The Constructor

What is Alkali-Activated Concrete (AAC)?

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Alkali-activated concrete (AAC) is defined as a class of cement-free concrete that is an alternative to Portland cement concrete (PCC). In AAC, the cement is entirely replaced by an alternative binder. Instead of using ordinary portland cement (OPC) and water, AAC uses other supplementary materials like blast furnace slag (BFS) or fly ash (FA) along with an alkaline activator.

The potential of AAC as a sustainable material to replace ordinary portland cement concrete is still being studied and researched. It is gaining popularity due to its low energy cost, high strength, and durability compared to ordinary portland cement. The features and applications of alkali-activated concrete have been explained in this article.

Composition of Alkali-Activated Concrete

The two main ingredients of alkali-activated concrete are:

  1. Alkaline activators
  2. Cementing components

1. Alkaline Activators

The two commonly used alkaline activators are caustic alkali and alkaline salts. Their composition can include caustic soda, silicates, non-silicate weak acid salts, aluminosilicate, aluminates, and non-silicate vital acid salts.

Alkaline activators are added to the slag either by dissolving in water, underground with slag or by blending it with ground slag before mixing it with water.

2. Cementing Components

The cementing components used in alkali-activated cement or concrete are blast furnace slag, silica fume, steel slag, coal fly ash, zeolite, etc.

Figure-1: Preparation of alkali-activated slag concrete hollow block (AASCHB) specimens Image Credits: Research Gate

Hydration of Alkali-Activated Concrete

The hydration reaction in alkali-activated concrete is more complex than ordinary portland cement. Its mechanism is not fully understood and is still under research. As the characteristics of slag used vary from source to source, the reaction and microstructure also vary.

The main hydration product of alkaline-activated slag cement at room temperature is calcium silicate hydrate gel (C-S-H). The characteristics of slag and the activator used also influence the formation of minor hydration products.

Figure-2: Microstructure of sodium silicate-activated slag cement after (a) 24 hours; (b) 7 days; and (c) 28 days of hydration

Future Prospects of Alkali-Activated Concrete

When comparing traditional portland cement and alkali-activated cement in terms of their hydration products, the former gives calcium silicate hydrate (C-S-H) and calcium hydroxide, while the latter provides poorly crystallized aluminosilicate gel that contains alkalis. The latter also provides enhanced durability to the concrete. Besides this, AAC plays an important role in protecting the environment by utilizing by-products and providing efficient waste management.

Challenges Faced by Alkali-Activated Concrete

Alkali-activated concrete faces many challenges which need to be addressed when used for mega construction projects.

  1. The leaching out of alkalis reacts with carbon dioxide exhibiting efflorescence in the form of alkali carbonates.
  2. AAC exhibits considerable drying shrinkage and cracking during dry conditions.
  3. The AAC reaction and mechanism can be studied entirely only through varied research and studies.
  4. The chemical admixtures that work with ordinary portland cement do not work well with AAC.
  5. The chemical and physical properties of raw materials, especially slag materials, vary based on the source that would finally affect the strength and properties of concrete.

FAQs

What is alkali-activated concrete (AAC)?

Alkali-activated concrete (AAC) is defined as a class of cement-free concrete that is an alternative to Portland cement concrete (PCC). In AAC, the cement is entirely replaced by an alternative binder. Instead of using ordinary portland cement (OPC) and water, AAC uses other supplementary materials like blast furnace slag (BFS) or fly ash (FA) along with an alkaline activator.

What is the elemental composition of alkali-activated concrete (AAC)?

The two main ingredients of alkali-activated concrete are:
1. Alkaline Activators
The two commonly used alkaline activators are caustic alkali and alkaline salts. Their composition can include caustic soda, silicates, non-silicate weak acid salts, aluminosilicate, aluminates, and non-silicate vital acid salts.
Alkaline activators are added to the slag either by dissolving in water, underground with slag or by blending it with ground slag before mixing it with water.
2. Cementing Components
The cementing components used in alkali-activated cement or concrete are blast furnace slag, silica fume, steel slag, coal fly ash, zeolite, etc.

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