Effect of Metakaolin and Biochar Addition on the Performance of 3D Concrete Printing: A Meta-Analysis Approach

Three-dimensional (3D) concrete printing (3DCP) is an emerging digital construction technology that enables geometrically complex structures with reduced labour, material waste, and formwork. However, the sustainability of 3DCP remains constrained by its heavy reliance on Portland cement, a major source of global CO₂ emissions. This study systematically examines metakaolin (MK) and biochar (BC) as sustainable additives for 3DCP, focusing on their independent effects on mechanical performance, printability, dimensional stability, and environmental impact. A comprehensive literature review (2015 to June 2025) identified 254 publications, of which 21 met the inclusion criteria for quantitative meta-analysis, contributing a total of 95 datasets for compressive and flexural strength. Pooled effect sizes were calculated using a random-effects model, supported by risk-of-bias and heterogeneity analyses. The results indicate statistically significant improvements in mechanical properties, with an overall pooled ratio of means (ROM) of 1.12 (95% CI: 1.06–1.20; I² = 48.9%), representing the overall mechanical performance effect across all datasets, while ROM for compressive and flexural strength was calculated separately in the main analysis. Meta-regression revealed that BC increased compressive and flexural strengths by 7% and 9%, respectively, while MK achieved greater enhancements of 21% and 13.4%. Optimum performance was observed at 15–20% MK for compressive strength and 10–15% for flexural strength, whereas BC performed best at 3–5% and 2–5%, respectively. BC contributed to CO₂ reductions of up to 43% through clinker substitution and biogenic carbon sequestration. These findings demonstrate that MK and BC are complementary eco-efficient modifiers capable of enhancing both structural and environmental performance in 3DCP. Future research should address long-term durability, standardisation of printing parameters, and cradle-to-grave life cycle assessments to strengthen practical implementation.