Civil Engineering Reference
In-Depth Information
12.15
Tabular sulphate-carbonate AFm phase of intermediate
composition (Ca
4
Al
2
O
6
(CO
3
)
0.67
(SO
3
)
0.33
. 11H
2
O) formed in thermally
treated bottom ash pastes hydrated under compression. After Qiao
et al. (2008a).
mixing. Two means of reducing the aluminium metal in bottom ash have
been proposed: direct oxidation of the metal by weathering or at elevated
temperature in air; or a reactive pre-treatment which will convert the metal
to its hydroxide. Qiao et al. (2008c) showed that heating MSWI bottom ash
in a rotary tube furnace at 600-800 °C reduced the aluminium content and
increased the content of hydraulic phases such as gehlenite (Ca
2
Al
2
SiO
7
),
and mayenite (Ca
12
Al
14
O
33
). They showed that compacted samples did not
suffer unduly from hydrogen evolution from the residual aluminium and
produced hydrated materials of adequate compressive strength (~12-15 MPa)
for re-use. rübner et al. (2008) demonstrated the problems of using untreated
MSWI bottom ash in concrete and showed how they may be overcome be
reacting the metallic aluminium with sodium hydroxide solution before use.
This 'lye' treatment (
sic.
) reduced the aluminium content to below 0.4% by
mass with the advantage that the washing step removes both sulphates and
chlorides by dissolution. The concretes they prepared in this way showed a
moderately lower strength than the control samples (-15%) which suggests
this pre-treatment is a suitable means of processing these solids. Pera et al.
(1997) concur with rübner et al. (2008) in the use of sodium hydroxide
to react the aluminium (15 days' exposure) and found acceptable concrete
could be made with 50% of the aggregate being processed MSWI bottom
ash with a similar quantity of natural aggregate.
Glass removal was achieved by Zwahr (2005) using a combination of
optical screening of the bottom ash particles and mechanical sorting, where
