Ground granulated blastfurnace slag (GGBS) is a hydraulic binder, i.e. a cement, which has been known and used for 150 years. It improves the quality and durability of concrete, and its production is virtually CO2-free. Yet its many advantages in producing sustainable, high-quality concrete remain underappreciated and underused.
In an increasingly resource-constrained and environmentally conscious world, all that is about to change.
Quality, sustainable concrete
Blastfurnaces produce pig iron, together with a slag by-product: a tightly controlled and stable material with the same constituents, though in different proportions, to normal cement. If the slag is vitrified by rapid quenching as it comes from the blastfurnace, its cementitious qualities are preserved.
Dried and ground to a fine powder, GGBS can be used to make quality, sustainable concrete. To ensure its activation, GGBS is most often used with normal cement. It will typically replace 30 - 70% of cement on an equal weight basis.
Reduction in CO2 emissions
The manufacture of normal cement (CEM I of EN 197) results in the emission of 930 kg of CO2/t of cement (British Cement Association, 2009): approximately 50% from decarbonation of the limestone raw material (process emissions), 40% from fossil fuel consumption, and 10% from generating the electricity used in the process.
GGBS manufacture typically releases 35 kg of CO2/t of GGBS: less than 4% of the carbon footprint of normal cement.
Concrete made with GGBS will have a high solar reflectance: studies in the US have shown increases of 20% in reflection of sunlight by concrete with GGBS. This will reduce the “heat island” effect in urban developments, as well as having other beneficial effects (reduced need for artificial lighting at night, safer roads from better visibility). Significantly, the reflected sunlight is not infrared radiation, and so will not be trapped by the greenhouse effect of the earth’s atmosphere.
Durability is essential to the long service life of concrete. In practice, concrete will deteriorate over time. The factors driving this deterioration can be internal (e.g. alkali-silica reaction) or due to external aggressive environments (e.g. sulfate attack, or the effects of chlorides on reinforced concrete).
GGBS substantially improves the ability of concrete to resist deterioration from all major threats to long service life. Requiring the use of GGBS is now established best practice where long service lives must be achieved, even in the most aggressive environments. GGBS concrete will even provide better fire resistance.
GGBS in concrete
GGBS can be used in concrete as GGBS or as CEM IIIs (combinations of cement and GGBS allowed under EN 197). There is no technical basis under which to prefer one option or the other: once in concrete, the GGBS behaves the same, irrespective of whether or not it was previously mixed with cement. Nevertheless, opinions are divided on the best approach. In the US and in certain EU countries (the UK, the Netherlands, Belgium and Ireland) GGBS is widely used. Elsewhere in the EU, CEM IIIs dominate.
Using GGBS has two advantages over CEM IIIs: the concrete manufacturer can optimise the proportion of GGBS according to the technical/environmental requirements to be met; and GGBS shipped directly to the concrete plant will have lower embodied CO2 (avoiding energy use in additional transport to/from the cement plant and mixing in a cement plant). For the lowest embodied CO2, GGBS use is to be preferred.
For the full report from Michel Pigeat, Ecocem France, please see the September 2009 issue of World Cement