The object of research is fly ash from Berezovskaya GRES. A feature of this fly ash is the high content of calcium oxide in the free state, the hydration of which occurs at a later age, when the bulk of the material has already hardened. The transition from CaO to Ca(OH)2 is accompanied by an increase in volume, which in turn can lead to the cracks in the material. The purpose of the work is to study the possibility of neutralizing the high-calcium fly ash expansion. Method. Tests for the expansion of fly ash paste with various additives in Le Chatelier mold, X-ray diffraction (XRF) and differential thermal (DTA) analyzes of the samples were carried out, their compressive strength was determined. Results. The Le Chatelier molds test showed that microsilica and silicic acid have the greatest influence on the fly ash expansion. When the microsilica content is about 40% by weight of fly ash, there is no expansion of the paste. The compressive strength of samples from the 1:5 fly ash-sand mortar at the age of 28 days was 1.47 MPa. Based on the results of X-ray diffraction analysis and DTA, it was found that in the absence of microsilica, free lime in the fly ash is completely hydrated in 11 days, giving the highest degree of expansion. In the presence of microsilica, the expansion is reduced, however, unslaked free lime remains in the system.