Belite-calcium aluminate as an additive

Use of a belite calcium aluminate obtainable by a method comprising the following steps:

• a) providing a starting material from one or more raw materials that contain CaO, SiO2, Al2O3 and Fe2O3 or other compounds of these elements, wherein the starting material has a molar Ca/(Si+Al+Fe) ratio from 1 to 3.5 and a molar Al/Si ratio from 100 to 0.1,
• b) mixing the raw materials to form a starting material mixture,
• c) hydrothermal treating of the starting material mixture produced in step b) in an autoclave at a temperature from 100 to 300° C. and a residence time from 0.1 to 24 h, wherein the water/solids ratio is 0.1 to 100, and
• d) tempering the intermediate product obtained in step c) at 350 to 600° C., wherein the heating rate is 10-6000° C./min and the residence time is 0.01-600 min as an accelerator of the early stiffening/setting and/or of the hardening of Portland cement.