Lecture 2a - Cement.pdf

CHAPTER 2a: Components of a Concrete (Cement)

Prepared by: Engr. NMSTabucal

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CHAPTER OUTLINE 2a.1 2a.2 2a.3 2a.4 2a.5 2a.6

Introduction Manufacturing Process of Cement Types of Cement Cement Chemistry Hydration of Cement Physical Properties of Cement

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2a.1 INTRODUCTION



Components are proportioned and mixed to produce concrete suited to a particular intent

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2a.1 INTRODUCTION Definitions: Powder

CEMENT WATER

Cement Paste

FINE AGGREGATES (FA) COARSE AGGREGATES (CA)

Mortar

CONCRETE

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2a.1 INTRODUCTION Cement  Pulverized material that binds with other materials due to reaction with water  Initially plastic and allows one to mold into desired shape  Chemical reaction (Hydration) and paste set concrete – producing concrete strength and stiffness

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2a.1 INTRODUCTION Cement  Can be classified into two:  Hydraulic Cement - hardens under water (hydration) i.e. Portland Cement (PC)  Nonhydraulic Cement - Should be kept dry to attain strength i.e. Slaked Lime

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2a.2 MANUFACTURING FiringPROCESS of Raw

Extraction of Raw Materials

Materials

Packaging and Shipment Grinding and Storage of Raw Materials Storage and Grinding of Cement

Source: http://www.lafarge-na.com/wps/portal/na/en/2_2_1-Manufacturing_process

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Extraction of Raw Materials

Raw Materials are Calcium & Silica Calcium: Limestone and chalk Silica: Clays and Shales

Source: http://www.lafarge-na.com/wps/portal/na/en/2_2_1-Manufacturing_process

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Grinding and Storage of Raw Materials Grinding Plant: initial milling before reducing to fine powder “Raw mix” - Raw materials (2/3 Calcium, 1/3 Clay) are stored in pre-homogenization pile

Source: http://www.lafarge-na.com/wps/portal/na/en/2_2_1-Manufacturing_process

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Firing of Raw Materials

Rotary Kiln: heating to a temperature of 1400˚C 1500˚C Combustion causes decarbonation Source: http://www.lafarge-na.com/wps/portal/na/en/2_2_1-Manufacturing_process

Clinker: hard granules formed from the fired materials and contains Hydraulic Calcium Silicates

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Storage and Grinding of Cement Other materials can be added such as Fly Ash and Blast Furnace Slag to produce a specific property of the cement Mixture is finely grinded to a size of 75 microns (sieve no. 200) After re-cooling, clinker is stored in the silos Gypsum is added to clinker

Source: http://www.lafarge-na.com/wps/portal/na/en/2_2_1-Manufacturing_process

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2a.3 TYPES OF CEMENT ASTM C150 (Standard Specification for Portland Cement) 

TYPE I - General purpose - special properties are not required



TYPE II - General purpose - Moderate sulfate resistance or moderate heat of hydration is desired

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2a.3 TYPES OF CEMENT 

TYPE III - High early strength is desired - C3A content is limited to 15%



TYPE IV - Low heat of hydration is desired



TYPE V - High sulfate resistance is desired - C3A content is limited to 5%

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2a.4 CEMENT CHEMISTRY  

Oxides and clinker compounds are expressed by their abbreviations Short Hand Notation  C (CaO, calcium oxide)  S ( SO3, sulfate)  A (Al2O3, alumina)  H (H2O, water)  S (SiO2, silica) 

Reactive Compounds  C3S (tricalcium silicate)  C2S (dicalcium silicate)  C3A (tricalcium aluminate) S  C H2 (gypsum)  C4AF (tetra-calcium alumino ferrite)

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2a.4 CEMENT CHEMISTRY 

Properties of Reactive Compounds Tricalcium Silicate

Hydrates and hardens rapidly Responsible for initial set and early strength development

Dicalcium Silicate

Hydrates and hardens slowly Contributes to age strength beyond 7days

C3A

Tricalcium Aluminate

Produces large amount of heat during first few days Contributes to early strength development

C4AF

Tetracalsium aluminoferrite

Reduces clinkering temperature Hydrates rapidly but contributes little to strength Color of hydrated cement (gray)

C3S

C2S

(Alite)

(Belite)

(ferrite)

Source: http://people.ce.gatech.edu/~kk92/hyd07.pdf

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2a.4 CEMENT CHEMISTRY 

These reactive compounds react with water and this reaction is called hydration



Hydration is mostly exothermic: reactions generate heat



Rate of heating dictates the rate of reaction of minerals with water

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2a.5 HYDRATION OF CEMENT 

Two primary mechanisms: 

