 Welcome to lecture number 4 of module 1 of advanced geotechnical engineering. In the previous lecture we have discussed about the particle arrangement and grain shapes and their impacts. In this lecture we are going to discuss about clay mineralogy and clay water interaction. In principle clay mineralogy I about the clay minerals prevalent in soils and clay particle water interaction. As we discussed in the previous lectures if you look into it, in this slide a typical silty sand in dry heap which is shown on the left hand side figure. On the right hand side a 500 times magnified SEM photograph which actually shows coarse sand particles and some fine silt particles. So this allows us to look into the different particle arrangements with respect to the SEM as concerned. So coming to today's discussion particularly clay mineralogy and clay water interaction. So we actually have discussed that the soils when they disintegrate they get into different shapes and sizes. They are classified based on the soil types as cobbles and boulders which size is actually more than 75 mm, gravels size is greater than 4.75 less than 75 mm, anything which is actually more than 19 mm is treated as cobble or boulder in principle. But between 4.75 mm to 19 mm is treated as the bulk size particles or the gravel particles and sand which is actually greater than 75 micron or 0.075 mm to 4.75 mm that is actually treated as sands and silt size greater than 2 micron to less than 75 micron. And clays which are actually fine particles we have introduced and we have told that particle diameter is less than 2 micron that is the clay particle can be as small as 0.002 mm. So most of the coarse particles are approximately equidimensional and most of those in clay size are far from the equidimensional. So most of the coarser particles are approximately equidimensional that is what we have discussed and most of those in clay size are from equidimensional. So we are going to discuss about what type of shapes these particles have and what type of minerals they consist and why the clays behave in this particular manner. Before discussing or introducing clay water interaction or clay mineralogy let us introduce to a particular term called specific surface area SSA. The smaller a particle size the larger its surface specific surface. In this slide if you consider an equivalent shape of a particle let us assume that you have got a particle of size say d units and which is cube in dimensions. So assume a soil with an equivalent shape of a particle as a cube in a total volume of 1 centimeter cube. So if length of the cube side is say 1 centimeter then we knew that the total surface area is 6 centimeter square and surface area per volume is 6. Similarly if you reduce this particle size cut into very very small cubes let us say that we consider 1 micro meter that is 10 to the power of minus 4 centimeters. In that case if you look into the total surface area works out to be 6 into 10 to the power of minus 4 centimeter square and surface area per volume is 6 into 10 to the power of 4. If you cut further is something like 1 nanometer is equal to 10 to the power of minus 7 centimeters the total surface area works out to be 6 into 10 to the power of 7 centimeter square. So this works out to be surface area per volume is 6 into 10 to the power of 7 centimeter to the power of minus 1 that means that if you observe here length of the side is decreased then the surface area is actually increasing. So in the previous slide we have observed that the surface area as the surface area we can see that the surface area goes up directly as the particle size or the cube size goes down that means that the number of particles in a given volume are increasing. So we can define with this background the SSA as SSA is defined as a total surface area of individual grain per dry mass of the grains. So SSA is nothing but surface area divided by volume into rho, rho is nothing but the mass density of the particular soil. So surface area if you indicate d is the size then 6d square divided by volume of that particular cube let us say that d is the size d cube into rho. That means that specific surface area gets simplified to 6 by rho d that units works out to be either meter square per gram or centimeter square per gram. So if you observe here the specific surface area is inversely proposed to the particle size that means that larger particles have low specific surface area and finer particles have higher specific surface area. So the observation is that the specific surface area is defined as the total surface of individual grains per dry mass of the grains and SSA is inversely proportional to the particle size. Now the property of the very fine soil fractions the most important grain property of fine drained soil materials is the mineralogical composition that we have been discussing and if the soil particles are less than about 2 micron the influence of force of gravity of each particles is insignificant. So we have discussed in the previous class that if the particles are less than 2 microns the influence of the force of gravity in each particle actually is insignificant or diminishes compared to that of the surface edges. That means that the surface charges dominate the finer particles. So the colloid particles of soil consist primarily of clay minerals the colloidal state is nothing but the domination of the surface charges. So what is a mineral a mineral is a naturally occurring chemical element or a compound formed by a geological process. Once the rocks disintegrate they possess certain minerals and they transfer that mineral property to the soil from which