New Insights into the Carbonation Process of Mortars and Concretes

The carbonation of cementitious materials leads to measurable changes in the material’s physicochemical properties: formation of carbonates, dissolution or decalcification of Ca-containing hydrate phases, changes in porosity and decrease of pore solution pH. The presence of supplementary cementitious materials leads to changes in the mineralogical composition and porosity of the hardened material and thus to changes in the carbonation process compared to pure Portland cement mixtures. In this study, the carbonation of concrete and mortar mixtures with varied clinker content under accelerated conditions was investigated. The carbonation progress was visualized via phenolphthalein (PHPHT) and quantitively using optical pH imaging techniques. Thermogravimetry and XRD was used to measure the portlandite content at individual depths of the carbonated samples. The investigations showed that the actual pH value of the clearly carbonated regions (= PHPHT colourless) of different samples can vary between 9.2 and 10.5. Thereby, with decreasing clinker content, a decreasing pH in the “PHPHT colourless” area was observed after the same time of accelerated carbonation. Measurable portlandite concentrations in the samples exhibiting higher pH in these regions support the findings. The obtained results highlight the strong impact of physicochemical material properties on the carbonation process and the corresponding risk of corrosion. The study shows that the PHPHT-test can only partially answer questions related to carbonation processes whereas the new optical imaging method allows for a better understanding of the carbonation state of cementitious systems.