Pozzolanic By-Products In Concrete

Marcia C. Belcher, MSCEMarcia C. Belcher, MSCE

Chloride Permeability of Concrete

Corrosion of reinforcing steelCorrosion of reinforcing steel Internal stressesInternal stresses Cracking and spalling of concreteCracking and spalling of concrete Loss of structural integrity and Loss of structural integrity and

serviceabilityserviceability

Water Permeabilty and Freeze Thaw Resistance

Concrete surfaces may be damaged by frost action Concrete surfaces may be damaged by frost action either before or after the concrete has set. either before or after the concrete has set.

The expansion of water while freezing exerts a force The expansion of water while freezing exerts a force sufficient to destroy the cohesion between particles.sufficient to destroy the cohesion between particles.

Pressures develop within the material as ice formation Pressures develop within the material as ice formation continues. continues.

Spalling, scaling and cracking occur when the concrete Spalling, scaling and cracking occur when the concrete cannot withstand the stress of the expansion and cannot withstand the stress of the expansion and contraction associated with freeze/thaw.contraction associated with freeze/thaw.

Pozzolans in Concrete Pozzolans are cementitious in presence of moisture and Pozzolans are cementitious in presence of moisture and

calcium hydroxide (Portland cement)calcium hydroxide (Portland cement)

All of the pozzolans used today are by-products of other All of the pozzolans used today are by-products of other manufacturing processes.manufacturing processes.

Use of them in concrete will:Use of them in concrete will: Reduce exothermic reactionReduce exothermic reaction Reduces thermal crackingReduces thermal cracking Produce finer microstructure Produce finer microstructure Reduce water permeabilityReduce water permeability Increases resistance to chloride penetrationIncreases resistance to chloride penetration

Manufacture of Portland Cement Each ton of cement manufactured consumes approx. 6.5 Each ton of cement manufactured consumes approx. 6.5

million .million .

Enough electricity to power the average American home for Enough electricity to power the average American home for 24 days.24 days.

Carbon dioxide emission equal to two months use of an Carbon dioxide emission equal to two months use of an automobile.automobile.

It is estimated that cement production contributes to about 7 It is estimated that cement production contributes to about 7 percent of carbon dioxide emissions from human sourcespercent of carbon dioxide emissions from human sources

Some Pozzolans Used in Concrete

Ground-granulated blast furnace slag Ground-granulated blast furnace slag (GGBFS)(GGBFS)

Fly ashFly ash Silica FumeSilica Fume

Fly Ash COAL COMBUSTION BY-PRODUCT COAL COMBUSTION BY-PRODUCT

Fossil fuels are used in modern power plants to produce Fossil fuels are used in modern power plants to produce electric energy. electric energy.

The inorganic residues that remains after pulverized coal The inorganic residues that remains after pulverized coal is burned are known as ‘coal combustion by-product'.is burned are known as ‘coal combustion by-product'.

One of these by-products is “fly ash”One of these by-products is “fly ash”

Captured in the exhaust of the boiler system.Captured in the exhaust of the boiler system.

Fly Ash

Coal combustion by-products rapidly accumulate and cause enormous Coal combustion by-products rapidly accumulate and cause enormous problems of disposal unless they can be utilizedproblems of disposal unless they can be utilized

In 2006, there were approx. 125 million tons of ash product produced in the In 2006, there were approx. 125 million tons of ash product produced in the USUS

Approx. 43% is recycledApprox. 43% is recycled

Use of coal combustion products has increased more than 50% in the past Use of coal combustion products has increased more than 50% in the past decadedecade

Fly Ash Fly Ash particlesFly Ash particles

250 micrometers in size250 micrometers in size spheroidalspheroidal

Fly Ash in Concrete

Pozzolanic Pozzolanic

Spherical particles increase workability Spherical particles increase workability

Reduces heat of hydrationReduces heat of hydration

Low early strengthLow early strength

Silica Fume

By-product from the production of metals in electric arc By-product from the production of metals in electric arc furnaces.furnaces.

When added to concreteWhen added to concrete

Produces finer microstructuresProduces finer microstructures Acts as a 'pozzolan,' reacting chemically to impart Acts as a 'pozzolan,' reacting chemically to impart

greater strength and durability to concretegreater strength and durability to concrete

Silica Fume Very fine gray powder of spherical particles Very fine gray powder of spherical particles

Particles average 0.1 to 0.3 microns in diameter Particles average 0.1 to 0.3 microns in diameter

Glassy surfaceGlassy surface

Surface area of 17 to 30 mSurface area of 17 to 30 m22/g/g

Ground Blast Furnace Slag

. •In the production of iron, two products are obtained from the furnace: molten iron and slag.

•Slag comes from the furnace at 2700 F

•Resembles molten lava, cools and solidifies

Ground Blast Furnace Slag

When ground and added to concrete:When ground and added to concrete:

pozzolanic pozzolanic improves workabilityimproves workability improves chloride resistanceimproves chloride resistance decreases water permeabilitydecreases water permeability increases compressive strengthincreases compressive strength

Test Method for Determining Chloride Ion Penetration

Rapid Chloride Permeability Applied Voltage Cell

0

0.05

0.1

0.15

0.2

0.25

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00

Time (Hours)

Cu

rren

t (A

mp

ere

s)

1682 Coulombs

Typical Rapid Chloride Permeability Test Result Curve

Rapid Chloride Test Results for GGBFS Group

0 1000 2000 3000 4000 5000

70% GGBFS

55% GGBFS

45% GGBFS

35% GGBFS

25% GGBFS

Control

Charge Passed (Coulombs)

28-Day90-Day

LowModerate

High

Very Low

Rapid Chloride Permeability Test Results for Fly Ash and Silica Fume Mixes

0 1000 2000 3000 4000 5000

23%Fly Ash 4.5% Silica

24% Fly Ash

Control

Charge Passed (Coulombs)

28-Day90-Day

Very LowLow

ModerateHigh

28-Day Compressive Strength Data for GGBFS Group

0

1000

2000

3000

4000

5000

6000

0 25 35 45 55 70

Percent GGBFS Replacement

Co

mp

ress

ive

Str

eng

th

(ps

i)

28-Day Compressive Strength Data for Fly Ash and Silica Fume Group

0

2000

4000

6000

8000

10000

Control 24% Fly Ash 23%Fly Ash/ 4.5 % Silica

FumeCo

mp

res

siv

e S

tre

ng

th (

psi

)

Conclusions

Benefits of pozzolans Benefits of pozzolans

Increased resistance to chloride Increased resistance to chloride penetration.penetration.

May increase compressive strength of May increase compressive strength of concrete in appropriate proportions.concrete in appropriate proportions.