Chapter III Concrete Making Materials · Advanced Concrete Technology - Zongjun Li 3 Portland...

Chapter IIIConcrete Making Materials

“Cementitious Binders”

Dr. Mert Yücel YARDIMCI

Advanced Concrete Technology - Zongjun Li 1

Classification of binders


Organic binders (Can easily burned and can not stand with fire)Polymer and asphalt

Inorganic bindersDifferent natural mineral

Non-hydraulic cementIt does not mean that it does not need water!Means only that such cement cannot harden and thus gain strength in water. Typical examples are gypsum and lime! They have been used since 6000 BC.

Hydraulic cementHydraulic cement can harden and gain strength in water.

The main difference in compositionbetween two types of inorganic cementsis that the hydraulic cement containssome amounts of clayey impurities(silicate composition).Examples of hydraulic cement :

• Hydraulic lime,• Pozzolan cement,• Portland cement.

Portland Cement (PC)


Portland cement (PC) concrete is the most popular and widely used buildingmaterial, due to the availability of the basic raw materials all over the world, andits ease of use in preparing and fabricating all sorts of shapes.

There are two major drawbacks with respect to sustainability:

• About 1.5 tons of raw materials is needed in the production of every ton of PC,while, at the same time, about 1 ton of carbon dioxide (CO2) is released intothe environment during the production.

• Concrete made of PC deteriorates when exposed to harsh environments, undereither normal or severe conditions. Cracking and corrosion have significantinfluence on service behavior, design life, and safety.

To overcome these problems, other different cementitious materials have been developedrecently. Two of them are geopolymer and magnesium phosphate cement (MPC).

Portland Cement (PC)


Summary of development of PC

1796 James Parker, England, Patent on a natural hydraulic cement.1813 Vicat, France, Artificial hydraulic lime.1824 Joseph Aspdin, England, Portland cement.

Portland cement was developed by Joseph Aspdin in 1824, sonamed because its color and quality are similar to a kind oflimestone, Portland stone (Portland, England).

Manufacture of Portland cement


Portland cement is made by blending an appropriatemixture of limestone and clay or shale together, andby heating them to 1450◦C in a rotary kiln.

The capability of a modern rotary kiln can reach 10,000metric tons daily.

Manufacture of Portland cement


The raw feed must have a uniformcomposition and be of fine enough sizethat reactions among the components canbe completed in the kiln.

The burned clinker is ground with gypsumto form the familiar gray powder known asPortland cement.

The basic raw materials used for manufacturing Portland cement are limestone, clay, and iron ore.

Reactions in calcination process


Clay is mainly providing silicates (SiO2) together with small amount of Al2O3 and Fe2O3.The decomposition of clay happens at a temperature around 600◦C

Limestone (CaCO3) is mainly providing calcium (CaO) and is decomposed at 1000◦C

Iron ore and bauxite provide additional aluminum and iron oxide (Fe2O3), which help the formation of calcium silicates at low temperature. They are incorporated into row mix.

There are different temperature zones in a rotary kiln.At various temperatures between 1000 and 1450◦C, different chemical compounds are formed. Theinitial formation of C2S occurs at a temperature of around 1200◦C.C3S is formed around 1400◦C.

The final product from the rotary kiln is called clinker.Pulverizing the clinker into small sizes (<75 μm) with addition of 3–5% gypsum or calciumsulfate produces the Portland cement.Gypsum added is to control fast setting caused by 3CaOAl2O3 (C3A)

How fine the cement is…


Major Compounds of Portland Cement


Major Compounds of Portland Cement


It should be indicated that C3S and C2S occupy 68 to 75% of Portland cement.

Since the primary constituents of Portland cement arecalcium silicates, we can define Portland cement as amaterial that combines CaO and SiO2 in such a proportionthat the resulting calcium silicate will react with water atroom temperature and normal pressure.


Developed by Bogue and adopted by ASTM C150

C, S, A, F, and S are weight percentage of corresponding oxide in a Portland cement such as what listed in Table 2-5.

valid only when A/F ≥ 0.64

Minor Components of PC


• Gypsum, • MgO• Alkali sulfates

Gypsum (2CaSO4 2H2O) is added by 4-5 % by mass in the last procedure of grinding the clinker to produce Portland cement.The reason for adding gypsum cement is to avoid the flash setting caused by fast reaction of C3A, because it can react with C3A and form a hydration product called ettringite on the surface of C3A to prevent the further reaction of C3A

Alkalies (MgO, Na2O, and K2O) can increase the pH value of concrete up to 13.5, which is good for reinforcing steel protection.

