Cement Paste Consistency and Cellulose Ether Dosage
Release Time:
2022-01-04
Source:
The consistency of cement paste exhibits a strong linear relationship with the dosage of cellulose ether; cellulose ether can significantly increase the viscosity of mortar, and the effect becomes more pronounced as the dosage increases. Aqueous solutions of high-viscosity cellulose ethers display pronounced thixotropy, which is one of their key characteristics. Aqueous solutions of cellulose-based polymers typically exhibit pseudoplastic, non-thixotropic flow below their gelation temperature, but transition to Newtonian flow at low shear rates. The degree of pseudoplasticity increases with rising molecular weight or concentration of the cellulose ether, independent of the type and degree of substitution. Consequently, cellulose ethers of the same viscosity grade—whether cellulose, HP-cellulose, or HE-cellulose—will consistently demonstrate identical rheological behavior as long as concentration and temperature are held constant. As temperature rises, a structural gel forms, leading to highly thixotropic flow. Even low-concentration, low-viscosity cellulose ethers exhibit thixotropy below their gelation temperature. This property is highly beneficial for adjusting the leveling and sag resistance of construction mortars during application. It should be noted, however, that while higher viscosity generally correlates with better water retention, higher viscosity also implies a higher molecular weight, which in turn reduces solubility—both of which can negatively impact mortar consistency and workability. Although increased viscosity enhances the thickening effect on mortar, the relationship is not strictly proportional. Certain modified cellulose ethers with medium to low viscosity perform even more effectively in improving the structural strength of wet mortars, and water retention improves as viscosity increases.
Retarding effect of cellulose ethers: The third function of cellulose ethers is to retard the hydration process of cement. Cellulose ethers impart a variety of beneficial properties to mortars and also reduce the heat of early cement hydration, thereby slowing down the kinetic processes of cement hydration. This is disadvantageous for mortar applications in cold regions. This retarding effect arises from the adsorption of cellulose ether molecules onto hydration products such as C–S–H gel and Ca(OH)2; moreover, the increased viscosity of the pore solution reduces the mobility of ions in the solution, further delaying the hydration process. The higher the concentration of cellulose ether in mineral gel materials, the more pronounced the hydration delay. Cellulose ethers not only retard setting but also postpone the hardening of cement mortar systems. The retarding effect of cellulose ethers depends not only on their concentration in the mineral gel system but also on their chemical structure: the higher the degree of methyl substitution in HE cellulose, the more effective the retarding action, with hydrophilic substitution exhibiting a stronger retardation than hydrophobic substitution. However, the viscosity of cellulose ethers has only a minor impact on the kinetics of cement hydration.
As the dosage of cellulose ether increases, the setting time of the mortar increases significantly. A strong nonlinear correlation is observed between the initial setting time and the cellulose ether dosage, while a strong linear correlation exists between the final setting time and the cellulose ether dosage. By adjusting the cellulose ether dosage, the workability of the mortar can be effectively controlled.
