Abstract
The addition of biochar to Portland cement composites has been proven to increase some of the material properties. The effect on alkali-activated materials has not been fully investigated. In this study, different recipes of metakaolin pastes at different biochar amounts are tested. Their physical and mechanical properties are analyzed to understand if any beneficial effects can be found even for alkali-activated binders. The results show that the addition of small amounts of biochar (<2 wt%) increases the compressive strength of metakaolin pastes (+15% after 28 days) and decreases the water absorption by capillarity, possibly leading to increased durability. Higher biochar content decreases the mechanical properties but provides higher dimensional stability and reduces the formation of efflorescence.