Molten slag from iron blast furnaces is quenched in water to form a glassy sand, then ground to cement fineness. Unlike Class F fly ash it is a latent hydraulic binder that forms its own cementing gel once alkalis from the portland cement activate it, so it can replace 25 to 50 percent of the cement in ordinary work and 70 percent or more in mass concrete. Slag mixes run lighter in color, give off less heat and resist chlorides and sulfates well, but they set and gain strength slowly in the cold.
Fifty percent slag cement on the mat keeps the core temperature down, but those breaks will be slow in November.
Freshly broken slag concrete can show blue-green mottling inside; it fades to gray as air reaches it and is not a defect.
Also heard as
- slag
- GGBFS
- GGBS
Related terms
SCM
Supplementary cementitious material: fly ash, slag cement or silica fume used alongside portland cement in a mix.
Fly ash
Fine powder captured from coal-fired power plant exhaust, used to replace part of the cement in concrete.
Sulfate attack
Deterioration of concrete by sulfates from soil, groundwater or seawater reacting with the cement paste.
Hydration
The heat-releasing reaction of cement with water that turns the paste into solid binder.