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Ground granulated blastfurnace slag (GGBS) is the slag from iron producing blastfurnaces that is rapidly quenched in water and then ground into a powder. Chemically it is similar to, but less reactive than, Portland cement (PC).
When mixed with water it will hydrate in a similar way to Portland cement. It is always used in combination with Portland cement, typically in the range 60% PC and 40% GGBS to 30% PC and 70% GGBS, depending on the application.
The blends can be supplied as pre-blended factory produced cements, or can be formed in the mixer by adding Portland cement and GGBS each from its own silo.
Concretes containing GGBS/PC blends are slower reacting than pure PC concretes, but may have improved durability.Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of ggbs on the rates of carbonation and chloride ingress.
Acknowledgement: The Concrete Society
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