Deep Purple and Rainbow Indicator
When carbon dioxide (CO2) in the air penetrates into concrete, it reacts with the calcium hydroxide (CaOH2) in the cement paste producing calcium carbonate (CaCO3). This reaction is called carbonation and causes the alkalinity of the paste to decrease. When the pH drops below 9, in the presence of moisture and oxygen, the embedded steel will corrode. This measurement of the depth of carbonation is important for Condition Assessment and Corrosion Evaluation of a reinforced concrete structure.
To measure the pH of the cement paste, a freshly broken piece of concrete or a newly cut core is sprayed with the indicator, and allowed to dry. The approximate pH of the paste is indicated by a color index which in turn indicates the depth of the carbonation front.
Carbonation indicator for concrete.