The natural seawater was used to carry out the experiment of microbial culture and microbial-induced calcium carbonate precipitation (MICP) to strengthen calcareous sand. First

the effect of seawater on microorganisms was studied through the growth and reproduction of microorganisms and the change of urease activity. Then

the effect of seawater on the strengthening effect of MICP was evaluated according to the changes of calcareous sand permeability and unconfined compressive strength (UCS) before and after MICP treatment. Finally

the mechanism of the effect of seawater on the strengthening of calcareous sand by MICP was analyzed by SEM and XRD tests. The results show that: 1) natural seawater leads to a lag in the growth of microorganisms

but the number of microorganisms and urease activity in the stable stage are not significantly different from those in fresh water environment; 2) the effect of using seawater to strengthen calcareous sand by MICP is less different than that under fresh water condition

the permeability coefficient of calcareous sand can be reduced by an order of magnitude

and the UCS value can reach 1.7 MPa; 3) under the condition of natural seawater

the MICP process is controlled by seawater composition

microorganisms

calcium ion concentration

urea concentration and pH. The main crystal form of calcium carbonate is calcite

which can fill the intergranular pores and make the sand particles cement as a whole

which is the main reason for the improvement of mechanical properties of calcareous sand.