Microbially Induced Calcite Precipitation Process Research Articles

Numerical optimisation of microbially induced calcite precipitation (MICP) injection strategies for sealing the aquifer's leakage paths for CO2 geosequestration application

Carbon capture and storage (CCS) in deep geological aquifers has shown to be the most viable option for mitigating the greenhouse gas effect of carbon dioxide (CO2) at a large scale. However, the underground formations often possess discontinuities in the caprocks, leaking the stored CO2. Potential leakage paths, such as abandoned wells, have been growing due to excessively unplugged oil and gas exploration wells. The leakage of CO2 from these wells is a major concern, considering their negative impact on the environment and compromising CO2 storage efficiency. Recently, microbially induced calcite precipitation (MICP) technology has proven to be an effective and sustainable method for reducing the permeability of geomaterials. Nevertheless, the MICP process involves intricate interactions among bio-chemo-hydraulics (BCH) domains to comprehend the reactive transport of biochemicals. The complex nature of the MICP process poses difficulties in setting the biochemical injection durations for a particular target distance at the given injection rate. Given this, the present study developed a coupled numerical model and employed it as a workable tool for optimising MICP injections to plug the abandoned well connecting two deep geological aquifers. Following that, the study evaluated the leakage of CO2 using flow migration rates in the untreated and MICP-treated leaky aquifer. The study proposed a novel optimisation strategy for biochemical injections under near and far-field leakage conditions. The sensitivity of biochemical injection durations on the attached bacterial amount and permeability in the leak was also determined. The observations from the present study indicated a complete reduction in the CO2 migration rates from the abandoned well due to a reduced permeability after MICP, thereby indicating the efficacy of the proposed optimisation methodology. Further, a cost analysis of the MICP treatment indicated a rational application cost with the target distance compared to the detrimental effects of CO2 leakage.

Experimental study on shield tunnel seepage control via microbially induced calcite precipitation

This study investigated the potential use of microbially induced calcite precipitation (MICP) to prevent seepage in shield tunnels with the aim of decarbonizing tunnel engineering. An apparatus was developed to conduct scale model tests to evaluate the effectiveness of using MICP for shield tunnel seepage control. To understand the MICP process and its induced change in seepage flow rate, a series of 1-g physical model tests were conducted using the designed apparatus to investigate the effect of injection methods, grouting pressure, and calcium carbonate (CaCO3) content produced as well as its distribution on the reduction of seepage flow rate for thephysical tunnel model with different backfills behind its linings. The variation law of the pore pressure near grouting hole of the tunnel segment was also revealed. Results indicated that when the amount of CaCO3 precipitation in sand-grout mixtures was 10.53% and 10.12%, water seepage flow rate for thephysical tunnel modelwith Fujian- and coarse-sand-grout backfill respectively reduced by 94.3% and 73.8% of their respective initial values, and S-wave velocity increased by 89.6% and 84.9% for Fujian- and coarse-sand-grout mixture, respectively. The grouting pressure needed to be controlled within a certain range to prevent the unstable CaCO3 precipitates from being washed away. The testing results also showed that the one-phase injection method was more effective in controlling seepage water into a shield tunnel. Based on the findings of the scale model tests, some vital considerations and suggestions were presented on the use of MICP approaches for shield tunnel seepage control.

A biotechnological approach for suspended solids removal in biogas slurry via microbially induced calcite precipitation (MICP)

