Ground granulated blast furnace slag

Blast furnace slag (BFS) is a by-product of iron manufacturing in the blast furnace. It is subjected to further processing to form ground granulated blast furnace slag (GGBFS). GGBFS is typically used as a supplementary cementitious material in concrete production, or as an activating agent for stabilising when combined with lime.

How is it processed?

BFS is rapidly quenched from the furnace, the heat energy contained within the slag causes it to explode and instantly form granulated blast furnace slag (GBFS). The GBFS is ground using a conventional cement clinker to form GGBFS.

Environmental, health and safety

The manufacture of the GGBFS produces no significant waste, utilising all the slag. GGBFS (or cement containing GGBFS) is handled and stored like conventional Portland cement.

GGBFS is a fine powder which can cause irritation to the eyes and respiratory system. Therefore, personal protective equipment such as safety googles, gloves, impervious clothing and suitable dust masks should be worn.

When used and disposed of as intended, there are no foreseen adverse environmental effects. In large quantities GGBFS in water will cause the pH to rise and reduce oxygen availability.

Applications

GGBFS is primarily utilised as a supplementary cementitious material in concrete production and cementitious stabilisation of soils and aggregates. These include:

general cementitious replacement, 25 – 50%
significant cementitious replacement, min 65% for long term durability
high cementitious replacement > 80% in special grout mixes
soils/subgrade stabilisation
railway concrete structures or components including sleepers, drainage pits/pipes/headwalls, panels and foundations

Specifications

Main Roads Western Australia (MRWA) and the Public Transport Authority currently has the following specifications which consider GGBFS:

Specification 820 for Concrete for structures
8880-450-020: Specification - Design of Concrete  
8880-450-021: Specification – Concrete durability 
8880-450-028: Specification – Sprayed Concrete

Benefits and challenges

GGBFS is considered to be more environmentally friendly than Portland cement. GGBFS accounts for only 17% of the CO₂ emissions compared to cement production
GGBFS enhances resistance to sulfate attack and chloride ingress, protecting steel reinforcement when used in correct proportions. It also mitigates alkali-silica reactions, which can lead to concrete degradation
On its own GGBFS has a slow hydration process, it needs an alkali activator, such as hydrated lime

Previous use in Western Australia

In concretes structures, MRWA has used blended cement comprising 32% type GP cement, 60% GGBFS and 8% percent silica fume in 50 MPa Class concrete.

GGBFS has successfully been used on Lancelin Road for stabilisation of the basecourse, where a 70/30 blend of GGBFS and quick lime was applied at 3% by mass to the upper 100 mm of a limestone basecourse.