UNIT-VI
PHASE RULE
The phase rule is an important generalization dealing with the equilibrium behavior of a heterogeneous system.
This heterogeneous equilibria was first discovered by an American physicist JW Gibbs ( 1874) and continued .
The change in the chemical composition or physical state or both may takesplace in chemical equilibrium. These reactions are reversible.
TERMINOLOGY
Systems are of 2 types. (1) Homogenous (2) Heterogonous.
Homogenous:- A system, which exhibit identical physical and chemical properties throughout is called as Homogenous .
Eg:- CuSO4--exhibits identical physical and chemical properties even under ultra microscope.
Heterogonous:- A system, which exhibit different physical properties is called as Heterogonous.
Eg:- (1) Ice-H2O –vapor system--3 portions, physically distinct and mechanically separable from one another.
(2) Milk---appears in uniform white colour, but it is a colloidal in presence of fat particles, seen in ultra microscope. Phase:- It is defined as an homogenous part of heterogonous system, physically distinct and mechanically separable portion of system, which is separated from other such parts of the system by definite boundaries.
Eg:- H2O consists of 3 phases at freezing point.
Ice (s) ↔Water (L)↔ Water vapours (g)
At below 00C water exhibit solid state---ice; as the temperature raise it melts to give water (L) then water vapours, reversible reaction.
Thus forming a system of 2 or 3 phases; that are homogenous; physically distinct and mechanically separable. It should be noted that each phase in a heterogonous system is homogenous in itself .
There are following phases.
One phase :-
A gaseous mixture, gases are completely miscible in all proportion ---- So form one phase .
Eg:- N2 and H2 forms one phase only .
Homogenous system consisting of only one phase .
If two liquids are miscible (alcohol and H2O) they will form only one phase.
The solution of substance in a solvent consists of one phase.
Eg: Glucose in H2O.
Two phases:-
If 2 liquids are immiscible, they will form 2 phases . Eg:-benzene and H2O.
Three phases :-
Two solid phases of Fe and FeO and one gaseous phase consisting of H2O(g) and
H2(g).
Heterogonous system consists of 2 or more phases .
Eg:- Ice(S) ↔ Water(L) ↔ Watervapour (g).
Component:-
The smallest number of independent variable constituents, necessary to describe the composition of each phase. Eg:- (1) H2O system
Ice (S) ↔ Water (L) ↔ Watervapour (g).
The chemical composition of the 3 phases is H2O. Hence it is one component system.
(2) The sulphur system consists of 4—phases rhombic, monoclinic, liquid and vapour. The Chemical composition of all phases is S. Hence it is one component system.
Two component system:-
A system of saturated solution of Nacl consists of solid salt ↔ salt solution ↔ H2O vapour
All these phases can be expressed in terms of NaCl and H2O. Hence it is a 2 component system.
(2) Thermal decomposition of CaCO3 (s) to give CaO (s) CO2 (g)
CaCO3 (s) ↔ CaO (s) + CO2 (g)
This 3 phase system can be expressed in terms of at least two of the independently variable constituents ( CaCO3 and CaO / CaO and CO2). Three component system :-
Fe(s) + H2O(g) ↔ FeO(s) + H2(g)
This 3 phase can be expressed in 3 component.
Number of component of a system :-
It is defined as the number of chemical constituents of the system minus the number of equations relating to these constituents in an equilibrium state.
Eg:- Dissociation of KClO3
2KClO3 (s) ↔ 2KCl(s) + 3O2(g)
2 3 [Kcl] [o2 ] 3 Keq: 2 =[0 2 ] [Kcl03 ]
So number of equations relating the concentrations of constituents =1
Hence the number of components = 3-1=2 i.e. 2 component system .
Eg:- (2) Dissociation of NH4cl in heated vessel.
NH4Cl (s) ↔ NH3 (g) + HCl (g)
[NH ] [Hcl] Keg = 3 [NH 4 cl]
The active mass of NH4Cl is constant.
Keg = [NH3] [HCl] =2
3 – 2 = 1
So, it is a single component system.
IV )Degree of freedom or variance :-(F)
A system in equilibrium is affected by the factors such as pressure, temperature and composition of the phases.