This study investigated the application of the microbially induced calcite precipitation (MICP) technique for removing suspended solids from biogas slurry. A series of laboratory tests on suspended solids content and colority determination was conducted to evaluate the feasibility of this technique and the removal performance of suspended solids. Combined with tests on residual calcium ions concentration and micro scale, the effects of crucial factors (including urease activity and CaCl2-Urea content) and the mechanisms of suspended solids removal were explored. The results unequivocally demonstrated the effectiveness of the MICP technique in removing suspended solids, achieving optimum removal efficiencies of 91.67 ± 1.68% for suspended solids and 96.50% for colority. The introduction of additional urease-producing bacteria via centrifugation-extraction increased the urease activity of bacterial solution and further enhanced the degree of MICP reaction, but undesirably led to a slight reduction in the removal efficiencies of suspended solids and colority due to the presence of bacteria themselves as suspended solids. Appropriately increasing the CaCl2-Urea content could offer a more adequate material supply to prolong the duration of the MICP process, significantly enhancing the removal efficiency. Nevertheless, the high salinity environment associated with excessive CaCl2-Urea addition (2.0 mol/L) would impair bacteria survival and impede the MICP process, negatively affecting suspended solids removal. The SEM-EDS analysis indicated that in the MICP process, suspended solids were tightly encapsulated by calcium carbonate (mainly calcite) to form strong immobilization aiding their gravity-driven removal. Due to the richness of organic components, they could be considered for reuse as soil amendments and fertilizers. This research provides an innovative insight into the removal of suspended solids from biogas slurry using a biotechnological approach. The MICP technique holds promise for practical applications due to its effectiveness and workability.

Effect of fibre‐reinforced microbial‐induced calcite precipitation on the mechanical properties of coastal soil

AbstractCoastal erosion is a global environmental concern, threatening infrastructure, human livelihoods and ecosystems. Recently, microbial‐induced calcite precipitation (MICP) has emerged as a promising ground improvement technique. The present study examined the effects of adding three different fibre reinforcements, namely carbon, basalt and polypropylene, on the physical and mechanical properties of coastal soil through MICP. The fibre content used was 0.20%, 0.40% and 0.60% of soil weight. A comprehensive biotreatment investigation was conducted using Sporosarcina pasteurii (S. pasteurii) in a 0.5 molar cementation solution. The samples prepared for this study had aspect ratios of 2:1 and 1:1. These samples were subjected to biotreatment, consisting of a 24‐h cycle for 9 and 18 days. Unconfined compressive strength (UCS), split tensile strength (STS) and ultrasonic pulse velocity (UPV) tests were conducted on the biotreated samples to evaluate the effect of fibre reinforcement on the mechanical properties of the biotreated samples. The amount of calcite precipitation, scanning electron microscope (SEM) and energy dispersive X‐ray spectroscopy (EDS) were used to interpret biocementation. Results suggest that adding fibres to the MICP process enhances the mechanical properties of coastal soil. The optimum fibre content for carbon and basalt fibre was 0.40%, whereas, for polypropylene, it stood at 0.20%. The maximum UCS, STS, UPV and average CaCO3 were observed in a basalt fibre‐reinforced biotreated sample with a fibre content of 0.40%, subjected to 18‐day biotreatment. Conversely, the sample without fibre‐reinforcement, biotreated for 9 days, exhibited the lowest values for these parameters. Samples subjected to 18 days of treatment have higher values of UCS, STS, UPV and CaCO3 content than 9‐day‐treated soil samples. SEM revealed the presence of CaCO3 precipitates on the surfaces of soil grains and their contact points, and the EDS spectrum corroborated this observation.

Microbial Induced Calcite Precipitate Improvement of Aeolian Soil for Use as a Construction Material

An environmentally friendly and successful technique of soil improvement known as Microbial Induced Calcite Precipitation (MICP) has attracted the attention of geotechnical engineers. This study was conducted to modify the engineering behaviour of an aeolian soil bio-treated with Bacillus brevis (B. brevis) to evaluate its suitability for use in construction works. Stepped B. brevis suspension density (i.e., 0.5, 2.0, 4.0, 6.0 and 8.0 McFarland standards equivalent to 0, 1.50 × 108, 6.0 × 108, 1.20 × 109, 1.80 × 109 and 2.40 × 109 cells/ml, respectively) and cementation reagent (i.e., 0.25, 0.5, 0.75 and 1.0 M), respectively, were utilized to activate the MICP process. Compaction was conducted using British Standard light (BSL) (or standard Proctor) energy. Tests results indicate that the engineering behaviour of the bio-treated non-plastic aeolian silt soil improved. The maximum dry density (MDD) and the angle of internal friction values marginally increased, while cohesion and optimum moisture content (OMC) decreased with higher B. brevis suspension density and cementation reagent concentration. Peak angle of internal friction and minimum cohesion were recorded at 1.5 x 108 cells/ml. Highest unconfined compressive strength (UCS) value of 215.58 kN/m2 was obtained for specimen treated with 25 % B. brevis (6.0 × 109 cells/ml): 75 % cementation reagent (0.75 M) mix ratio. The results recorded show that the improved aeolian soil could be used in engineering works.

