LECTURE No. 03

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Civil Engineering Material

CEMENT

Cement Cement is a binder, a substance that sets and hardens

independently, and can bind other materials together.

“Cement is a crystalline compound of calcium silicates and other calcium compounds having hydraulic properties” (Macfadyen, 2006).

Cements in general are adhesive and cohesive materials which are capable of bonding together particles of solid matter into compact durable mass.

For civil engineering, they are restricted to calcareous cements containing compounds of lime as chief constituent to bind the fine and coarse aggregate particles together.

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Cement HistoryClay was used as cementing material – by

Assyrians and BabyloniansLime and gypsum were used as binder material –

by Egyptians in pyramidsCalcareous cements, like limestone was used– by

RomansJoseph Aspedin of Yorkshire (UK) in 1824 formed

Portland cement by heating a mixture of limestone & fine clay expelling carbonic acid gas

Isaac C. Johnson in 1845 invented cement by burning limestone and clay to form clinker.

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History

Lime and clay have been used as cementing material on constructions through many centuries.

Romans are commonly given the credit for the development of hydraulic cement, the most significant incorporation of the Roman’s was the use of pozzolan-lime cement by mixing volcanic ash from the Mt. Vesuvius with lime. Best known surviving example is the Pantheon in Rome

In 1824 Joseph Aspdin from England invented the Portland cement

History of cement and concrete

• The early days:

• Setting stone blocks without cementing them

• Mud mixed with straw is the oldest cementing material used to bind dried bricks together

• Pyramid of Cheops.

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• Cheops, Giza• Stones were brought from Aswan and Tura using

the Nile river• Built around 2566 B.C.• It would have taken over 2,300,000 blocks of

stone with an average weight of 2.5 tons each• Total weight of 6 million tons• 30 years and 100,000 slaves to build it • Has a height of 482 feet (140m)• It is the largest and the oldest of the Pyramids of

Giza • Mortars made by calcining impure gypsum

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History of cement and concrete• Non-hydraulic cements

• Gypsum and lime• Cements based on compounds of lime (calcareous

cements)• Gypsum

• Calcining impure gypsum at 130 °C• Add water calcined gypsum and water recombine• Cannot harden under water because gypsum is quite

soluble.• Pyramid of Cheops (3000 B.C.)

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Cement

• Portland cement is so named because a paste of cement with water, after it sets hard, resembles in color and hardness a Portland stone, a limestone quarried in Dorset (a county in South West England).

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• Uses of cement• Masonry work, plastering, pointing, joints for pipes &

drains.• Used in concrete for laying floors, roofs, constructing

lintels, beams, stairs, pillars/columns etc.• Used in manufacturing of precast pipes, piles, fencing posts

etc.• Important engineering structures e.g. bridges, culverts,

dams, tunnels, etc.

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Chemical Composition of Cement

Oxide %age FunctionLime (CaO) 60-65 Controls strength and soundnessSilica (SiO2) 20-25 Gives strength. Excess causes slow

settingAlumina (Al2O3) 4-8 Quick setting. Excess lowers strength

Iron oxide (Fe2O3) 2-4 Color. Helps in fusion of ingredients

Magnesium oxide (MgO)

1-3 Color and hardness. Excess causes cracking

Na2O+K2O, TiO2, P2O5

0.1-0.5 Residues. Excess causes cracking and effloresence

Sulphur trioxide (SO3)

1-2 Makes cement sound 10

Functions of ingredients of cement

• It is the major constituent of cement . Its proportion is important.• The excess makes the cement unsound and causes the cement to

expand and disintegrate.• In case of deficiency, the strength of cement is decreased and

cement sets quickly.• The right proportion makes cement sound and strong.

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Lime (CaO)

Functions of ingredients of cement

• It imparts strength to the cement due to formation of dicalcium silicate (2CaO SiO2 or C2S) and tricalcium silicate (3CaO SiO2 or C3S).

• Silica in excess provides greater strength to the cement but at the same time it prolongs its setting time.

