SLAG base cement – new horizon for sustainable & durable construction | The Daily Star
12:00 AM, September 17, 2015 / LAST MODIFIED: 08:02 PM, September 17, 2015

SLAG base cement – new horizon for sustainable & durable construction

Innovational History of SLAG Cement

The most recent development that has further increased the value and usefulness of slag cements is the high costs and prospective shortages of energy. The slag does not need to be burned in a kiln. Pointing out that slag is the only cement component other than Portland cement clinker that can be used either as a secondary or main constituent of the cement, which slag has long been used to blend with Portland cements and in so doing has not deleteriously affected its strength.

SLAG                                        

Slag is a byproduct of smelting ore to separate the metal from the unwanted materials. It can usually be considered to be a mixture of metal ores and silicon dioxide. The most common sources of slag are from manufacture of pig iron. Slag cement is most widely used in concrete, either as a separate cementitious component or as part of a blended cement.  It works synergistically with portland cement to increase strength, reduce permeability, improve resistance to chemical attack and inhibit rebar corrosion.

SLAG Cement

In recent years, "slag cement" has been commonly used to refer to either combinations of Portland cement and ground slag or to the ground slag alone. It is possible at times to find varying opinions as to the proper classification for the latter - cement or mineral admixture. "Slag cement" is also used at times to refer to a very specific material, as it is in ASTM Standards C2191 and C5952. In C219, slag cement is defined as a "blend of granulated blast-furnace slag and hydrated lime in which the slag constituent is more than a specified minimum percentage." C595 gives the minimum percentage applicable in that particular specification.

Bashundhara Group is the most renowned and a pioneer name in the Cement Industry of Bangladesh in the last two decades from its inception in 1992. This company proves its superiority by continuous advancement in the technologies and qualities of product. Bashundhara Group continuing its advancement by introducing world's most advanced Vertical Roller Mill (VRM) and slag base cement to ensure highest grinding and superior quality of product.

Slag cement is used in virtually all concrete applications:

●  Concrete pavements

●  Structures and foundations

● Mass concrete applications, such as dams or retaining walls

●  Precast and pre-stressed concrete

● Concrete exposed to harsh environments, such as wastewater treatment and marine applications

●High-performance/high-strength concrete, such as high-rise structures or 100-year service life bridges

● Slag cement is also used in non-concrete applications such as soil-cement and hazardous waste solidification

● Improved Project Life Cycle

Improve hardened properties of concrete

Slag cement improves many of the strength and durability properties of hardened concrete.  Slag cement is a hydraulic binder that, like portland cement, reacts with water to form cementitious material (calcium-silicate hydrate or CSH).  It also, similar to a pozzolan, consumes by-product calcium hydroxide from the hydration of Portland cement to form additional CSH.  The resulting cement paste is stronger and denser, thus improving the concrete.

●Improved compressive and flexural strength  

● Reduced permeability, and resistance to chloride intrusion and corrosion

● Ability to mitigate moderate to severe sulfate attack

● Ability to mitigate alkali-silica reaction with reactive aggregates

● Reduced thermal stress in mass concrete through lower heat generation

Resistance to permeability and corrosion

Low concrete permeability is essential for long-term durability, especially with regard to corrosion resistance of reinforcing steel.  The additional CSH formed and denser cement paste in slag cement concrete reduce pore size and lower concrete permeability, often by several orders of magnitude.  Low permeability reduces the ingress of harmful substances (such as chlorides and sulfates) and the availability of water to catalyze harmful chemical reactions within concrete. It also dramatically lowers the rate of chloride ion diffusion and carbonation, thus significantly enhancing the corrosion protection offered by the concrete to the reinforcing steel.

Reduce heat and cracking in mass concrete

Mass concrete applications require limitations on the temperature differential between the surface and center of concrete to guard against thermal cracking.  Slag cement reduces the rate of heat rise in proportion to its quantity in a mixture.  Generally, high replacement rates are required (50 to 80%) to meet low heat of hydration requirements for mass applications.

Mitigate alkali-silica reaction

ASR occurs when the alkalis in portland cement react with certain reactive aggregates to form an expansive gel that causes the concrete to crack, swell and prematurely deteriorate. Slag cement mitigates ASR by reacting with the alkalis in portland cement and making them unavailable for reaction.  It also lowers the permeability of the concrete, reducing the water available for reaction and, in some cases, lowers the total alkali content of the cement paste.

Improve plastic and early age properties of concrete

Slag cement geneally improves workability, finishability and pumpability of plastic concrete.  It may provide a small decrease in water demand. Slag cement will tend to increase time of initial set, which is often a benefit in warm weather.  In cooler weather, accelerators, heated materials or lowering the percentage of slag cement in a mixture (as a portion of cementitious material) can be employed to decrease times of set.  Early age strengths (through 7 to 14 days) of slag cement tend to be lower while later age strengths will be higher.

Environmental benefits

Production of slag cement creates a value-added product from a material—blast furnace slag—that otherwise might be destined for disposal. Not only does the making of slag cement lessen the burden on landfills, but it also reduces air emissions at steel plants through the granulation process (as compared to the traditional air cooling process).  Use of slag cement in concrete reduces the environmental impact of concrete by:

●Reducing greenhouse gas emissions by eliminating approximately one ton of carbon dioxide for each ton of portland cement replaced.

●Reducing energy consumption, since a ton of slag cement requires nearly 90% less energy to produce than a ton of portland cement.

●Reducing the amount of virgin material extracted to make concrete.

●Reducing the “urban heat island” effect by making concrete lighter in color thus reflecting more light and cooling structures and pavements with exposed concrete.

Considering versatile benefits of slag cement, Bashundhara Group launched slag based PCC cement in market. Cement Sector, Bashundhara Group, over the last two decades maintain highest quality cement and proved its consistency again and again by supplying cement to the most iconic projects of Bangladesh. Projects like-Padma Multipurpose Approach Road Project, Padma Main Bridge Project, Padma River Training Projects, Seventh China Bangladesh Friendship Bridge, Mogbazar Flyover, Mega Power Plant Projects, and Others Government Residential/Commercial projects are using Bashundhara Cement exclusively which proves its superiority over others.

Bashundhara Group with its continuous product improvement, modification, technological improvement and years of experience, ensures customers satisfaction and the group is fully committed to sustain this reputation in the future.

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Engr. Saroj Kumar Barua

Deputy General Manager-Technical Support

Cement Sector, Bashundhara Group

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