Influence of Magnesia Addition on the Rheological Properties of Mullite Suspensions

Water‐based suspensions of mullite that are used to form porous bodies through the impregnation of sponges were investigated. The rheological properties of the suspensions are strongly influenced by the amount of magnesia added, as well as the solid loading and the preparation procedure. Amounts of magnesia higher than 0.15 wt% change both the steady shear behavior from nearly Newtonian to plastic (with an increasing apparent yield stress) and time dependent, as well as the viscoelastic properties (the storage modulus increases more than 5 orders of magnitude). Similar effects are shown for long milling procedures, or solids loadings higher than 25 vol%, while the elastic properties are not strictly related to the total volume fraction. The results of the rheological characterization, under either continuous or dynamic conditions, are discussed in relation to the properties of the structure built up within the suspension. The density of the cast bodies increases with decreasing magnesia content.