|
Ground Granulated Blastfurnace Slag (GGBS) cement is a by-product of the steel industry. Molten slag lying on top of the molten iron in the blastfurnace comprises silicates (glass), and is the raw material for GGBS cement. The molten slag - of no use to the steel making process - is cooled and then finely ground to form GGBS cement.
GGBS cement has exactly the same chemical constituents as ordinary Portland cement, only in slightly differing proportions. Thus it can replace Portland cement by up to 85% in making ready-mixed concrete. GGBS cement is widely used in the UK and on the continent for making concrete. It has been used in the UK for over seventy years where the annual usage is now about 20% of the total cement market. In continental Europe approximately 17.7 million tonnes of GGBS cement are used annually in making concrete. The leading user of GGBS is the Netherlands, where 54% of cement used in making concrete comprises GGBS cement. Belgium (25%), Italy (27%) and Germany (14%) also have high GGBS cement usage levels.
GGBS cement is a recycled product and it replaces the use of Portland cement in concrete on a one-for-one basis. By replacing Portland cement it replaces a product that has very high CO2 emissions, has high embodied energy and involves a significant amount of quarrying. Used appropriately, GGBS cement will reduce the CO2, NOX and SO2 emissions of concrete by over 50%, it will reduce the embodied energy in concrete by over 40%, and it will significantly increase the service life of concrete, enhancing sustainable development.
How GGBS cement is used
GGBS cement is 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 cementitous 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. GGBS cement is used for its environmental benefits, improved appearance for exposed concrete and technical benefits, including resistance to sulphate and chloride attack and reduced heat of hydration.
The use of GGBS cement in concrete in Ireland is covered in the new Irish concrete standard IS EN 206-1:2002. This standard establishes two categories of additions to concrete along with ordinary Portland cement: nearly inert additions (Type I) and pozzolanic or latent hydraulic additions (Type II). GGBS cement falls in to 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
Appearance
GGBS cement is a near-white powder. It imparts a lighter, brighter colour to concrete, in contrast to the stoney grey of concrete made with Portland cement. The colour of the finished concrete is lighter, the concrete has a smoother surface finish, and the addition of GGBS to the mix prevents the formation of efflorescence.
The near-white colour 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 (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 GGBS continues to gain strength over time, and has been shown to double its 28 day strength over periods of 10 to 12 years (1).
Continue to Page 2
|
|
|
|
