It is an object of the present invention to provide a
cement admixture which can show not only a high dispersing ability and a
slump loss preventing effect but also a
viscosity reducing effect and can show these characteristics even in the high water
reduction ratio range, and a
cement composition in which this admixture is used. A
cement admixture comprising, as three essential constituents, a
copolymer (A), an unsaturated (poly)alkylene glycol
ether monomer (a) and an unpolymerizable (poly)alkylene glycol (B) not containing an alkenyl group, wherein the content of the unsaturated (poly)alkylene glycol
ether monomer (a) is 1 to 100% by
mass relative to the
copolymer (A) and the content of the unpolymerizable (poly)alkylene glycol (B) not containing an alkenyl group is 1 to 50% by
mass relative to the
copolymer (A), the copolymer (A) comprising, as essential constituent units, a constituent unit (I) derived from the unsaturated (poly)alkylene glycol
ether monomer (a) and a constituent unit (II) derived from an unsaturated
carboxylic acid monomer (b), with the constituent unit (I) and constituent unit (II) each accounting for not less than 1% by
mass based on all the constituent units, and the unsaturated
carboxylic acid monomer (b) being an unsaturated monocarboxylic acid monomer.