All about Cement

Definition: Cement is a  powdery substance that sets and hardens independently, and can bind other materials together.

Colour: Grey

Function:  When cement is mixed with water, it can bind sand and gravel into a hard, solid mass called concrete. 

Composition:  Portland cement consists essentially of compounds of lime (calcium oxide, CaO) mixed with silica (silicon dioxide, SiO₂) and alumina (aluminum oxide, Al₂O₃). The lime is obtained from a calcareous (lime-containing) raw material, and the other oxides are derived from an argillaceous (clayey) material. Additional raw materials such as silica sand, iron oxide (Fe₂O₃), and bauxite (containing hydrated aluminum, Al[OH]₃) may be used in smaller quantities to get the desired composition.

Manufacturing process:

The quarry is the starting point
Cement plants are usually located closely either to hot spots in the market or to areas with sufficient quantities of raw materials. The aim is to keep transportation costs low. Basic constituents for cement (limestone and clay) are taken from quarries in these areas.

A two-step process
Basically, cement is produced in two steps: first, clinker is produced from raw materials. In the second step cement is produced from cement clinker. The first step can be a dry, wet, semi-dry or semi-wet process according to the state of the raw material.

Making clinker
The raw materials are delivered in bulk, crushed and homogenised into a mixture which is fed into a rotary kiln. This is an enormous rotating pipe of 60 to 90 m long and up to 6 m in diameter. This huge kiln is heated by a 2000°C flame inside of it. The kiln is slightly inclined to allow for the materials to slowly reach the other end, where it is quickly cooled to 100-200°C.
Four basic oxides in the correct proportions make cement clinker: calcium oxide (65%), silicon oxide (20%), alumina oxide (10%) and iron oxide (5%). These elements mixed homogeneously (called “raw meal” or slurry) will combine when heated by the flame at a temperature of approximately 1450°C. New compounds are formed: silicates, aluminates and ferrites of calcium. Hydraulic hardening of cement is due to the hydration of these compounds.
The final product of this phase is called “clinker”. These solid grains are then stored in huge silos. End of phase one.

From clinker to cement
The second phase is handled in a cement grinding mill, which may be located in a different place to the clinker plant. Gypsum (calcium sulphates) and possibly additional cementitious (such as blastfurnace slag, coal fly ash, natural pozzolanas, etc.) or inert materials (limestone) are added to the clinker. All constituents are ground leading to a fine and homogenous powder. End of phase two. The cement is then stored in silos before being dispatched either in bulk or bagged.

 

 

Cement Test:

(a) Soundness Test: It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken.

Procedure to determine soundness of cementi) Place the mould on a glass sheet and fill it with the cement paste formed by gauging cement with 0.78 times the water required to give a paste of standard consistency.
ii) Cover the mould with another piece of glass sheet, place a small weight on this covering glass sheet and immediately submerge the whole assembly in water at a temperature of 27 ± 2^oC and keep it there for 24hrs.
iii) Measure the distance separating the indicator points to the nearest 0.5mm (say d₁ ).
iv) Submerge the mould again in water at the temperature prescribed above. Bring the water to boiling point in 25 to 30 minutes and keep it boiling for 3hrs.
v) Remove the mould from the water, allow it to cool and measure the distance between the indicator points (say d₂ ).
vi) (d₂ – d₁ ) represents the expansion of cement.


(b) Setting Time: Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times [Ref. Fig. 1.5]. Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.

Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely. If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

Initial Setting Time: 300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the experiment is noted as initial setting time.

Final Setting Time: The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed
between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.


(c) Soundness Test: This test is conducted to find free lime in cement, which is not desirable. Le Chatelier apparatus shown in Fig. 1.6 is used for conducting this test. It consists of a split brass mould of diameter 30 mm and height 30 mm. On either side of the split, there are two indicators, with pointed ends. The ends of indicators are 165 mm from the centre of the mould.

Properly oiled Le Chatelier mould is placed on a glass plate and is filled completely with a cement paste having 0.78 times the water required for standard consistency. It is then covered with another glass plate and a small weight is placed over it. Then the whole assembly is kept under water for 24 hours. The temperature of water should be between 24°C and 50°C. Note the distance between the indicator. Then place the mould again in the water and heat the assembly such that water reaches the
boiling point in 30 minutes. Boil the water for one hour. The mould is removed from water and allowed to cool. The distance between the two pointers is measured. The difference between the two readings indicate the expansion of the cement due to the presence of unburnt lime. This value should not exceed 10 mm.

