Ground granulated blast-furnace (GGBF) slag is a nonmetallic byproduct derived from blast furnaces during iron making. It is rapidly chilled into granules and processed to produce glassy particles that are dried and ground to a fineness exceeding portland cement.

Three performance-based classifications, Grades 80, 100, and 120, have been identified in ASTM C 989, the material specification for GGBF slag. These grades correspond to the percentage of compressive strength achieved by a slag-modified portland cement mortar cube over a portland cement mortar cube.

ACI 233R, Ground Granulated Blast-Furnace Slag as a Cementitious Constituent in Concrete, reports the following changes in concrete using GGBF slag:

• Improved workability and placeability of fresh concrete
• Reduced bleeding. This raises the risk of premature finishing of concrete slabs unless allowed for by the Contractor.
• Extended setting time of concrete.
• Reduced early age strength. Grade 120 reduced from 1 to 3 days, Grade 100 reduced from 1 to 21 days, but subsequent strength gain exceeds that of concrete made from portland cement. Grade 80 results in early age and long-term strength reduction.
• Must cure for as long as possible.
• Reduced concrete permeability with corresponding improvement in durability.
• Increased sulfate resistance
• Reduced expansion damage from alkali-silica reactions with aggregates.