The main ingredient of Portland cement is hydraulic calcium silicates.
According to ASTM C-150, a standard specification for Portland cement, it is defined as a hydraulic cement produced by pulverizing clinker. This clinker consists essentially of hydraulic calcium silicates. It usually also contains one or more types of calcium sulfate (such as gypsum) added during the grinding process.
Understanding Hydraulic Calcium Silicates
The term "hydraulic" means that these silicates react chemically with water. This reaction, known as hydration, is what causes the cement paste to harden and gain strength. The clinker, which forms the base of the cement powder, is produced by heating a mixture of raw materials like limestone (providing calcium oxide) and clay or shale (providing silica, alumina, and iron oxide) in a kiln at very high temperatures.
Key Components in Cement Clinker
While hydraulic calcium silicates are the main component, the clinker also contains other compounds. These include:
- Calcium Silicates: Predominantly Dicalcium Silicate (C₂S) and Tricalcium Silicate (C₃S). These are the primary compounds responsible for strength development, especially Tricalcium Silicate for early strength and Dicalcium Silicate for later strength.
- Calcium Aluminate (C₃A): Contributes to early strength development but also affects setting time and heat of hydration.
- Calcium Aluminoferrite (C₄AF): Influences the color and contributes slightly to strength.
A simplified view of the composition of a typical Portland cement clinker might look like this:
| Component | Typical Percentage Range | Function |
|---|---|---|
| Calcium Silicates | ~70-85% | Primary source of strength (hydraulic) |
| Calcium Aluminate | ~5-15% | Early strength, setting control |
| Calcium Aluminoferrite | ~5-15% | Color, minor strength contribution |
| Free Calcium Oxide, etc. | < several % | Minor components |
Note: Percentages are approximate and vary based on cement type.
Calcium sulfate (typically 3-5% gypsum) is added during the final grinding process to control the setting time of the cement when mixed with water. Without it, the cement would set too quickly.
In essence, when you mix Portland cement with water, the hydraulic calcium silicates in the clinker react to form a stable, stone-like material that binds aggregates (like sand and gravel) together, creating concrete or mortar.
