How is GGBS used?
GGBS (Ground Granulated Blastfurnace Slag) is used all over the world as a direct replacement for Portland cement. It is added to the concrete mixer along with ordinary cement, aggregates and water.
The normal ratios and proportions of aggregates and water to cementitious material in the mix remain unchanged. Mixing times are the same as for ordinary cement. Both wet mixing and dry mixing processes can be used for making concrete with GGBS.
What is the substitution rates of GGBS in concrete?
Usually, replacement rates for GGBS vary from 30% to up to 85%, depending on the application and technical requirements. Typically 50% is used in most instances. Higher replacement rates up to 85% are used in specialist applications such as in aggressive environments and to reduce heat of hydration.
According to the EU code “EN-197-1-2000 Cement. Composition, specifications and conformity criteria for common cements“, slag in Blastfurnace cement CEM III/C can be used at a substitution rate as high as 95%.
Below are some typical Substitution Rates
50% GGBS is used for most mixes.
30% to 50% GGBS primarily depending on concrete curing conditions.
Mass concrete typically has at least 70% GGBS for temperature control. Concrete in aggressive environments usually contains from 50% to 70% GGBS for enhanced durability performance.
Better performance of slag concrete exposing to high temperatures
Concrete made with GGBS cement is much better than concrete made with 100% OPC at maintaining its compressive strength when exposed to high temperatures.
OPC specimens heated to 400 °C or above have been shown to exhibit severe cracking to the point of disintegration after a few days. However, it was found that concrete made with blends of 35%, 50% and 65% GGBS performed much better. In the GGBS specimens there was no visible cracking after exposure to the higher temperatures. The concrete with 100% OPC degraded to powder over the following year, while the concrete made with GGBS maintained its strength over the same period.
Further studies showed that after exposure to 900 °C concrete with 0% GGBS maintained 6% of its original strength, concrete with 30% GGBS maintained 54% of its original strength and concrete with 70% GGBS maintained 70% of its original strength.
Date source: Ecocem.
How does slag improve the resistance to fire damage?
Why concrete made with GGBS has better resistance to fire damage? It is generally agreed that above 400 °C Ca(OH)2 which is the hydration products of OPC decomposes into CaO and H2O, then on cooling CaO and left H2O rehydrates into Ca(OH)2 which needs more space than CaO causing the concrete to crack and degrade. With the addition of GGBS into concrete, hydration product Ca(OH)2 is generally reduced compared with concrete made with 100% OPC, thus improving the performance of resisting fire damage.
Reference: Ecocem report–Technical Performance