Understanding Bleeding in Concrete

Bleeding is a form of segregation in concrete where water separates from the mix and rises to the surface. This happens when solid particles (cement and aggregates) settle downwards because of their higher density. The water, being lighter, gets pushed up. Excessive bleeding can lead to problems like reduced strength, increased permeability, and poor surface finish (laitance) when the bleed water evaporates.

Concrete Admixtures and Their Role

Admixtures are materials added to concrete, besides cement, water, and aggregates, just before or during mixing. They are used to modify the properties of fresh or hardened concrete. Different admixtures are used for different purposes, such as accelerating or retarding setting time, improving workability, enhancing durability, or controlling bleeding.

Analyzing the Given Admixtures for Checking Bleeding

Let's look at the options provided and see which one is used to control or check bleeding in concrete:

• Calcium Sulphate: Calcium sulphate (commonly in the form of gypsum) is a key component in Portland cement clinker grinding. It controls the initial setting time of cement. It is not primarily used as an admixture to check bleeding in ready-mix concrete production.
• Potash Soap: Soaps are surfactants. While some surfactants might be used in admixtures (like air-entraining agents), potash soap specifically is not a standard admixture for controlling bleeding. Surfactants can sometimes increase bleeding if not formulated correctly within a superplasticizer or other admixture.
• Red Oxide: Red oxide is an iron oxide pigment used to impart a red color to concrete. It is a coloring admixture and has no effect on controlling bleeding. Its purpose is purely aesthetic.
• Paraffin Wax: Paraffin wax, or more specifically, emulsions containing paraffin wax, are used as pore-blocking or hydrophobic admixtures. When added to concrete, the wax particles can help reduce the capillary pores through which water rises during bleeding. It can also act as a water repellent, reducing water movement within the concrete matrix. This mechanism helps in checking or reducing bleeding.

Why Paraffin Wax Checks Bleeding

Admixtures that control bleeding often work by:

• Increasing the cohesion of the mix.
• Reducing the rate at which solid particles settle.
• Reducing the capillary movement of water.

Paraffin wax admixtures primarily work by the third mechanism - reducing the capillary flow of water to the surface. As the concrete sets and bleed water evaporates, the wax can also help in internal curing by trapping some moisture, although its primary function related to this question is bleeding reduction.

Considering the functions of the given admixtures, Paraffin wax is the one specifically used to check bleeding in concrete.