- Also called ‘Supplementary Cementing Materials’
- Used when special performance is needed: Increase in strength,
reduction in water demand, impermeability, low heat of hydration, improved
durability, correcting deficiencies in aggregate gradation (as fillers),
- Result in cost and energy savings: Replacement of cement
leads to cost savings; energy required to process these materials is
also much lower than cement
- Environmental damage and pollution is minimized by the use
of these by-products – about 6 – 7% of total CO2 emission
occurs from the production of cement
- Usage depends on supply and demand forces, as well as the
market potential and attitudes
|% by mass
PC: Portland cement, GGBFS: Ground granulated blast furnace slag,
F-FA: Type F fly ash, C-FA: Type C fly ash, SF: Silica fume
Figure 1 shows the compositions of cement and supplementary cementing
materials plotted on a C-S-A ternary diagram.
Figure 1: Composition of
cement and mineral admixtures
Classification of mineral admixtures by RILEM
- Highly pozzolanic: Silica fume, Rice husk ash (controlled
- Normally pozzolanic: Class F fly ash
- Cementitious and pozzolanic: GGBFS, Class C fly ash
An additional category is also suggested by researchers –
Weak pozzolans, such as slowly cooled and ground slag, bottom ash, and
field-burnt rice husk ash
Pozzolans are siliceous or aluminous materials, which possess
by themselves little or no cementitious properties, but in finely divided
form react with calcium hydroxide in the presence of moisture at ordinary
temperatures to form compounds possessing cementitious properties (definition
according to ASTM C595).
Portland-pozzolan cement reaction:
C3S/C2S + H2O —›C-S-H
CH + Reactive SiO2 —›Pozzolanic C-S-H
The pozzolanic C-S-H is generally more porous than the normal
C-S-H, and also has a lower C/S ratio. A pozzolan may also have reactive
Al2O3, in which case the reaction with CH leads
to the formation of C-A-H, which can give rise to problems in sulphate
Pozzolanic activity is evaluated using the Pozzolanic Activity
Index test, which defines the index as:
PAI (%) = Strength (PC/pozzolan mixture)*100 / Strength (PC mixture)
In this test, the mix design is done using a volumetric replacement of
cement by the pozzolan (ASTM C311) as opposed to the Slag Activity Index
test (ASTM C989) where a mass replacement is used.
Characteristics of pozzolanic reaction:
- Lime consuming
- Pore refiner (although there is not much decrease in the
total pore volume)
- Interface refiner (leads to higher strengths)
- Slow rate of reaction; low heat of hydration. Time to attain
same strength as PC may be termed as teff, or effective time. Figure
2 shows some typical values of teff for various mineral admixtures
- Better durability, due to reduced permeability and reduced
- Alkalis and gypsum accelerate the pozzolanic reaction
Figure 2: Strength of pozzolanic
Fly ash is a by-product obtained during the combustion
of coal in thermal power plants. Nearly 73% of India’s electricity
generation is through coal-burning thermal power stations. About 120 million
tons of coal ash now being generated annually; could reach 200 million
tons in 2010. Only about 10% of fly ash is being utilized in various industries;
remaining amount gets dumped in landfills. According to World Bank estimates,
by 2015, the disposal of coal ash would require 1000 square kilometers
or one square meter of land per person. Thus, the utilization of fly ash
is a matter of primary concern.
The quality and composition of fly ash depends on the type of
coal being burnt. The rank (or purity) of coal increases in this order:
5. Lignite (brown coal)
4. Sub-bituminous coal (70 – 80% C)
3. Bituminous coal (80 – 90% C) – Soft coal, used for ordinary
2. Semi-bituminous coal – Good heating value, has a smokeless flame
1. Anthracite (90 – 95% C) – hard coal; high temperature needed
to burn it
Low rank coals contain impurities such as clay, shale, quartz,
carbonates, and sulfides. It is these impurities which give fly ash its
Fly ash use is not very common for the following reasons:
- Difficult quality assurance
- Poor marketing
- Conservative attitudes
- Storage problems
- Presence of toxic chemicals inside fly ash
- It is called a ‘waste’ instead of pozzolan or
Collection of fly ash
During combustion of coal, 75 – 80% of the
ash flies out with the flue gas, and is thus called ‘fly ash’.
The ash that doesn’t fly out is called ‘bottom ash’.
This can be processed as aggregate, but is generally not used in concrete.
