A REVIEW OF MICROBIAL CATALYST–DRIVEN STRENGTH ENHANCEMENT OF REGUR SOIL THROUGH BIO-MINERALIZATION

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072

A REVIEW OF MICROBIAL CATALYST–DRIVEN STRENGTH

ENHANCEMENT OF REGUR SOIL THROUGH BIO-MINERALIZATION

Bheem Gautam1, Mr. Ushendra Kumar2

1Master of Technology, Civil Engineering, Lucknow Institute of Technology, Lucknow, India

2Head of Department, Department of Civil Engineering, Lucknow Institute of Technology, Lucknow, India

Abstract -Regur (black cotton) soil presents significant geotechnical challenges due to its high swelling–shrinkage behavior, low shear strength, and moisture sensitivity, which adversely affect the performance of civil infrastructure. Conventional stabilization methods using cement and lime, although effective, are associated with high carbon emissions and long-term environmental concerns. In recent years, microbial catalyst–driven bio-mineralization has emerged as a sustainable alternativeforimprovingtheengineeringproperties of problematic soils. This review critically examines the current state of research on bio-mineralization–based stabilization of Regur soil, with particular emphasis on microbial-induced and enzyme-induced carbonate precipitation mechanisms. The review synthesizes findings from laboratory and limited field studies addressing strength enhancement, swelling control, durability, and microstructural modifications. The influence of microbial activity, treatment protocols, and environmental conditions on performance isdiscussed indetail.Environmental, economic, and scalability aspects are also evaluated to assess practical applicability. The review identifies key research gaps and highlightsfuturedirectionsforadvancingbio-mineralization as an eco-friendly ground improvementtechniqueforexpansive soils.

Keywords: Regur soil; Bio-mineralization; Microbialinduced carbonate precipitation; Expansive soil stabilization; Sustainable ground improvement; Geotechnical engineering

1. INTRODUCTION

1.1EngineeringChallengesofRegur(BlackCotton) Soil

Regur soil, commonly referred to as black cotton soil, is a highly expansive clay soil predominantly found in central andsouthernregionsofIndia.Itischaracterizedbyahigh content of montmorillonite minerals, which impart pronouncedswell–shrinkbehaviorundervaryingmoisture conditions.Seasonalmoisturefluctuationscausesignificant volumetric changes, leading to differential settlement, cracking, and structural distress in pavements, embankments,andshallowfoundations(Chen,1988;Nelson andMiller,1992).

From an engineering perspective, Regur soil exhibits high plasticity,lowshearstrengthinsaturatedconditions,poor bearing capacity, and excessive compressibility. These

properties pose serious challenges for infrastructure development,particularlyforlow-volumeroads,residential foundations,andlightlyloadedstructures.Theunpredictable nature of its volume change behavior necessitates soil improvementorstabilizationpriortoconstructiontoensure long-term serviceability and safety (Sivapullaiah et al., 2000).

1.2 Limitations of Conventional Stabilization Methods

Conventionalstabilizationtechniquessuchaslime,cement, and fly ash stabilization have been widely adopted to improvetheengineeringpropertiesofexpansivesoils.While these methods are effective in reducing plasticity and increasing strength, their application is associated with severaltechnical,environmental,andeconomiclimitations, particularly in the context of sustainable geotechnical engineering(Bell,1996;Sherwood,1993).

1.2.1 Environmental and Sustainability Concerns

The production of cement and lime is highly energyintensive and contributes significantly to global carbon dioxideemissions.Cementmanufacturingaloneaccountsfor approximately7–8%ofglobalCO₂emissions,raisingserious concerns regarding the environmental sustainability of cement-based soil stabilization (Scrivener et al., 2018). In addition,excessiveuseofchemicalstabilizersmayaltersoil chemistryandnegativelyimpactsurroundingecosystems. InexpansivesoilssuchasRegursoil,conventionalstabilizers maynotalwaysprovidedurablelong-termperformance.The effectivenessoflimeorcementtreatmentisinfluencedby factors such as mineralogy, sulphate content, curing conditions,andmoistureingress.Sulphate-inducedheaving, leaching of stabilizing agents, and strength degradation under cyclic wetting and drying have been reported in severalstudies(Little,1995;Puppalaetal.,2006).Moreover, the brittle nature of cement-treated soils can lead to cracking, which compromises durability and increases maintenancerequirements.

