Slag

Summary

Slag cement is blast furnace slag quenched in water or steam, dried, and ground to a fine powder. It replaces 30 to 85 percent of portland cement in concrete, improving durability and lightening color, with slower early setting.

Cols

Image

GGBFS: ground granulated blast-furnace slag
Slag cement: the name used for GGBFS in North American concrete specifications
Supplementary cementitious material: cement replacement such as slag, fly ash, silica fume

Composition and production

Raw materials

Molten iron slag from a blast furnace
CaO 30 to 50 percent, SiO2 28 to 38, Al2O3 8 to 24, MgO 1 to 18, MnO also present
Glass content 90 to 100 percent for slags suitable for blending with portland cement

Manufacturing process

Slag must be quenched below 800°C (1,470°F) to prevent merwinite and melilite from crystallizing
Dried and ground to a fine powder

Properties

Physical

Color: near white, versus stony grey for portland cement concrete; smoother, more defect-free surface

Durability

Reduced risk of alkali-silica reaction; better resistance to chloride ingress and sulfate attack
Lower peak temperature and less total heat of hydration than portland cement
Continues to gain strength over time

Standards

Material specifications

ASTM C989: slag cement for use in concrete and mortars (C989/C989M-25)

Uses in buildings

Building elements

Replaces 30 to 85 percent of cementitious material in all concrete elements, typically 40 to 50 percent, one for one by weight
About 50 percent for chloride protection; 50 to 70 percent (up to 85) for lighter finishes; 50 to 60 percent to prevent efflorescence

Pros and cons

Pros

Slightly less expensive than portland cement
Sulfate resistance and lower heat of hydration
Lighter, more uniform finish

Cons

Sets more slowly, which may affect schedules needing quick setting

Sustainability

Recycled content and end of life

By-product of iron making; recognized by LEED and BEAM Plus for improving project sustainability

Manufacturing and cost

Main manufacturers and suppliers

Trade association: Slag Cement Association.

Sources

Wikipedia: Ground granulated blast-furnace slag, https://en.wikipedia.org/wiki/Ground_granulated_blast-furnace_slag
ANSI: ASTM C989/C989M-25, https://webstore.ansi.org/standards/astm/astmc989c989m25
Wikimedia Commons: Pieces of slag from glazing process MET vs22.1.1925b3.jpg, photo by unknown (CC0), https://commons.wikimedia.org/wiki/File:Pieces_of_slag_from_glazing_process_MET_vs22.1.1925b3.jpg