The collection of fly ash is done using the following two types of precipitators
(see Figure 3):
- Bag-house precipitator
- Electrostatic precipitator
The bag-house precipitator is found to be more efficient, and
removes out very fine material.
Sometimes, beneficiation of fly ash is done where coarse particles
are ground to a fineness compatible with the intended use.
Figure 3: Electrostatic
precipitator for fly ash collection
Uses of fly ash in concrete
- As a mineral admixture
- As a synthetic aggregate: Fly ash aggregate can be produced
by sintering. The resultant aggregate can be used for lightweight concrete.
However, it is very expensive. Aggregate can also be synthesized by
agglomeration using lime or cement as binder, as in ‘cold bonding’.
Types of fly ash (ASTM classification)
- Type C: This is also called High Calcium fly ash, and possesses
both cementitious and pozzolanic properties. 10 – 15% of the material
has a particle size greater than 45 µm, and the fineness (Blaine)
is 300 – 400 m2/kg. The particles are primarily solid
spheres with a smooth texture. The average particle size is less than
- Type F: This is also called Low Calcium fly ash, and is a
normally pozzolanic material. 15 – 20% of the material is larger
than 45 µm, and the fineness is 200 – 300 m2/kg.
Particles are solid spheres with a smooth texture, and the average particle
size is 20 µm.
Apart from solid spherical particles, there also may exist hollow
spheres. The small hollow spheres with entrapped gas are called cenospheres,
while the large hollow spheres with solid spheres inside them are called
plerospheres (see Figure 4).
Figure 4. Structure of fly
The loss on ignition of fly ash can represent the
amount of unburnt carbon present. Too much of unburnt carbon can interfere
with the air-entraining agent, leading to poor air void parameters. Restrictions
are also placed on the sulfate (SO3) content, MgO content,
alkali content, and moisture content of fly ash.
Effect on fresh concrete properties
- The setting time is increased when fly ash is used.
- Workability and flow of concrete are increased due to the
spherical shape of the fly ash particles, which lends a ball-bearing
type effect on the concrete mixture.
- Bleeding and segregation are usually reduced for well-proportioned
fly ash concrete.
- The paste volume is increased when mass replacement of cement
by fly ash is done.
Effect on hardened concrete properties
- Strength gain of fly ash concrete is slower than normal concrete,
as stated in the previous sections. The potential for thermal cracking
is much reduced compared to ordinary PC concrete. Ultimate strengths
are usually improved when fly ash is used.
- Pozzolanic activity is proportional to the amount of particles
under 10 µm in diameter.
- Creep and shrinkage of fly ash concrete are typically higher
than normal concrete, because of the increased amount of paste in the
concrete (when mass replacement is done).
- More air-entraining admixture is needed to entrain air in
- The results on the effects of fly ash on sulphate resistance
- Expansions during alkali aggregate reaction are reduced by
the use of fly ash, because of the dilution of Portland cement (implying
there are lesser alkalis available).
- For properly cured fly ash concrete, the rate of chloride
diffusion is reduced compared to ordinary PC concrete.
Use of fly ash in specialized applications
- In high strength concrete, as an additional cementitious material.
- In roller-compacted concrete. fly ash is good for bonding
in-between the layers of this concrete.
- In controlled low-strength materials (CLSM), which are flowable
mortars used as backfill
- As a synthetic aggregate
- High volume fly ash concrete: Concrete with 50% of the Portland
cement replaced by Class F fly ash. Has a low water content, generally
less than 130 kg/m3. For slumps of 150-200 mm, the use of a superplasticizer
is mandatory. The range of characteristic compressive strengths that
can be achieved using HVFA concrete is 20-50 MPa. This concrete possesses
excellent pumpability, and exhibits little bleeding (therefore, prone
to plastic shrinkage cracking) and low drying shrinkage. It has applications
in mass concrete blocks, building columns and foundations, caissons
and piles, dams, highways, shotcrete and self-compacting concrete.
According to ACI 116R, silica fume is a very fine
amorphous (noncrystalline) silica produced in electric arc furnaces as
a byproduct of the production of elemental silicon or alloys containing
silicon; also known as condensed silica fume or microsilica. Despite previous
knowledge about the benefits of silica fume, its use as mineral admixture
for concrete really picked up only in the 80s.
