Civil Engineering Reference
In-Depth Information
the glass has been changed at high temperature. Moreover, this use suggests
that no appreciable change in strength (with respect to reference mortars)
was seen at loadings of up to 20% by volume.
This is an intriguing result, suggesting that although some pozzolanic
reaction may be responsible for maintaining the strength, the glass is unaffected
by alkali silica reaction. It is possible that on melting, a substantial quantity
of alkali metal has been subject to volatile loss, depleting the system in
an essential reactant necessary for ASR or that the modified glass is not
very susceptible to ASR and this is worthy of further study. Notably, these
authors find no evidence of ettringite-induced expansion. If the hypothesis
of Bethanis et al. (2002) is correct, that sulphate is depleted from these
materials at high temperature, this becomes a compelling explanation of
Ferraris's findings: insufficient sulphate remains in these materials to promote
ettringite formation. Conversely, it is possible that the gas evolution at high
temperature is (at least in part) caused by some reduction of iron III in the
melt, liberating oxygen as described by Sandrolini and Palmonari (1976) in
their work on vitrified ceramics. Appenido et.al. (2004) allude to this in their
discussion, but it would seem at the elevated temperatures they consider, the
thermal decomposition of crystalline sulphates would occur (~1000-1150 °C)
alongside this reduction.
Mangialardi (2002) treated MSWI fly ash in a similar way, following
washing in water for 30 minutes. This step effectively removed the soluble
salts (28-45% by mass of the original solids depending on source) which
were shown to be sodium and potassium chlorides, calcium sulphate and
and the double salt aphthitalite ((Na,K)
3
Na(SO
4
)
2
). The remaining dried
solids were re-wetted (8% water by mass) and compacted into cylindrical
pellets prior to sintering at between 1090 and 1140 °C for an hour. Control
samples of unwashed MSWI fly ash were treated similarly and all were
subject to detailed thermal analysis (differential scanning calorimetry and
thermogravimetry). Distinct mass losses due to thermal decomposition were
seen at ~600 °C (siderite, FeCO
3
) ~700 °C (calcite CaCO
3
) and volatile loss
of the alkaline sulphates around 1200 °C, this latter representing 6.8-12.5%
of the initial mass. In the case of the unwashed fly ashes, volatilization of the
alkali halides was seen between 800 and 1140 °C. Based on these findings,
they suggest that the vast majority of sulphates and chlorides would be lost
or decomposed by sintering at 1140 °C. Mechanical testing of the sintered
products showed a marked increase in compressive strength between the
unwashed and washed fly ashes, due to the loss of the mechanically weak
alkali halides and sulphates. In addition, they note that washing relatively
enriches the remaining solid in glass network formers (Si and Al) which
increase greatly the integrity of the sintered product. Leach testing of the
sintered products show levels of heavy metals released (Cd, Cr, Cu, Pb) to be
well below the required Italian specifications and that the final compressive
