Low-alkali one-part hybrid cements from recycled concrete and waste glass: Microstructure, mechanisms, compositional window, and eco-efficiency

Limestone-aggregate recycled concrete powder (CaO-rich hardened concrete, HC) is an underexplored reactive precursor for low-alkali alkali-activated and hybrid cement binders. Here, one-part hybrid hydraulic binders were developed by combining HC (50–75%), Portland cement (20–50%), and a waste-glass-derived sodium silicate activator (SWG, 5–15 wt%) and cured under ambient temperature. Despite the very low external alkali input (0.1–0.34% Na 2 O N, introduced as NaOH during SWG synthesis), mixtures with 35%PC-60%HC-5%SWG reached 53 MPa at 28 days, while 50% PC-45%HC-5%SWG attained 73 MPa at 90 days. XRD, TGA, 29 Si MAS NMR, and SEM/EDS indicate coupled PC hydration and alkaline activation of HC, producing an integrated C–S–H/N–C–S–H gel network consistent with hybrid binding. The alkalinity was sustained by Na-bearing silicate phases formed during SWG synthesis and gradual Na release from residual waste glass, enabling activation at minimal external alkali input; however, higher SWG contents promote alkali saturation and strength loss in PC-rich systems. Comparative eco-efficiency screening indices (per MPa) suggest ∼65–70% lower CO 2 emissions than 100% PC reference, with 35%PC–60%HC–5%SWG providing the best 90-d balance of performance and impact. These results define a composition–alkali window linking microstructure and performance for circular, low-alkali cementitious composites.