Hybrid Cements Based on Fly Ash and Blast Furnace Slag with Portland and Calcium Sulfoaluminate Cements: Hydration, Microstructure, and Performance

This study examined hybrid cement systems that incorporated high percentages of ground granulated blast furnace slag (GGBFS) and fly ash (FA), as well as ordinary Portland cement (OPC) and calcium sulfoaluminate cement (CSA). The addition of CSA significantly reduced the clinker content. A blend containing 15% OPC, 15% CSA, 35% FA, and 35% GGBFS achieved compressive strengths of 35.5 MPa after 28 days and 40.8 MPa after 90 days. These results are comparable to those of a 50:50 OPC–CSA reference mix (42.2 MPa after 90 days), despite containing 70% less Portland cement clinker. Two alkaline activation systems were employed: one using sodium sulfate (4 wt.% Na₂O) and another using a combination of sodium hydroxide and sodium silicate (6 wt.% Na₂O). All mixtures were cured at 25°C. Selected samples were also exposed to 65°C for 24 hours to evaluate the impact of the initial curing temperature on reactivity. Strength development was monitored up to 90 days. X-ray diffraction confirmed clinker phase consumption and ettringite formation. Scanning electron microscopy (SEM) revealed the presence and transformation of calcium silicate hydrate (C-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gels into mixed calcium-(sodium)-aluminosilicate hydrate (C-(N)-A-S-H) and calcium-aluminosilicate hydrate (C-A-S-H) structures. Infrared (IR)-attenuated total reflectance (ATR) spectroscopy provided additional evidence of these gels. These results demonstrate that hybrid systems based on GGBFS and FA can match the mechanical performance of conventional binders while significantly reducing CO₂ emissions, thus supporting their use in low-carbon construction.