1.3 Emergence of Microbial Catalyst–Based Soil Improvement

Inresponsetothelimitationsofconventionalstabilization techniques,microbialcatalyst–basedsoilimprovementhas emergedasaninnovativeandsustainablealternativewithin the field of bio-geotechnical engineering. This approach

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exploitsthemetabolicactivityofmicroorganismstoinduce theprecipitationofcementitiousminerals,primarilycalcium carbonate, within the soil matrix a process commonly referred to as microbial-induced carbonate precipitation (MICP)(DeJongetal.,2006;IvanovandChu,2008).

Microorganisms act as natural biochemical catalysts, facilitatingmineralprecipitationunderambientconditions withouttheneedforenergy-intensiveindustrialprocesses. The precipitated minerals bind soil particles, reduce pore spaces,andenhancemechanicalstrengthwhilepotentially improving durability and resistance to environmental loading. Due to its eco-friendly nature and low carbon footprint,microbialsoilimprovementhasgainedincreasing attentionasasustainablegroundimprovementtechnique.

1.4 Need for a Focused Review on BioMineralization of Regur Soil

Although extensive research has been conducted on biomineralization in granular soils such as sands, the application of microbial techniques to fine-grained and expansive soils remains relatively limited. Regur soil presentsuniquechallengesformicrobialtreatmentduetoits lowpermeability,high surfaceactivity,andcomplex clay–microbeinteractions(MitchellandSantamarina,2005).

Existing studies on microbial treatment of Regur soil are scattered, often limited to laboratory-scale investigations, and vary widely in terms of microbial species, treatment protocols, and reported performance outcomes. A comprehensive and critical synthesis of this literature is therefore necessary to evaluate the true potential, limitations, and applicability of microbial catalyst–driven bio-mineralizationspecificallyforRegursoil.Suchafocused review can help bridge the knowledge gap between laboratoryresearchandpracticalfieldimplementation.

1.5 Objective of the Review

Theprimaryobjectiveofthisreviewistocriticallyexamine existing literature on microbial catalyst–driven biomineralization techniques for strength enhancement of Regur soil. The review synthesizes findings related to microbialmechanisms,treatmentmethodologies,strength improvement,durability,andsustainabilityaspects.

2. REGUR SOIL: ENGINEERING CHARACTERISTICS AND STABILIZATION REQUIREMENTS

2.1

Origin, Mineralogy, and Physico-Chemical Properties

Regursoil,commonlyknownasblackcottonsoil,isderived primarily from the weathering of basaltic rocks of the DeccanTrapformation.Itisextensivelydistributedacross central and southern India, particularly in Maharashtra, Madhya Pradesh, Gujarat, Karnataka, and parts of Andhra Pradesh. The soil develops under semi-arid to sub-humid

climaticconditionsandischaracterizedbydarkcoloration due to the presence of iron and titanium oxides (Murthy, 2002;Gidigasu,1976).

2.1.1 Mineralogical Composition

The dominant clay mineral present in Regur soil is montmorillonite,amemberofthesmectitegroup,whichis responsibleforitshighswellingpotential.Minorproportions of illite, kaolinite, quartz, feldspar, and calcite are also commonlyreported(Grim,1968;Sivapullaiahetal.,2000). Thehighspecificsurfaceareaandweakinterlayerbonding of montmorillonite enable significant water adsorption, makingthesoilhighlysensitivetomoisturevariations.

Fromaphysico-chemicalstandpoint,Regursoilexhibitshigh cationexchangecapacity(CEC),typicallyrangingbetween 40 and 80 meq/100 g, which contributes to its strong interactionwithporefluidsandstabilizingagents.Thesoil generallyshowsalkalinetoneutralpHvaluesandcontains exchangeablecationssuchascalcium,magnesium,sodium, and potassium. These properties strongly influence its response to chemical and bio-mediated stabilization techniques(MitchellandSoga,2005).

