| The bound water in clays is the result of interaction between clay minerals such as montmorillonite, kaolinite, illite etc. and water vapor or water liquid, which has been the key study focus no matter in soil science, soil mechanics, engineering geology, environmental geology, colloid chemistry or in mineralogy. The physical-mechanic and electrochemical properties of clays largely depend on the characteristics of bound water specifically for categories, diffusivity, fluidity and permeability. In other words, it is evident that properties of liquid and plasticity, hydration swelling, dispersion, shrinkage, specific surface area, pore structure, micro-structure, soil water characteristic curve, strength, deformation, PH, conductivity, adsorption heat, Zeta potential, categories of exchangeable cations and cation exchange capacity are controlled by the properties of bound water. Due to having a great impact on the physical-chemical-mechanic properties of clays, the bound water is the key factor to lead to many problems of engineering geology involved in clays. There are large number of researches on bound water of clay, however, the study on universal model and theory of water absorbed are rather limited. Thus, it is quite crucial to study kinetic model of adsorption-desorption bound water at different conditions such as clay in water vapor, in the interface between terracotta panels and water liquid and in electrolyte solution. The purpose of this paper is to try to establish the model and theory of water adsorption-desorption on clay that are able to interpret and predict the adsorptive behaviors of water absorbed by clay at various conditions.The experimental materials in this investigation consists of five categories of clays. Montmorillonite, pure kaolinite, sliding zone soil obtained from Huang Tupo Landislide, bentonite acquires in Henan and red clay in Wuhan, respectively. The basic physical-mechanic indexes, mineral component, microchemistry component and electrochemical parameters are acquired using a variety of test instruments. Then to reveal the adsorptive mechanism of clay at water vapor, the interface between terracotta panels and water liquid and in electrolyte solution, the isothermal adsorption and desorption and kinetic experiments are accomplished using AutoSorb iQ sorbing instrument, mercury injection apparatus (Poremaster33, made in America), soil moisture equipment and unsaturated soil direct shear apparatus (GDS, made in British). The major studies are shown as follows.(1) The chemical analysis test demonstrates that all of five kinds of clay, Montmorillonite, pure kaolin, red clay in WuHan, sliding zone soil of Yellow Slope and bentonite in Henan, contain swelling clay minerals such as montmorillonite and kaolinite. Due to the surface of clay minerals having a certain surface energy and a lot of pore in the dry state, they can absorb different types of bound water whether it is in the water or in the interface between soil and water as long as there is water molecular. For example different categories of bound water in Montmorillonite correspond to different hydration interaction. And the analysis of Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) could reflect the relationship between the temperature and hydration interaction. Therefore, various categories of bound water would be desorbed at corresponding temperature ranges such as the two endothermic peaks in curve of DSC at106.6℃and150.5℃, respectively. Actually, these temperature points are the borders of bound water and power bound water. The shapes and area of endothermic peaks indicate that the peak area and peak width of bound water is larger than ones of power bound wate, which is considered that the affecting range of the hydration interaction of bound water is less than one of power bound water. Hydration interaction to adsorbing different kinds of bound water was studied using a variety of initial water content of montmorillonite. According to the curves of TG and DSC, initial moisture content does not have an impact on locations of bound water and power bound water. But the thermal weightlessness of different initial moisture content is quite different and higher initial moisture content can lead to higher mass percentage of thermal weightlessness. From the point of DSC, different initial moisture content does not change its position of endothermic peaks which is still at106.6℃and150.5℃.