There are numerous variants of this highly pozzolanic material
Condensed silica fume, microsilica, silica flour, fume silica (a white
fluffy material produced from vapour phase hydrolysis of chlorosilanes
such as SiCl4 in the flame of hydrogen and oxygen, used in
the paint industry as filler), silica gel, and precipitated silica.
Silica fume is a by-product of the ferrosilicon industry. The
purity of silica fume depends on the ferrosilicon alloy from which Si
metal is being extracted (see Table 1).
Table 1. Silica purity in
various ferrosilicon alloys
||18 – 48%
||44 – 48%
||72 – 77%
||84 – 88%
|Silicon metal (98%)
||93 – 98%
Figure 5 depicts the process of collection of silica fume. After
being collected over the furnace, the silica fume is transferred, cooled,
and physically trapped. Cyclones are used to remove oversize and other
unwanted materials. Similar to fly ash, silica fume is also collected
using baghouse precipitator.
Figure 5: Process of silicon
Silica fume is available in the following states
(see Figure 6):
- As is bulk powder: Due to the low specific gravity of silica
fume (~2.2), the bulk powder becomes very bulky and difficult to handle
and transport. It is difficult to handle pneumatically; it is sticky
and self agglomerating with a tendency to create small weak lumps. Furthermore,
its low density yields small loads in bulk tankers. It is primarily
used in bagged products (e.g., grouts, pre-mixed mortars)
- Dry-densified silica fume: Densification is performed by
aerating and tumbling the silica fume powder with compressed air, until
the desired bulk density is reached due to the agglomeration caused
by electrostatic charges that develop. An efficient superplasticizer
is required to deflocculate and cause a good dispersion of the silica
fume in concrete. Unlike the as is powder, the densified version is
easy to handle and transport, and flows well pneumatically.
- Slurry: 50% water + 47% silica fume + 3% chemical agent,
that keeps the particles in suspension and prevents gelling. The slurry
form is susceptible to gelling in cold climates. However, it is a very
efficient way of dispensing silica fume. Also, storage space can also
Figure 6: Different forms
of silica fume
- Specific gravity: 2.2
- Typical fineness: 20000 m2/kg (average particle
size ~ 0.1 – 0.5 µm)
- Bulk density: As-produced - 130 to 430 kg/m3,
slurry - 1320 to 1440 kg/m3, densified - 480 to 720 kg/m3
- Colour: light grey to dark grey (lighter implies purer)
- Cost: almost 10 times as much as PC
- Typically used at 5 – 15% replacement level
- Benefits from silica fume are due to the pozzolanic reaction
that produces additional C-S-H, as well as due to the particle packing
(filler effect) of the fine silica fume particles
Effects on fresh concrete properties
- Because of its high fineness, the use of silica fume causes
an increase in the water demand of concrete. Typically it is always
used in conjunction with a superplasticizer.
- Silica fume causes the mix to be sticky and cohesive. Also,
concrete mixes with silica fume are prone to slump loss problems. Because
of its cohesiveness, a higher slump is needed to place silica fume concrete.
- Bleeding is reduced drastically. In fact, most silica fume
mixes do not show any bleeding. In dry areas, if the evaporation rate
exceeds the rate at which concrete sets, plastic shrinkage may occur.
Figure 7 shows the increase in capillary pressure with a decrease in
the pore diameter. Silica fume concrete is especially susceptible to
this problem in case curing is not done properly. A simple calculation
of the capillary tension due to plastic shrinkage can be shown as follows:
Pc (capillary tension) = 0.001YS/(w/c), where Y = surface tension of
water = 0.0073 N/m, and S = surface area of particles (20000 m2/kg
for SF, 350 m2/kg for cement). Assuming a w/c of 0.35,
For PC concrete, Pc = 0.07 MPa
For SF concrete, Pc = 4.20 MPa
(for more information on plastic shrinkage problems with silica fume,
see the paper: M. D. Cohen, J. Olek, and W. L. Dolch, “Plastic
Shrinkage Cracking in Portland Cement and Portland Cement-Silica Fume
Paste and Mortar,” Cement and Concrete Research Vol. 20, 1990,
pp. 103 – 119.
Figure 7: Relation between
capillary pressure and diameter
Effects on hardened concrete properties
- Pore size refinement and reduction in permeability occurs
when silica fume is used. Due to a combined effect of silica fume as
a highly reactive pozzolan and filler, the transition zone between aggregate
and paste is strengthened.
- Compressive and flexural strengths are increased, while the
chloride permeability and diffusion are reduced significantly compared
to ordinary PC concrete.
