The role of cellulose ether in mortar
In wet mortar, the water in the mortar evaporates, and the cellulose ether concentrates on the surface, forming a film on the surface of the mortar within 5 minutes, reducing the speed of subsequent evaporation.
In wet mortar, the water in the mortar evaporates, and the cellulose ether concentrates on the surface, forming a film on the surface of the mortar within 5 minutes, reducing the speed of subsequent evaporation. As more water flows from the thicker migrating part to the thinner part of the mortar, the film part formed begins to partially dissolve, and the migration of water will allow more cellulose ether to accumulate on the surface of the mortar.
The formation of cellulose ether on the surface of the mortar has a great influence on the properties of the mortar: First, the film formed is too thin and will dissolve twice, which cannot limit the evaporation of water and reduce the strength. Second, the formation of the film layer is too thick, and the concentration of cellulose ether in the liquid gap in the mortar is very high. It can be seen that the film-forming property of cellulose ether has a great influence on the opening time. The type and degree of etherification (degree of substitution) of cellulose ethers (HPMC, HEMC, MC, etc.) directly affect the film-forming properties, film hardness and toughness of cellulose ethers. 2. Drawing strength Cellulose ethers, in addition to imparting various beneficial properties mentioned above, also retard the hydration kinetics of cement. This retarding effect is mainly due to the adsorption of cellulose ether molecules on various mineral phases in the hydrated cement system, but it is generally believed that cellulose ether molecules are mainly adsorbed on water, such as CSH and calcium hydroxide. It is rarely adsorbed on the original mineral phase of the clinker. In addition, cellulose ether reduces the activity of ions (Ca2+, SO42-, ...) in the pore solution due to the increased viscosity of the pore solution, further hindering the hydration process. Viscosity is another important parameter that shows the chemical properties of cellulose ethers. As mentioned above, viscosity mainly affects water retention capacity and has a significant effect on the workability of fresh mortar.
However, experimental studies have found that the viscosity of cellulose ethers has little effect on the hydration kinetics of cement. Molecular weight has little effect on hydration, with a maximum difference of only 10 minutes between different molecular weights. Therefore, molecular weight is not a key parameter to control cement hydration. "Use of cellulose ethers in cement-based dry mix mortar products" clearly states that the deceleration of cellulose ethers depends on their chemical structure. And to summarize the general trend is that for MHEC, the higher the degree of methylation, the lower the resistance of the cellulose ether. In addition, hydrophilic substitutions (such as substitution of HEC) have greater retardation than hydrophobic substitutions (such as substitution of MH, MHEC and MHPC). The retardation of cellulose ethers is mainly affected by two types of parameters, such as the type and amount of substituents. Our systematic experiments also found that the content of substituents plays an important role in the mechanical strength of tile adhesives.