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Carbonation Test
Carbon dioxide from air can react with the calcium hydroxide in concrete to form calcium carbonate. This process is called carbonatation, which is essentially the reversal of the chemical process of calcination of lime taking place in a cement kiln. Carbonatation of concrete is a slow and continuous process progressing from the outer surface inward, but slows down with increasing diffusion depth.

Cement paste contains 25-50 wt% calcium hydroxide (Ca(OH)2), which mean that the pH of the fresh cement paste is at least 12.5. The pH of a fully carbonated paste is about 7.
The concrete will carbonate if CO2 from air or from water enters the concrete according to:
Ca(OH)2 + CO2        --->   CaCO3 + H2O

When Ca(OH)2 is removed from the paste hydrated CSH will liberate CaO which will also carbonate. The rate of carbonation depends on porosity & moisture content of the concrete.

The carbonation process requires the presence of water because CO2 dissolves in water forming H2CO3. If the concrete is too dry (RH <40%) CO2 cannot dissolve and no carbonation occurs. If on the other hand it is too wet (RH >90%) CO2 cannot enter the concrete and the concrete will not carbonate. Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).

Carbonatation has two effects: it increases mechanical strength of the carbonated concrete, but it also decreases alkalinity, which is essential for corrosion prevention of the reinforcement steel. Below a pH of 10, the steel's thin layer of surface passivation dissolves and corrosion is promoted. The pH of carbonated concrete drops to about 7, thus, carbonatation provides a favourable condition for corrosion.  

It can be tested by applying phenolphthalein solution, a pH indicator, over a fresh fracture surface, which indicates non-carbonatated and thus alkaline areas with a violet colour.  Carbonation rate is high in dry weather, but possibility of corrosion is less due less moisture content
Purple colour indicates Un-Carbonated concrete and Colourless Concrete is a Carbonated Concrete