Closing the loop: enabling 100% sand replacement in mortars through in-situ PVAc–silicate stabilization of untreated demolished concrete

Natural sand depletion and the rapid accumulation of construction and demolition waste (CDW) have become major barriers to sustainable construction and circular resource management. Although recycled fine aggregates have been widely studied, current approaches typically allow only partial sand replacement or rely on energy-intensive pretreatment methods, such as thermal activation, carbonation, acid washing, or nanomaterial modification, to achieve acceptable performance. Here, we introduce a simple in-situ stabilization strategy based on polyvinyl acetate (PVAc) and sodium silicate (SS) that enables the use of 100% untreated CDW as the sole fine aggregate in cement mortars without any prior processing. A control mortar containing 100% untreated CDW without additives was used to isolate the contribution of the proposed PVAc-SS system. The hybrid additive significantly enhanced mortar performance, increasing compressive and flexural strengths by 26.1 and 24.0%, respectively, and reducing water absorption by 37% relative to the untreated CDW control. The optimized mortar achieved a 28-day compressive strength of 19.8 MPa and a flexural strength of 5.08 MPa, met the requirements for ASTM C270 type M structural mortar, and attained a W2 capillary water-absorption classification. These findings demonstrate that the limitations traditionally associated with fully recycled fine aggregates can be overcome through a low-dosage, in-situ polymer-silicate stabilization approach, eliminating the need for energy-intensive pretreatment. This work provides a practical pathway toward complete natural sand replacement, advancing circular construction while offering a scalable solution for sustainable reconstruction in resource-constrained and post-conflict environments.