rate-controlling process. However, the accelerated reaction kinetics
of calcium carbonate precipitation in the supercritical process
was chemically controlled by the detachment of calcium ions from
solid portlandite or C–S–H gel. Under 99.995% high CO2 concentrations,
the concrete carbonation did not increase obviously. Houst
[19] and Wittmann [20] think that the role of water depends on
the CO2 concentration and on the porosity of hcp. It has been
shown that an increase in the CO2 concentration does not always
induce an acceleration of the carbonation rate, because the release
of water blocks capillary pores, thus decreasing CO2 diffusion.
Since there are different diffusion mechanisms between high
and low CO2 concentrations, employing an accelerated carbonation
test with high CO2 concentration (more than 20%) may not represent
the natural concrete carbonation and thus care should be
taken when using accelerated carbonation tests.