<p>This study evaluates the mechanical and durability performance of sustainable two-stage concrete incorporating recycled fine and coarse aggregates obtained from building demolition waste. Four two-stage concrete mixtures were produced: NN with natural coarse and fine aggregates, RR with recycled coarse and fine aggregates, NM with natural coarse aggregate and mixed fine aggregate, and RM with recycled coarse aggregate and mixed fine aggregate. The mixed fine aggregate consisted of 50% natural sand and 50% recycled sand by mass. A conventional concrete mixture was also prepared as an additional reference. The experimental program included grout flexural and compressive strength, concrete compressive strength, ultrasonic pulse velocity, X-ray-based pore structure assessment, and water penetration under pressure. The results showed that recycled fine aggregate can be used effectively in grout production under the investigated conditions. The recycled sand grout reached a 90-day compressive strength of 49.25 MPa, slightly higher than the natural sand grout, although its flexural-strength response was lower and more variable. The fully recycled RR mixture achieved a 28-day compressive strength of 39.40 MPa, corresponding to 94.8% of the natural aggregate reference mixture. X-ray analysis showed that RR had a relatively low total air content of 0.82%, which supported its mechanical performance. RR also showed the lowest mean water penetration depth, 50.7 mm; however, this result was highly variable and should be interpreted as a mixture-specific observation governed by grout filling, aggregate packing, pore connectivity, and interface continuity rather than as a general improvement caused by recycled aggregates. Overall, the findings indicate that demolition-waste-derived recycled aggregates can be incorporated into two-stage concrete when aggregate quality, grading, grout rheology, and curing conditions are carefully controlled.</p>