2.2 Swell–Shrink Mechanisms and Strength Limitations

The swell–shrink behavior of Regur soil is primarily governed by the hydration and dehydration of montmorillonite minerals. During wet seasons, water moleculesentertheinterlayerspacesoftheclayminerals, leadingtovolumetricexpansion.Conversely,moistureloss during dry periods causes shrinkage and the formation of deepsurfacecracks(Chen,1988;NelsonandMiller,1992).

This cyclic volumetric change adversely affects the mechanical strength of the soil. In saturated conditions, Regursoilexhibitslowshearstrengthandbearingcapacity due to reduced effective stress and increased pore water pressure.Unconfinedcompressivestrength(UCS)valuesare typicallylow,andsignificantstrengthlossisobservedupon repeatedwetting–dryingcycles.Theseinherentlimitations necessitatestabilizationpriortoitsuseasafoundationor subgradematerial(HoltzandKovacs,1981).

2.3 Performance Requirements for Infrastructure Applications

Forsafeanddurableinfrastructureperformance,soilsused insubgrades,embankments,andfoundationsystemsmust satisfy minimum criteria related to strength, stiffness, volume stability, and durability. In the case of pavement subgrades, parameters such as California Bearing Ratio (CBR),resilientmodulus,andresistancetomoisture-induced softeningarecritical(IRC:37,2018).

Regur soil in its natural state rarely meets these performance requirements due to excessive swelling pressure,lowsoakedCBRvalues,andhighcompressibility.

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Forlightlyloaded foundations,differential settlementand heave pose serious serviceability concerns. Consequently, regulatoryguidelinesanddesignstandardsrecommendsoil stabilization or replacement when expansive soils are encountered at construction sites (IS:1498, 1970; Bell, 1996).

3. FUNDAMENTALS OF MICROBIAL CATALYST–DRIVEN BIO-MINERALIZATION

3.1ConceptandClassificationofBio-Mineralization Processes

Property

Liquidlimit(%) 50–100 Chen (1988); Sivapullaiah et al. (2000)

Plasticityindex(%) 25–60 Murthy(2002)

Free swell index (%) 50–120 IS:2720 (Part 40), 1977

Cation exchange capacity(meq/100 g) 40–80 Mitchell and Soga (2005)

SoakedCBR(%) 1–4 Bell(1996)

Dominant clay mineral Montmorillon ite Grim(1968)

2.4 Suitability of Bio-Mineralization for Expansive Clay Soils

Bio-mineralization techniques, particularly microbialinduced carbonate precipitation (MICP), have emerged as promisingalternativesforimproving problematicsoils.In expansive clays like Regur soil, bio-mineralization offers potential advantages by inducing calcium carbonate precipitationthatbindsclayparticles,reducesporespace, andmodifiessoilfabric(DeJongetal.,2006;IvanovandChu, 2008).

Unlikeconventionalstabilizers,microbialprocessesoperate underambientconditionsandhavealowercarbonfootprint. Additionally,bio-mineralizationhasbeenreportedtoreduce swellingpotentialbylimitingwateringressandalteringthe diffusedoublelayersurroundingclayparticles(Soonetal., 2013). However, challenges such as low permeability, nutrienttransport,anduniformdistributionofprecipitates must be addressed to ensure effective application in finegrainedsoils.Despitetheselimitations,theinherentphysicochemicalpropertiesofRegursoilmakeitaviablecandidate for bio-mediated stabilization, warranting focused investigationandcriticalreview.

Bio-mineralization refers to the process by which living organismsinducetheformationofmineralphasesthrough metabolicactivitiesorenzymaticreactions.Ingeotechnical engineering, microbial catalyst–driven bio-mineralization exploitsthesenaturalprocessestoimprovesoilproperties by precipitating cementitious minerals within the pore spacesofsoils(DeJongetal.,2006).Themicroorganismsact as biochemical catalysts, accelerating mineral formation withoutbeingconsumedinthereaction.

3.1.1 Classification Based on Biological Control

Bio-mineralization processes are broadly classified into biologicallycontrolledmineralization(BCM)andbiologically inducedmineralization(BIM).InBCM,organismsregulate mineralnucleationandgrowththroughcellularmechanisms, whereasBIMinvolvesindirectmineralprecipitationdriven by metabolic by-products such as carbonate ions (LowenstamandWeiner,1989).

