In ordinary Portland cement, the first one to react with water is:
🔬 The Race of Reactions: Bogue's Compounds
When water is added to cement, a series of complex chemical reactions, known as hydration, begins. The four main compounds in cement, called Bogue's compounds, all participate in this process, but they react at vastly different speeds. The sequence and rate of these reactions dictate the cement's properties, such as setting time, rate of strength gain, and heat evolution.
The question asks which compound "wins the race" and reacts first, initiating the entire hardening process.
📝 Detailed Analysis of the Options
(b) C₂S (Dicalcium Silicate)
This is the slowest reacting compound. It hydrates and hardens very slowly, contributing mainly to the strength gain at later stages (after 7 days). Its slow reaction also means it produces very little heat.
(c) C₃S (Tricalcium Silicate)
This compound reacts fairly quickly and is the primary contributor to the early strength of concrete (within the first week). While it's fast, it is not the very first to react.
(d) C₄AF (Tetracalcium Aluminoferrite)
This compound reacts fairly quickly upon adding water, but its contribution to strength is minimal. Its reaction is not as vigorous or immediate as that of C₃A.
(a) C₃A (Tricalcium Aluminate)
This is the correct answer. Tricalcium aluminate is the most reactive of the four Bogue's compounds. It reacts almost instantaneously with water, releasing a large amount of heat. This rapid reaction is responsible for the initial set of the cement. In fact, the reaction is so fast that if left uncontrolled, it would cause a "flash set," making the concrete unworkable. Gypsum is added to the cement specifically to regulate this initial, vigorous reaction of C₃A.
📊 Hydration Reaction Speed of Bogue's Compounds
| Compound | Reaction Speed | Primary Role |
|---|---|---|
| C₃A (Tricalcium Aluminate) | Fastest | Initial Set / Flash Set |
| C₄AF (Tetracalcium Aluminoferrite) | Fast | Flux / Color (little strength) |
| C₃S (Tricalcium Silicate) | Moderate | Early Strength (1-7 days) |
| C₂S (Dicalcium Silicate) | Slowest | Ultimate Strength (>7 days) |
💡 Study Tips
- A for Aluminate, A for Action: Think of C₃A as the compound for immediate "Action." It's the first to react and gets the setting process started.
- The Sprinter vs. The Marathon Runners: C₃A is the sprinter that starts the race. C₃S is the middle-distance runner providing strength for the first week. C₂S is the marathon runner, providing strength slowly and steadily for the long haul.
- Heat is a Clue: The fastest reaction (C₃A) also produces the most heat in the shortest time. This is why it's the main cause of the initial high heat of hydration.