Acetic acid has been used to leach Portland cement paste, and the effect of acid concentration, liquid/solid ratio, temperature, and time on leaching of Ca, Al, Fe and Si is reported. The silica-rich solid residue (SR) remaining after leaching was washed, dried, and ground. This amorphous aluminosilicate has comparable pozzolanic reactivity to commercial supplementary cementitious materials (SCM) such as coal fly ash. The 28-day compressive strength of 0.5 w/b mortar containing 20% CEM 1 replaced with SR extracted from cement paste (63.6 MPa) was higher than the reference (51.2 MPa). The leachate from acid leaching contains high levels of Ca2+ ions, and these can sequester CO2 under controlled conditions to form vaterite, a spherical polymorph of CaCO3. Cement paste present in waste concrete can therefore be used to form a reactive silica-rich SCM in a process that sequesters carbon. The research has implications for the development of circular concrete.