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Silica (SiO2)

Functions of ingredients of cement

• It imparts quick setting quality to the cement.• It acts as a flux (rate of flow of energy) and lowers the clinkering

temperature.• Alumina in excess reduces the strength of cement.

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Alumina (Al2O3)

Functions of ingredients of cement

• It provides color, hardness and strength to cement.• It also helps the fusion of raw materials during manufacture of cement.

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Iron oxide (Fe2O3)

Harmful ingredient of cement

• Alkali oxides (K2O and Na2O): if the amount of alkali oxides exceeds 1%, it leads to the failure of concrete made from that cement.

• Magnesium oxide (MgO): if the content of MgO exceeds 5%, it causes cracks after mortar or concrete hardness.

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•Cements are considered hydraulic because of their ability to set and harden under or with excess water through the hydration of the cement’s chemical compounds or minerals.

•There are two types:Those that activate with the addition of waterAnd pozzolanic that develop hydraulic properties when the interact with

hydrated lime Ca(OH)2

Pozzolanic: any siliceous material that develops hydraulic cementitious properties when interacted with hydrated lime.

HYDRAULIC CEMENTS:Hydraulic lime: Only used in specialized mortars. Made from calcination of

clay-rich limestones.

Natural cements: Misleadingly called Roman. It is made from argillaceous limestones or interbedded limestone and clay or shale, with few raw materials. Because they were found to be inferior to Portland, most plants switched.

Types of Cement

Portland cement: Artificial cement. Made by the mixing clinker with gypsum in a 95:5 ratio.

Portland-limestone cements: Large amounts (6% to 35%) of ground limestone have been added as a filler to a Portland cement base.

Blended cements: Mix of Portland cement with one or more SCM (supplementary cemetitious materials) like pozzolanic additives.

Pozzolan-lime cements: Original Roman cements. Only a small quantity is manufactured in the U.S. Mix of pozzolans with lime.

Masonry cements: Portland cement where other materials have been added primarily to impart plasticity.

Aluminous cements: Limestone and bauxite are the main raw materials. Used for refractory applications (such as cementing furnace bricks) and certain applications where rapid hardening is required. It is more expensive than Portland. There is only one producing facility in the U.S.

Cement TypesPortland Cement

• Ordinary Portland Cement (Type I)• Moderate Sulphate Resistance Cement (Type II)• Rapid Hardening or High Early Strength Portland Cement (Type III)• Low Heat Portland Cement (Type IV)• Sulphate Resistant Portland Cement (Type V)• Water Repellent Portland Cement• Water Proof Portland Cement• Air Entraining Portland Cement (Type I-A, II-A, III-A)• Pozzolana Portland Cement

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Modified Portland Cement

• This cement on setting develops less heat of generation than OPC.

• It is best suited in hot climate for civil works construction.

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Rapid Hardening or High Early Strength Cement (Type III)

• Gains strength faster than OPC. In 3 days develops 7 days strength of OPC with same water cement ratio

• After 24 hours – not less than 160 kg/cm2 (2276 psi)• After 72 hours – not less than 275 kg/cm2 (3391 psi)

• Initial and final setting times are same as OPC

• Contains more tri-calcium silicate (C3S) and finely ground

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• Emits more heat during setting, therefore unsuitable for mass concreting

• Lighter and costlier than OPC. Short curing period makes it economical

• Used for structures where immediate loading is required e.g. repair works

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Quick Setting Cement

• Sets faster than OPC• Initial setting time is 5 minutes• Final setting time is 30 minutes• Used for concreting in underwater or running

water• Mixing and placing has to be faster to avoid initial

setting prior to laying.

