Cement
Topics under Cement • Introduction • History • Chemical composition • Manufacturing • Cement Properties • Setting and Hardening of Cement • Testing
Topics under Cement • Introduction • History • Chemical composition • Manufacturing • Cement Properties • Setting and Hardening of Cement • Testing
Cement • Clay as cementing material – material – Assyrians and Babylonians • Lime and gypsum as binder material – material – Egyptians used in pyramids • Calcareous cements, limestone material – material – Romans • Joseph Aspedi Aspedin n of Yorkshire Yorkshire (UK) in 1824 formed 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
Cement • 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.
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). • 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.
Chemical Composition of Cement Oxide
%age
Function
Lime (CaO)
60-65
Controls strength and soundness
Silica (SiO2)
20-25
Gives strength. Excess causes slow setting
Alumina (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 Sulphur trioxide (SO3)
0.1-0.5 Residues. Excess causes cracking 1-2
Makes cement sound
Functions of ingredients of cement Lime (CaO) • 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.
Functions of ingredients of cement Silica (SiO2) • 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.
Functions of ingredients of cement Alumina (Al2O3) • 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.
Functions of ingredients of cement Iron oxide (Fe2O3) • It provides color, hardness and strength to cement. • It also helps the fusion of raw materials during manufacture of cement.
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.
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.
Wet process Raw slurry
elevators
Correction silos
Lime slurry Clay slurry
Calcareous materials ( limestone)
Argillaceous materials ( clay)
Preliminary crushing
Wash mills
Elevators (storage bins)
Elevators (storage bins)
Hoppers Water Wet grinding (Ball mills)
Wet process (contd.)
Gypsum Gypsum hopper
Correction silos
Fuel-coal
Rotary kilns
Crushing & grinding (Ball mill)
Clinker
Pulverized coal
Elevators (Clinker storage) Clinker grinding (Cement grinding mills)
Elevator (Cement silos)
Cement Manufacturing Process
Cement Manufacturing Process
Rotary Kiln (Furnace)
Wet process • Collection 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%
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.
Zones of Rotary Kiln
Wet process (contd.) • Burning – When slurry moves down the kiln, it encounters progressively high temperatures. – At first water is driven off, and CO 2 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
Clinker
Clinker
Gypsum
Source: PCA, 2003 9/16/2014
CE-115 Civil Engineering Materials - Concrete
23
Wet process (contd.) • Grinding – The cool clinker is crushed with 3-4% of gypsum (CaSO 4) 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.
9/16/2014 Source:
PCA, 2003 CE-115 Civil Engineering Materials - Concrete
25
Cement Clinker Composition • Tri-calcium silicate – 3CaO SiO2 or C3S (Alite) – Best cementing material. About 40%. Main cause of hardness and early strength
• Di-calcium silicate – 2CaO SiO2 or C2S (Belite) – About 32%. Resistance to chemical attacks. Hardens slow and takes long to add to strength
• Tri-calcium aluminates – 3CaO Al2O3 or C3A (Celite) – About 10%. Rapidly reacts responsible for flash set, volume changes, cracking and high heat of hydration
• Tetra-calcium alumino ferrite – 4CaO Al2O3 Fe2O3 or C4AF (Felite) – About 9%. Responsible for flash set.
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
Properties of cement components Component
Rate of reaction
Heat liberated
Ultimate cementing value
Tri-calcium silicate, C3S
Medium
Medium
Good
Di-calcium silicate, C2S
Slow
Small
Good
Tri-calcium aluminate, C 3A
Fast
Large
Poor
Tetra-calcium alumino ferrite, C4AF
Slow
Small
Poor
Setting and hardening of cement • The 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.
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.
Setting and hardening of cement • C3S 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 on concrete. It is more resistant to sulphate attacks.
Setting and hardening of cement • Tri-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 sulphoaluminate which causes expansion during setting.
Setting and hardening of cement • C4AF (Felite) – It is comparatively inactive and contributes little to the strength of concrete and the heat of hydration.
Hydration • The chemical reaction between cement and water is known as hydration of cement. • The reaction takes place between cement components (C4Af,C3A,C3S &C2S) and water. • The chemical reaction is as follows:
Hydration • The product C-S-H gel represents the calcium silicate hydrate also known as tobermorite gel. • The C-S-H phase makes up 50-60% of the volume of solids in a completely hydrated Portland cement paste
Testing of Portland cement • Fineness test • Consistency test • Setting time test • Soundness test • Tensile strength test • Compressive strength test
Fineness of cement • Finer 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.
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
Fineness test • Sieve 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).
Consistency test
Cement Types • Portland Cement – Ordinary Portland Cement – *Rapid Hardening or High Early Strength Portland Cement – * Low Heat Portland Cement
– Sulphate Resistant Portland Cement – Water Repellent Portland Cement – Water Proof Portland Cement – *Air Entraining Portland Cement – * Pozzolana Portland Cement
Cement Types • Other Varieties of Cement – * High Alumina Cement – * Quick Setting Cement – * Blast Furnace Slag Cement
– * White Cement – * Colored Cement – Expanding Cement
– Hydrophobic Cement
Cement Types • Rapid Hardening or High Early Strength Cement – 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 • After 72 hours – not less than 275 kg/cm2
– Initial and final setting times are same as OPC – Contains more tri-calcium silicate and finely ground – 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
Cement Types • 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
Cement Types • High Alumina Cement – 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
Cement Types • Portland Slag 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
Cement Types • Low Heat Cement – Low percentage (5%) of tri-calcium aluminates and silicate and high (46%) of di-calcium silicate to keep heat generation low – 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
Cement Types • Air Entraining 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
Cement Types • 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
Cement Types • 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
Puzzolona
Cement Types • Sulphate Resistant Portland Cement – Percentage of tricalcium aluminate 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, siphons, etc