Variance is defined as the number of independent intensive variables such as temperature, pressure and concentration, which must be fixed in order to define the system completely.
(The mass dependent properties like weight, volume etc --- extensive)
A system having one, two, 3 or 0 degrees of freedom are usually called univariant (F=1), bivariant (F=2), Trivariant(F=3) and invariant(F=0) respectively.
Eg:- (1) A pure gas---component =1, P =1 (T and P are fixed )
F = C - P + 2 = 1 – 1 + 2 = 2
A pure gas has 20 s of freedom .
(2) A gaseous system consists of 2 gases :-
C = 2, P = 1, F = 2 – 1 + 2 = 3
So, this system has 3 degrees of freedom.
A gaseous system consists of 3 gases.
C = 3, P = 1 F = 3 – 1 + 2 = 4(Tetra variant).
Saturated NaCl solution in contact with solid and vapour
C = 1, P = 2 F = 1 – 2 + 2 = 1
Ice – H2O - vapour system ( invariant ).
C = 1 , P = 3 F = 1 – 3 + 2 = 0 Phase Rule:-
It may be stated as equilibrium is dependent only on temperature, pressure and concentration of the phases, is not influenced by gravity/electrical/ magnetic forces etc, then the number of degrees of freedom(F) of the system is related to the number of components( C )and of phases (P) by 2.
F = C – P + 2 F = degree of freedom
C=components of the system
P =number of phases of the system.
Phase diagram:-
A diagram of substance, which illustrates the conditions of equilibrium between various phases of a substance is called as phase diagram / equilibrium diagram.
When system goes from one phase to another phase without any change of chemical composition is called as phase transition.
Eg:- Melting (Solid ↔ Liquid), freezing (Liquid↔ Solid)
Boiling (L ↔ g) and Condensation (g ↔ L)
These different phases of system may be represented as phase diagram as a function of the temperature and pressure.
Generally temperature is along the horizontal axis and pressure is along the vertical axis.
Phase diagram exhibit following contents.
(1) Lines (2) Areas (3) Triple point.
Lines /Curves :- There 3 lines separating the areas / regions.
Three curves shows the conditions of equilibrium any 2 of the 3 phases.
Melting / fusion curve represents equilibrium between liquid ↔ solid.
Vaporization curve represents equilibrium between liquid ↔ vapour .
Sublimation curve represents equilibrium between vapour ↔ solid.
These curves are termed as phase boundaries ;since along these curves the 2 phases are in equilibrium.
Areas / Regions:- The diagram divided in to 3 regions or areas which are labeled as solid, liquid and vapour. In the diagram these are represented as A O B, A O C, B O C.
Each of the 3 areas shows the conditions of temp and pressure under which the respective phase can exist.
Triple point:-
The three boundary lines intersect at a common point called Triple point .At which all the 3 phases (L, S and v) can co exist in equilibrium.
At this point both temp and pressure on the diagram are fixed. If temperature or pressure is change the equilibrium will be disturbed.
One component system (Water system ):-
The equilibrium conditions in a one–component system may be conveniently represented by pressure – temperature (P-T) diagrams. This diagram includes
Areas:- Represent bivariant system because the temperature and pressure should be fixed to the define system .
A line :- Represent a monovariant system, the equilibrium conditions, at any point on the line depends on the conditions of temperature or pressure .
A point :- Represent an invariant system, where all the 3 phases are in contact with each other; because it is completely defined by itself and any information regarding pressure or temperature is unnecessary.
Eg:- Water system .
Water is example for one component and 3 phases.It exhibits following phases.
Ice (s) ↔ water (l) ↔ water vapour (g)
In all these phases can be represented by one chemical entity H2O. The number of phases which can exist in equilibrium any time depends on the conditions of temperature and pressure.
The phase diagram consists of
The curves OA, OB and OC
The areas AOC, AOB, BOC
The triple point O.
The curves OA, OB, OC:-
These 3 curves meet at the point O and divide the diagram into 3 regions/areas.
The curve OA:- It is a vapour pressure curve of liquid water at different temperature. Along this curve OA, the 2 phases water and water vapour co-exit in equilibrium. Above this curve the liquid water alone exist and below it only water vapour exist.
