The invention relates to a kit of parts for preparing a glass ionomer composition for dental use, the kit of parts comprising a Powder Part P and a Liquid Part L, Powder Part P comprising: acid-reactive inorganic filler, Liquid Part L comprising: water, complexing agent, polyacid, either the Powder Part P or the Liquid Part L or the Powder Part P and the Liquid Part L comprising non-aggregated nano-sized particles based on silica or alumina, the composition obtained by combining the components of Powder Part P and Liquid Part L before hardening comprising the components in the following amounts: non-aggregated nano-sized particles: from 0.1 to 15 wt.-%, acid-reactive filler in an amount from 50 to 75 wt.-%, polyacid: 7 to 20 wt.-%, complexing agent: 0.5 to 3 wt.-%, water: 5 to 18 wt.-%, wt.-% with respect to the weight of the whole composition.