Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay

The cement industry is a major contributor to global CO₂ emissions. This has driven the need for low-carbon, locally adaptable binder systems. This study develops and evaluates a fully clinker-free pozzolana–lime cement formulated using sawdust ash (SDA) and calcined kaolinitic clay (metakaolin, MK) as complementary supplementary materials. SDA and MK were thermally activated at 600°C to enhance pozzolanic reactivity and combined with hydrated lime to form ternary binders. Mortars were prepared at a constant binder-to-sand-to-water ratio of 1:3:0.8 by blending the powdered constituents, followed by controlled mechanical mixing and casting into 40 × 40 × 160 mm prisms. Specimens were demolded after 48 h and cured initially under high-humidity conditions and subsequently in ambient air to promote both pozzolanic reactions and natural carbonation. Chemical and mineralogical characterization was performed using X-ray fluorescence, X-ray diffraction, and loss on ignition, while pozzolanic activity was assessed through electrical conductivity measurements in saturated calcium hydroxide solutions. Hardened mortars were evaluated in terms of density, water absorption at 28 days, and flexural and compressive strength at 28 and 90 days. Results showed that both SDA and MK exhibit measurable pozzolanic activity, with MK providing stable aluminosilicate reactivity and SDA contributing high silica content and alkali-assisted dissolution. Blended mortars demonstrated significantly improved mechanical performance compared with pure lime, achieving compressive strengths up to 7.1 MPa at 90 days for an optimal 50:25:25 (lime:SDA:MK) formulation. Moderate pozzolan contents refined pore structure and reduced water absorption, while excessive replacement reduced density and strength due to calcium deficiency. The findings demonstrate a synergistic interaction between SDA and MK, enabling the formation of strength-contributing phases and supporting the development of a sustainable, low-emission, lime-based binder suitable for structural and non-structural applications.