The parameters of α-C₂SH hydrothermal synthesis from a non-stoichiometric composition of CaO and SiO₂·nH₂O mixtures (the molar ratio CaO/SiO₂ = 1.75) in unstirred suspensions were determined and a sequence of intermediary compounds formation was examined and explained. It was evidenced that α-C₂SH is formed already after 4 h of hydrothermal treatment at 200 °C. However, together with this compound a fair amount of some intermediary compounds (C-S-H(I) and C-S-H(II)) are formed. It was determined that after 24 h of hydrothermal treatment, α-C₂SH starts to recrystallized into kilchoanite which dominates in the products after extending the duration (48-72 h) of synthesis.