International Slag Valorisation SymposiumLeuven6-7/4/2009 - Third ...

pressure at water-saturated and -unsaturated conditions. The pH of the treated slag
showed a short-lived improvement, starting at 9.5 but with a rebound to 10.5-11 for K1
and back to the original pH of 12.5 for K3 slag after 48 hrs of water contact. 1) Initially,
the surface carbonation was effective in neutralising the pH, as observed for the more
finely ground (K3-type) particles carbonated in the autoclave (Figure 3).
Improved pH goes along with higher V-leaching, also similar to the results shown in
Fig. 3. The K1 slag shows a particularly enhanced V-leaching up to 10-100 mg/kg,
while the K3 slag remains within acceptable limits (< 1 mg/kg). A clear distinction is
visible between the pH and vanadium leaching of the K1 and K3 slag. The K1 slag does
not comply with the limit values for a category 1 application in the Dutch Building
Materials Decree (open application without isolation measures), while the K3 slag does
comply with these criteria. However, vanadium leaching does also increase substantially
after carbonation of K3 slag. Comparing the samples carbonated under unsaturated
(lower degree of carbonation; see Fig. 2) with those carbonated under water-saturated
conditions (higher degree of carbonation) shows that increasing the degree of
carbonation is paralleled with further increasing vanadium leaching.
The pH of the untreated K1 slag, at each L/S ratio, is substantially lower than the pH of
the K3 slag, which is caused by the absence of CaO in the K1 slag, while free lime is
present in the K3 slag. As Figure 4 shows, carbonation of the K1 slag reduces the pH of
the material, while the pH of the K3 slag is not significantly influenced. Carbonation
was found to occur only at the surface of the grains in both the K1 and K3 slag. During
the carbonation treatment, portlandite is also partly formed from CaO inside the grains
of K3 slag. This process results in the formation of cracks due to volume expansion,
resulting in a slow (diffusive) release of alkalinity from unreacted portlandite in the
inner grains. Since the K1 slag does not contain CaO, this material does not crack and
largely maintains its reduced pH after carbonation.
Microscopic characterisation of the spatial mineral patterns in carbonated K1 and K3
slag have shown that the calcite and amorphous silicate reaction products of the CaO
(K3) and C2S (K1 and K3) conversion do not sufficiently envelope the slag grains to
improve the environmental properties with respect to pH and V-leaching in a
sustainable manner. 1) The carbonation treatment of particularly the K1 slag (without
CaO) is still insufficient to comply with the criteria of the Dutch Building Materials
Decree. Further research is currently focusing on options to optimise the carbonation
treatment of steel slag and to evaluate the development of the pH and leaching
properties of steel slag in specific environmental utilisation scenarios. The geochemical
modelling approach described above forms an important tool for both of these
objectives. We are further developing this approach on the basis of microscopic
characterisation of steel slag mineral phases and the implementation of thermodynamic
1 st International Slag Valorisation Symposium│Leuven│6-7/4/2009
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