Hydration of Cement


Hydration of cement is the reaction between cement particles and water, including chemicaland physical processes. The properties of fresh concrete, such as setting and hardening, arethe direct results of hydration.

The rate of hydration during the first few days is in the order of C3A > C3S > C4AF > C2S

Hydration of Cement


The two peaks in the curve represent the dominant effect of C3S or C3A correspondingly and their order of occurrence can be reversed.

Stage I: DissolutionStage II: Dormant Stage III: AccelerationStage IV: DecelerationStage V: Steady state


Traditionally, it is believed that theinitial set of cement correspondsclosely to the end of the induction(dormant) period, 2–4 h aftermixing.


On first contact with water, calcium ions and hydroxide ions are rapidly released from the surface of each C3S grain

the pH values rises to over 12 within a few minutes.

The induction (dormant) period is caused by the need to achieve a certain concentration of ions in solution, before crystal nuclei form, from which the hydration products grow.

At the end of the dormant period, CH starts to crystallize from the solution with the concomitant formation of C–S–H, and the reaction of C3S again proceeds rapidly (the third stage, acceleration, begins).

CH crystallizes from the solution, while C–S–H develops on the surface of C3S and forms a coating covering the grain.

As hydration continues, the thickness of the hydrate layer increases and forms a barrierthrough which water must flow to reach the unhydrated C3S and through which ions must diffuse to reach the growing crystals.

Eventually, movement through the C–S–H layer determines the rate of reaction, and hydration becomes diffusion controlled and moves into the 5th stage, the steady-state stage.


The initial set of cement corresponds closely to the end of the induction period, 2–4 hafter mixing. The initial set indicates the beginning of gel formation. It is controlledprimarily by the rate of hydration of C3S.

The final set occurs 5–10 h after mixing, which indicates that sufficient hydrationproducts are formed and the cement paste is ready to carry some external load.

It should be noted that the initial and the final set have a physical importance. However,there is no fundamental change in the hydration process for these two different sets.

The rate of early hardening, which means a gain in strength, is primarily determined bythe hydration of C3S, and the strength gain is roughly proportional to the area under theheat peak in the calorimetric curves of Portland cement.

The strength development is mainly derived from the hydration of silicates.

Hydration of Portland Cement(Highlights)

Materials of Construction-Cement 19

Chemical reactions with water are called ‘hydration’

Each compound in the cement reacts with water independently

during hydration.

The main hydration product of calcium silicate compounds are

C3S2H3 and CH.

tobermorite gel calsium hydroxide

Calcium-silicate hydrate gels are called C-S-H gels.

C-S-H gels are poorly crystalline (nearly amorphous)

C3S and C2S are the two compounds that contribute to strength as

they hydrate. C-S-H gels provide the binding characteristics of

cement!

Hydration of Portland Cement(Highlights)


The reaction of C3A with water is very rapid leading to

the formation of crystalline alumino hydrates.

The rapid hydration of C3A causes immediate stiffening

of the cement known as ‘quick set’ or ‘flash set’.

In order to prevent this flash set, a small amount of

gypsum rock is added to the clinker during the grinding

process of the cement production.

Types of PC


According to ASTM standards

According to EN standards

22

K: Portland Cement clinker

S: Ground granulated blast furnace slag

P: Natural puzolan

V, W : Fly ash (V=F type, W= C type)

L, LL : Limestone powder (L: organic carbon content max %0.5, LL: organic

carbon content max %0.20 by weight)

D: Silica fume

P, Q : Puzolans

General designation of Portland Cement (European Norms)

Example: CEM II A-M (P-L) 42.5R

Cement typeIIIIIIIV

Amount of clinkerA: HighB: ModerateC: Low Composition Strength class (MPa)

32.542.552.5

Strength developmentN : NormalR: Rapid (High early strength)


From the information provided in Table 2-8, we can evaluate the behavior of each type ofcement. The various behaviors provide the basic justification in selecting cement forengineering practice. For instance, for massive concrete structures, hydration heat is a bigconsideration because too much heat will cause a larger temperature gradient, thermalstress, and cracking.Hence, type IV cement should be the first candidate and type III should not be used. For amarine structure, high sulfate resistance and lower ettringite are needed; thus, type Vshould beselected. If high early strength is needed, type III will be the best choice. Generally, type I isthe most popular cement used in civil engineering.

