Civil Engineering Reference
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then sintering at 900-100 °C produced dense glassy ceramics containing
gehlenite and wollastonite of extremely high strengths (~4.5 GPa). The authors
suggest that such materials may have applications as high-value construction
products; however, it would seem a wasted resource to incorporate them in
concrete as an aggregate.
Most of the work on APC residues described above requires the removal of
chloride by washing as a first processing step. Singh et al. (2008) turned the
chloride in these materials to their advantage, through imaginative thinking
to produce alinite. Alinite cements were initially developed at the Scientific
research Institute of Building Materials in Tashkent, during the Soviet era.
Stoichiometrically (and to an extent structurally) Alinite resembles alite
(tricalcium silicate, 'C
3
S') with some substitution of oxygen for chlorine. The
phase was initially described by Ilyukhin et al. (1977) whose composition
was revised by Noudelman et al. (1980) as Ca
11
(Si,Al)
4
O
18
Cl which better
describes the composition, which varies considerably between limits and is
subject to considerable substitution. Magnesium plays a particularly important
role in stabilizing the structure (~3-4%) and in reducing the energy required
for the synthesis and this was provided from MgO additions. The bulk oxide
requirement for alite synthesis from both MSWI fly ash and bottom ash
were adjusted in mixtures with bauxite and limestone. The mass fraction of
chloride was 1% in the bottom as and 19% in the fly ash, so a stoichiometric
chloride content was provided by additions of CaCl
2
. After firing for four
hours at 1100-1200 °C, the product was cooled, ground and characterized,
then used to make cement pastes. Subsequent mechanical testing showed
them to have similar (indeed slightly better) compressive strengths than the
control samples made from Portland cement at around 32 kPa. It should be
noted, however, that due to the chloride content of alinite cements, their
applications must avoid steel-reinforced concrete, owing to the potential for
chloride-induced corrosion.
12.7 Future trends
The combustion products of municipal waste incineration have some real
potential to be used in concrete production, but this must be done with a
clear and confident understanding of their composition. As these materials
are not currently included in cement or concrete standards for structural use,
their initial applications are likely to focus on mass-pours such as 'blinding'
concrete, void fill, hard standing and similar uses. In this way, a body of
knowledge will evolve which increases our understanding of their durability
and environmental impact over the longer term. The area which seems
most likely to see rapid growth in the future, is the area of manufactured
aggregates. Both high temperature sintering or plasma processing and low
temperature accelerated carbonation offer great potential in making efficient
