.75, 1.5, 3, 6, 12, 24, 48 and 96 hours. Besides, a
subset of samples carbonated for 24 hours using this method
was then cured under ambient conditions for 7, 28 and 90 days.
Figure 1.The triaxial cell used for MgO-stablised soil carbonation.
In addition, an incubator, with 20% CO
2
concentration (1 bar)
at 20±2
℃
and relative humidity of 95±3%, was also used to
carbonate MgO-stabilised soils for comparison purposes. The
carbonation periods were 12, 24, 48, 96 and 168 hours (7 days).
A subset of samples carbonated for 7 days using this method
were then cured under ambient conditions for 28 and 90 days.
All the samples were tested in triplicate for their unconfined
compressive strength (UCS) at a constant displacement rate of
1.14 mm/min. X-ray diffraction analysis (XRD) and scanning
electron microscopy (SEM) were conducted for selected mixes.
3 RESULTS AND DISCUSSIONS
3.1 Unconfined compressive strength
Figure 2 shows the UCS of MgO-stabilised soils carbonated
using different carbonation methods. The MgO-stabilised soils
carbonated in an incubator, which is generally used to carbonate
the MgO-based construction blocks (Liska 2009; Unluer 2012),
took ~96 hours of carbonation to reach a maximum UCS value
(~8 MPa). While those carbonated in triaxial cell with 200 kPa
CO
2
stationary showed similar strength development behavior
but with much faster rate, whereby after ~3 hours the stabilised
soil also reached a similar maximum UCS value. The UCS of
the MgO-stabilised soils carbonated in triaxial cell with 20 kPa
CO
2
flow through for 45 minutes was much lower than those
with 200 kPa CO
2
stationary, mainly due to the lower CO
2
pressure of the former (Yi et al. 2012). However, the attained
strength is approximately the same regardless of the CO
2
pressure applied and its concentration. Figure 2 indicates that
there is no need to keep the CO
2
flowing through the sample
during the carbonation process, which causes CO
2
leakage.
1
10
100
0
2
4
6
8
10
12
Unconfined compressive strength (MPa)
Carbonation period (hours)
Carbonated in:
incubator (20% CO
2
concentration, 1 bar)
triaxial with 20 kPa CO
2
flow through
triaxial with 200 kPa CO
2
stationary
Figure 2. UCS of MgO-stabilised soils carbonated in incubator, triaxial
cell with 20 kPa CO
2
flow through and 200 kPa CO
2
stationary.
Figure 3 presents the UCS of uncarbonated MgO- and PC-
stabilised soils and carbonated MgO-stabilised soils cured under
ambient conditions. It is evident that the UCS values of the
uncarbonated MgO-stabilised soil are much lower than those
PC-stabilised soils, and both of the two mixes took ~28 days to
finish most of their strength development. Comparing of Figure
2 and Figure 3 indicates that the carbonation significantly
increased the UCS of MgO-stabilised soils in a very short time,
and the UCS of highly carbonated MgO-stabilised soils was
close to that of the 28-day PC-stabilised soils, which was ~10
times that of 28-day uncarbonated MgO-stabilised soils.
However, the ambient curing period did not affect the strength
of carbonated MgO-stabilised soil significantly, indicating the
carbonated MgO-stabilised soil could be used to support a
structure just after the completion of the carbonation procedure.
