In ready-mixed mortar, only a little cellulose ether can significantly improve the performance of wet mortar. It can be seen that cellulose ether is the main additive that affects the working performance of the mortar.
Choosing different types of cellulose ethers with different viscosities, different particle sizes, different degrees of viscosity and added amounts will also have different effects on the improvement of the performance of drymix mortar. At present, many masonry and plastering mortars have poor water retention performance. The water and slurry will separate after a few minutes of standing. So it is very important to add cellulose ether to cement mortar.
Let's take a closer look at the role of cellulose ether in cement mortar!
1. Cellulose ether--Water retention
Water retention is an important performance of methyl cellulose ether, and it is also a performance that many domestic dry-mix mortar manufacturers, especially those in southern regions with high temperatures paying attention to. In the production of building materials, especially dry powder mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), it is an indispensable and important component.
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 dosage of cellulose ether can greatly improve the water retention of mortar. When the dosage reaches to 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; Cellulose ether with lower degrees of substitution 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;Thewater 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 free water, entangled water, and improving the water retention of cement slurry; Cellulose ether improves the fresh cement slurry’s rheological properties, porous network structure and osmotic pressure or the film-forming properties of cellulose ether hinder the diffusion of water.
2. Cellulose ethers - Thickening and Thixotropy
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-sag 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 and segregation. It 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.
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, whether it is HPMC or HEMC, will always exhibit 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.
3. Cellulose ether - Air-entraining effect
Cellulose ether has obvious air-entraining effect on fresh cement-based materials. Cellulose ether has both hydrophilic groups (hydroxyl groups, ether groups) and hydrophobic groups (methyl groups, glucose rings), and is a surfactant with surface activity, thus having an air-entraining effect. The air-entraining effect of cellulose ether will produce a "ball" effect, which can improve the working performance of the freshly mixed materials, such as increasing the plasticity and smoothness of the mortar during operation, which is conducive to the spreading of the mortar; it will also increase the output of the mortar , reduce the production cost of mortar; but it will increase the porosity of the hardened material and reduce its mechanical properties such as strength and elastic modulus.
As a surfactant, cellulose ether also has a wetting or lubricating effect on cement particles, which increases the fluidity of cement-based materials together with its air-entraining effect, but its thickening effect will reduce fluidity. The effect of fluidity is a combination of plasticizing and thickening effects. Generally speaking, when the content of cellulose ether is very low, the main performance is plasticization or water reduction; when the content is high, the thickening effect of cellulose ether increases rapidly, and its air-entraining effect tends to be saturated. So it shows as a thickening effect or an increase in water demand.
4. Cellulose Ether - Retardation
Cellulose ether will prolong the setting time of cement paste or mortar, and delay the hydration kinetics of cement, which is beneficial to improve the operability time of freshly mixed materials, improve the consistency of mortar and the loss of concrete slump over time, but may also cause delay the construction progress.
Post time: Jun-02-2023