Generally speaking, the higher the viscosity, the better the water retention effect of gypsum mortar. However, the higher the viscosity, the higher the molecular weight of cellulose ether, and the corresponding decrease in its solubility will have 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 it is not directly proportional.
The higher the viscosity, the more viscous the wet mortar will be. During construction, it is manifested as sticking to the scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of the wet mortar itself. In addition, during construction, the anti-sag performance of wet mortar is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
Building wall materials are mostly porous structures, and they all have strong water absorption. However, the gypsum building material used for wall construction is prepared by adding water to the wall, and the water is easily absorbed by the wall, resulting in the lack of water necessary for hydration of the gypsum, resulting in difficulties in plastering construction and reduced bond strength, resulting in cracks, Quality problems such as hollowing and peeling. Improving the water retention of gypsum building materials can improve the construction quality and the bonding force with the wall. Therefore, water retaining agent has become one of the important admixtures of gypsum building materials.
Plastering gypsum, bonded gypsum, caulking gypsum, gypsum putty and other building powder materials are used. In order to facilitate construction, gypsum retarders are added during production to prolong the construction time of gypsum slurry. Because gypsum is mixed with Retarder, which inhibits the hydration process of hemihydrate gypsum. This type of gypsum slurry needs to be kept on the wall for 1 to 2 hours before it sets. Most of the walls have water absorption properties, especially brick walls and aerated concrete. Wall, porous insulation board and other lightweight new wall materials, so water retention treatment should be carried out on the gypsum slurry to avoid the transfer of part of the water in the slurry to the wall, resulting in water shortage and incomplete hydration when the gypsum slurry is hardened. Cause the separation and peeling of the joint between gypsum and wall surface. The addition of water-retaining agent is to maintain the moisture contained in the gypsum slurry, to ensure the hydration reaction of the gypsum slurry at the interface, so as to ensure the bonding strength. Commonly used water-retaining agents are cellulose ethers, such as: methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), etc. In addition, polyvinyl alcohol, sodium alginate, modified starch, diatomaceous earth, rare earth powder, etc. can also be used to improve water retention performance.
No matter what kind of water-retaining agent can delay the hydration rate of gypsum to varying degrees, when the amount of retarder remains unchanged, the water-retaining agent can
generally retard setting for 15-30 minutes. Therefore, the amount of retarder can be appropriately reduced.