Dear Experts,We havea problem that when ever we mak the paste of the cement after setting, on the surface a white color is appearing. It is due to free lime or some heat of hydration or what? waiting for your kind coments
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re cement color
This sounds like efflorescence. (see the attached document)
Efflorescence can be caused by the leaching out of Ca(OH)2 after the mortar or concrete has cured or during curing itself. Ca(OH)2 is a product of the hydration reactions of C3S and C2S with water. Normally this Ca(OH)2 crystallises within the voids in the mortar or concrete. However sometimes, under certain conditions, the Ca(OH)2 is dissolved and moves towards the surface of the curing cement where it reacts with CO2 in the air, leaving an insoluble white layer of CaCO3.
Another cause of efflorescence can be a high potassium or sodium content in the cement. These alkalis can also migrate towards the surface of the cement paste where they too react with CO2 to produce white alkali carbonates.
Does the white colour appear on mortar bars after water tank curing, or only on cement paste cured in dry air?
re cement color
Yes, salt water spray can accelerate efflorescence due to its high salinity, which causes the water in the cement containing dissolved Ca(OH)2 and alkali salts to move more rapidly towards the surface, due to osmosis.
In cold, dry areas, repeated application of water during curing of the concrete is more likely and this wetting and drying cycle will also accelerate efflorescence.
I have attached a link below to another good document on the subject, which might be useful to you.