Preparation And Cementing Mechanism Of Bio-Phosphate Cement And Composite Cement

Bio-cement, based on microbial induced mineralization, can consolidate the loose particles and would be a novel and sustainable cementing material which is applied to foundation reinforcement, dust control and other fields. The bio-calcite cement, as one kind of bio-cement, has been investigated widely. However, ammonia will be released in the cementation process of bio-calcite cement, which has a negative impact on the ecological environment. This paper presents a new kind bio-cement based on phosphate-mineralization microbe, and the bio-phosphate minerals, which have cementation, are prepared by the bacteria reacting with the substrate. Ammonia/ammonium can be changed into environment-friendly struvite when the soluble phosphate is added to bio-carbonate cement. After that, struvite and carbonate, which can be considered as composite cement, are applied to cement loose particles. Two types of bio-cement are environmentally friendly, which have important application prospect.Closely abounding bio-phosphate cement, composite and its cementing mechanism in this paper, the type of phosphate ions was studied by phosphate-mineralization microbe hydrolyzing the substrate. Bio-phosphate was prepared by adding different metal ions to the mixture solution of phosphate-mineralization microbe and the substrate, and then the properties of bio-phosphate were defined. Mixing-precipitation process was proposed to use based on injecting process. Mechanical properties of the bio-sandstones cemented by bio-barium phosphate, bio-magnesium phosphate and bio-ferric phosphate were studied. The influence of standing time and content of the slurry on the properties of the bio-sandstones were researched according to bio-barium phosphate. Ammonia was produced during the formation of bio-carbonate cement and could be effectively become into struvite by phosphate. The fixation ammonia ratio was researched by Mg/P molar ratio, reaction temperature, pH and CO32-. The performance of the bio-sandstones cemented by three formulations of composite cement was compared. Finally, cementing mechanism of bio-phosphate cement and composite cement was revealed. Mainly research conclusion and innovation results of this article were listed as follows five points.(1) Characteristics of different types of bio-phosphateBio-barium phosphate, magnesium phosphate and ferric phosphate were synthesized by bio-mineralization method. Firstly, the substrate was hydrolyzed by alkaline phosphatase secreted via phosphate-mineralization microbe, and then PO43- ions were obtained. Micro and nano-scale MHPO4, MPO4 or M3(PO4)2 are prepared by PO43- ions reacting with different types of metal cations. Structure, morphology and decomposition temperature of bio-barium phosphate, magnesium phosphate and ferric phosphate were studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric-differential scanning calorimetry analysis (DSC-TGA), and these properties were differ from chem-phosphate. (1) The morphology of bio-BaHPO4 is dumbbell, however, chem-BaHPO4 is quadrilateral. The average particle size of bio-BaHPO4 is bigger than chem-BaHPO4. The decomposition temperature of bio-BaHPO4 is lower than chem-BaHPO4, and the mass loss of bio-BaHPO4 is slightly higher than chem-BaHPO4. (2) The bio-Mg3(PO4)2·5H2O like a honeycomb, but chem-Mg3(PO4)2·5H2O is irregular. The average particle size of bio-Mg3(PO4)2·5H2O is less than chem-Mg3(PO4)2·5H2O. The decomposition temperature of bio-Mg3(PO4)2·5H2O is higher than chem-Mg3(PO4)2·5H2O, and the mass loss of bio-BaHPO4 is slightly higher than chem-BaHPO4.3) Bio-Fe3(PO4)2·8H2O can be prepared by microbial deposition, but chem-Fe3(PO4)2·8H2O cannot be synthesized by a simple chemical reaction.(2) Influence of standing time and content of bio-phosphate on compressive strength of the bio-sandstonesThe standing time of bio-BaHPO4 slurry has a greater influence on the compressive strength of the bio-sandstones when mixing-precipitation process was innovatively adopted. The compressive strength of bio-sandstones is increasing with the increases content when the quality of bio-BaHPO4 slurry accounts for 10%-50% of the total mass of quartz sand. However, the compressive strength of the bio-sandstone is decreased at content 60%. The porosity of the bio-sandstones with the increasing content of the slurry inclines to decrease, but reducing the range is difference. SEM images show that the filler in the bio-sandstones are increasing with increases in bio-BaHPO4 slurry content.(3) Ammonia treatment in bio-carbonate cement and formulation of composite cementlmol of ammonia was produced and released into the air when lmol of calcium carbonate was prepared by microbial induced precipitation. Ammonia/ammonium can effectively be converted into environment-friendly struvite, which has cementation, when carbonate-mineralization microbe contained K2HPO4·3H2O. Composite cement was obtained thereby, and its formulation (molar ratio) was bio-carbonate cement:MgCl2:K2HPO4·3H2O=1: 2:2. The fixation ammonia ratio was 62.75% in the formulation. However, the fixation ammonia ratio was 75.12% when the formulation was applied to bind sand column. The optimum fixation ammonia ratio, which was 88.52%, was determined by the composite of sodium carbonate and pH in the formulation.(4) Influence of cementation process and content of composite cement on compressive strength of the bio-sandstonesBased on mixing-precipitation process, the injection process was adopted to bind loose sand particles. Permeability, porosity, compressive strength and internal microstructure of the bio-sandstones cemented by composite cement were determined under different number of injections. Mixing-precipitation process was inferior to injection process according to compressive strength of the bio-sandstones caused by the particle size and morphology of composite cement. Permeability, porosity, compressive strength and fixation ammonia ratio of the bio-sandstones were compared when three different formulations of composite cement (CJ1, CJ1.5 and CJ2) were adopted to bind sand columns. The results show that the CJ2 has the best overall performance. The molar ratio of K2HPO4·3H2O and urea was 2:1 in the CJ2 formulation.(5) Cementing mechanism of bio-phosphate cement and composite cement between loose sand particlesThermal properties, Si-O bond, Si(2p) electron binding energy, micro-structure, etc. of the sand columns were determined by FTIR, XPS, DSC-TGA, SEM and TEM techniques. Results show that bio-barium phosphate and composite cement can interact with quartz sand to generate van der Waals bond and intramolecular hydrogen bond respectively, which play a role in binding function between loose sand particles and bio-cement. The difference of cementing mechanism between bio-phosphate cement and composite cement was caused by the products and organic matrix.