3.2 Microbial-Induced Carbonate Precipitation (MICP)

Microbial-induced carbonate precipitation (MICP) is the most extensively studied bio-mineralization technique for soil stabilization. It relies on ureolytic bacteria capable of hydrolyzing urea to produce carbonate ions, which subsequently react with calcium ions to form calcium carbonate(CaCO₃)precipitates(Stocks-Fischeretal.,1999).

Figure-1: Microbial-Induced Carbonate Precipitation (MICP)

3.3 Enzyme-Induced Carbonate Precipitation (EICP)

Enzyme-induced carbonate precipitation (EICP) is a bioinspired alternative to MICP that utilizes free urease enzymesratherthanlivingmicroorganismstocatalyzeurea hydrolysis. The absence of bacterial cells simplifies treatment protocols and reduces uncertainties related to microbialsurvivalandtransportinsoilmedia(Whiffinetal., 2007).

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3.3.1 Advantages over MICP

EICP offers improved control over reaction rates and uniformity of treatment, particularly in fine-grained soils where bacterial mobility is limited. Since enzymes are smallerthanbacterialcells,theycanmoreeasilypenetrate low-permeabilityclaysoilssuchasRegursoil(Hamdanand Kavazanjian,2016).

3.4 Other Microbial Mineral Precipitation Mechanisms

Beyondcarbonateprecipitation,othermicrobialprocesses capableofinducingmineralformationhavebeenexplored for soil improvement. These include microbial-induced calciumphosphateprecipitation(MICPP),sulfatereduction–inducedmineralization,andironoxideprecipitation(Achal andPan,2014).

3.4.1 Calcium Phosphate Precipitation

MICPPinvolvesmicrobialmetabolicpathwaysthatrelease phosphate ions, which react withcalciumto formcalcium phosphate minerals. These precipitates exhibit higher chemicalstabilityandlowersolubilitycomparedtocalcium carbonate, making them attractive for long-term stabilization(ZhuandDittrich,2016).

4.LITERATUREREVIEWONBIO-MINERALIZATION FOR SOIL STRENGTH ENHANCEMENT

4.1 Evolution of Bio-Geotechnical Engineering

Bio-geotechnical engineering emerged as an interdisciplinary field integrating microbiology, geochemistry, and geotechnical engineering with the objective of improving soil behavior using biologically mediated processes. Early research was primarily exploratory, focusing on understanding how microbial activitycouldinfluencemineralprecipitationandsoilfabric modification(IvanovandChu,2008).

4.1.1 Early Developments in Microbial Soil Improvement

Initialstudiesonmicrobialsoilimprovementcanbetraced backtomicrobiologicalresearchoncalciteprecipitationby ureolyticbacteria.Stocks-Fischeretal.(1999)demonstrated that bacteria such as Sporosarcina pasteurii could induce calciteprecipitationthroughureahydrolysis.Thesefindings were later translated into geotechnical applications by DeJong et al. (2006), who established microbial-induced carbonate precipitation (MICP) as a viable ground improvementtechnique.

4.2 Literature on Bio-Mineralization in Clayey and Expansive Soils

The application of bio-mineralization to clayey and expansive soils has been investigated to a lesser extent compared to granular soils. However, existing studies

indicatethatmicrobialtreatmentscaneffectivelyalterclay fabric, reduce swelling potential, and improve strength undercontrolledconditions(Soonetal.,2013).

4.2.1 Reported Challenges in Low-Permeability Soils

Low permeability in clayey soils significantly restricts the transportofbacteria,enzymes,andcementationsolutions. Studies have reported uneven calcium carbonate distribution,localizedclogging,andreducedtreatmentdepth in expansive clays (Van Paassen, 2009). Additionally, the high surface charge and cation exchange capacity of clays can interfere with microbial activity and precipitation efficiency.

4.3StudiesonMicrobialTreatmentofRegur(Black Cotton) Soil

ResearchonmicrobialtreatmentofRegursoilisrelatively limitedbutgrowing,withmostinvestigationsconductedat thelaboratoryscale.Thesestudiesprimarilyaimtoevaluate the feasibility of MICP and EICP in mitigating the adverse propertiesofexpansiveblackcottonsoil.

