2. Lecture1

Concrete TechnologyHydraulic Cements

Professor Kamran M. NematiWinter Quarter 2015 1

Concrete Technology

HYDRAULIC CEMENTSAND THEIR PROPERTIES

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Concrete Basics

Portland cement, water, sand, and coarse aggregate are proportioned and mixed to produce concrete suited to the particular job for which it is intended.



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Definitions

Cement Powder

Cement + Water Cement Paste

Cement Paste + Fine Aggregate (FA) Mortar

Mortar + Coarse Aggregate (CA) Concrete

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Concrete is initially plastic, allows one to mold into desired shape.

Chemical reaction (hydration) and paste set of concrete - producing strength and stiffness.

Definitions



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Cement is a pulverized material that develops binding forces due to a reaction with water

Hydraulic Cement Stable under water

Nonhydraulic Cement Products of hydration are not resistant to water (i.e. limestone)

Cement

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Cements that harden by reaction with water and form a water-resistant product.

Portland Cement (P.C.) Portland cement is a hydraulic cement

capable of setting, hardening and remains stable under water. It is composed of calcium silicates and some amount of gypsum.

Hydraulic Cements



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Cement Chemistry

Short Hand Notation

C (CaO, calcium oxide)

A (Al2O3, alumina)

S (SiO2, silica)

S (SO3, sulfate)

H (H20, water)

Reactive Compounds

C3S (tricalcium silicate)

C2S (dicalcium silicate)

C3A (tricalcium aluminate)

CSH2 (gypsm)

C4AF (tetra-calcium alumino ferrite)

In cement chemistry, the individual oxides and clincker compounds are expressed by their abbreviations

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C3S 3CaO . SiO2

C2S 2CaO . SiO2

C3A 3CaO . Al2O3

C4AF 4CaO.Al2O3.Fe2O3

C4A3S 4CaO.3Al2O3.SO3

ferrite aluminate mTetracaciu AFC

aluminate Tricalcium A C

Silicate Dicalcium SC

Silicate Tricalcium SC

4

3

2

3

Compounds of Portland Cement



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Hydration Reactions 2C3S + 6H C-S-H + 3CH (120 cal/g)

2C2S + 4H C-S-H + CH (62 cal/g)

C3A + 3CSH2 +26H C6AS3H32 (300 cal/g)

2C3A + C6AS3H32 + 4H 3C4ASH12

C4AF + 10H + 2CH C6AFH12

C3S2H3 (C-S-H gel) CH (calcium hydroxide) C6AS3H32 (ettringite) C4ASH12 (monosulfate)

Cement Chemistry

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Hydration of Portland Cement

Cal/g 1203CHHSC6HS2C3 Cal/g 26CHHSC4HS2C2

e)(Ettringit HSACHSC AF,C A,C 43

Compound Composition Morphology Amount (% Vol.)

C-S-HVariableC/S 1.5 to 2

Poorly crystallineHigh surface area: higherbonding energy

50 – 60%

CH Ca(OH)2

Large hexagonal crystals,low surface area, and poorbonding energy

20 – 25%

C-A-S-HC6AS3H32

Ettringite

C4ASH12-18

Monosulfate

Long, well crystallizedneedles

Hexagonal – smallcrystals

15 – 20%

Hydration: Reaction with water



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Calcium silicates are the primary constituents of portland cement.

Raw material for P.C. Calcium & Silica Calcium: Limestone, chalk, etc (CaO+CO2) Silica: Clays and shales (SiO2+Al2O3+Fe2O3+H2O)

Clay 1/3

Calcium 2/3

ment heat treat

thebefore dhomogenize

wellbe shouldmix Raw

Manufacturing Process

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Aerial Photo of a Cement Manufacturing Plant(Colorado)



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Raw Mill Feed

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Kiln Line Overview

This image cannot currently be displayed.



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T = 1400CRaw

(Limestone+Clay)

Clinker+Gypsum

Grind

Portland CementGrind Mill

AFC

AC

SC

SC

O.FeO4CaO.Al

O3CaO.Al

2CaO.SiO

3CaO.SiO

OHOFeOAlSiOClay

COCaOLimestone

4

3

2

3

3232

32

2

2

232322

2


Rotary Kiln

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Reactivity of cement with water is a function of its fineness.

Generally, the finer a cement, the more rapidly it will react, and the strength development will be enhanced (expensive).

Fineness



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Types of Portland CementASTM C 150, Standard Specifications for Portland Cement

Type I: General purpose. For use when the special properties specified for any other types are not required.

Type II: For general use, more specially when moderate sulfate resistance or moderate heat of hydration is desired.

Type III: For use when high early strength is desired. (limit the C3A content of the cement to maximum 15%)

Type IV: For use when low heat of hydration is desired. Type V: For use when high sulfate resistance is desired.

(Maximum limit of 5% on C3A)

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In classic research from over seventy years ago Bouge and Lerch* found that of the four portland cement phases only C3S and C2S developed appreciable compressive strength when pure samples of each were hydrated.

The compressive strength found by Bogue and Lerch** are plotted in the next Fig. as a function of age. Compressive strengths of C3A and C4AF, hydrated alone “A” and have not been plotted explicitly.

