Steel slag aggregate
Processed slag from steelmaking used in place of part of the natural stone aggregate in bituminous layers. It is compatible with existing pavement testing and specification frameworks.
Research & Innovation · Case Study
Building stronger, lower-impact highways with steel slag
How AIPL evaluated industrial by-products as road-building materials, and tested processed steel slag in place of natural stone aggregate on a national highway project in Odisha.
Technical feasibility study · Field validation pending
Project
NH-53 / NH-149 / NH-55, Odisha
Material
Processed steel slag aggregate
Application
Bituminous pavement layers
Stage
Laboratory validation complete
01 · The Challenge
India is building roads faster than ever, and that takes enormous resources
Conventional road construction depends on quarried stone and petroleum bitumen, much of it imported. At the same time, steel plants produce millions of tonnes of slag each year that mostly ends up stockpiled. We asked whether one problem could help solve the other.
6.67M
km road network, the second largest in the world
~34 km/day
of national highways built in FY 2023–24 (12,349 km)
~8.8 Mt
of bitumen used in FY 2023–24, about half of it imported
19 Mt / yr
of steel slag generated, projected to reach 60 Mt by 2030
02 · What We Studied
Three sustainable technologies, one common yardstick
We reviewed published research and assessed three options that could cut virgin-resource use or emissions on AIPL projects. Each was scored on the same practical criteria, because a technology only matters if it can be built, approved and paid for.
Sugarcane molasses bio-binder
Partial replacement of bitumen with raw molasses. A 2023 laboratory study found 25% replacement optimal, with about 21% lower binder cost. It still needs field trials before highway use.
Solar-powered site infrastructure
Solar PV and battery storage for site offices, labour camps and lighting. Technically proven; the main constraint is recovering the capital cost within a project's duration.
Technical feasibility
Cost & lifecycle economics
Environmental benefit
Supply-chain availability
Regulatory compliance
Constructability
Scalability
03 · Why Steel Slag
The strongest balance of performance, practicality and impact
Steel slag offered the best combination for a near-term pilot. It turns an industrial by-product into a pavement resource and reduces quarrying, and it can be tested and approved within the engineering frameworks our teams already use.
Its main risk, variability between sources and potential expansion from free lime, can be managed through processing, ageing and source-specific testing.
Local availability
Steel plants near the project make supply and transport practical.
Fits existing specifications
Can be designed and checked with standard Marshall mix-design
methods and MoRT&H criteria.
Resource and waste benefit
Replaces virgin stone with a material that would otherwise
be stockpiled or landfilled.
Manageable risk
Quality issues are controllable through testing rather than
being fundamental unknowns.
04 · The AIPL Pilot
Testing slag on a live national highway project
Before any material goes into the road, it has to meet the same mix-design requirements as conventional aggregate. We ran a side-by-side laboratory investigation comparing a steel slag mix against a conventional stone aggregate mix.
1
Characterised the slag: specific gravity, water absorption, impact value, flakiness and elongation.
2
Designed both mixes with VG-40 bitumen, compacted at 75 blows per face.
3
Compared stability, flow and volumetrics against MoRT&H / project requirements.
Project Details
Project
Four-laning, Godibandha – Balla Har Chhak –
Banarapal – start of Anugul Bypass
Stretches
NH-53 Km 314.130–321.035 · NH-149 Km 75.200–84.000 ·
NH-55 Km 84.000–85.750
Model
NH(O), Hybrid Annuity Mode, Odisha
Slag source
Fulapada Stone Quarry / TATA Steel & Power source
identified for the project
Binder
VG-40 bitumen
Control
Conventional stone aggregate mix
05 · Key Results
The slag mix met every specification, using less bitumen
≈13% less
bitumen needed: 4.54% vs 5.23% for the conventional mix
2,130 kg
Marshall stability, more than twice the 900 kg minimum
5 of 5
volumetric and flow criteria within specified limits
0.37–0.61%
water absorption of slag, vs 1.50–1.94% for stone
| Parameter | Steel slag mix | Stone aggregate mix | Requirement | Slag mix |
|---|---|---|---|---|
| Bitumen content (%) | 4.54 | 5.23 | — | — |
| Marshall stability (kg) | 2,130 | 2,370 | Min. 900 | Pass |
| Marshall flow (mm) | 3.50 | 3.42 | 2.0–4.0 | Pass |
| Air voids, VIM (%) | 3.73 | 4.62 | 3–5 | Pass |
| VMA (%) | 13.40 | 17.17 | Min. 12.0 | Pass |
| VFB (%) | 72.19 | 73.05 | 65–75 | Pass |
| Fines / bitumen ratio | 0.98 | 0.85 | 0.6–1.2 | Pass |
An honest reading:
the slag mix's stability was about 10% lower than the stone control mix,
so this trial shows slag is adequate and specification-compliant,
not stronger. The lower binder content is promising but needs confirming
through repeated job-mix trials.
06 · What We Learned
Practical lessons for using industrial by-products in roads
01
Every slag source is different
Properties depend on the steelmaking process, cooling, ageing and stockpiling. Results apply to the tested source and must be re-checked for any other.
02
It is a controlled substitution, not a swap
Slag must be processed, graded and tested before use, with attention to free lime, volumetric stability and binder compatibility.
03
Published results don't automatically transfer
Our stability result differed from some published studies, which is why project-specific testing matters.
04
Economics need the full picture
Real savings must account for processing, transport, screening, testing and handling, not just lower binder use.
08 · Research Team
The people behind the study
SG
09 · Download Full Brief
Read the complete research report
Full methodology, technology assessments, laboratory data, limitations and references.
Sources
- Ministry of Road Transport and Highways. (2024, January 5). Year end review 2023 [Press release]. Press Information Bureau. pib.gov.in
- Press Trust of India. (2024, April 9). Morth builds over 12,000 km of highways in FY24, 2nd highest so far. Business Standard. business-standard.com
- Argus Media. (2024, August 13). India's bitumen demand to rise by 14pc in FY2024-25; CW Team. (2024, August 8). Govt approves 35% bio-bitumen mixing. Construction World. argusmedia.com
- Ministry of Steel. (2023, July 19). Steel slag road technology fulfilling the Prime Minister's 'Waste to Wealth' mission [Press release]. Press Information Bureau. pib.gov.in
- Saboo, N., Sukhija, M., Mehta, D., Haswanth, K., Srivastava, A., & Patil, A. (2023). Use of raw sugarcane molasses as a partial replacement of asphalt binder. Construction and Building Materials, 369, 130541. doi.org
- Ministry of Road Transport and Highways. (2013). Specifications for road and bridge works (5th rev.). Indian Roads Congress.
This case study summarises a technical feasibility study.
Pilot results are from laboratory Marshall mix-design testing of
material from a single source and have not yet been validated through
field trials or long-term performance monitoring. Use of alternative
materials remains subject to approval by the Engineer/Authority and
compliance with applicable MoRT&H and IRC specifications.