Process for the manufacture of a solid additive with a suitable dissolving rate for water-hardening binders

Water-hardening binding agent such as concrete, mortar, cement and the like are modified with a solid additive in the form of aggregates consisting of fine particle having a grain size of from 0.1 to 10 mm, preferably in pelletized or pressed grain form. These additives have an adjustable rate of dissolution and permit controlling the speed and manner of hardening of said water-hardening binding agent.