3.9.1. Types of Deleterious Substances

• Clay and silt: Weakens bond with cement, increases water demand.

Clay and silt are small particles that can mix with aggregates. When these materials are present, they can interfere with how well the cement binds to the aggregates. This weak bond can cause the concrete structure to become less strong over time, and since clay and silt absorb water, they can also increase the overall water requirement in the concrete mix.

Imagine trying to mix sand into a pile of mud. The mud (representing clay and silt) doesn’t allow the sand (the aggregates) to blend well, resulting in a weaker mixture. This is similar to how clay and silt disrupt the bond between the aggregates and cement in concrete.

• Organic impurities: Retards setting and hardening.

Organic impurities, such as plant matter or decaying substances in aggregates, can slow down the chemical process that allows cement to harden. This means that the concrete takes longer to set, which can be problematic especially in construction timelines. If these impurities aren’t removed, the integrity of the concrete can be compromised, affecting its durability.

Think of making a cake. If you add rotten fruit to the batter, it can spoil the whole cake. Similarly, organic impurities in concrete can delay its hardening process and ultimately weaken the final product.

• Soft fragments: Reduces strength.

Soft fragments in aggregates are weak materials that do not provide sufficient structural support. When soft fragments are present, the overall strength of the concrete is lowered because they can break down easily under pressure. This can lead to structural failures in concrete works, especially those that need to bear heavy loads.

Imagine trying to build a tower using marshmallows instead of bricks. As soon as you put weight on it, the marshmallows squish and collapse, failing to support the structure. That’s what happens with soft fragments in concrete.

• Salts (chlorides/sulfates): Corrosion of steel, efflorescence.

Salts like chlorides and sulfates can cause significant damage to concrete. They can lead to the corrosion of any steel reinforcements within the concrete, which weakens the structural integrity. Additionally, these salts can cause efflorescence, which is a white powdery substance on the surface of the concrete, indicating ongoing chemical reactions that can deteriorate the concrete over time.

Think about how salt can cause damage to roads and vehicles in winter. Just as road salt can corrode a car's metal body, chlorides from salts can corrode the steel inside concrete, leading to major problems down the line.

• Coal and lignite: Discoloration and poor durability.

Coal and lignite are types of coal that, when present in aggregates, can cause discoloration of concrete and negatively affect its overall durability. These materials may not bond well with cement and can lead to patches and stains on the surface, making the finished concrete look unprofessional and weak.

Imagine painting a wall with different shades of paint. If you add coal dust, it would not only change the nice color you aimed for but might also weaken the wall material. Similarly, coal and lignite affect the appearance and strength of concrete.

• Alkali-reactive particles: Causes expansion and cracking.

Alkali-reactive particles in aggregates, particularly those containing reactive silica, can react with hydroxides in cement when moisture is present. This reaction produces a gel that expands over time, leading to cracking and deterioration in the concrete. The presence of these particles must be carefully managed to prevent long-term damage to concrete structures.

Think of a sponge immersed in water. As it absorbs moisture, it expands. Similarly, alkali-reactive particles cause concrete to swell, leading to cracks, much like if the sponge pushed against other objects and caused damage.

• Permissible Limits (IS: 383):
  • Clay lumps: <1%
  • Soft particles: <3%
  • Organic impurities: Should pass color test with NaOH solution.

The Indian Standard (IS: 383) sets specific permissible limits for various deleterious substances to ensure quality in concrete mixes. Clay lumps should not exceed 1%, soft particles should be less than 3%, and any organic impurities must pass a color test when mixed with a sodium hydroxide solution. Adherence to these limits helps maintain the integrity and performance of concrete.

Consider following safety regulations when driving a car. Just like there are speed limits and other rules to keep you safe on the road, these permissible limits ensure that the concrete used in construction remains strong and durable, safeguarding structures.

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