Performance Comparison of Bacterially Immobilized Natural Fibers for Self-Healing in Cement Mortar

Abstract The efficiency of self-healing cementitious materials is governed not only by the choice of microbial strain but also by the carrier medium that sustains bacterial viability under the highly alkaline cement environment. This study examines the suitability of natural fibers, hemp, jute, and coir, as sustainable carriers for Bacillus licheniformis KGEB16 and Bacillus subtilis VEB4 to facilitate microbially induced calcium carbonate precipitation in cement mortar. The fibers were alkali-treated to enhance the surface roughness and exposure of functional groups, enabling improved bacterial attachment and retention. Mechanical and healing assessments showed that bacterially immobilized hemp fiber provided the most favorable microenvironment for spore protection, metabolic activation, and uniform mineral deposition. Mortar specimens containing hemp-immobilized Bacillus licheniformis and Bacillus subtilis exhibited 22.9% and 20.6% higher compressive strength, respectively, along with complete crack healing. Jute and coir supported moderate to lower healing efficiency, reflecting differences in bacterial retention, moisture preservation, and mineral deposition. Morphological and mineralogical analysis revealed the formation of dense and continuous calcium carbonate layers, with Bacillus licheniformis immobilized hemp fiber producing the most compact polycrystalline calcite structures. These findings demonstrate that fiber-assisted microbial immobilization effectively improves autonomous crack sealing and mechanical recovery, establishing a durable and sustainable pathway for next-generation self-healing cementitious materials.