6.6.3. Effects on Concrete

• Improves workability and pumpability • Reduces water demand

The use of mineral admixtures like fly ash in concrete leads to better workability. This means that concrete can be mixed, poured, and molded more easily. Moreover, they also help reduce the amount of water needed to achieve the desired consistency, which is crucial for maintaining strength. When less water is used, the concrete becomes denser and stronger in the long run.

Imagine trying to mix a thick batter for a cake. The more difficult it is to stir, the harder it is to incorporate air and achieve a fluffy result. Similarly, when concrete mixes are easier to work with, they can form a more uniform structure, leading to a stronger end product.

• Enhances long-term strength

Adding mineral admixtures contributes to the development of additional calcium silicate hydrate (C-S-H) over time. This compound strengthens the concrete as it cures. While the initial strength of concrete might be lower when mineral admixtures are used, over time, the strength continues to improve significantly, making the concrete more durable.

Think of a tree that takes time to grow strong. Initially, it might seem small and weak, but as months go by and the roots deepen, it becomes sturdy. Similarly, concrete that incorporates mineral admixtures develops strength gradually, resulting in a robust structure over time.

• Reduces permeability

The use of mineral admixtures results in a denser microstructure, which reduces the size and number of pores within the concrete. This effect minimizes water and chloride ion penetration, enhancing resistance to environmental conditions. A concrete mix with lower permeability is much less susceptible to damage from freeze-thaw cycles or chemical attacks.

Imagine a sponge. A densely packed sponge absorbs less water than a looser one. In the same way, concrete with reduced permeability repels water and harmful substances better, leading to longer-lasting structures.

• Slower early strength gain

One of the drawbacks of using mineral admixtures is that they can slow down the rate at which concrete gains its initial strength. This means that while the long-term strength improves, the concrete might take longer to reach its early strength compared to concrete without admixtures. This factor needs to be considered in construction schedules.

Consider making a loaf of bread: if you use whole grains, it might take longer to rise compared to using regular flour. Similarly, while the concrete made with mineral admixtures will ultimately be stronger, it may take more time before it firms up enough for early use.

• Reduces heat of hydration

Mineral admixtures help lower the temperature increase that occurs when cement hydrates. This is particularly beneficial in mass concrete pours, where high heat can lead to cracking. By reducing the heat of hydration, the risk of thermal stress is diminished, promoting structural integrity.

Think about how a large pile of compost generates heat as it decomposes. If you mixed it in smaller batches, it would generate less heat. Similarly, by using mineral admixtures, large concrete projects 'cool down' during hydration, reducing the risk of damage.

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