In ready-mixed mortar, just a little bit of cellulose ether can significantly improve the performance of wet mortar. It can be seen that cellulose ether is a major additive that affects the construction performance of mortar. Choosing cellulose ethers of different varieties, different viscosities, different particle sizes, different viscosity degrees and added amounts also have different effects on improving the performance of dry mortar. At present, many masonry and plastering mortars have poor water retention properties. The water slurry will separate after leaving it alone for a few minutes. Therefore, it is very important to add cellulose ether to cement mortar. Let’s take a detailed look at the functions of cellulose ether in cement mortar! 

1.Cellulose ether-water retention 

Water retention is an important property of cellulose ether, and it is also a property that many domestic dry-mix mortar manufacturers, especially those in the southern region with higher temperatures, pay attention to. In the production of building materials, especially dry-mix 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 amount of cellulose ether can greatly improve the water retention rate of mortar. When the dosage reaches a certain level, the trend of increasing water retention rate slows down; the water retention of cellulose ether usually decreases with the increase of ambient temperature, but some modified cellulose ethers also have good water retention under high temperature conditions; cellulose ethers with lower substitution degree have better water retention performance.

The hydroxyl groups on the cellulose ether molecules and the oxygen atoms on the ether bonds will form hydrogen bonds with water molecules, turning free water into bound water, thus playing a good role in water retention; the mutual diffusion between water molecules and cellulose ether molecular chains allows water molecules to enter the interior of the cellulose ether macromolecular chain and be subject to strong constraints, thus forming free water and entangled water, thereby improving the water retention of cement slurry; cellulose ether improves the rheological properties, porous network structure and osmotic pressure of fresh cement slurry, or the film-forming properties of cellulose ether hinder the diffusion of water.

2. Cellulose ether-thickening and thixotropy

Cellulose ether gives the wet mortar 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, tile bonding mortar and exterior wall insulation systems. The thickening effect of cellulose ether can also increase the dispersion resistance and homogeneity of fresh 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 the cellulose ether solution. Under the same conditions, the higher the viscosity of the cellulose ether, the better the viscosity of the modified cement-based material. However, if the viscosity is too high, it will affect the fluidity and operability of the material (such as sticking to the plaster knife). Self-leveling mortars and self-compacting concrete that require high fluidity require a low viscosity of cellulose ether. In addition, the thickening effect of cellulose ethers increases the water demand of cement-based materials and increases mortar yields.

High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of methylcellulose generally have pseudoplastic, non-thixotropic flow properties below their gel temperature, but exhibit Newtonian flow properties at low shear rates. Pseudoplasticity increases with the increase in molecular weight or concentration of cellulose ether, regardless of the type and degree of substitution of the substituent. Therefore, cellulose ethers of the same viscosity grade, whether MC, HPMC, or HEMC, will always exhibit the same rheological properties as long as the concentration and temperature are kept constant. When the temperature is increased, a structural gel is formed and high thixotropic flow occurs.

High concentrations and low viscosity cellulose ethers exhibit thixotropy even below the gel temperature. This property is of great benefit in adjusting the leveling and sagging properties of building mortar during construction. It should be noted here that the higher the viscosity of the cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of the cellulose ether, and its solubility decreases accordingly, which has a negative impact on the mortar concentration and construction performance.

3. Cellulose ether-air entraining effect

Cellulose ether has a significant 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). It is a surfactant with surface activity and thus has an air entraining effect. The air entraining effect of cellulose ether will produce a “ball” effect, which can improve the working performance of freshly mixed materials, such as increasing the plasticity and smoothness of mortar during operation, which is beneficial to the paving of mortar; it will also increase the output of mortar and reduce the production cost of mortar; but it will increase the porosity of hardened materials and reduce their mechanical properties such as strength and elastic modulus.

As a surfactant, cellulose ether also has a wetting or lubricating effect on cement particles, which together with its air entraining effect increases the fluidity of cement-based materials, but its thickening effect will reduce the fluidity. The effect of cellulose ether on the fluidity of cement-based materials is a combination of plasticization and thickening effects. Generally speaking, when the cellulose ether dosage is very low, it mainly manifests as plasticizing or water reducing effect; when the dosage is high, the thickening effect of cellulose ether increases rapidly, and its air entraining effect tends to saturation, so it manifests as thickening effect or increased water demand.

4. Cellulose ether-retarding effect

Cellulose ether will prolong the setting time of cement paste or mortar and delay the cement hydration dynamics, which is beneficial to increase the operability time of the new mix material and improve the time-dependent loss of mortar consistency and concrete slump, but it may also delay the construction progress. 

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