(d) Crushing Strength Test: For this 200 gm of cement is mixed with 600 gm of standard sand confirming to IS 650–1966. After mixing thoroughly in dry condition for a minute distilled potable water (P/4)+ 3 percentage is added where P is the water required for the standard consistency. They are mixed with trowel for 3 to 4 minutes to get uniform mixture. The mix is placed in a cube mould of 70.6 mm size (Area 5000 mm2) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds. Then the mould is placed on a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute. A hopper is secured at the top and the remaining mortar is filled. The mould is vibrated for two minutes and hopper removed. The top is finished with a knife or with a trowel and levelled. After 24 ± 1 hour mould is removed and cube is placed under clean water for curing.
After specified period cubes are tested in compression testing machine, keeping the specimen on its level edges. Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 11.5 N/mm2 and that at the end of 7 days not less than 17.5 N/mm2.
 

Mechanical properties of Solids

Mechanical Properties of Solids:

From engineering stand point, the most important properties of engineering materials are:
Elasticity, Plasticity, Ductility, Brittleness, Malleability, Toughness and Hardness.

Here we need to remember that, a particular material can't possess all of these simultaneously because of these are in opposition.


Now we learn about these engineering properties of materials in brief:

Elasticity:
The property by which a body returns to its original shape after the removal of external load is called Elasticity.
Note: 1. A material is said to be isotropic, if it is equally elastic in all directions.
         2. Steel, Copper, Aluminum, Stone etc., may be considered to be perfectly elastic, within certain limits
      

Plasticity:
The property by which no strain disappears when it is relieved from the stress.
Note: Plasticity is converse of elasticity.

(Meanings: con·verse - To engage in a spoken exchange of thoughts, ideas, or feelings, to be familiar; associate)
Ductility:
The property of a material by which it can be drawn out by tension to a small section. In a ductile material, therefore, large deformation is possible before the absolute failure or rupture takes place.
Note: During ductile extension, a material shows a certain degree of elasticity, together with a considerable degree of plasticity.

Brittleness:
A material is said to be brittle when it can't be drawn out by tension to smaller section.
Note: 1. Lack of ductility is brittleness
         2. In a brittle material, failure takes place with a relatively small deformation
         3. Brittleness is generally considered to be highly undesirable.

Malleability:
It is a property by which a material can be uniformly extended in a direction without rupture.
Note: 1. Malleable material possesses a high degree of plasticity.
         2. This property is utilised in many operations, such as forging, hot rolling, drop stamping etc.

Toughness:
This is a property of a material which enables it to absorb energy without fracture.
Note: This property is very desirable in components subject to cyclic or shock loading.

Hardness:
Ability of a  material to resist indentation or surface abrasion.

Stress, Strain, Bending moment & Shear force

Stress:

It is a force that tends to deform the body on which it acts per unit area.
It is measured in N/m2 and this unit is specifically called Pascal (Pa). A bigger unit of stress is the mega Pascal (MPa).

1 Pa = 1N/m2,
1MPa = 106 N/m2 =1N/mm2.

Compressive stress tends to squeeze a body, tensile stress to stretch (extend) it, and shear stress to cut it.

Three Basic Types of Stresses
Basically three different types of stresses can be identified. These are related to the nature of the deforming force applied on the body. That is, whether they are tensile, compressive or shearing.

Tensile Stress

Consider a uniform bar of cross sectional area A subjected to an axial tensile force P. The stress at any section x-x normal to the line of action of the tensile force P is specifically called tensile stress pt . Since internal resistance R at x-x is equal to the applied force P, we have,

pt = (internal resistance at x-x)/(resisting area at x-x)

=R/A

=P/A.

Under tensile stress the bar suffers stretching or elongation.

Compressive Stress If the bar is subjected to axial compression instead of axial tension, the stress developed at x-x is specifically called compressive stress pc. pc =R/A = P/A. Under compressive stress the bar suffers shortening.

Shear Stress Consider the section x-x of the rivet forming joint between two plates subjected to a tensile force P as shown in figure.

The stresses set up at the section x-x acts along the surface of the section, that is, along a direction tangential to the section. It is specifically called shear or tangential stress at the section and is denoted by q. q =R/A =P/A.

Normal or Direct Stresses
When the stress acts at a section or normal to the plane of the section, it is called a normal stress or a direct stress. It is a term used to mean both the tensile stress and the compressive stress.

2.3. Simple and Pure Stresses
The three basic types of stresses are tensile, compressive and shear stresses. The stress developed in a body is said to be simple tension, simple compression and simple shear when the stress induced in the body is (a) single and (b) uniform. If the condition (a) alone is satisfied, the stress is called pure tension or pure compression or pure shear, as the case may be.

