Ground granulated blast furnace slag (GGBS or GGBFS) is obtained by quenching molten iron slag (a by-product of iron and steel-making) from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder.

Ground granulated blast-furnace slag is used to make durable concrete structures in combination with ordinary portland cement and other pozzolanic materials. Ground granulated blast-furnace slag has been widely used in Europe, and increasingly in the United States and in Asia, extending the lifespan of buildings from fifty years to a hundred years.

Two major uses of Ground granulated blast furnace slag are in the production of quality-improved slag cement,it namely Portland Blast furnace cement (PBFC) and high-slag blast-furnace cement (HSBFC), with Ground granulated blast-furnace slag content ranging typically from 30 to 70%; and in the production of ready-mixed or site-batched durable concrete.

Concrete made with Ground granulated blast-furnace slag cement sets more slowly than concrete made with ordinary Portland cement, depending on the amount of GGBS in the cementitious material, but also continues to gain strength over a longer period in production conditions. This results in lower heat of hydration and lower temperature rises, and makes avoiding cold joints easier, but it may also affect construction schedules where quick setting is required.

How Ground granulated blast-furnace slag cement is used

Ground granulated blast-furnace slag cement can be added to concrete in the concrete manufacturer's batching plant, which is along with Portland cement, aggregates and water. The normal ratios of aggregates and water to cementitious material in the mix remain unchanged. Ground granulated blast furnace slag is used as a direct replacement for Portland cement, on a one-to-one basis by weight. Replacement levels for Ground granulated blast-furnace slag vary from 30% to up to 85%. Typically 40 to 50% is used in most instances.

The use of Ground granulated blast-furnace slag in addition to Portland cement in concrete in Europe is covered in the concrete standard EN 206:2013. This standard establishes two categories of additions to concrete along with ordinary Portland cement:Ground granulated blast-furnace slag cement was in the latter category. As Ground granulated blast-furnace slag cement it is slightly less expensive than Portland cement, concrete made with Ground granulated blast-furnace slag cement will be similarly priced to that made with ordinary Portland cement.