The free lime does not contribute to strength. When combined with carbon dioxide, it forms a soluble salt, which leaches through the concrete causing efflorescence, a familiar architectural problem. Concrete is also more vulnerable to chemical attack and detoriation. When it is added, the following reaction takes place:
Ca(OH)2 + SiO2 → H20 + CSH
The reaction reduces the amount of calcium hydroxide in the concrete.
2. Micro filler effect : It is an extremely fine material, with an average diameter much finer than cement. It gets well distributed in the paste, thus blocking the bleed-water channels.