Coconut shell biochar–Bacillus cereus DKBovi-5 based biocomposite as a sustainable additive for cement mortar: Effect of pyrolysis temperature on characterization, strength, hydration, and healing

Although biochar–bacteria composites have been explored for self-healing in cementitious materials, the influence of pyrolysis temperature on microbial compatibility and healing performance has been insufficiently investigated. This study addresses this gap by examining how pyrolysis temperature affects the physicochemical properties of coconut shell biochar and its effectiveness as a microbial carrier in mortar. Biochar produced at 300 °C, 400 °C, and 500 °C was characterized, and Bacillus cereus DKBovi-5 was immobilized onto it to form biocomposites. The biocomposites were incorporated into mortar to evaluate mechanical, microstructural, and crack healing performances. Characterization of biochar showed enhanced crystallinity at 500 °C as indicated by XRD, development of primary and secondary pores confirmed by FESEM, and increased micronutrient concentrations due to thermal enrichment observed through ICP-MS. Compressive strength restoration increased from 80.21 % to 91.23 % between 300 °C and 500 °C temperatures. TGA analysis, interpreted using Bhatty's method, indicated an increase in the degree of hydration from 61.65 % to 65.33 %. Rietveld refinement of XRD data revealed a rise in calcite content from 24 % to 51 %. FESEM imaging confirmed the deposition of hydration products within the biochar pores. Healing evaluation showed closure of cracks up to 0.762 mm and 0.920 mm in mortars with 300 °C and 500 °C biocomposites, respectively, corresponding to healed areas of 92.49 % and 100 %. The healed products in all biocomposites were confirmed as calcite through FESEM-EDS and XRD analyses. Optimized pyrolysis at 500 °C yielded a biocomposite with superior microbial healing performance, establishing its suitability as a self-healing admixture in bio-mortar applications.