Sir I was reading a note in which stated that if C3A is in orthorombic form it will be more reactive than cubic C3A,
Thus orthorobmic C3A will form ettringite faster and increased sitffness of paste i.e increased water consistency with orthorombic C3A
can you please tell me more about this and if C3A is in orthorombic how it can be converted in cubic C3A
Regards
Dilip Yadav
Hello Dilip,
Orthorhombic C3A (sometimes called alkali-aluminate) is formed when alkalis particularly Na) become are incorporated into the C3A crystal lattice. This occurs when there is an excess of alkalis over sulphur in the raw materials (incl. fuel ash and waste streams)
As you rightly pointed out, this form of C3A is more reactive than its cubic counterpart and, if too much orthorhombic C3A is present, this can cause problems with setting times and workability.
(See http://www.springerlink.com/content/d63w6761l7l23559/fulltext.pdf )
There is no way to convert C3A from orthorhombic to cubic, once it has crystallised in the clinker. The trick is to prevent or minimise the formation of orthorhombic C3A in the first place.
As mentioned earlier, this can be done by controlling the ratio of alkalis to sulphur in the total material inputs to the kiln.
If the clinker alkali/sulphur balance is =< 1.0 then almost all of the alkalis will be present as alkali sulphates, which are immicible in the normal clinker liquid phase, thus making it very difficult for the alkalis to be incorporated into the C3A.
If the alkali/sulphur balance is >1.0, then the alkalis that are in molar excess of the sulphur are free to become incorporated into orthorhombic C3A.
Regards,
Ted.