S8 - Low-E cements.
Pozzolans
Dr Sara Pavía
Dept of Civil Engineering
Trinity College Dublin
Energy efficiency of cement production
E-efficient machinery
Waste materials as fuel (paint residues, used oil, scrap tyres)
Modify product composition:
• replace the cement with supplementary cementitious materials
such as FA [fly ash], GBS [granulated blastfurnace slag],
manufactured pozzolans.
• modify clinker composition to high belite cement -requires
lower energy in pyroprocessing but higher energy for grinding.
• substitute the high T alite for the lower T Ca sulpho aluminate
phase.
Use pozzolans
Industrial and agricultural by-products with pozzolanic activity
such as blast-furnace slag , fly ash, silica fume and rice husk ash
have been used for the production of high performance PC
concrete with a view to improve the durability and service life
of concrete structures.
Partial binder replacement (up to
60%!!!)
Economic and environmental benefits:
• recycling waste whose disposal poses a threat for the environment,
• reducing cement content in concrete, with the subsequent drop in:
• energy consumption,
• non-renewable natural raw material consumption and
• CO2 emissions- climate change.
The replacement of PC by waste material lowers the cost of
construction simultaneously conserving energy and natural
resources, thus reducing the negative impact of building on the
environment.
Pozzolans
• PFA- fly ash, silica fume,
• CKD-cement kiln dust,
• RHA-rice husk ash,
• GGBS-granulated blastfurnace slag,
• MS- microsilica,
• CSF-condensed silica fume,
• Ceramic dust,
• thermally activated clays,
• natural pozzolans.
Leca : Light Expanded Clay Aggregate.
•lightweight, bloated particles of burnt clay
•thousands of small, air-filled cavities
•strength and thermal insulation properties.
•A plastic clay is pretreated and then heated and expanded in a rotary
kiln.
•Finally, it is burned at 1100 °C.
•used to produce light weight concrete, blocks for wall construction,
lowers dead load of a structure
Metakaolin- dehydroxylated form of the clay
mineral kaolinite.
• thermally activated (530-630 °C)
• china clay or kaolin, traditionally used in the
manufacture of porcelain
• admixture for concrete/cement applications.
PC replacement e.g. 8–20% (by weight)
• favorable engineering properties, including:
filler effect, acceleration of hydration,
pozzolanic reaction between 3 and14 days.
Ceramic dust- RBD (red brick dust); YBD
(yellow brick dust); Tile (tile dust)
• Variable composition
• Recycled- control contamination
Microsilica (silica fume)-amorphous (noncrystalline) polymorph of SiO2-silica. It is an
ultrafine powder collected as a by-product of the
silicon and ferrosilicon alloy production.
Blastfurnace slag (BS) by product of the steel industry.
It results from the combination of iron ore with limestone flux.
It is obtained in the manufacture of pig iron- an intermediate form of iron
produced from iron ore subsequently worked into steel or wrought iron.
When BS is quenched by water it forms a glassy material known as
granulated blastfurnace slag (GBS).
GGBS = Ground Granulated Blastfurnace Slag
RHA- agricultural by product
When rice is harvested and milled, roughly 78%
of the paddy is rice and bran, the remaining 22%
is the husk.
In countries of large rice industries such as India
or Brazil, the rice harvested is par-boiled in mills
which are fuelled by burning the husks.
On combustion, the cellulose-lignin matter in the
husk burns away, leaving a porous silica skeleton
which is grinded into fine particles with a large
surface area known as rice husk ash.
Each tonne of rice harvested produces roughly
220 kg of husk, leaving 55 kg of ash after
burning.
Rice husk is used as biomass, many power
plants in Asia are fuelled with rice husk.
RHA is very siliceous, therefore its disposal
poses a threat to the environment.
CKD - cement kiln dust
• by-product of the manufacture of Portland cement, therefore an industrial
waste.
• produced during the pyroprocessing process: when air moves into the kiln with
the fuel to provide oxygen for combustion, this gas flow picks up partly burnt
raw material from the kiln.
CKD
• Over 30 million tons of CKD are produced worldwide per
year.
• In some European countries such as Spain and Ireland, CKD is
recycled as a raw feed substitute in cement manufacturing
• in other countries such as UK and the USA a significant proportion is
land filled.
• The US cement industry generates approximately 4.1 million tons of CKD
per year, 3.3 million of which is land filled.
• The UK cement industry claims that over 200,000 tonnes a year of landfill
space could be saved if the surplus CKD is recycled back into the clinkermaking process or if alternative uses are found.
