When mixing concrete with Supplementary Cementitious Materials (SCMs), the ideal mix varies based on the project type—foundations, tall structures, and roadways—due to different load, durability, and environmental requirements. SCMs like fly ash, slag, etc enhance properties such as strength, durability, and sustainability.
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Formulas and concrete mixing are a worldwide science in gaining the best outcome for your project. As industry standards, we put together some of the common uses for SCM in concrete based on the project’s purpose.
What is SCM?
Supplementary Cementitious Material (SCM) refers to materials that, when used in combination with Portland cement, contribute to the properties of the hardened concrete through hydraulic or pozzolanic activity. SCMs either react chemically with water (hydraulic) or with calcium hydroxide released during cement hydration (pozzolanic), improving the strength and durability of the concrete.
Examples of SCMs include:
Ground Granulated Blast-Furnace Slag (GGBFS): A byproduct of steel production.
Fly Ash: A byproduct of coal combustion in power plants.
Silica Fume: A byproduct of silicon and ferrosilicon alloy production.
Natural Pozzolans: Materials such as volcanic ash or calcined clay
SCMs are commonly used to:
- Reduce the environmental impact of concrete by lowering the amount of Portland cement required.
- Enhance durability, workability, and resistance to chemical attacks.
- Improve long-term performance and sustainability of concrete structures.
Use and Mixing Concrete with SCM
Foundational Structures
Goal: Strength, low permeability, and durability.
Best Mix:
- Cement: Use Ordinary Portland Cement (OPC) blended with Fly Ash (15-25%) or GGBS (Ground Granulated Blast Furnace Slag) (20-50%).
- Water-Cement Ratio:4 – 0.5 for optimal strength.
- Aggregate: Well-graded coarse and fine aggregates.
- Admixtures: Plasticizers/superplasticizers for workability.
Why: Fly ash improves long-term strength and reduces heat of hydration, minimizing cracking in large sections like foundations.
Tall Structures (High-Rise Buildings)
Goal: High early strength, reduced weight, and durability under load.
Best Mix:
- Cement: OPC with Silica Fume (5-10%) + Fly Ash (15-20%) or GGBS (up to 30%).
- Water-Cement Ratio: ≤ 0.4 (low w/c ratio ensures high compressive strength).
- Admixtures: Superplasticizers for flowability and air-entrainers for freeze-thaw durability.
- Aggregate: Lightweight aggregates for structural elements to reduce self-weight (optional).
Why: Silica fume significantly increases compressive strength and reduces permeability, critical for tall structures under high stress.
Roadways (Pavements)
Goal: Durability, resistance to wear/abrasion, and crack prevention.
Best Mix:
- Cement: OPC with Fly Ash (15-25%) or GGBS (30-50%) for improved workability and resistance to alkali-silica reaction (ASR).
- Water-Cement Ratio:4 – 0.45 for optimal durability.
- Admixtures: Water reducers, air-entrainers for freeze-thaw resistance.
- Aggregate: Coarse aggregates with high abrasion resistance.
Why: SCMs like GGBS and fly ash improve flexural strength and extend the life of pavements exposed to weather and traffic.
General SCM Considerations:
- Fly Ash (Class F or C): Improves workability and long-term strength. Ideal for foundations and roadways.
- GGBS: Reduces heat of hydration and enhances durability. Suitable for all applications.
- Silica Fume: Best for achieving high-strength concrete. Ideal for tall structures.
- Metakaolin: Improves early strength and durability but is costlier.
Energy-intensive Clinker
The term energy-intensive clinker refers to the clinker produced during the cement manufacturing process, which is one of the most energy-demanding and CO2-intensive stages of cement production. Portland cement creates astronomical amounts of energy needed to manufacture this additive which is why slag is an excellent alternative with more benefits.
What is Clinker?
Clinker is a nodular material created by heating a mixture of limestone, clay, and other materials in a kiln at temperatures around 1,450\u00b0C (2,642\u00b0F). This process releases significant amounts of CO2 due to:
Chemical Reactions: The calcination of limestone (CaCO3) releases CO2.
Energy Usage: High-temperature kilns require large amounts of fuel, typically derived from fossil sources.
Why is Clinker Energy Intensive?
- The kiln operates at extremely high temperatures, requiring substantial energy inputs.
- The calcination process itself inherently releases CO2.
- Impact on Carbon Emissions
- Reducing clinker usage in cement production is one of the most effective ways to lower its carbon footprint.
This is where supplementary cementitious materials (SCMs) like slag come in. By partially replacing clinker with SCMs, manufacturers can significantly reduce the environmental impact of concrete.
Key Points:
- Always design mixes based on project-specific load, exposure conditions, and durability requirements.
- Conduct trial mixes to validate performance.
- Ensure compliance with standards like ASTM C595, ACI 318, or EN 206.
