5.2.1　Mud Material Composition and Basic Propertie...

5.2.1　Mud Material Composition and Basic Properties

5.2.1.1　Mud Material Composition

1）Bentonite

Bentonite is a layered aluminosilicate clay mineral with montmorillonite as the main component.Its cell is composed of two layers of silica tetrahedron and a layer of aluminum oxide octahedron.The oxygen atoms shared between the tetrahedron and the octahedron form a highly ordered quasi-two-dimensional layer，which has a large specific surface area.The silicon and aluminum ions can be replaced by low-valent ions such as magnesium，iron and lithium，resulting in unsaturated interlayer electricity price of bentonite and permanent negative charge between layers.This negative charge is usually balanced by interlayer exchangeable hydrated cations.Therefore，bentonite has a series of valuable characteristics such as good ion exchange，water absorption，expansion，adhesion and adsorption.Using its good ion exchange performance，the natural Ca-bentonite is sodiumized，and its hydration performance is optimized，which can be used as the material of drilling mud and slurry shield mud.The evaluation items and test methods of bentonite are as follows.

（1）Montmorillonite content：Montmorillonite is a kind of silicate-aluminate clay mineral.Montmorillonite is a unit crystal layer composed of two layers of silica tetrahedron layer and one layer of aluminum hydroxide octahedron layer.Tetrahedron and octahedron are connected by shared oxygen atoms.Its molecular formula is Al2（Si4O10）（OH）2·nH2O，which is expressed as 4SiO2·Al2O3·nH2O with oxides.The corresponding theoretical contents are 66.9％，28.3％and 5.0％，respectively.Bentonite with high montmorillonite content is easy to disperse，large cation exchange capacity，high pulping rate and good stability due to its mineral structure and lattice substitution characteristics.Therefore，montmorillonite content is the most important index for evaluating pulping bentonite.At present，the commonly used method for the determination of montmorillonite content is the blue absorption method，as detailed in“Benton”（GB/T 20973—2007）.It uses methylene blue to form monovalent organic cations in aqueous solution，which can react with bentonite by cation exchange to form organic bentonite complex and dye it.

（2）Expansion index：The expansion index of bentonite refers to the volume of 2 g bentonite after 24 h expansion in water.The test method is detailed in GB/T 20973—2007.The greater the expansion index of bentonite，the better its dispersion，the higher the pulping rate.In common pulping bentonite，the expansion index of sodium bentonite is higher than that of calcium bentonite and acid bentonite.This is because the water absorption and expansion process of bentonite is the continuation of the interlayer cation hydration process.The interlayer hydration process is carried out step by step，and roughly forms 1-4 layers of water molecules.The types of interlayer cations are different，and the hydration energy is also different.Ca2+hydration energy is larger，Ca2+hydration film is thin and irregular，and Na+hydration film is thick and arranged regularly，which inevitably leads to the expansion of sodium bentonite.For the same type of bentonite，the more montmorillonite content is，the higher expansion index is.Expansion index is one of the technical indexes for evaluating bentonite quality.

（3）Cation exchange capacity：Cation exchange capacity of bentonite CCEC refers to the total amount of cations（Na+，K+，H+，Ca2+，Mg2+，etc.）adsorbed by bentonite at pH=7（meq/100 g soil），including exchangeable hydrogen and exchangeable base.The size of CCEC is related to the hydration，expansion and electrification of bentonite，which is not only an important indicator for identifying the mineral types of bentonite，but also an important basis for evaluating the quality of pulp bentonite.Bentonite can be divided into sodium bentonite，calcium expansive soil and alkali treated calcium expansive soil according to the type and quantity of adsorbed ions.The exchangeable cations adsorbed were mainly sodium bentonite，and calcium expansive soil，etc.The exchangeable cations adsorbed by bentonite are different，and the pulping performance will be greatly different.In terms of pulping rate，only sodium bentonite can have high pulping rate，and other calcium bentonite can have high pulping rate only after pure alkali treatment.