This curve terminates at A, critical point (218 atm and 3740c) beyond which the liquid and vapour phases merge into each other resulting in a homogenous phase. When vapour pressure is equal to 1 atm, the corresponding temperature is 1000c (i.e. B.P of
H2O).
(B) Curve OB:-
Is the sublimation curve of ice; this curve separates the ice region from vapour region.
This curve represents the equilibrium between ice and vapour.
At the lower limit, the curve OB terminates at absolute zero (-2730c) where no vapour exists.
(C )Curve OC :- This is melting point curve /fusion curve of ice. This curve divides the solid ice region from the liquid water region .Where ice and water exists in equilibrium.This curve slopes to the left indicates that the melting point of ice is lowered by raise of pressure. Since ice melts with decrease in volume by Le-chatelier‘s principle( M.P is decreased by an increase of pressure. Along the curves OA, OB, OC there are 2 phases (L ↔ G/ G ↔ L/ L↔ S) in equilibrium and one component.
According to phase rule
F =C – P + 2 = 1 – 2 + 2 = 1 So, the system is univariant.
Hence each two– phase system
H2O / H2O vapour represented by OA.
Ice / H2O vapour represented by OB.
Ice / H2O represented by OC.
II) Areas AOC, AOB, BOC:-
The areas/ region between the curves show the conditions of temperature and pressure under which a single phase exist (Ice, H2O or vapour ). Thus, Area AOC ----represents conditions for the one phase system of water.
Area AOB ----represents condition for the one phase system of water vapour.
Area BOC represents conditions for the one phase system Ice. In all these areas there being one–phase and one component.
F = C – P + 2
= 1 – 1 + 2 = 2
Thus each system H2O , water vapour or ice has 2 degrees of freedom i.e. the system is bivariant .
III) Triple point :-
The curves OA, OB, OC meet at the triple point ‗O. Where all 3 phases occurs in equilibrium. This occurs at 0.00760C and a vapour pressure of 4.58 mmHg. Since there are 3 phases and one component.
F = C – P + 2
= 1 – 3 + 2 = 0
The system at the triple point is invariant. Thus, if either pressure or temperature is changed, the 3 phases would not exist and one of the phases would disappear.
Iv) Metastable system:- super cooled water / vapour:-
The vapour pressure curve of water AO can be continued past the triple point O as shown by the dashed line OA1. The water can be super cooled (does not always freeze at O0C) by carefully eliminating solid particles.
This OA1 curve represents the metastable (unstable) system and this curve represents the vapour pressure curve of super cooled water. On slight disturbance, the super cooled water spontaneously changed in to solid ice.
It may be noted that metastable vapour pressure of super cooled water is higher than the vapour pressure of ice.
Two component system (lead – silver system):-
In two component system, minimum number of possible phases could be taken as unity. So, this corresponds to 3 degree of freedom.
F = C-P + 2 = 2 – 1 + 2 = 3
Hence to illustrate the 2 component system we require to define at least 3 physical variables such as temperature , pressure and composition .The phase behavior of this system may be represented by a 3- dimensional diagram ,which cannot be conveniently shown on paper .
However to represent the phase - diagram as a simple manner usually consider only 2 variable and another one is constant.
If the pressure is constant –Isobaric phase diagram,
If the temperature is constant -- Isothermal phase diagram and
If the composition is constant –Isoplethal phase diagram.
Eg:- For solid ↔ liquid equilibrium no gas phase exist and the effect of pressure on the equilibrium is very small, so it is ignored. Such solid/ liquid system with no gas phase is called as condensed system.
The degrees of freedom for such system are reduced to one. Thus condensed phase rule equation may be written as
F = C – P + 1 =2 –p+2 = 3 –p. This is known as the ―reduced (condensed) phase rule‖, which is more convenient to apply to solid / liquid 2 component condensed system.
Two variables like temperature and composition are need for such systems hence these are called as temperature-composition (T-C) diagrams.
Eg:- Lead – Silver system :- This two component system has 4 possible phases
(1)Solid silver (2) solid lead (3) solution of silver and pb (4) vapour. Since the boiling point of 2 metals are very high and so, the pressure has no effect on equilibrium.
