Research On The Strength Improvement And Controllability Of Bio-Cmented Sand Materials

Biomineralization technology especially the microbially induced calcium carbonate precipitation(MICP)technique has wide applications in the field of civil engineering.The efficient preparation of bio-cemented sand materials and in-depth study of macroscopic mechanical properties as well as microstructures of bio-cemented sand materials pave the way for promoting the engineering application of MICP technique.This paper focuses on the strength improvement of bio-cemented sand materials and aims at achieving strength controllability of bio-cemented sand materials.The main conclusions of this paper are as follows:(1)The mechanism of growth and transformation of different calcium carbonate crystal polymorphs during MICP reactions is clarified.The main influencing factors of the transformation process of amorphous calcium carbonate to vaterite to calcite or calcite and aragonite during the MICP reaction are revealed.Studies indicate that increasing the reaction time,reducing the amount ratio of bacterial suspension to cementation solution,and the silica sand particle environment are beneficial to the transformation of calcium carbonate products to more stable crystal polymorphs.Additionally,calcium acetate acting as the calcium source can slow down the MICP reaction rate,reduce the size of precipitated calcite,and generate more aragonite crystals.(2)Without increasing the number of grouting and the amount of cementation solution,the utilization rate of calcium ions can be improved by combining calcium sources(calcium chloride and calcium acetate),adding additives(magnesium ions,watersoluble matrix extracted from nacreous layer of shell),further improving the strength of bio-cemented sand materials.(3)The mechanism of calcium carbonate crystal polymorph on the strength improvement of bio-cemented sand materials is revealed.By analyzing microstructures of sand particle materials cemented by different calcium carbonate crystal polymorphs,it is found that materials cemented by calcite-aragonite crystal polymorph have denser bonds between calcium carbonate crystals and sand particles,also with higher modulus and hardness,denser pore structure,and larger specific surface area at the contact interface,contributing to higher mechanical strength.(4)The cementation effects along the grouting direction of 1m bio-cemented sand columns after one batch of grouting is investigated.Experimental results indicate that when using well-graded coarse particles as aggregates,using calcium acetate as the calcium source,and increasing the grouting rate of bacterial suspension or cementation solution can increase the transmission distance of unhydrolyzed urea and free calcium ions,improve the uniformity of calcium carbonate distribution along the grouting direction,and reduce the strength differences among the bio-cemented sand materials.(5)The strength controllability of bio-cemented sand materials on the71mm×71mm×71mm brick is achieved.The bio-cemented sand bricks(abbreviated as bio-brick)with 5 MPa,7.5 MPa,10 MPa,15 MPa,20 MPa,and 25 MPa are successfully prepared,and the preparation methods of bio-bricks corresponding to different strength grades are provided.Through further analysis of the mechanical behaviors of bio-bricks,it is found that compared to clay bricks,mortar,and concrete,bio-bricks have a smaller modulus and larger peak strain.