Write short notes on (a) Plaster of Paris (b) Cement

Plaster of Paris is one of those materials that you have probably come across in your art class or when learning about quick-fix medical casts. Essentially, it's a quick-setting chalky white powder that becomes a hard solid called gypsum when mixed with water. This transformation occurs through an exothermic reaction, which means it gives off heat as it changes states.

Here's where things get scientific and fun: Plaster of Paris is made by heating gypsum to about 150 degrees Celsius (\(150 ^\text{o}C\)). This process drives off a portion of the water contained in the gypsum, resulting in a fine powder. When water is added to this powder, it rehydrates and goes back to its original gypsum state, hardening as it goes. Because it sets fast, usually in about 10 to 15 minutes, it's ideal for making molds, sculptures, and casts. However, be careful when mixing as it can cause burns due to the heat released during setting.

Now let's talk about cement, which is often confused with concrete but is actually a component of concrete. Cement is a powdery substance made from calcined lime and clay. It acts as a hydraulic binding material, which means it can set and become adhesive even under water. When mixed with water, cement undergoes a chemical reaction called hydration and forms a paste that sets and hardens over time.

There are various types of cement, but the most commonly used is Portland cement, named after the stone it resembles from the Isle of Portland in England. Its unique properties make it a fundamental ingredient in concrete, which is used to build everything from homes to highways. Cement's binding ability is attributed to the formation of calcium silicate hydrate (C-S-H) during the reaction with water, which provides the strength and durability properties needed in construction.

Gypsum might not get the same attention as other construction materials, but it's a mineral with numerous applications in the industry. You might recognize it as the raw material for Plaster of Paris. Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate ((CaSO4·2H2O)).

Used in its natural state, it's a handy material for creating drywall or plasterboard for interior walls and ceilings. Additionally, gypsum has a role in agriculture as a soil additive, helping to improve drainage and soil structure. As a chemical, gypsum acts as a fertilizer providing vital sulfur and calcium to plants. In the construction world, gypsum is lauded for its fire resistance, which is why it's a popular choice for protective wall and ceiling coverings.

So, why are binding materials important in construction? These substances, like cement, lime, and even asphalt, are the 'glue' that holds building materials together. Binding materials are characterized by their adhesive and cohesive properties, allowing them to join structural components effectively and maintain integrity under various conditions.

The main role of binding materials is to provide strength and stability. Cement, for instance, binds aggregate to form concrete, which can withstand immense loads. Lime is another binder, often used in mortars and plasters for its workability and gradual setting time. Binders need to adhere well to the surfaces they are applied to and resist environmental factors such as moisture, temperature changes, and chemical attacks. Their performance is crucial, as it impacts the durability and safety of buildings and infrastructure.