HPMC stands for Hydroxypropyl Methylcellulose, which is a non-ionic cellulose ether. In mortar, HPMC is primarily used as a thickener, water retention agent, and workability enhancer. HPMC can help to improve the performance of mortar by providing the following functions:

1. Water retention: HPMC can hold water in the mortar, which can help to prevent the mortar from drying out too quickly, leading to a better bond and stronger final product.

2. Improved workability: HPMC can improve the workability of mortar by reducing the need for excessive mixing and allowing for better spreadability and leveling.

3. Increased adhesion: HPMC can help to improve the adhesion of the mortar to the building material by enhancing the bond strength.

4. Reduced shrinkage: HPMC can help to reduce the shrinkage of the mortar during the curing process, which can help to prevent cracks from forming.

5. Improved sag resistance: HPMC can help to improve the sag resistance of the mortar, which is particularly important in vertical applications, such as wall rendering.

HPMC can provide several benefits to mortar, including improved workability, adhesion, water retention, reduced shrinkage, and improved sag resistance.