Cement Hydration Compounds

Summary

This presentation dives into the science of hydration compounds in concrete, focusing on both unhydrated and hydrated Portland cement. It covers key compounds like C3S, C2S, calcium silicate hydrate (CSH), calcium hydroxide, ettringite, and AFM. The talk explains the structure and role of these compounds in concrete's strength and durability. The speaker uses analogies, such as coral and human health, to explain the complex processes. Scanning electron microscope images illustrate these compounds, offering insights into their formation and their critical role in modern concrete.

Takeaways

• 🧱 The talk focuses on compounds involved in the hydration of Portland cement, starting with unhydrated compounds like C3S, C2S, C3A, C4AF, gypsum, and limestone.
• 💧 Hydrated Portland cement consists of key compounds like calcium silicate hydrate (CSH), calcium hydroxide (CH), ettringite (AFT), and calcium monosulfoaluminate (AFM).
• 🌊 CSH (calcium silicate hydrate) is the main strength-giving material in hydrated concrete. It forms from reactions of C3S or C2S with water and looks like coral at the micro-level.
• 🧪 Calcium hydroxide (CH) is a byproduct of CSH formation, and it helps maintain a high pH level in concrete, protecting steel reinforcement from corrosion.
• 🌱 Ettringite (AFT) forms early in hydration from C3A, gypsum, and water. It contributes to early stiffening and strength gain and is later converted into AFM.
• 🌹 AFM (calcium monosulfoaluminate) forms later in hydration and is dependent on the sulfur content in the solution. It has a rose-like appearance under a microscope.
• 🔬 At the nano scale, concrete compounds like CSH, CH, AFT, and AFM have unique structures visible through x-ray diffraction or electron microscopes.
• 🏗️ The hydration process in concrete reduces porosity as CSH, CH, and other compounds grow, but complete hydration is rare, with most concrete achieving only about 75% hydration.
• 🌿 CSH is the workhorse of concrete, providing strength and resistance to external elements. Its structure is complex and changes based on conditions.
• 📊 The presentation includes graphs showing the relationships between various compounds during hydration, demonstrating the interplay between porosity, time, and hydration degree.

Q & A

What are the primary compounds found in unhydrated Portland cement?

The primary compounds in unhydrated Portland cement include C3S (alite), C2S (belite), C3A (tricalcium aluminate), C4AF (tetracalcium aluminoferrite), and gypsum (CaSO4·2H2O). Limestone is also added for specific purposes.

What is C-S-H and why is it important in concrete?

C-S-H (calcium silicate hydrate) is the main compound responsible for the strength and durability of concrete. It forms from the reaction of C3S and C2S with water, giving concrete its mechanical strength and preventing external elements from penetrating the structure.

How does calcium hydroxide (CH) form, and what role does it play in concrete?

Calcium hydroxide (CH) forms as a byproduct of the reaction between C3S or C2S and water. It helps maintain a high pH level in the pore solution, protecting steel reinforcement from corrosion by creating a passive layer around it.

What happens when calcium hydroxide is lost in concrete?

When calcium hydroxide is dissolved or removed, usually by external factors like acid or water, it creates voids that weaken the concrete and make it easier for external harmful substances to penetrate, potentially reducing strength and durability.

What are Aft and AFM, and how do they form?

Aft (Ettringite) and AFM (calcium monosulfoaluminate) are compounds formed from the hydration of C3A in the presence of gypsum and water. Ettringite forms early and contributes to initial stiffening and strength gain, while AFM forms later as Ettringite gets consumed.

Why is C-S-H considered difficult to characterize?

C-S-H has an amorphous structure, resembling a network of disordered sheets. Its chemical composition and structure can vary based on when and how it's measured, making it challenging to fully understand and characterize.

What role does porosity play in the hydration process of concrete?

As hydration occurs, the porosity of concrete decreases because hydration products like C-S-H, CH, Aft, and AFM fill the voids. Reduced porosity leads to increased strength and durability of the concrete.

How does the formation of Aft (Ettringite) affect early age strength in concrete?

Aft (Ettringite) crystals grow during the early stages of hydration, leading to the initial stiffening of concrete and contributing to its early age strength.

What is the significance of hydration products in protecting steel reinforcement in concrete?

Hydration products like calcium hydroxide maintain a high pH in the pore solution, which helps create a protective layer around steel reinforcement, preventing corrosion and enhancing the concrete's durability.

How do hydration compounds appear under different scales of magnification?

Under a macro scale, compounds like C-S-H appear as solid rock-like structures. On a micro scale, C-S-H looks like coral, and on a nano scale, it resembles disordered sheets with water molecules between them. Similarly, CH appears as hexagonal plates on a micro scale.