PFA - pulverised fuel ash
• a by product of (typically coal) fired power stations
• around 500 million tons of coal ash (of which 80% is PFA) are generated per annum
world wide- approx 35% reused.
• Coal is pulverized into a fine powder, mixed with heated air and burned to produce
electricity.
• The carbon in the coal burns off and the inorganic part of the coal –minerals such as
clays and feldspar- melt and form fused droplets that, on rapid cooling, solidify as the
spherical glassy particles that comprise the coal ash. Some crystalline phases are also
present.
• The lighter glass spheres are carried in the flue gasses and extracted by (electrical,
mechanical or cyclone) precipitators
• The resultant material is used as engineering fill and as a component for concrete and
concrete block production. The blocks are lightweight and have excellent thermal
insulation properties.
PFA
When newly produced the dust is strongly alkaline; a pH as high as 11 is
known, and >9 is normal.
Luke in Bernsted and Barnes eds 2002
UK Concrete Centre, Specifying Sustainable Concrete, 2011
Figures for the embodied CO2 of various cements:
Cement Type
Embodied CO2
CEM III/A (36-65% GGBS)
610-360kg CO2/tonne
CEM IIA/L
750-880kg CO2/tonne
CEM I
930kg CO2/tonne
Pozzolans
• materials with amorphous SiO2 or SiO2/ Al2O3 that react with
Ca(OH)2 in the presence of water to form cementitious
hydrates
• calcium silicate hydrates (CSH) and calcium silicate
aluminate hydrates (CSAH) = hydration of PC clinker
• can result in faster setting times, higher mechanical strength,
lower permeability, greater durability…
• Resultant properties depend on the reactivity of pozzolan
Pozzolanic reaction
(similar to the hydration of PC clinker)
• Pozzolan + PC clinker + water =
1st- The clinker minerals quickly react with water (hydration) to
form hydrates (cements) of which calcium silicate hydrate (C-SH) and portlandite (lime=Ca(OH)2) are the most abundant.
2nd – POZZOLANIC REACTION: THE POZZOLANS (SiO2 or SiO2/
Al2O3) REACT WITH THIS PORTLANDITE (Ca(OH)2) FORMING
ADDITIONAL CEMENTS.
The hydrates formed are the same as those occurring on PC hydration.
• CSH - calcium silicate hydrate
• CH – Portlandite= Ca(OH)2
• AFt – ettringite (calcium sulfoaluminate hydrate= CšAH)
• AFm - hexagonal calcium aluminate hydrates= CAH
However in different %s (since the chemical composition of the
pozzolan is different than that of the PC)
Depending on the level of pozzolan
replacement, CH can be partially or entirely
combined producing hydrates.
•Preferably all CH should be combined to
avoid flaws (large crystals-hexagonal plates) in
the microstructure e.g. in the transition zone.
– they reduce the width of the interfacial
zone between the paste and the
aggregate (weakest area of concrete),
reinforcing the microstructure of the
transition zone Zhang et at 1996.
http://tcdlocalportal.tcd.ie/pls/public/staff.detail?p_unit=civil_engineering&p_name=pavias
Effect of pozzolans in composites
• physical filler effect, increasing density by enhancing
packing of the composite particles;
• they reduce the amount of pores and cause a
refinement of the pore structure.
• The pozzolan particles act as nucleation points for the
precipitation of hydrates: the CSH that forms by pozzolanic
reaction between the calcium hydroxide in the paste and the
silica in pozzolan fills the pores and strengthens the
microstructure of the concrete, particularly around the coarse
aggregate.
• this pore refinement transforms a composite from an
opened to a closed pore system.
• the diameter of the mesopores is reduced (thinner and more segmented
pores)- lower permeability.
• the overall porosity may increase (there may be a higher content of
relatively large pores -macropores).
Microstructure of the hardened
pozzolan cement
• The main characteristic of the paste is the absence of
large CH crystals and the presence of layers of hydrates
covering residual pozzolan particles.
• The groundmass of the paste does not appreciably differ
from that of PC
Research
Properties of pozzolan
• Particle Size
• Specific Surface Area
• Chemical Composition
• Mineralogy
• Amorphousness
Behaviour of composite
• Water Demand
• Reactivity
Compressive Strength
Chemical Conductivity
• Setting Time
• Porosity
Methods - Physical characteristics pozzolans
• Particle Size – laser diffraction - Malvern Mastersizer 2000
• Particle Surface Area – BET method - Quantachrome Nova 4200e
• Chemical Composition - % oxide - XRF
• Mineralogy and amorphousness - XRD
Methods – Behaviour / properties of pozzolan pastes
•
•
•
•
Lime: pozzolan pastes at 1:1 and 1:3 ratios and water content to a specific
flow
Water Demand – Initial flow
Setting Time – VICAT test
Reactivity –Mechanical method: Compressive strength; Chemical method: Conductivity.
