<p>This study explores the effectiveness of internal curing (IC) in lightweight mortar prepared using recycled autoclaved aerated concrete (AAC) with building block debris as fine aggregate and its influence on the strength characteristics of brick masonry. Lightweight sand (LWS), derived from crushed AAC waste, was used as a partial and full replacement for river sand (RS) at replacement levels ranging from 0, 25, 50, 75, and 100%. Mortar (1:3 mix) was prepared and subjected to conventional water curing (WC) and combined WC-IC regimes. The experimental evaluation included the compressive strength (CS) of mortar, masonry prism strength, flexural bond strength (FBS), and diagonal shear strength (DSS) of wall panels constructed with two types of clay bricks (CB-1 and CB-2). The results demonstrate that the incorporation of LWS significantly reduced the mortar density, resulting in the production of lightweight mortar with up to 50% lower density at full replacement. Under combined curing regimes, particularly 21 days of WC + 7 days of IC, mortar and masonry samples exhibited strengths comparable to those of water-cured samples. An optimum replacement level of 50% LWS was identified, beyond which the strength reduction became significant because of weaker interfacial bonding. By supplying water from saturated LWS, internal curing sustained hydration, improving later-age strength. The proposed empirical relationships between the masonry prism strength and the measured strengths of the constituent materials aligned well with the experimental observations. AAC debris can be effectively utilized as a sustainable lightweight aggregate for internal curing in masonry applications without compromising structural performance.</p>