actually it has been generated. The classification of minerals based on the nature and arrangement of atoms carbonates the typically carbonates, phosphates, oxides, hydroxides and silicates. Carbonates some soils they do consist of carbonates calcite and dolomite basically used to make cement, phosphates mixing for petalizers and hydroxides sheets in clay minerals and silicates most abundant and more than 90% soils consist of this silicate minerals. So silicates present in more than 90% of soils. So minerals are classified based on the nature and arrangement of the atoms as carbonates, phosphates, oxides and hydroxides and silicates. Most abundant and most prevalent mineral is silicate. So what is this silicate? Silicates are nothing but groups of minerals with a structural unit called the silica tetrahedral. So in this case you have seen one silicon and the four oxygen anions. So the central silica cation is surrounded by four oxygen anions and one of each corner of the tetrahedral. So Si plus 4 and O4 minus 8 with that the net negative charge is minus 4 and this cannot exist alone in the sense that the net charge is actually neutral. So it tries to net charge is not neutral and it actually has got a net negative charge of minus 4. So it tries to combine with other minerals. So this primary valence bonding which is prevalent here and here the plan view of the arrangement of the oxygen anions and the silicon cation which is actually shown and this is the expanded side view. So silicates are a group of minerals with a structural unit called silica tetrahedral. A central silica cation is surrounded by four oxygen anions, one of each corner of the tetrahedral. So net negative charge is minus 4 and they cannot exist alone. So because of this they form tetrahedrons units link up in hexagonal pattern and form tetrahedral layer. So combining silicon oxygen tetrahedron gives silicate sheet. So if you have several silicon oxygen tetrahedrons and it forms silicate sheet and even with the formation of silicate sheet still it is not neutral and continues to form octahedral sheets. So tetrahedron units linked up in hexagonal pattern and form tetrahedral layer. So tetrahedral layer is nothing but represented by a silicate sheet. So the silicate sheet is actually represented as in this particular shape which is actually shown here. So combining silicon and oxygen tetrahedron gives this silicate sheet. So the silicate frameworks and composition of granular materials if you look into is tetrahedral form 3D array so that all oxygen are shared and one Si per 2 oxygen anions. Very resistant to weathering and it form it actually has large particles. And here some typical minerals are shown here quartz, K feldspar, plegeoclase and calcite dolomite and muscovite. So if you look into here the order which is actually shown here indicates the relative abundance in the sand. That means that the quartz is actually most abundantly available in the sand then followed by feldspar and plegeoclase, calcite, dolomite and then followed by muscovite. So tetrahedral form the 3D array so that all oxygen are actually shared and one silica per 2 oxygen and they are very resistant to weathering and have tendency to have large particles. The other fundamental unit we have discussed about the silicate sheet. Now in order to discuss the other fundamental unit that is actually called octahedron. Here there are two here in this particular slide an expanded side view which is actually shown here where you have got adroxiles at each corner and either aluminium or magnesium cation which is actually shown here. So here this shows aluminium octahedron wherein you have 3 hydroxiles with minus 3 in A2 charge, one aluminium with plus 3 and 3 OH that is with minus 3 valence here. So if you have a central cation which is aluminium then that is called aluminium octahedron. If you have a central cation magnesium then it is called magnesium octahedron. So in this slide a fundamental unit of octahedron is introduced wherein we have aluminium octahedron or magnesium octahedron. When you link up these several octahedron layers then it forms a octahedron layer. If the anions of octahedral sheet or hydroxiles and 67% of the cation positions are filled with aluminium then it is called gipsite. So it is indicated as a rectangular sheet which is actually known as gipsite. Combining aluminium oxygen tetrahedron gives gipsite, if you happen to find or you have combining magnesium oxygen tetrahedron then it gives a mineral called brusite. So combining aluminium oxygen tetrahedron that is called a gipsite and combining magnesium oxygen tetrahedron gives brusite. So the clay minerals when silicates or hydroxide hydroxides that is gipsite sheets are combined with the help of primary valency bond to form a layer and how these layers are glued to form particles and they form different clay minerals. Chemical weathering results in the formation of groups of crystalline particles of colloidal size less than 2 microns they are basically defined as clay minerals and these clay minerals are formed when you have silica and silicate and gipsite sheets are glued to form a layer and these are actually resulting into the different clay minerals. And these sheets are bonded primarily by predominantly by three different types of bonds they are actually primary valence that is covalent, ionic and metallic very strong which has actually has a capacity of 15 to 100 kilocalories per molecule and then it is followed by hydrogen bonding which is intermediate in nature and which actually has got 4 to 5 kilocalories per mole for H2O and hydrogen cation fluctuates between O2O-2 and