In pre-mixed mortar, even a small amount of cellulose ether can significantly improve the performance of wet mortar, demonstrating that cellulose ether is a key additive affecting mortar workability.
The selection of different types, viscosities, particle sizes, adhesive properties, and dosages of cellulose ether has varying impacts on the performance of dry-mix mortar. Currently, many masonry and plastering mortars suffer from poor water retention, leading to water separation after just a few minutes of standing. Therefore, adding cellulose ether to cement mortar is crucial.
1. Cellulose Ether – Water Retention
Water retention is a key property of methyl cellulose ether and a major concern for many dry-mix mortar manufacturers, especially in regions with high temperatures, such as southern China. In the production of building materials, particularly dry-mix mortar, cellulose ether plays an irreplaceable role, and it is an essential component in the production of specialty (modified) mortars.
The water retention performance of cellulose ether is influenced by its viscosity, dosage, environmental temperature, and molecular structure. Under the same conditions:
Higher viscosity leads to better water retention.
Increased dosage improves water retention, though once a certain amount is reached, the rate of improvement slows down.
Higher temperatures generally reduce water retention, but some modified cellulose ethers maintain good performance even at elevated temperatures.
Lower substitution degree results in better water retention.
Cellulose ether enhances water retention by forming hydrogen bonds between hydroxyl and ether oxygen atoms in its molecular structure and water molecules, effectively binding free water. Additionally, the interaction between water molecules and cellulose ether chains allows water to enter the polymer structure, where it is more strongly constrained, forming bound water that enhances cement paste water retention. Furthermore, cellulose ether improves the rheology of fresh cement paste, creating a porous network structure and affecting permeability or film formation, which hinders water diffusion.
2. Cellulose Ether – Thickening and Thixotropy
Cellulose ether provides wet mortar with excellent viscosity, significantly enhancing adhesion to substrates and improving anti-sagging properties. It is widely used in plastering mortars, tile adhesives, and external thermal insulation systems. The thickening effect of cellulose ether also increases the cohesion and homogeneity of fresh materials, preventing segregation, bleeding, and layering. It is commonly used in fiber-reinforced concrete, underwater concrete, and self-compacting concrete.
The thickening effect of cellulose ether on cementitious materials arises from the viscosity of its aqueous solution. Under the same conditions:
Higher viscosity leads to greater thickening effects, improving mortar consistency. However, excessive viscosity may negatively impact material flowability and workability (e.g., making it harder to spread with a trowel).
Low-viscosity cellulose ether is preferred for highly flowable materials like self-leveling mortar and self-compacting concrete.
Increased thickening effect leads to higher water demand, increasing mortar yield.
High-viscosity cellulose ether solutions exhibit significant thixotropy, a key characteristic of cellulose ethers. Below their gelation temperature, methyl cellulose solutions typically display pseudoplastic, non-thixotropic flow behavior but behave as Newtonian fluids at low shear rates. The degree of pseudoplasticity increases with molecular weight and concentration but is independent of substitution type and degree. At higher temperatures, the solution forms a structural gel, exhibiting strong thixotropic flow behavior.
Even at temperatures below the gelation point, high-concentration, low-viscosity cellulose ether solutions exhibit thixotropy. This property is beneficial for adjusting the leveling and anti-sagging properties of construction mortar. However, while higher viscosity improves water retention, it also increases molecular weight, reducing solubility and negatively affecting mortar consistency and workability.
3. Cellulose Ether – Air-Entraining Effect
Cellulose ether has a noticeable air-entraining effect on fresh cementitious materials. As a surfactant, it contains both hydrophilic (hydroxyl and ether groups) and hydrophobic groups (methyl and glucose rings), allowing it to introduce air into the mixture.
The air-entraining effect creates a “ball-bearing” effect, enhancing mortar workability by increasing plasticity and smoothness, facilitating application and spreading. It also increases mortar yield and reduces production costs. However, excessive air entrainment increases porosity in hardened materials, reducing strength and elastic modulus.
As a surfactant, cellulose ether also has wetting and lubricating effects on cement particles, further enhancing fluidity. However, its thickening action counteracts this effect. The overall impact on workability is a balance between plasticizing and thickening effects:
At low dosages, cellulose ether primarily acts as a plasticizer or water reducer.
At higher dosages, thickening effects become dominant, while air-entraining effects reach saturation, increasing water demand.
4. Cellulose Ether – Retardation Effect
Cellulose ether extends the setting time of cement paste or mortar by delaying the hydration kinetics of cement. This improves workability time, maintains mortar consistency, and reduces slump loss in concrete. However, it may also slow down construction progress.
