Hydration Mechanism Of Clay Modified By Adapting Ionic Soil Stabilizer

Clay is of strong hydrophilicity,illustrated by that it is apt to hydrate with water,which is closely associated with its own physic-chemical properties and could affect engineering properties of clay.To deal with the problems on geological engineering which arise from clays,this paper focuses on original causes of hydration and investigates the mechanism of this phenomenon.Besides,dominant factors in each processes of hydration are determined and types as well as properties of absorbed water of clay are analyzed quantitatively.Based on this,ionic soil stabilizer?cottonseed oil-based sulfonated oil,short for ISS?which is developed independently by Pro.Wei Xiang group was adapting to adjust and control the dominant factors during the whole hydration progress to change the surface hydration properties of modified clay and then regulate its hydration or water retention characteristics,improve its mechanical properties and lower the swelling potential to improve the engineering properties of clay.To illustrate the mechanism that how the ionic stabilizer modifies clay,and since the interaction processes in clay-water-ISS system are hard to be analyzed,many types of micro-test technologies should be used during the whole project in terms of the physic-chemical properties of clay such as mineral and chemical composition,crystal structure,absorbed behavior,electrochemical,spectral and thermal characteristics,etc.In this paper,firstly,the hydration mechanisms of clay are investigated by means of gas absorption method,XRD,cation exchange test and literature review on relative field.Then,the water retention equation in high suction range of clay is inferred and established based on the above hydration mechanisms.Meanwhile,to investigate how ionic stabilizer modifies hydration mechanisms and what the water retention model is,the above three test methods combined with infrared spectrum?IR?analysis,thermogravimetric-differential scanning calorimeter?TG-DSC?test and scanning electron microscope?SEM?are used.In addition,association analysis of pores and hydration mechanism is implemented through pore analysis methods such as XRD,gas absorption method,mercury intrusion method,SEM and macroscopic volume measurement to investigate the associated evolutionary mechanism of“hydration-pore”of modified clay.Finally,based on the full understanding how ionic stabilizer modifies the clay properties,unconfined compression strength and uniaxial tensile strength tests are carried out and how the mechanical strength of modified clay change are explained.The main contents and researching results are as follows:1.Basic physic-chemical properties of montmorillonite and kaoliniteShandong Montmorillonite?SMT?,Greek Montmorillonite?GMT?and Guangdong Kaolinite?GKT?act as the objects to be studied to find out the mechanism of clay hydration and how ionic stabilizer works.The basic physical and chemical properties of clay,including mineral and chemical composition,crystal structure,electrochemical characteristics,specific surface area?SSA?and Atterberg limits are analyzed through a series of laboratory tests,which shows that:?1?Montmorillonite has crystal structure of2:1 type,in which the layers are connected by oxygen-oxygen bond with weak connection and it is apt to occur isomorphous substitution.Moreover,it has interlamellar structure and can absorb a large number of positive ions with larger specific area.What is more,as for montmorillonite,water molecules tend to entry interlayer during hydration process,resulting in expansion of crystal layer,hence montmorillonite is a kind of expanding clays.?2?Kaolinite has crystal structure of 1:1 type,in which the layers are mainly connected by hydrogen bond with strong connection and it is not apt to occur isomorohous substitution.Besides,it has no interlamellar structure and can absorb few cations.As for expansibility,kaolinite cannot swell and is one type of non-swelling clays.?3?Clay minerals contain a variety of hydration active sites and can be divided into four types,which are complex triangular cell on basial surface?oxygen?,oxygen and hydrogen on basial surface,exchangeable cation and imbalanced ion at the edge of a mineral respectively.The first two,crystal basial surface and exchangeable cation,are the main hydration active centres,while the ion at edge account for less?5%-10%?and hence will not be discussed in this paper.?4?According to results of oriented slide analysis,the clay minerals studied are of high purity and the type of mineral of each kind of clay is single.Based on this and X Ray Fluorescence?XRF?results,the chemical component expressions of clays were deduced.?5?The surface physicochemical parameters of clay have strong correlations among each other,such as specific surface area,cation exchange capacity,layer charge density and Zeta potential,which are all closely related to the hydrophilic ability of clay.2.The hydration mechanism of clay in high suction range and properties of adsorbed waterProcess SMT by interlayer cation purification treatment via sodium chloride and calcium chloride,obtaining the homoionic montmorillonites of sodium type and calcium type,noted as Na-SMT and Ca-SMT respectively.With the homoionic montmorillonites?Na-SMT and Ca-SMT?,natural montmorillonites?SMT and GMT?,natural kaolinite?GKT?as research objects,the vapor absorption-desorption curves of these five kinds of clays are obtained through vapor adsorption method.Meanwhile,adsorption velocity curves,d₀₀₁ variation with P/P₀,and evolution of infrared spectrum peak are analyzed to figure out the dominant factors of hydration during the whole water-absorbing process.Besides,boundaries of each hydration processes are raised and the properties of absorbed water in high suction range are determined according to above tests.Based on BET multilayer absorption theory and starting from Na-SMT as well as Ca-SMT,two types of generalized hydration models are proposed and computing methods for