then followed by a van der Waals is relatively weak bonding where which is the strength is 1 by 10th of the hydrogen bond. So these silicate sheets and gipsite sheets are connected either by two sheets or three sheets and they are connected by three bonds predominantly they are primary valence bond which is actually consists of covalent, ionic and metallic and very strong bond and which actually has got a strength of 15 to 100 kilocalories per molecule followed by hydrogen bonding which is intermediate in nature and which is actually relatively weak compared to primary valence but it has actually got an energy of 4 to 5 kilocalories per molecule for water and then van der Waals bonding which is one-tenth of the hydrogen bonding as far as the strength is concerned. So the three basic clay minerals are kaolinite basically these minerals are found in sedimentary and residual soils and then illite which is actually found in stiff clay and shales as well as postglacial marine and liquid stream soft clay and silk deposits. So this illite can actually occur in some marine environment and it is common in the stiff clays and shales as well as in the postglacial marine and liquid stream soft clay and silk deposits. Montevallite dominant clay mineral in some clays and shales in some residual soils derived from the volcanic ash. So the other minerals are helosite, chloride, atopulgite, allopane. So there are some other minerals also but we discuss primarily in this lecture about the kaolinite, illite and matrimonite. Here in this particular slide what you see is a kaolinite, its scientific formula is nothing but Al4, Si4, O10 and OH8. Basically repeating layers of one silicate sheet and one gipset sheet or alumina sheet. So it actually has got repeating layers of one silica tetrahedron that is the silicate sheet and alumina. So when they are glued, when they are formed, when they are actually attached with the bonding then it forms actually alumina sheet and silicate sheet will get attached. So the structure of the kaolinite particle is actually written with several layers like this. And this is actually about 7.2 angstrom thickness, one angstrom is equal to 10 to power of minus 10 meters is equal to 0.1 nanometers. So when the kaolinite layers join to form the kaolinite particle, so here the strong glue between the layers hydrogen bonding and van der Waals produce large hexagonal platey shaped particles which is actually typically shown here in this particular slide. This is a platey shaped particle. So one of the common clay mineral which we have discussed is kaolinite basically which actually has got repeating layers of silicate sheet, one silicate sheet and one gipset sheet and which is about 7.2 angstrom thickness and the strong glue between the layers which actually there is a strong bonding is prevalent between the high that is nothing but hydrogen bonding and van der Waals produce, van der Waals bonding produce large hexagonal platey shaped particles. The kaolinite particle if you look into the length and thickness if this is the length dimension and if the thickness dimension formed by typically 7 to 100 elementary layers, the ratio of thickness to length is about 1 by 10 that means that the thickness to length ratio is about 1 by 10. A kaolinite crystal or particle then consists of a stack of several layers of the basic 7.2 angstrom meter thick layers. Or the kaolinite crystal or particle consists of a stack of several layers of basic 7.2 angstrom thick layers. So this forms a strong hydrogen bonds between the hydroxyls of gipset sheet and oxygens of the silicate sheet and little tendency to the interlayers and allow water and to swell. So these particular characteristics of this mineral is that there is a little tendency to allow water and then so there is a little possibility to swelling of a soil which consists of this mineral. So the thickness to length ratio is about 1 by 10. Here in this a photomicrograph of kaolinite is shown, the scanning electron microphotograph of the kaolinite is shown as can be seen here, a cluster of the plates they resembling a pages of a book which actually is the analogous example for the kaolinite particle. Montamelite is another mineral where you have got this is basically a 2 is to 1 clay mineral is called a 3 sheet mineral and is also known as a smactite and is 2 is to 1 clay mineral. So here we have 1 gipset sheet and 2 silicate sheets and they are separated by a several water layers. So we have a structure which is actually formed by 2 silicate sheets and 1 gipset sheet or 1 alumina sheet or 2 silicate sheets forms a structure like this and which are actually strongly attached and the montamelite actually has got several layers of such type of this thing and they have a tendency to actually keep the availability of the water up to 400 angstroms thick of water that means that between each inter layers between each inter layer they can actually keep or have a tendency to store up to 400 angstroms thickness. This indicates that this is actually due to weak bonding due to the weak van der Waals forces we have discussed that because of the weak bonding due to weak nature of the van der Waals forces the weak bonding is prevalent. So the spacing between silicate sheet and gipset and silicate sheet depends upon the amount of water available to occupy the space. So in case of availability of the water there is a tendency for the water to enter in between these inter layers. So montamelite is nothing but a 2 is to 1 clay mineral which actually has got 2 sheets of silicate and 1 gipset sheet separated by the possibility of can have a several water layers. So pure