What is the minimum fineness required for GGBS by IS 16715?

A minimum specific surface area of 320 m²/kg is required (Table 2).

What is the maximum permissible chloride content in GGBS?

The maximum chloride content is 0.1 percent by mass (Table 1).

What is the Slag Activity Index?

It is a measure of the reactivity of GGBS, determined by comparing the compressive strength of a mortar made with a GGBS-cement blend to a reference mortar made with only cement (Clause 4.2.3).

Why is 'glass content' important for GGBS?

A high glass content (amorphous state) is essential for the pozzolanic reactivity of GGBS. The minimum required is 85% (Table 1).

IS 16715

: 2018

Ground Granulated Blastfurnace Slag for use in Cement, Mortar and Concrete - Specification

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This standard specifies the requirements for Ground Granulated Blast-furnace Slag (GGBS), a pozzolanic material used as a partial replacement for cement. It covers essential chemical and physical properties, such as glass content, fineness, and strength activity index, to ensure its suitability for use in cement, mortar, and concrete.

Specifies requirements for ground granulated blastfurnace slag (GGBS) for use as an active mineral admixture or cement replacement in cement, mortar, and concrete.

Quick Reference Values

Minimum Glass Content85%

Maximum Magnesium Oxide (MgO) Content17.0%

Maximum Sulphide Sulphur (S) Content2.0%

Maximum Chloride Content0.1%

Minimum Fineness (Specific Surface)320 m²/kg

Minimum 7-day Slag Activity Index75%

Practical Notes

Using GGBS generally slows down the setting time and early strength gain of concrete, but significantly improves long-term strength and durability, especially against sulphate and chloride attacks.

The replacement percentage of cement with GGBS typically ranges from 30% to 70%, depending on the application and desired properties. Higher replacement levels are common in mass concrete to control heat of hydration.

Always obtain a manufacturer's test certificate for each batch of GGBS, paying close attention to glass content and activity index, which are critical indicators of its reactivity.