Hydroxypropyl methylcellulose HPMC endows wet mortar with excellent viscosity, which can significantly increase the adhesion between wet mortar and the base layer, and improve the anti-sagging performance of mortar. in mortar. The thickening effect of cellulose ether can also increase the homogeneity and anti-dispersion ability of fresh cement-based materials, prevent delamination, segregation and bleeding of mortar and concrete, and can be used in fiber-reinforced concrete, underwater concrete and self-compacting concrete .
Hydroxypropyl methylcellulose increases the viscosity of cement-based materials from the viscosity of cellulose ether solution. The viscosity of cellulose ether solution is usually evaluated by the index “viscosity”. The viscosity of cellulose ether generally refers to a certain concentration (such as 2%) of cellulose ether solution at a specified temperature (such as 20°C) and shearing The viscosity value measured by a specified measuring instrument (such as a rotational viscometer) under the condition of a speed (or rotation rate, such as 20 rpm).
Viscosity is an important parameter to evaluate the performance of cellulose ether. The higher the viscosity of hydroxypropyl methylcellulose solution, the better the viscosity of the cement-based material, the better the adhesion to the substrate, the better the anti-sagging and anti-dispersing ability. Strong, but if its viscosity is too large, it will affect the fluidity and operability of cement-based materials (such as sticking plastering knives during plastering mortar construction). Therefore, the viscosity of cellulose ether used in dry-mixed mortar is usually 15,000~60,000 mPa. S-1, self-leveling mortar and self-compacting concrete that require higher fluidity require lower viscosity of cellulose ether.
In addition, the thickening effect of hydroxypropyl methylcellulose increases the water demand of cement-based materials, thereby increasing the yield of mortar.
The viscosity of the hydroxypropyl methylcellulose solution depends on the following factors:
Cellulose ether molecular weight (or degree of polymerization) and concentration, solution temperature, shear rate, and test methods.
1. The higher the degree of polymerization of cellulose ether, the larger the molecular weight, and the higher the viscosity of its aqueous solution;
2. The higher the dosage (or concentration) of cellulose ether, the higher the viscosity of its aqueous solution, but attention should be paid to selecting the appropriate dosage when using it, so as to avoid excessive dosage and affect the working performance of mortar and concrete;
3. Like most liquids, the viscosity of cellulose ether solution will decrease with the increase of temperature, and the higher the concentration of cellulose ether, the greater the effect of temperature;
4. Cellulose ether solutions are usually pseudoplastics with shear thinning properties. The higher the shear rate during the test, the lower the viscosity.
Therefore, the cohesion of the mortar will be reduced due to the action of external force, which is beneficial to the scraping construction of the mortar, so that the mortar can have good workability and cohesion at the same time. However, when the concentration of cellulose ether solution is very low and the viscosity is very small, it will show the characteristics of Newtonian fluid. When the concentration increases, the solution will gradually show the characteristics of pseudoplastic fluid, and the higher the concentration, the more obvious the pseudoplasticity.
Post time: Oct-10-2022