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Low Heat Cement

• Low percentage (5%) of tri-calcium aluminates (C3A) and silicate (C3S) and high (46%) of di-calcium silicate (C2S) to keep heat generation low

• It has low lime content and less compressive strength.• Initial and final setting times nearly same as OPC• Very slow rate of developing strength• Not suitable for ordinary structures

• Shuttering required for long duration so cost will increase• Prolonged curing is required• Structure utilization will be delayed

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Sulphate Resistant Portland Cement

• Percentage of tricalcium aluminate (C3A) is kept below 5% resulting in increase in resisting power against sulphates

• Heat developed is almost same as Low Heat Cement• Theoretically ideal cement. Costly manufacturing because of

stringent composition requirements• Used for structures likely to be damaged by severe alkaline

conditions like bridges, culverts, canal lining etc

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Water Repellent Portland Cement

• It contains a small percentage of water-proofing material with the cement and is manufactured under the name “Aqua-crete”.

• The cement is prepared with ordinary or rapid hardening cement and white cement.

• It is used in to check moisture penetration in basements etc.

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Water Proof Portland cement• It is prepared by mixing ordinary or rapid hardening cement and

some percentage of some metal stearate ( Ca, Al etc).• It is resistant to water and oil penetration.• It is also resistant to acids, alkaline and salt discharged by industrial

water.• It is used for water retaining structure like tanks, reservoir,

retaining walls, pool, dam etc

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◦ Black chocolate color cement produced by fusing bauxite and limestone in correct proportion, at high temperature

◦ Resists attack of chemicals, sulphates, seawater, frost action and also fire. Useful in chemical plants and furnaces

◦ Ultimate strength is much higher than OPC◦ Initial setting time is 2 hours, followed soon by final set◦ Most heat emitted in first 10 hours so good for freezing

temperatures in cold regions (below 18°C)◦ Develops strength rapidly, useful during wartime

emergency◦ Unsuitable for mass concrete as it emits large heat on

setting 27

High Alumina Cement

• Produced by mixing Portland cement clinker, gypsum and granulated blast furnace slag

• Cheaper than OPC, blackish grey

• Lesser heat of hydration. Initial setting 1 hour and final setting 10 hours

• Better resistance to soils, sulphates of alkali metals, alumina, iron and acidic waters

• Suitable for marine works, mass concreting

• Due to low early strength, not suitable for RCC

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Portland Slag Cement

• OPC with small quantity of air entraining materials (resins, oils, fats, fatty acids) ground together

• Air is entrained in the form of tiny air bubbles during chemical reaction

• Concrete is more plastic, more workable, more resistant to freezing

• Strength of concrete reduces somewhat

• Quantity of air entrained should not be more than 5% to prevent excess strength loss

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Air Entraining Cement

White Cement• OPC with pure white color produced with white chalk or clay free

from iron oxide• Instead of coal, oil fuel is used for burning• Much more costlier than OPC

Colored Cement• Suitable pigments used to impart desired color• Pigments used should be chemically inert and durable under

light, sun or weather

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Portland Pozzolana Cement

• OPC clinker and pozzolana (calcined clay, surkhi and fly ash) ground together

• Properties same as OPC• Produces less heat of hydration and offers great resistance to

attacks of sulphates and acidic waters• Used in marine works and mass concreting• Ultimate strength is more than OPC but setting timings are same

as OPC

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Other Varieties of Cement

• High Alumina Cement• Quick Setting Cement• Blast Furnace Slag Cement• White Cement• Colored Cement• Expanding Cement• Hydrophobic Cement

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Cement Manufacturing• Steps

• Grinding & mixing . Raw materials are ground and mixed in right proportions.

• Burning. Burning at 1300 to 1900oC and obtaining clinker from kilns.

• Grinding. Grinding of clinker to fine powder.

• Processes• Dry process. Dry mixing and grinding of constituents. Difficult for

composition control, slow, costly.• Wet process. Wet mixing and grinding into slurry which turns

into clinker on burning in kiln.33

Wet process

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Calcareous materials( limestone)

Argillaceous materials( clay)

Preliminary crushing Wash mills

Elevators (storage bins) Elevators (storage bins)

Water

Hoppers

Wet grinding(Ball mills)

Raw slurry

elevators

Correction silos

Lime slurryClay slurry

Wet process (contd.)

