| Microbial induced calcium carbonate precipitation(MICP)is a green and sustainable technology to change or improve the physical and mechanical properties of soil.Microbial reinforcement technique is a new technique for rock and soil mass reinforcement which has been developed rapidly recently.By inputting a bacterial solution and a nutrient salt used as a cementing solution into the soil,calcium carbonate is produced in a short time between the cracks of the soil by microbial mineralization,and calcium carbonate is used as the carrier to consolidate the gel soil,so as to improve the physical and mechanical properties of the soil.With the development of microbiology,geochemistry,civil engineering and other interdisciplinary cooperation,microbial induced calcium carbonate precipitation technology(MICP)has been widely paid attention to and studied.In this paper,active adsorbent materials are added to this technology to assist microorganisms in inducing calcium carbonate precipitation,so as to achieve stronger and more significant properties,which is more conducive to the practical engineering application value of this technology.A total of three materials were used for comparative analysis,respectively: silica powder,zeolite and vermiculite.The excellent activity and adsorbability of these materials are used to improve the efficiency of microbial reinforcement technology,so as to increase the yield and precipitation efficiency of calcium carbonate to strengthen the soil.The specific research contents and conclusions of this paper are as follows:(1)Silica powder has good pozzolanic activity and filling ability.Four different silica powder contents(0%,1%,2%,3% by weight)were used in combination with MICP technology.The effect of silica powder on the physical and mechanical properties of biotreated medium and coarse soil was studied by improving the precipitation efficiency of microbial induced calcium carbonate with the good activity of silica powder.The biotreated samples were evaluated for bacterial retention(the amount of bacteria that can be retained in the soil after bacterial injection),calcium carbonate precipitation,microstructure(the morphological structure of the above additional samples was examined by scanning electron microscopy.Mineral composition is examined by X-ray diffraction),unconfined compressive strength(UCS)and durability are tested and evaluated.The results showed that with the increase of silica powder content,the bacterial activity retention rate increased,the calcium carbonate content increased,the UCS increased,the durability improved.Overall,waste silica powder can be effectively reused in MICP technology because it improves the performance of sand treated by MICP.(2)Zeolite has strong adsorption and ion exchange properties,is now widely used in agriculture,environmental protection,pharmaceutical,petroleum technology and construction and many other fields.Combining zeolite powder with MICP technology,aiming to study the effect of zeolite on the performance of biotreated coarse-grained soil,samples were prepared by mixing five different zeolite contents(0%,1%,3%,5% and 7% by weight,respectively)with medium and coarse-grained sand from Chinese standard sand.Bacterial retention capacity,calcium carbonate precipitation and microstructure(SEM and XRD)were analyzed.The mechanical properties of the biotreated samples were evaluated in terms of unconfined compressive strength(UCS),Brazilian split tensile strength(BTS)and permeability.It was found that with the increase of zeolite content,bacterial activity retention rate increased,calcium carbonate content increased,UCS and BTS increased,permeability decreased significantly.The influence mechanism of adsorption and ion exchange of zeolite on MICP was emphatically analyzed,and the effects of calcium carbonate morphology and bridging methods on the solidification strength of sand were analyzed in detail from a microscopic perspective.(3)The purpose of adding vermiculite as auxiliary material is to study the effect of vermiculite on coarse sand in MICP treatment.Samples were prepared by mixing 6 different vermiculite contents(0%,1%,2%,3%,4% and 5% by weight)with medium and coarse sand from Chinese standard sand.Bacterial retention capacity,calcium carbonate precipitation and microstructure(SEM and XRD)were analyzed.The mechanical properties of the biotreated samples were evaluated in terms of unconfined compressive strength(UCS)and permeability.The physical properties including water absorption and softening coefficient were evaluated.Vermiculite also has excellent adsorption and ion exchange properties,which also play a positive effect on MICP.(4)In this paper,three kinds of materials have been studied,which have promoted the MICP technology respectively.Mainly from the microscopic morphology and physical and mechanical properties of the analysis.The microscopic morphology is SEM and XRD.The increase of calcium carbonate content can be clearly seen from the pictures of microscopic morphology,and the change of mineral composition can be seen from the XRD curve.The physical and mechanical properties analyzed in this paper are as follows: unconfined compressive strength(UCS),Brazilian splitting tensile strength(BTS),permeability,water absorption and durability.No matter what kind of material,all improve the physical properties of rock and soil reinforcement,different materials caused by different effects.In general,the effect of activated adsorbent materials assisting microorganisms to induce calcium carbonate precipitation is very obvious,which plays a role in this process with half the effort and brings practical value for practical engineering. |