This system is illustrating the following phase diagram.
Phase diagram of Ag – pb system
In this diagram point A represents the melting point of pure Ag and point B represent the melting point of pure pb. It can be observed from the diagram that addition of pb to pure Ag lowers the M.P of Ag and vice versa. This diagram shows following parts. (1) Freezing point curve OA and freezing point curve OB
(2) Eutectic point O (3) Areas AOB, AOC, BOD, COE, DOE
(1) Freezing point curve OA :- If represents the melting point curve or freezing point curve of Ag (961oC).This curve shows the addition of Pb lowers the melting point of Ag till the silver melt gets saturated with Pb. At ‗O‘ the Ag solution fully saturated with Pb and simply separates out as solid Pb. This saturation limit is represented by ‗O‘ is called as Eutectic point.
Freezing point curve OB:- It represents the melting point curve or freezing point curve of pure Pb(327oC). By the gradual addition of Ag; the M.P of Pb falls till the Pb melt gets saturated with Ag. Any further addition of lead beyond O, simply separate out as solid silver.
Applying the reduced phase rule equation
F= C – P + 1 = 2 – 2 + 1 = 1 Both OA and OB curves represent univarient system.
(2) Eutectic point 0 :- The curves OA andOB intersect at a point ‗O‘ at a temperature of 303oC.This point is called as Eutectic point. In Greek eutectos means ―easy melting―.
At eutectic point the system composition is 2.6 Ag and 97.4 Pb. This is called Eutectic composition. At this point, solid Ag, solid Pb and liquid co- exist in equilibrium.
Hence F = C – P +1 = 2 – 3 + 1 =0 System is invariant.
Both variables temperature (3030C) and composition (97.5 Pb and 2.5 Ag) are fixed. If you raise the temperature above the eutectic temperature, the solid phases Ag and Pb disappear and if you cool it below the eutectic temperature, you will land in the solid Ag/Pb area , where solution phase is non- existent . The Areas:- The following areas are observed in this phase diagram .
(1) AOB (2)AOC (3)BOD (4)COE (5)DOE
AOB:- It represents the a liquid solution of Ag and Pb (liquid alloy) or melt of Ag and Pb are present .
If a sample of Pb containing is taken say L, when cooled, the temperauture gradually falls without any change in composition, till point L is reached on the curve BO.
On lowering the temperature, Pb begin to separate out and the composition varies along L1O, till point ‗O‘ reached .On further cooling, the whole mass solidifies en-block to the eutectic composition. This process is called pattinson‘s process, which is used to desilverzation of Pb.
The area AOC:- Solid Ag and liquid melt in equilibrium. The composition of the liquid melts directly corresponding to the line of intersection point on the AO curve.
System is univariant F = 2 – 2 + 1 = 1
Area BOD:- The area BOD has solid Pb and liquid melt in equilibrium, system is univariant.
Area COE:- It has solid eutectic and solid Ag in equilibrium, system is univariant .
Area DOE:- It has solid eutectic and solid Pb in equilibrium, system is univariant.
Eutectic system:- A binary system in which two components are miscible in all proportion in the liquid state but do not react chemically is known as Eutectic system. (Eutectic --- easy melting).
Eg:- A mixture of Pb and Ag comprises of such a system. Charecterstics of Eutectic point :-
A Eutectics are only mixtures of components which resemble compounds in some properties. At above Eutectic temperature solid phase disappears and at below Eutectic temp liquid phase disappear. At the Eutectic point , 3 phases co-exist.
F1 = C – p + 1 = 2 – 3 + 1 =0, so, system is invariant.
This invariant system has definite temp and composition.
At this point, the components solidify in the form of small crystals, So they have higher strength than their components.
Application of Eutectic system:-
Eutectic mixtures are composed of alloys are used as fail safe devices in boilers, as plugs in automobiles and other safety devices.
Because of their low M.P eutectics are also used in preparing solders, which are used in joining 2 metal pieces. Eg:- Pb-Sn solders.
Also used in safety fuses, used in buildings to protect them against fire hazards.
