CEMENT
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Cement
Cements may be defined as adhesive substances capable of uniting fragments or masses of solid matter to a compact whole in general terms.
Plastic materials employed to produce adhesion between stones, bricks, etc. in the construction of buildings and engineering works is cement in specific context.
Term 'cements' in this restricted sense then becomes equivalent to 'calcareous cements.
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Historical Development
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Manufacture of Cement
William Aspdin's preserved beehive kiln at Northfleet, Kent, which operated from 1847 to 1850. This is the oldest surviving Portland cement kiln in the world. Meso-Portland cement was manufactured in this kiln.
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Chemical Composition of Cement
Main constituents Cement Chemist's shorthand SiO2 S Al2O3 AFe2O3 F CaO C
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Raw Material for Cement
Lime stone S 3.3
% A 0.7 F 0.2 C
53.5
Clay or Shale S
52.8 % A
14.2 F
8.7 C
1.0
Raw S = 13.2% A = 3.4%Feed F = 1.9% C = 43.0%04/13/2023
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Typical composition of Clinker
Shorthand Chemical Mineral Typical level Typical range
nomenclature formula name (mass %) (mass %)
C3S 3CaO·SiO2 or Ca3SiO5 Alite 57 45–65 C2S 2CaO·SiO2 or Ca2SiO4 Belite 16 10–30 C3A 3CaO·Al2O3 or Ca3Al2O6 Aluminate 9
5–12 C4AF 4CaO·Al2O3·Fe2O3 or Ca4Al2Fe2O10 Ferrite 10
6–12
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Compounds in Clinker
Sequence of reactions taking place during the formation of Portland cement clinker.( Source: Reproduced by courtesy of KHD Humbolt Wedag AG.)
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Cement Manufacturing
Steps Mixing. Limestone and clay are ground and
mixed in right proportions. Burning. Obtaining of 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, now obsolete process.
Wet process. Wet mixing and grinding into slurry which turns into clinker on burning in kiln.
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Cement Manufacturing Process
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Cement Manufacturing Process
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Cement ManufacturingModern Process
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Cement ManufacturingModern Process
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Cement Clinker Composition
Tri-calcium silicate (Alite) Best cementing material. About 57%. Main cause
of hardness and early strength Di-calcium silicate (Belite)
About 16%. Resistance to chemical attacks. Hardens slow and takes long to add to strength
Tri-calcium aluminates (Celite) About 9%. Rapidly reacts responsible for flash set,
volume changes, cracking and high heat of hydration
Tetra-calcium alumino ferrite (Felite) About 10%. Responsible for flash set.
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Hydration of Cement
When water is added to cement , a chemical reaction starts and new products are formed which bind the material together in a strong solid mass giving us concrete. This process is known as hydration. It is complex process due to presence of multiple compounds in varying proportions. Hydration of each compound can be studied separately to arrive at total picture. 04/13/2023
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C3S + 4.3H ⇒ C1.7SH3 + 1.3CH Major component of cement Produces C-S-H jel which glues aggregates
together. Hydrate s 50% by 3days and 80% by 28 days. Dormant, or induction, period after mixing.
C2S + 3.3H ⇒ C1.7SH3 + 0.3CH Second major component Produces C-S-H jel which glues aggregates
together. No significant reaction till 14 days.
Hydration of cement compounds
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Hydration of cement compounds
Hydration of C3A and C4AF
C3A Reacts rapidly to form C2AH6 resulting in immediate set.
When gypsum (CaSO4·2H2O) is added, the reaction becomes
C3A + 3C + 3 S + 32H ⇒ C3A·3CS ·32H (ettringite). Avoid immediate set. On consumtion of Gypsum the reaction turns to
(ettringite). C3A·3CS· 32H + 2C3A + 4H ⇒ 3(C3A·CS ·12H).C4AF is variable but generally slow.
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Hydration of Cement
Mixing Period
Different ions are dissolved .
Process is rapid and Exothermic
C-S-H is formed on surface of cement.
Ettringite is formed by Cellite.
Reaction slows down due to protective layer
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Dormant Period Reaction slows down between
1-2 hours.
Silicates (alite
And belite) slowly
dissolve, releasing calcium ions in
solution
Hydration of Cement
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Hardening (2–4 hours) The solution eventually becomes
supersaturated with calcium ions, triggering the formation of new compounds.
(C-S-H, fiber-like particles), which adheres to aggregate and gives concrete its strength.
Calcium hydroxide (CH, crystals). “Initial set” occurs4 when enough
C-S-H and CH form to mesh together, causing the mix to stiffen
Final set” occurs when the concrete achieves a defined stiffness, about when the concrete is hard enough to walk on.
needle-like solid (ettringite) that contributes somewhat to early strength.
Hydration of Cement
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Hydration of Cement
Cooling Soon after final set, the
buildup of C-S-H and CH begins to limit access of water to undissolved cement. Silicate reactions slow. Heat peaks and begins to drop.
Sulfate will be depleted. Any remaining aluminate then reacts . This has little effect on concrete properties except for a brief increase in heat.
concrete contracts or shrinkage.
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Densification or Hardening
Alite hydrate and provide early strength.
Belite dissolves slowly than alite and start to have an impact on concrete strength after 10-14 days.
Hydration of Cement
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Setting of cementStiffening of cement paste to defined consistency is called setting.
Flash Set
True Set
False Set
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Heat of Hydration
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Heat of Hydration
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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 Pozzolan Portland Cement
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Cement Types
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 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
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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
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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
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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
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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
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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
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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
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Cement Properties Chemical composition as shown earlier 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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