Dispersion-retaining agent for hydraulic composition

A dispersion-retaining agent for a hydraulic composition, comprising a copolymer obtained by polymerizing monomers, said copolymer comprising monomer 1 represented by formula (1):

• wherein R1 to R3 each represent a hydrogen atom or a methyl group,
• AO represents an oxyalkylene group having 2 carbon atoms,
• n represents the number of AO units added on the average per molecule and is a number of 5 to 50,
• R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms,
• q represents an integer of 0 to 2, and
• p represents 0 or 1, and
• monomer 2 represented by formula (2):
• wherein R5 represents a hydroxyethyl group optionally containing a heteroatom, wherein:
• in the constituent monomers of the copolymer, the proportion of the monomer 1 is 28 to 78% by weight when n in formula (1) is 5 to 50,
• in the constituent monomers of the copolymer, the total proportion of the monomers 1 and 2 is 90% by weight or more,
• at least a part of the monomer 2 are monomer 2 wherein R5 represents the hydroxyethyl group optionally containing a heteroatom and has an OH group at the β-position of R5,
• in the constituent monomers of the copolymer, the proportion of monomer 2 is 22 to 72 wt %, and
• in the constituent monomers of the copolymer, the proportion of monomers having at least one group selected from the group consisting of a carboxylic acid group, a phosphoric acid group and a neutralized group thereof is 2.5% by weight or less.