CEMENT HYDRATION

AND ADMIXTURES

Engr. RAFIA FIRDOUS

PRC-I

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HYDRATION

Series of irreversible exothermic chemical reactions

between cement and water

Cement-water paste sets and hardens, “gluing” the

aggregate together in a solid mass

Formation of hydration products over time leads to:

Stiffening (loss of workability)

Setting (Solidification)

Hardening (Strength gain)

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WHY IS IT IMPORTANT?

Understanding the basics of hydration is

important to

• Ensure the strength and durability of concrete

• Recognize and mitigate the thermal stresses

to prevent cracking

• Appreciate the importance of good curing

and construction practices

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CEMENT-Hydration

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CEMENT-Hydration

COMPOSITION OF CEMENT CLINKER

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CEMENT-Hydration

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CEMENT-Hydration

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CEMENT-Hydration

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CEMENT-Hydration

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CEMENT-Hydration

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Pozzolanic Materials

Pozzolanic cements are those cements in which we add artificially some

pozzolans (ASTM C-618)

Pozzolans are natural or artificial material containing silica in a reactive

form.

Pozzolans are usually added to improve certain characteristics of concrete

resulting in improved durability.

Commonly used pozzolan in the world are:

Fly Ash

Metakaoalin

Silica Fume

Rice husk Ash

Blast furnace slag etc

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ADMIXTURES

Admixtures are chemicals added to concrete, mortar or grout at he time of

mixing to modify the properties either in the wet state immediately after

mixing or after the mix has hardened.

The dosage may be expressed as liters or “kg” per 100 kg of cement

Important types of Admixtures are:

Normal Plasticizing/ water reducing (WRA)

Super plasticizing/ high range water reducing (HRWRA)

Retarders

Accelerators

Air entraining

Water retaining

etc

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ADMIXTURES

Admixtures act in the following ways

Chemical interaction with the cement hydration process

Adsorption onto cement surfaces, typically causing better particle

dispersion

Affecting the surface tension of the water, typically resulting in

increased air entrainment.

Affecting the rheology of the water usually resulting in an increased

plastic viscosity or mix cohesion.

The benefits obtained from the using the admixture are often in the

hardened properties. The properties affected by affecting the following:

Affect water demand

Change the stiffening rate

Change the air content and plastic viscosity.