MCQs on Lime
15 questions. Tap an option to check it instantly, then read the solution.
Answer: C. Plaster of Paris
The chemical formula for gypsum is CaSO₄·2H₂O. When it is heated to about 393 K (120°C), it loses three-quarters (75%) of its water of crystallization. The resulting product is calcium sulphate hemihydrate (CaSO₄·½H₂O), which is commonly known as Plaster of Paris.
Answer: A. CaO
Lime, also known as quicklime or unslaked lime, is chemically calcium oxide. Its correct chemical formula is CaO. It is produced by heating limestone (calcium carbonate, CaCO₃).
Answer: B. Fat lime
Fat lime is a high-purity lime (95% or more calcium oxide) that is produced by burning very pure sources of calcium carbonate. The materials listed, such as marble, white chalk, and seashells, are all rich in pure calcium carbonate, making them ideal for manufacturing fat lime.
Answer: B. Clay
Hydraulicity is the property of lime that allows it to set and harden under water. This property is imparted by the presence of clay (containing silica and alumina) in the limestone before burning. During calcination, these impurities react with calcium oxide to form compounds that are not water-soluble, allowing the lime to set underwater.
Answer: A. quick lime
Calcination is the process of heating a substance to a high temperature. When pure limestone (CaCO₃) is heated, it decomposes, driving off carbon dioxide and leaving behind calcium oxide (CaO). This resulting product, CaO, is known as quick lime or unslaked lime.
Answer: A. hydraulic lime
Hydraulic lime is the only type of lime that can set and harden in the presence of water or in damp conditions. This property, known as hydraulicity, makes it essential for constructing foundations, bridge piers, and other masonry work that is submerged or exposed to moisture.
Answer: D. slaking
The process of adding water to quicklime (CaO) is called slaking. It's an exothermic reaction that converts calcium oxide into calcium hydroxide (Ca(OH)₂), also known as hydrated lime or slaked lime. While "hydration" is chemically correct, "slaking" is the specific industrial and construction term for this process.
Answer: B. Gypsum
Plaster of Paris is produced by heating gypsum (calcium sulphate dihydrate) to a temperature of about 150 °C. This process, known as calcination, drives off some of the water of crystallization to form calcium sulphate hemihydrate, the main component of Plaster of Paris.
Answer: C. Lump lime
Quicklime (CaO) that is obtained directly from the kiln after burning limestone exists in the form of irregular lumps. This is why it is commonly referred to as lump lime before it is processed further by slaking or grinding.
Answer: D. All options are correct
Lime putty is a wet, plastic paste made by slaking quicklime. While it's typically made from fat lime, hydraulic lime can also be used. The process involves adding lime to water (or vice versa). Due to its properties, it is best used within a few days of preparation. Therefore, all the statements can be considered correct in a general context.
Answer: D. All options are correct
Lime concrete has specific properties that differ from modern cement concrete. The values given are typical for lime concrete used in traditional construction:
- A slump of 50-75 mm indicates a workable mix.
- A 90-day compressive strength of 1.5 N/mm² and flexural strength of 0.2 N/mm² are characteristic values, reflecting its slower setting time and lower strength compared to Portland cement concrete.
Therefore, all the options are correct representations of lime concrete properties.
Answer: D. All of the above
Lime with a high percentage of calcium oxide (typically >95%) is known by several names. It is called fat lime or rich lime because it slakes vigorously and its volume increases significantly. It is also called white lime due to its pure white color. Therefore, all the given options are correct.
Answer: A. Fat lime
Fat lime is the most suitable lime for finishing works like plastering and whitewashing. Its high purity results in a bright white finish, and its high plasticity makes it easy to apply smoothly. Hydraulic lime is used for structural work, while quicklime must be slaked before use.
Answer: B. Calcining
The process of heating limestone (CaCO₃) to a high temperature (around 900°C) to drive off carbon dioxide (CO₂) and produce quicklime (CaO) is known as calcination or simply calcining.
Answer: C. Hydraulic lime
Hydraulic lime is used to make lime mortar because of its ability to set and harden in the presence of water. This makes it suitable for structural masonry work, whereas fat lime (which does not set underwater) is primarily used for plastering and whitewashing.