HPMC (Hydroxypropyl Methyl Cellulose) is a common additive used in cement mortar and other construction materials. It serves various purposes such as improving workability, water retention, and adhesion. Here's how it works in cement mortar:

1. Water Retention: HPMC helps in retaining water in the mortar mix, which is crucial for the hydration of cement particles. This ensures that the mortar remains workable for a longer time, allowing for better application and finishing.

2. Improved Workability: By controlling the water content and enhancing the rheological properties of the mortar mix, HPMC improves workability. This means the mortar is easier to spread, mold, and shape, which is particularly useful for applications such as plastering or rendering.

3. Enhanced Adhesion: HPMC can improve the adhesion of mortar to various substrates, including bricks, concrete, and other surfaces. This is important for ensuring the durability and strength of the mortar joints.

4. Reduced Sagging: In vertical applications such as wall rendering, HPMC helps reduce sagging or slumping of the mortar, allowing it to adhere better to vertical surfaces without sliding down.

5. Control of Air Content: HPMC can also help control the air content in mortar, which is important for preventing cracking and improving freeze-thaw resistance.

When using HPMC in cement mortar, it's important to follow manufacturer's guidelines regarding dosage and mixing procedures. Typically, it is added to the dry mix and thoroughly mixed before water is added. The exact dosage may vary depending on factors such as the desired properties of the mortar and environmental conditions.

Overall, HPMC is a versatile additive that can significantly improve the performance and workability of cement mortar, making it a popular choice in the construction industry.