A method for treating an inorganic slurry to preserve the slurry against bacterial contamination, including (I) providing a slurry in a substantially homogeneous phase, and then (II) adding to the slurry an effective amount of a composition containing: (a) a tetrakis(hydroxyorgano)phosphonium salt selected from the group consisting of tetrakis(hydroxymethyl)phosphonium sulphate, tetrakis(hydroxymethyl)phosphonium chloride, tetrakis(hydroxymethyl)phosphonium phosphate, tetrakis(hydroxymethyl)phosphonium nitrate and tetrakis(hydroxymethyl)phosphonium oxalate; and (b) a dispersant selected from the group consisting of (i) a phosphonated compound containing at least one tertiary nitrogen atom and (ii) a homopolymer of an unsaturated acid; and (III) preserving the slurry against bacterial contamination, while avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis(hydroxyorgano)phosphonium salt.