Bio-cementation for tidal erosion resistance improvement of foreshore slopes based on microbially induced magnesium and calcium precipitation

In most coastal and estuarine areas,tides easily cause surface erosion and even slope failure,resulting in severe land losses,deterioration of coastal infrastructure,and increased floods.The bio-cementation technique has been previously demonstrated to effectively improve the erosion resistance of slopes.Seawater contains magnesium ions(Mg^(2+))with a higher concentration than calcium ions(Ca^(2+));therefore,Mg^(2+)and Ca^(2+)were used together for bio-cementation in this study at various Mg^(2+)/Ca^(2+)ratios as the microbially induced magnesium and calcium precipitation(MIMCP)treatment.Slope angles,surface strengths,precipitation contents,major phases,and microscopic characteristics of precipitation were used to evaluate the treatment effects.Results showed that the MIMCP treatment markedly enhanced the erosion resistance of slopes.Decreased Mg^(2+)/Ca^(2+)ratios resulted in a smaller change in angles and fewer soil losses,especially the Mg^(2+)concentration below 0.2 M.The decreased Mg^(2+)/Ca^(2+)ratio achieved increased precipitation contents,which contributed to better erosion resistance and higher surface strengths.Additionally,the production of aragonite would benefit from elevated Mg^(2+)concentrations and a higher Ca^(2+)concentration led to more nesquehonite in magnesium precipitation crystals.The slopes with an initial angle of 53°had worse erosion resistance than the slopes with an initial angle of 35°,but the Mg^(2+)/Ca^(2+)ratios of 0.2:0.8,0.1:0.9,and 0:1.0 were effective for both slope stabilization and erosion mitigation to a great extent.The results are of great significance for the application of MIMCP to improve erosion resistance of foreshore slopes and the MIMCP technique has promising application potential in marine engineering.

Compressibility characteristics of bio-cemented calcareous sand treated through the bio-stimulation approach

Calcareous sand is widely present in coastal areas around the world and is usually considered as a weak and unstable material due to its high compressibility and low strength.Microbial-induced calcium carbonate precipitation(MICP)is a promising technique for soil improvement.However,the commonly adopted bio-augmented MICP approach is in general less compatible with the natural soil environment.Thus,this study focuses on the bio-stimulated MICP approach,which is likely to enhance the dominance of ureolytic bacteria for longer period and thus is deemed more efficient.The main objective of this paper is to investigate the compressibility of calcareous sand treated by bio-stimulated MICP approach.In the current study,a series of one-dimension compression tests was conducted on bio-cemented sand pre-pared via bio-stimulation with different initial relative densities(D r).Based on the obtained compression curves and particle size distribution(PSD)curves,the parameters including cementation content,the coefficient of compressibility(a v),PSD,relative breakage(B r),and relative agglomeration(A r)were discussed.The results showed that a v decreased with the increasing cementation content.The bio-cemented sand prepared with higher initial D r had smaller(approximately 20%e70%)a v values than that with lower initial D r.The specimen with higher initial D r and higher cementation content resulted in smaller B r but larger A r.Finally,a conceptual framework featuring multiple contact and damage modes was proposed.

Cementation of Loose Sand Particles based on Bio-cement

Loose sand particles could be cemented to sandstone by bio-cement（microbial induced magnesium carbonate）. The bio-sandstone was firstly prepared, and then the compressive strength and the porosity of the sandstone cemented by microbial induced magnesium carbonate were tested to characterize the cementation effectiveness. In addition, the formed mineral composition and the microstructure of bio-sandstone were analyzed by X-ray diffraction（XRD） and scanning electron microscopy（SEM）, respectively. The experimental results show that the feasibility of binding loose sand particles using microbial induced magnesium carbonate precipitation is available and the acquired compressive strength of bio-sandstone can be excellent at certain ages. Moreover, the compressive strength and the porosity could be improved with the increase of microbial induced magnesium carbonate content. XRD results indicate that the morphology of magnesium carbonate induced by microbe appears as needles and SEM results show that the cementation of loose sand particles to sandstone mainly relies on the microbial induced formation of magnesium carbonate precipitation around individual particles and at particle-particle contacts.

Unconfined compressive strength of MICP and EICP treated sands subjected to cycles of wetting-drying,freezing-thawing and elevated temperature:Experimental and EPR modelling

Microbial-induced carbonate precipitation(MICP)and enzyme-induced carbonate precipitation(EICP)are two bio-cementation techniques,which are relatively new methods of ground improvement.While both techniques share some similarities,they can exhibit different overall behaviours due to the differences in urease enzyme sources and treatment methods.This paper presented 40 unconfined compressive strength(UCS)tests of MICP and EICP treated sand specimens with similar average calcium carbonate(CaCO3)content subjected to cycles of wetting-drying(WD),freezing-thawing(FT)and elevated temperature(fire resistance test e FR and thermogravimetric analysis e TG).The average CaCO3 content after a certain number of WD or FT cycles(ACn)and their corresponding UCS(qn)reduced while the mass loss increased.The EICP treated sand specimens appeared to exhibit a lower resistance to WD and FT cycles than MICP treated specimens possibly due to the presence of unbonded or loosely bonded CaCO3 within the soil matrix,which was subsequently removed during the wetting(during WD)or thawing(during FT)process.FR test and TG analysis showed a significant loss of mass and reduction in CaCO3 content with increased temperatures,possibly due to the thermal decomposition of CaCO3.A complete deterioration of the MICP and EICP treated sand specimens was observed for temperatures above 600C.The observed behaviours are complex and theoretical understanding is far behind to develop a constitutive model to predict qn.Therefore,a multi-objective evolutionary genetic algorithm(GA)that deals with pseudo-polynomial structures,known as evolutionary polynomial regression(EPR),was used to seek three choices from millions of polynomial models.The best EPR model produced an excellent prediction of qn with a minimum sum of squares error(SSE)of 2.392,mean squared error(MSE)of 0.075,root mean square error(RMSE)of 0.273 and a maximum coefficient of determination of 0.939.

Experimental Study on Urease Activity and Cementation Characteristics of Soybean

A new and more ecologically sound cementing material known as“bio-cement”has been found to have the capacity to consolidate loose gravel into sand columns offering a certain degree of strength,and to fill and repair cracks in concrete to restore resilience.The typical representative is the microbial induced calcium carbonate deposition technology(MICP)and enzyme induced calcite precipitation(EICP).As part of this research,EICP with soybean urease as the core was studied.The test results show that soybean urease activity is significantly affected by pH and urea concentration values,while the external nickel source is not found to impair a stimulating effect on activity.When the concrete specimens were immersed in the composite solution of soybean urease,urea,and calcium chloride after having been subjected to a high temperature,a continuous layer of white precipitations quickly appeared on the surface of the specimens.Measured using a metalloscope,the thickness of the precipitations was found to reach up to 2.0 mm,while the surface water absorption rate was reduced by 70%.The effects of this combined outcome are believed to significantly protect and improve the durability of the concrete specimens previously subjected to a high temperature.At the same time,the composite solution is shown to be capable of cementing fly ash,with the cubic strength of the finished samples reaching 4.0 MPa after 3 days.Results from the use of a scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD),reveal that both the white precipitations on the surface of the concrete specimens and the cement binding the fly ash particles are calcite crystals.It is concluded from these preliminary study results that the use of soybean urease as a bio-cement had proved successful.