4.3.1 Laboratory-Scale Investigations

Laboratorystudieshaveemployedmixing-basedtreatment methods to ensure uniform distribution of microbial solutions in Regur soil. Researchers have reported noticeable improvements in strength and reductions in swelling potential following microbial treatment (Gowthamanetal.,2019;SharmaandReddy,2021).

4.4ComparativeReviewofMICPandEICPinRegur Soil

Comparative assessments of MICP and EICP indicate that both techniques are capable of improving the strength characteristicsofRegursoil,albeitwithdifferingoperational efficienciesandlimitations.

4.4.1 Treatment Efficiency and Uniformity

MICPoftenresultsinhigherpeakstrengthduetosustained microbialactivity;however,uniformityoftreatmentremains achallengeinclayeysoils.Incontrast,EICPprovidesmore homogeneousmineralprecipitationduetothesmallersize andmobilityofureaseenzymes(Neupaneetal.,2015).

4.5 Influence of Key Parameters Reported in Literature

Theeffectivenessofbio-mineralizationinRegursoilishighly sensitive to several interacting parameters, as reported acrosstheliterature.

4.5.1 Bacterial Strain and Urease Activity

Ureolytic activity directly governs the rate of carbonate precipitation. Studies consistently report superior performance when using Sporosarcina pasteurii due to its highureaseactivityandalkalinitytolerance(Stocks-Fischer etal.,1999).

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5. MECHANISMS OF STRENGTH ENHANCEMENT IN REGUR SOIL: A SYNTHESIS OF LITERATURE

5.1

Particle Bonding and Cementation Effects

One of the primary mechanisms responsible for strength enhancement in Regur soil treated through biomineralization is particle bonding induced by mineral precipitation.Microbialorenzyme-catalyzedreactionsresult in the formation of calcium carbonate crystals that act as cementingagents,creatinginterparticlebondsandreducing thedegreeoffreedomofclayparticles(DeJongetal.,2006).

Figure-2: Soil Pore Filling and Cementation Mechanism

5.1.1 Nature of Cementation in Expansive Clay Soils

Unlike granular soils where calcite forms point-to-point contacts, in fine-grained soils such as Regur soil, precipitation occurs along clay plate surfaces and within micro-pores.Thisleadstosurfacecoating,edgebonding,and partial pore filling, contributing to increased apparent cohesionandstiffness(Soonetal.,2013).

5.2 Alteration of Clay Fabric and Diffuse Double Layer

Beyond cementation, bio-mineralization induces fundamental changes in clay fabric and interparticle physicochemicalinteractions.Theprecipitationofcalcium carbonatealterstheelectricalenvironmentofclayparticles, affecting the thickness of the diffuse double layer (DDL) (MitchellandSoga,2005).

5.2.1 Role of Multivalent Calcium

Ions

Theintroductionofcalciumionsduringbio-mineralization promotescationexchange,replacingmonovalentionssuch as sodium with divalent calcium. This process reduces repulsive forces between clay particles and encourages flocculationandaggregation(Grim,1968).

5.3 Reduction in Swelling Potential and Moisture Sensitivity

Swelling control is a critical requirement for stabilizing Regur soil, and bio-mineralization has been shown to significantly reduce swelling potential by modifying both mechanicalandphysicochemicalsoilbehavior.

5.3.1

Restriction of Water Ingress

Calciumcarbonateprecipitationreducesporeconnectivity andformsprotectivecoatingsaroundclayparticles,limiting water absorption. This restricts the entry of water into interlayerspacesofmontmorillonite,therebyreducingswell pressureandfreeswellindex(Soonetal.,2013).

5.4 Correlation between Calcite Content and Strength Gain

Astrongcorrelationbetweencalciumcarbonatecontentand mechanical strength improvement has been consistently reportedintheliterature.Calcitecontentisoftenquantified using acid digestion or thermogravimetric analysis and is consideredakeyindicatoroftreatmenteffectiveness.

5.4.1

Quantitative Relationships Reported in Literature

Studies indicate that strength gain increases with calcite content up to an optimum level, beyond which additional precipitationmayleadtobrittlenessorlocalizedclogging. Forexpansivesoils,relativelylowercalcitecontentisoften sufficient to achieve significant improvements due to enhancedclay–mineralinteractions(DeJongetal.,2013).

