SUpplementary cementitious materials
What are SCMs and how can you use them
to your advantage?
This is the second of a two-part
series covering Supplementary Cementitious Materials.
Part 1 discussed pozzolanic SCMs, and this, the final
part, focuses on hydraulic SCMs. Part 1 appeared in
the September/October 2004 issue of MC Magazine.
Part
II: Hydraulic SCMs
By
Adam D. Neuwald
The Resource Conservation and Recovery
Act (RCRA) requires U.S. agencies using federal funds
to purchase products composed of the highest percentage
of recovered materials practical. In response, the
Environmental Protection Agency (EPA) created the
Comprehensive Procurement Guideline (CPG), which identifies
various items that are or can be made with recovered
materials. The items are broken down into eight categories.
Manufactured concrete products are an ideal solution
for the following categories: construction products,
landscaping products, park and recreation products,
transportation products and miscellaneous products.
The EPA’s Comprehensive Procurement Guideline
recommends the use of various supplementary cementitious
materials (SCMs) in the production of concrete when
adhering to the guidelines set forth in the RCRA.
The government of Canada has also played a strong
role in promoting sustainable development. The preamble
of the Canadian Environmental Assessment Act states
“The Government of Canada seeks to achieve sustainable
development by conserving and enhancing environmental
quality, and by encouraging and promoting economic
development that conserves and enhances environmental
quality.”
Both the U.S. and Canadian governments are promoting
the use of supplementary cementitious materials in
the production of concrete.
SCMs can be divided into two categories based on the
type of reaction they undergo: hydraulic or pozzolanic.
Hydraulic materials react directly with water to form
cementitious compounds, while pozzolanic materials
chemically react with calcium hydroxide (CH), a soluble
hydration product, in the presence of moisture to
form compounds possessing cementitious properties.
In addition to meeting government requirements, supplementary
cementitious materials are often used to reduce cement
contents and improve the workability of fresh concrete,
increase strength and enhance durability of hardened
concrete. Part I of this article addressed the use
of pozzolanic SCMs in addition to presenting a brief
overview on the use and history of SCMs. As a supplement
to Part I, the following is a brief overview of two
of the more common hydraulic SCMs used in the manufactured
concrete products industry as well as a review of
blended cements and the benefits associated with the
use of these materials.
Slag
According to the Slag Cement Association, a record
3.1 million metric tons of slag cement were shipped
for use in concrete and construction applications
in 2003. This figure represents both slag cement shipped
as a separate product (conforming to ASTM C 989) and
as a component of blended cement (conforming to ASTM
C 595).
Slag, as commonly used in the manufactured concrete
products industry, is often referred to as Ground
Granulated Blast-Furnace Slag (GGBFS), which refers
to the manner in which the material is processed.
Slag is a byproduct from the production of iron and
must be periodically removed from the blast furnace.
The chemical properties of the material are controlled
by the input materials used in the production of iron,
while slag’s physical properties are influenced
by the manner in which the molten slag is cooled.
To form a hydraulic cementitious material, the slag
must be rapidly quenched, or cooled, producing a reactive
amorphous vitreous glass that is ideal for use in
concrete construction. Quenching with water is the
most common process. High-pressure water jets are
used at a water-to-slag ratio of about 10 to one by
mass. The blast furnace slag is broken up and instantaneously
cooled, producing slag with a high glass content.
Slag can also be cooled utilizing the pelletization
process, which uses less water. During pelletization
the slag passes over a vibrating table where it is
expanded and cooled by water jets. The material then
passes over a rotating drum that throws the slag into
the air where it is further cooled, producing spherical
glassy pellets of various sizes as illustrated in
Figure 1. The resulting material from both of these
processes is generally referred to as granulated blast-furnace
slag (GBFS).