seams of montamelite one of the examples is winomic bentonite. So here a typical structure of the montamelite elementary layer is shown wherein you have actually got silicate sheet and silicate sheet attached actually between by using this gipset sheet. So you have got 2 silicate sheets and 1 gipset sheet. In between there is a possibility of lot of water molecules to enter. So that is here at this junction at this surface and at this surface it can actually have what several inter layers of water. So here this particular slide shows the photo micrograph of the montamelite. Among clay minerals sodium montamelite has the smallest and filmy particles. So what you can see is a gel type of tendency. So which actually shows that among the clay minerals sodium based montamelite that means that the montamelite which actually has got sodium ion has the smallest and filmy particles. If it is shown here the length ratio that is 1000 to 5000 angstroms the thickness is 10 to 50 angstroms. So if you look this one the T by L ratio is about 1 by 100. That means that the T by L is 1 by 100 for montamelite. Another mineral which is illite which is also called as hydrous mica which actually has got repeated layers of gipset sheet sandwiched between 2 silicate sheets. It is similar to montamelite except that the adjacent silicate layers are bonded with potassium ions instead of water. So this presence of potassium ions actually bonds these 2 silicate sheets strongly. So these potassium ions which are actually shown and this is particularly nothing but 2 is to 1 clay mineral which is nothing but very similar to montamelite. But the adjacent silicate sheets are actually bonded by with the help of potassium ions with a strong bond. In case of montamelite adjacent silicate sheets have a possibility to have several layers of water that is water molecules can be stored. And the T by L the thickness to length ratio is about 1 by 30. So potassium bond bond the 2 negative surfaces of the silicate layers. So the potassium ion bonds the 2 negative surface of the silicate sheets. So this ensures that if you look the swelling tendency montamelite swells higher and if you compare illite it is followed by illite and then the swelling tendency of the kaolinite will be much more low. Now having discussed about the 3 minerals then if you discuss along with the kaolinite then we can actually also link with whatever we have defined the specific surface area. We also discussed that the specific surface area is inversely proportional to the particle size. Now here according to Lambien-Wittman 1969 the kaolinite actually has got specific surface area of 0.03 meter square per gram and the percentage water absorbed is 1.5 into 10 to the power of minus 4 kaolinite 0.5 percent illite 5 percent and montamelite 50 percent. So the percentage water absorbed absorbed can be calculated as follows for say for example for kaolinite which is nothing but specific surface area into thickness of the layer of water into rho w the mass density of water. So here specific surface area of kaolinite is 10 meter square per gram into 5 into 10 to the power of minus 10 meter that is the thickness of the layer of the water and into 10 to the power of 6 gram per meter cube which is nothing but 5 into 10 to the power of minus 3 or 0.5 percent. So here one thing can be observed is that the kaolinite as we go towards quartzite, kaolinite, illite and montamelite the specific surface area is increasing and percentage water absorbing tendency is also increasing. Now the specific surface area the water absorption is a function of the specific surface area and the specific surface area is a function of the particle size. So a specific surface area of montamelite is 100 times that of kaolinite. If you look into this the specific surface area of montamelite 100 times that of the kaolinite. So this can be visualized when one realizes that 6 grams of montamelite has approximately has a surface area of an entire football field. So a specific surface area of 25 meter square per gram has also been suggested as the lower limit for the colloidal range. So if you take a 6 grams of montamelite its surface area is equivalent to the that of a football field. So the specific surface area of the montamelite is 100 times that of the kaolinite. So if you look these grain size and specific surface area as the grain size decreases the specific surface of the soil which is simply the total surface of the individual grains per dry mass of the grains increases exponentially. So if you compare here in this slide the quartzite, kaolinite and montamelite pictures are shown, micrographs are shown. So in this direction with an increase in specific surface area there is actually decrease in the particle size. So here this is prevalent in sandy type of soils, this is in certain type of clay soils and then this is actually nothing but in montamelite type of soil. So comparison of the size, shape and surface area of several clay particles if you look into it as we discuss kaolinite it can have a specific surface area in the range from 10 to 15 meter square per gram and then you have chloride and clay mica montamelite which actually has got the tendency of 60 and 80 and 800 meter square per gram. So clay particles are plate shaped particles which is evident from the discussion here and the layer lattice structure results in a strong bonding along the two axis but weak bonding between the layers and variation specific surface attributed to different thickness of the plate shaped particles. So the specific surface area is actually varying drastically because of the decrease in thickness of the plate