Water-soluble copolymer

A water-soluble copolymer, which is a water-soluble copolymer containing the following structural unit (1) derived from an unsaturated monocarboxylic monomer, the following structural unit (2) derived from an unsaturated dicarboxylic monomer, and the following structural unit (3) derived from an unsaturated alcoholic monomer, wherein:

• the structural unit (1) is
• (in general formula (1) above:
• A1, A2, and A3 each independently represent a hydrogen atom or a methyl group;
• and X1 represents a hydrogen atom, a monovalent metal, a divalent metal, an ammonium group, or an organic amine group);
• the structural unit (2) is
• (in general formula (2) above:
• A4 and A each independently represent a hydrogen atom, a methyl group, or —COOX3;
• and A6 represents a hydrogen atom, a methyl group, or —CH2COOX4;
• and further, only any one of A4, A5, and A6 is —COOX3 or —CH2COOX4;
• and X2, X3, and X4 each independently or all together represent a hydrogen atom, a monovalent metal, a divalent metal, an ammonium group, or an organic amine group);
• and
• the structural unit (3) is
• (in general formula (3) above:
• R1, R2, and R4 each independently represent a hydrogen atom or a methyl group;
• and R1 (or R4) and R2 are not simultaneously the methyl groups;
• and R3 represents —CH2—, —(CH2)2—, or —C(CH3)2—;
• and the total number of the carbon atoms in R1, R2, R3, and R4 is in the range of 1 to 3;
• and Y represents an alkylene group having 2 to 18 carbon atoms;
• and n is an integer of 1 to 300);
• with the water-soluble copolymer having:
• a structural unit (3) content of 0.1 to 40 weight % (but not including 40 weight %);
• and
• a product of not less than 200 of the calcium-ion-binding ability and the clay dispersibility (which is measured in an aqueous solution having a calcium ion content of 50 ppm in terms of calcium carbonate) under conditions of solid component content=40 weight % and pH=7.5.