Cellulose ether can significantly improve the performance of wet mortar, and is a main additive that affects the construction performance of mortar. Reasonable selection of cellulose ethers of different varieties, different viscosities, different particle sizes, different degrees of viscosity and added amounts will have a positive impact on the improvement of the performance of dry powder mortar. At present, many masonry and plastering mortars have poor water retention performance, and the water slurry will separate after a few minutes of standing. Water retention is an important performance of methyl cellulose ether, and it is also a performance that many domestic dry-mix mortar manufacturers, especially those in southern regions with high temperatures, pay attention to. Factors that affect the water retention effect of dry powder mortar include the amount of addition, viscosity, fineness of particles, and the temperature of the use environment.

Water retention of cellulose ether

In the production of building materials, especially dry powder mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), it is an indispensable and important component. The important role of water-soluble cellulose ether in mortar mainly has three aspects, one is excellent water retention capacity, the other is the influence on the consistency and thixotropy of mortar, and the third is the interaction with cement. The water retention effect of cellulose ether depends on the water absorption of the base layer, the composition of the mortar, the thickness of the mortar layer, the water demand of the mortar, and the setting time of the setting material. The water retention of cellulose ether itself comes from the solubility and dehydration of cellulose ether itself. As we all know, although the cellulose molecular chain contains a large number of highly hydratable OH groups, it is not soluble in water, because the cellulose structure has a high degree of crystallinity. The hydration ability of hydroxyl groups alone is not enough to cover the strong hydrogen bonds and van der Waals forces between molecules. Therefore, it only swells but does not dissolve in water. When a substituent is introduced into the molecular chain, not only the substituent destroys the hydrogen chain, but also the interchain hydrogen bond is destroyed due to the wedging of the substituent between adjacent chains. The larger the substituent, the greater the distance between the molecules. The greater the distance. The greater the effect of destroying hydrogen bonds, the cellulose ether becomes water-soluble after the cellulose lattice expands and the solution enters, forming a high-viscosity solution. When the temperature rises, the hydration of the polymer weakens, and the water between the chains is driven out. When the dehydration effect is sufficient, the molecules begin to aggregate, forming a three-dimensional network structure gel and folded out.

Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity and the higher the molecular weight, the corresponding decrease in its solubility will have a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the more viscous the wet mortar will be, that is, during construction, it is manifested as sticking to the scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of the wet mortar itself. During construction, the anti-sag performance is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.

Thickening and Thixotropy of Cellulose Ether

There is also a good linear relationship between the consistency of cement paste and the dosage of cellulose ether. Cellulose ether can greatly increase the viscosity of mortar. The larger the dosage, the more obvious the effect. High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether.

Thickening depends on the degree of polymerization of cellulose ether, solution concentration, shear rate, temperature and other conditions. The gelling property of the solution is unique to alkyl cellulose and its modified derivatives. The gelation properties are related to the degree of substitution, solution concentration and additives. For hydroxyalkyl modified derivatives, the gel properties are also related to the modification degree of hydroxyalkyl. For low viscosity MC and HPMC, 10%-15% solution can be prepared, medium viscosity MC and HPMC can be prepared 5%-10% solution, while high viscosity MC and HPMC can only prepare 2%-3% solution, and Usually the viscosity classification of cellulose ether is also graded by 1%-2% solution. High molecular weight cellulose ether has high thickening efficiency. In the same concentration solution, polymers with different molecular weights have different viscosities. High degree. The target viscosity can only be achieved by adding a large amount of low molecular weight cellulose ether. Its viscosity has little dependence on the shear rate, and the high viscosity reaches the target viscosity, and the required addition amount is small, and the viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of the solution) and solution viscosity must be guaranteed. The gel temperature of the solution also decreases linearly with the increase of the concentration of the solution, and gels at room temperature after reaching a certain concentration. The gelling concentration of HPMC is relatively high at room temperature.

Retardation of Cellulose Ether

The third function of cellulose ether is to delay the hydration process of cement. Cellulose ether endows mortar with various beneficial properties, and also reduces the early hydration heat of cement and delays the hydration dynamic process of cement. This is unfavorable for the use of mortar in cold regions. This retardation effect is caused by the adsorption of cellulose ether molecules on hydration products such as C-S-H and ca(OH)2. Due to the increase in the viscosity of the pore solution, the cellulose ether reduces the mobility of ions in the solution, thereby delaying hydration process. The higher the concentration of cellulose ether in the mineral gel material, the more pronounced the effect of hydration delay. Cellulose ether not only delays setting, but also delays the hardening process of the cement mortar system. The retarding effect of cellulose ether depends not only on its concentration in the mineral gel system, but also on the chemical structure. The higher the degree of methylation of HEMC, the better the retarding effect of cellulose ether. The ratio of hydrophilic substitution to water-increasing substitution The retarding effect is stronger. However, the viscosity of cellulose ether has little effect on cement hydration kinetics.

In mortar, cellulose ether plays the role of water retention, thickening, delaying cement hydration power, and improving construction performance. Good water retention capacity makes cement hydration more complete, can improve the wet viscosity of wet mortar, increase the bonding strength of mortar, and adjust the time. Adding cellulose ether to mechanical spraying mortar can improve the spraying or pumping performance and structural strength of the mortar. Therefore, cellulose ether is being widely used as an important additive in ready-mixed mortar