micro-parameters?exchangeable cation,specific surface area,etc.?are established.The research results illustrate that:?1?In the initial phase of absorption,the type and hydration energy of interlayer cation determine the initial position where hydration occurs.As for homoionic montmorillonites with low hydration energy,such as Na-montmorillonites,the absorption of water molecules starts from external surface of clay minerals and then gets into the interlayer with the hydration process.As for homoionic montmorillonites with high hydration energy,such as Ca-montmorillonites,the hydration process starts from the interlayer directly.?2?In the process of hydration in interlayer of montmorillonites,firstly,the interlayer cations experience hydration to become hydrated cations and expand the crystal layers since the hydration energy of cations is higher than the absorption energy of basial surface of crystal layers.Then water molecules are absorbed by internal surface of crystal layer or fill the space between hydrated cations,during which crystal layers will not expand.?3?The process of absorbing bound water for natural Ca-montmorillonites?SMT and GMT?can be divided into several hydration processes,and each process is controlled by related dominant factors of hydration.Firstly at the range of0<P/P₀<0.15⁰.25,the interlayer cations hydrate to form a monolayer“hydration shell”.And then 0.15⁰.25<P/P₀<0.45⁰.55,the interlayer cations hydrate further to form a bilayer“hydration shell”.With further hydration sequence when 0.45⁰.55<P/P₀<0.8⁰.9,the basial surface of crystal layers dominates the hydration of montmorillonites,and water molecules are absorbed to form an integrated bilayer“hydration shell”with hydrated cations.After that,when P/P₀ exceeds the boundary range?0.8⁰.9?,energy of the water lower,becoming weak adsorbed water.?4?Natural kaolinite?GKT?has the same boundary humidity range?0.45⁰.55,0.8⁰.9?as that of SMT and GMT.At the range of0<P/P₀<0.45⁰.55,cations dominate the hydration process and can be hydrated to form a monolayer“hydration shell”on condition that P/P₀?0.2.After that,water molecules start to fill the space among hydrated cations.When 0.45⁰.55<P/P₀<0.8⁰.9,the basial surface of crystal layers dominates absorption.And when P/P₀ exceeds 0.8⁰.9,water transform into weak adsorbed water.?5?The values of exchangeable cation capacity and specific surface area calculated from characteristic relative humidity?based on BET curve?can match measured values well.Meanwhile,the maximum error does not exceed 10%,which shows that these calculation methods is reasonable and accurate.3.The water retention model of clay in high suction rangeAccording to the hydration mechanism of clay in high suction segment,the dominant factors over the hydration are basial surfaces of crystal layers and cations and hence the equation of water retention curve can be determined from the following two aspects,one is hydration energy of cations to water dipoles,the other is the adsorbed energy generated by basial surface of crystal layer.Meanwhile,predictions about water retention curves of clay are made via this model,which show that:?1?The model based on the hydration energy produced by electrostatic field from cations to adsorb a single water molecule dipole can be used to forecast the water retention curve of clay in extremely high suction range??>200MPa?effectively and this model illustrates the impacts of cation and its own characteristics on the macroscopic water retention properties of clay quantitatively.?2?The microscopic water retention model established based on the intermolecular forces between polarity plate?i.e.basial surface of crystal layer?and boundary of water molecular film can forecast the water retention curve in the moderately high suction segment?15MPa<?<200MPa?effectively.Moreover,the model can demonstrate influences on water retention properties of clay quantitatively,which result from basial surface of crystal layer.?3?Express the microscopic equation of water retention curve over the whole high-suction sections??>15MPa?via dividing them into extremely and moderately high suction range.Consequently,this model can predict the water retention curve well and illustrate the qualification of water retention behavior of clays in high suction range.4.Properties of adsorbed water and water retention model of modified clay by adapting ISSModify the natural montmorillonite and kaolinite via ISS developed by Pro.Wei Xiang group and then implement the vapor absorption test on modified montmorillonite and kaolinite.The hydration mechanisms of modified soil and characteristics of absorbed water can be determined through absorption rate curve based on the hydration mechanisms of natural soil.Meanwhile,how ISS changes the properties of absorbed water is studied by means of several test methods,such as IR,TG-DSC,exchangeable cations,XRF,SSA analysis and Energy Dispersive Spectrometer?EDS?.On the basis of this,the hydration mechanism of modified clay is analyzed through water retention behavior.Besides,the deduced water retention equations in the overall high suction range are used to analyze the effects of ISS on the surface hydration properties and the water retention content of montmorillonite and kaolinite and related results show that:?1?The hydration and water retention mechanisms of modified montmorillonite and kaolinite do not change intrinsically,which are the same as that of natural clay,considering relative humidity range 0.15⁰.25,0.45⁰.5 and 0.8⁰.9 as boundaries of each stage of the whole hydration process.In other words,when 0<P/P₀<0.15⁰.25,the exchangeable cations interact with water molecules directly,forming a monolayer“hydration shell”.And then0.15⁰.25<P/P₀<0.45⁰.55,the exchangeable cations of montmorillonite hydrate further to form a bilayer“hydration shell”.As for cations of kaolinite,they also hydrate further and water molecules are absorbed to surround the hydrated cations.With further hydration sequence when 0.45⁰.55<P/P0<0.8⁰.9,the basial surface of crystal layers