An admixture for cement comprises phosphorous acid and/or a salt thereof or a waste fluid originated from an electroless nickel-plating process which contains phosphorous acid and/or a salt thereof. The admixture is used as a setting accelerator, a rust proofing agent for reinforcing steel or a frost resistive agent for concrete and mortar as hydraulic cement blends. The admixture not only permits improvement of the resulting concrete in the setting-accelerating effect, but also can maintain the compressive strength at a high level while maintaining the bleeding rate at a low level.