6.12. Hydration Reactions of Mineral Admixtures

Most mineral admixtures undergo a secondary hydration process known as pozzolanic reaction, where they react with calcium hydroxide (Ca(OH)₂)—a by-product of cement hydration—to form additional calcium silicate hydrate (C-S-H), the main strength-giving compound in concrete.

Basic Reaction (Simplified):
SiO₂ + Ca(OH)₂ + H₂O → C-S-H

In this chunk, we explore the pozzolanic reaction, which is a vital chemical reaction that occurs when mineral admixtures like fly ash or silica fume react with calcium hydroxide from hydrated cement. This reaction produces calcium silicate hydrate (C-S-H), which is the primary compound that contributes to the strength and durability of concrete. The equation presented illustrates how silicon dioxide (SiO₂) from the mineral admixture, water (H₂O), and calcium hydroxide (Ca(OH)₂) combine to create C-S-H. This process increases the overall strength of the concrete mix.

You can think of this reaction like cooking. Just as you need certain ingredients to make a dish, such as flour, water, and yeast to make bread, the pozzolanic materials need the right conditions (like the presence of Ca(OH)₂ and water) to react and form a strong concrete structure.

• Refinement of pore structure: Leads to lower permeability and higher density.
• Reduction in Ca(OH)₂ crystals: Minimizes leaching and efflorescence.
• Increased volume of C-S-H gel: Improves long-term strength and durability.

This chunk outlines the beneficial effects of the pozzolanic reaction on the microstructure of concrete. First, by refining the pore structure, the reaction reduces the size and number of pores, making the concrete denser and less permeable. This means that water and harmful substances can't easily seep through the concrete, enhancing its durability. Additionally, as the pozzolanic reaction reduces the crystals of calcium hydroxide, it mitigates issues like leaching (where materials are washed away) and efflorescence (the formation of white, powdery deposits on the surface). Finally, the increase in the volume of C-S-H gel strengthens the concrete over time, ensuring it remains robust and reliable under various conditions.

Imagine a sponge. A sponge with small, tightly packed pores can hold more liquid and is less likely to leak than one with large, scattered holes. Similarly, the refinement of pore structure in concrete helps it resist water penetration and makes it stronger over time.

In the presence of water and alkaline activators (like calcium hydroxide from OPC), GGBS undergoes hydration similar to Portland cement, forming:

GGBS + H₂O → C-S-H + C-A-H (calcium aluminate hydrates)

GGBS (Ground Granulated Blast Furnace Slag) performs a different reaction in the presence of water and alkaline activators like calcium hydroxide, resulting in hydration akin to that of traditional Portland cement. Here, not only does it produce C-S-H (about which we have already learned) but also calcium aluminate hydrates (C-A-H). This dual reaction further contributes to the strength and durability of concrete, making GGBS an effective mineral admixture in construction.

Think of this as a mix of various ingredients in a recipe. When you combine necessary activators (like the water and alkaline environment) with GGBS, it is like adding yeast to dough; it triggers the mix to rise and gain structure, making it stronger and more resilient.

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