(2) At the0.05～0.95relative pressure, the hydration interaction of the unit of montmorillonite mass dried at105℃is more than one dried at150℃. When the heated temperature is less than105℃, the amount of adsorption bound water increases with the rise of heat treatment temperature. However, when heating temperature is greater than105℃, the amount of adsorption bound water of montmorillonite decreases with raising heat treatment temperature. But this is opposite with the experience of drying water absorption regularity and it is generally believed when montmorillonite lost strong bound water, it will respectively absorb strong bound water and weakly bound water as a result that the amount of adsorption bound water of montmorillonite is supposed to be higher than the montmorillonite that lost weakly bound water, but the test result is just the opposite. The research results show that when weakly bound water of water and crystal layer surface in the hydrated cation desorbed, the electrostatic potential among different electrical ion, the electrostatic potential of exchangeable cation and crystal layer, hydrogen bonding potential of interlayer water molecules and crystal layer in montmorillonite crystal intracellular are enhanced and under the effect of combined action potential, montmorillonite crystal will reach a new balance, making the entirety show decrease of water molecules adsorption energy to the outside.(3) When relative pressure is less than0.3, the hydration interaction of montmorillonite dries at105℃is more than that dried at150℃. In the stage of initial linear adsorption, the slope of the fitted curve of montmorillonite adsorption dried at105℃is significantly greater than one dried at150℃. In this interval, the energy and amount of bound water adsorbed of montmorillonite dried at105℃are greater than one dried at150℃. From the difference of adsorption between the two kinds of drying conditions, when the relative pressure is less than0.6, the single point and cumulative adsorption of drying montmorillonite of105℃are greater than150℃, and at a relative pressure of0.6, amount of bound water adsorbed of two drying condition is almost the same. After the relative pressure is greater than0.6, while the single point and cumulative adsorption of drying montmorillonite of105℃are not less than150℃, its hydration interaction is significantly lower than the relative pressure of smaller than0.6. This indicates that as long as the proper relative pressure and enough free water molecules, in static state, the point of the same water adsorbed under two different drying states can be found in the montmorillonite after dehydration, called "equal binding energy" adsorption point. Although adsorption energy of Na-montmorillonite between the two kinds of drying conditions is different, and the single point of the adsorption has difference, it can be seen that when nearly saturated state (0.95), the amount of bound water adsorbed of montmorillonite is nearly350times its quality.(4) Test results of10adsorption-desorption cycles of pure kaolin show that when the relative pressure is less than0.52, the surface adsorption of pure kaolin after dried at105癈is relatively large at water bath environment dried at20℃, and the maximum difference between1st and2nd the amount of water adsorbed of per unit mass pure kaolin can be up to7.4184cc/g. The other nine adsorption test results show that difference of adsorption is at an average of2.36cc/g. It is concluded that pure kaolin after heating of105℃, when the relative pressure is0.048305. the first largest adsorption of bound water of pure kaolin after heating of105℃is just3.3233cc/g, water molecules are not fully covered with surface of kaolin at this time, after the second adsorption, the adsorption of bound water can reach11.165cc/g, while bound water is fully covered with surface of kaolin. The difference between the bound water adsorbed of the maximum desorption and maximum adsorption is10.7417cc/g for kaolin, as bound water adsorbed by binding energy in20-105℃. And due to hydrogen bond and van der Waals of kaolin, adsorption potential of bound water is very high, and this part of bound water won’t take off because of dry-wet circulation at20℃.(5) The adsorption-desorption cycles experiments results indicate that when the relative pressure is below0.76, the hydration interaction of sliding zone soil is quite evident and large. The maximum adsorptive difference occurs to the first and second cycles and is6.7762cc/g, whereas the sorption variances of the other cycles are relatively small and their mean difference is1.03cc/g. After samples are heated at105℃for24hours, the largest amount of water absorbed in the first cycle is only0.25cc/g at0.047278relative pressure, which is designed as the process that only the surface with lager sorption ability appears to water adsorption. The amount of water absorbed in the second cycle, however, is up to6.7762cc/g, which is considered as stage that the surface of sliding zone clay has been absolutely occupied by water molecule. Besides, the difference between the largest amount of water desorbed and the largest amount of water adsorbed is6.703cc/g, which is designed as the bound water adsorbed by adsorption capacity located in20-105℃. When the relative pressure is over0.80, the variance between amount of adsorption and desorption is quite little, which suggests that owing to the bound water film having completely formed, the majority of water adsorbed or desorbed is water molecular indirectly link to surface.