26

The oxide composition of three clinkers are givenabove. Calculate the compound composition of theclinkers. Determine the most suitable clinker for theproduction of cement suitable for the followingapplications:

A) cold weather concreting

B) hot weather concreting

C) retaining wall exposed to moderate sulfate attack

D) concrete pipe exposed to sewage attack

(Cements will be produced with the same fineness)

% in clinker

Oxide A B C

CaO 64.5 63 66

SiO2 21 22 20

Al2O3 6 7.7 5.5

Fe2O3 2.5 3.3 4.5

others 6 4 4

Calculations for clinker A (calculate for clinker B and C)

% C3S = 4.071(64.5) - 7.6(21) - 6.718(6) -1.43(2.5)=59

% C2S = 2.867(21) - 0.7544(59)= 16

%C3A = 2.650(6) - 1.692(2.5)=12

% C4AF = 3.043(2.5)=8

Example

Since we have made calculations for clinker only, the gypsum rock does not count in the calculations!

27

Similarly the compound composition of other clinkers can be calculated as follows:

Cement produced from clinker C is the most suitable one for cold weather concreting due to the

highest C3S content among clinkers. Clinker B is suitable for the cement for hot weather concreting

since C3S is minimum and C2S is max.

Cement from clinker C is the most suitable one for moderate sulfate attack since C3A <8%.

For sewage attack (severe sulfate attack), sulfate resistance cement with C3A content less than 5% is

required). Thus, none of the clinkers is suitable from this point of view. However, if there is no other

choice, cement produced from clinker C is the most suitable one among the clinkers because it has

the lowest C3A content.

clinker

compound A B C

C3S 59 33 73

C2S 16 38 2

C3A 12 15 7

C4AF 8 10 14

28

HomeworkTwo Portland cements with the

following oxide compositions are

available. Compare the cements

from following points of view:

A) rate of hydration

B) heat of hydration

C) early cementitious value

D) ultimate cementitious value

(Fineness of cements are similar)

Cements

Oxide (%) A B

CaO 62.8 61.8

SiO2 20.4 21.4

Al2O3 5.2 5.2

Fe2O3 3.0 3.0

MgO 4.0 4.0

SO3 2.5 2.5

The role of water on hydration


The water is necessary for the hydration of cement.

The water added in the mix is usually much higher than whatthe chemical reaction needs due to the fluidity requirementof concrete for placing. Thus, we can distinguish the threekinds of water in cement paste according to their roles :

• Chemically reacted water• Absorbed water• Free water



The chemically reacted water or chemically bonded water is the waterthat reacts with C, S, A, F, and S to form a hydration products such as C–S–H, CH, and AFt.This type of water is difficult to remove from cement paste and acomplete decomposition happens at a temperature about 900◦C.

Absorbed water is the water molecules inside the layers of C–S–H gel.The loss of absorbed water causes shrinkage, and the movement ormigration of absorbed water under a constant load affects the creep.

Free water is the water outside the C–S–H gel. It behaves as bulk water and creates capillary pores when evaporated, and can influence the strength and permeability ofconcrete.



Porosity is a major component of the microstructure that is mainlycaused by loss of water.

The size of the capillary pores formed due to the loss of free water isin the range of 10 nm to 10 μm.The size of the gel pores involved in absorbed water is in the rangeof 0.5 to 10 nm.A knowledge of porosity is very useful since porosity has such astrong influence on strength and durability.

According to experiments, the gel porosity for all normally hydrated cements is a constant, with a value of 0.26.



The total volume of the hydration products (cement gel) is given by

where α represents the degree of hydration.

The capillary porosity can then be calculated by;

where w is the original weight of water, c is the weight of cement, and w/c is the water tocement ratio.

With an increase of w/c, the capillary pores increase!



The gel/space ratio (X) is defined as

The gel/space ratio reflects the percentage of solid materials in a cement paste.

The higher the ratio, the more solid the materials and hence the higher the compressive strength.

The gel/space ratio is inversely proportional to the w/c. It can be deduced that a higher w/c leads to a low compressive strength of cement paste or concrete.