Larger pellets, which are porous and partially crystalline,
are often used as lightweight aggregate, while smaller
pellets are ground to produce blended cements meeting
the requirements of ASTM C 595, “Standard Specification
for Blended Hydraulic Cements,” or ground separately
for use as a supplementary cementitious material meeting
the requirements of ASTM C 989, “Standard Specification
for Ground Granulated Blast-Furnace Slag for use in
Concrete and Mortars.” Granulated blast-furnace
slag is often ground finer than portland cement, thereby
increasing its reactivity at early ages. The grinding
of GBFS requires more energy than the grinding of
portland cement clinker. Thus, GGBFS is often similar
in price to portland cement. (From here on, the term
“slag” will refer to ground granulated
blast-furnace slag. The price of slag is also influenced
by transportation costs. A majority of slag granulation
and grinding facilities are centered around the U.S.
iron industry, which is concentrated east of the Mississippi
River.
There are numerous factors simultaneously contributing
to the cementitious performance of slag, including
chemical composition, glass content, fineness, alkali
concentration within the hydrating system and temperature
during the initial stages of hydration. ASTM C 989
merely sets a numerical limit on the amount of sulfide
sulfur (S) and sulfate (SO3) present while classifying
the material based on its performance in comparison
to a reference cement meeting the requirements of
ASTM C 150. The slag activity index (SAI), as defined
in ASTM C 989, is a ratio of the compressive strength
of a 50-50 slag blend mortar to the compressive strength
of a reference cement mortar at seven and 28 days.
The slag is then given one of the following designations:
Grade 80, Grade 100 or Grade 120. A Grade 120 slag
can be expected to produce higher 28-day compressive
strengths than a concrete produced with just the reference
cement, while a Grade 80 slag can be expected to reduce
the compressive strength of a concrete when compared
to the reference cement.
By now you may be asking yourself: Why use slag if
it’s roughly the same price as cement and may
even produce lower compressive strengths? One reason
is that you may be required by the Resource Conservation
and Recovery Act to incorporate the use of SCMs when
bidding on federally funded projects. In addition,
you may come to realize the numerous advantages associated
with the use of slag. Improved workability, reduced
water contents, reduced permeability, alkali silica
reaction (ASR) mitigation and improved resistance
to chemical attack can all be reasonably expected
when slag is used as a supplementary cementitious
material.
Slag has been found to improve the workability and
cohesion of a concrete mix. The glassy surface characteristic
of slag creates smooth slip planes within the paste,
which increases workability, while the presence of
an impervious coating of amorphous silica and alumina
absorb little if any water during the initial mixing
period. The water content can often be reduced by
3 percent to 5 percent when slag is used as a cement
replacement. Research has also shown that concretes
produced with slag require less energy to adequately
consolidate, leading to less wear and tear on forms
and vibration equipment used for both wet-cast and
dry-cast production operations. Bleeding can also
be expected to decrease for slag ground finer than
portland cement, while the opposite can be expected
for coarser slag.
There are various factors that must be considered
when designing a concrete mixture containing slag.
From a production standpoint, proper measures must
be taken to ensure the concrete gains sufficient strength
to be stripped and handled the following day. Fortunately,
slag is extremely sensitive to temperature, and adequate
strengths typically can be obtained by using slag
activators, accelerators or elevated curing temperatures.
The reaction of slag is a two-part reaction, first
reacting with the available alkalis in the system
and later with the reaction product calcium hydroxide
(CH). The alkalis present in portland cement are typically
sufficient to activate the hydration of slag. In addition,
the solubility of alkali hydroxides from portland
cement will increase as the curing temperature is
increased, promoting the early reaction of slag. The
hydration of slag produces calcium silicate hydrate
(CSH), the same reaction product formed during the
hydration of portland cement.
Research has found that a 40 percent to 50 percent
cement replacement by mass with slag will produce
the greatest 28-day strength. Typical dosage rates
for slag in the manufactured concrete products industry
range from 25 percent to 50 percent by mass, depending
on production operations, strength requirements and
durability requirements. Slag can greatly improve
concrete’s resistance to sulfate attack, but
replacement levels may need to be as high as 50 percent
when using a Type I cement. The conversion of calcium
hydroxide to calcium silicate hydrate throughout the
concrete’s pore structure will reduce the overall
permeability, preventing the ingress of harmful sulfates.
In addition, the tricalcium aluminate (C3A) content
of the overall system will be reduced, since less
cement is being used. Research has established a clear
correlation between the C3A content of portland cement
and its susceptibility to sulfate attack, which is
why Type II and Type V cements, which contain lower
C3A contents, are often specified.
