Hello Stephen,
A change from 0.2 to 0.8% SO3 in clinker should not affect the burnability of the clinker too much. If anything, it should slightly improve the burnability, as long as the additional SO3 becomes part of the liquid phase, but that would depend on the amount
of alkalis present.
Normally, sulphur will first combine with K2O and Na2O to form alkali sulphates, then as the molar SO3/(K2O + Na2O) ratio starts to rise significantly above ~1, excess sulphur combines with CaO and K2SO4 to form calcium langbeinite, Ca2K2(SO4)3. In the
past, it was thought that any sulphur in excess of that required to form alkali sulphates and calcium langbeinite was present in clinker as anhydrite (CaSO4), but numerous studies have shown this to be incorrect. In fact, as the SO3/(K2O + Na2O) ratio rises higher
than that necessary to form calcium langbeinite, sulphur begins to be incorporated into the clinker minerals instead, especially belite (C2S). (Belite can contain up to about 2% of SO3 in solid solution, while alite only about 0.5 - 0.8%.)
Only when alkali sulphate/calcium langbeinite formation and clinker mineral SO3 saturation have occurred would any excess sulphur present form calcium sulphate anhydrite. In addition, this anhydrite is not normally in a sufficiently soluble form to be of
much use in controlling set times.
The effect of this increase in SO3 on your clinker quality therefore depends on the alkali content, more specifically the molar SO3/(K2O + Na2O) ratio.
If the excess sulphur is present as soluble sulphates, then you would require proportionately less gypsum in your cement. However, if the excess sulphur is incorporated into belite, then it might have the following effects;-
1. Reduction in alite content of the clinker, since SO3 stabilises the belite crystal structure hindering the uptake of CaO during clinkerisation and thus retarding the conversion of belite to alite. (NOTE: This effect is counteracted by the addition of
a proportional amount of fluoride)
2. Since belite is stabilised by SO3 it also retards the hydration of belite.
The result of both of these effects is that your early cement strengths might be lower. Whether this reduction in strengths is significant or not is another thing.
If you let me know the SO3, Na2O, K2O, C3S and C2S content of your clinker I might be able give you an estimate of the theoretical distribution of the sulphur phases.
Regards,
Ted.