The minimum w/c ratio for complete hydrationis assumed to be 0.36 to 0.42.Complete hydration never happens and thatresidual anhydrate cement is beneficial forattaining a high ultimate strength.

Basic tests of Portland cement


• Fineness (surface area / weight)• Normal consistency• Time of setting• Soundness• Strength• Heat of hydration test• Other experiments, including sulfate expansion

and mortar air contentThe detailed test method can be found in the textbook and the laboratory manuals which were provided the students in lecture CIV204 .

Fineness


It is an important quality index and represents the average size of thecement grains.

It controls the rate and completeness of hydration due to theexposure surface of cement particles to the water.

The finer the cement particles, the more rapid the reaction, thehigher the rate of heat evolution, and the higher the early strength.

However, finer cement particles can lead to high hydration heat, highpossibility of early age cracking, and possible reduced durability.

Fineness


The fineness of Portland cement can be measured by different methods. Twocommon methods are the Blaine air permeation method defined in ASTM C204and sieving (ASTM C194-94).

In Blaine method, cement particles are placed on a porous bed and then a givenvolume of fluid (air) is passed through the bed at a steady diminishing rate. After allthe air is passed, the time (t ) for the process is recorded. The specific surface ofthe cement can be calculated using

where K is a constant. In practice, S (or K) can be determined by comparing the sample to the known surface area issued by the U.S. National Institute of Standards and Technology.


Blaine apparatus


TS EN 196-3 Methods of testing cement- Part 3: Determination of setting time and soundness

Consistency is measured by the Vicat apparatus, using a 10 mm diameterplunger with the total mass of moving parts of 300g.

A trial paste of 500g cement and 125 g water is mixed and placed in themold having an inside bottom diameter of 80 mm and a top diameterof 70 mm and a height of 40 mm.

The plunger is then brought into contact with the top surface of paste andreleased. Under the action of its weight the plunger will penetrate thepaste, the depth depending on the consistency. When the plungerpenetrates the paste to a point 6±1 mm from the bottom of the mold,the paste is considered to be at normal consistency.

The water content of the paste is expressed as a percentage by weight ofdry cement, the usual range of values being between 26% and 33%.


Vicat ApparatusPlunger

Length : 10 mm

Diameter : 5 mm

Starting of test Normal Consistency

6 ± 1

mm

40

±0.2

mm

Total weight of

moving part is

300 g

The required water content is adjusted to obtain normal

consistency for determining of setting time of cement.


Setting time test is conducted by using Vicat apparatus, exceptthat a 1-mm diameter needle is used for penetration, themould is filled with a paste of normal consistency. The test isconducted at 10 min intervals.

The initial setting time is defined as the length of time betweenthe preparation of the paste and the time when the distancebetween needle and base plate is 4±1 mm.

The final setting time is defined as the length of time between thepreparation of the paste and the time when the needle nolonger sinks visibly into the paste.


Needle

Length : 10 mm

Diameter : 1 mm

Starting of test The initial setting time is defined as the

length of time between the preparation of

the paste and the time when the distance

between needle and base plate is 4±1 mm.

The test is conducted at 10 min intervals.

4 ± 1

mm

40

±0.2

mm

Total weight of

moving part is

300 g

Cement paste

(cement + water)

in normal

consistency


Soundness

Soundness is defined as the volume stability of the cement paste. The cement

paste should not undergo large volume changes after it has set. However,

when excessive amounts of free CaO or MgO are present in the cement,

these overburned oxides can slowly hydrate and cause expansion of the

hardened cement paste.

The soundness test is conducted by using either Le Chatelier apparatus or

Autoclave method. Le Chatelier method determines the unsoundness due

to free lime. The apparatus consists of a small brass cylinder split along its

generatrix. Two indicators with pointed ends are attached to the cylinder

on either side of the split.


SoundnessA cement paste of normal consistency is placed in Le Chatelier mould

and covered with glass plate. After being cured at 98%RH for 24 hrs,the distance (A) between the indicator points is measured and thenthe mould is heated gradually to boiling during (30±5) min and thewater bath is maintained at boiling temperature for 3h ± 5 min.

At the end of the boiling period and after cooling to 20oC the distance(C) between the indicator points is measured again.

For each specimen, the measurements A and C and the difference C-Aare calculated.

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