A Study on Forced Carbonation of Limestone Calcined Clay Cement and Concrete

There is a growing need for research exploring innovative solutions to reduce CO₂ emissions in the construction sector while maintaining concrete performance. This research examines the incorporation of sodium bicarbonate (NaHCO₃) in ordinary Portland cement (OPC) and limestone calcined clay cement (LC³) to investigate its effects at both the cement and concrete scales. LC³ cement was developed by partially substituting traditional clinker with calcined clay and limestone, known for their lower carbon footprint and comparable strength properties to OPC. Various dosages of NaHCO₃ (0%, 1%, 3%, and 5%) were incorporated to assess their impact. Tests conducted include calorimetry, slump, compressive and tensile strength, density, water absorption, pH evolution, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Results demonstrated that moderate dosages of NaHCO₃ (1% for LC³ and 3% for OPC) enhanced early-age hydration and mechanical properties by promoting internal carbonation, increasing nucleation sites for hydration products, and improving microstructure. However, higher concentrations negatively impacted overall performance due to excessive carbonation. Comparatively, LC³ demonstrated superior strength and durability properties at optimal NaHCO₃ dosages, evidenced by higher compressive and tensile strengths, reduced water absorption, and a denser microstructure than OPC, owing to the pozzolanic activity of calcined clay and limestone. These findings highlight LC^3’s potential as a sustainable alternative capable of providing strong and durable concrete with a significantly reduced environmental impact. Optimizing NaHCO₃ dosage is critical to maximizing performance benefits while minimizing structural integrity risks.