- 1. Introduction
- 2. Pozzolanic & Hydraulic Mechanism
- 3. Types of SCMs & Their Characteristics
- 4. Key Performance & Quality Parameters
- 5. Benefits of Using SCMs
- 6. Common Mix Design Considerations
- 7. Troubleshooting & Field Notes
- 8. Codes & Standards (Consultant Reference)
- 9. Consultant Takeaways
- 10. Example SCM Selection Matrix
- 11. Practical Testing & QC
- Disclaimer
At a glance
- SCM types with replacement ranges and benefits.
- Key performance parameters and mix design notes.
- Yellow callout below captures common failures and remedies.
How to use this page
- Use the table of contents to jump to sections.
- Start with Types and Key Parameters.
- Review Troubleshooting and QA before finalizing specs.
1. Introduction
Supplementary Cementitious Materials (SCMs) are finely divided mineral materials that either possess pozzolanic or latent hydraulic properties.
When mixed with Portland cement and water, they react with calcium hydroxide released during cement hydration to form additional Calcium Silicate Hydrate (C–S–H) gel — the true binder in concrete.
SCMs:
- Improve durability, workability, and impermeability.
- Reduce heat of hydration, CO₂ footprint, and cost.
- Enhance long-term strength and resistance to sulphate & chloride attack.
2. Pozzolanic & Hydraulic Mechanism
Pozzolanic Reaction (slow):
CH + S + H → C–S–H
(CH = Calcium Hydroxide; S = Reactive Silica; H = Water)
Latent Hydraulic Reaction (fast, with activation):
C₂S + H → C–S–H
Thus, SCMs either consume CH or generate C–S–H directly, densifying the matrix and enhancing durability.
3. Types of SCMs & Their Characteristics
SCM | IS / ASTM Code | Nature | Typical Replacement | Key Benefits | Typical Applications |
Fly Ash (Class F / C) | IS 3812 (Part 1): 2013 / ASTM C618 | Pozzolanic | 15–35 % of cement | Improves workability & long-term strength, reduces permeability | Mass concrete, RMC, pavements |
Ground Granulated Blast Furnace Slag (GGBFS) | IS 12089: 1987 / ASTM C989 | Latent hydraulic | 30–70 % | High sulphate & chloride resistance, low heat | Marine / coastal / underground structures |
Silica Fume | IS 15388: 2003 / ASTM C1240 | Highly pozzolanic | 5–10 % | Ultra-high strength, impermeability, chemical resistance | Bridges, high-performance & self-compacting concrete |
Metakaolin | ASTM C618 (Type N) | Pozzolanic (aluminous) | 5–15 % | Reduces efflorescence & shrinkage, boosts early strength | Precast, white concrete, repairs |
Rice Husk Ash (RHA) | — | Pozzolanic (amorphous SiO₂) | 10–20 % | Improves microstructure & sustainability | Rural / eco projects |
Natural Pozzolans (Volcanic Ash, Calcined Clay) | IS 1344 / ASTM C618 | Pozzolanic | 10–30 % | Locally available, economical | Low-cost housing, non-structural works |
4. Key Performance & Quality Parameters
Property | Test / Standard | Desired Value / Range |
Pozzolanic Activity Index (PAI) | IS 1727 / ASTM C311 | ≥ 75 % of OPC control strength |
Reactive Silica (SiO₂) | IS 3812 / ASTM C311 | ≥ 45 % |
Loss on Ignition (LOI) | IS 1727 | ≤ 6 % |
Specific Surface (Fineness) | IS 4031 (Pt 2) | 300–400 m²/kg |
Moisture Content | — | ≤ 1 % |
Glass Content (for GGBFS) | ASTM C989 | ≥ 85 % amorphous phase |
Sulphide & Sulphate (for GGBFS) | — | ≤ 2 % each |
Bulk Density (Silica Fume) | — | 200–600 kg/m³ |
5. Benefits of Using SCMs
Benefit | Mechanism |
Durability | Reduced permeability & chloride ingress |
Workability | Fly ash improves lubrication effect |
Strength (long-term) | Continued pozzolanic reaction forms extra C–S–H |
Sustainability | Reduced clinker factor → lower CO₂ |
Thermal control | Lower heat of hydration in mass concreting |
Aesthetic improvement | Lighter color, smoother finish (with GGBFS) |
6. Common Mix Design Considerations
Parameter | SCM Influence | Design Guidance |
Water Demand | ↓ with Fly Ash / GGBFS, ↑ with Silica Fume | Adjust W/B ratio |
Setting Time | Slightly delayed | Monitor for early stripping |
Strength Gain | Slower early → higher 28-day | Adjust curing time |
Admixture Compatibility | PC-based admixtures recommended | Verify dosage |
Curing | Critical for pozzolanic reaction | 7–14 days minimum |
7. Troubleshooting & Field Notes
Troubleshooting and common failures — quick view
- Slow strength gain with high fly ash or slag content → extend curing and adjust replacement levels.
