Mineral Admixtures - Fly Ash

Mineral Admixtures | Fly Ash (Part 1) | Introduction

Supplementary Cementitious Materials (SCMs): Definition and Purpose

• Definition of SCM: Supplementary Cementitious Materials (SCMs) are fine materials that, when used with Portland cement, improve the final concrete properties through their inherent chemical reactivity (hydraulic or pozzolanic activity).

• Why SCMs are Added:

  1. Enhance Performance: They improve properties like long-term strength, durability (resistance to chemical attack), and make the concrete less permeable.

  2. Environmental Benefits: They partially replace cement clinker, which requires a large amount of energy to produce, thereby lowering the concrete's carbon footprint.

  3. Cost Reduction: Using industrial byproducts like fly ash or slag can lower the material cost of the concrete mix.

  4. Optimized Chemistry: SCMs react with the calcium hydroxide released during cement hydration, forming additional compounds that contribute to strength.

The Significance of Portland Cement Components

Portland cement is made from essential oxides. The main chemical components and their significance are:

• Calcium Oxide: This is the most significant component, giving cement its hydraulic property. It allows the cement to react with water, set, and harden on its own.

• Silicon Dioxide: A major component of the resulting hardened cement paste structure.

• Aluminum Oxide: Contributes to the early strength development of the cement.

• Iron Oxide: Acts as a flux in the kiln during manufacturing, helping to lower the temperature needed for production.

The Significance of SCMs in Layman's Terms

Imagine concrete hardening as a chemical reaction that creates "glue" (Calcium Silicate Hydrate or C-S-H).

• Cement: Creates the primary, fast-forming glue. As a byproduct of this process, it releases a "waste product" called lime (calcium hydroxide).

• SCMs (like fly ash): These are materials rich in Silicon Dioxide and Aluminum Oxide. They react with the cement's lime waste product and convert it into more, dense, long-lasting glue (additional C-S-H gel).

The benefits are:

1. Stronger, Denser Concrete: The new glue fills microscopic gaps, making the concrete stronger and more resistant to water and chemicals.

2. Greener Process: You replace a portion of the manufactured cement with recycled materials, reducing resource consumption and pollution.

Lecture Notes: Mineral Admixtures | Fly Ash (Part 1)

I. SCM Evaluation and Characterization

• Systematic Evaluation: Any new SCM must be evaluated using a systematic methodology with available standards (e.g., ASTM) for characterization.

• Evaluation Steps:

  1. Characterization of the material itself.

  2. Determination of its reactivity.

  3. Understanding necessary processing methodologies (e.g., grinding or sorting) to optimize performance.

  4. Final performance tests on cement paste and concrete.

II. Compatibility with Chemical Admixtures

• Complexity: Using SCMs complicates the interaction between cementitious materials and chemical admixtures (like superplasticizers).

• Crucial Assessment: Compatibility must be assessed alongside strength development.

• Examples of Incompatibility:

  • Finer SCMs: Can increase the superplasticizer demand due to higher surface area.

  • Spherical SCMs (like fly ash): Can decrease the superplasticizer demand due to the ball-bearing effect, which improves flow.

  • Layered SCMs (like calcined clay): Can cause incompatibility because Superplasticizer molecules may get intercalated (absorbed) between the layers, requiring a higher dosage.

• Pre-Project Assessment: Compatibility must be checked before a project begins to ensure consistent workability.

III. The Future of Cement and Alternatives

• Optimized Cement: Portland cement is already highly optimized, utilizing the most abundant elements in the earth's crust: Silicon Dioxide, Aluminum Oxide, Iron Oxide, and Calcium Oxide.

• Scale Limitation: Alternative cements (e.g., magnesium-based) cannot be produced at the necessary scale (over 4 billion tons annually) due to the limited availability of required raw materials.

• Sustainability Strategies: The greatest environmental impact is achieved by:

  1. Using blended cements (which incorporate SCMs) to reduce clinker content.

  2. Improving concreting practices.

  3. Shifting to performance-based specifications.

IV. Geopolymers and Alkali-Activation

• Pozzolanic Reaction: Amorphous alumino-silicates react with calcium hydroxide from cement to form short-chain Calcium Alumino Silicate Hydrate (C-A-S-H) gel.

• Geopolymer Reaction (Alkali-Activated): Alumino-silicates are activated using strong alkaline solutions (rich in sodium or potassium). They form a 3-dimensional network called Sodium/Potassium Alumino Silicate Hydrate.

• Sustainability Note: While geopolymers replace cement, the high energy input required for manufacturing the necessary alkaline chemicals must be considered when evaluating their true sustainability.

Fly Ash: An Essential Mineral Additive

I. Source, Production, and Consumption

• Source: Fly ash is obtained from coal-fired thermal power plants as a byproduct of burning coal.

• Collection: Collected using Electrostatic Precipitators (ESPs), which charge the fine particles and collect them on plates.

  • Fields: ESPs have different collection fields where the fineness and properties of the collected fly ash may vary. Consistency in collection point is crucial.

• Usage: Fly ash is the most common mineral additive globally. It is largely consumed by the cement industry to manufacture blended cements.

• Bottom Ash: Coarser ash collected at the bottom of the boiler. It is less reactive but can sometimes be processed for use as aggregate.

II. Fly Ash Composition and Types

• Composition Order: In fly ash, Silicon Dioxide and Aluminum Oxide are typically the largest fractions.

• Classification by Source Coal:

  1. Lignite (Brown Coal): Impure coal that yields High Calcium Fly Ash (often called Class C). This ash may react with water on its own due to high calcium content.

  2. Sub-Bituminous and Bituminous Coals: Purer coals that yield Low Calcium Fly Ash (often called Class F). This ash is primarily pozzolanic and requires external lime for reaction.

• Quality Variability: Fly ash quality can vary greatly depending on the type of coal burnt and the collection process. Direct use in concrete requires careful quality control to ensure consistent performance.

III. Alternative Uses of Fly Ash

• Filler: Used to occupy volume and fill voids in the concrete mix.

• Synthetic Aggregate: Fly ash particles can be processed (pelletized and sintered or chemically bonded) to create aggregate-sized pellets for use in concrete.

Reference video: https://www.youtube.com/watch?v=4jDQ1ydgYeo