Effects of Chemical Composition of PortlandCements on Strength

* T.C. Powers, “The Non-Evaporable Water Content of Portland Cement Paste: Its Significance for Concrete Research and Its Method of Determination,” ASTM Bulletin, No. 158, (May 1949) pp. 68-76.** R H Bouge and W Lerch “Industrial Engineering, Chem. 26 873 (1934)



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The compressive strength found by Bouge and Lerch** for hydrated samples of the pure cement phases C3S and C2S are plotted as a function of age. The compressive strengths of C3A and C4AF, hydrated along and with gypsum, fall within the cross-hatched region labeled “A” and have not been plotted explicitly. The time scale is linear. The time scale is logarithmic, which has the effect of expanding the early ages, and this shows the differences between strength gain of C3S and C2S pastes.

Effects of Chemical Composition of Portland Cements on Strength

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The Structure of Concrete The type, amount, size, shape & distribution of

phases present in a solid material constitute its structure.

Concrete Consists of aggregates, paste and voids. The macrostructure of concrete is shown below:

A polished section of concrete



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The structure of the aggregates in concrete is important but it can be characterized as a macrostructure which is visible to the human eye.

The limit of resolution of the unaided human eye is approximately 1/5 millimeter which is 200 microns.

The Microstructure of Portland Cement Concrete

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The use of both light and electron microscopes allows the study of the microstructure of concrete at the submicron level.

The microstructure of concrete can be divided into regions:

Cement Paste Transition Zone between Aggregate and Cement paste




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Structure of “un-damaged” Concrete Macrostructure

Aggregates (CA, FA) Hydrated cement paste (hcp) Entrapped air voids

Microstructure Hydrated cement paste

(Hydration products:C-S-H, ettriginite; monosulfate; porosity: gel, capillary pores entrained/ entrapped air voids)

Transition zone (TZ)

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Microstructure of Concrete(Hydration products)

CH C-S-H



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Ettringite

Microstructure of Concrete(Hydration products)

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Characteristics of the TZ Large crystals of Ettringite and

CH with preferred orientation Porous Structure

Microstructure of Concrete(Transition Zone)



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One way to view cement paste is to consider the hydration of one grain of cement.

The partial hydration of one grain of cement is schematically represented in the next slide.

There are many details in this process that are not yet understood, but there is sufficient information available to allow a consistent mental picture to be considered.


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The hydration products formed inside and outside the cement grain are schematically represented.

The multiple nature of the cement grain is neglected and assumed to be a single phase that shows two types of products.

P1 refers to the “primary” portlandite which appears early in the originally water-filled space.



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The hydration of a number of cement grains is schematically represented in the next slide at different degrees of hydration.

The fresh paste (i.e., the initial combination of water and cement grains) is drawn to approximately represent the 0.4 water/cement ratio, and thus there are not enough hydration products to fill the originally-water-filled space and a capillary porosity remains in the final microstructure.

The Microstructure of Portland Cement Paste

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A schematic representation of the hydration of a number of cement grains. The multiphase nature the cement grains has been neglected as this is like the hydration of tricalcium silicate alone. (a) Fresh paste of water-to cement ration of 0.4 is shown cement grains in the originally water-filled space. (b) After 33% hydration, the cement grains now have inner hydration regions and outer products which form a “columnar zone” around each grain. (c)After 67% hydration, the un-hydrated cores are clearly surrounded by thick “rims” of inner hydration products and the columnar zone of outer products is growing on the surface of each grain. The primary portlandite, P1, is shown with the dendrite morphology. (d) At 100% hydration, the un-hydrated cement has been consumed but the shape of the original cement grains can be distinguished if the inner product differs from the columnar zone of outer products. The intergrowth of the columnar zones from two different grains is shown at several points, but this would be larger at low water/cement rations.

Originally water-filled space = clear, unhydrated cement = , inner hydration products = ,outer hydration products = , & primary portlandite = P1.

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The Microstructure of Portland Cement Paste



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The originally-water-filled space within the cement paste becomes the capillary pores which act as stress concentrations and reduce the strength significantly.

The strength of most engineering materials is increased with a decrease in porosity, and by controlling the water/ cement ratio the engineer is assured that the basic porosity of the paste is also controlled. That is not to say the other sources of porosity will not occur, but at least the cement paste will have a given porosity.

Capillary Porosity

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Figure below shows a graphical representation of the relative volumes of hydration products during hydration.

Graphical representation of the relative volumes of hydration products during hydration. The initial w/c is 0.5, and one unit of cement is shown to produce two volumes of hydration products.

Capillary Porosity



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If cement paste specimens are prepared with a range of w/c ratios it is apparent that the density of high w/c samples is much lower than low w/c samples.

This is illustrated in the next slide in a presentation originally given by T.C. Powers*.

*T.C. Powers, “The Non-Evaporable Water Content of Portland Cement Paste: Its Significance for Concrete Research and Its Method of Determination,” ASTM Bul., No. 158, (May 1949) pp. 68-76.

Capillary Porosity Over a Range of W/C Ratios

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Composition of Cement Paste at different stages of hydration. The percentage indicated applied only to paste with enough water-filled space to accommodate the products at the degree of hydration indicated.

Capillary Porosity Over a Range of W/C Ratios

2. Lecture1

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