Cement is one of the most important building material of the present time. It is a dirty greyish heavy powder containing calcium aluminates and silicates. The important raw materials neede for the manufacture of cement are limestone, clay and gypsum. the main step in the manufacture of cement is the heating of raw meal or slurru in the rotary kiln at a very high temperature `1400-1600^(@)C`. Finally `2% or 3%` gypsum is added.
When cement is mixed with water and left as such for sometime, it becomes a hard mass. This is known as setting of cement. It is believed that varous aluminates and silicates present in the cement form hydrates with water which seperate in the form of gel. the gel formed start losing water partly by evaporation and partly by forming hydrates with unhydrated constituents. this results in the formation of a hard mass.
Concrete is a mixture of

Options

A. Cement, sand, gravel and water
B. Cement, sand limestone and water
C. Cement, slaked lime and water
D. Cement, sand and water

Answer

To solve the question regarding what concrete is a mixture of, we can break down the information provided in the paragraph and the video transcript step by step.

### Step-by-Step Solution:

1. **Understanding Cement**:
- Cement is described as a dirty greyish heavy powder that contains calcium aluminates and silicates. These components are crucial for its properties and functionality in construction.

**Hint**: Remember that cement is primarily made up of calcium compounds and silicates.

2. **Raw Materials for Cement**:
- The main raw materials needed for the manufacture of cement include limestone, clay, and gypsum. These materials undergo a specific process to produce cement.

**Hint**: Identify the key raw materials that contribute to the making of cement.

3. **Manufacturing Process**:
- The manufacturing involves heating the raw meal or slurry in a rotary kiln at very high temperatures (1400-1600°C). After this process, 2% to 3% of gypsum is added to the cement.

**Hint**: Focus on the high-temperature process and the addition of gypsum as a crucial step in cement production.

4. **Setting of Cement**:
- When cement is mixed with water, it undergoes a process called setting, where it becomes a hard mass. This occurs due to the formation of hydrates from the aluminates and silicates present in the cement.

**Hint**: Understand the chemical reaction that occurs when cement interacts with water.

5. **Formation of Concrete**:
- Concrete is defined as a mixture of cement, sand, gravel (small pebbles), and water. This combination results in a strong building material used in construction.

**Hint**: Remember the components that make up concrete, as they are essential for its properties.

### Final Answer:
Concrete is a mixture of **cement, sand, gravel, and water**.

### Summary:
- Cement is made from limestone, clay, and gypsum.
- It is heated to high temperatures and mixed with water to set and harden.
- Concrete is formed by combining cement with sand, gravel, and water.