pozzolans - mert yücel...

Materials of Construction-Pozzolans 1

Pozzolans

Chapter 07

Wikipedia.org


Pozzolan - Historical background

The oldest concrete-like material discovered so

far is in a place on the halfway between Tel-Aviv

(capital of Israel) and Ramallah (a city in Palestine)

and dates from about 7000 BC.

Remains of a wall of a Natufian house

It was used for flooring, and consists of quicklime, made by burning limestone, mixed with water and stone which set into a hardened material.

Construction materials – IIlston & Domone

http://en.wikipedia.org/wiki/File:Natufian-SupportingWall-Elwad.jpg


Pozzolan – Historical background

Mortars made from lime, sand and gravels dating from about 5000 BC. have been found in Western Europe, and similar mixtures were used by the ancient Egyptians and Greeks some three to four thousand years later.

Early concrete-like materials produced by

Romans were also of this type, but during the second century BC, it was the Romans who first made hydraulic binders, i.e. one which reacts chemically with the mix water, and is therefore capable of hardening under water and is subsequently insoluble.




The binding material (cement at that time) was a mixture of lime and volcanic ash from a source near Pozzuoli, a small town near Vesuvius Volcano, Italy.

This ash contained silica and and alumina in an active form which combined chemically with the lime; the term pozzolana is still used to describe such materials.




The dome of the Pantheon seen

from the hill of Janiculum

Mortars produced by combining binding material with aggregates were used in many of the great Roman structures, for example in the foundations and columns in aquaducts, and, in combination with pumice (a lightweight aggregate) in the arches of the Colosseum and in the dome of the Pantheon in Rome.



Pozzolan

Pozzolan is a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, react with calcium hydroxide to form compounds having hydraulic cementitious properties.

In addition to high silica and alumina contents, pozzolans contain small amounts of iron oxide, calcium oxide, magnesium oxide, and alkalies.

The artificial pozzolans contain some amount of carbon.


Classification of Pozzolans

Natural: Volcanic ash, volcanic tuff,

pumicite, diatomaceous earth...

Artificial: Fly ash, silica fume, blast

furnace slag, burned clay….


Volcanic Ash, volcanic tuff

Volcanic ashes are bits of pulverized rock and glass created by volcanic eruptions, less than 2 millimetres in diameter.

After falling to the ground after the eruption, the ash deposited on the ground becomes known as an ashfall. Ashfall may become cemented together to form a solid rock called tuff.

Composed of mainly alumino silicates, quick cooling of magma results in amorphous phases.


Pumicite (Pumice)

Pyroclastic, highly

porous, amorphous,

lightweight material

http://en.wikipedia.org/wiki/Image:Teidepumice.jpg


Diatomaceous Earth

Diatomaceous earth

consists of fossilized

remains of diatoms,

a type of hard-

shelled algae.

These organic

residuals can lead to

silica contents up to

94%.

http://en.wikipedia.org/wiki/Image:Mineraly.sk_-_diatomit.jpg


Artificial Pozzolans

Fly Ash: a by-product from the burning

of pulverized coal in power plants.

Silica fume: (often called microsilica)

extremely fine particles of silica

condensed from the waste gases given off

in the production in silicon metal industry


• Granulated blast furnace slag (GBFS)

obtained by rapid cooling of slag that is a

by-product of pig iron production.

• Burned (calsined) clay or shale

A clay or shale heated, and rapidly cooled

and ground.

• Rice husk ash

obtained by burning rice husks

Artificial Pozzolans


Pozzolanic Reactions

Silis + Calcium Hydroxide + Water → Calcium-Silicate-Hydrate

(C-S-H)

C-S-H provides the hydraulic binding property of the

material.

Pozzolanic Activity: Capacity of pozzolan to form

alumino-silicates with lime to form cementitious

products.


Factors Affecting Pozzolanic

Activity

1) SiO2 + Al2O3 + Fe2O3 content

2) The degree of amorphousness of its

structure

3) Fineness of its particles


SiO2 + Al

2O

3 + Fe

2O

3

The greater amount of these, the greater its activity.

ASTM C 618 & TS 25 → SiO2+Al2O3+Fe2O3 for natural pozzolans > 70%

Fly Ash - ASTM

Class C→ from lignitide or subbituminous coals (SiO2+Al2O3+Fe2O3>50%) (CaO>10%)

Class F→ from bituminous coals and SiO2+Al2O3+Fe2O3>70%

Silica fume → SiO2 ≈ 85-98%

Blast Furnace Slag→ SiO2 ~ 30-40%

Al2O3 ~ 7-19%

CaO ~ 30-50%


Chemical Analysis of Typical Fly Ash, Slag, Silica Fume, Calcined

Clay, Calcined Shale, and Metakaolin

Class F

fly ash

Class C

fly ash GBFS

Silica

fume

Calcined

clay

Calcined

shale

Volcanic

Tuff

SiO2, % 52 35 35 90 58 50 53

Al2O3, % 23 18 12 0.4 29 20 43

Fe2O3, % 11 6 1 0.4 4 8 0.5

CaO, % 5 21 40 1.6 1 8 0.1

SO3, % 0.8 4.1 9 0.4 0.5 0.4 0.1

Na2O, % 1.0 5.8 0.3 0.5 0.2 — 0.05

K2O, % 2.0 0.7 0.4 2.2 2 — 0.4

Total Na

eq. alk, % 2.2 6.3 0.6 1.9 1.5 — 0.3

Natural Pozzolans Artificial Pozzolans


Amorphousness

For chemical reaction → pozzolans must be amorphous

Volcanic ash, volcanic tuff, fly ash, silica fume are all amorphous by nature.