Coupled bio-chemo-hydro-mechanical modeling of microbially induced calcite precipitation process considering biomass encapsulation using a micro-scale relationship

Geomaterials with inferior hydraulic and strength characteristics often need improvement to enhance their engineering behaviors. Traditional ground improvement techniques require enormous mechanical effort or synthetic chemicals. Sustainable stabilization technique such as microbially induced calcite precipitation (MICP) utilizes bacterial metabolic processes to precipitate cementitious calcium carbonate. The reactive transport of biochemical species in the soil mass initiates the precipitation of biocement during the MICP process. The precipitated biocement alters the hydro-mechanical performance of the soil mass. Usually, the flow, deformation, and transport phenomena regulate the biocementation technique via coupled bio-chemo-hydro-mechanical (BCHM) processes. Among all, one crucial phenomenon controlling the precipitation mechanism is the encapsulation of biomass by calcium carbonate. Biomass encapsulation can potentially reduce the biochemical reaction rate and decelerate biocementation. Laboratory examination of the encapsulation process demands a thorough analysis of associated coupled effects. Despite this, a numerical model can assist in capturing the coupled processes influencing encapsulation during the MICP treatment. However, most numerical models did not consider biochemical reaction rate kinetics accounting for the influence of bacterial encapsulation. Given this, the current study developed a coupled BCHM model to evaluate the effect of encapsulation on the precipitated calcite content using a micro-scale semiempirical relationship. Firstly, the developed BCHM model was verified and validated using numerical and experimental observations of soil column tests. Later, the encapsulation phenomenon was investigated in the soil columns of variable maximum calcite crystal sizes. The results depict altered reaction rates due to the encapsulation phenomenon and an observable change in the precipitated calcite content for each maximum crystal size. Furthermore, the permeability and deformation of the soil mass were affected by the simultaneous precipitation of calcium carbonate. Overall, the present study comprehended the influence of the encapsulation of bacteria on cement morphology-induced permeability, biocement-induced stresses and displacements.

Optimization of Injection Methods in the Microbially Induced Calcite Precipitation Process by Using a Field Scale Numerical Model

Microbially induced calcite precipitation (MICP) is a promising, more eco-friendly alternative method for landslide prevention and foundation reinforcement. In this study, we investigated the optimization of injection methods within the MICP process in porous media to enhance calcite mass and consolidation effect. The results demonstrated that staged injections with considerable advantages significantly improved precipitated calcite mass by 23.55% compared with continuous injection methods. However, extended retention times in staged injections reduced reinforcement effects. Moreover, setting the additional time in all injection methods can improve the consolidation area and effect without added injections. Apart from the injection methods, the changes in porosity and substance concentration also directly affected calcite masses and the reinforcement effect. Both the total calcite mass and the reinforcement effect should be taken into account when selecting appropriate injection methods. In terms of influencing factors on the total calcite mass, substance concentration ≫ average porosity ≫ additional time > retention time in staged injection. For the consolidation effect, substance concentration ≫ retention time in staged injection > average porosity ≫ additional time. The 5 h retention time in staged injections was recommended as the optimum injection method in the geotechnical conditions for average porosity from 0.25 to 0.45, with the changes in different reactant concentrations.

