Abstract: A use of a thiosulfate as a retarder for a cement paste comprising a magnesium phosphate cement.

Abstract: A method for conditioning an acid waste by cementation, wherein the acid waste is chosen among liquids having a pH of no more than 4, semi-liquids having a pH of no more than 4, solids of which the partial or full dissolution in water leads to a solution or suspension having a pH of no more than 4, and mixtures thereof, which method comprises the steps of: a) preparing a cement paste having as components at least: a magnesium phosphate cement and the acid waste, and b) hardening the cement paste thus obtained, and is characterised in that in step a), the cement paste is prepared without subjecting beforehand the acid waste to any treatment consisting in reducing the acidity thereof.

Abstract: A process for immobilizing a mercury-containing waste, which comprises: —stabilizing the mercury of the waste by precipitating the mercury as mercury (II) sulfide; then —encapsulating the waste by cementation, the cementation comprising coating the waste in a cement paste obtained by mixing a composition comprising a powder of at least one binder chosen from hydraulic cements, alkali-activated cements and acid-activated cements, with an aqueous mixing solution, then hardening the cement paste; and which is characterized in that the precipitation of the mercury as mercury (II) sulfide is obtained by reacting the mercury with a thiosulfate in a basic aqueous medium, while stirring and in the presence of a sulfide of an alkali metal, the molar ratio of the thiosulfate to the mercury being at least equal to 1.

Abstract: A process for immobilizing a mercury-containing waste, which comprises: —stabilizing the mercury of the waste by precipitating the mercury as mercury (II) sulfide; then—encapsulating the waste by cementation, the cementation comprising coating the waste in a cement paste obtained by mixing a composition comprising a powder of at least one binder chosen from hydraulic cements, alkali-activated cements and acid-activated cements, with an aqueous mixing solution, then hardening the cement paste; and which is characterized in that the precipitation of the mercury as mercury (II) sulfide is obtained by reacting the mercury with a thiosulfate in a basic aqueous medium, while stirring and in the presence of a sulfide of an alkali metal, the molar ratio of the thiosulfate to the mercury being at least equal to 1. Applications: immobilization of any mercury-containing waste regardless of the origin thereof and, in particular, of mercury-containing waste from nuclear facilities.