Abstract The process of transition zone modification in silica fume mortars was studied in this work. The experimental program was developed to distinguish that process from the modification of bulk paste in the same mortars. Gap graded mortars with varying aggregate size, and with constant aggregate volume content, were made. A change of aggregate size could induce a change of the transition zone volume content in mortar. Thus, it was possible to quantify the modification of the transition zone alone, and to investigate separately the effects of silica fume on the transition zone and on the bulk paste.