What Are The Functions Of Cellulose Ether In Cement Mortar?
In ready-mixed mortar, only 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.
The selection of cellulose ethers with different varieties, different viscosities, different particle sizes, different degrees of viscosity and added amount will also have different effects on the improvement of the performance of dry mortar. At present, many masonry and plastering mortars have poor water retention performance, and water slurry separation occurs after a few minutes of standing still. Therefore, it is very important to add cellulose ether to cement mortar.
Water retention is an important performance of methyl cellulose ether, and it is also a performance that many domestic dry mortar manufacturers, especially those in areas with higher temperatures in the south, pay attention to.
In the production of building materials, especially dry mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), it is an indispensable and important part.
The viscosity, dosage, environmental temperature and molecular structure of cellulose ether have a great influence on its water retention performance. Under the same conditions, the higher the viscosity of the cellulose ether, the better the water retention; the higher the content, the better the water retention. Usually, a small amount of cellulose ether can greatly increase the water retention of the mortar, and when the content reaches a certain level When the temperature is higher, the increasing trend of water retention rate slows down; as 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; fibers with lower substitution degree Plain ether 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, so that the free water becomes bound water, thereby playing a good water retention effect; the water molecule and the cellulose ether molecular chain Interdiffusion allows water molecules to enter the macromolecular chain of cellulose ether and are subject to a strong binding force, thereby forming free water, entangles the water, and improves the water retention of cement slurry; cellulose ether improves the fresh cement slurry The rheological properties, porous network structure and osmotic pressure or film-forming properties of cellulose ether hinder the diffusion of water.
Cellulose ether gives the wet mortar excellent viscosity, can significantly increase the bonding ability of the wet mortar and the base layer, and improve the sag resistance of the mortar. It is widely used in plastering mortar, tile 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, 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, but if the viscosity is too large, it will affect the fluidity and operability of the material (such as sticky plaster 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 output of mortar.
The 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 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 MC, HPMC, or HEMC, always exhibit the same rheological properties as long as the concentration and temperature remain constant. When the temperature increases, a structured gel is formed, and a high thixotropic flow occurs.
High-concentration and low-viscosity cellulose ethers exhibit thixotropy even below the gel temperature. This property is very beneficial to the construction of building mortar to adjust its leveling and sagging properties. 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 the corresponding reduction in its solubility, which will have a negative impact on the mortar concentration and construction performance.
Cellulose ether has obvious air-entraining effect on fresh cement-based materials. Cellulose ether has both hydrophilic group (hydroxyl group, ether group) and hydrophobic group (methyl group, glucose ring). It is a surfactant with surface activity and 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 the mortar during operation, which is conducive to the paving of mortar; it will also increase the output of mortar. , Reduce the production cost of mortar; but it will increase the porosity of the hardened material, reduce its strength and elastic modulus and other mechanical properties.
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 fluidity. Cellulose ether has an effect on cement-based materials. The influence of fluidity is a combination of plasticizing and thickening effects. Generally speaking, when the content of cellulose ether is very low, it is mainly manifested as plasticizing or water-reducing effect; when the content is high, the thickening effect of cellulose ether increases rapidly, and its air-entraining effect tends to become saturated. Therefore, it has a thickening effect or an increase in water demand.
Cellulose ether will prolong the setting time of cement paste or mortar and delay the hydration kinetics of cement. This will help increase the operating time of fresh materials and improve the consistency of mortar and the loss of concrete slump over time, but it may also delay the construction progress.
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