BCH modelling studies on biocementation process in mitigating leaks from a CO2 sequestrated aquifer

BCH modelling studies on biocementation process in mitigating leaks from a CO2 sequestrated aquifer

Insights in MICP dynamics in urease-positive Staphylococcus sp. H6 and Sporosarcina pasteurii bacterium

Microbially induced calcite precipitation (MICP) is an efficient and eco-friendly technique that has attracted significant interest for resolving various problems in the soil (erosion, improving structural integrity and water retention, etc.), remediation of heavy metals, production of self-healing concrete or restoration of different concrete structures. The success of most common MICP methods depends on microorganisms degrading urea which leads to the formation of CaCO3 crystals. While Sporosarcina pasteurii is a well-known microorganism for MICP, other soil abundant microorganisms, such as Staphylococcus bacteria have not been thoroughly studied for its efficiency in bioconsolidation though MICP is a very important proccess which can ensure soil quality and health. This study aimed to analyze MICP process at the surface level in Sporosarcina pasteurii and a newly screened Staphylococcus sp. H6 bacterium as well as show the possibility of this new microorganism to perform MICP. It was observed that Staphylococcus sp. H6 culture precipitated 157.35 ± 3.3 mM of Ca2+ ions from 200 mM, compared to 176 ± 4.8 mM precipitated by S. pasteurii. The bioconsolidation of sand particles was confirmed by Raman spectroscopy and XRD analysis, which indicated the formation of CaCO3 crystals for both Staphylococcus sp. H6 and S. pasteurii cells. The water-flow test suggested a significant reduction in water permeability in bioconsolidated sand samples for both Staphylococcus sp. H6 and S. pasteurii. Notably, this study provides the first evidence that CaCO3 precipitation occurs on the surface of Staphylococcus and S. pasteurii cells within the initial 15–30 min after exposure to the biocementation solution. Furthermore, Atomic force microscopy (AFM) indicated rapid changes in cell roughness, with bacterial cells becoming completely coated with CaCO3 crystals after 90 min incubation with a biocementation solution. To our knowledge, this is the first time where atomic force microscopy was used to visualize the dynamic of MICP on cell surface.

Investigation on the Microstructure, Unconfined Compressive Strength, and Thermal Conductivity of Compacted CDG Soil by MICP Treatment during Curing

As an eco-friendly treatment method, the microbially induced calcite precipitation (MICP) approach has been widely adopted in sand, but its use on compacted fine-grained soils remains scarce. In this study, compacted completely decomposed granite (CDG) soils at both the dry and wet side of the optimum water content (dry- and wet-of-optimum) were treated by the MICP approach, using a bacterial suspension and chemical reagents at fixed concentrations. After various curing periods, the microstructure (by scanning electron microscopy testing), the produced calcium carbonate content, the unconfined compressive strength (UCS), and the thermal conductivity were analyzed through a series of laboratory tests. The testing results indicate that the sample at dry-of-optimum exhibits a flocculated microstructure, while a dispersed microstructure is shown for the sample at wet-of-optimum. The calcium carbonate content increases with the curing period until reaching a stable value at around 6 days’ curing. After 6 days’ curing, the MICP process is relatively static, without significant amounts of calcium carbonate produced. With this treatment, the UCS of the sample is improved, where higher efficiency was observed for the sample at wet-of-optimum conditions. The variation of UCS for the MICP-treated samples with respect to the curing period follows the same trend as that of calcium carbonate content. At dry-of-optimum, the air-solid phases control the heat transfer in the MICP-treated samples with a discontinuous water phase, leading to insignificant effects of the MICP treatment on the thermal conductivity. In contrast, the water-solid phases dominate the heat transfer in the MICP-treated samples at wet-of-optimum, resulting in variation of the thermal conductivity with the curing period similar to that for the calcium carbonate content.

