Fineness is the dispersion characteristic of fine-grained materials, in particular, inorganic air binder - gypsum .
Depending on the degree of grinding, gypsum binders of coarse, medium and fine grinding are distinguished with a maximum residue on a sieve with a mesh size of 0.2 mm of no more than 23%, 14% and 2%, respectively.
The finer the gypsum is ground, the greater is its reaction surface and the faster setting and further hardening takes place, but at the same time water demand increases.
The fineness of grinding is of particular importance for high-calcining binders, for them the maximum residue on sieve No. 008 is no more than 15%.
Determination of fineness:
To determine the fineness of gypsum grinding, weigh 50 g of gypsum, previously dried in an oven for 1 hour at 105-110 ° C, and sieved through a sieve with mesh No. 02.
Gypsum sieving is considered completed if within 1 min. no more than 0.1 g passes through the sieve. The fineness of gypsum grinding in percent is calculated as twice the weight in grams of gypsum residue on the sieve.
Grinding gypsum is the most important technological operation providing a developed surface of its particles, which is of particular importance for gypsum.
Since the process of 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 binder consumption in the manufacture of products. Grinding of gypsum, lime or clinker-free binders in an open cycle is carried out in one- or two-chamber mills, and clinker - in multi-chamber.
The open cycle is also successfully used for wet grinding.
Application of secondary grinding of gypsum:
Some enterprises use secondary grinding of gypsum after digesters in ball or rod mills with limited air access, in which, under the influence of heat developing from friction and impact of balls, gypsum ripening processes are accelerated.
In the languishing chamber, the process of dehydration of the dihydrate remaining in the central part of the grain is difficult, requires a long time, since moisture needs to penetrate through the surface layers of other gypsum modifications.
When grinding, the two hydrate is exposed and its dehydration process is significantly accelerated.
Grinding freshly burned hot gypsum in ball mills is more effective than in drum mills. When grinding in ball mills, a product of a more uniform quality is obtained, with reduced water demand and, consequently, increased strength.
Ball mills sometimes used for secondary grinding of gypsum after digesters, which helps to align the composition of the product. In addition, during secondary grinding, gypsum particles take the form of flakes, which improves the ductility, molding properties of gypsum and increases its activity.