Concrete admixture

A concrete admixture comprising a copolymer-mixture obtained by copolymerizing at least one monomer (A) represented by the formula (a) and at least one monomer (B) represented by the formula (b), changing a molar ratio of (A) to (B) at least one time in the copolymerization course:

• wherein
• each of R1 and R2 represents a hydrogen atom or a methyl group,
• m is a number selected from zero to 2,
• R3 is a hydrogen atom or —COO (AO) nX,
• p is zero or 1,
• AO represents an oxyalkylene having 2 to 4 carbon atoms or an oxystyrene group,
• n is a number selected from 2 to 300, and
• X represents a hydrogen atom or an alkyl group having 1 to 18 carbon atoms;
• wherein
• each of R4, R5 and R6 represents a hydrogen atom, a methyl group or —(CH2)m1COOM2, in which (CH2)m1COOM2 may be cooperated with COOM1 or another (CH2)m1COOM2 to produce an anhydride, in this case, M1 and M2 in these groups are not present,
• each of M1 and M2 represents a hydrogen atom, an alkali metal, an alkaline earth metal, an ammonium group, an alkylammonium group or a substituted alkylammonium group, and
• m1 is a number selected from zero to 2;
• wherein the monomer (A) comprises a monomer (A-1) represented by the formula (a-1) and a monomer (A-2) represented by the formula (a-2):
• wherein
• R7 represents a hydrogen atom or a methyl group,
• AO represents an oxyalkylene group having 2 to 4 carbon atoms or an oxystyrene group,
• n1 is a number selected from 12 to 300, and
• X1 is a hydrogen or an alkyl having 1 to 18 carbon atoms;
• wherein
• R8 represents a hydrogen atom or a methyl group,
• AO represents an oxyalkylene group having 2 to 4 carbon atoms or an oxystyrene group,
• n2 is a number selected from 2 to 290 provided that n1>n2 and that (n1-n2)≧10 in relation with n1 of the formula (a-1), and X2 represents a hydrogen atom or an alkyl having 1 to 18 carbon atoms;
• and
• wherein an average ratio of the monomer (A-1) to the monomer (A-2) by weight is 0.1 to 8.