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.
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