The invention relates to an additive which can be used as a hardening accelerator for hydraulically setting compositions, comprising a) at least one polymeric dispersant comprising structural units having anionic or anionogenic groups and structural units having polyether side chains, b) at least one sulfonic acid compound of the formula (I) in which A1 is NH2, NHMe, NMe2, N(CH2—CH2—OH)2, CH3, C2H5, CH2—CH2—OH, phenyl, or p-CH3-phenyl and Kn+ is an alkali metal cation, or one equivalent of a cation selected from Ca2+, Mg2+, Sr2+, Ba2+, Zn2+, Fe2+, Fe3+, Al3+, Mn2+, or Cu2+, and c) calcium silicate hydrate particles.