A curing accelerator composition for building chemical mixtures comprises a mineral constituent and a polymeric water-soluble dispersant. The mineral constituent comprises a semi-ordered calcium silicate hydrate having an apparent crystallite size of 15 nm or less and less than 35% by weight of crystalline phases other than the semi-ordered calcium silicate hydrate. The composition displays a more pronounced accelerating effect than comparative compositions in which the mineral component comprises a calcium silicate hydrate having a higher degree of crystallinity.