Hydroxypropyl methylcellulose (HPMC) is a nonionic cellulose ether, a semi-synthetic polymer compound produced from natural cellulose (such as cotton pulp or wood pulp) through a series of chemical reactions, including alkalization and etherification. In building plaster mortars, HPMC is not a binder (like cement), but rather a crucial functional additive. While typically added in very low amounts (approximately 0.1%-0.3% of the total mortar weight), it can significantly improve mortar performance. When dissolved in water, it forms a colloidal solution that physically influences various properties of the mortar system, making it an indispensable component in the production of high-quality ready-mixed mortars and on-site mixed mortars.