Abstract: To provide a chemical agent whereby adjustment of air volume is easier, particularly in a hydraulic composition having fly ash blended therein. An admixture for a hydraulic composition, having a structure indicated by general formula (1) and including a carbon blocker including either one type of compound selected from a group comprising phosphate esters or at least two mixtures; and a water-reducing agent. (In the formula, R1 indicates a hydrogen atom, an alkyl group, an alkenyl group, or a (meth)acryloyl group, A1O indicates a C2-3 oxyalkylene group, n indicates an average added molar number for the oxyalkylene group A1O of 2-150, m indicates an integer between 1 and 3, M indicates a hydrogen atom, an alkali metal atom, a group II metal atom, an ammonium group, or an organic ammonium group.