Results – Specific Surface Area and Particle Size
Metakaolin, RHA, GGBS and PFA are the finest
pozzolans (finer than lime).
MS particles flocculated -finer than determined
by laser. (60% of the MS particles are sized under 1 µm,
therefore the finest).
MS, Metakaolin and RHA - much greater
specific surface area than any of the other
pozzolans.
600
Demand
500
• It depends on:
• pozzolan’s particle size;
• specific surface area;
• lime:pozzolan ratio
• surface area has the
greatest influence on
water demand
Water Content (g)
Results – Water
400
Ratio 1:1
300
Ratio 1:3
200
100
0
0.00
5.00
10.00
15.00
20.00
25.00
Surface Area (m2/g)
Water demand (amount of mixing water) affects workability, strength
and shrinkage therefore, it impacts concrete/mortar quality and
durability.
Results – Water demand of the pozzolans
• Water content determines initial flow
of a paste/mortar.
• Measuring the amount of water
required for a 1:1 (lime:pozzolan)
paste to flow to a specific diameter
provides the water demand of each
specific pozzolan.
• Pozzolans/aggregate or any other
components of mortars and concrete
must not rise water demand as this
can lead to strength, fracturing and
other problems.
Flow table and conical
mould.
Results –
Reactivity
Both reactivity indices positioned the
pozzolans in the same order of reactivity:
Meta/GGBS/MS/RHA/Leca/PFA/YBD/Tile/R
BD
Mechanical Index
•
Strength depends on the
amount, type and
microstructure of hydrates
formed: S hydrates contributing
more to strength than Al
hydrates.
45
Pozzolanic Index (Mech)
• Increasing silica content
results in a higher
mechanical index
• The more abundant the
hydrates [CSH] formedthe higher the strength.
40
GGBS
35
Leca
30
Meta
25
MS
20
PFA
15
RBD
10
RHA
5
Tile
0
YBD
-5 0
20
40
60
% Silica
80
100
Reactivity and Strength Development
45
Pozzolanic Index (Mech)
40
GGBS
35
Leca
30
Meta
MS
25
PFA
20
RBD
15
RHA
10
Tile
YBD
5
0
increasing ----------------------------------------------------------►
Amorphous Content
a relationship between increasing amorphous content and reactivity clearly evident;
amorphous materials –
• greater mobility and superficial location of their atoms
• noncrystalline solids in which the atoms and molecules are not organized in a
definite lattice pattern.
• e.g. glass, plastic and gels [CSH].
• Metastar, GGBS, RHA and MS were found to be
the most reactive pozzolans
high specific surface area
high amorphouness
small particle size
Setting time
• The Vicat test determines the
rate of stiffening by dropping a
needle from a fixed height and
measuring its penetration.
• The stiffness is related to the
formation of hydrates and the
rate of carbonation.
• The initial and final setting
times (at 35mm and 0.5mm
respectively) are standard
references which provide
comparative data between
samples.
Results – Setting Time
all pozzolans speed up the initial set of the lime paste except for PFA
and MS
all pozzolans reduced the final setting time of the lime paste by at
least 40%
no clear relationship between reactivity and setting time
a small increase in water content (5%) significantly slowed down the setting
Depth of Penetration (mm)
40
GGBS
35
Leca
30
Meta
25
MS
PFA
20
RBD
15
RHA
10
Tile
YBD
5
Lime
0
0
20
40
60
Time (hours)
80
100
production parameters also determine reactivity
• temperature, combustion environment, grinding.
E.g. RHA –Rice Husk Ash- Effect of production parameters on
reactivity
Uncontrolled burning produces crystalline RHA, while controlled
burning at lower temperatures produces RHA containing amorphous
SiO2-greater reactivity.
Combustion time, temperature and environment affect both specific
surface area (Nehdi et al. 2003) and carbon content
RHA produced by uncontrolled burning is usually high in carbon, and
this adversely affects the pozzolanic activity of the ash (Nair et al. 2006)
and the rheology of the mortar or concrete (Chagas Cordeiro et al.
2009).