shaped particles. Now having discussed about the clay minerals present in soil, typical minerals present in soils now let us discuss that clay and water interaction. Now we have discussed that the clay particle surface which is actually nothing but a natively charged surface. So the water which is actually dipole in nature has actually has got positive which is indeed as a dipole nature. Water though neutral has its oxygen and hydrogen atoms based in such a manner that the center of gravity of the positive and negative electric charges do not coincide hence water molecule is said as dipolar. And molecule of water is like a bar magnet with the positive and negative charges at the opposite ends. So the molecule of water is analogous to the bar magnet where positive and negative charges at the opposite ends. So here which is actually shown a typical dipole nature model of water molecule where you have got two hydrogen cations and one oxygen anion and which is actually with the distance of 1.4 angstroms between hydrogen and oxygen 0.97 angstroms and this polar representation of the water molecule is indicated as a typical bar magnet. So clear particles have a net negative charge on the both the edges and is a tendency that on the edges they can have the positive charges or when the particles break then they have a tendency of having also at particular breakage points they have a tendency of having the positive charges. So the reasons for this charge accumulation is one is because of the isomorphous substitution which actually takes place. One I said another one is that the breakage of particles and dissociation this association of hydroxyl OH- radicals. So the reason for having clay negative charge is that one predominant reason is that isomorphous substitution then second one is the breakage of particles and the dissociation of the hydroxyl radicals. The process or the isomorphous substitution it is nothing but replacement of a cation in the mineral structure by another cation of lower valence but of the same physical size. So the isomorphous substitution is nothing but replacement of a cation in the mineral structure by another cation of lower valence but of the same physical size. So this may lead to different clay minerals with different physical properties because of this isomorphous substitution there is a tendency to form different clay minerals with different physical properties. So consider for example replacement of silicon that is plus 4 valence ion in a tetrahedral unit by aluminum plus 3 ion which could happen when aluminum ions are more abundantly available in water. When aluminum cations are more abundantly available then this process of this isomorphous substitution can take place. Because of this the typical clay particle which is actually shown here has a negative surfaces on these edges and some positive on these along these surfaces and along this at the breakage points the positive charges are actually shown and the polar water molecule if it is represented as positive and negative. So it is actually attracted towards the clay particle surface because of the negative charge here and the positive charge here and the cations which are actually present in water are the positively charged cations and charged water they are also attracted towards the clay particle. So the negatively charged charge on the clay surface is actually shown here and here these are the positively charged cations present in the water and this is because of the polar water molecules they are also arranged in this because of this attraction. So the mechanism of clay and water interaction can be explained by these three heads. One is attraction between negatively charged faces of clay and positive ends of dipoles that is attraction between negatively charged clay surface and positively positive end of the dipoles that is negatively charged clay surface and positive end of the dipole and attraction between cations in the water and negatively charged end of dipoles that is attraction between cations in the water, cations in the water and negatively charged ends of the dipoles and sharing of the hydrogen atoms in water molecules by hydrogen bonding, sharing of this hydrogen atoms in water molecules in the form of the hydrogen bonding. So this actually explains the general mechanism of the clay water attraction. So if you look into this the negative charge is to be neutralized by opposite charges leading to the formation of a adsorbed water layer and double layer. So here because of this particular observation what we made in the previous slides, so the water molecules are arranged in some fashion here and here up to certain distance you will see that the positive cations and these are actually attracted and attached closely and this particular zone is actually called adsorbed water layer or a double water layer. So water in this zone is adsorbed water, water attracted and bound to the clay particle very very strongly and the water, this is the boundary of that water layer and this water is actually nothing but which is called free water or a the adsorbed water and this is called adsorbed water the water which is actually strongly attached to the clay particle surfaces. So here the typical cross section of the or the you know more explanation which actually given here where you have got clay particle, a cross section which of the clay particle is shown here and here there is natively charges and here the polar water molecules which are actually arranged and then this particular water is, this is the adsorbed water and this is weakly oriented or low viscous or free water molecules. In this case it is said