In our experiments, mortar samples were exposed to a CO2-enriched atmosphere (CO2-series) and to a CO 2 + SO2-enriched atmosphere (SO2-series). Because of the gas concentrations (0.7 vol.-% CO2; 0.06 vol.-% CO2) used, the dominant process consists in the reaction of CO 2 with the alkaline components of the mortar. This transformation which reduces the initial alkalinity of the mortar is generally called carbonation.
With specimens exposed to a CO 2 + SO 2 mixture it is, however, more correct to use the term neutralization since, apart from the carbon dioxide, the sulphur dioxide also contributes to the reduction of the akalinity of the cement matrix.
It is well known that is measured with phenolphthalein the actually pH-value. Pink color disappears when the pH-value is below 9 and this is generally taken as an indication for the carbonated zone. In fact it is not totally correct to refer to it as carbonate zone because the whole procedure is based on pH measurement.
The reason why we did not find any sulfite salts is the presence of liquid water, because the mortar prisms were soaked in water once a week for half an hour. Under these conditions the initial product -sulfite -is quickly converted to sulfate.