CEMENT:Cement is a dirty greenish heavy powder, used as a building material. The original name of cement is concrete. It is a material which posses adhesive & cohesive properties to bind rigid masses like stones, bricks, Building blocks etc. It has the property of setting & hardening in the presence of water i.e. hydraulic in nature. Chemically it is composed of mixture of calcium silicate & Calcium aluminate in which Calcium materials are called as calcarious&Aluminium and silica are called as argillaceous. The first cement factory in India was started in 1904 in Chennai, by the South India Industrial Ltd.which existed for a short period . Classification of cement:Cements are classified into following types. (1)Natural cement (2) Pozzolana cement (3) Slag cement. (4) Port land cement (5) Expanding cement (6) Quick getting cement
(1)Natural cement: - This is obtained by calcining& pulverizing natural rocks consisting of clay & limestone. During heating, silica & alumina present in the clay react with lime to produce calcium silicate & Calcium aluminates. Natural cement is usually used for construction of big structure such as dams. Properties: - (1) It is hydraulic in nature with low strength (2)Its setting time is very less.
(2) Pozzolanacement: - It is obtained by volcanic ash. (The place Puzzouli in Italy), which consist of silicates of Ca, Fe, Al mixed with lime on heating forms pozzolana cement. Properties: - (1) Hydraulic in nature. (2) Mixed with port land cement for different applications.
(3) Slag cement: - It is prepared by mixing hydrated lime + mixture of Ca, Al silicates(Blast furnace slag) in a stream of cold H2O. It is dried & then pulverized to fine powder.Sometimes accelerators like clay or caustic soda are added for hardening process.Mainly used in making concrete in bulk construction. Properties: (1) Decreased strength. (2)The time required for setting & hardening is more i.e. a week.
(4)Portland cement:- It is obtained by heating a mixture of lime stone &clay &crushing the resulting product to a fine powder. It is also known as magic powder. It is a mixture of
Calciumsilicates &aluminates with small amount of gypsum. William Aspidin (1824) was father of modern Portland cement Industry. It is a type of cement & not a brand name.
(5)Expanding cements: Portland cement + sulphoaluminate (expanding agent) This is obtained by burning a mixture of gypsum, bauxite & Lime stone.
(6)Quicksetting cement: - This cement contains high percentage of
Alumina (Al2O3) & takes only 5 minutes for initial& 30 seconds for final setting. This cement is mostly used for under water constructions.
CHEMICAL COMPOSITION OF PORTLAND CEMENT: It is a finely powdered mixture of calcium silicate and aluminates of varying compositions. Ratio of % of lime (CaO) to that of silica (SiO2), Alumina (Al2O3) and iron oxide is calculated by the formula
Cao
2.8 SiO2 + 1.2 Al2O3 + 0.65 Fe2O3
A good sample of Portland cement has the composition of.
Ingredient Percentages(%)
1. CaO 60-70
2. Silica 20-29
3. Alumina 5-7.5
4.. Magnesia 2-3
5.Ferric oxide 1-2.5 6. SO3 1-1.5
7. Na2O 1
8.K2O 1
Manufacture of Portland cement:- Raw materials for manufacturing of Portland cement (1)Calcareous: - They are rich in lime such as limestone, chalk, and cement rock. The limestone should not contain more than 5% of MgO, it leads to cracking. (2) Argillaceous: - Those contain silica, alumina & iron oxide Eg: -Clay, blast furnace slag (ashes). (3)Gypsum: - It is added during the final grinding & it control the ratio of setting & hardening.
Methods of manufacturing process: - These are two methods for manufacturing
(1) Dry method (2) Wet method.
The two methods differ only in the treatment of raw material. In dry process water is not added to the material during grinding.The following steps are involved in manufacture of Portland cement. (1) Mixing of Raw material (2) Burning (3) Grinding (4) Packing
(1)Mixing of Raw material;-
Dry process: This process is employed if the lime stone and clay are hard. Initially lime stone is crushed in to pieces and then it is mixed with clay in the proportion of 3:1. This mixture is pulverized to a fine powder and is stored in storage bins and later on it is introduces in to the upper end of the rotary kiln. Wet process: This process is performed when the raw materials are soft. The clay is washed with water in wash mills to remove any foreign material or organic material. Powdered lime stone is then mixed with the clay paste in a proper proportion 3:1. The mixture is then finally ground and homogenized to form slurry containing about 40% water and stored in storage bins.