（4）Particle gradation：In soil mechanics，the soil particle content of a certain particle group in the soil is defined as the ratio of soil particle mass to total dry soil mass in the particle group，expressed by percentage.The relative content of soil particles in grain groups of various sizes in soil is called soil gradation.In soil mechanics，the particles with particle size greater than 75μm are defined as sand，the particles with particle size of 5-75μm are silt，the particles with particle size less than 5μm are clay，and 2μm is also used as the upper limit particle size of clay in foreign countries.In colloidal chemistry，particles with particle size less than 0.1μm are called colloidal particles.In fact，particles with particle size of 0.1-5μm have some properties of colloidal particles.In addition，the smaller the particle size is，the larger the specific surface area is.For clay soil，due to the small particle size of clay particles，it has a large specific surface area.When clay particles meet with water，it will produce complex physical and chemical changes and have some characteristics of colloidal dispersion system.The higher the clay content in the soil，the greater the adsorption and hydration ability，the stronger the dispersion ability，the thicker the particles，and the worse the pulping performance.Therefore，the content of impurities such as sand can be determined by measuring the particle size distribution of bentonite，and the content of clay beneficial to pulping can be measured.Therefore，the gradation curve of bentonite can be used as a basis for evaluating the quality of bentonite.

2）CMC

As an important part of mud materials，carboxymethyl cellulose（CMC）plays a major role in regulating the viscosity of mud.CMC is the largest，most widely used and most convenient product of cellulose ethers，commonly known as“industrial monosodium glutamate”.CMC is an important cellulose ether and an anionic cellulose compound with good water solubility obtained from chemical modification of natural fibers，which is easily soluble in hot and cold water.CMC is a macromolecular chemical substance that can absorb water and expand.When swelling in water，it can form a transparent viscous glue，which is neutral in pH.Solid CMC is stable to light and room temperature，and can be stored for a long time in dry environment.CMC is white or yellow powder，granular or fibrous solid，odourless，tasteless，non-toxic.The important characteristics of CMC are the formation of colloids and solutions with high viscosity，which have the characteristics of adhesion，thickening，flow，emulsification and dispersion，shaping，water retention，protective colloids，film forming，acid resistance，salt resistance and turbidity，and are physiologically harmless.

3）Mud filling materials such as clay and silt

Clay，silt and fine sand with different particle size ranges are used as mud filling materials.The main function is to adjust the particle size of solid particles in the mud and the density of the mud.The filling materials with different particle size ranges are added to adjust the mud gradation，and different amounts are added to adjust the density of the mud.

4）Other additives

Sometimes in order to increase the stability of bentonite mud，will also be added to the mud sodium carbonate Na2CO3，play a scattered，uniform role，the principle can be called salt immersion principle.Ingredients such as calcium silica and magnesium in the monoshide can react with alkaline substances，and the alkali is designed to destroy the lattice structure of the monosacis，making the main components of the monomorphic state of polysetic sodium hydrochloric acid and sodium polysilicate and partial al-aluminic acid.Because the monosacis contains the gender substance Al2O3，generally in high temperature conditions，will react with NaOH or Na2CO3 under the melting state，thus destroying the cell of the montiline，when the Al2O3 dissolution in the montiline reaches 75％-85％，the lattice structure is often destroyed.Coupled with the solubility of other ions，the amorphous silicon in the lattice structure is converted into active silicon，thus achieving the function of activation.Sodium carbonate，an additive，can solve a certain amount of Na+and polymerized anions.Ca2+on the surface of Na+and clay is an ion exchange，due to the intervention of the hydration of sodium ions，increased the thickness of the diffusion layer，increased the potential ξ of the clay particles，in addition to the polymerion of anion can be partially adsorbed by the positively charged clay particles end surface，so that the end of the charge modification，breaking the bond structure formed by the side of the clay particles，Release part of the free water，so can play a certain dilution，dispersion role.

At present，some special pulping agents have been developed in the engineering field.NSHS-1，NSHS-2，NSHS-3，NSHS-4，NSHS-5 and other five kinds of pulping agents provided by North China Oilfield are composed of several products，and their structures are complex.Due to the use of a variety of polymer，and the same polymer has different molecular weight，substitution degree and polymerization degree，the actual effect of each pulping agent is still uncertain.Through analysis and research，the main functions of five kinds of mud are determined.The main components and functions of different pulping agents are shown in Table 5.1.

