The SO3 determined in the cement is the sum of SO3 of the different constituents:
• Mineral components (Fly ash ,Slag etc)
• Calcium sulfate added for finish grinding .
The SO3 from the different constituents does not react in the same manner. This reaction can considerably influence the cement properties in terms of the following :
• Setting Time (Initial & Final )
• Rheology of the cement paste
• Kinetics of hardening
• Dimensional variations
• Admixture Compatibility in the fresh concrete
In case of clinker the SO3 is integrated in the crystal lattice. The contribution of sulfur by the raw meal or by the fuels influences the final SO3 content in the clinker.
The SO3 of clinker varies mainly as a function of the fuels used during the burning process Attention has to be given to obtain sufficient SO3 in form of alkali sulfates in order to avoid fast setting of the cement. It is not possible to calculate in a quantitative manner the effect of all these phenomena without a purely experimental study applied to the cement. This is more indispensable since not all the cements have exactly the same composition, the same burning conditions, the same MIC, the same particle size distribution, etc….
There are certain mathematical models to determine the optimum SO3 in the cement. None of them gives satisfactory results, as the effect of particle size distribution is not taken into account.
In the Plant lab Ball mill grinding carried out by making cements at different SO3 contents is not an ideal tests primarily because the lab ground cement have different particle size distribution.