When slag, a by-product of the iron-making process, is quenched with water and rapidly chilled, it forms a glassy granulated material of sand-like consistency. Because of its high calcium silicate content, it has excellent cementitious properties. When finely ground and combined with a suitable activator, slag sets in a manner similar to Portland cement.

Four important methods for use of granulated slag, where it maintains its separate identity, and is a cementitious component, are: Ground glassy slag is mixed with hydrated lime as the activator; the glassy slag is ground with burned gypsum or anhydrite and small amounts of Portland cement or lime; ground glassy slag and Portland cement are combined in various proportions to form Portland blast-furnace slag cements; or, separately ground slag may be combined with the Portland cement and other ingredients of the mortar or concrete at the mixer.

The primary effect of ground slag admixtures on the properties of freshly mixed concrete is to provide better workability and finishability. As a result, lower water-cement ratios may be used in many cases. Properly proportioned slag-Portland cement concretes have the following properties compared to regular Portland mixes: Higher ultimate strengths with a tendency toward lower early strengths; higher ratio of flexural to compressive strengths; improved refractory properties; lower coefficients of variation in strengths; improved resistance to sulfates and seawater; lowered expansions from alkali-silica reactions; lower temperature rise due to lower heat of hydration; better finish and lighter color; equivalent durability in freezing and thawing; and decreased porosity and chloride penetration.