(6) Experiments of9adsorption-desorption cycles of bentonite show that expansive soil after dried at105℃has relatively bigger surface adsorption energy and the amount of adsorbed bound water’s maximum difference in the first and second adsorption of unit mass of expansive soil can reach171.6823cc/g at the20℃water bath environment when the relative pressure is smaller than0.3. While the other eight adsorption experiments results show that the average of adsorption amount’s difference is30cc/g. It can be concluded that after dried at105℃, the first largest adsorption amount of bound water is just262.8088cc/g and after the second adsorption, the largest adsorption amount of bound water can reach497.7739cc/g when the relative pressure is0.052882and at this moment, bound water is fully covered with expansive soil’s surface. The difference between expansive soil’s maximum desorption of bound water and maximum adsorption of bound water is434.4911cc/g as the expansive soil’s amount of bound water which is adsorbed by adsorption energy in the range of20-105℃.(7) Experiments of9adsorption-desorption cycles of red clays show that red clay after dried at105℃has more surface adsorption energy and the maximum difference of amount of bound water in the first and second adsorptions can reach6.3285cc/g when the relative pressure is smaller than0.68. However, the other eight adsorption experimental results show that the average difference of adsorption amount is0.828cc/g. When the relative pressure is0.047278, largest adsorption amount of bound water in the first adsorption is only12.2122cc/g, whereas the largest adsorption amount of bound water in second adsorption can reach20.0101cc/g. The above phenomena results from whether the surface of red clay is completely covered by bound water. additionally, the difference between the largest amount of water desorbed and the largest amount of water adsorbed is23.0784cc/g, which is designed as the bound water adsorbed by adsorption capacity located in20-105℃.(8) The experiments of adsorption kinetics show that the curves of adsorption kinetics are rather different, which is dependent on the categories of clay and conditions of water relative pressure. But it is quite consistent that the all of coefficients of hydration interaction under five seconds decrease and the equilibrium time to adsorbing bound water grows up then decrease with increasing the relative pressure. Above phenomena first result from that the hydration interaction is greatly large before the monomolecular water layer occurs in particle of clays. Once the monomolecular water layer or multi-molecular is accomplished, however, the hydration interaction gradually lowers and capillary interaction in gradual plays the controlling role. Therefore, the equilibrium time to adsorbing bound water gradually decreases.(9) The specific surface area calculated upon BET theory indicates that the specific surface area using water vapor desorption isotherm is greatly larger than one using nitrogen desorption isotherm, which results from that due to water molecular being polar and having hydrogen bound, it is more easily to be absorbed by cations of surface. Additionally, the phenomenon that surface fractal dimension calculated from water vapor adsorption-desorption isotherms is larger than one obtained from nitrogen adsorption-desorption isotherms implies that other than pore that nitrogen molecular could come in, water molecular is able to go to the pore located in the rougher surface. Comparison to freeze-dried samples, the hydration interaction of heated-dried samples is less, which is designed that the heating treatment results in the water molecular directly link to surface and exchangeable cation desorbed. Moreover, under the synactic interaction of electrostatic interactions and hydrogen bonds, the particle of clay becomes closer and chemical bonds redistribute, which results in that pore becomes smaller. These changes lead that it is more difficulty for water molecular to come into the pore and finally that the amount of water absorbed by per gram clay decreases.(10) Integrated water vapor adsorption method and constant speed mercury injection method are able to use to measure large size of aperture and reveal wider scope of the aperture. The ink bottle shape pore and slit-shaped structure pore in the dry clay sample can make water molecules and clay minerals form hydrogen bond easily and prevent further adsorption of bound water, which has contributed to less specific surface area and pore volume of measurement than freeze-dried sample and smaller surface fractal dimension than freeze-dried sample, showing that pore surface roughness decrease under the action of attraction. The suitable aperture range which is evaluated by the water vapor adsorption experiment is micro-porous range which is from0.825to17.925nm. But the suitable aperture ranges which is evaluated by constant speed mercury injection experiment are mesoporous and macroporous range. In the micro-porous and mesoporous range, the pore diameter and pore volume of freeze-dried clay are both bigger than drying clay’s. Because of the volume dehydration shrinkage of drying clay sample, the connection among particles become closer and the pellets diameter is bigger than freeze-dried sample’s under the effect of molecular bond and hydrogen bond. |
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