The aim of this work was to observe the impact of the milling technique employed by the DESI 11 disintegrator on the properties of fly ash. This type of mill is a high-speed pin mill with two counter rotors. The device selected for study allows the use of rotors with different working tools. In this case, two types of rotors were selected, identified as BR-AR and OR rotors. The OR rotor has cylindrical teeth, while the teeth on the AR-BR rotor can be described as rhomboidal. The fly ash was ground by 1, 2, 3, 5, and 10 passes through the mill. The Blaine specific surface area, particle size and particle size distribution were measured for each sample. The pozzolanic activity was also determined via a modified Chapelle test, and its influence on the shape of the grains was assessed by SEM. The results were compared together and with the original fly ash. There was a steep increase in the specific surface area and pozzolanic activity after the first pass through the mill. The second pass of ash through the mill did not increase the specific surface area due to strong aggregation, which gradually changes into agglomeration. However, the pozzolanic activity still increased during the aggregation phase. This phenomenon was clearly observable for both types of selected rotors. Based on the results, we can say that in the case of fly ash, a high-speed disintegrator can be a promising means of improving its properties via grinding and mechanical activation. Keywords: high speed grinding, fly ash, pozzolanic activity Namen dela je opazovati posledice uporabljene tehnike mletja, v DESI 11 mlinu, na lastnosti lete~ega pepela. To je mlin z veliko hitrostjo, z dvema nasprotnima rotorjema. Naprava, izbrana za {tudij, omogo~a uporabo rotorjev z razli~nimi orodji. V tem primeru sta bila izbrana dva razli~na rotorja, ozna~ena kot AR-BR in OR rotor. OR rotor ima cilindri~en zob, medtem ko je zob na AR-BR rotorju ozna~en kot romboidalen. Lete~i pepel je bil zmlet z 1, 2, 3, 5 in 10 prehodi skozi mlin. Za vsak vzorec so bile izmerjene Blainova specifi~na velikost povr{ine ter velikost in razporeditev delcev. Pucolanska aktivnost je bila dolo~ena z modificiranim Chapelle testom. Vpliv na obliko zrn je bil dolo~en s SEM. Rezultati so bili primerjani z originalnim lete~im pepelom. Po prvem prehodu skozi mlin se je skokovito pove~ala specifi~na povr{ina in pucolanska aktivnost. Drugi prehod pepela skozi mlin ni pove~al specifi~ne povr{ine zaradi mo~ne agregacije, ki se je postopoma spremenila v aglomeracijo. Vseeno pa je pucolanska aktivnost {e nara{~ala med fazo agregacije. Ta pojav je bil jasno opa`en pri obeh vrstah rotorjev. Na osnovi teh rezultatov lahko re~emo, da je v primeru lete~ega pepela dezintegrator z veliko hitrostjo obetajo~e sredstvo, da z mletjem in mehansko aktivacijo izbolj{amo njegove lastnosti. Klju~ne besede: mletje z veliko hitrostjo, lete~i pepel, pucolanska aktivnost