The results of physico-chemical studies of the hydration processes and the transformation of the hardened products of modified magnesia binders with high strength and water resistance are presented. Secondary raw materials were used as modifying additives: microsilica and calcined rock. X-ray phase analysis methods were used to study the hardening processes, phase and chemical composition of hydration products after 3, 7, 14, 28 and 90 days of ageing under normal conditions. It was found that in the early stages of hardening of magnesia binders without and with pozzolanic additives, the main hardening products are magnesium hydroxide formed in the process of magnesium oxide hydration and magnesium hydroxychlorides. Strength growth of magnesia binder samples without additives practically stops between 14–28 days. At later stages of setting (from 28 to 90 days), microsilica and calcined rock are actively involved in chemical interaction with magnesium hydroxide, oxide and chloride, forming complex hydroxosilicates as well as hydroxoaluminates and ferruginous magnesium compounds.