| Microbial induced calcite precipitation（MICP）,as a new environmentally friendly sand curing technology,has been widely studied by domestic and foreign scholars since Whiffin was proposed in 2004.As the most common calcium source,CaCl2 has been defined as the mainstream calcium source of research since the rise of MICP technology,but there are obvious drawbacks in the process of investing in many applications such as improving the strength of reinforced concrete.As is known to all,the passivation and electrochemical corrosion of steel bar will occur when chloride ion penetrates concrete through cracks,which may have adverse effects on steel bar and even seriously affect the durability of reinforced concrete.Ca（CH3COO）2 and Mg（CH3COO）2 have been used in various tests by domestic and foreign scholars as calcium sources to replace CaCl2due to their weak corrosion of acetate ions and stronger curing effect of MICP.Based on the MICP method,this paper mainly used Sporosarcina pasteurii to cement the standard sand by secondary injection.During the test,tests such as microbial activation culture preservation,determination of insoluble salt calcium carbonate content,X-ray diffraction,scanning electron microscopy,and unconfined compressive strength were carried out.We tested the macro mechanical properties of the cemented sand column treated by CaCl2,Ca（CH3COO）2 and Mg（CH3COO）2 respectively,a series of micro-structural analysis were performed to prove it and find the optimal calcium source.On this basis,the optimal concentration of the calcium source was also selected,providing basic data for the practical application of MICP solidified soil.The main research results of this paper are as follows:（1）Although the microbe-induced cementation products of urea/calcium acetate and urea/calcium chloride were CaCO3,they mainly existed in the forms of aragonite and calcite,while the cementation products of urea/magnesium acetate were magnesite magnesium carbonate（rodlike）and sepiolite magnesium silicate（fibrous）.（2）When the nutrient salt is urea/magnesium acetate,the microbial sand column relies on the above two cementing products to cement the loose particles into a whole,and has a certain compressive strength.However,the density of cemented particles is far less than that of the samples treated with urea/calcium acetate nutrient salt.and its compressive strength is also significantly weaker than the latter.Of the three,urea/calcium acetate as nutrient salt samples had the best cementation effect.（3）A series of comparative tests with a nutrient salt concentration of 0.3 and0.5mol/L proved:with the increase of the nutrient concentration,a large number of aragonite crystal clusters are generated,showing different crystal morphologies,and their deposited calcium carbonate content is also increasing.The mechanical properties will be improved due to the close contact between standard sands,showing better and better cementation effect.（4）Excessive nutrient concentration（0.7mol/L）will result in uneven distribution of cementitious substances and may not achieve the desired effect.It is further proved that the difference of macroscopic mechanical properties and cementation effect of the samples is not entirely dependent on the content of calcium carbonate,but also related to the size and distribution of calcium carbonate. |
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