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Correction silos Fuel-coal

Rotary kilns Crushing & grinding (Ball mill)

Clinker Pulverized coal

Elevator (Cement silos)

Elevators(Clinker storage)

Clinker grinding(Cement grinding mills)

Gypsum

Gypsum hopper

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Cement Manufacturing Process

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Cement Manufacturing Process

Rotary Kiln (Furnace)

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Wet processCollection of raw materials:

◦ Calcareous materials (limestone, marl, chalk, etc.) are quarried by blasting. Argillaceous materials (clay, slate, etc.) are transported to the site.

Crushing, grinding & mixing of raw materials: ◦ Chalk: chalk is finely broken up and dispersed in water in a wash mill.

The clay is also broken up and mixed with water in similar wash mill. The mixture is passed through a series of screens. The resulting cement slurry flows into storage tanks.

◦ Limestone: limestone is crushed & fed into a ball mill with the clay dispersed in water. After grinding, the resulting slurry is pumped into storage tanks.

◦ Slurry: it is a liquid of creamy consistency, with water content between 35-40%

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Wet process (contd.)Crushing, grinding & mixing of raw materials:

◦ Slurry is kept in storage tanks. The sedimentation of suspended solid is prevented by mechanical stirrers or bubbling of compressed air.

◦ The slurry is passed into silos where proportioning is finely adjusted to ensure correct chemical composition.

Burning◦ Slurry is pumped into upper end of the rotary kiln set at a slight

gradient. It is 4 m in diameter and upto 150 m long. It rotates slowly about its axis. The slurry is fed at the upper end while pulverised coal is thrown in by an air blast at the lower end. Oil and natural gas can also be used instead of gas.

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Zones of Rotary Kiln

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Wet process (contd.) Burning

◦ When slurry moves down the kiln, it encounters progressively high temperatures.

◦ At first water is driven off, and CO2 is liberated. The material becomes dry.

◦ The dry material undergoes a series of chemical reactions until finally, in the hottest part of kiln, 20-30 % of the material becomes liquid, and lime, silica and alumina recombine.

◦ The mass fuses into balls, 0.3-2.5 cm dia, known as clinker. ◦ The clinker drops into coolers.◦ A large kiln can produce 700 tonnes of cement a day

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Process Of Clinker Production From Raw Feed To The Final Product

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Clinker

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Clinker

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Gypsum

Source: PCA, 2003

Wet process (contd.)• Grinding

• The cool clinker is crushed with 3-4% of gypsum (CaSO4) in order to prevent flash setting of cement.

• Once the cement has been satisfactorily ground it is ready for packing in bags.

• Each bag contains 50 kg of cement.

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Source: PCA, 2003

Cement Clinker CompositionTri-calcium silicate – 3CaO SiO2 or C3S (Alite)

◦ Best cementing material. About 40%. Main cause of hardness and early strength (7 days strength)

Di-calcium silicate – 2CaO SiO2 or C2S (Belite)◦ About 32%. Resistance to chemical attacks. Hardens slow and takes

long to add to strengthTri-calcium aluminates – 3CaO Al2O3 or C3A (Celite)

◦ About 10%. Rapidly reacts with water and is responsible for flash set, volume changes, cracking and high heat of hydration. Rapidity is regulated by the addition of gypsum.

Tetra-calcium alumino ferrite – 4CaO Al2O3 Fe2O3 or C4AF (Felite)◦ About 9%. Responsible for flash set. 48

Properties of cement components

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Component Rate of reaction

Heat liberated

Ultimate cementing

valueTri-calcium silicate, C3S Medium Medium Good

Di-calcium silicate, C2S Slow Small Good

Tri-calcium aluminate, C3A Fast Large Poor

Tetra-calcium alumino ferrite, C4AF

Slow Small Poor

Cement Properties

Fineness: Finer cements react quicker with water and increase shrinkage and cracking of concrete.

Soundness: Change in volume of concrete after setting. It may cause cracks, distortion and disintegration of concrete.

