Man-made deposits of coal ash – a solid by-product of coal combustion - accompany practically every thermal power station and many industrial plants. The total annual production of coal ash worldwide (mainly fly ash) is700–800 million tons with less than 30 % being reused. The disposed coal ash has become a matter of serious environmental concern because of its heavy metal contents which may leach out causing serious pollution of natural water bodies and soil. The mineral composition of coal ash is represented mainly by the glass phase, similar to volcanic glass, as well as by quartz, mullite, magnetite, etc. In recent years, intensive research has been carried out in the use of fly ash for the synthesis of geopolymer materials. Geopolymers are a subclass of alkali activated materials prepared by the reaction of low-calcium, natural and industrially produced aluminosilicates, with an alkaline agent (e.g. alkali metal hydroxide solution or liquid glass) at near ambient temperatures. Due to their energy savings, environmentally friendly processing and high performance, geopolymers are gaining attention in the construction industry as a promising replacement for traditional Portland cement. In this work, we focused on the effect of natural dolomite addition to fly ash and mechanical activation of this two-component mixture on the geopolymerization process. The influence of dolomite dosage and mechanical activation time in a planetary mill on the reactivity of the raw material in relation to sodium hydroxide solution and on the geopolymer strength was investigated.