Carbonation is effectively the reversal of calcination (the
heating of limestone to form cement materials), and starts as soon at the
concrete is poured. It increases the mechanical strength of concrete, but at the
same time, lowers the pH. Because a high pH is essential for the protection of
steel reinforcement from corrosion, this makes the steel reinforcement
Some signs that extensive carbonation has occurred in concrete
- There may be some bright orange
discolouration on the surface of the concrete if the steel has already started
- Putting phenolphthalein
onto the surface will show any heavily carbonated areas up.
- If a sample is looked at through
an optical microscope, the presence of calcite crystals and the absence of
calcium hydroxide indicate that carbonation has occurred.
causes shrinkage and a slight increase in the mass of a concrete element.