Presence of Mg-calcite and its influence on MICP and EICP processes

Microbial induced carbonate precipitation (MICP) and enzyme induced carbonate precipitation (EICP) processes can be affected by many factors. The influence of magnesium on the MICP and EICP based soil improvement was studied in this paper across different scales ranging from micro, pore to macro. Results obtained from microfluidic chip tests indicate that the presence of a little amount of Mg ions in the cementation solution can reduce the bacterial cell coagulation and promote a more uniform distribution of crystals in the reaction channel. Aqueous phase tests were performed by controlling the concentration of calcium (Ca) to magnesium (Mg) ratio to vary from 1.00 : 0 to 0 : 1.00. The results show that magnesium could delay the precipitation process and increase the quantity of the precipitates. As the magnesium content increases, the crystal morphology of precipitates changes from calcite to Mg-calcite, vaterite, rosette and nesquehonite. Cementation effect in the Ca-rich group is superior to that in the Mg-rich group. In terms of unconfined compressive strength of the treated sand, the contribution of Mg is much less significant in Mg-rich groups. The performance of the sand treated with both MICP and EICP based methods under the presence of Mg was evaluated and discussed. All samples exhibited strength improvement after biotreatments. Among all the four groups, the EICP 1-phase group with Ca: Mg of 0.90 : 0.10 and 0.75 : 0.25 exhibited the largest strengths of 4.5 MPa and 4.7 MPa, respectively.