Experimental study of enhancing the effects of microbial-induced calcite precipitation treated sand using carbonic anhydrase

This study aimed to enhance the efficiency of microbial-induced carbonate precipitation (MICP) for reinforcing sandy soil by inspiring natural processes involving microbial-induced carbon cycling and carbonation. The experiment focused on enhancing MICP curing of sandy soil using carbonic anhydrase (CA), which significantly increases the reaction rate of CO₂ hydration (10⁸ times faster) and facilitates the rapid hydration of CO₂ (produced by urease (UA) decomposition of urea) to form a substantial amount of carbonate. The effect of carbonic anhydrase on MICP-reinforced sandy soil and its underlying mechanism were systematically examined through a combination of macroscopic physical and mechanical tests and microfabrication tests. The results showed that: (1) CA significantly increases the production of cement during the microbial consolidation of sandy soils, and the optimum dose of carbonic anhydrase producing bacteria is reached at about 4%, which increases the production of cement by 105.3%, compared with conventional MICP. (2) The incorporation of CA improves the compressive strength and resistance of the cured body. In the range 0.25−4.00%, the unconfined compressive strength of the solidified soil sample increases with the increase of the CA bacteria content. The strength of the cured soil sample reaches 1.915 MPa when the content is 4%, which is 8.54 times the strength of the conventional MICP cured sample. (3) CA does not change the product of the MICP process, it is still calcite, but after adding CA, the grain size of the calcite is larger, the shape of the hexahedron is more standardised, and the mechanical properties are improved. (4) In the process of MICP, urease and CA co-precipitate calcium carbonate-cured sandy soil. CA can significantly accelerate the rate of urea-generated CO₂ hydrate and form <inline-formula id="M1"> <math id="mathml_M1" display="inline" overflow="scroll"> <msubsup> <mrow class="MJX-TeXAtom-ORD"> <mtext>HCO</mtext> </mrow> <mn>3</mn> <mo>−</mo> </msubsup></math></inline-formula> and <inline-formula id="M2"> <math id="mathml_M2" display="inline" overflow="scroll"> <msubsup> <mrow class="MJX-TeXAtom-ORD"> <mtext>CO</mtext> </mrow> <mn>3</mn> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> <mo>−</mo> </mrow> </msubsup></math></inline-formula>, providing more favourable conditions for mineralisation.