Civil Engineering Reference
In-Depth Information
6.9% and the loss of ignition (lOI) between 0.8% and 7%. The surface
areas measured were between 2 and 148 m
2
/g and the average particle size
between 1.8 and 35 mm. The reactivity of these MgOs, using the citric acid
test, varied significantly from as little as 9 seconds (extremely reactive) to
>2.5 hours (highly unreactive). The equilibrium pH of the MgOs also varied
significantly between 10.0 and 12.5, mainly related to the CaO content.
A degree of hydration of 40-80% was observed with a limit of 80-95%;
the degree and rate of hydration were enhanced by the presence of certain
hydration agents similar to those used with PC. Commercial trial production
of magnesia from magnesite in China under controlled calcination conditions
demonstrated that highly reactive MgO can be produced which is far more
reactive than those currently available commercially in China (li, 2012).
Fundamental studies of reactive MgO, alone and in blends, using pastes
and mortars, in terms of hydration behaviour, microstructure and carbonation
behaviour (Vandeperre et al., 2006, 2007, 2008a, 2008b; Vandeperre and
Al-Tabbaa, 2007; liska et al., 2006, 2008; liska, 2009; Unluer, 2012; li,
2012; Jin, forthcoming) showed that in the presence of water and under
ambient CO
2
levels, MgO hydrates to form Mg(OH)
2
, or brucite, according
to:
MgO + H
2
O Æ Mg(OH)
2
[19.1]
The solubility of brucite in water is quite low at 0.009 g/L at 18 °C and the
equilibrium pH of a saturated solution of pure magnesium hydroxide is 10.5.
brucite has a layered structure and its morphology varies depending on the
magnesium source and formation conditions (e.g. Henrist et al., 2003; Gao
et al., 2008). The binding ability of brucite as a cement was found to be
limited. brucite is far less soluble and far less mobile than Portlandite and
its pH is stable in the long-term and is hence far less susceptible to attack
by aggressive chemicals.
In the presence of sufficient CO
2
in the curing atmosphere (5-20%)
and water, brucite was found to carbonate to form one or more hydrated
magnesium carbonates:
Mg(OH)
2
+ CO
2
+2H
2
O Æ MgCO
3
·3H
2
O (nesquehonite) [19.2]
and/or
5Mg(OH)
2
+ 4CO
2
+ H
2
O Æ Mg
5
(CO
3
)
4
(OH)
2
·5H
2
O (dypingite)
[19.3]
and/or
5Mg(OH)
2
+ 4CO
2
Æ Mg
5
(CO
3
)
4
(OH)
2
·4H
2
O (hydromagnesite)
[19.4]
