IS 16715 : 2018Ground Granulated Blastfurnace Slag for use in Cement, Mortar and Concrete - Specification

IS 16715:2018 is the specification for Ground Granulated Blast-furnace Slag (GGBS) for use in cement, mortar and concrete — the standard that lets GGBS be used as a *separately-added* supplementary cementitious material at the concrete mixer (as opposed to factory-blended Portland slag cement). It is central to durable and low-carbon concrete design in India.

It is read with the concrete-materials stack:

• IS 456 — plain and reinforced concrete (permits and governs mineral admixtures/SCMs)
• IS 10262 — concrete mix proportioning (with SCM)
• IS 455 — Portland slag cement (the factory-blended route) · IS 269 — OPC (the clinker partner)
• IS 3812 — fly ash · IS 16714 — GGBS-related references; IS 9103 — chemical admixtures

GGBS is the glassy, rapidly-quenched, finely-ground by-product of iron-making. IS 16715 fixes the quality that makes it a reliable cement replacement:

• Chemical limits — glass content, the slag activity/basicity ratio, magnesia, sulphide and chloride limits
• Fineness and the slag activity index (strength of a GGBS–OPC mortar vs the OPC control at 7/28 days) — the headline performance measure
• Soundness and consistency

Why engineers specify it: at typical replacement levels (commonly 30–65%, up to ~70% for mass/marine work) GGBS gives lower heat of hydration (mass-concrete crack control), much lower permeability and chloride diffusivity (marine/coastal durability), better sulphate resistance, and a large embodied-carbon reduction — at the cost of slower early-age strength gain and a greater sensitivity to curing and cold weather.

Brief: a coastal raft foundation, M35, durability-critical (severe/very-severe exposure), large pour (heat a concern).

Step 1 — replacement level: select 50% GGBS (cement = 50% OPC + 50% GGBS) — high enough for low permeability + low heat, not so high that strength/curing risk is unmanageable.

Step 2 — mix design (IS 10262): proportion total cementitious for M35 at the IS 456 durability minimums for the exposure (min cementitious content, max w/c) — durability, not strength, governs the floor here.

Step 3 — strength timeline: design/strip and load on 56-day (not 28-day) strength where the structure allows — GGBS concrete keeps gaining strength well beyond 28 days; specify the acceptance age explicitly.

Step 4 — thermal: 50% GGBS markedly cuts peak hydration temperature → lower thermal-gradient cracking risk in the thick raft (still check ΔT and use the IS 456 thermal precautions).

Step 5 — curing: specify extended moist curing (≥ 10–14 days) — GGBS concrete is curing-sensitive; under-curing kills the durability benefit you bought it for.

1. Judging GGBS concrete on 28-day strength. It gains strength later — use 56-day acceptance where appropriate, or you will wrongly reject sound concrete (or over-cement it).

2. Under-curing. The permeability/durability gain is entirely curing-dependent; the common 3-day curing that 'works' for OPC leaves GGBS concrete porous — specify and enforce extended curing.

3. High replacement in cold weather / fast-cycle work. Slow early strength + low temperature = stripping and early-loading problems; cap the replacement level for fast-track or winter pours.

4. Double-counting with slag cement. Don't add GGBS to IS 455 Portland slag cement and assume the same dosing logic — start from OPC (IS 269) when using separately-added GGBS.

5. No source quality control. Activity index, fineness and glass content vary by source — accept GGBS to IS 16715 per supply, not on a one-time certificate.

• IS 456 — plain and reinforced concrete (SCM use, durability tables)
• IS 10262 — concrete mix proportioning
• IS 455 — Portland slag cement (factory-blended slag route)
• IS 269 — OPC (clinker partner for separately-added GGBS)
• IS 3812 (parts) — pulverised fuel ash (the other major SCM)
• IS 9103 — concrete admixtures · IS 516 / IS 9013 — strength tests

IS 16715:2018 is a current, important standard — it formalised separately-added GGBS at the batching plant, which is now mainstream for durable infrastructure (metro, marine, bridges, large rafts) and is a primary lever for low-embodied-carbon concrete in green-rated and net-zero-aligned projects. Used well it is one of the best value durability investments in concrete; used carelessly it produces under-strength, porous concrete.

The single practitioner rule that matters: GGBS concrete is a curing-and-age contract. If you specify high GGBS replacement, you must also specify the acceptance age (often 56 days), extended moist curing, and cold-weather/early-loading limits, and QA the slag source to IS 16715 per supply. Designers who change the binder but leave the curing spec and 28-day acceptance untouched routinely turn a durability upgrade into a defect.