The Relationship Between the Viscosity of Cellulose Ether and the Water Retention Performance of Gypsum Mortar

Viscosity is a crucial parameter in determining the performance of cellulose ether. In general, a higher viscosity is associated with better water retention in gypsum mortar. However, as viscosity increases, the molecular weight of cellulose ether also rises, resulting in a corresponding decrease in its solubility. This has a negative impact on the strength and construction performance of the mortar. While higher viscosity enhances the thickening effect on the mortar, it does not exhibit a proportional relationship.

Increased viscosity makes wet mortar more adhesive during construction, leading to higher adhesion to tools and substrates. However, this does not significantly contribute to the structural strength of the wet mortar itself. Additionally, it may not demonstrate a noticeable improvement in the mortar’s resistance to sagging during application. On the contrary, some cellulose ethers with medium to low viscosity but modified properties show excellent performance in enhancing the structural strength of wet mortar. Reputable companies recommend:

Most building wall materials have a porous structure with strong water absorption properties. Gypsum building materials used for wall construction, when mixed with water and applied to the wall, can easily lose moisture to the wall, leading to a lack of the necessary water for gypsum hydration. This can result in difficulties in plastering construction and a decrease in bonding strength, leading to quality issues such as cracks, hollows, and peeling. Improving the water retention of gypsum building materials can enhance construction quality and increase bonding strength with the wall. Therefore, water-retaining agents have become one of the important additives for gypsum building materials.

For plaster gypsum, bonding gypsum, jointing gypsum, gypsum putty, and other building powder materials used in construction, gypsum retarders are added during production to extend the construction time of the gypsum slurry. As these gypsum materials contain retarders, inhibiting the hydration process of semi-hydrated gypsum, these slurries need to be retained on the wall for 1 to 2 hours before setting. Most walls have water-absorbing properties, especially lightweight new wall materials such as brick walls, aerated concrete walls, and porous insulation boards. Therefore, water retention treatment of gypsum slurries is necessary to prevent some of the water in the slurry from transferring to the wall, causing insufficient water during the hardening of the gypsum slurry, incomplete hydration, and separation or peeling at the interface between the gypsum and the wall. Water-retaining agents, commonly cellulose ethers such as methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), are used for this purpose. Other materials like polyvinyl alcohol, sodium alginate, modified starch, diatomaceous earth, and rare earth powder can also be used to improve water retention.

Regardless of the water-retaining agent used, it can delay the hydration rate of gypsum to some extent. When the amount of retarder remains constant, water-retaining agents generally delay setting by 15 to 30 minutes. Therefore, it is possible to reduce the amount of retarder appropriately.