Cement paste (dried at 105 °C)

With our first result, we wanted to check the grindability of the sample in general. We used a 250 ml bowl made of hardmetal tungsten carbide and 15x 20 mm Ø tungsten carbide grinding balls.

With 100 grams of bulk sample volume, the bowl was filled to its maximum. We ground the sample with 450 rpm (max. speed for 20 mm Ø grinding balls) and emptied the bowl afterwards. Indeed, no intact rocks of cement where spotted. The sample already started sticking.

Sticking typically occurs when majority of sample was comminuted < 30 µm. With this level of fineness, interacting forces will get bigger as a particles own g-force. The particles do not fall back on the bottom of a grinding vessel; they attached to all available surfaces. By longer dry grinding, those fine particles will get compressed by the grinding tools to clusters which also can contain bigger particles (typically up to 150 µm) which will not be ground any further. A longer dry grinding is not recommended when such a level of fineness is present.

We checked the level of fineness by static light scattering and found a d50 of ~ 60 µm. Such an early sticking can be promoted by moisture etc.

Just for demonstration, we proceeded dry grinding for a total grinding time of 3 minutes and checked the particle size distribution again. As expected, only a minimal improvement was found by dry grinding.

We added 40 grams of isopropyl alcohol and proceeded grinding in suspension. The mill was programmed for comminution in cycles and we ground for one minute, followed by two minutes of programmed pausing time. After a total grinding time of 10 minutes (3 min dry grinding and 7 min wet grinding), the fineness was checked again by static light scattering. With a d90 < 30 µm, desired fineness was achieved. This is a typical result which can be achieved when 20 mm Ø grinding balls will be used. To improve the result, it is possible to switch to smaller grinding balls.

For our second grinding trial, we comminuted the desired amount of sample with steel equipment instead of hardmetal tungsten carbide. With a density about 7,8 g/ml instead of 14,9 g/ml, less impact force will be applied during the grinding process.

A 500 ml bowl made of hardened stainless steel was equipped with 25x 20 mm Ø grinding balls. 200 grams of sample were placed on top and the bowl was placed into our mill.

After one minute of dry grinding, still some large fragments were spotted. We proceeded grinding for a total grinding time of three minutes. After three minutes, also the large piece we found after one minute were ground successfully.

The sample was sticking similar to result one dry grinding result. We packed a small amount of sample for demonstration and added isopropyl alcohol to comminute in suspension. For this trial, we used more isopropyl alcohol to produce a slurry with thinner viscosity (a motor oil like viscosity should achieve best grinding results).

After a total grinding time of 10 minutes, the level of fineness was checked again. The fineness after 10 minutes was similar to result one. Intensive cleaning was performed rapidly by comminution with sand and water for two minutes. Afterwards the equipment was flushed with fresh water and dried to prevent corrosion.