This paper deals with water requirement determination of fine mineral admixtures for concrete by a simple and rapid test based on "single-drop" agglomeration. Eighteen fine and very fine mineral admixtures for concrete, different in origin and characteristics are tested. The influence of cement and high-range water-reducing admixtures are also studied. The "single-drop" test results are compared with standard tests proposed in concrete technology. The water-fine mineral admixture ratio of the formed agglomerate is proportional to the water requirement of the mineral admixture determined by the standard tests for concrete mixtures. However, the frictional forces between particles are not taken into account by the "single-drop" test and consequently the dispersing effect of high-range water-reducing admixtures. Round- and angular-shaped particles present a linear function between the water-fine mineral ratio and fineness, which is determined by the BET specific surface area or the average particle size of the minerals. Irregular-shaped particles present two to three times higher water-fine mineral ratio than that of round-shaped particles with equivalent fineness. In this case, the very low granular packing of mixtures exclude any use in concrete. When there is low cement content, the water-fine mineral-plus-cement ratio and consequently the porosity of binary blended mixtures increase significantly because of interaction phenomena between cement particles and water.