Mitigating deterioration of cement-treated clay by microbe-based calcite precipitation

Cement is a crucial raw material widely used in ground improvement applications. However, on exposure to seawater, cement deteriorates due to the exchange of calcium and magnesium. Previous studies have proposed the countermeasure of microbiologically induced calcium carbonate precipitation (MICP) in an effort to mitigate this deterioration process. However, most MICP studies are laboratory based and address sand instead of clay. This study presents the preparation of samples of cement-treated clay with added urea and urease-producing bacteria (Sporosarcina aquimarina). Seawater exposure tests were conducted in both laboratory and in situ. The results show that S. aquimarina remained active, hydrolysing urea in cement-treated soil under highly alkaline conditions. In MICP-treated samples, the leaching of calcium and absorption of magnesium were reduced. Moreover, the exposed surfaces were replenished with calcite crystals, thereby mitigating the deterioration. In situ, the deterioration proceeded ∼1.5 times faster than in the laboratory, whereas in the deep sea, the deterioration was considerably slower. Nevertheless, the deterioration mitigation effect was confirmed at all depths. Therefore, the approach of premixing S. aquimarina and urea into cement applies to clay, and the proposed technique was demonstrated in situ.