The flocculant injection control system efficiency was evaluated on-site in an aggregate quarry, by means of laser diffraction and analysis of the size and texture of flocs. The configuration of the feed tank and the laser particle size measurement cell installed at the facility (280,000 t/year of aggregates) allowed characterization of flocs with particle size between 4.2 and 3473 pm under hydrodynamic conditions that were highly favorable for the examination of large and very fragile flocs. Two days of analysis of the floc formation process along the path followed by the slurry showed that flocculation was optimal during standard operation of the facility when the flow rate of waste fines and concentrations of solids were close to those used to calibrate the flocculant injection control system. Conversely, when the concentration of solids in the flocculator feed slurry dropped by 57.3%, the flocculant dosing fluctuated during stabilization of the mechanism, the kinetics of flocculation slowed, the mean size of flocs arriving at the settling tank dropped by 69%, and the mode of smaller flocs shifted from 77.8 to 10.4 mu m in relation to normal operation. On-site analyses confirmed that the measurements made with laser diffraction (using a methodology developed in the laboratory) allow determination of the effects of conditioning on the characteristics of flocs in terms of particle size, porosity, density, and volume fraction in the slurry. Evolution of these characteristics according to the local parameters of conditioning (mean detention time, mean slurry velocity, and mean velocity gradient) provides a significant part of basic information necessary to a diagnosis of the operation of an industrial circuit of flocculation. (C) 2007 American Institute of Chemical Engineers.