•RHA- agricultural by product
• The use of rice husk ash in concrete was patented in 1924.
• As the silica content of RHA is so high (85 – 90%), it is considered a
‘super-pozzolan’ and used in high performance concrete to
enhance workability, strength and resistance to chemical attack
and chloride corrosion of steel reinforcement.
• RHA increases compressive strength due to the capacity of the
pozzolan of fixing the Ca (OH)2 generated during PC hydration to
form CSH and CASH.
• RHA’s reactivity reaches a high IPA value (Index of Pozzolanic Activity),
over the limits stipulated by the standards in order to qualify as a
pozzolan.
Effect of cement replacement by RHA on the properties of
concrete:
• increase of compressive strength with low level replacements:
• a 5% cement replacement by RHA achieved a compressive strength 24% higher
than that of a PC control mix.
• 8 and 10% level replacements achieved higher compressive strength at 28 and
91 days, that a PC control mix.
• A 10% cement replacement has been reported to achieve excellent
performance leading to an increase in compressive strength, decrease in
permeability, chloride penetration and decreased heat of cement
hydration [Zhang et at 1996; Singhania N. P. 2004 [1] Nair et al. 2006
Ganesan et al. 2008].
0% RHA
25% RHA
50% RHA
75% RHA
Compressive Strength [MPa]
14
11.72
12
10
8.05
8
6.95
6
4
2
1.404
0
1
Effect of cement replacement by RHA on the properties of concrete:
• Acceleration of rate of hydration of cement (Zhang et at 1996);
• Reduction of alkali-aggregate reaction and
• Reduction of hydration heat (Hasparyk et al 2004);
• increase resistance to sulfate attack (Chindaprasirt 2007).
Compressive strength increases with RHA content up to 15% replacement
At 30% replacement, c.s. equivalent to that of control mix
Ganesan et al 2008
RHA
Ganesan et al 2008
The water required for a standard consistency linearly increases
with the %RHA
Up to 15%, increasing RHA content increases the initial setting
time. At higher level replacement (20-25-30 and 35%) there is a
decrease in the initial setting time
PFA is pozzolanic: it reacts with CH
(Ca (OH)2) and water to form
insoluble hydrates (CSH and CASH)
• There is considerable variation on
the rate and kinetics of reaction;
• Generally agreed that PFA’s pozz
reaction becomes apparent from 314 after starting of hydration.
• Delay may be related to the
dissolution of the spheres.
• In the pozzolanic reaction, CH
formed during PC clinker hydration
is consumed by the dissolution
products of the PFA glassy
component.
• The consumption of CH is gradual
over time, the rate differs
depending on the composition of
the PFA.
Luke in Bernsted and Barnes eds 2002
Luke in Bernsted and Barnes eds 2002
PFA fineness vs compressive strength of PC/PFA
pastes
Luke in Bernsted and Barnes eds 2002
PC/PFA paste hydrated for 3
months showing ettringite as
binder. Top-100%PFA; lower
image 80/20 (PFA/PC)
Chemical composition of CKD and typical composition of
CKD and PC - standard content range in brackets
CKD (%)
Chemical
composition
typical composition CKD
(%)
Maslehuddi
n et al. 2008
Peethamparan
et al. 2008
PC (%)
CaO
42.0
38-50
37-55
63.7 (64)
S1O2
11.6
11-16
12-16
20.3 (22)
Al2O3
4.5
3.0-6.0
2.0-5.0
5.3 (5)
Fe2O3
2.9
1.0-4.0
1.7-2.3
3.9 (3)
MgO
1.3
0.0-2.0
1.2-2.7
2.1 (1)
K2O
0.6
3.0-13
1.4-7.0
0.1 (<1)
Na2O
0.1
0.0-2.0
0.1-0.8
0.4 (<1)
SO3
0.2
4.0-18
4.2-14.6
2.7 (3)
Cl-1
-
0.0-5.0
0.3-0.7
0.03 (<0.1)
35.5
5.0-25
4.0-29.6
Loss
ignition
on
The chemical and
physical
characteristics of the
CKD are determined
by the raw feed
material, type of kiln
operation, the dust
collection system and
fuel type.
In particular, the
chemical composition
of CKD depends on
the raw materials used
to produce the clinker
and the type of kiln
fuel.
CKD from dry-process
kilns tends to be
higher in calcium
content than the dust
arising from wet kilns
CKD contains alkalis and is a
caustic material (the typical pH of
CKD water mixtures is
approximately 12).
If used in concrete, corrosion of
metal reinforcement may occur.
