Civil Engineering Reference
In-Depth Information
MgO and brucite have also been employed in nuclear waste containment
(Xiong and lord, 2008). Periclase type MgO, is the only engineered barrier
certified by the USEPA (Environmental Protection Agency) for emplacement
in the USA Waste Isolation Pilot Plant (WIPP) used for the permanent
disposal of defence-related transuranic waste (US DOE, 2009), and brucite is
employed as an engineered barrier in the Asse repository in Germany (Xiong
and lord, 2008). As a result extensive research has been conducted on the
performance of periclase and brucite in this context in relevant geological,
usually bedded salt, formations in terms of their hydration of the former
and carbonation of the latter (Xiong and lord, 2008). The low pH cements
mentioned above were also investigated with nuclear waste immobilisation
in mind (Iyengar, 2008) and are currently being investigated further in a
large industry-academia collaboration.
A number of patented special cement formulations have been developed
for applications in specific extreme environments, such as CO
2
-rich
environments, high salinity, extreme acidic conditions, high temperatures
and high pressures to name a few, as encountered in many sectors including
civil, petroleum and chemical processing (Schlumberger, 2008; Halliburton,
2009). In geothermal applications, such as oil production, geothermal energy
generation and geological CO
2
sequestration, the long-term integrity of the
cement linings in the respective wellbore shafts has always been a major
concern (Nygaard, 2010), with the recent Deepwater Horizon oil spill being
such an example (Anon, 2010). The expansive nature of MgO hydration and
carbonation and the durability of the resulting products implies potential
significant enhancement of thermal shrinkage compensation and resistance to
such aggressive chemical environments in such applications. Research work
is ongoing to assess potential advantages in the incorporation of reactive
MgO in such cements and comparing their performance with commercially
available CO
2
-resistant cements (Mackay, 2012; lim, 2012).
19.6 Sustainable production of reactive magnesia
cement
The two current production routes of MgO, which are through the calcination
of magnesium carbonates and from seawater, are currently not particularly
sustainable. For the magnesite calcination process, the total theoretical energy
requirement is 2415 MJ/t (Shand, 2006) and, in theory, the production of
1 t MgO would require 2.08 t of MgCO
3
and would result in the emission
of 1.4 t of CO
2
. The corresponding values for the production of PC are
1760 MJ/t (Taylor, 1990), with 1.5 t raw material required for the production
of 1 t PC resulting in emissions of 0.85 t CO
2
. The production route for
MgO from seawater is currently a very energy-intensive process with a total
theoretical energy requirement of 6700 MJ/t (Shand, 2006). The potential for