6. REVIEW OF TREATMENT TECHNIQUES AND APPLICATION METHODS

6.1

Mixing-Based Laboratory Treatment Methods

Mixing-based treatment is the most commonly adopted approachinlaboratory-scalestudiesonbio-mineralization of clayey and expansive soils, including Regur soil. In this method,soilisthoroughlymixedwithbacterialorenzyme solutions along with cementation reagents to achieve uniformdistributionofreactants(Gowthamanetal.,2019).

6.1.1

Rationale for Mixing-Based Approaches

Due to the low permeability and high surface activity of expansiveclays,directpercolationorinjectionofmicrobial solutionsisoftenineffectiveatthelaboratoryscale.Mixing ensuresintimatecontactbetweensoilparticles,microbesor enzymes, and calcium sources, thereby enhancing precipitationefficiencyandrepeatabilityofresults(Soonet al.,2013).

6.2

Injection and Percolation Techniques

Injection and percolation techniques are more representativeoffield-scalebio-mineralizationandinvolve

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delivering bacterial or enzyme solutions into soil through injectionwellsorsurfacepercolationsystems.

6.2.1

Injection-Based Delivery Methods

Injectionmethodstypicallyinvolvelow-pressureinjectionof microbial and cementation solutions into the subsurface. These techniques have been successfullyapplied in sandy soils; however, their effectiveness in expansive clays is limited by low hydraulic conductivity and rapid clogging nearinjectionpoints(VanPaassen,2009).

6.3

Curing Regimes and Environmental Control

Curing conditions play a critical role in governing the kinetics of bio-mineralization and the stability of precipitatedminerals.Temperature,moisturecontent,and curing duration directly influence microbial activity and enzymeefficiency.

6.3.1 Effect of Temperature and Moisture

Most ureolytic bacteria exhibit optimal activity in the temperaturerangeof25–35°CandalkalinepHconditions. Insufficient moisture restricts reactant mobility, whereas excessive moisture may dilute reactants and reduce precipitationefficiency(Stocks-Fischeretal.,1999).

7. DURABILITY AND LONG-TERM PERFORMANCE: LITERATURE INSIGHTS

7.1

Wet–Dry and Cyclic Durability Studies

Durability under cyclic environmental loading is a critical performance criterion for stabilized expansive soils, particularly in regions experiencing seasonal moisture variations.Severalstudieshaveinvestigatedtheresponseof bio-mineralizedsoilstorepeatedwettinganddryingcycles toassessthestabilityofmicrobial-inducedcementationover time(DeJongetal.,2010).

7.1.1 Effects of Wet–Dry Cycling on Bio-Treated Soils

Wet–drycyclinginducesvolumetricchanges,microcracking, anddegradationofbondinginuntreatedexpansivesoils.In contrast,bio-mineralizedsoilsexhibitimprovedresistance duetothepresenceofcalciumcarbonatecementation,which restrictsparticlemovement andreducesmoistureingress (Soon et al., 2013). However, partial strength loss after multiple cycles has been reported, particularly when cementationisnon-uniform.

7.2 Resistance to Environmental Degradation

Environmental degradation mechanisms such as chemical leaching, temperature fluctuations and microbial activity losscanadverselyaffectthelong-termperformanceofbiomineralized soils.Understandingthese factors is essential for evaluating the feasibility of bio-based stabilization in real-worldapplications.

7.2.1 Chemical Stability and Leaching Resistance

Calcium carbonate precipitates formed during biomineralizationaregenerallystableunderneutraltoalkaline conditions. However, acidic environments can dissolve calcite,leadingtoareductionincementationandstrength. Laboratory leaching studies indicate that bio-treated soils exposedtomildlyacidicconditionsmayexperiencegradual strength reduction, highlighting the importance of sitespecificchemicalassessment(VanPaassen,2009).

8. ENVIRONMENTAL, ECONOMIC, AND SUSTAINABILITY ASPECTS

8.1 Environmental Implications of Microbial Treatments

Microbial catalyst–driven soil improvement has attracted attention primarily due to its potential environmental advantages over conventional chemical stabilization techniques.Bio-mineralizationprocessesrelyonnaturally occurring biochemical reactions operating at ambient temperature and pressure, thereby avoiding the energyintensive manufacturing processes associated with traditionalbinderssuchascementandlime(DeJongetal., 2006).

