Grinding fineness is a dispersion characteristic of fine-grained materials, in particular, of the inorganic air-setting binder — construction gypsum plaster.
Gypsum binders can be of coarse, medium and fine grinding – with their maximum residue on a sieve with a mesh size of 0.2 mm in the clear not exceeding 23%, 14%, and 2%, respectively.
The finer the grinding, the larger the reaction surface of the gypsum and the faster the setting and further hardening. At the same time, binders with a finer grind have higher water requirement.
Grinding fineness is of particular importance for hard-burnt binders. For them, the maximum residue on sieve No.008 should not exceed 15%.
Determination of grinding fineness:
To determine the grinding fineness, specialists weigh 50 g of gypsum, previously dried in an oven for 1 hour at 105–110°C and sift it through a sieve No. 02.
Sifting of gypsum is considered complete if no more than 0.1 g of substance passes through the sieve within 1 minute. The gypsum grinding fineness (in percent) is calculated as twice the mass in grams of the gypsum residue on the sieve.
Gypsum grinding is the most important technological procedure that optimizes the surface of its particles.
Since the process of its hydration ends quickly, larger particles do not have time to react with water, and part of the binder remains unused. This leads to an increase in the consumption of binder during the manufacture of products. The open-cycle grinding of gypsum, lime, or clinker-free binders is carried out in one- or two-chamber mills. Hard-burnt brick (clinker) is ground in multi-chamber mills.
The open cycle is also successfully implemented in wet grinding.
Application of secondary grinding of gypsum:
Some production facilities use secondary grinding of gypsum obtained in ball or rod mills with limited air access after the kettle treatment. The heat developed from friction and impact of balls in the mill accelerates the processes of gypsum maturation.
In the holding chamber, dehydration of gypsum dihydrate remaining in the central part of the grain is constrained and takes a long time, since moisture must penetrate the surface layers of other gypsum modifications.
During grinding, the gypsum dihydrate is exposed and the process of its dehydration is significantly accelerated.
Grinding freshly burnt hot gypsum in ball mills is more efficient than in impact mills. Ball mills produce gypsum of a more uniform quality with reduced water requirement, and, consequently, increased strength properties.
Ball mills are sometimes used for secondary grinding of gypsum after kettle treatment, which helps to even out the composition of the finished product. In addition, during the secondary grinding, gypsum particles take the form of flakes, which improves the plasticity and molding properties of gypsum and increases its activity.