- Elastic modulus is increased (ESFC ~ 1.15 EPCC), or, in other
words, concrete becomes stiffer with the use of silica fume.
- Creep and shrinkage are increased at high replacement levels
(10 – 15%) because of an increase in the volume of the paste.
However, due to the higher stiffness, the resistance to creep and shrinkage
deformation is higher.
- Amount of air-entraining agent required for a particular
volume of air is increased in silica fume concrete. Freeze-thaw resistance
is reduced slightly compared to normal concrete, but damage is usually
limited owing to the extremely low permeability of SFC.
- In most cases, silica fume concrete shows better resistance
to chemical attack (exceptions being ammonium sulphate and magnesium
sulphate attack), owing to the decreased permeability, as well as due
to reduced CH in the paste.
- Expansions due to ASR are reduced in silica fume concrete.
- Corrosion rate is reduced with the use of silica fume. This
is because of two reasons: the low permeability of SFC causes a lower
availability of moisture and oxygen at the cathodic sites, and the high
resistivity of SFC makes the flow of electrons difficult.
- Carbonation depth is generally lowered.
- SFC has very good abrasion and erosion resistance. This makes
it an ideal choice for industrial flooring.
- Fire performance of SFC is not very good. This is a consequence
of the low permeability of silica fume concrete. When a fire occurs,
the free water inside concrete transforms to steam and escapes through
the interconnected voids. When this escape is prevented to the dense
microstructure, significant pressures get built up inside, which ultimately
cause the concrete to explode and spall. This type of failure occurred
in the late 1990s in the English Channel tunnel.
Ground granulated blast furnace slag
Blast furnace slag is a by-product of the extraction
of iron from iron ore. Coke and limestone are added as fluxes inside the
blast furnace. The impurities in iron ore combine with the lime and rise
up to the surface of the blast furnace (shown in Figure 8), while the
molten iron, which is heavier, stays at the bottom.
The use of blast-furnace slag as a cementitious material is very
old. 1892 was the first time that Portland-blast furnace slag cement was
manufactured. In the present day scenario, slag is used almost in every
country to varying degrees.
The reactivity of slag depends on the rate of cooling. In increasing
order of reactivity, the cooling processes may be ranked as: Slow cooling
(in air), Rapid cooling (by water spray), and Quenching (dipping in water).
Amongst mineral admixtures, slag possesses the highest specific
gravity (~ 2.8 – 3.0). Because it is a processed material, the fineness
can be controlled to any desired degree. However, for most typical applications,
slag fineness is only slightly higher than cement fineness.
Figure 8: Blast furnace
used in iron extraction
Types of slag
- Air cooled slag: Low reactivity slag that finds use as aggregate.
The strength and toughness of this aggregate makes it a very suitable
material for railroad ballast.
- Expanded or foamed slag: Low reactivity slag that is foamed
with air. Makes a very good lightweight aggregate, and is used for thermal
- Granulated: This is a high reactivity slag, and is usually
quenched. The hardened matter is then ground to a fineness similar to
cement. Thus the name: Ground Granulated Blast Furnace Slag (GGBFS).
Pelletized slag: The reactivity is similar
to GGBFS, but the process of pelletization is a complex one. The schematic
diagram in Figure 9 shows the process of pelletization. Typically,
this type of slag is not used as much as GGBFS
Figure 9: Process of pelletization
Factors determining cementitious properties of
- Chemical composition of GGBFS – The amount of CaO in
slag determines its cementitious properties. Since slag is almost entirely
amorphous, the reactivity of the CaO will determine the overall slag
- Alkali concentration of reacting system – When the
alkali content is higher, the system will be more reactive.
- Glass (reactive SiO2) content of GGBFS – Glassy SiO2
causes the pozzolanic reaction to take place with the hydrated lime.
- Fineness of GGBFS and PC – Higher fineness implies
a faster reaction.
- Temperature during early phase of hydration – Slag
hydration is greatly enhanced at high temperatures.
Hydration of slag
GGBFS is a mineral admixture with both cementitious
and pozzolanic properties. In fact, it is classified as a hydraulic cement
in most codes. However, an activator is necessary to hydrate the slag.
The activation of slag hydration can be done in the following ways:
- Alkali activation: e.g. by caustic soda (NaOH), Na2CO3, sodium
silicate, etc. The products formed are C-S-H, C4AH13 and C2ASH8 (Gehlenite).