8.1.1

Environmental Benefits of Bio-Mineralization

Theprincipalenvironmentalbenefitofmicrobialtreatments lies in their reduced greenhouse gas emissions during material production and application. Calcium carbonate precipitationoccursinsitu, eliminatingthe need forhightemperature calcination processes. Additionally, biomineralization can utilize locally available materials and indigenous microorganisms, further reducing transportation-relatedemissions(IvanovandChu,2008).

8.2 Comparison with Cement and Lime Stabilization

Cementandlimestabilizationhavebeenwidelyadoptedfor improvingexpansivesoilsduetotheirrapidstrengthgain and well-established design practices. However, these methods are increasingly scrutinized due to their environmentalanddurabilitylimitations.

8.2.1

Performance and Environmental Trade-Offs

Whilecementandlimetreatmentsprovidesubstantialshorttermstrengthimprovement,theirproductionisassociated withsignificantCO₂emissions.Cementmanufacturingalone contributesapproximately0.8–0.9tonnesofCO₂pertonne ofcementproduced(Scriveneretal.,2018).Incontrast,biomineralizationsignificantlyreducesembodiedcarbon,albeit oftenwithslowerstrengthdevelopment.

8.3 Cost-Effectiveness and Carbon Footprint Considerations

Economic feasibility is a critical factor influencing the adoption of any soil improvement technique. At present,

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microbial treatments are generally more expensive at the laboratory scale due to costs associated with nutrients, enzymes,andcontrolledapplicationprocedures.

8.3.1 Life-Cycle Cost Perspective

Several authors argue that a life-cycle cost assessment provides a more realistic comparison between biomineralization and conventional stabilization. Although initialcostsmaybehigher,potentialsavingscanbeachieved through reduced material usage, lower environmental compliance costs, and extended service life of treated infrastructure(DeJongetal.,2010).

9. CONCLUSIONS

Thisreviewcriticallysynthesizedtheexistingliteratureon microbial catalyst–driven bio-mineralization for strength enhancement of Regur (black cotton) soil. The findings indicatethatbio-mineralization,particularlymicrobial-and enzyme-inducedcarbonateprecipitation,offersapromising and sustainable alternative to conventional chemical stabilization methods. Reported studies consistently demonstrate improvements in strength, stiffness, and durability, along with significant reductions in swelling potentialandmoisturesensitivity.Microstructuralevidence confirms that calcium carbonate precipitation alters clay fabric, enhances interparticle bonding, and restricts volumetric instability. Compared to cement and lime stabilization, bio-mineralization exhibits lower carbon footprint and improved environmental compatibility, making it attractive for sustainable geotechnical applications. Although challenges related to treatment uniformity and scalability remain, the reviewed literature highlights the strong potential of bio-mineralization for mitigatingtheengineeringproblemsassociatedwithRegur soil. With continued research and field-scale validation, microbial catalyst–based techniques can contribute meaningfullytoeco-friendlygroundimprovementpractices inexpansivesoilregions.

10. LIMITATIONS OF THE REVIEWED STUDIES

Despite encouraging outcomes, the literature on biomineralization of Regur soil exhibits several limitations. Moststudiesareconfinedtolaboratory-scaleinvestigations employingmixing-basedtreatmentmethodsthatdonotfully replicatefieldconditions.Limitedfieldtrialsandlong-term monitoring data restrict the assessment of in-situ performanceanddurability.Variabilityinmicrobialstrains, treatmentprotocols,curingregimes,andtestingprocedures makes direct comparison between studies difficult. Environmentalconcernsrelatedtoammoniumby-products areoftenacknowledgedbutrarelyquantifiedormitigated systematically. Additionally, economic feasibility and scalabilityunderIndianfieldconditionsremaininsufficiently addressed. These limitations highlight the need for standardized methodologies, comprehensive life-cycle assessments, and pilot-scale demonstrations to enable

reliabletranslationofbio-mineralizationfromresearch to practice.

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