HPMC (Hydroxypropyl Methyl Cellulose) is commonly used as an additive in cement-based plaster and mortar. Here are some of the ways HPMC can improve the properties of cement-based plaster/mortar:

1. Water retention: HPMC can absorb and retain water, which can help to improve the workability and setting time of cement-based plaster/mortar. This is because the water retention properties of HPMC help to keep the mix moist and pliable, which can make it easier to apply and reduce the risk of cracking.

2. Improved adhesion: The use of HPMC in cement-based plaster/mortar can improve the adhesion of the plaster/mortar to the substrate. This is because the water retention properties of HPMC help to keep the plaster/mortar in contact with the substrate for a longer period of time, allowing for better bonding.

3. Increased durability: HPMC can improve the durability of cement-based plaster/mortar by reducing shrinkage and cracking. This is because the water retention properties of HPMC help to keep the mix moist and pliable, which reduces the risk of cracking.

4. Enhanced workability: The use of HPMC in cement-based plaster/mortar can improve the workability of the mix. This is because HPMC helps to keep the mix moist and pliable, which makes it easier to apply and shape.

Overall, the use of HPMC cellulose ether in cement-based plaster/mortar can help to improve the workability, adhesion, durability, and overall quality of the mix.