Setting time: Initial setting time is that stage after which any cracks that may appear do not reunite. Final setting is that stage when it has attained sufficient strength and hardness.

Compressive strength of cement and sand mortar should not be less than◦ 115 kg/cm2 after 3 days◦ 175 kg/cm2 after 7 days

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Setting and hardening of cementThe chemical reaction between cement and water is called

hydration of cement.When cement is mixed with water (25-35% by weight), a stiff

and sticky paste is formed which remains plastic for a short period.

With passage of time, the plasticity disappears and the cement past becomes stiff due to initial hydration of cement. This phenomenon of plastic cement changing into a solid mass is known as setting of cement.

On setting, cement binds the aggregates into a solid mass which gains strength as the time passes, till hydration of cement is complete.

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Setting and hardening of cement• The phenomenon by virtue of which the cement paste,

which is finally set, develops strength is known as hardening of cement.

• Heat of hydration: The reaction of cement with water evolves heat known as heat of hydration.

• The rate of setting and hardening of cement, the rate of evolution of heat and resistance to chemical attack are affected by the proportions of different cement components.

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Setting and hardening of cementC3S and C2S constitute about 70-80% of all Portland

cements.

Tri-calcium silicate (C3S): C3S hydrates more rapidly than C2S and develops strength in concrete for first 28 days. It also generates more heat.

Di-calcium silicate (C2S): C2S is next to hydrate but it hydrates slowly and is responsible for the ultimate strength. C2S takes 2-3 years for its complete hydration which contributes towards ultimate strength of cement mortar or concrete. It is more resistant to sulphate attacks. 53

Setting and hardening of cementTri-calcium aluminate (C3A): When cement reacts with water,

C3A is the first to react with water and causes the initial set. It generates great amount of heat and is easily affected by sulphates.

C3A contributes little to the strength of concrete.

C3A is rendered ineffective by addition of gypsum during grinding of clinkers.

Gypsum reacts with C3A and turns it into calcium sulpho-aluminate which causes expansion during setting.

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Setting and hardening of cement

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• C4AF (Felite)– It is comparatively

inactive and contributes little to the strength of concrete and the heat of hydration.

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Sequence of Hydration• Ettringite (needle shape prismatic crystals or rod like shape)

make appearance with few minutes of hydration• A few hours later large crystals of CH form• Simultaneously very small fibrous of C-S-H start to fill the

gaps

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Identification of Various Phases

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Identification of Various Phases

CH crystals

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Identification of Various Phases

CH crystals

Etteringite

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Identification of Various Phases

CH crystals

C-S-H fibres

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ASTM Portland cements• Type I - General purpose• Type II - Moderate heat of hydration and sulfate

resistance (C3A < 8%). General construction, sea water, mass concrete.

• Type III - High early strength (C3A < 15%). Emergency repairs, precast, winter construction.

• Type IV - Low heat (C3S < 35%, C3A < 7%, C2S > 40%). Mass concrete

• Type V - Sulfate resistant (C3A < 5%). Sulfate in soil, sewers

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ASTM Portland cements

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ASTM also has Type I-A, II-A, III-A cements with air entrainment

ASTM Portland cements

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Testing of Portland cement• Fineness test• Consistency test• Setting time test• Soundness test• Tensile strength test• Compressive strength test

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Fineness of cementFiner cements will

◦ React more quickly◦ React more completely◦ Improve mix cohesion (or make ‘sticky’)◦ Reduce bleeding◦ Deteriorate more quickly◦ Be more susceptible to cracking◦ Generally require more water

Fineness of cement does not alter the total quantity of heat liberated but it changes the rate of development due to change in surface areas.

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Fineness test• Excessive fineness is not desirable because

• Cost of grinding to achieve fineness increases• Excessively fine cement deteriorates more quickly• Greater fineness requires more gypsum for proper retardation• Water required for standard consistency increases for finer

cements.

