To protect coastlines from coastal erosion, we consider making artificial rock modeled after beachrock as a novel countermeasure with low environmental impact. Beachrock is a coastal rock composed of coastal sediments cemented mainly by CaCO3, and exists mainly on tropical and subtropical beaches. The formation period of beachrock is much shorter than that of other sedimentary rocks. According to one of our previous studies, a beachrock in Okinawa, Japan could have been formed by bacterial ureolysis and/or seawater evaporation. In this work, we studied the effects of the ureolysis and seawater evaporation on beachrock formation by solidification tests in a laboratory. For the solidification test by bacterial ureolysis, we added a solution of cultivated urease-positive bacteria sampled from sand near a beachrock in Okinawa, and the CaCl2 and urea solution, into coral sand. After cultivation, we conducted a unconfined compression test, scanning electron microscopy? energy-dispersive X-Ray analysis, and microbial population count of the specimens. For the evaporation test, we repeated two processes in which the coral sand was soaked in artificial seawater with added CaCl2, and dried at 60 °C.
We obtained a specimen that was solidified to 6.7 MPa of unconfined compressive strength, after 28 days, by the bacterial ureolysis. Moreover, the cement in the specimen was aragonite. However, in the evaporation test, only the surface of the coral sand specimen was solidified. These results indicate that both bacterial ureolysis and seawater evaporation influence beachrock formation in Okinawa.