5.2.1.2　The Basic Nature of the Mud

1）Density

Mud density characterizes the content of solid particles in mud，which has an important impact on the penetration of mud in the formation.It is one of the most important properties of mud and an important indicator of mud management in slurry shield construction.The mud density is small，the mud pump will not overload operation，but the density is small，the content of soil particles in the mud is small，the speed of mud penetrating into the formation will be slow，which is unfavorable to the stability of the excavation face.The density of mud is relatively large，that is，there are many solid particles in the mud.Solid particles are easy to deposit at the bottom of the mud drainage pipeline，which not only causes great wear and tear of the pipeline，but also increases the driving power of the equipment，and makes the mud pump overload operation.So in determining the density of mud，not only to consider the stability of excavation face，but also to consider the bearing capacity of equipment.

The commonly used instrument for measuring mud density is a simple and easyto-operate mud specific gravity scale.The ratio of mud specific gravity to 4℃pure water is tested，and the reading on the scale is mud specific gravity.The specific gravity scale used is 1002 type slurry specific gravity scale，as shown in Fig.5.1.It is composed of slurry cup，beam，moving weight and support，and there are weight regulating tubes and horizontal bubbles on the beam.

2）Viscosity

Mud viscosity represents the viscosity of mud flow and reflects the fluid characteristics of mud.When the liquid flows，the measurement of the resistance to shear deformation rate is called the viscosity of the liquid（referred to as viscosity）.Viscosity is the nature of the liquid itself，and does not change with the shear rate.Viscosity is expressed by viscosity.The greater the viscosity of the mud，the better the physical stability of the mud，and the mud is not easy to segregation.It is not easy for particles to settle when transporting slag.However，the viscosity of the slurry is too large，and there are not only many viscosifiers needed in pulping，but also high economic costs.When the slurry is pumped，the pumping capacity of the slurry pump is also limited.The mud viscosity is small，the physical stability of the mud is poor，and the mud is prone to segregation.The combination of particles and water in the mud is weak，and the carrying capacity of the mud is poor.Although the particles in the mud plug the formation pores，the water in the mud is easily pressed out under the action of mud pressure，leaving the particles in the mud plug on the surface of the formation，which makes the formed mud film thick and loose，and is not conducive to the stability of the excavation face.

Mud viscosity can be divided into two kinds：one is more intuitive and simple funnel viscosity，which characterizes the macroscopic viscosity of the mud.The funnel viscometer is used to measure the viscosity of the mud.The commonly used funnel viscometers are Sue funnel viscometer（Fig.5.2）and Mae funnel viscometer（Fig.5.3）.The other is the plastic viscosity that reflects the properties of slurry fluid，representing the ability of slurry to hinder the flow.It is one of the basic parameters of hydrodynamic properties.The rotational viscometer is used to measure the commonly used rotational viscometers，including six-speed rotational viscometer（Fig.5.4）and fifteen-speed rotational viscometer（Fig.5.5）.However，in order to simplify the measurement in site construction，only the funnel viscosity related to yield is usually measured（in a certain range，the apparent viscosity of mud can be characterized by the funnel viscosity of mud，and finally the viscosity and fluidity of mud can be reflected）.

（1）Funnel viscosity.The slurry viscosity commonly used in engineering is basically funnel viscosity，because the funnel viscometer is cheap，compact，portable，and easy to operate in the field.It measures the viscosity of slurry at low flow rate.At present，the mud viscosity used in engineering is mainly Sue funnel viscosity and Mae funnel viscosity，and the conversion relationship between them is shown in Table 5.2.If there is no special description of the funnel viscosity of mud in this book，it is measured by the funnel viscometer commonly used in the construction site（the Soviet field funnel viscometer）.Method for determination of funnel viscosity（taking the soxhlet funnel viscometer as an example）：the funnel is vertical，gripped by hand and blocked by index finger，and then poured 200 and 500 mL mud into the funnel at both ends of the measuring cylinder.Put the measuring cylinder 500 mL upward under the funnel，open the index finger，and start the stopwatch to record the time needed to fill the 500 mL mud，that is，the viscosity of the measured mud.

