Hydroxypropyl methylcellulose (HPMC) is often added to mortar as a functional additive. It is a versatile compound derived from cellulose, a natural polymer found in the cell walls of plants. HPMC is commonly used in various industries, including construction, pharmaceuticals, and food.

In the context of mortar, HPMC serves several purposes:

1. Water Retention: HPMC is hydrophilic, meaning it has an affinity for water. When added to mortar, it helps to retain water within the mixture. This increased water retention allows for better hydration of cement, leading to improved workability and prolonged setting time. It prevents premature drying of the mortar, especially in hot or dry conditions, which can negatively affect its strength and bonding properties.

2. Improved Workability: HPMC acts as a thickener and rheology modifier, enhancing the workability of mortar. It imparts a more cohesive and flexible texture to the mixture, making it easier to spread, shape, and trowel. The improved workability enables better adhesion to surfaces and facilitates the formation of smooth, uniform finishes.

3. Reduced Sagging and Slumping: Mortar containing HPMC exhibits reduced sagging and slumping tendencies, particularly in vertical applications. The additive provides increased viscosity and thixotropic properties to the mortar, preventing excessive flow or deformation when applied on walls or overhead surfaces. This ensures that the mortar stays in place during curing, enhancing the overall quality of the construction.

4. Enhanced Bonding: HPMC promotes better adhesion between mortar and various substrates. It forms a thin film on the surface, improving the bond strength between the mortar and the materials it is applied to, such as bricks, tiles, or stones. This increased bonding capability helps to create more durable and long-lasting structures.

5. Crack Resistance: The addition of HPMC to mortar can improve its resistance to cracking. The increased water retention and improved workability result in a more homogeneous and cohesive mixture. This homogeneity reduces the likelihood of shrinkage cracks, which can occur as the mortar dries and cures.

It's important to note that the specific properties and effects of HPMC in mortar can vary depending on the grade and dosage used, as well as other factors such as the composition of the mortar mix and environmental conditions. Manufacturers and construction professionals consider these factors when determining the appropriate type and amount of HPMC to be added to mortar for desired performance characteristics.