Cellulose ether has excellent water retention, which can prevent the moisture in the wet mortar from evaporating prematurely or being absorbed by the base layer, and ensure that the cement is fully hydrated, thereby finally ensuring the mechanical properties of the mortar, which is especially beneficial to thin-layer mortar and water-absorbing base layers Or mortar constructed under high temperature and dry conditions. The water retention effect of cellulose ether can change the traditional construction process and improve the construction progress. For example, plastering construction can be carried out on water-absorbing substrates without pre-wetting.
The viscosity, dosage, ambient temperature and molecular structure of cellulose ether have a great influence on its water retention performance. Under the same conditions, the greater the viscosity of cellulose ether, the better the water retention; the higher the dosage, the better the water retention. Usually, a small amount of cellulose ether can greatly improve the water retention of mortar. When the dosage reaches a certain When the degree of water retention increases, the trend of water retention rate slows down; when the ambient temperature rises, the water retention of cellulose ether usually decreases, but some modified cellulose ethers also have better water retention under high temperature conditions; fibers with lower degrees of substitution Vegan ether has better water retention performance.
The hydroxyl group on the cellulose ether molecule and the oxygen atom on the ether bond will associate with the water molecule to form a hydrogen bond, turning the free water into bound water, thereby playing a good role in water retention; the water molecule and the cellulose ether molecular chain Interdiffusion allows water molecules to enter the interior of the cellulose ether macromolecular chain and is subject to strong binding forces, thereby forming bound water and entangled water, which improves the water retention of cement slurry; cellulose ether improves the fresh cement slurry. The rheological properties, porous network structure and osmotic pressure or the film-forming properties of cellulose ether hinder the diffusion of water.
Cellulose ether endows the wet mortar with excellent viscosity, which can significantly increase the bonding ability between the wet mortar and the base layer, and improve the anti-sagging performance of the mortar. It is widely used in plastering mortar, brick bonding mortar and external wall insulation system. The thickening effect of cellulose ether can also increase the anti-dispersion ability and homogeneity of freshly mixed materials, prevent material delamination, segregation and bleeding, and can be used in fiber concrete, underwater concrete and self-compacting concrete.
The thickening effect of cellulose ether on cement-based materials comes from the viscosity of cellulose ether solution. Under the same conditions, the higher the viscosity of cellulose ether, the better the viscosity of the modified cement-based material, but if the viscosity is too high, it will affect the fluidity and operability of the material (such as sticking a plastering knife). Self-leveling mortar and self-compacting concrete, which require high fluidity, require low viscosity of cellulose ether. In addition, the thickening effect of cellulose ether will increase the water demand of cement-based materials and increase the yield of mortar.
The viscosity of cellulose ether solution depends on the following factors: molecular weight of cellulose ether, concentration, temperature, shear rate and test method. Under the same conditions, the greater the molecular weight of cellulose ether, the higher the viscosity of the solution; the higher the concentration, the higher the viscosity of the solution. When using it, care should be taken to avoid excessive dosage and affect the performance of mortar and concrete; cellulose ether The viscosity of ether solution will decrease with the increase of temperature, and the higher the concentration, the greater the influence of temperature; cellulose ether solution is usually a pseudoplastic fluid with the property of shear thinning, the greater the shear rate during the test , the smaller the viscosity, therefore, the cohesion of the mortar will decrease under 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; because the cellulose ether solution is non-Newtonian For fluids, when the experimental methods, instruments and equipment or test environments used to test the viscosity are different, the test results of the same cellulose ether solution will be quite different.
Cellulose ether molecules can fix some water molecules of the fresh material at the periphery of the molecular chain, thus increasing the viscosity of the solution. The molecular chains of cellulose ether are intertwined to form a three-dimensional network structure, which will also make its aqueous solution have good viscosity.
High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of methyl cellulose usually have pseudoplastic and non-thixotropic fluidity below its gel temperature, but show Newtonian flow properties at low shear rates. Pseudoplasticity increases with the molecular weight or concentration of cellulose ether, regardless of the type of substituent and the degree of substitution. Therefore, cellulose ethers of the same viscosity grade, no matter mc, HPmc, HEmc, will always show the same rheological properties as long as the concentration and temperature are kept constant. Structural gels are formed when the temperature is raised, and highly thixotropic flows occur. High concentration and low viscosity cellulose ethers show thixotropy even below the gel temperature. This property is of great benefit to the adjustment of leveling and sagging in the construction of building mortar. It needs to be explained here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, and the corresponding decrease in its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not completely proportional. Some medium and low viscosity, but the modified cellulose ether has better performance in improving the structural strength of wet mortar. With the increase of viscosity, the water retention of cellulose ether improves.
Post time: Feb-28-2023