The high alkali content of CKD
together with its sulphur and
chlorine can enable
crystallization of disruptive alkali
sulphates and chlorides.
• CKD is finer than PC -further surface available for reaction.
• 5% CKD replacement yielded the highest 90 days compressive
strength of 42 MPa - Al-Jabri et al.
• compressive strength drops with increased CKD replacement,
however, 5%-10% replacement do not significantly reduced
compressive strength - Ali et al.
• Siddique: comprehensive review of CKD in concrete concluding
that, concrete containing low percentages of CKD replacement
(5%) achieves almost equal compressive strength, flexural
strength, toughness and freezing and thawing resistance than
100%PC mixes.
• blends containing as little as 70% PC can still exhibit adequate
strength if only CKD is used as the blending waste material.
• Shoaib et al.:
• optimum quantity of CKD which could be recycled in concrete types without
major strength loss.
• the chloride in the CKD leads to crystallization of hydration products which
open the pore system of the hardened concrete leading to a strength
reduction.
Al Harty et al. 2003
Al Harty et al. 2003
Blastfurnace cements
• Blastfurnace slag (BS) by product of the steel
industry.
• GGBS = Ground Granulated Blastfurnace Slag
• 1853 - early slag-lime cements
• 1909 - 1st standard for BS cement
• same constituents as PC but in different
amounts.
Chemical comp. %
PC
GGBS
SiO2
20.10
35.04
Al2O3
4.15
13.91
Fe2O3
2.50
0.29
CaO
61.30
39.43
MgO
3.13
6.13
K2O
0.39
0.39
Na2O
0.24
0.34
TiO2
0.24
0.42
P2O5
<0.90
<0.10
MnO
-
0.43
SO3
4.04
2.43
Chemical composition of BS
from different countries-ref Lea
GGBS contains clinkers so it is not strictly a pozzolan. However, the
rate of reaction is slow and needs alkalis and sulphates to activate.
When mixed with PC, as PC hydrates, it releases alkalis and sulphates
which serve as a activators for the BS.
Slag reaction
products in
the presence
of different
activators.
Microphotograph
of
SEM
showing the morphology and
size of the GGBS particles.
EDX spectrum of quantitative chemical analysis of GBS.
Bernsted and
Barnes eds 2002
• The final properties of GBS concrete are determined
by the reactivity of the slag (pozzolanicity- hydraulic
activity) which control the amount of hydrates
produced and the properties of the concrete.
• Reactivity depends on:
• the reactive glass (amorphous) content: roughly linear
relation between strength and glass content. Increasing
crystalline components reduce cementing properties.
• the chemical and mineralogical composition,
• type of activator and
• fineness of GBS (Ganesh Babu and Rama Kumar 2000).
• In general, the more basic
slags are, the greater their
hydraulic activity in the
presence of alkalis.
• To ensure high alkalinity, without
which slag would be hydraulically
inactive, European Standard EN
197-1:1992, recommends that
the ratio of CaO + MgO to SiO2
exceeds 1.
• at constant basicity, strength
of concrete increases with the
Al2O3 content.
• hydraulic activity is enhanced
with an increase in Al2O3, CaO
and MgO while an increase in
SiO2 diminishes hydraulic
activity- Frearson (1986)
Influence of Al2O3 in the
development of strength- ref Lea
• GBS improves the general performance of PC composites
• decreasing chloride diffusion and chloride ion permeability (Luo et al.
2003, Yun Yeau and Kyum Kim 2005);
• reducing creep and drying shrinkage (Jianyong and Yan 2001);
• increasing sulfate resistance (Higgins 2003, Binici and Aksogan 2006);
• enhancing the ultimate compressive strength (Barnett et al. 2006)
• reducing the heat of hydration and bleeding (Wainwright and Rey 2000).
• GGBS also improves concrete workability due to its high specific
surface, marketed at 375-435 m3/kg with a fineness of
approximately 460 Blaine (m2/kg.min).
• this makes GGBS finer than PC (typically, PC is 300 m3/kg).
• this leads to increased workability and a better performance in bleeding,
setting times and heat evolution -Swamy (1986).
• Strength is determined by the specific surface area of
the slag
Compressive strength of mortars made with CEM III/B, with 75%
slag of specific surface areas 3000-4500 cm2/g – ref Lea
Effect of slag on
concrete
permeability
decrease
chloride
diffusion and
chloride ion
permeability
Increase
sulfate
resistance
Bernsted and
Barnes eds 2002