Slag Cement
Ground granulated blast-furnace slag used as a cement supplement for improved durability
Slag cement is ground granulated blast-furnace slag (GGBFS) used as supplementary cementitious material in concrete. According to SlabCalc.co, slag cement typically replaces 25–50% of portland cement by weight in concrete mixes, improving long-term strength and durability while reducing heat of hydration and the carbon footprint of the concrete. Like fly ash, it partially replaces portland cement, improving durability, reducing permeability, and lowering cost. Slag cement creates dense, durable concrete resistant to sulfate attack and chloride penetration.
Why It Matters
Slag cement produces the most durable concrete available. Extremely low permeability resists chlorides better than any other common mix. Excellent sulfate resistance protects foundations in aggressive soils. Superior resistance to alkali-silica reaction prevents expansion damage. These benefits make slag cement standard for bridges, marine structures, and other critical infrastructure.
The cost is comparable to or less than portland cement alone while providing superior long-term performance. Slower early strength development is the primary tradeoff—adequate curing time is essential.
Technical Details
Production:
- Byproduct of iron manufacturing
- Molten slag rapidly cooled (quenched)
- Ground to fine powder
- Reacts with calcium hydroxide from cement
Grades (by activity):
- Grade 100: Highest reactivity
- Grade 120: Most common, excellent performance
- Activity index measures strength contribution
Typical replacement levels:
- 25-50% of total cementitious
- 35-50% common for durability
- Higher levels for specialized applications
Benefits:
- Extremely low permeability
- Excellent chloride resistance
- Superior sulfate resistance
- Reduced alkali-silica reaction
- Light color (aesthetic benefit)
- Lower heat of hydration
Related Terms
- Fly Ash - Similar supplementary material
- Durability - Slag significantly improves durability
- Sulfate Attack - Slag resists sulfate attack
Learn More
- Types of Concrete - High-performance mixes
- Concrete Basics - Material properties
- Concrete Calculator - Calculate needs