Bleaching a particulate calcium carbonate aqueous slurry with a reductive bleaching agent such as formamidine sulfinic acid increases the viscosity of the slurry. Combining the slurry with a metal material such as strontium hydroxide reduces any increase in viscosity of the bleached calcium carbonate slurry. Addition of the reductive bleaching agent and/or the metal material takes place prior to, during or after the manufacture of the slurry or the bleaching step. Metal materials in one embodiment include those that form substantially water insoluble precipitates of the reductive bleaching reaction products.