A coupled bio-chemo-hydro-wave model and multi-stages for MICP in the seabed

As an environmentally friendly technology, the microbially induced calcite precipitation (MICP) has been gradually applied for the seabed reinforcement. However, there are few models for the simulation of the MICP process in marine environment. A coupled bio-chemo-hydro-wave model for MICP in the seabed was proposed in this paper. This model can capture the bacterial behaviour (growth, decay, attachment, diffusion), the evolutions of chemical substances (urea, calcium, ammonium, calcium carbonate), and the variations of porosity and permeability. It is indicated that (1) distributions of the bacteria and chemical substances show obvious heterogeneity, and are related to the injection rate and injection time; (2) The calcium carbonate content, porosity, and permeability are significantly affected by the initial bacterial concentration, injection rate, injection time and cementing solution concentration. The reduction ratios of porosity and permeability show significant linear increasing trend with the increase of calcium carbonate content; (3) Although MICP process increases the vertical excess pore pressure gradient in the shallow seabed, the seabed can remain stable under the large waves due to the increasing of soil weight and inner cohesive force.

Investigation of the effect of microbial-induced calcite precipitation treatment on bio-cemented calcareous sands using discrete element method

Owing to the bio-geological origin, calcareous sands are characterized by irregular particle shapes and abundant internal pores. These weak soil structures undergo extensive particle breakage under specific stress conditions. In recent years, microbial-induced calcite precipitation (MICP), a green soil treatment technique, has been widely utilized to improve the mechanical properties of calcareous sands. The treatment effect of MICP on calcareous sands and the underlying mechanism are still relatively unclear owing to the complex microstructures of the sands. This study investigated the micromechanism of MICP treatment on calcareous sands via the discrete element method (DEM). First, a group of calcareous sand particles were individually treated via a proposed MICP process. Single-particle crushing tests were conducted on the MICP-treated calcareous particles and a control group of untreated calcareous particles. The experimental results were further used to verify and calibrate the DEM models and parameters. The DEM model of MICP-treated calcareous particles was established via a novel modeling technique using the commercial PFC3D platform, which considered the inter-particle bridging and intra-particle pore filling effects by MICP treatment. Through this method, a series of MICP-treated calcareous sand samples subjected to uniaxial compression were simulated. The effects of calcite content and its spatial non-uniformity on the compression behaviors of the bio-cemented calcareous sand samples were studied. The micromechanics in terms of crack propagation and distribution as obtained from DEM were investigated to elucidate the mechanism of the MICP treatment of calcareous sands.

The Impact of Calcium Chloride in Cementation Solution on Microbial Induced Calcite Precipitation: A Systematic Review

This review aims to quantify the impact of calcium chloride in cementation solutions on Microbial Induced Calcite Precipitation (MICP). Specific soil strength properties, such as the Unconfined Compressive Strength (UCS) test, permeability (k) and calcium carbonate content of the soil, form the basis of quantifying the test results. Relevant articles from various online databases such as Scopus, Science Direct, ProQuest Dissertations and Theses Global (PQDT), Mendeley and Google Scholar are obtained with search strings of suitable keywords. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) were used to screen and select related articles based on exclusion and inclusion characteristics. This review shows a positive correlation between calcium concentrations and soil strength properties, where higher concentrations of calcium solutions induce stronger bonding between soil particles due to better calcite precipitation. However, we also note a reversed correlation when the concentration of calcium solutions is higher than 1 M. This review also verifies that the MICP process enhances soil strength using optimum calcium chloride concentration to avoid soil brittleness. This result benefits other fields, such as agricultural and soil engineering.

State-of-the-Art Review of Utilization of Microbial-Induced Calcite Precipitation for Improving Moisture-Dependent Properties of Unsaturated Soils

Unsaturated soil is a form of natural soil whose pores are filled by air and water. Different from saturated soil, the microstructure of unsaturated soil consists of three phases, namely, the solid phase (soil particle), vapor phase, and liquid phase. Due to the matric suction of soil pores, the hydraulic and mechanical behaviors of unsaturated soils present a significant dependence on the moisture condition, which usually results in a series of unpredictable risks, including foundation settlement, landslide, and dam collapse. Microbial-induced calcite precipitation (MICP) is a novel and environmentally friendly technology that can improve the water stability of unsaturated soft or expansive soils. This paper reviews the microscopic mechanisms of MICP and its effect on the mechanical properties of unsaturated soils. The MICP process is mainly affected by the concentration of calcium ions and urea, apart from the concentration of bacteria. The moisture-dependent properties were comparatively analyzed through mechanical models and influence factors on the experimental data among various unsaturated soils. It suggests that the variations in resilient modulus and permanent deformation are strongly related to the extent of MICP applied on unsaturated soils. Finally, the problems in the MICP application, environmental challenges, and further research directions are suggested.

