The hydroxyl groups on 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, thereby forming free water and entangled water, which improves 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.
The water retention of cellulose ether itself comes from the solubility and dehydration of cellulose ether itself. The hydration capacity of hydroxyl groups alone is not enough to pay for the strong hydrogen bonds and van der Waals forces between molecules, so it only swells but does not dissolve in water. When substituents are introduced into the molecular chain, not only do the substituents destroy the hydrogen chains, but also the interchain hydrogen bonds are destroyed due to the wedging of the substituents between adjacent chains. The larger the substituents, the greater the distance between molecules, and the greater the effect of destroying hydrogen bonds. After the cellulose lattice swells, the solution enters, and the cellulose ether becomes water-soluble, forming a high-viscosity solution, which then plays a role in water retention.
Factors affecting water retention performance:
Viscosity: The greater the viscosity of cellulose ether, the better the water retention performance, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, and its solubility decreases accordingly, which has a negative impact on the concentration and construction performance of mortar. Generally speaking, for the same product, the viscosity results measured by different methods are very different, so when comparing the viscosity, it must be carried out between the same test methods (including temperature, rotor, etc.).
Addition amount: The greater the amount of cellulose ether added to the mortar, the better the water retention performance. Usually, a small amount of cellulose ether can greatly improve the water retention rate of mortar. When the amount reaches a certain level, the trend of increasing water retention rate slows down.
Particle fineness: The finer the particles, the better the water retention. When large particles of cellulose ether come into contact with water, the surface immediately dissolves and forms a gel to wrap the material to prevent water molecules from continuing to penetrate. Sometimes, even long-term stirring cannot achieve uniform dispersion and dissolution, forming a turbid flocculent solution or agglomeration, which greatly affects the water retention of cellulose ether. Solubility is one of the factors for selecting cellulose ether. Fineness is also an important performance indicator of methyl cellulose ether. Fineness affects the solubility of methyl cellulose ether. Coarser MC is usually granular and can be easily dissolved in water without agglomeration, but the dissolution rate is very slow and it is not suitable for use in dry mortar.
Temperature: As the ambient temperature rises, the water retention of cellulose ethers usually decreases, but some modified cellulose ethers also have good water retention under high temperature conditions; when the temperature rises, the hydration of polymers weakens, and the water between the chains is expelled. When the dehydration is sufficient, the molecules begin to aggregate to form a three-dimensional network structure gel.
Molecular structure: Cellulose ethers with lower substitution have better water retention.
Thickening and thixotropy
Thickening:
Effect on bonding ability and anti-sagging performance: Cellulose ethers give wet mortar excellent viscosity, which can significantly increase the bonding ability of wet mortar with the base layer and improve the anti-sagging performance of mortar. It is widely used in plastering mortar, tile bonding mortar and external wall insulation system 3.
Effect on material homogeneity: The thickening effect of cellulose ethers can also increase the anti-dispersion ability and homogeneity of freshly mixed materials, prevent material stratification, segregation and water seepage, and can be used in fiber concrete, underwater concrete and self-compacting concrete.
Source and influence of thickening effect: 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 modified cement-based materials, but if the viscosity is too high, it will affect the fluidity and operability of the material (such as sticking to the plastering knife). Self-leveling mortar and self-compacting concrete with high fluidity requirements require very low viscosity of cellulose ether. In addition, the thickening effect of cellulose ether will also increase the water demand of cement-based materials and increase the output of mortar.
Thixotropy:
High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. The aqueous solution of methyl cellulose usually has pseudoplasticity and non-thixotropic fluidity below its gel temperature, but exhibits Newtonian flow properties at low shear rates. Pseudoplasticity increases with the increase of cellulose ether molecular weight or concentration, and has nothing to do with the type of substituent and degree of substitution. Therefore, cellulose ethers of the same viscosity grade, whether MC, HPMC, or HEMC, always show the same rheological properties as long as the concentration and temperature remain constant. When the temperature rises, a structural gel is formed, and a high thixotropic flow occurs. Cellulose ethers with high concentration and low viscosity show thixotropy even below the gel temperature. This property is very beneficial for adjusting the leveling and sagging of building mortar during construction.
Air entrainment
Principle and effect on working performance: Cellulose ether has a significant air entrainment 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, thus having an air entrainment effect. The air entrainment effect 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 spreading of mortar; it will also increase the output of mortar and reduce the production cost of mortar.
Effect on mechanical properties: The air entrainment effect will increase the porosity of the hardened material and reduce its mechanical properties such as strength and elastic modulus.
Effect on fluidity: 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 plasticizing and thickening effects. Generally speaking, when the cellulose ether dosage is very low, it mainly manifests as plasticizing or water reducing effects; when the dosage is high, the thickening effect of cellulose ether increases rapidly, and its air entraining effect tends to be saturated, so it manifests as thickening or increasing water demand.
Post time: Dec-23-2024