Disclosed is a polycondensation product comprising A) an aromatic or heteroaromatic compound containing 5 to 10 C atoms or heteroatoms, said compound being provided with an average of 1 to 300 oxyethylene groups and/or oxypropylene groups per molecule which are linked to the aromatic or heteroaromatic compound via an O atom or N atom, and optionally B) an aromatic compound selected among the group comprising phenols, phenol ether, naphthols, naphthol ether, anilines, furfuryl alcohols, and/or an aminoplast former selected among the group comprising melamine (derivatives), urea (derivatives), and carboxylic acid amides, and C) an aldehyde selected among the group comprising formaldehyde, glyoxylic acid, and benzaldehyde or mixtures thereof, the benzaldehyde optionally containing acid groups in the form of COOM.sub.a, SO.sub.3M.sub.a, and PO.sub.3M.sub.a, wherein M=H, akali metal or alkaline earth metal, ammonium, or organic amine radicals and a=1/2, 1, or 2. Surprisingly, the inventive polycondensation product causes very good liquefaction of hydraulic binding agents, e.g. cement, resulting in substantially improved liquefaction of the construction material at a lower dose while pourability can be maintained over a longer period of time as opposed to naphthalene sulfonates or melamine sulfonates.