Self-repairing of shrinkage crack in mortar containing microencapsulated bacterial spores

The purpose of this research work was to evaluate the efficacy of microbially induced calcite precipitation (MICP) method in shrinkage crack repairing. Portland cement was combined with sand, bacterial spores, water, and nutrients to prepare self-healing composites. The microencapsulated bacterial spores were employed as an additive substance at ratios of 0%, 0.5%, and 1% by weight of cement. Specimens were kept in a climate-controlled room after casting to induce shrinkage crack. Crystalline phases formed in the specimen were identified using the X-ray diffraction (XRD) technique. The scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) method was used to investigate the morphology of precipitated calcite (CaCO3) crystals. Additionally, the specimen's permeability and compressive strength were also determined. When the bacterial spores were added to produce MICP procedure, the results demonstrated that it was successful in healing the shrinkage cracking. Within three days, the specimen with 1% spores could completely seal the shrinkage crack. According to the XRD patterns, the CaCO3 crystals appeared to promote the production of the calcite phase. Based on SEM/EDS investigation, the MICP process leads to concentrations of calcite crystal in certain regions, especially at the top surface of the crack. The 28-day compressive strength, on the other hand, decreased, owing to the addition of nutrients.

A discrete element simulation considering calcite crystal shape to investigate the mechanical behaviors of bio-cemented sands

Microbial-induced calcite precipitation (MICP) is an effective way to stabilize soils, but the microscopic mechanism is unclear. Discrete element method (DEM) models of the mechanical behaviors of bio-cemented sands offer a way to understand this process. This paper used DEM simulations considering calcite crystal shape to understand the microscopic behavior of MICP-cemented sands. Simulation results showed that macrocracks caused by biaxial tests are mainly caused by shear failure. Decreasing calcite content increases the number of high-amplitude acoustic emissions events. The magnitude of the contact force becomes more homogenous with increased calcite. Sand particles initially sustain the load, then calcite particles contribute more to the strong contact force chain. The calcite concentration also affects the magnitude and direction of anisotropy. The proposed DEM model demonstrates its potential to analyze the underlying mechanisms of MICP process.

Combining microbially induced calcite precipitation (MICP) with zeolite: A new technique to reduce ammonia emission and enhance soil treatment ability of MICP technology

The byproduct ammonia limits the wide application of the urea-hydrolysis based microbially induced calcite precipitation (MICP) technique. Here, we proposed a new technique termed the “MICP-zeolite technique” to reduce ammonia emission, which combined ammonia removal by zeolite with the MICP process. To understand the new technique, 1) the ability of the used zeolite to adsorb bacteria and remove ammonia was tested; 2) the MICP and MICP-zeolite techniques were applied to soils via both a mixing way and an injecting way to evaluate their performance in ammonia emission, soil cementation, and permeability reduction; 3) many properties of the zeolite and treated/untreated sands were characterized including surface charge, microstructure, surface area, pore size distribution. The results present that the zeolite can adsorb Sporosarcina pasteurii strongly, due to its larger specific surface area, rougher and outwardly-protruding surface, and less negative charge. Contact time, pH, temperature, and zeolite dosage affected ammonia removal efficiency. The MICP-zeolite technique resulted in less ammonia emission, greater soil strength, and lower permeability, and the injecting way is more effective in soil cementation. In soils treated by the proposed technique, the clogged pores were more, the precipitated crystals were more and smaller, the number of micropores and macropores reduced more, and the zeta-potential of particles increased more. Collectively, the MICP-zeolite technique is superior to the MICP technique in treatment effect and environmental friendliness.

Microbial induced calcite precipitation can consolidate martian and lunar regolith simulants.

