Microbial induced calcium carbonate precipitation under extreme environmental conditions: impacts on biomineralization process and mechanical performance of concrete

Cracks in concrete structures can significantly decrease their lifespan by exposing reinforcement to the outside environment, leading to concrete degradation. To address this issue, bio-healing techniques have been developed, including biomineralization, where bacteria are employed to initiate microbially induced calcium precipitation (MICP).

This study examines the mechanisms of bio-healing in concrete under extreme environmental conditions, focusing on variations in pH and salinity. It utilizes immobilized cells of Bacillus paramycoides strain G10 for this investigation. Mineralization and the concrete strength were investigated using five distinct pH values (3.0, 5.0, 7.0, 9.0, and 11.0) and various salinity levels (0, 1, 2.5, and 5%).

At very high pH, rapid urea hydrolysis leads to the precipitation of mixed phases such as calcite and amorphous calcium carbonate, while soluble ions also promote the growth of long ettringite needles that help fill cracks. On the other hand, precipitation usually shifts toward metastable polymorphs such as aragonite or vaterite, or even amorphous phases with distorted and irregular morphologies, under extremely high salinity, which inhibits the production of stable calcite. Compressive strength improvements in 15 specimens containing bacteria immobilized on nano eggshells at pH 3, 5, 7, 9, and 11 were 25.5, 15.2, 13.5, 16.2, and 17.6%, respectively, in comparison with the corresponding control specimens without bacteria cured under identical conditions. Under various salinity concentrations of 0, 1, 2.5, and 5%, improvements were 5.2, 1.4, 3.3, and 6.7%, respectively, for specimens containing bacteria immobilized on nano eggshells when compared to their relative controls.

The combined action of calcite, which provides long-term stability, and ettringite, which offers early crack filling, enhances the strength and durability of concrete, making pH control essential in MICP systems. Understanding the interplay between environmental factors and bacterial sustainability is crucial in enhancing concrete infrastructure.