Clays → contain high amounts of silica & alumina but have a crystallic structure! (Do not possess pozzolanic activity) • However, by heat treatment, such as calcining ~700-

900°C crystallic structure is destroyed & a quasi-amorphous structure is obtained


Amorphousness

Clay → does not possess pozzolanic property

Burned clay → possess pozzolanic property

Blast furnace slag → contain high amounts of

silica, alumina & lime.

However, if molten slag is allowed to cool in air, it

gains a crystal structure. * do not possess pozzolanic

property.

However, if it is cooled very rapidly by pouring it into

water, it becomes a granular material & gains

amorpousness. * possess pozzolanic property.


Fineness

Pozzolanic activity increases as fineness increases.

Volcanic ash, rice husk ash, fly ash, condensed silica fume are obtained in finely divided form.

Volcanic tuff, granulated blast furnace slag & burned clay must be ground.


Determination of Pozzolanic

Activity

Pozzolanic activity is determined by “strength

activity indexes”

Six mortar cubes are prepared (ASTM)

→”Control Mixture” 500 g portland cement+1375 g

sand+242 ml water

→”Test Mixture” 400g of portland cement+100g of

pozzolan+1375g of sand+some water for the

same consistency


Strength Activity Index

Compressive testing at 7 or 28 days

Strength Activity Index (SAI) =A/B*100

A=f’c of test mixture

B=f’c of control mixture

ASTM C 618 → SAI ≥ 75%


Example

Determine the 7 and 28 day SAI of a natural pozzolan (trass) using the following data. Specimens are

50 mm cubes.

The specimens are tested for each type of mixture at each testing age.

For control specimen B7-D=81000 N/2500 mm2=32.4 MPa, B28-D=116700 N/2500 mm2=46.7 MPa

For test specimen A7-D=59700 N/2500 mm2=23.9 MPa, A28-D=86700 N/2500 mm2=34.7 MPa

Thus,

7-D SAI=23.9/32.4x100=73.7%<75% Not satisfactory

28-D SAI=86.7/116.7x100=74.3%75% Satisfactory

Mixture 7-day crushing loads (kN) 28-day crushing loads (kN)

Control mixture 78, 81, 84 112, 118, 120

Test mixture 58, 60, 61 86, 86, 88

Mixture Average 7-day crushing load

(kN)

Average 28-day crushing load

(kN)

Control mixture 81 116.7

Test mixture 59.7 86.7


Chemical Composition of

Pozzolans

Silica Fume is mostly SiO2

GBFS→ high amounts of CaO (self-

cementitious)

Class C Fly Ash has CaO (self-cementitious)


Chemical Analysis of Typical Fly Ash, Slag, Silica Fume, Calcined

Clay, Calcined Shale, and Metakaolin

Class F

fly ash

Class C

fly ash GGBFS

Silica

fume

Calcined

clay

Calcined

shale

Volcanic

Tuff

SiO2, % 52 35 35 90 58 50 53

Al2O3, % 23 18 12 0.4 29 20 43

Fe2O3, % 11 6 1 0.4 4 8 0.5

CaO, % 5 21 40 1.6 1 8 0.1

SO3, % 0.8 4.1 9 0.4 0.5 0.4 0.1

Na2O, % 1.0 5.8 0.3 0.5 0.2 — 0.05

K2O, % 2.0 0.7 0.4 2.2 2 — 0.4

Total Na2Oeq

alk, % 2.2 6.3 0.6 1.9 1.5 — 0.3

Natural Pozzolans Artificial Pozzolans


SILICA FUME FLY ASH

GRANULATED BLAST FURNACE SLAG


Uses of Pozzolans

1. By mixing the finely divided pozzolan with calcium hydroxide,

2. By intergrinding pozzolan with portland cement clinker and thus producing portland-pozzolan cements (as an addition in cement production).

3. By adding the finely divided pozzolan to the concrete mixture during or before the batching operation (as a concrete admixture).


Direct Use by Mixing Ca(OH)2

This procedure was extensively used in ancient times for producing pastes; pozzolanic concrete was obtained by including aggregate or small pieces of crushed rock in the paste.

However, this method of using is not very common at the present time (pozzolan + lime mixtures are still used in pavement subbases for small applications).


Use of Pozzolan in Blended

Cement Production

Blended cements are produced by replacing some amount of clinker with a pozzolan, and grinding the "clinker + pozzolan + gypsum" together.

When the blended cement is mixed with water, the PC portion of it immediately starts to hydrate, so the calcium silicate compounds in the portland cement portion of the blended cement begin to produce calcium-silicate hydrate and calcium hydroxide.

The function of pozzolan in blended cement begins with the production of calcium hydroxide due to the hydration of calcium silicate compounds of the portland cement portion. The pozzolan reacts with this calcium hydroxide and thus new calcium-silicate-hydrate gels are produced.


Use of Pozzolan as a Mineral

Admixture for Concrete

Fly ash, ground granulated blast-furnace slag and ground natural pozzolans are commonly used mineral admixtures for concrete. In making concrete, usually 30 to 50% by weight of the cement is replaced with mineral admixtures, and a pozzolanic concrete is obtained by mixing the "cement + pozzolan + aggregates + water" together.

In pozzolanic concrete (similar to blended cements) pozzolan reacts with calcium hydroxide produced upon hydration of calcium-silicate compounds of portland cement and provides additional calcium-silicate hydrate.

The use of finely divided mineral admixtures improves many properties of the fresh and hardened concrete; it provides economy.

pozzolans - mert yücel...

Documents