Mechanical activation of three natural acidic igneous rocks, including granite, pumice, and perlite, was studied using a planetary mill. Positive effects of milling were demonstrated with respect to the compressive strength of mortars with the partial substitution of cement by mechanically activated acidic powders. A set of mortar specimens was made with 0%, 10%, 20%, and 30% activated powder replacement. X-ray diffraction and mineralogy studies revealed that the initial granite consists of fully crystalline phases, while pumice and penile contain not only amorphous but also crypto-crystalline phases. It was found that pumice and perlite rapidly lost their crystalline nature through milling, remaining in solely amorphous phases after 15 min of milling, whereas granite maintained its crystalline nature even after 45 min. Milling generates a larger specific surface area, much more remarkable for pumice than for granite and perlite, but due to reactive agglomeration, there was a limitation on the specific surface area of pumice. All three mechanically activated acidic powders had significant effects on strength development especially at the early ages of solidification. But in the long term their effects depended on material type and grinding condition. The greatest strength improvement was achieved by mechanically activating perlite for 30 min, at 10% substitution in cement mix. Cement consumption can be reduced by up to 30% in weight using mechanically activated powders in cement mix while keeping the 28 day compressive strength at a satisfactory level. In addition, ultrasonic pulsation of mechanically-activated perlite indicated that the specific surface area was mostly influential and available in the early stages of solidification, while structural changes appeared to be so at later stages of hardening. (C) 2014 Published by Elsevier B.V.