Cement

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


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


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


Cement Manufacturing Process


Cement ManufacturingModern Process


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.


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


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



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


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


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

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

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


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


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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Engineering

Cement