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GROUND GRANULATED BLAST - FURNACE SLAG
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.
The optimum percentage of GGBF slag used in concrete to produce the greatest increase in compressive strength is 50%. For high-sulfate resistance, a minimum of 50% replacement of portland cement is necessary. Amounts less than 50% are recommended when early strength of concrete is needed. Concrete made from GGBF slag is a lighter color.
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