that the viscosity is so high or isolic structure where the water molecules are strongly attached to the clay surface. So the adsorbed water cannot be removed by oven drying at 105 to 110 degree centigrade and may therefore be considered as a part of the solid soil green and this adsorbed water is generally held to the surface of the particle by powerful forces of electrical attraction and virtually in a solid state and this is actually this thickness of this adsorbed layer on one side of the clay particle or one edge from the one edge of the clay particle is said as about 10 angstroms thickness. So this cannot be removed by oven drying and if the temperature actually increases then there is a possibility that the structure of the soil has a tendency to get distorted or change. So here in this adsorbed water and here this is the edge of the clay particle and here there is a rigid layer which is actually has got properly oriented cations and water molecules which are actually shown and here disoriented water molecules which are actually there which is actually shown here. So if you see from here the distance from the clay particle and the field intensity which decreases from the edge of the clay particles to when you go away from the clay particle distance. So the cations distribute themselves around the neatly charged surface and of the clay particles with the greatest density near the surface the intensity will be very high and as you go away from the clay particle it actually decreases. The similar situation prevails on the other side of the clay particle also. So the nature of the electric double layer affects the structure of the aggregates of the clay particles and hence the physical properties of the soil. Here again one angstrom is equal to 10 to the power of minus 10 meters or equivalent to 0.1 nanometers. So the thickness of the it is said that the viscosity said to be very high close to the clay particle. So here this is the edge of the clay particle. Let us say this is the edge of the clay particle as you go away from that you actually have got adsorbed layer and some double layer of water up to 400 angstrom thickness and then followed by a free water. So as if this is one side of the particle then other side of the particle also it actually has got the similar distribution of viscosity. So at close to the clay particle there is a tendency that the water which is said that has got a ice like tendency with very viscosity very high viscosity. Close to the clay particle. So in this particular slide a typical kaolinite particle with 10000 into 1000 times angstrom with adsorbed layer and double layer water is shown here. So here this is the kaolinite crystal which is relatively large and here we have 10 angstroms and then the double layer thickness which is actually shown here. So this is that adsorbed water layer surrounding the kaolinite particle and typical in this slide a typical matrimonite particle which is 1000 into 10 angstrom. So here the double layer water which is actually amounting to about 200 angstroms and the adsorbed water layer surrounding the clay particle is about 10 angstrom thickness where actually it possess very high viscosity shown in this slide. So having discussed about clay minerals clay water particle interaction clay particle and water interaction and we also have defined the specific surface area and there is another parameter which is actually called cation exchange capacity which is called CEC which is referred here as CEC the ability of a clay particle to adsorb ions on its surface or edges is called cation exchange capacity. The CEC is basically measured in milli equivalents per 100 grams of dry soil particle in is a measure of the net native charge on the soil particles resulting from the isomorphous substitution and broken bonds at the boundaries. So CEC is function of the mineral structure of the clay and size of the particles. So the CEC nothing but which actually measures the net native charge on the soil particles resulting from the isomorphous substitution or broken bonds at the boundaries. So CEC is a function of the mineral structure of the clay and the size of the particle. So the cation exchange capacities in the units of milli equivalents of 100 grams of dry soil of quartzite, kaolinite, illite and montanolite are given here and where we have got very small due to fine particles and broken bonds. This is very very small and kaolinite actually has got 328, illite actually has got 40 and montanolite is 80. The CEC of montanolite is 10 times the CEC of kaolinite. Similarly, the specific surface area of the montanolite is 100 times the specific surface of the kaolinite but when you look into the specific surface of montanolite it is about the 10 times of the CEC of the kaolinite. So this indicates that the smaller amount of montanolite is required than kaolinite to impart the properties of clay to mixed grain soil. So if a smaller amount of montanolite is actually mixed and that can actually impart its properties than a larger fraction of clay kaolinite. So this indicates that the smaller amount of montanolite is required than kaolinite to impart properties of clay to the mixed grain soil. See the cation exchange capacity, the exchangeable cations are the positively charged ions from salts in the pore water which are attracted to the surface of the clay particles to balance this need to be charged. The cations can be arranged in a series in terms of their affinity for attraction as follows, aluminum 3 plus CA2 plus and Mg2 plus NH4 plus K plus H plus Na plus and lithium plus. This indicates that for example, aluminum 3 plus ion can replace calcium 