Blends of portland clinker and gypsum plaster exhibit fast setting and early strength development if mixed with water. An excessive expansion of such systems may be controlled by the use of a “sulfate resistant” rather than “ordinary” clinker and by adding condensed silica to the system. The water resistance of such hardened pastes is significantly improved as compared to plain gypsum pastes, mainly due to the formation of the C-S-H-and AFt-phases.