4.2.1. Stages of Hydration

• Upon contact with water, cement compounds begin to dissolve.
• C₃A reacts rapidly unless gypsum is added to control flash setting.

The first stage of hydration, known as Initial Hydrolysis, occurs immediately when cement is mixed with water. During this stage, the chemical compounds in the cement start to dissolve in the water. The first compound to react is Tri-calcium Aluminate (C₃A), which reacts very quickly. However, to control this fast reaction and prevent flash setting (which can occur when cement hardens too quickly), gypsum is often added. This helps to regulate and slow down the setting process.

Think of Initial Hydrolysis like making a hot cup of instant coffee. When you add hot water, the coffee granules quickly dissolve and release flavor. If you were to add too much coffee too quickly, it could become too strong, just like how cement can set too fast without gypsum.

• Reaction slows down.
• Concrete remains plastic and workable for about 2–4 hours.
• Ideal time for mixing, transporting, and placing.

Following the Initial Hydrolysis, the Induction or Dormant Period occurs. During this phase, the hydration reaction slows considerably, allowing the concrete to remain in a workable, plastic state for about 2 to 4 hours. This period is crucial as it gives builders adequate time to mix, transport, and pour the concrete into forms or molds before it begins to set.

Imagine making a cake batter. After you mix the ingredients, there’s a period when you can still pour or adjust the batter before it starts to rise and harden in the oven. Similarly, this dormant period allows for adjustments in the concrete work before it becomes unworkable.

• C₃S and C₂S react to form calcium silicate hydrate (C–S–H) and calcium hydroxide (Ca(OH)₂).
• Initial set occurs; heat evolution increases.

The Acceleration Period is characterized by the vigorous reaction of C₃S (Tri-calcium Silicate) and C₂S (Di-calcium Silicate) with water, leading to the formation of Calcium Silicate Hydrate (C–S–H) and Calcium Hydroxide (Ca(OH)₂). This indicates the beginning of the concrete setting process, where it begins to harden significantly. During this phase, heat is generated as a byproduct of the reactions, which is important for early strength development.

Think of this phase like the rising of dough in baking. As the yeast reacts and generates gas, the dough expands and changes texture. In the same way, when C₃S and C₂S react with water, the concrete begins to harden and gain strength.

• Hydration continues at a slower pace.
• Strength development continues for months, primarily due to C₂S.

In the Deceleration and Steady-State Period, the hydration process continues but at a much slower rate compared to the acceleration phase. This is mainly due to the continued reaction of C₂S, which contributes to long-term strength development. Even though the initial set has occurred, hydration does not stop; it can persist for months, gradually enhancing the concrete's strength over time.

Consider growing a plant. After planting the seed (like the initial hydration), it takes time for the roots to grow and establish itself in the soil, leading to a stronger plant over time. Similarly, the ongoing hydration in the concrete strengthens it as time goes on.

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