During a cement mortar plastering of a brick wall, the ratio of 1 : 4 corresponds to ______.
🔬 Understanding Cement Mortar for Plastering
Plastering is the process of applying a layer of mortar to cover the rough surfaces of walls, columns, and ceilings. This provides a smooth, durable, and clean finish, hiding imperfections in the masonry and protecting it from the atmosphere.
Cement Mortar: For plastering, the mortar is a workable paste made by mixing a binding material, a fine aggregate, and water.
• Binding Material: This is what holds everything together. For cement mortar, this is obviously cement.
• Fine Aggregate: This provides the bulk and volume to the mortar. For plastering, this is always sand.
• Water: This is added to create a workable paste and to enable the chemical hydration process of the cement.
⚖️ Detailed Analysis of the Options
The ratio 1:4 refers to the proportion by volume of the dry ingredients before water is added.
(c) Cement : Sand
Why it's correct: This is the standard convention for expressing a mortar mix ratio. The first number always represents the binder (cement), and the second number represents the fine aggregate (sand). A 1:4 ratio means one part cement is mixed with four parts sand. This is a common and suitable mix for plastering on brick walls.
(a) Cement : Coarse aggregate
Why it's incorrect: Coarse aggregate (gravel, crushed stone) is used to make concrete, not mortar for plastering. Plaster requires a smooth finish, which can only be achieved with fine aggregate (sand).
(b) Lime : Cement
Why it's incorrect: This describes a ratio within a composite binder, not the full mortar mix. A mortar that includes lime, cement, and sand would be expressed with three numbers, such as 1:1:6 (Lime : Cement : Sand).
(d) Cement : Water
Why it's incorrect: The ratio of cement to water is known as the water-cement ratio and is usually expressed as a decimal (e.g., 0.5). It is a critical factor for strength but is not represented by a simple volumetric ratio like 1:4 for the dry components.
📋 Common Plaster Mix Ratios (Cement : Sand)
The choice of mix ratio depends on the location and the required strength and durability of the plaster.
- 1:2 : A very rich and strong mix, used for areas with high wear and tear, such as door jambs.
- 1:3 : A strong, durable mix commonly used for external plastering and for ceiling plaster, where good adhesion is critical.
- 1:4 : A standard, versatile mix suitable for general brick wall plastering.
- 1:6 : A leaner, more economical mix often used for internal walls where the exposure conditions are less severe.