Cellulose Ether as a Water Retention Agent and Thickener in Cement-Based Materials
Cellulose Ether as a Water Retention Agent and Thickener in Cement-Based Materials
Due to the formation of hydrogen bonds between hydroxyl and ether oxygen atoms with water molecules, free water is converted into bound water. This enables cellulose ether to function as a water retention agent. The water-retaining effect of cellulose ether prevents excessive and rapid absorption of water by the substrate and slows down water evaporation, ensuring that sufficient water is available for cement hydration.
For instance, in plastering operations, when ordinary cement slurry is applied to a substrate, the dry and porous substrate rapidly absorbs a large amount of water from the slurry. As a result, the cement slurry layer near the substrate loses the water necessary for hydration, preventing the formation of a cement gel with bonding strength. This may lead to issues such as warping, water seepage, and easy detachment of the cement layer. If the applied cement layer is thin, cracks may form throughout the entire material. Traditionally, wetting the substrate with water before application was used to mitigate this issue, but this method is labor-intensive, time-consuming, and difficult to control in terms of quality.
In general, the water retention capacity of cement slurry increases with the cellulose ether content. Higher viscosity (molecular weight) of the added cellulose ether leads to better water retention. Additionally, cellulose ether enhances liquid viscosity, making it an effective thickener. It plays a crucial role in coatings, latex paints, and cement grouts. In fact, cellulose ether also functions as a protective colloid, preventing solid particle flocculation. Cement slurry containing cellulose ether exhibits higher viscosity, reducing the tendency for phase separation. This improves workability and reduces the mixing time and labor required for application. Furthermore, the presence of cellulose ether imparts a certain yield stress to the flowing mortar, enhancing its anti-sagging properties. This helps reduce the sliding of tiles on vertical walls and decreases the amount of tile adhesive needed.
The water retention and thickening effects of cellulose ether mainly depend on its viscosity (i.e., molecular weight) and dosage in cement slurry. The viscosity of cellulose ether is typically measured using a Brookfield viscometer at 20°C with a 2% mass fraction aqueous solution at 20 rpm. The higher the degree of polymerization (or molecular weight), the greater the viscosity of the cellulose ether.
The viscosity of freshly mixed mortar containing cellulose ether is influenced not only by the molecular weight (viscosity) and dosage of cellulose ether but also by its dissolution characteristics. The viscosity develops gradually over time as the cellulose ether fully dissolves. The time required to reach the final viscosity decreases as the dissolution rate of the cellulose ether increases. This is why many dry-mix mortar instructions recommend allowing the mortar to stand (mature) for a certain period after mixing before use.
Commercially available cellulose ethers are typically white powders, and their fineness affects the rate at which viscosity develops in cement slurry. The smaller the powder particles, the better the dispersion and solubility of the cellulose ether in water, reducing the time required to reach the final viscosity and thus shortening the necessary maturation time.
Additionally, temperature and pH also influence the effectiveness of cellulose ether. It is more soluble in cold water than in hot water. As water temperature increases, water molecules gradually detach from hydroxyl groups. When the temperature reaches a certain point, the water molecules become highly active and no longer interact stably with cellulose ether, causing the latter to form polymer entanglements. This temperature is known as the gel point. Below the gel point, the lower the solution temperature, the higher its viscosity, resulting in better water retention and thickening effects. Some types of cellulose ether, such as carboxymethyl cellulose ether and hydroxyethyl cellulose ether, dissolve in both cold and hot water and do not exhibit a gel point.
Within a pH range of 3–11, the viscosity of cellulose ether remains relatively stable due to its non-ionic nature. However, in strongly acidic or strongly alkaline conditions, cellulose ether undergoes dehydration, leading to a decrease in viscosity.
