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

weathering solid wastes, it was necessary to take this process into account when
assessing long-term treatment options. The effect of carbonation on leaching of heavy
metals from incinerator residues became a focal point of the group, both with respect to
solid waste as such and solid waste incorporated in cement matrices. When it became
clear that in some conditions, carbonation can have a positive effect on heavy metal
leaching (i.e. decrease leaching), accelerated carbonation was investigated as a
treatment method to enhance environmental properties.
Over 20 years of investigation has led to various papers, conference communications
and PhD dissertations in the field of carbonation. A list of the most important ones is
provided in the references. 1-26) In the future the groups will collaborate to investigate
mineral carbonation systematically, with a focus on processing conditions, product
properties (structural, mineralogical and environmental) and in-situ analysis.
This conference paper reviews the most important findings at the K.U.Leuven relative
to mineral carbonation, by focusing on the three domains of expertise: processing
conditions, product properties and in-situ analysis. At the end, future directions of the
research are described.
Contributions by K.U.Leuven
The carbonation reaction in mortars and other alkaline materials is a rather complex
mechanism composed of diffusion of the CO2 through the pore structure and its
dissolution in the capillary pore water where its reaction with calcium hydroxide occurs
with the precipitation of calcium carbonate crystals. For a better understanding of this
reaction mechanism in mortars, research has been carried out to clarify the diffusion
term and the reaction term. Based on gas diffusion in a porous medium, Van Balen 1)
provided the very first one-dimensional modelling of the lime mortar carbonation,
taking into account the combined CO2 diffusion with water (vapour) transfer. This
research has identified the interrelationship between CO2 diffusion and water content in
the mortar, considering the water produced from the carbonation reaction itself.
Numerical models show that the carbonation reaction starts quickly on the outer surface
of the lime mortar and the CO2 diffusion controlled phase starts when the mortar has
dried enough. The quick start and fast blockage is due to the limited diffusion resistance
at start and due to the blockage of diffusion resulting from the water produced by
carbonation. 1,2)
The carbonation reaction model in lime mortars was improved with further research
based on experimental studies on the diffusion term and the reaction term. The effective
diffusivity of CO 2 in carbonated lime mortars was studied using a Wicke and
Kallenbach type of set-up composed of a diffusion cell and a CO 2-gas analyzer. 3) The
CO2 diffusion coefficient was determined in relation to different water contents.
1 st International Slag Valorisation Symposium│Leuven│6-7/4/2009
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