This paper presents the results of a study of the effect of synthetic zeolite addition on the formation of tobermorite in a burnt lime-quartz mixture. Synthetic zeolite was obtained by thermal activation of halloysite followed by boiling in sodium hydroxide solution. In this study, the formation of tobermorite was investigated at a constant CaO/SiO₂ molar ratio of 0.86 under autoclaving conditions corresponding to saturated steam pressure at 180ºC. The synthesis times were 4 and 12 hours. Synthetic zeolite was added at 5%, 15% and 30%. Studies have shown that synthetic zeolite infl uences the formation of hydrated calcium silicates with poor crystallinity. After 4 hours of autoclaving for samples with synthetic zeolite, a small peak of tobermorite can be found on the XRD pattern. In samples where the molar ratio of Al/(Al+Si) > 0.15 was exceeded, hydrogarnets of the hibschite-katoite group appear. The paper shows that hydrogarnets of the hibschite-katoite group appear for samples with 15% and 30% zeolite after 4 hours of hydrothermal treatment. The intensity of the katoite reflection decreases with increasing autoclaving time and disappears for the sample with 15% zeolite after 12 hours of autoclaving. The presence of portlandite was also demonstrated in these samples.