A hardener composition for an addition-polymerisation-curable synthetic fixing mortar system for embedding anchoring means in mortar in holes or crevices, wherein the hardener composition includes oligomeric siloxanes having on average per molecule at least one organic radical that carries one or more secondary and/or primary amino and/or thiol groups that react with isocyanate or epoxy groups in the addition reaction and having on average per molecule one or more hydrolysable groups, and, in addition, can include one or more further customary additives.