Generally, the rate of CO2 diffusion increases with an increase in
concentration of CO2 in the pore solution. However, this is not the
case when the concrete is under high CO2 concentration level (i.e.
20% or above). The main reason for this is that both the CO2 diffusion
and reaction velocity are reduced as a result of the progressive
carbonation processes. In fact, other researcher’s study results can
also support this phenomenon. Castellote et al. [17] studied chemical
changes and phase analysis of OPC pastes exposed to accelerated
carbonation using different concentrations of CO2 (3%, 10%
and 100%) and compared with those of natural carbonation
(0.03%). Their test results showed that the carbonation of the samples
has resulted in a progressive polymerisation of calcium silicate
hydrate (CSH) that leads to formation of a Ca-modified silica gel
and calcium carbonate. The carbonation of CSH and portlandite
occurs simultaneously and the polymerisation of the CSH after carbonation
increases with the increase in concentration of CO2.
When carbonating at 10% and 100% of CO2, the CSH gel completely
disappears. Based on experimental results of this study, mechanisms
of CO2 gas diffusion in concrete for different CO2 concentrations
can be proposed. The formation of a dense carbonated
concrete structure retards the carbonation process as illustrated
in Fig. 8a. With the sufficient supply of CO2 gas, the pores of the
outermost layer of concrete are filled and clogged up with particles.
So, concrete is less permeable to CO2 diffusion because of
smaller pores and low connectivity of the pores. This explains
why there is no significant difference in carbonation depths when
concrete samples are exposed to CO2 concentration of more than
20%. In the case of concrete samples exposed to low CO2 concentration
levels (i.e. 20% or below), the CO2 diffusion and reaction velocity
are not significantly reduced due to the low concentration of
CO2 in the pore solution. Fig. 8b shows the mechanism of CO2
gas diffusion in concrete under low CO2 concentration. Since carbonation
products are mainly located near the pore surface the
pores are partly filled. When the CO2 concentration increases (up
to 20%), more CO2 can diffuse into concrete and thus increase the
concrete carbonation depth.
Generally, the rate of CO2 diffusion increases with an increase inconcentration of CO2 in the pore solution. However, this is not thecase when the concrete is under high CO2 concentration level (i.e.20% or above). The main reason for this is that both the CO2 diffusionand reaction velocity are reduced as a result of the progressivecarbonation processes. In fact, other researcher’s study results canalso support this phenomenon. Castellote et al. [17] studied chemicalchanges and phase analysis of OPC pastes exposed to acceleratedcarbonation using different concentrations of CO2 (3%, 10%and 100%) and compared with those of natural carbonation(0.03%). Their test results showed that the carbonation of the sampleshas resulted in a progressive polymerisation of calcium silicatehydrate (CSH) that leads to formation of a Ca-modified silica geland calcium carbonate. The carbonation of CSH and portlanditeoccurs simultaneously and the polymerisation of the CSH after carbonationincreases with the increase in concentration of CO2.When carbonating at 10% and 100% of CO2, the CSH gel completelydisappears. Based on experimental results of this study, mechanismsof CO2 gas diffusion in concrete for different CO2 concentrationscan be proposed. The formation of a dense carbonatedconcrete structure retards the carbonation process as illustratedin Fig. 8a. With the sufficient supply of CO2 gas, the pores of theoutermost layer of concrete are filled and clogged up with particles.So, concrete is less permeable to CO2 diffusion because ofsmaller pores and low connectivity of the pores. This explainswhy there is no significant difference in carbonation depths whenconcrete samples are exposed to CO2 concentration of more than20%. In the case of concrete samples exposed to low CO2 concentrationlevels (i.e. 20% or below), the CO2 diffusion and reaction velocityare not significantly reduced due to the low concentration ofCO2 in the pore solution. Fig. 8b shows the mechanism of CO2gas diffusion in concrete under low CO2 concentration. Since carbonationproducts are mainly located near the pore surface thepores are partly filled. When the CO2 concentration increases (upto 20%), more CO2 can diffuse into concrete and thus increase theconcrete carbonation depth.
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