Civil Engineering Reference
In-Depth Information
and Al-Tabbaa, 2012). The ambient cured MgO blocks showed the highest
resistance, and although they had lower initial strength, they showed no change
in the strength even after 1 year of exposure, as the formed brucite did not
enter into any detrimental reactions with the acid or sulphate. The carbonated
MgO blocks showed good resistance, which although not as good as that
of the ambient cured blocks, was much better than the performance of the
corresponding PC blocks. There were no reaction products visible and it is
possible that some of the nesquehonite formed converted to hydromagnesite/
dypingite, which have lower strengths, leading to the partial loss in strength
observed. The corresponding PC blocks showed 30% and 100% loss in
strength due to acid and sulphate attack from their extensive reaction with
the cement hydration products. blending the PC with MgO presented some
advantages particularly in terms of reducing the sulphate attack. Resistance
to sulphuric and hydrochloric acids by blended cement concrete with up to
10% reactive MgO content showed that PC-slag concrete performed far
better than PC and PC-ash concretes. The presence of 4-7% MgO enhanced
the resistance of the PC-slag concrete to both acids (Tsang, 2010) for which
the microstructure was denser, with brucite forming both on the outside
and inside of the samples surrounding the portlandite and with very little
ettringite being present.
In terms of waste and contaminant immobilisation, cement and lime
are commonly used for heavy metals in particular, through the resulting
high pH of ~12-13 in the pore solution, leading to the precipitation of the
metal hydroxides, as well as some incorporation within hydration products.
However, as the solubility of many heavy metals is lowest at pH of ~10,
which is closer to that of brucite, MgO has been found to reduce metal
concentrations much further (Iyengar and Al-Tabbaa, 2008; Al-Tabbaa
et al., 2009, 2012b, 2012c). In addition, the lower pH will provide a far
more appropriate environment for microbial activities which will facilitate
biodegradation of organic contaminants (Al-Tabbaa et al., 2007; Kogbara et
al., 2010; Kogbara, 2011). Concentrations of heavy metals lower than their
solubility limits were found to leach out in some tests, suggesting factors,
other than pH, play a major role in the immobilisation process (Iyengar, 2008).
In order to better understand the performance mechanisms, the behaviour of
MgO in comparison with PC, in contaminated aqueous solutions as well as
pastes, was investigated. In general, the MgO performed better, and in some
cases with up to three orders of magnitude reduced leachability. The results
showed that MgO and PC do not significantly interact in metal removal,
and that their effect is metal-specific. The hydration of the MgO was found
to be accelerated by some heavy metals and retarded by others. It was also
found that the metals were incorporated into the formed brucite. A detailed
investigation of the immobilisation mechanisms of MgO and MgO blends
is currently underway (Wang, forthcoming).