Slag can also be used to mitigate concrete expansion
caused by reactive aggregates. While slag will not
prevent the expansion of the aggregates themselves,
it will greatly reduce the expansion of concrete containing
highly reactive aggregates. Research has shown that
replacement levels between 35 percent and 60 percent
have been used successfully to reduce expansion caused
by alkali-silica reaction (ASR). According to the
American Concrete Institute (ACI), slag’s effect
in preventing or reducing the detrimental expansion
of concrete containing reactive aggregates can be
attributed to the following: reduced permeability,
change of the alkali-silica ratio, dissolution and
consumption of the alkali species, direct reduction
of available alkali in the system and reduction of
calcium hydroxide needed to support the reaction.
Slag can also be used to prevent the corrosion of
reinforcing steel by decreasing the permeability of
the concrete and ultimately reducing the penetration
of chloride ions and the depth of carbonation, which
ultimately promotes the corrosion of reinforcing steel.
The use of slag creates additional calcium aluminate
hydrates within the system, improving concrete’s
chloride binding effect, thus immobilizing chloride
ions and reducing the potential for corrosion.
Slag can be purchased in bulk or smaller quantities
and should be stored in the same manner as cement.
Because slag is a hydraulic SCM it should be kept
dry to prevent hydration of the material. Trial batches
should always be cast, and mix proportioning should
be done in accordance with ACI 211.1, “Standard
Practice for Selecting Proportions for Normal, Heavyweight
and Mass Concrete.” Concrete mixes with high
percentages of fine particles tend to become “sticky”
when finishing, so the coarse-to-fine aggregate ratio
may need to be increased slightly. It should also
be noted that concretes containing slag may turn a
greenish-blue color within the first few days after
casting. The color will fade as the concrete oxidizes
with the atmosphere. The color will remain if the
concrete is continuously exposed to water or sealed
prior to oxidation.
Class C Fly
Ash
As a quick review, fly ash is a byproduct from the
combustion of coal used in electric power plants.
It is a fine residue of mineral impurities that melt
and recrystallize within the air stream moving through
the combustion boiler. The material is then collected
from exhaust gases using electrostatic precipitators
or filters. The composition of fly ash is controlled
by the chemical composition of the coal used by the
power plant. Class C fly ash, as defined in ASTM C
618, “Standard Specification for Coal Fly Ash
and Raw or Calcined Natural Pozzolans for Use in Concrete,”
is typically found west of the Mississippi River where
lignite or subbituminous coals are used.
The chemical composition analysis as presented in
ASTM C 618 does not address the nature or reactivity
of the material and is merely presented as a quality
control measure. Class C fly ashes have higher calcium
contents and lower carbon contents than Class F fly
ashes. Class C fly ashes, which contain calcium-rich
glassy phases, are considerably more reactive than
Class F fly ashes, meaning Class C fly ashes have
both pozzolanic and hydraulic properties and often
exhibit higher reaction rates at early ages. This
may be advantageous for use in the manufactured concrete
products industry, making it possible to meet production
schedules.
The carbon content (as measured by loss on ignition)
of Class C fly ash is often less than 1 percent and
will not have an adverse effect on the use of air-entraining
admixtures in comparison to a Class F fly ash. Lower
dosage rates of air entraining admixture may even
be expected when using a Class C fly ash. Class C
fly ashes often contain higher percentages of particles
finer than 10 µm, contributing to improved workability
while reducing the potential for bleeding in fresh
concrete.
Although the preceding information expresses the advantages
of using a Class C fly ash over a Class F fly ash,
one must be aware of the shortcomings of Class C fly
ash. Research has suggested that some Class C fly
ashes will actually reduce concrete resistance to
sulfate attack, especially when used with cements
having a high tricalcium aluminate (C3A) content.
In addition, fly ashes containing high concentrations
of sulfur, as measured by SO3, should be checked for
the potential for efflorescence. Although efflorescence
is not a structural concern, it may cause problems
in architectural products. One must also be cautious
when using fly ash that contains calcium oxide (CaO)
in the form of free lime, especially in low water-to-cement
ratio mixes. If there is not enough moisture present
to hydrate the free lime prior to initial set, delayed
hydration may cause detrimental volume changes.