dominates the hydration process.Else if P/P₀ exceeds 0.8⁰.9,adsorbed water molecules transform into weak adsorbed water.?2?ISS affects the surface hydration properties of clay minerals thus weakening the macroscopic water retention capacity via influencing the dominant factors?cation and basial surface of crystal layer?of each sequence of the whole hydration process.When cation dominates,ISS lowers the number of hydrated cations through wrapping and fixing and occupying.In this way,the adsorbed energy between cations and water molecule is reduced,lowering the macroscopic water retention capacity of clay on this sequence.When basial surface of crystal layer dominates,ISS aggregates the clay minerals?or particles?via connecting and attachment processes,eliminating the chance of absorbing water molecules for some basial surfaces and lowering the SSA as well as surface hydration energy.In this way,the macroscopic water retention capacity of clay is weakened on this sequence.?3?The water retention equations of modified clays can be used to predict the measured water retention curve well,to show the intrinsic relationship between microscopic hydration parameters and macroscopic water retention capacity quantitatively,and to reveal the regulate mechanisms of ISS further.5.Associated evolutionary mechanism of“hydration-pore”for montmorillonite by adapting ionic soil stabilizerBoundary relative humidity for pores in different scales adsorbing water was stated based on the association analysis of“hydration-pore”through void ratio variation with P/P₀ and the hydration mechanism of raw and modified clay in high suction range.In addition,associated evolutionary mechanism of“hydration-pore”for clay conducted by ISS was indicated by different pore size analysis method such as XRD,nitrogen adsorption,mercury injection test and SEM.The results show that:?1?A certain hydration process occurs in pores with corresponding size.For montmorillonite,the cations interact with water molecules firstly at the range of 0<P/P₀<0.45⁰.55,accompanying with the expansion of micropores?interlamellar space?.And kaolinite is different,adsorbing water in the spaces between lamellas without expansion.Secondly,with hydration on the surface of montmorillonite and kaolinite minerals at the range of 0.45⁰.55<P/P₀<0.8⁰.9,water mainly adsorbed into the interparticular mesopores.Followed by further hydration sequence when P/P₀>0.8⁰.9,weak adsorbed water get into macropores sostenuto.?2?Ionic soil stabilizer regulate the characteristics of adsorbed water by changing the physic-chemical parameters?cations and basal surface of crystal layer?to influence the hydration processes of pores.In the sequence that cations hydrate,micropores of clay adsorb water and ISS adjust this pores by dwindling the hydration energy of cations especially for montmorillonite.Then in the process that basal surface of crystal layers hydrate,mesopores of clay adsorb water and ISS regulate this pores by decreasing the hydration energy of surface.With further hydration that osmotic swelling occurs,macropores adsorb water and ISS modified this pores by influencing hydration energy of cations and surface collectively.?3?The pore measurements display that width and volume of micropores,mesopores and macropores of montmorillonite all reduce after the modification of ISS,which indicates tight crystal layers,clay agregats and an outward shrinkage.?4?As to the modified kaolinite,its mesopores size decrease while macropores size increase inversely,which state a tight agregats and inward shrinkage.6.The mechanical strength modified by ISSUnconfined compressive test and tensile test were implemented to investigate the behavior of mechanical strength by adapting ISS in different hydration state which is conducted by water content.To measure the tensile strength,a digital image processing technology-based soil tensile device was developed.In this part,the results of compressive and tensile test were discussed and the mechanism of modification by using ISS to clay was expounded based on the conclusion before about regulation mechanism of ISS for surface properties.It shows that:?1?The tensile test device designed reasonably which guarantee a tensile failure for soil.Also the stress and strain information recorded by this device displayed the differences of samples and express a regularity,which demonstrates that the developed tensile apparatus satisfy the accuracy requirement.?2?Because of the transformation of properties and forms of adsorbed water in different hydration states?water content?,the modified effect of ISS to mechanical strength of clay is distinctly different.With high water content,integrated water film is formed and the mechanical behavior is improved by adapting ISS.However,bound water among clay particles is rare when water content is low and the mechanical strength of modified clay decreases.?3?When water content is closed to or exceeds plastic limit of clay,diffused double layer on the surface of clay particles forms and the connection among particles is condensative contaction which get stronger with thinner weak adsorbed water film,hence the mechanical strength of modified clay increasing because of the adjustion of ISS to adsorbed water.Furthermore,as an organic macromolecule,ISS generate cementing cohesion which can enhance the connection among clay particles so that mechanical behaviors are improved.The cementing cohesion of ISS is particularly important for modified kaolinite to improve strength because its weak adsorbed water film nearly decrease.?4?When water content is low and the hydration state is closed to the state of adsorption equilibrium,adsorbed water is rare on the surface of clay particles.On the condition that test sample was made of watered powder clay,more strong adsorbed water makes more transitional contaction?or point contaction?which conducts higher mechanical strength.As a consequence,the modified clay with less strong adsorbed water has less transitional contaction so that the mechanical strength is weaken.