（2）Plastic viscosity（PV）.Under laminar flow conditions，the slope value when the shear stress is linearly related to the shear rate is called the plastic viscosity of the slurry，and the measurement unit is“mPa s”.Plastic viscosity reflects the strength of internal friction between suspended solid particles，between solid particles and liquid phase，and within continuous liquid phase when the damage and recovery of grid structure in slurry is in dynamic equilibrium under laminar flow.

Plastic viscosity is measured by rotary viscometer，which consists of four parts：dynamic part，variable speed part，measurement part and support part.The power part provides power for rotary viscometer by motor.Variable speed part variable six speed，respectively，3 r/min，6 r/min，100 r/min，200 r/min，300 r/min，600 r/min.The measurement part is composed of a torsion spring，a dial and an inner and outer cylinder.The inner cylinder is matched with the taper of the shaft，and the outer cylinder is connected with the chuck.The bracket part adopts the tray to lift the tested vessel.

The working principle of the rotary viscometer is as follows.The liquid is placed in the annular space of two concentric cylinders.The motor drives the outer cylinder to rotate at constant speed through the transmission device.With the help of the viscosity of the measured liquid，it acts on a certain torque of the inner cylinder to drive the inner cylinder connected with the torque spring to rotate.The size of the rotation angle is proportional to the viscosity of the liquid，so the viscosity measurement of the liquid is converted to the measurement of the inner cylinder rotation angle.

The use of rotary viscometer（taking NXS-11A rotary viscometer shown in Fig.5.5 as an example）：The appropriate measurement system is selected according to the estimated viscosity value of the material.The selection basis is that in the 15 speed measurement，the scale reading cannot be all less than 25 or all greater than 75，and then the corresponding inner cylinder is inserted into the coupling of the measuring head to tighten the screw fixation.Appropriate amount of measured material is added to the outer cylinder，and the outer cylinder is installed on the measuring head，and fixed with a screw sleeve，so that the material in the outer cylinder is just immersed in the inner cylinder.Level the measuring head with leveling bubble，turn on the power supply，adjust 15 speed in turn，record the corresponding scale of each speed.The rheological characteristic curve was drawn according to the corresponding rotational speed and the shear force value converted by the viscometer reading.

3）Particle grading

The mud gradation reflects the size and distribution range of solid particles in the mud，which is an important parameter affecting the permeability of the mud in the formation.Studies have shown that for high permeability formation，the mud contains a certain amount of sand，vermiculite and other coarse particles can change the permeability form of mud.Therefore，the slurry gradation is another important parameter reflecting the slurry properties.Generally，for strata with permeability coefficient exceeding 10-2 cm/s，such as coarse sand，gravel，pebbles or strong weathered strata with cracks，mud is easy to escape and mud pressure cannot be maintained，which is easy to cause the collapse of excavation face to lose stability.At present，the practice and theoretical research in Nanjing Yangtze River Tunnel and Nanjing Wei San Road Crossing Channel have proved that the requirements of slurry shield construction on slurry performance in this stratum need to consider the gradation of particles in the slurry to ensure that this slurry can form a stable and dense mud film on the excavation surface.The traditional soil sieve is mainly suitable for testing the gradation of sand（＞75μm），and the gradation of particles with particle size less than 75 μm cannot be measured.Since there are many fine particles in the slurry，the commonly used test instrument for slurry gradation is laser particle size analyzer.The particle size distribution curve of mud is often measured by MS2000 laser particle size analyzer（Fig.5.6）.

4）Sand content

The sand content of bentonite refers to the percentage of sand volume that cannot pass through the No.200 sieve（equivalent to the diameter greater than 0.075 mm）in the bentonite slurry，which reflects the amount of coarse particles in the slurry.For the bentonite slurry，the sand content can also be called the impurity content，reflecting the advantages and disadvantages of the grinding process of a bentonite ore.For high permeability formation，a certain sand content of mud is conducive to plugging the pores of the formation.The determination of sand content in this experiment mainly adopts LNH mud sand content tester，as shown in Fig.5.7.The instrument is mainly composed of filter tube，funnel and glass measuring tube.

