Accelerator mixture for a hydraulic binder, comprising: - at least a first superplasticiser comprising side chains connected to a main chain, less than 10% of the side chains separating from the main chain in basic medium at 20°C; - a water-soluble calcium salt, present in quantities of between 50 and 90% by dry weight (percentages calculated from the dry extracts of the constituents of the accelerator mixture); and - at least a second superplasticiser which is different from the first superplasticiser, comprising a main chain and pendant groups connected to the main chain, at least 10% of the side chains separating from the main chain in basic medium at 20°C, said second superplasticiser comprising at least one polymer which is hydrolysable in basic medium selected from the hydrolysable polymers obtained either by radical polymerisation: • of at least one ionic or ionisable monomer of the phosphonic, sulfonic or carboxylic type; • of at least one monomer of the type polyethylene glycol (meth)acrylate, polyethylene glycol maleate, polyethylene glycol vinyl ether or polyethylene glycol allyl, in particular PEG methyl ether (in the terminal position), the molecular weight of which is, for example, between 100 and 10,000; and • optionally one or more other hydrolysable or non-hydrolysable monomers; the hydrolysable monomers representing from 5 to 95% molar of the totality of the monomers used, or in an equivalent manner by a so-called post-esterification process by reacting alcohols or polyoxyalkylated amines with a polymer having carboxylic functions in order to permit grafting. - at least a first superplasticiser comprising side chains connected to a main chain, less than 10% of the side chains separating from the main chain in basic medium at 20°C; - a water-soluble calcium salt, present in quantities of between 50 and 90% by dry weight (percentages calculated from the dry extracts of the constituents of the accelerator mixture); and - at least a second superplasticiser which is different from the first superplasticiser, comprising a main chain and pendant groups connected to the main chain, at least 10% of the side chains separating from the main chain in basic medium at 20°C, said second superplasticiser comprising at least one polymer which is hydrolysable in basic medium selected from the hydrolysable polymers obtained either by radical polymerisation: • of at least one ionic or ionisable monomer of the phosphonic, sulfonic or carboxylic type; • of at least one monomer of the type polyethylene glycol (meth)acrylate, polyethylene glycol maleate, polyethylene glycol vinyl ether or polyethylene glycol allyl, in particular PEG methyl ether (in the terminal position), the molecular weight of which is, for example, between 100 and 10,000; and • optionally one or more other hydrolysable or non-hydrolysable monomers; the hydrolysable monomers representing from 5 to 95% molar of the totality of the monomers used, or in an equivalent manner by a so-called post-esterification process by reacting alcohols or polyoxyalkylated amines with a polymer having carboxylic functions in order to permit grafting. • of at least one ionic or ionisable monomer of the phosphonic, sulfonic or carboxylic type; • of at least one monomer of the type polyethylene glycol (meth)acrylate, polyethylene glycol maleate, polyethylene glycol vinyl ether or polyethylene glycol allyl, in particular PEG methyl ether (in the terminal position), the molecular weight of which is, for example, between 100 and 10,000; and • optionally one or more other hydrolysable or non-hydrolysable monomers; the hydrolysable monomers representing from 5 to 95% molar of the totality of the monomers used,