- Bleeding or segregation in hot weather → reduce water, improve grading, and add viscosity modifier if needed.
- Sticky mixes with silica fume → optimize superplasticizer dosage and sand gradation.
- Color variability across pours → lock a single SCM source and lot, and control moisture.
- Cold‑weather delayed set → consider accelerators and warm mixing water.
Issue | Cause | Corrective Action |
Slow Strength Gain | High SCM %, poor curing | Extend curing, lower replacement |
Bleeding / Segregation | Excess Fly Ash | Adjust fines content |
Sticky Mix | High Silica Fume % | Add superplasticizer |
Color Variation | Mix of SCM types | Uniform source selection |
Delayed Set in Cold Weather | SCM hydration slower | Use accelerators or warm water |
8. Codes & Standards (Consultant Reference)
Standard | Title |
IS 3812 (Part 1): 2013 | Fly Ash for use in cement, mortar & concrete |
IS 12089: 1987 | GGBFS for use in cement |
IS 15388: 2003 | Silica Fume specifications |
IS 456: 2000 | General concrete durability guidance |
IS 10262: 2019 | Mix design with SCM integration |
ASTM C618 / C989 / C1240 | Pozzolanic and slag materials |
EN 197-1 / EN 206 | European composite cements standards |
9. Consultant Takeaways
Use Fly Ash / GGBFS for durability & workability, Silica Fume / Metakaolin for strength & impermeability.
For marine or sulphate environments → prefer PSC or PPC mixes.
Always confirm source test reports (PAI, LOI, SiO₂ content).
Modify curing period and W/B ratio to compensate for SCM reactivity.
Ensure compatibility with superplasticizers (check slump retention).
For precast or fast-set concrete → limit SCM < 25 %.
10. Example SCM Selection Matrix
Requirement | Recommended SCM | Replacement % | Key Code |
High Strength Concrete (M60 +) | Silica Fume + GGBFS | 10 + 25 % | IS 15388 + IS 12089 |
Durable Marine Structure | GGBFS + Fly Ash | 50 + 20 % | IS 455 / IS 3812 |
Sustainable Low-Carbon Concrete | Fly Ash + RHA | 25 + 10 % | IS 3812 / IS 10262 |
Repair Mortar | Metakaolin + Silica Fume | 5 + 5 % | ASTM C618 / C1240 |
High Temperature Exposure | Metakaolin | 10 % | ASTM C618 |
High Workability PCC | Fly Ash | 25 % | IS 3812 |
11. Practical Testing & QC
Field / Lab Test | Purpose | Frequency | Remarks |
Chemical Analysis | Confirm SiO₂, Al₂O₃, LOI | Source approval | Verify each lot |
Strength Activity Index | Pozzolanic reactivity | Every 3 months | ≥ 75 % of control |
Particle Size Analysis | Fineness check | Each delivery | Affects workability |
RCPT / Permeability | Durability index | Design phase | < 1000 Coulombs for HPC |
Trial Mix | Compatibility & strength | Project start | Adjust dosage & curing |
Disclaimer
SpecX is an industry initiative & a neutral resource, compiled from industry references and best practices. It is not brand‑specific. Always cross‑check with project requirements and local codes before finalizing specifications.