Civil Engineering Reference
In-Depth Information
∑
light burned MgO, also called caustic magnesia, caustic-calcined
magnesite (CCM) or reactive magnesia: manufactured at temperatures of
~700-1000 °C, has the highest reactivity, least crystallinity and highest
surface area and its applications include plastics, rubber, paper and pulp
processing, adhesives and acid neutralisation to name a few and is the
main ingredient of the cement presented in this chapter.
MgO has been used in the formation of cements or as an additive in
concrete since the mid-nineteenth century when Sorel in 1867, shortly after
the invention of PC, developed magnesium oxychloride cements (MOC),
also called Sorel cements, by combining light burned MgO with magnesium
chloride solution (Sorel, 1867). Despite many technical advantages, Sorel
cements suffer from poor water resistance, which has prevented their
widespread use. Since then other similar chemically bonded cements have
emerged including magnesium phosphate cements, which are the results
of the reaction between MgO, either hard burned or dead burned, and acid
phosphate salts (Wagh, 2004), and which have applications as rapid hardening
repair cements. Since the mid-1960s hard burned MgO has been used for
shrinkage compensation in concrete dams in China (Du, 2006). Much more
recently, reactive MgO cements (Harrison, 2008), which are blends of light
burned MgO and PC, emerged as a more sustainable alternative to PC and
with anticipated superior technical performance. Extensive coverage of the
chemistry and technology of magnesia is given in Shand (2006).
19.2 Overview, history and development of
reactive magnesia cements
Reactive MgO cement formulations were developed and patented just over
a decade ago by the Australian scientist John Harrison (2003, 2008) and are
blends of PC and reactive MgO (sometime also incorporating fly ash) in
different proportions, depending on the intended application, ranging from
structural concrete to porous masonry units. They have been developed
with strong emphasis on a range of sustainability advantages over PC and
have received significant publicity including coverage in the
New Scientist
(Pearce, 2002) and the
Guardian
(Dyer, 2003). Advantages include: (i)
manufacture at a much lower temperatures (650-800 °C), potential complete
recyclability of the MgO, (iii) uptake of significant quantities of CO
2
and
(iv) insensitivity to impurities. Three different composition ranges have been
proposed (Harrison, 2003):
∑
tec-cements with MgO << PC mainly for concrete applications to enhance
durability and strength through increased density, reduced permeability,
internal dryness and long-term pH and volume stability;
∑
enviro-cements
with MgO ~ PC for enhanced waste immobilisation