• The fineness of cement is tested by• Sieve test• Surface area test

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Fineness testSieve test: The maximum residue after sieving through 90-

micron sieve should be limited to◦ 10% by weight for ordinary Portland cement◦ 5% by weight for rapid hardening Portland cement

Sieve test does not give any idea of smaller grains retained on sieve.

Surface area test: also known as specific surface test.◦ Specific surface is the total surface of all particles of cement per unit

weight.◦ Determined by air permeability method or Wagner’s turbidimeter.◦ Less than 2250 cm2/g of cement (air permeability) or 1600 cm2/g of

cement (Wagner’s method).

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Consistency test• Consistency test is conducted to determine the

percentage of water required for preparing cement pastes of standard consistency for other tests (e.g., setting time, soundness and compressive strength)

• Vicat’s apparatus consists of metal frame with a movable rod (300 g). Attachments to this apparatus are:• Square needle: used for initial setting time test• Plunger: used for consistency test• Annular collar: used for final setting time test

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Vicat’s apparatus

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Consistency test• Method: Consistency is measured by Vicat apparatus by

using a 10 mm dia plunger fitted to needle holder• A trial past of cement (600 g) and water (e.g. 30% by weight or

180 g) is mixed and placed in the mould.• The plunger is brought into contact with top surface of paste

and released.• If plunger penetrates paste to a point 30+-(1) mm from top of

the mold, it is termed as standard consistency.• Usual range is between 26 and 33%.

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Initial setting time test Initial setting time: this value is necessary for various

operations such as mixing, transportation, placing and compaction of cement mortar or concrete.

Method:◦ Cement paste is filled in the Vicat mould.◦ A round or square needle with cross-sectional area of 1 mm2 is used.

Needle is released at regular intervals and penetration is noted.◦ If needle penetrates to a point 25 mm from top, the initial set is said

to have taken place.◦ Initial set is expressed as time elapsed since the mixing water was

added till required needle penetration.◦ Initial set is 30 minutes for ordinary cement. 76

Final setting time test• Final setting time: is the time after which the cement

mortar or concrete gains strength.• Method:

• Cement paste is filled in the Vicat mould.• The needle with annular collar is used.• Final set is said to have taken place when the needle, gently

lowered to the surface makes an impression on it but the circular cutting edge fails to do so.

• The final setting is reckoned from the moment when mixing water was added to the cement.

• This time is about 10 hours for ordinary cement. 77

Soundness test• objective: this test is

performed to detect the presence of uncombined lime and magnesia in cement.

• Le Chatelier apparatus: consists of brass cylinder split along its generatix.

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Soundness testMethod : ◦ The cement paste is prepared. The percentage of water is taken as

determined in consistency test.◦ The cylinder is placed on a glass plate and filled with cement paste.

The top of cylinder is covered with another glass plate and a small weight is placed on top.

◦ The whole assembly is immersed in water at 24-35 oC for 24 hours. After 24h, distance between the indicators is measured.

◦ The mould is immersed in water again and brought to boil in 30 minutes. It is boiled for one hour, and mould is removed.

◦ After cooling, distance between the indicators is again measured. ◦ The increase in distance is expansion of cement.◦ It should not exceed 10 mm for ordinary cement.

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Tensile strength test• Method • A 1:3 cement sand mortar with a water content of 8% of the

weight of solids is mixed and molded into a briquette of shape shown.

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– The briquettes are cured for 24 hours at 27 oC.

– The briquettes are tested for direct tension. The pull is applied through special jaws engaging wide ends of briquette.

– Average strength is 2.0 N/mm2 (after 3 days) and 2.5 N/mm2 (after 7 days).

Compressive strength test• Method

• 1:3 mortar cubes (50x50x50 mm) or cylinders (150x300 mm) are prepared.

• Mortar composition: Cement = 185 g, sand = 555 g, water = 74 g.

• The cubes/cylinders are tested in compression.• Average compressive strength should not be less than 11.5

N/mm2 (after 2 days) and 17.5 N/mm2 (after 7 days).

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Questions ?

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