Difference between dry & wet process: Dry process Wet process
1. This process is adopted when the raw 1. This method is performed when the materials are quite hard. raw materials are soft.
2. It is a slow and costly process 2. Comparatively cheaper and fast process. 3. The final consumption is low, hence smaller kiln is used. 3. Fuel consumption is high so longer kiln is used. 4. The process is not suitable if the raw material has moisture content of 15% or 4. This process can be adopted even in more. wet conditions.
(2) Burning: - Rotary kilns are used for burning of cement. A Rotary kiln is an inclined steel cylinder, length is about 150—200 & diameter is about 10feet & it is lined with fire bricks. The kiln can be rotated at desired speed,(usually 0.5 to 2 rotations/minute)as it is mounted on rollers. As the kiln rotates the raw materials passes slowly from the upper end of rotary kiln to lower end, while the burning fuel (pulverized coal, oil / natural gas)Escape after the removal of dust. As the mixture or slurry gradually descends with increasingtemperature the following reactions take place in rotary kiln. (a) Drying zone: -The upper part of the kiln, where the temp around 4000 C & the slurry gets dried because of hot gases. The clay is broken as Al2O3, SiO2& Fe2O3. (b) Calcination zone / decarbonating zone :- It is the central part of the kiln ,where the temp is around 10000C. In this lime stone of dry mix /slurry completely decomposed into quick lime (CaO) & CO2.
CaCO3 CaO +CO2 (c) Clinkering zone / Burning Zone:- The lower part of the rotary kiln, the temperature is between 15000C to 17000C. In this zone lime & clay reacts to each other forming aluminates & silicates.
2 CaO + SiO2 2 CaO. SiO2 Dicalcium silicate
2 CaO + SiO23 CaO.SiO2 Tri calcium silicate
3CaO + Al2O3 3CaO.Al2O3 Tri calcium Aluminate
4CaO + Al2O3 + Fe2 O34 CaO. Al2O3. Fe2O3 Tetracalcium Aluminoferite. These aluminates & Silicates of calcium fuse together to form hard greenish stones Called clinkers. This reaction is exothermic.The cooled clinkers are collected in trolleys.
(3) Grinding / mixing of cement clickers with gypsum :- The cooled clinkers are pulverized to form powered mixture . This finely powdered clinkers set quick rapidly . So, the cement mortar will unworkable.
Therefore its setting decreases usedretarders. Commonly gypsum (CaSO4.2H2o) is used as retarder. Usually about 2-6% gypsum is mixed with clinkers in long tube. Mixture of clinkers + 2-6% gyps portland cement. (4) Packing :- The resulting Portland cement is stored in silos
Flow chart of manufacture of cement:- Raw materials
Lime stone Clay
Crushing Washing (Wet method)
Proportioning
Mixing
Dry mix Slurry
Rotary kiln
Pulverized coal
Clinkers
Cooled clinkers
Gypsum
Grinding
Portland cement Packing.
Setting & Hardening of cement:
When Portland is mixed with H2O, it is converted in to aplastic mass called Cement paste (initial set ) which slowly loses its plasticity & becomes stiff & ultimately a rocky mass is obtained (final set ).This process is called as setting.The time for the initial set should not be less than 45min and for the final set should not be more than 10 hours. After hydration, anhydrous compounds become hydrated, which have less solubility. Hence they are precipitated as insoluble gels or crystals. These have ability to surround sand, crushed stones, other inert materials & bind them strongly. Development of strength due to crystallization is called as Hardening, which continue to increase for years. The process of setting & hardening are collectively called as solidification.
Hardening of cement can be explained on the basic of 2 theories. Crystallinetheory(given by Le – chatlier):- Constitutional compounds after hydration form crystalline products,which undergo interlocking which is responsible for hardening of cement. Colloidal theory (given by Michaelies):- According to this theory hardening of cement is due to the hardening of the silicate gels, which are formed during hydration . The main constituents of the common cement are Name Formula Abbreviatio % Setting n time
Tri calcium silicate 3CaO. SiO2 C3s 45 7days
Di calcium silicate 2 CaO.SiO2 C2s 25 28days
Tricalcium Aluminate 3 CaO. Al2O3 C3A 10 1day
Tetra calcium alumino 4CaO.Al2O3. Fe2O3 C3AF 10 1day ferrite
The remainder consists of free MgO& small amounts of K2O & Na2O & various combination of lime, Al2O3, SiO2& SO3.
