The formation of groups or clusters of particles (aggregates) in a fluid. In water or in water-base drilling fluid, clay particles form aggregates in a dehydrated, face-to-face configuration. This occurs after a massive influx of hardness ions into freshwater mud or during changeover to a lime mud or gyp mud. Aggregation results in drastic reductions in plastic viscosity, yield point and gel strength. It is part of wastewater cleanup and water clarification. Alum or polymers cause colloidal particles to aggregate, allowing easier separation.