The method includes subjecting the pozzolans in crushed state to a high energetic mechanical processing by grinding in a grinding equipment, whereby the pozzolan particles receive mechanical impulses, and the grinding is carried out for a predetermined time resulting in a compressive strength of a 2 inch side cube of mortar having 80% Portland cement and 20% natural pozzolan in a ratio of 1:2.75 to standard sand and in addition water required to obtain a flow of the mortar according to American standard ASTM C 109, which has been properly compacted under vibration and hardened at +20° C. in sealed condition, which after 28 days is ≧75% of the compressive strength of a 2 inch side cube, treated as the cube, of a mortar having a ratio of Portland cement:sand of 1:2.75 and water corresponding to 48.5% of the weight of Portland cement.