Hydroxypropyl methyl cellulose(abbr.HPMC) is always divided into non-surface treated one and surface treated one.

Non-surface treated HPMC always disperse in hot water and dissolve in cold water(whose temperature must be under HPMC's gelling temperature) but cannot disperse in cold water without lump.As reverse, the surface treated HPMC can easily disperse in cold water without caking or lumping but will take some minutes to dissolve (which depends on the treatment time).

As the surface treated HPMC always dissolve in alkaline products and reach the calibration viscosity; Due to the lower pH and the acidity of the gypsum, the viscosity grows very slowly in the gypsum-based mortar. And what's more, the sulfation of gypsum will react with the treated HPMC so it will lose its properties.