The results of the investigation of the influence of reactive mineral admixtures on the resistance of cement systems to alkali-silica reaction (ASR) and delayed ettringite formation (DEF) are presented. The study was conducted to address the problem of premature failure of reinforced concrete sleepers in Ukraine due to progressive crack formation within several years after production. Such a problem began to be observed after Ukrainian cement producers had to switch from gas to coal fuel with an increase in Na + /K + ions in the clinker. The combined mechanism of premature failure due to ASR and DEF is likely to be as follows: in the early stages of hardening and/or during heat treatment, the high alkali metal content prevents the formation and/or causes the decomposition of primary ettringite in the concrete. At the same time, in the presence of reactive silica in the aggregates, a slow ASR starts, concentrating the alkali metals in the ASR gel and reducing their concentration in the pore solution. This in turn creates the conditions for delayed ettringite formation from the monocarbonate (monosulfate) and secondary gypsum present in the system. This research has shown that the use of pozzolanic admixtures containing reactive Al2O3, such as metakaolin, minimises the risk of ASR and DEF, probably by binding Na + /K + ions and reducing their content in the pore solution at early stages of hydration. This predetermines the prevention of ASR and increases the ettringite stability during heat treatment.