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, whilst avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis (hydroxyorgano) phosphonium salt.
