Ground granulated blastfurnace slag
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.