2643
Technical Committee 211 /
Comité technique 211
This is confirmed by the results of laboratory-scale auger
installed soil-MgO deep mixed columns (Yi et al. 2013).
0
20
40
60
80
0
2
4
6
8
10
12
100
Unconfined compressive strength (MPa)
Curing period (days)
PC: uncarbonated
MgO: uncarbonated
MgO: carbonated in triaxial with 200kPa
CO
2
stationary for 24 hours
MgO: carbonated in incubator for 7 days
Figure 3. UCS of uncarbonated MgO- and PC-stabilised soils and
carbonated MgO-stablised soils versus ambient curing period.
3.2 X -ray diffraction
Figure 4 shows the XRD diffractograms of the MgO-stabilised
soils ambient cured for 28 days (uncarbonated) and carbonated
in incubator for 12 hours and 7 days, the MgO-stabilised soils
carbonated in triaxial cell with 20 kPa CO
2
flow through for 45
minutes, 3 hours and 6 hours, as well as the MgO-stabilised
soils carbonated in triaxial cell with 200 kPa CO
2
stationary for
45 minutes, 3 hours and 4 days. Two strong brucite (Mg(OH)
2
)
peaks and a weak MgO peak are detected for the 28-day
ambient cured MgO-stabilised soil (Figure 4a). For the
carbonated samples, the disappearance of the brucite peak
together with the gradually weakening of the MgO peak suggest
the carbonation of the MgO resulting in the formation of
nesquehonite (MgCO
3
·3H
2
O). With no other changes in the
mineralogy, this correlates with the rapid and significant
strength development of carbonated MgO-stabilised soils.
Additionally, the MgO peak was detected in all the
carbonated MgO-stabilised soils, including those carbonated in
an incubator for 7 days and in a triaxial cell with 200 kPa CO
2
stationary for 4 days, which were much stronger than that in the
28-day ambient cured sample. These results indicated that the
water content (10%) was not enough for all the MgO (13%) to
carbonate to form nesquehonite. Hence the MgO content should
be linked to the water content of the soil, as well as its porosity.
(a)
(b)
(c)
Figure 4.XRD diffractograms of MgO-stabilised soils: (a) ambient
cured for 28 days and carbonated in incubator; (b)carbonated in triaxial
cell with 20 kPa CO
2
flow through; and (c) carbonated in triaxial cell
with 200 kPaCO
2
stationary. The abbreviations stand for: B-Brucite, C-
Calcite, Mg-MgO, N-Nesquehonite, Q-Quartz.
3.3 Scanning electron microscopy
Typical SEM micrographs of the MgO-stabilised soils ambient
cured for 28 days (uncarbonated), carbonated in triaxial cell
with 200 kPa CO
2
stationary for 45 minutes and 4 days, as well
as that carbonated in an incubator for 7 days are shown in
Figure 5.
The 28-day ambient cured sample (Figure 5a) exhibits the
presence of brucite, which has a very limited binding ability due
to its poorly interconnected structure (Liska 2009; Unluer 2012).
All the carbonated MgO-stabilised soils (Figure 5b, c and d)
display formation of nesquehonite, consistent with the XRD
results. Besides, brucite was also observed in Figure 5b
confirming that there was only partial carbonation at 45 minutes,
and which then disappeared at 4 days (Figure 5c) confirming
that the carbonation proceeded to a high degree.
