The paper presents a study of structure formation during the hardening of a new water resistant hydraulic magnesium cement. Improvement of water resistance and the manifestation of hydraulic properties in magnesium cement are associated with the use of a fundamentally new mixing liquid involving a magnesium bicarbonate solution. Use of this solution makes possible for the magnesium composition to harden not only in air, but also in water. The Coefficient of resistance is 1.0-1.8. When mixing the magnesium cement with an aqueous solution of magnesium bicarbonate, the main products of hardening are magnesium hydroxide and magnesium hydrocarbonates. The increase of the hydrocarbon phase contributes to the strengthening characteristics of magnesia compositions. Magnesium hydrocarbonates are insoluble in water, which provides the increased water resistance of magnesia compositions and the opportunity to harden and gain strength in water.