The mechanism of formation of silica scales from cooling water was studied by chemical analyses of the cooling water and silica scales, characterization of the aluminum in the silica scales by Al-27 magic angle spinning NMR, the relationship between size distribution of particles in the cooling water and their Al/Si atomic ratios and zeta potentials, and the adsorption properties of the particles on the surface of silica gel powder as a mimic of silica scale. From our results, we determined that aluminum is concentrated from the cooling water into silica scales during their formation, 6-coordinate aluminum is preferentially adsorbed on the surface of the solid, and various particles with differing sizes, surface charges, and Al/Si atomic ratios are formed in the cooling water after addition of polyaluminum chloride. The formation mechanism for silica scales in the cooling water system is proposed based on the electrostatic interaction. The formation of aluminum hydroxide particles smaller than 0.2 mu m with positive charges, consisting of 6-coordinate aluminum, and their subsequent adsorption on the surface of the solid are the most important factors contributing to the formation of silica scales. (C) 2009 Elsevier Inc. All rights reserved.