9.13. Chemical Attack on Concrete

Concrete is vulnerable to various aggressive chemical environments that deteriorate its matrix and reduce its durability.

Concrete is a strong material, but it can be damaged by certain chemicals found in the environment. When these aggressive chemicals come in contact with concrete, they can react with the materials in the concrete and cause degradation. This process weakens the concrete structure, making it less durable over time.

Think of concrete as a protective shell around a fragile treasure. If you expose that shell to corrosive liquids, like acid or saltwater, it can start to break down and compromise the treasure inside. Just like how a shell might crack or degrade, concrete can suffer similar fates when subjected to harmful chemicals.

9.13.1 Sulfate Attack

• Sources: Groundwater, sewage, industrial waste.
• Effect: Formation of expansive products like ettringite and gypsum causing cracking and expansion.
• Types:
  • External Sulfate Attack (ESA)
  • Internal Sulfate Attack (ISA)
• Prevention:
  • Use sulfate-resistant cement (SRC)
  • Reduce permeability
  • Adequate cover and compaction

Sulfate attack occurs when sulfates in the environment, like those found in groundwater or industrial waste, penetrate concrete. When these sulfates react with the components of concrete, they form new compounds such as ettringite and gypsum, which can expand. This expansion creates internal pressures that lead to cracks and structural damage. There are two types of sulfate attack: external, where sulfates come from outside, and internal, where sulfates are generated inside due to certain aggregates. To prevent sulfate attack, it's essential to use sulfate-resistant cement and ensure a tightly packed, low-permeability concrete mix.

Imagine putting a sponge into saltwater. Over time, the salt crystals might expand, causing the sponge to crack and break apart. Similarly, when sulfate-containing water enters concrete, it causes compounds to expand and ultimately disrupt the integrity of the concrete structure.

9.13.2 Acid Attack

• Mechanism: Acids react with calcium hydroxide and C-S-H gel, forming soluble calcium salts.
• Examples: H₂SO₄, HCl, HNO₃, acetic acid.
• Symptoms: Surface erosion, loss of mass, exposure of aggregates.
• Protection:
  • Protective coatings
  • Use of pozzolans to reduce free lime content
  • Silica fume and fly ash improve acid resistance

Acids can significantly harm concrete by reacting with its components, particularly calcium hydroxide, which is a major part of the concrete's structure. This reaction produces soluble salts, leading to surface erosion and loss of concrete mass. For instance, strong acids like sulfuric acid or hydrochloric acid can make the surface of concrete look pitted and worn down, while exposing the aggregates within. To protect concrete from acid attacks, special coatings can be applied, and using supplementary materials like silica fume or fly ash can also help improve resistance.

Consider how lemon juice, which is quite acidic, might erode a metal surface if left on for too long. In the same way, when acids in the environment interact with concrete, they can wear it down, leading to structural damage over time.

9.13.3 Alkali-Aggregate Reaction (AAR)

• Includes:
  • Alkali-Silica Reaction (ASR): most common
  • Alkali-Carbonate Reaction (ACR)
• Effect: Expansion and cracking due to reactive aggregates.
• Control Measures:
  • Use of non-reactive aggregates
  • Use of lithium salts or pozzolanic materials
  • Low-alkali cement

Alkali-Aggregate Reaction occurs when alkalis in cement react with certain reactive types of silica and carbonate found in some aggregates. This reaction can result in significant expansion within the concrete, leading to cracks. The most common type of this reaction is the Alkali-Silica Reaction (ASR). To mitigate these reactions, it's important to use aggregates that are known to be non-reactive, or to incorporate lithium salts or other pozzolanic materials that can counteract these reactions, along with using low-alkali cement.

Imagine a sponge that expands when exposed to water. If that sponge were confined in a tight space, its expansion might lead to cracks in the walls of that space. Similarly, when reactive aggregates in concrete swell due to alkali reactions, they can create stress within the concrete structure, resulting in visible cracking.

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