Technical, economical and environmental benefits of blastfurnace cement

What is granulated blast furnace slag?

Blastfurnace slag is a byproduct of the iron and steel manufacturing process. In the production of steel, iron ore – a mixture of oxides of iron, silica, and alumina – together with a fuel consisting of coke, natural gas, oxygen and pulverised coal and also limestone as a fluxing agent, are fed into a blastfurnace, which consists of a large vertical chamber through which large volumes of hot air are blasted. Generally a blastfurnace operates on a continuous basis and produces approximately 250 – 300 kg of slag/t of iron produced.

The chemical reaction results in two products: molten iron metal and molten blastfurnace slag. At this stage molten blastfurnace slag can be converted into two different products; (1) blastfurnace slag aggregate (BFS) or (2) granulated blastfurnace slag (GBFS). To produce GBFS, liquid slag must be rapidly quenched using large volumes of high-pressure fresh water sprays, instantly cooling the molten material to produce a sandy material with a maximum size of 6 mm.

Typically BFS is then collected from the manufacturing source or blastfurnace granulation bays and transported to the nominated processing facilities. The GBFS is de-watered and dried to reduce the moisture content before further processing (grinding) in a traditional cement clinker grinding plant, then ground to a fine powder. At this point the ground granulated blastfurnace slag or GGBFS is stored in silos awaiting delivery to customers.

The reactive properties of GBFS depend upon a number of factors, including:

• Temperature of slag before granulation.
• Chemical composition.
• Condition during granulation (flowrates and temperature).
• Fines content.

GBFS is mostly used in the production of quality-improved slag cement, namely blastfurnace cement (BFC).

Blast Furnace Cement (BFC)

Blast furnace cement (BFC) is created through the addition of granulated blastfurnace slag with Portland cement clinker and gypsum. Three types are available, namely A, B and C class, which meet Standard EN 197-1:2000.

Technical benefits of using BFC

BFC is used to make durable concrete structures around the world. BFC has been widely used in Europe and increasingly in the US and in Asia (particularly in Japan and Singapore), extending the lifespan of buildings from 50 years to 100 years.

Concrete containing BFC is less permeable, has lower hydration, higher ultimate compressive strengths, is resistant to sulfate-acid attack and aggressive chemicals, resistant to many forms of deleterious attack, to alkali-silica reaction and has better workability and finish ability than normal concrete.

Concrete made with BFC sets more slowly than concrete made with ordinary Portland cement, depending on the amount of GGBS in the cement, 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 also means that quick setting is required.

The use of BFC significantly reduces the risk of damage caused by alkali-silica reaction (ASR), provides higher resistance to chloride ingress, reducing the risk of reinforcement corrosion, and provides higher resistance to attacks by sulfate and other chemicals.

Durability

BFC has now effectively replaced sulfate resistant cement (SRC) because of its superior performance and greatly reduced cost compared to SRC. BFC is more resistant against sulfate attack than Portland cement (OPC) and sulfate resistant cement (SRC) according to tests performed by Quality Control Laboratory of Turkish Cement Manufacturers’ Association (TCMA).

28-day strength loss due to the effects of sulfate was observed at 30% for OPC, at 20% for SRC and at just 18% for BFC.

Instances of chloride attack occur in reinforced concrete in marine environments and in road bridges where the concrete is exposed to splashing from road deicing salts. In most projects BFC is now specified in structural concrete for bridge piers and abutments for protection against chloride attack.

Test results of pressure strength, chloride and water penetration (for 28-day aged concrete)

BFC is also routinely used to limit the temperature rise in large concrete pours. The more gradual hydration of BFC generates both lower peak and less total heat than Portland cement. This reduces thermal gradients in the concrete, which prevents the occurrence of micro cracking. This can weaken the concrete and reduce its durability.

In contrast to the grey stone colour of concrete made with Portland cement, the whitish colour of BFC permits architects to achieve a lighter appearance for exposed fair-faced concrete finishings, at no extra cost. BFC 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. BFC 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 BFC has a higher ultimate strength than concrete made with Portland cement. It has a higher proportion of the strength-enhancing calcium silicate hydrates (CSH) than concrete made with Portland cement only, and a reduced content of free lime, which does not contribute to concrete strength. Concrete made with BFC continues to gain strength over time, and has been shown to double its 28-day strength.

Economical and environmental benefits

In recent years, there has been a significant growth in the production and sale of BFC. The manufacture of BFC requires 75% less energy than that needed for the production of OPC, making it cheaper to produce and a more viable option in a downturn economy.

A comparison of OPC and BFC savings is shown in the following tables.

Reducing cement’s clinker content by replacing with blastfurnace slag is one way to reduce cement’s environmental footprint and therefore meet increasingly stringent regulations.

References

• U.S. Federal Highway Administration. "Ground Granulated Blast-Furnace Slag".
• Civil and Marine Company. "Frequently Asked Questions".
• "Ground Granulated Blast furnace Slag (GGBS)". Archived from the original on 2007-01-22.  
• Construct Ireland. "Ground Granulated Blast furnace Slag (GGBS)". 

Written by Ismail Yüksek, Iskenderun Plant Manager, OYAK Adana Cement Inc., Turkey.

Read the article online at: https://www.worldcement.com/europe-cis/09072012/benefits_of_blastfurnace_cement_1159/