Fly ash typically has a lower density than portland
cement, thus its bulk density should be used when
ordering and checking inventories. Fly ash will require
about 30 percent to 40 percent more storage space
when compared to an equivalent mass of portland cement.
Because fly ash is spherical in shape, the material
tends to flow quite easily. Storage bins should be
separate from those used for cement and other materials.
Bins should be maintained and checked for flaws that
may lead to cross contamination or loss of material
and positive shut-off valves should be installed to
prevent over-batching.
Blended Cements, Ternary Blends
and Quaternary Blends
Blended cements have been used for the production
of concrete for more than 100 years. Blended cements
are produced by intergrinding portland cement clinker
with various SCMs or by blending portland cement with
varying quantities of SCMs. The advantage of blending
materials is that each material is ground to its optimum
fineness prior to blending. Blended cements may contain
quantities of slag, fly ash, silica fume or natural
pozzolans depending on local availability. The advantage
of using blended cements is that they can be designed
for a specific application. By varying proportions
of the blend, attributes such as increased sulfate
resistance, ASR mitigation or strength gain can be
attained. Chemical admixtures such as air-entrainment
admixtures or accelerators/retarders can also be incorporated.
Blended cements allow producers with limited storage
capacity the ability to capitalize on the numerous
advantages associated with the use of SCMs.
The disadvantage of using blended cements is that
you minimize the flexibility to change your mix design
to accommodate various product lines or project specifications.
The use of blended cements is more common throughout
Europe, while North American producers tend to incorporate
SCMs separately during batching at the mixer. This,
however, requires additional storage bins or silos.
Ternary systems consist of portland cement and two
additional SCMs, while quaternary systems contain
portland cement and three SCMs. Before discussing
the use of multiple SCMs we will quickly review some
of the more common pozzolanic SCMs covered in Part
I of this article.
Silica fume is an industrial byproduct of silicon
metal or ferrosilicon alloy production. Silica fume
particles are extremely small and spherical in shape
resulting in enhanced particle packing and, ultimately,
higher compressive strengths and enhanced durability.
Silica fume is typically used as a 5 percent to 10
percent replacement for cement. High-range water-reducing
admixtures are often used to offset increased water
demand and improve particle dispersion for concretes
containing silica fume. Other pozzolanic SCMs include
Class F fly ash and raw or processed natural pozzolans
such as shale, clay, metakaolin and rice husk ash
(RHA). Natural pozzolans are either ground and used
in their natural state or require calcining. Calcining
is the process of altering the composition or physical
state by heating a material below the temperature
of fusion. Metakaolin and RHA are highly reactive
and often used in the same manner and proportions
as silica fume.
There are numerous advantages to using multiple SCMs
in blended cements or as a separate additive in ternary
and quaternary systems. According to the Portland
Cement Association (PCA), silica fume can be used
to offset the early strength gain associated with
the use of Class F fly ash, while fly ash and slag
can also be used to increase the long-term strength
gain of silica fume concrete. Silica fume can also
be used to reduce the levels of fly ash or slag required
for sulfate resistance and alkali silica reaction
mitigation. Fly ash, and to a lesser extent slag,
can be used to offset the increased water demand associated
with the use of silica fume. Overall, the use of multiple
SCMs will greatly improve the resistance of concrete-to-chloride
ion penetration.
There are numerous benefits to incorporating the use
of supplementary cementitious materials into your
mix design. Not only will SCMs allow cement contents
to be reduced, they will also improve both the fresh
and hydrated properties and performance of your concrete.
Plus, by using SCMs, you are helping to preserve and
protect our environment. The use of byproducts such
as fly ash, slag or rice husk ash in the production
of concrete keeps these materials from the land fill.
In addition, the use of SCMs may also lead to a reduction
in cement consumption, further helping our environment.
The production of a ton of cement releases roughly
a ton of carbon dioxide into the atmosphere.
We must continue to investigate and use environmentally
friendly materials that improve the strength and performance
of manufactured concrete products.
Related
Article
SCMs
Part I: Pozzolanic SCMs