- Sulphate activation: e.g. by gypsum, hemihydrate, anhydrite,
phosphogypsum, etc. The products formed are C-S-H, ettringite, and aluminium
- Mixed activation: When both alkali and sulphate sources are
present, such as in a cement system.
Effects on fresh and hardened concrete
- Apart from delaying the initial set, slag does not significantly
alter the fresh concrete properties. The workability of slag concrete
is similar to an equivalent PC concrete, primarily because slag possesses
the same level of fineness as PC.
- The rate of strength gain is slowed down considerably when
cement is replaced by slag. The delay increases with increasing replacement.
100% slag concrete is also possible, although the curing duration to
produce the required strengths would need to be substantially increased
- The ultimate strengths with slag are generally improved;
the durability is also improved with the replacement of cement by slag.
- Slag is the ideal admixture for marine concrete, as slag
concrete shows excellent resistance to chemical attack and corrosion
- Slag concrete is reported to have higher carbonation rates
compared to ordinary PC concrete
Other mineral admixtures
Rice husk ash (RHA)
This is a high reactivity pozzolan obtained by controlled
calcination of rice husk. Field-burnt rice husk is almost crystalline
in nature, and makes a weak pozzolan. Thus, to obtain a high degree of
pozzolanicity, a good control is needed while burning. RHA usually contains
a large amount of unburnt carbon which might adversely affect air entrainment.
RHA is a fine material, with particle sizes less than 45 µm, and
a surface area of 60000 m2/kg. The particles are typically
cellular. A high amount of reactive silica is present in the system (>
This is obtained from calcination of kaolinite clay
in the range of 740 – 840 oC. The crystalline clay loses
its structure at this temperature by the loss of bound water. Burning
should strictly be done in this range, since beyond 1000 oC, recrystallization
of the clay occurs.
A general formula of metakaolin can be written as AS2. This
aluminosilicate compound reacts with CH produced during cement hydration
in the following form (suggested by Murat – in Cement and Concrete
Research, Vol. 13, 1983):
AS2 + 6CH + 9H —›C4AH13
C-S-H formed in this reaction is aluminous, with a C/S ranging from 0.83
(for crystalline forms of C-S-H) to > 1.5 (for amorphous and semi-crystalline
forms of C-S-H).
The content of C-S-H and its formation rate depends on the mineralogical
characteristics of the kaolin precursor. Metakaolin has a performance
comparable to silica fume as a mineral admixture in concrete. Since MK
is not a by-product, its processing is an expensive affair. Thus the marketability
of MK is not as good as silica fume, which is a proven by-product.
Proportioning methods for mineral admixtures
- Simple replacement method: This is the traditional method
of proportioning. Replacement of cement can be done either on a volume
basis or a mass basis. Volume replacement does not change the overall
volume of the paste. However, when mass replacement is done, volume
of the paste increases, and this increase is usually compensated by
a decrease in the volume if sand. Mixture characteristics can be adversely
affect by the removal of sand if the volume removed is substantial (which
can happen for large amounts of low specific gravity admixtures such
as silica fume). For slow reacting pozzolans like Type F fly ash, this
method results in low early age strengths. Another disadvantage of this
method is that it does not account for the variations in characteristics
of the mineral admixtures. The advantages of this method are its simplicity
and positive effects on workability (when replacing material is fly
- Addition method: This method involves a direct addition of
the mineral admixture to the concrete without replacing any part of
the cement. In high performance concrete, this is the method of choice,
since it increases the cementitious content. This increase is compensated
by a decrease in the fine aggregate content. Addition method usually
results in higher strengths.when fine materials such as silica fume
are used, this method can cause a substantial increase in the water
- Modified replacement method: In this case, part of the admixture
is added, and part of it is used as a replacement. The quantity of mineral
admixture put into the mix is greater than the quantity of cement removed.
This method is typically used to obtain sufficiently high early age
strengths with fly ash. However, workability and water demand can be
difficult to control in this method.
- Rational method: This is an efficient method of proportioning
admixtures. It quantifies the influence of the admixture using a factor
K, which is the ‘cementing efficiency factor’. This factor
qualifies the mineral admixture as a lower grade or higher grade cement.
In other words, K represents the amount of the mineral admixture that
can replace 1 unit of Portland cement in the mixture to achieve similar
properties. This method is able to overcome the slow early age strength
development for fly ash concrete.