Low-carbon composite binders with high contents of Icelandic volcanic pozzolan and silicomanganese slag: Hydration kinetics, microstructure and strength development

Reducing clinker factor is essential for lowering the environmental impact of cement-based materials, but maintaining mechanical performance at high replacement levels remains challenging. This study presents a systematic long-term comparison of low-carbon binary binders containing Icelandic volcanic pozzolan (VPI) and ternary binders containing VPI and silicomanganese slag (SiMn) at total cement replacement levels of 50–90%. Paste mixtures prepared at a water-to-binder ratio of 0.35 with 1% superplasticiser were investigated by compressive strength, isothermal calorimetry, and compositional phase and microstructural characterisations up to 365 days. Increasing cement replacement level delayed hydration and reduced early-age strength. At 28 days, binders with 50% total replacement achieved strengths comparable to or even exceeding control with ordinary Portland cement (OPC) reaching 75–85 MPa. Mixtures with 70% and 90% replacement attained 78–84% and 46–58% of the OPC strength, respectively, and continued to gain strength with curing. Binary binders showed greater portlandite consumption and more sustained formation of secondary reaction products than the corresponding ternary binders. Nevertheless, incorporation of 20% SiMn slag contributed to the long-term performance of VPI-rich systems despite its low intrinsic reactivity, and strengths remained, on average, within ±5% of the corresponding binary binders up to 70% replacement. The most favourable ternary response was obtained at 50% replacement. These results clarify stand alone and combined effect of VPI with SiMn slag in high-volume cement replacement binders and establish long-term relationships between hydration, microstructure, and strength that support the design of low-carbon binders with competitive mechanical performance.