Using hydroxypropyl methylcellulose (HPMC) as a water-retaining agent and retarding agent for cement mortar can make the mortar pumpable. In adhesive mortars, HPMC improves spreadability and extends operation time. Its water retention prevents the paste from cracking due to rapid drying after coating and enhances the strength after hardening. Water retention is an important property of HPMC and a key performance indicator for many wet-mixed mortar manufacturers in China. Factors affecting the water retention effect of wet-mixed mortar include HPMC dosage, HPMC viscosity, particle fineness, and ambient temperature.

The important roles of HPMC in wet-mixed mortar are mainly reflected in three aspects: excellent water retention performance, influence on the consistency and thixotropy of wet-mixed mortar, and interaction with cement. The water retention effect of cellulose ethers depends on the water absorption of the base layer, mortar composition, mortar layer thickness, mortar water demand, and the setting time of the cementitious materials. The higher the transparency of HPMC, the better the water retention.

Factors affecting the water retention of wet-mixed mortar include the viscosity of cellulose ethers, dosage, particle fineness, and service temperature. The higher the viscosity of cellulose ethers, the better the water retention. Viscosity is an important parameter of HPMC performance. For the same product, the viscosity results measured by different methods vary greatly, sometimes even twice the difference. Therefore, when comparing viscosities, it must be between the same test methods, including temperature, rotor, etc.

Generally, the higher the viscosity, the better the water retention effect. However, the higher the viscosity, the higher the molecular weight of HPMC, and its solubility will decrease accordingly, which has a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but not proportionally. The higher the viscosity, the higher the viscosity of the wet mortar, which will show stickiness to the trowel and high adhesion to the substrate during application. However, it is not helpful to increase the structural strength of the wet mortar itself. During construction, the anti-sagging performance is not obvious. On the contrary, some low-viscosity but modified HPMC has excellent performance in improving the structural strength of wet mortar.

The more HPMC added to the wet-mixed mortar, the better the water retention, and the higher the viscosity, the better the water retention. Fineness is also an important performance index of HPMC.

The fineness of HPMC also has a certain influence on its water retention. Generally, for HPMC with the same viscosity but different fineness, the finer the fineness, the better the water retention effect.