2 plus and calcium 2 plus can replace sodium ion. So this process is called cation exchange. So the practical example for the cation exchange is that which is used is that the stabilization of sodium based clay soil using lime. If you look into the sodium clay montanolite base and if it is added with calcium chloride then there is a calcium clay and sodium chloride is formed. So here what happens is that the calcium 2 plus ions replace Na plus ions and reduce the swelling of sodium montanolite because of this the adsorbable water layer would become thinner and undergoes a structural distortion. So the affinity for keeping water decreases because of this. So this is actually used or this is how the concept of the lime stabilization which is actually used for strengthening as a stabilization measure used in practice for soils which actually contain montanolite based soils in the field. So here the structure of the clay soils which is actually shown the fine grained soils. The forces between clay mineral particles, if the two particles basically we have discussed that the clay particles are plate clay particles and they have a net negative charges. So there is always a possibility that these two particles or plate clay particles have a tendency to get repulsive tendency they can actually have got repulsive tendency or if they are actually have got the positive charges then there is a possibility of the attraction. So if the two particles plate clay shaped approach each other in suspension the forces acting on them are the Van der Waals forces of attraction the repulsion between the two positively charged ionized adsorbed water layers. So the Van der Waals forces of attraction or the repulsion between the two positively charged ionized adsorbed water layers. So here it is a typical lately charged clay surface which is actually shown here. So because of this particular phenomenon one of the clay structure the soil structure which we are not discussed. So previously we have discussed that if you have got a large particles there is a possibility of the bulk structure or what we said is that bulk structure then we also discussed for some certain silt particles there is a possibility that you have got a honeycomb structure but we actually have not discussed about the type of the structure which is actually prevalent in fine-grained soils. So as we have actually discussed that the fine-grained soils actually has negatively charged clay particles and which are basically have a plate shaped particles which basically they are plate shaped particles. So if you look into this here you have got two plate shaped particles which are actually surrounded by these adsorbed and double layers and there is always a tendency of the net repulsion. So at very small separations the Van der Waals forces are always larger and the particles which approach sufficiently closely will head air however the Van der Waals forces decrease rapidly with increasing separation. So if the adsorbable layer is thick the repulsion will be large at the distances from the surface at which the Van der Waals forces are small. So the particles will remain dispersed and settle independently. So if you have the structure of the clay soils particularly in the fine-grained soils you have got say you have got particles which are actually have edge to face orientation then they have a tendency of the flocculation. So if the adsorbable water layer is thin and there will be little or no net repulsion at any distance and the random movements of particles will be enough to bring them into contact and this process is called flocculation. So what we said is that because of the repulsive forces there is a possibility of some structure which is actually can occur is because of the dispersed structure is called dispersed structure and because of the thinner adsorbable layer and absence of any repulsion then there is a possibility of the flocculation. So net force if it is attraction then you have a flocculation the groups of particles they actually settle together. So this is actually explained that net force between the two particles in suspension if you look into this net repulsion where you have got a low ion concentration in soil water in the soil then there is a possibility of repulsion which actually takes place and when you have got high ion concentration in the soil water there is net attraction takes place. So this particular type of environment makes actually two different types of structures particularly called flocculent and dispersed structures. So in this particular lecture what we try to understand is that we defined specific surface area then we also discussed about the how the specific surface area or cation exchange capacity are varied with reference to the three fundamental three clay minerals and we also discussed about different types of clay minerals and the basic structures which are actually like the gipside sheet and the silicate sheet. So kaolinite is nothing but you have got a two sheet mineral and montaminate is nothing but two is to one mineral it is called two sheets of silicate and one gipside sheet and similarly you have got and illite mineral which is actually formed by also nothing but two is to one mineral but it actually has got two adjacent silicate sheets are bonded with potassium cation strongly. So in the next lecture we are going to discuss about the you know introduce ourselves to this particular flocculent dispersed structures and then we will discuss further on the different types of clay minerals particularly other than the kaolinite, montaminate and illite like some structures of yellow site, muscovite etc.