Reactions involved in setting & Hardening:- When cement mixed with water, the paste becomes quite rigid within a short time known as Initial set / flash set. This is due to rapid hydration of C3A.
3CaO. Al2O3 + 6H2O 3CaO. Al2O3. 6 H2O+ 880 Kj /Kg (crystals) However these crystals present in the hydration reactions of other constitutional compounds form barrier over them. To decrease or retard the flash set, gypsum or plaster of paris is added during the pulverization of cement clinkers.
Gypsum retards the dissolution of C3A. When it interacts with C3A forms insoluble complex of sulphoaluminate which does not have quick hydrating property.
3 CaO.Al2O3 + x H2O + Y CaSO4.2H2O 3 CaO. Al2O3. CaSO4. 2 H2O. + 880 KJ / Kg. The tetra calcium alumino ferrite (C4 AF) then reacts with water forming both gels & crystalline compounds.
4 CaO. Al2O3. Fe2O3 + 7H2O 3 CaO. Al2O3. 6H2O + CaO.
Fe2O3 H2O gels +420 Kj/Kg.crystals (gels)
Now C3S & C2S undergo further hydration & hydrolysis reactions increase the development of greater strength .The initial setting and hardening of cement paste is due to the formation of tobermonite gel.
2(2 CaO.SiO2) + 4 H2O 3CaO.2SiO2. 3H2O + Ca (OH)2+250KJ/Kg
The final setting & hardening of cement paste is due to formation of tobermonite gel and crystallization of Ca(OH)2.
2(3 CaO.SiO2) + 6H2O 3CaO.2 SiO2 3 H2O + 3 Ca (OH)2+500 KJ/Kg. Sequence changes during setting & Hardening of cement:-
Cement+ 1 day Hydration of 7 days 28days Gelation of
H2O paste &A&C4 AFC2s C2s&C3s
REFRACTORIES An inorganic material, which does not melt easily i. e. which can withstand very high temp without softening or deformation in shape, is called as ―Refractories‖. Or Any substance that is difficult to fuse / melt is a Refractory & used as constructing material.
CHARACTERISTICS: They are chemically inert by the action of Corrosive gases, molten metal‘s, slag‘s etc. Resistant towards corrosion, abrasion etc. Does not change their size at operating temperature. The main role of a refractory is to continue heat in it. Expand & contract uniformly at increasing & decreasing temperature.
APPLICATIONS:Used for the construction of lining in furnaces, retorts, kilns, crucibles etc., which are employed in metallurgical & industrial purposes i.e. Ferrous, non- ferrous, glass, ceramic, power generation, oil refining & cement.They are also used in the manufacture of rocket nozzles, launch pads & for domestic heating.Refractories are available in different shapes & sizes, as bricks, crucibles, tubes, granules etc..
CLASSIFICATION:- Refractories are classified into following categories based
(I) Ontheir chemical properties
1. Acid refractories: These refractories consist of acidic materials such as Al2O3,
Silica (SiO2). They can with stand acidic materials but easily attacked by basic materials like CaO, MgO etc… Eg: - Alumina, Silica & Fine clay refractories.
2. Basic refractories:These refractories consists of basic materials like CaO ,MgOetc& are easily attacked by acid materials. Eg :-Magnesite , dolomite bricks , chomomagnesite.
3. Neutral Refractories: They made of weakly acidic / basic materials like carbon , zircon(ZrO2) & chromite (FeO.CrO2). They show resistance to the action of acidic & basic materials & also show good chemical stability. Eg: - Graphite, Zirconia &Carborundum (SiC).
