Depth-Dependent Crack Healing in Concrete Using MICP: A Transparent Mold Technique and the Role of Supplementary Cementitious Materials

This study investigates the depth-dependent crack-healing behavior of microbially induced calcium carbonate precipitation (MICP) using a novel transparent acrylic mold that enables non-destructive, real-time observation of precipitation across the crack depth in a controlled visualization model. Mortar specimens incorporating fly ash (30%), silica fume (10%), and crystalline admixture (2%) were evaluated to assess the influence of binder composition on MICP-related calcium accumulation. The crack healing process was conducted and monitored over a period of 28 days, and later, calcium accumulation was quantified using an EDTA titration method. The results show that MICP exhibits non-uniform, depth-dependent behavior, with precipitation initiating near the crack opening and progressing toward deeper regions over time. Limited oxygen availability and sustained moisture in deeper zones appear to restrict in situ MICP precipitation, suggesting a possible transition from reaction-controlled to transport-limited conditions. Among the mixtures, fly ash mortar demonstrated the highest calcium accumulation, while silica fume mortar showed the lowest. The findings show an important role of calcium availability and environmental conditions on the performance of MICP and the proposed technique, and they also provide more detail in observing the behavior of MICP across the crack depth.