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.