2.4. Volumetric Stress
Three mutually perpendicular like direct stresses of same intensity produced in a body constitute a volumetric stress. For example consider a body in the shape of a cube subjected equal normal pushes on all its six faces. It is now subjected to equal compressive stresses p in all the three mutually perpendicular directions. The body is now said to be subjected to a volumetric compressive stress p.

Volumetric stress produces a change in volume of the body without producing any distortion to the shape of the body.

Strain:

Measure of the extent to which a body deforms under stress.

Linear Strain
Linear strain of a deformed body is defined as the ratio of the change in length of the body due to the deformation to its original length in the direction of the force. If l is the original length and dl the change in length occurred due to the deformation, the linear strain e induced is given by e=dl/l.

Linear strain may be a tensile strain, et or a compressive strain ec according as dl refers to an increase in length or a decrease in length of the body. If we consider one of these as +ve then the other should be considered as –ve, as these are opposite in nature.

Lateral Strain Lateral strain of a deformed body is defined as the ratio of the change in length (breadth of a rectangular bar or diameter of a circular bar) of the body due to the deformation to its original length (breadth of a rectangular bar or diameter of a circular bar) in the direction perpendicular to the force.

Volumetric Strain Volumetric strain of a deformed body is defined as the ratio of the change in volume of the body to the deformation to its original volume. If V is the original volum and dV the change in volume occurred due to the deformation, the volumetric strain ev induced is given by ev =dV/V Consider a uniform rectangular bar of length l, breadth b and depth d as shown in figure. Its volume V is given by, This means that volumetric strain of a deformed body is the sum of the linear strains in three mutually perpendicular directions.

Shear Strain Shear strain is defined as the strain accompanying a shearing action. It is the angle in radian measure through which the body gets distorted when subjected to an external shearing action. It is denoted by *. Consider a cube ABCD subjected to equal and opposite forces Q across the top and bottom forces AB and CD. If the bottom face is taken fixed, the cube gets distorted through angle * to the shape ABC’D’. Now strain or deformation per unit length is Shear strain of cube = CC’ / CD = CC’ / BC = * radian

Bending Moment & Shear Force

Beam: Beam is a structural member which is acted upon by a systme of external loads at right angle to its axis.

Bending: Bending implies deformation of a bar produced by loads perpendicular to its axis as well as forece-couples acting in a plane passing through the axis of the bar.

Plane Bending: If the plane of loading passes through one of the principal centroidal axis of inertia of the cross-section of the beam, the bending is said to be plane or direct.

Oblique Bending: If the plane of loading does not pass through one of the principal centroidal axes of inertia of the cross-section of the beam, the bending is said to be oblique.

Bending moment: Algebric sum of the moments of all vertical forces either to the left or to the right of a section.

Shear force: Algebric sum of all vertical forces either to the left or to the right hand side of a section.
 

Tension & Compression

Tension and compression are directional terms to identify how forces are acting upon or within a member. If a member (for example, a truss or a guide rod) is in tension, then the overall forces are pulling away from it; if the member is under compression, the forces acting upon it are directed toward the member. Tension can be likened to pulling on the ends of a rod, whereas compression can be likened to pushing on the ends of the rod toward the middle.

Tension and compression tests are used to determine the strength of a material and to develop the material's stress-strain diagram, which shows the relationship between the stress placed on the material and the strain experienced by the material.

Tension and compression are directional terms to communicate how forces are acting upon or within a member. If a member (for example, a truss or a guide rod) is in tension, then the overall forces are pulling away from it; if the member is under compression, the forces acting upon it are directed toward the member. Tension can be likened to pulling on the ends of a rod, whereas compression can be likened to pushing on the ends of the rod toward the middle. Tension and compression tests are used to determine the strength of a material and to develop the material's stress-strain diagram, which shows the relationship between the stress placed on the material and the strain experienced by the material.

Place a flexible object like an eraser, sponge, or small piece of bread between your thumb and index finger. Press your fingers together. One side of the object will bend inwards and shorten while the other will bend outwards and lengthen. The shorter side has been compressed, while the other side is under tension.

http://www.cement.org/tech/cct_cement_characteristics.asp

http://www.buildinglime.org/Tate_Property.pdf

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/stress.cfm

http://www.youtube.com/watch?v=HzDacklKnNc


http://blog.mechguru.com/machine-design/create-bending-moment-diagrams-in-four-simple-steps/