Civil Engineering Reference
In-Depth Information
made to accelerated tests of dimensional stability as a means of assessing
the reactivity of the glass fraction. Aubert et al. (2004) compares methods
for measuring expansion (total weight gain, X-ray diffraction (XRD) etc.)
and ASTM methods C227 and C342 detail conventional expansion tests
using mortar bars.
12.6 Use in concrete: examples
12.6.1 Conventional applications
Siddique (2010) reviews numerous examples of the use of MSWI ashes in
concrete production and their re-use as other engineering materials such as
ceramics and unbound fill. He concludes that the fly ashes have some potential
as supplementary cementitious materials (SCM) but that leaching of heavy
metals remains problematic. Bottom ash (he notes) has potential value as an
aggregate, given the caveat that aluminium must first be removed.
It would seem, however, from the examination of the chemistry of the
materials considered here, that no distinct division can be made between
materials suited to aggregate use and those suited as ingredients of blended
cements. Bottom ashes contain numerous reactive components which, if
ground finely, hydrate either alongside the cement clinker minerals or through
pozzolanic action. Where the bottom ash is coarse, this hydration is expected
to be confined to its surface layers, presumably increasing the aggregate-paste
bond strength and accounting for its behaviour as an acceptable aggregate
(although somewhat weaker than many conventional aggregates). In any
eventuality, the content of free sulphate and free (elemental) aluminium
must be measured and reduced before use.
Numerous examples of the use of MSWI fly ash and bottom ash have
been reported, where the combustion products are used as either cement
replacement material or as aggregates. Hamernik and Frantz (1991) note the
retarding effect of fly ash on the hydration of cement in concrete, as do Lin
et al. (2004) who used the material as a partial replacement of commercial
cements (types I, III and V). They found considerable variation in the rates
of strength development at 28 days, but surprising similarities after 90
days. Their recommendation is that up to 20% by mass of cement may be
replaced by fly ash without adverse effect on strength, confirming their earlier
findings (Lin et al. 2003). Collivignarelli and Sorlini (2002) pre-treated their
fly ashes by water washing and stabilisation with Portland cement, sodium
silicate and slaked lime, milling the hydrated product to <1.5 mm. This
processed fraction was used as a substitute for fine aggregate in concrete
with compressive strengths of around 15 MPa. Pavlik et al. (2010) used
untreated fly ash as a direct replacement for cement in concrete (2-15%)
but concluded its performance was not sufficient to justify wider use. This