Through solutions - Dissolution of anhydrous (without water) to their ionic constituents - Hydrates are formed



Topochemical - Solid-state hydration - Reaction at the surface of anhydrous cement compounds without going into solution

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2a.5 HYDRATION OF CEMENT 

Series of chemical reactions occur during hydration

C3 A + 3C S H 2 + 26 H  → C6 AS3 H 32 , ∆H Tricalcium + gypsum Aluminate

+ water

 →

Ettringites

+ heat

 Ettringites    

Needle like crystals Take up much water which contributes to stiffening of mixture Stable in a solution with gypsum Does not contribute to strength of cement paste

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2a.5 HYDRATION OF CEMENT 2C3 S + 6 H  → C3 S 2 H 3 + 3CH , ∆H Alite + water

 →

Calcium Silicate + Lime Hydrate

+ heat

 Calcium Silicate Hydrate (CSH)    

Short- network fiber structure Poorly crystalline to amorphous High surface area thus, higher bonding energy Contributes greatly to high early strength development

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2a.5 HYDRATION OF CEMENT C2 S + 4 H  → C3 S 2 H 3 + CH , ∆H Belite + water

  

 →

Calcium Silicate + Lime Hydrate (CSH)

+ heat

Reaction generates less heat and proceeds at a slower rate Belite has slow rate of initial strength development Belite is responsible for long term strength of cement

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2a.5 HYDRATION OF CEMENT 

Once all gypsum are consumed:

2C3 A + 3C6 AS3 H 32 + 22 H  → 3C4 ASH 18 Tricalcium + ettringite aluminate

+ water

 →

Monosulfate aluminate hydrate

 Monosulfate Aulimate Hydrate   

Hexagonal small crystals, smaller than ettringites Stable in sulfate deficient solution Increase in size causes cracking when subjected to sulfate attack

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2a.5 HYDRATION OF CEMENT 

Ferrite undergoes two progressive reaction with gypsum

C4 AF + 3C S H 2 + 3H  → C6 ( A, F ) S 3 H 32 + ( A, F ) H 3 + CH Ferrite + gypsum

+ water

 → Ettringite +

Ferric aluminum hydroxide

+ lime

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2a.5 HYDRATION OF CEMENT C4 AF + C6 ( A, F ) S3 H 32 + 2CH + 23H  → 3C4 ( A, F ) S H18 + ( A, F ) H 3 Ferrite + ettringite

+lime

+water

 →

Garnets

 Garnets  

Take up space Do not contribute to the strength of concrete

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2a.5 HYDRATION OF CEMENT 

Hydrates

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2a.5 HYDRATION OF CEMENT 

Stages of Hydration (Summary) Hydrolysis  

Breaking of molecule bonds using water Occurs rapidly with an increase in temperature

v v v

Source: http://ciks.cbt.nist.gov/~garbocz/cell1994/node4.htm

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2a.5 HYDRATION OF CEMENT 

Stages of Hydration (Summary) Dormancy/Induction Period  v v

Source: http://ciks.cbt.nist.gov/~garbocz/cell1994/node4.htm

 

Evolution of heat slows down dramatically (one to three hours) Concrete is plastic Critical for transportation v

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2a.5 HYDRATION OF CEMENT 

Stages of Hydration (Summary) Deceleration Acceleration

 

Concrete starts to harden Heat evolution increases(C3S)

v v v

Source: http://ciks.cbt.nist.gov/~garbocz/cell1994/node4.htm

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2a.5 HYDRATION OF CEMENT 

Stages of Hydration (Summary)  

Slow formation of hydrates Continues as long as water and unhydrated silicates are present

v v v

Steady State Source: http://ciks.cbt.nist.gov/~garbocz/cell1994/node4.htm

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2a.6 PHYSICAL PROPERTIES 

Portland cement are characterized by their physical properties for quality control.



Key Parameters:    

Setting Time Soundness Fineness Strength

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2a.6 PHYSICAL PROPERTIES 

Setting Time -



Affected by fineness, water-cement ratio, chemical content and admixtures Caused by C3A and C3S Defined into two:  Initial Set – paste begins to stiffen  Final Set – Cement hardened

Soundess (Volume Stability) -

Ability if hardened cement paste to retain its volume after setting without delayed expansion Expansion is caused by free lime or magnesia

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2a.6 PHYSICAL PROPERTIES 

Fineness -



Affects hydration rate, therefore affects rate of strength development Smaller particle size : greater SA-to-V ratio : larger watercement interaction per unit volume Coarser particles: hydration on surface and incomplete hydration : low strength and low durability

Strength -

Defined in three ways: compressive, tensile, and flexural Affected by w/c ratio, c/FA ratio, type and grading of FA, curing conditions, and age

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