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Effect of high valence counter-ions (HVCIs) on rheological properties of alumina suspensions was investigated via the addition of bivalent ions (Ca2+, Mg2+ and Cu2+) and trivalent ions (Al3+, Y3+, Cr3+, and Fe3+) into alumina suspensions of 50% (volume fraction) with ammonium citrate as a dispersant, respectively. The results show that the viscosity of suspension increases when the HVCIs addition is higher than the critical coagulation concentration (CCC). The bivalent ions such as Ca2+, Mg2+ and Cu2+ produced the coagulation in the ceramic suspensions more effectively, compared to the trivalent ions of Al3+, Y3+, Cr3+ and Fe3+, which was inconsistent with the Schulze–Hardy rule. This was mainly because the most of trivalent ions reacted with OH- and citrate, decreasing the coagulation behavior. Compared to other bivalent ions, Ca2+ had a lower CCC and an increased viscosity. The CCC of HVCIs increased with increasing the amount of dispersant and decreased with increasing the solids content of suspension. The HVCIs led to a strong coagulation and an increased viscosity of the suspension, which could be promising for the effective ceramic solidification.
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