5）Water loss of mud

In the field of oil drilling，the research on drilling mud filtration is relatively mature，and there are currently test standards.Since the temperature and pressure difference have a great influence on the filtration of drilling mud，the filtration of drilling mud can be divided into low temperature and low pressure filtration（or API filtration）and high temperature and high pressure filtration.At present，API filtration is usually used to evaluate the filtration of drilling mud in China and abroad，that is，under the specified pressure difference（0.69 MPa），a certain volume（240 mL）of mud is measured by the filtration of a certain area of infiltration medium within 30 min（mL/30 min）.Commonly used instruments are ZNS 1 mud dehydrator，as shown in Fig.5.8.The special filter paper produced by American Baroid Company is used in the filtration medium，the filter paper area is（45.6±0.5）cm2，and the pore size is 0.5-51μm.Because the API filter loss is measured under the same pressure and the same permeability medium，and the formation is changeable and the mud pressure is different，the same mud has different permeability types in different formations and forms different forms of particle accumulation.However，according to the API filter loss test standard，one mud has only one API filter loss.Therefore，the API filter loss of mud can only reflect the quality of mud itself and cannot be used as a standard to evaluate the permeability process of mud in the formation.

6）Physical stability and colloidal rate of mud

Physical stability refers to the ability of solid particles in mud to maintain the physical state of floating suspension，which is usually described by the interface height.After a certain amount of slurry is placed in the measuring tube for a period of time，some soil particles will lose the suspension characteristics and precipitate.Clear water appears on the surface of the slurry，soil particles appear at the bottom，and mud is still in the middle.Clear water and mud appear at the interface.The precipitation degree of soil particles in the slurry can be identified by observing the variation of the boundary height between clear water and slurry over time.The slurry colloidal ratio is commonly used to measure the physical stability of slurry in engineering.Colloid ratio represents the degree of dispersion and hydration of clay particles in bentonite，which is a rough indicator of the degree of dispersion and hydration of clay particles in mud.It characterizes the hydration ability of mineral components in mud.It is mainly the process of water absorption of clay particles，colloidal particles and other particles，the stretching of molecular bonds，and the thickening of the binding water film around clay particles.Macroscopically，it shows that the particles absorb water and disperse into uniform colloids.The higher the colloid ratio is，the more stable the physical properties of the mud are，and the mud is not easy to segregation and water precipitation，so the formed mud is relatively uniform.Therefore，the colloid rate is the basic index to characterize the physical stability of mud.The test of mud colloid rate is shown in Fig.5.9.

Test method：1000 mL mud was poured into 1000 mL measuring cylinder，and glass was placed on the cover for 2 h or 24 h.The volume of the upper clarification liquid and the sedimentation volume of the lower sand particles were observed.If the upper clarification rate was 3％and the lower sedimentation rate was 2％，the slurry colloid rate was 95％.

7）The pH of the mud

The pH value is mainly used to reflect the acidity and alkalinity of bentonite mud.The main measuring tool is pH meter（Fig.5.10）.The pH value of the test mud indirectly reflects the chemical stability of the mud.Chemical stability refers to the chemical deterioration of the mud film-forming function when positive impurities（including Ca2+，Na+，Mg2+，etc.）are mixed into the mud.The reason is that clay particles are negatively charged.When positive ions are encountered，clay particles change from suspended state to condensed state，and the number of suspended clay particles in mud is sharp.Less，resulting in difficulty in forming mud film.Studies have shown that the pH range of the slurry without positive ion pollution is 7-10，showing weak alkaline.When the sludge is polluted by positive ion impurities，the pH value is far more than 10.Therefore，the degree of deterioration caused by positive ions can be determined by measuring pH value，so as to identify the chemical stability of sludge.The mud with pH 8-10 has good chemical stability.