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

leaching (mg/kg dry matt er)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0 25 50 75 100 125 150 175
carbonation time (h)
1 st International Slag Valorisation Symposium│Leuven│6-7/4/2009
Sb
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Cu
0 25 50 75 100 125 150 175
carbonation time (h)
Figure 7: Leaching of antimony and copper as a function of carbonation time. The
different data points at the same carbonation time indicate different processing
conditions (temperature, CO 2 concentration, moisture content).
In blended waste-cement monoliths the carbonate introduction and pH decrease become
decoupled. 6,9) Because of the presence of Calcium-Silicate-Hydrate, a key component in
hydrated cement with a high pH buffering capacity, progressive carbonation first
introduces carbonate in the pore water without decreasing pH. As a result, calcium
leaching decreases. Only at a more advanced stage of carbonation does the pH decrease
and carbonates are converted into bicarbonates, the salts of which are highly soluble.
Therefore, calcium leaching from almost completely carbonated with mildly acidic
leachant is higher than leaching from partially carbonated samples. Metals for which the
carbonate salt is more soluble than the hydroxide go through a different scheme. Lead,
for example, is soluble at high pH as a lead hydroxide complex. The very limited pH
decrease in the first stage of carbonation shifts the equilibrium to the precipitated lead
hydroxide, thus lowering lead leaching. With more advanced carbonation and
significantly decreasing pH, lead hydroxide is converted into the soluble lead
bicarbonate salt, thereby increasing lead leaching again.
Besides the effects on mineral speciation and pH, carbonation also affects porosity.
Carbonation leads to an average decrease of porosity and a modified particle size
distribution. 10) In blended waste-cement mortars it was found that the relative
importance of small capillary pores ( 0.1 µm) increases. The effect of carbonation-induced porosity
decrease on leaching was shown to be significant for pH independent components such
as sodium and, to a lesser extent, potassium. However, for heavy metals the pH effect of
carbonation will be much more important.
With these studies, significant insights have been gained on the carbonation reaction
mechanism in different materials such as lime mortars, lime-based hydraulic mortars,
alkaline wastes and waste-cement blends. It has been confirmed that the role of water is
one of the key parameters in the carbonation process in a porous system, which requires
leaching (mg/kg dry mat ter)
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