Ground-granulated blast-furnace slag is obtained by quenching molten iron slag
from a blast furnace in water or steam, to produce a glassy, granular product
that is then dried and ground into a fine powder.
Production and composition The chemical composition of a slag
varies considerably depending on the composition of the raw materials in the
iron production process. Silicate and aluminate impurities from the ore and
coke are combined in the blast furnace with a flux which lowers the viscosity
of the slag. In the case of pig iron production the flux consists mostly of a
mixture of limestone and forsterite or in some cases dolomite. In the blast
furnace the slag floats on top of the iron and is decanted for separation.
Slow cooling of slag melts results in an unreactive crystalline material
consisting of an assemblage of Ca-Al-Mg silicates. To obtain a good slag
reactivity or hydraulicity, the slag melt needs to be rapidly cooled or
quenched below 800 °C in order to prevent the crystallization of merwinite
and melilite. To cool and fragment the slag a granulation process can be
applied in which molten slag is subjected to jet streams of water or air
under pressure. Alternatively, in the pelletization process the liquid slag is
partially cooled with water and subsequently projected into the air by a
rotating drum. In order to obtain a suitable reactivity, the obtained
fragments are ground to reach the same fineness as Portland cement.
The main components of blast furnace slag are CaO, SiO2, Al2O3, and MgO. In
general increasing the CaO content of the slag results in raised slag basicity
and an increase in compressive strength. The MgO and Al2O3 content show the same
trend up to respectively 10-12% and 14%, beyond which no further improvement can
be obtained. Several compositional ratios or so-called hydraulic indices
have been used to correlate slag composition with hydraulic activity; the
latter being mostly expressed as the binder compressive strength.
The glass content of slags suitable for blending with Portland cement typically
varies between 90-100% and depends on the cooling method and the temperature
at which cooling is initiated. The glass structure of the quenched glass largely
depends on the proportions of network-forming elements such as Si and
Al over network-modifiers such as Ca, Mg and to a lesser extent Al. Increased
amounts of network-modifiers lead to higher degrees of network
depolymerization and reactivity. Common crystalline constituents of
blast-furnace slags are merwinite and melilite. Other minor components which
can form during progressive crystallization are belite,
monticellite, rankinite, wollastonite and forsterite. Minor amounts of reduced
sulphur are commonly encountered as oldhamite.
Applications GGBS is used to make durable concrete
structures in combination with ordinary portland cement and/or other pozzolanic
materials. GGBS has been widely used in Europe, and increasingly in the United
States and in Asia for its superiority in concrete durability, extending the
lifespan of buildings from fifty years to a hundred years.
Two major uses of GGBS are in the production of quality-improved slag
cement, namely Portland Blastfurnace cement and high-slag blast-furnace
cement, with GGBS content ranging typically from 30 to 70%; and in the
production of ready-mixed or site-batched durable concrete.
Concrete made with GGBS cement sets more slowly than concrete made with ordinary
Portland cement, depending on the amount of GGBS in the cementitious material,
but also continues to gain strength over a longer period in production
conditions. This results in lower heat of hydration and lower temperature
rises, and makes avoiding cold joints easier, but may also affect construction
schedules where quick setting is required.
Use of GGBS significantly reduces the risk of damages caused by alkali–silica
reaction, provides higher resistance to chloride ingress — reducing the risk of
reinforcement corrosion — and provides higher resistance to attacks by sulfate
and other chemicals. How GGBS cement is used
GGBS cement can be added to concrete in the concrete manufacturer's batching
plant, along with Portland cement, aggregates and water. The normal ratios
of aggregates and water to cementitious material in the mix remain unchanged.
GGBS is used as a direct replacement for Portland cement, on a one-to-one basis
by weight. Replacement levels for GGBS vary from 30% to up to 85%. Typically 40
to 50% is used in most instances. The use of GGBS in addition to Portland
cement in concrete in Europe is covered in the concrete standard EN 206:2013.
