Yield stress modification of suspensions of irregularly shaped particles by addition of spherical colloidal silica

Magnesium hydroxide (Mg(OH)2) suspensions are encountered in the nuclear industry as legacy waste that is to be packaged for long-term storage. It is desirable to increase the solids content of the waste to minimize the total volume, yet this would produce high yield stress fluids that are difficult to process. However, high solids content suspensions of low yield strength are desired. Blending nano-silica (nano-SiO2) into a high yield stress suspension of 27 vol% Mg(OH)2, the suspension yield stress was reduced from 86 Pa to 47 Pa. X-ray CT imaging revealed that approximately two-thirds of the 3 vol% nano-SiO2 added to the Mg(OH)2 suspension was well-dispersed, with the rest forming large clusters that had minimal interaction with the Mg(OH)2 network. SEM images showed small aggregates/individual particles of nano-SiO2 dispersed between Mg(OH)2 particles. We conclude that the finely dispersed nano-SiO2 act like ball bearings to lubricate contacts between the irregularly shaped Mg(OH)2 particles, thus decreasing yield stress. When aging the samples for several days, the yield stress of the binary suspension increased, reversing the yield stress reduction that was observed within the first day. The network stiffening is attributed to the formation of magnesium silicate hydrate (MSH) due to a reaction between soluble Mg2+ and Si(OH)4. The MSH precipitates onto the nano-SiO2 to fuse particles together, thus reducing their effectiveness as flow modifiers. While this is a side effect of this particular binary suspension, the initial yield stress drop remains encouraging to provide the desired fluid conditions to process legacy wastes.