Optical pH imaging reveals what phenolphthalein conceals – A carbonation study on blended cements

In this study, optical pH imaging was compared to phenolphthalein coloration and mineralogical analyses to enhance the comprehension of carbonation mechanisms in blended cements. Mortar mixes with four different binders were cured both sealed and underwater before being tested at 2 % CO₂, 65 % RH for up to 91 days. With its high spatial resolution capability, pH imaging facilitated the quantification of pH within the range of 6.5 to 12.0. The optical pH imaging revealed that the pH in the carbonated regions ranged from <9.5 to 11.9 which indicates significant differences in the corrosion risk. Additionally, the pH at the phenolphthalein endpoint (= colour change) was higher than commonly presumed within a range of 10.3 to 11.9. The correlations obtained between mineralogy and pH changes during carbonation highlight the combined effect of the clinker content and the curing regime not only on the carbonation rate but also on the carbonation degree.