In this study, compressive strength of high strength mortar using cement-based powder extracted from sludge separated from ready-mixed concrete was examined for the purpose of assisting effective treatment of returned concrete. As far as the mortar with a W/B of 30% was compared, the mortar with a high recycled cement replacement rate of over 75% took a longer time to flow down and tended to have a higher viscosity than the other mortars. And, it was found that the compressive strength of mortar using 100% recycled cement was about 0.7 to 0.8 that of mortar using ordinary Portland cement.
