Kerstin Eckstein wrote:
> On the other hand, a lot of modern refractories are very rich in
> CaO. High melting points can be achieved, as Tim states, in the
> SiO2-rich corner of systems, but equally on the other end with CaO
> or MgO. The reason for these seemingly inconsistent statements such
> as: CaO lowers the melting point, or: it increases it, are the
> principles in complex systems.
This is, of course, true, but the original question was about clay -
rather than refractories in general. I don't have all the appropriate
phase diagrams to hand just now, but I would have thought that
within the range of compositions exhibited by "normal" clay
deposits, elevating the Ca content would reduce melting point. For
a clay SiO2 + Al2O3 is going to be well over 50%. Along the SiO2
- CaO system quoted, increasing the CaO content reduces melting
points all the way up to the 47% mark (not that any of these
compositions are actually clays - we would need to consider a
significant Al2O3 component too).
Within the constraint that we were asked about clays, we can
simplifiy the complicated general statements. We only have a fairly
small compositional range to play with after all - we have a system
dominated by alumino-silicates, with CaO, MgO and K2O as minor
components
> Furthermore, every further compound in a system, even in traces,
> generally lowers the melting point. Refractories should thus have
> simple compositions rich in either CaO/MgO, or SiO2, or Al2O3,
> little alkaline group metals, hydroxide and iron.
Absolutely- but extreme CaO, MgO or Al2O3 compositions are not
clays.
Tim
Dr Tim Young
Email: [log in to unmask]
Web: http://www.geoarch.demon.co.uk/
Phone: 029 2074 7480
Fax: 08700 547366
Mobile: 07802 413704
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|