We demonstrate that Microbial Induced Calcite Precipitation (MICP) can be utilized for creation of consolidates of Martian Simulant Soil (MSS) and Lunar Simulant Soil (LSS) in the form of a 'brick'. A urease producer bacterium, Sporosarcina pasteurii, was used to induce the MICP process for the both simulant soils. An admixture of guar gum as an organic polymer and NiCl2, as bio- catalyst to enhance urease activity, was introduced to increase the compressive strength of the biologically grown bricks. A casting method was utilized for a slurry consisting of the appropriate simulant soil and microbe; the slurry over a few days consolidated in the form of a 'brick' of the desired shape. In case of MSS, maximum strength of 3.3 MPa was obtained with 10mM NiCl2 and 1% guar gum supplementation whereas in case of LSS maximum strength of 5.65 Mpa was obtained with 1% guar gum supplementation and 10mM NiCl2. MICP mediated consolidation of the simulant soil was confirmed with field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and thermogravimetry (TG). Our work demonstrates a biological approach with an explicit casting method towards manufacturing of consolidated structures using extra-terrestrial regolith simulant; this is a promising route for in situ development of structural elements on the extra-terrestrial habitats.

The effects of biomineralization on the localised phase and microstructure evolutions of bacteria-based self-healing cementitious composites

Microbially induced calcite precipitation (MICP) is one of the most effective mechanisms to achieving self-healing abilities in cementitious composites. However, there has only been limited understanding of the effect of the MICP process on the mineralogy and microstructure of the cementitious matrix closely mixed with the healing products. This study systematically assessed the effect of biomineralization on the localised cementitious binders at micro and atomic level combining different characterisation techniques (i.e. XRD, FTIR and μCT). The results show that, in addition to the formation of CaCO3 polymorphs that close the crack space, the MICP process will also modify the phase assemblages near the healed cracks. For the first time we observed that when the most common source of calcium for the MICP process (calcium hydroxide) is limited, ettringite and C–S–H can also act as the providers of the calcium for the biomineralization process to take place. The detailed microstructure characterisations support that, apart from the dense thin layer (around 0.5 mm) of healing products formed on the surface of the cracks, loose particle-like calcium carbonate crystals can also form in pores and voids, suggesting that healing can also be generated in deeper sections of the crack. The outcomes of this study advance the fundamental understanding of the MICP process in Portland cement binders, and will also assist the further evaluation of the durability performances of these self-healed cementitious composites.

Effect of different fibers on small-strain dynamic properties of microbially induced calcite precipitation–fiber combined reinforced calcareous sand

Microbially induced calcite precipitation (MICP)–fiber combined reinforcement through the addition of fibers during the MICP process is a novel, natural, and green technique for foundation improvement, which can effectively enhance the mechanical properties of cemented soil. However, the small-strain dynamic properties of this method remain unexplored. The effects of different polyester fiber and hemp fiber contents (0%, 0.1%, 0.2%, and 0.4%) on the dynamic properties of the MICP–fiber combined reinforced calcareous sand under small-strain conditions were studied using resonant column tests, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The maximum dynamic shear modulus of the MICP–fiber combined reinforced calcareous sand under small-strain increases with the fiber content; the stiffness degradation rate of the dynamic shear modulus ratio of the test sample increases with the addition of two types of fibers, and the addition of hemp fibers greatly boosts the increase rate of the damping ratio. The SEM results indicate that excessive fibers distributed between the sand particles occupy the nucleation sites of bacteria, hinder the bonding between sand particles, and ultimately affect the dynamic shear modulus parameters of the sample. The AFM results show that the hemp fiber has much greater surface roughness than the polyester fiber. Therefore, the change rate of the damping ratio of the hemp fiber-doped MICP–fiber combined reinforced calcareous sand is much greater than that of the polyester fiber-doped sample. This is the first report on the basic mechanism of the small-strain dynamic properties of the MICP–fiber combined reinforced calcareous sand with different fiber contents and types. Furthermore, it provides a new theoretical basis for related research on improving the dynamic properties of sand thus reinforced.