The carbonation of cement with additions of clinoptilolite and silica fume during the first minutes up to the 24thh of hydration is studied. The crystallization of calcite on the surface proceeds directly between the calcium ions from the solution and CO2 from air without formation of portlandite and ettringite. The crystallite size of calcite up to 120min is about 50–60nm and then to 480min it lowers (20–30nm). The mixture with clinoptilolite displays slowed process of gypsum dissolution. The thermal reactions in the range 480–700°C are related to dehydration–decarbonation of the unstable, imperfectly crystallized Ca-carbonates. In the range 700–850°C the main event is decomposition of stable carbonate phases.