This standard establishes two categories of additions to concrete along with
ordinary Portland cement: nearly inert additions and pozzolanic or latent
hydraulic additions. GGBS cement falls in the latter category. As GGBS cement
is slightly less expensive than Portland cement, concrete made with GGBS cement
will be similarly priced to that made with ordinary Portland cement.
Architectural and engineering benefits = Durability =
GGBS cement is routinely specified in concrete to provide protection against
both sulphate attack and chloride attack. GGBS has now effectively
replaced sulfate-resisting Portland cement on the market for sulfate
resistance because of its superior performance and greatly reduced cost
compared to SRPC. Most projects in Dublin's Docklands, including Spencer
Dock, are using GGBS in subsurface concrete for sulfate resistance.
To protect against chloride attack, GGBS is used at a replacement level of 50% in
concrete. Instances of chloride attack occur in reinforced concrete in marine
environments and in road bridges where the concrete is exposed to splashing
from road de-icing salts. In most NRA projects in Ireland GGBS is now
specified in structural concrete for bridge piers and abutments for
protection against chloride attack. The use of GGBS in such instances will
increase the life of the structure by up to 50% had only Portland cement been
used, and precludes the need for more expensive stainless steel reinforcing.
GGBS is also routinely used to limit the temperature rise in large concrete
pours. The more gradual hydration of GGBS cement generates both lower peak
and less total overall heat than Portland cement. This reduces thermal
gradients in the concrete, which prevents the occurrence of microcracking
which can weaken the concrete and reduce its durability, and was used for this
purpose in the construction of the Jack Lynch Tunnel in Cork.
= Appearance = In contrast to the stony grey of
concrete made with Portland cement, the near-white color of GGBS cement permits
architects to achieve a lighter colour for exposed fair-faced concrete
finishes, at no extra cost. To achieve a lighter colour finish, GGBS is usually
specified at between 50% to 70% replacement levels, although levels as
high as 85% can be used. GGBS cement also produces a smoother, more defect
free surface, due to the fineness of the GGBS particles. Dirt does not adhere to
GGBS concrete as easily as concrete made with Portland cement, reducing
maintenance costs. GGBS cement prevents the occurrence of efflorescence, the
staining of concrete surfaces by calcium carbonate deposits. Due to its much
lower lime content and lower permeability, GGBS is effective in
preventing efflorescence when used at replacement levels of 50% to 60%.
= Strength = Concrete containing GGBS cement has a
higher ultimate strength than concrete made with Portland cement. It has a
higher proportion of the strength-enhancing calcium silicate
hydrates than concrete made with Portland cement only, and a reduced
content of free lime, which does not contribute to concrete strength.
Concrete made with GGBS continues to gain strength over time, and has been
shown to double its 28-day strength over periods of 10 to 12 years.
= Sustainability = Since GGBS is a by-product of steel
manufacturing process, its use in concrete is recognized by LEED etc. as
improving the sustainability of the project and will therefore add points
towards LEED certification. In this respect, GGBS can also be used for
superstructure in addition to the cases where the concrete is in contact with
chlorides and sulfates. This is provided that the slower setting time for casting
of the superstructure is justified. Notes
Literature The Concrete Society, Cementitious
Materials: The effect of ggbs, fly ash, silica fume and limestone fines on the
properties of concrete Cementitious materials
References U.S. Federal Highway Administration.
"Ground Granulated Blast-Furnace Slag". Retrieved 2007-01-24.
Civil and Marine Company. "Frequently Asked Questions". Retrieved 2007-01-24.
EnGro Corporation Ltd. "Ground Granulated Blastfurnace Slag". Archived
from the original on 2007-01-22. Retrieved 2007-01-24.
Construct Ireland. "Ground Granulated Blastfurnace Slag". Retrieved
2008-02-21. Ecocem. "Ground Granulated Blastfurnace
Slag". Retrieved 2013-05-27.