Phenolphthalein is the most common pH indicator used to measure the
carbonation depth of cementitious materials. It is simple and quick to use, but according
to some authors, is not an appropriate tool. Moreover, this substance is in the
candidate list as a substance of very high concern by European Chemicals Agency.
This article discusses the use of thymolphthalein rather than phenolphthalein. Accelerated
carbonation tests were performed on cement mortars and high volume fly ash
mortars. Once this kind of mixtures has low carbonation resistance additional alkalinity
was provided by adding hydrated lime during its fresh mixture, and by curing
the samples immersed in a calcium hydroxide solution. The results showed that the
best indicator was the thymolphthalein, mainly for blends with lower carbonation
depth. It was also checked that adding alkalinity can be useful for increasing the
carbonation resistance.