Influence of black liquor on concrete performance: a sustainable approach

The integration of industrial by-products into concrete mixtures has gained significant attention as a sustainable approach to improving material performance while reducing environmental impact.This study investigates the effects of black liquor (BL), a lignin-rich by-product of the paper industry, as a novel admixture in concrete.Concrete samples with varying BL concentrations (0-4 wt.%) were prepared and evaluated for mechanical strength, workability, density, porosity, and microstructural modifications.The results indicate that the incorporation of 2 wt.%BL optimally enhances concrete properties, yielding a 25% increase in compressive strength (from 40 MPa to 50 MPa) at 28 days.Furthermore, splitting tensile and flexural tensile strengths peaked at 3.6 MPa and 15 MPa, respectively, at this concentration.Workability improved significantly, as evidenced by a 200% increase in slump value compared to BL-free concrete.Additionally, bulk density reached its maximum at 2.28 kg/L, while apparent porosity exhibited a notable decline to 9%, indicating matrix densification.Scanning electron microscopy (SEM) confirmed the refinement of pore structure and enhanced cementitious bonding at 2 wt.%BL.However, excessive BL content (3 wt.%) led to reduced performance due to increased porosity and disruption of cement hydration.These findings highlight the potential of black liquor as an effective and sustainable concrete admixture, offering enhanced mechanical properties and improved durability while promoting industrial waste reutilization.