2641
Initial investigation into the carbonation of MgO for soil stabilisation
Premières investigations sur la carbonatation de MgO utilisé pour la stabilisation des sols
Yi Y.L.
Institute of Geotechnical Engineering, Southeast University, China, and Department of Engineering, University of
Cambridge, Cambridge, UK
Liska M., Unluer C., Al-Tabbaa A.
Department of Engineering, University of Cambridge, Cambridge, UK
ABSTRACT: While Portland cement (PC) is the most widely used binder for soil stabilisation, there are significant environmental
impacts associated with its production in terms of high energy consumption and CO
2
emission. Hence, the development of new low
carbon foot-print alternative cements has been encouraged. In this paper, reactive magnesia (MgO) was used as a soil stabilisation
binder and the MgO-stabilised soils were carbonated by gaseous CO
2
to improve the mechanical properties of the soil and reduce the
CO
2
emission. The mechanical and microstructural properties of the carbonated MgO stabilised soils were investigated by using
unconfined compressive testing, x-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the
strength development rates of carbonated MgO-stabilised soils were much faster than those PC- and MgO-stabilised soils, and the
unconfined compressive strength of highly carbonated MgO-stabilised soils was close to that of 28-day ambient cured PC-stabilised
soils. The XRD and SEM results indicated that nesquehonite (MgCO
3
·3H
2
O) was the main product of the carbonated MgO-stabilised
soils and responsible for the significant strength development.
RÉSUMÉ : Alors que le ciment Portland est le liant le plus utilisé pour la stabilisation des sols, il y a d'importants impacts
environnementaux associés à sa production en termes de consommation d'énergie élevée et d'émission de CO
2
. Par conséquent, le
développement de nouveaux ciments alternatifs à basse teneur en carbone a été encouragé. Dans cet article, la magnésie réactive
(MgO) a été utilisée comme liant pour la stabilisation des sols et les sols stabilisés à la magnésie ont été carbonatés par du CO
2
gazeux
afin d'améliorer les propriétés mécaniques des sols et de réduire les émissions de CO
2
. Les propriétés mécaniques et microstructurelles
des sols stabilisés à la magnésie et carbonatés ont été étudiées en utilisant des essais de compression simples, la diffractométrie de
rayons X (DRX), et la microscopie électronique à balayage (MEB). Les résultats montrent que le développement de la résistance des
sols stabilisés à la magnésie et carbonatés était beaucoup plus rapide que celui du ciment Portland avec prise à l'air ambiant et celui
des sols stabilisés à la magnésie. Ils ont également montré que la résistance à la compression uniaxiale des sols stabilisés à la
magnésie et carbonatés était proche de celle des sols stabilisés au ciment Portland avec prise à l'air ambiant pendant 28 jours. Les
résultats des DRX et MEB ont indiqués que la nesquehonite (MgCO
3
·3H
2
O) était le produit principal des sols stabilisés à la magnésie
et carbonatés, et responsable de la forte augmentation de la résistance.
KEYWORDS: soil stabilisation, reactive MgO, carbonation, unconfined compressive strength, microstructure.
1 INTRODUCTION
Soil-cement mix technology is one of the most widely used
ground improvement methods, with Portland cement (PC) being
the most commonly employed binder (Sherwood 1993, Bergado
et al. 1996, Al-Tabbaa, 2003). However, there are significant
environmental impacts associated with the production of PC in
terms of high energy consumption and CO
2
emissions (0.85t
CO
2
/t PC), and hence is responsible for 5-8% of anthropogenic
CO
2
emissions worldwide (WBCSD, 2002; IPCC, 2004).
In order to reduce the usage of PC, new alternative
cements have been encouraged. Reactive magnesia (MgO)
cements recently emerged as a more sustainable alternative to
PC (Harrison 2008). Reactive MgO is generally calcinated from
magnesite (MgCO
3
) at temperatures of ~700-800
℃
and should
not be confused with dead burned MgO manufactured at a
temperature higher than 1400
℃
, which is known to cause an
unsoundness problem in PC due to its delayed hydration
behavior (Shand, 2006).
Extensive research has been conducted at the University of
Cambridge since 2004 into the reactive MgO cements, as
detailed in Al-Tabbaa (2013). Reactive MgO hydrates, much
faster than dead burned MgO, to form brucite (Mg(OH)
2
).
Although brucite has a very limited binding ability, it could
carbonate to form one or more of the hydrated magnesium
carbonates,
namely
nesquehonite
(MgCO
3
·3H
2
O),
hydromagnesite (Mg
5
(CO
3
)
4
(OH)
2
·5H
2
O) and/or dypingite
(Mg
5
(CO
3
)
4
(OH)
2
·4H
2
O). The hydration of MgO and
carbonation of brucite both are expansive reactions, which
significantly fill available pores. These hydrated magnesium
carbonates also form well ramified networks of massive crystals
with a very effective binding ability resulting in substantial and
rapid strength increase. For example, in porous construction
blocks, they significantly outperformed corresponding PC
blocks with strengths of 200-300% higher (Liska 2009; Unluer
2012). High levels of carbonation have been achieved in full-
scale porous blocks trial production (Liska et al. 2012a and b),
reabsorbing most of the CO
2
generated during the
decomposition of the magnesite.
In this paper, reactive MgO was initially used as a soil
stabilisation binder and the MgO-soil samples were carbonated
by CO
2
gas to improve the mechanical properties of the soil and
reduce the CO
2
emission. This initial work is thereafter
complemented by investigating the impact of relevant variables
including soil type, soil water content, binder content and
carbonation method (Yi et al., 2012), as well as by using a
laboratory-scale auger to model the installation of carbonated
soil-MgO deep mixed columns (Yi et al. 2013).