CRITERIA/PROPERTIES OF REFRACTORIES:
1. Refractoriness:It is the ability of a material to with stand high temp without appreciable deformation or softening under working conditions .It is usually measured by the softening temperature of the material. The material to be used as refractory, should have a softening temp much higher than the operating temperature of the furnace in which it is to be used as refractory .The softening temperature of the material is usually determined by the ‗pyrometric cones test‘ or ‗Seger cones test‘ . Measurement of Refractoriness / Pyromtric cones (Seger cone) :- Refractoriness is usually determined by comparing the softening behavior of test cone with that of standard cone (seger cone). The refractoriness is expressed in terms of pyrometric cone equivalent (PCE). These cones are small, pyramid – shaped, 38 mm high, 19 mm long sides with a triangular base. Test cone along with standard cones are heated under standard conditions of 10oc / minute. Each standard cone is made of particular refractory with a definite softening temperature and are arranged with increasing softening temperature. Standard cone fuses (melts) along with the test cone , the temperature at which the fusion of the test cone occurs is indicated by its apex (tip ) touching the base . If test cone softens earlier than one standard cone but later than the next cone , the PCE value of test cone is taken as the average value of the 2 standard cones . Eg : - Silica bricks – PCE no –32 & softening temperature 1710oC Magnesite bricks -- PCE no--- 38 & softening temperature 1850oC 2. Refractories under load / strength: - This property gives an idea of strength of refractory. Refractories should have high load bearing capacity under operating temperature, which can be measured by RUL test. This test is done in a rectangular container by applying a load of 1.75 Kg /cm2 on to the refractory & heating at a constant rate of 10oC / minute. During this process the material will soften & its height will decrease under the load. This decrease in height is measured and when there is 10 % decrease to that of original height the temperature is noted. The RUL is then expressed as the temperature at which this 10% deformation occurs.
3.Chemical inertness: - The refractory selected for a specific purpose should be chemically inert & not react with the slags, furnace gases etc. It is always advisable not to employ an acidic refractory in a basic furnace vice versa. 4.Dimensional stability: - It is defined as the resistance of material to any volume changes which may occur because of its exposure to high temperature over a prolonged period of time .These changes may be reversible/ irreversible. Further the irreversible change may result in contraction/ expansion. A good refractory should have high dimensional stability. 5. Thermal expansion& contraction: - The refractory tends to expand when temperature increases& contract when temperature decreases which affect the properties of the refractory. So a good refractory should have low thermal expansion& contraction. 6. Thermal conductivity: - The conductivity of refractory primarily depends on its chemical composition & porosity. As porosity increases thermal conductivity decreases because the entrapped air in the pores function as insulator. Depending on the type of furnaces low thermal conductivity and high thermal conductivity refractories are used. 7. Abrasion resistance: - A good refractory should resist the abrasion action of fuel gases, flames, slags etc... 8. Porosity: - Refractories usually contain pores due to manufacturing defects etc. It is the important property of a refractory , which affect many physical & chemical properties. It is the ratio of pore volume to the bulk volume.
W D P = 100 W A P = porosity
W = saturated weight of specimen (with H2O) D = Dry weight of the specimen A = saturated weight+ moisture content of specimen. Good refractory should have low porosity. 9. Electrical conductivity:- In general refractories are poor conductors of electricity (except graphite). However electrical conductivity of refractories increases with increasing temperature. 10. Thermal spalling: - It is the property of breaking, cracking / fracturing of the refractory due to rapid fluctuations in temperature causing uneven stresses & strains in the body of the refractory .A good refractory must show good resistance to thermal spalling. It is avoided by i. Avoiding sudden fluctuations in temp ii. By modification of furnace design iii. Low porosity & low co-efficient of expansion.
CAUSES FOR THE FAILURE OF REFRACTORIES :- Following reasons are responsible for failure of refractories . (1)The most common cause for failure of refractory is chemical reaction with the environment in which it is operating. Eg:- An acidic refractory should not be used in furnaces using basic fluxes , slag etc & vice versa. (2)The porosity of refractory plays an important role in the chemical reaction. They possess more porous nature so slag will penetrate in these pores &destroy the refractory. (3) The deposition of carbon from CO in fire clay refractoriness in a blast furnace is an imp cause of its failure . (4)Thermal spalling is also responsible for failure. i.e Rapid changes in temp of the furnace, leads to destroy the refractory. (5) Using a refractory of less refractoriness than that of operating temperature.