Shortage of natural sand is severely affecting the global construction industry. The construction sector also generates a substantial amount of construction and demolition (C&D) waste of which masonry wastes constitute a major fraction. Using a circular economy approach, this article explores the feasibility of using treated masonry waste fines (MWF) to replace 75 % of natural sand in Portland cement-based mortars. The treatment consists of a combination of acid washing and CO2 curing to improve the engineering properties of MWF-cement mortars. 0.1 mol/L and 0.5 mol/L (M) H2SO4 were used for chemical treatment of the MWF followed by CO2 curing at 50,000 ppm for 4 hours. Mortars containing treated MWF (MWF-0.1 M-C and MWF-0.5 M-C) were subject to conventional curing (moist curing followed by dry curing at 30 ◦C and 65 % RH) and carbon sequestration via accelerated carbonation curing. Experimental findings suggest that combined treatment with acid and CO2 reduces the pore volume by 13 – 27 % in the pore size range of 10 – 100 nm in MWF. Consequently, mortars with MWF-0.1 M-C and MWF-0.5 M-C show 29 – 30.50 % higher compressive strength at 28-d age and a 29 – 36 % reduction in total shrinkage compared to mortars with similar dosages of as-received MWF. This ensures statistically similar strength and shrinkage as that of the plain mortar (with 100 % natural sand), demonstrating the potential for 75 % sand replacement using treated MWF. Furthermore, carbon sequestration of 16.40 – 19 % by mass of Portland cement is achieved, which contributes to a 15 % enhancement in early strength and reduces the mesoporous volume and shrinkage by up to 43 % and 15–19 % respectively. In summary, the research offers a pathway to recover a “new sand” from C&D wastes , which could be used to manufacture masonry construction products with reduced demand for natural sand.