Carbon neutrality in alkali-activated slag (AAS): The role of biochar in AAS under carbonation curing

Abstract

This study aims to develop a carbon-neutral alkali-activated slag (AAS) composites by incorporating biochar. To achieve this, biochar dosages of 12 % and 15 % were incorporated, with corresponding AAS mixes containing 5 % and 7 % Na₂O dosages by weight of slag, respectively. The samples were subjected to accelerated carbonation curing for first three days; thereafter, kept at room temperature until testing. The AAS mortar specimens were evaluated for compressive strength following 3, 28, and 90 days of curing. Furthermore, paste samples were analyzed using various techniques, including chemical extraction analysis, TGA, FTIR, XRD, and BSE imaging, to gain insights into the microstructural characteristics. The results showed that incorporating biochar increased the compressive strength of AAS batches, with the mix containing 5 % Na₂O and 12 % biochar exhibiting a 53 % improvement after 90 days compared to the control at the same activator dosage. Microstructural studies showed that 5 % Na₂O concentration increased CaCO₃ formation, resulting in C-A-S-H decalcification. Due to the highly porous nature of biochar, the precipitated CaCO₃ occupies the empty pore space, leading to pore refinement, thereby improving the strength. In contrast, 7 % Na₂O concentration batches, with or without biochar, exhibited less CaCO₃ formation compared to 5 % Na₂O batches. Although the CaCO₃ formation in the 7 % Na₂O batch was similar regardless of the presence of biochar, the enhanced C-A-S-H formation from biochar incorporation improved the compressive strength of these samples. Therefore, this study presents a novel approach for incorporating biochar into AAS samples, resulting in a carbon-neutral cementitious composite with enhanced compressive strength.