Cellulose ether in ready mixed mortar

The important role of cellulose ether in ready mixed mortar:

In the ready-mixed mortar, the cellulose ether added amount is very low, but can significantly improve the performance of wet mortar, mortar construction performance is a major additive. Reasonable selection of cellulose ether with different varieties, different viscosity, different particle size, different viscosity degree and addition amount has a positive effect on the improvement of dry mortar properties. At present, many masonry and plastering mortars have poor water retention performance, and water slurry separation will occur after a few minutes of standing.

Water retention is an important performance of methyl cellulose ether, but also a lot of domestic dry mortar manufacturers, especially in the southern area of higher temperature manufacturers concerned about the performance. The factors affecting the water retention effect of dry mortar include the amount of MC, MC viscosity, particle fineness and ambient temperature.

Cellulose ether is a synthetic polymer made of natural cellulose as raw material by chemical modification. Cellulose ether is a derivative of natural cellulose, cellulose ether production and synthetic polymer is different, its most basic material is cellulose, natural polymer compounds. Due to the particularity of natural cellulose structure, cellulose itself has no ability to react with etherifying agent. However, after the treatment of swelling agent, the strong hydrogen bonds between molecular chains and within the chain were destroyed, and the activity of hydroxyl group was released into alkali cellulose with reaction ability, and cellulose ether was obtained through the reaction of ETHERifying agent -OH group into -OR group.

The properties of cellulose ethers depend on the type, number and distribution of substituents. The classification of cellulose ether is also based on the type of substituents, degree of etherification, solubility and related application can be classified. According to the type of substituents on the molecular chain, it can be divided into single ether and mixed ether. MC is usually used as a single ether, while HPMC is a mixed ether. Methyl cellulose ether MC is a natural cellulose glucose unit on the hydroxyl methoxide replaced by the product structure formula is [COH7O2 (OH) 3-H (OCH3) H] X, hydroxypropyl methyl cellulose ether HPMC is a unit on the hydroxyl part of the methoxide replaced by hydroxypropyl, another part of the product is replaced by hydroxypropyl, The structural formula is [C6H7O2 (OH) 3-M-N (OCH3) M [OCH2CH (OH) CH3] N] X and hydroxyethyl methyl cellulose ether HEMC, which is widely used and sold on the market.

From the solubility can be divided into ionic type and non-ionic type. Water-soluble non-ionic cellulose ether is mainly composed of alkyl ether and hydroxyl alkyl ether two series of varieties. Ionic CMC is mainly used in synthetic detergent, textile, printing, food and petroleum exploitation. Non-ionic MC, HPMC, HEMC and other mainly used in building materials, latex coatings, medicine, daily chemistry and other aspects. As thickening agent, water retention agent, stabilizer, dispersant, film forming agent.

Cellulose ether water retention: in the production of building materials, especially dry mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), but also an indispensable part. The important role of water-soluble cellulose ether in mortar mainly has three aspects, one is excellent water retention ability, the second is the influence of mortar consistency and thixotropy, and the third is the interaction with cement. Cellulose ether water retention, depends on the base of hydroscopicity, composition of mortar, mortar layer thickness, mortar water demand, condensation material condensation time. The water retention of cellulose ether comes from the solubility and dehydration of cellulose ether itself. It is well known that cellulose molecular chains, although they contain a large number of highly hydrated OH groups, are insoluble in water because of their highly crystalline structure. The hydration ability of hydroxyl groups alone is not enough to pay for the strong intermolecular hydrogen bonds and van der Waals forces. When substituents are introduced into the molecular chain, not only the substituents destroy the hydrogen chain, but also the interchain hydrogen bonds are broken due to the wedging of substituents between adjacent chains. The larger the substituents are, the greater the distance between molecules is. The greater the destruction of hydrogen bond effect, cellulose lattice expansion, the solution into the cellulose ether becomes water-soluble, the formation of high viscosity solution. As the temperature rises, the hydration of the polymer decreases and the water between the chains is driven out. When the dehydrating effect is sufficient, the molecules begin to aggregate and the gel folds out in a three-dimensional network.

The factors affecting the water retention of mortar include cellulose ether viscosity, dosage, particle fineness and service temperature.

The greater the viscosity of cellulose ether, the better the water retention performance. Viscosity is an important parameter of MC performance. At present, different MC manufacturers use different methods and instruments to measure the viscosity of MC. The main methods include Haake Rotovisko, Hoppler, Ubbelohde and Brookfield. For the same product, the results of viscosity measured by different methods are very different, some are even multiple differences. Therefore, when comparing viscosity, it must be carried out between the same test method, including temperature, rotor, etc.

Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity is, the higher the molecular weight of MC is, and the dissolution performance will decrease correspondingly, which has a negative impact on the strength and construction performance of mortar. The higher the viscosity, the more obvious the thickening effect of mortar, but it is not proportional to the relationship. The higher the viscosity, the wet mortar will be more sticky, both construction, the performance of the sticky scraper and high adhesion to the base material. But it is not helpful to increase the structural strength of wet mortar. During construction, the anti-sag performance is not obvious. On the contrary, some low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.

The more cellulose ether is added to the mortar, the better water retention performance, the higher the viscosity, the better water retention performance.

For particle size, the finer the particle, the better the water retention. Large particles of cellulose ether contact with water, the surface immediately dissolve and form a gel to wrap up the material to prevent water molecules from continuing to penetrate, sometimes long time stirring can not be evenly dispersed dissolved, the formation of a muddy flocculent solution or agglomerate. The solubility of cellulose ether is one of the factors to choose cellulose ether. Fineness is also an important performance index of methyl cellulose ether. MC for dry mortar requires powder, low water content, and fineness of 20%~60% particle size less than 63um. Fineness affects the solubility of methyl cellulose ether. Coarse MC is usually granular and can be easily dissolved in water without agglomerating, but the dissolution speed is very slow, so it is not suitable for use in dry mortar. In dry mortar, MC is dispersed between aggregate, fine fillers and cementing materials such as cement, and only powder that is fine enough can avoid clumping of methyl cellulose ether when mixing with water. When MC adds water to dissolve agglomerate, it is very difficult to disperse and dissolve it. MC with coarse fineness not only wastes, but also reduces the local strength of mortar. When such dry mortar is constructed in a large area, the curing speed of local dry mortar is significantly reduced, resulting in cracking caused by different curing time. For mechanical spraying mortar, because of the short mixing time, the fineness is higher.

The fineness of MC also has a certain influence on its water retention. Generally speaking, for methyl cellulose ether with the same viscosity but different fineness, the finer the water retention effect is better under the same amount of addition.

The water retention of MC is also related to the temperature used, and the water retention of methyl cellulose ether decreases with the rise of temperature. But in the actual material application, many environments of dry mortar often will be in high temperature (higher than 40 degrees) under the condition of construction in hot substrate, such as summer insolation of the exterior wall putty plastering, which often accelerated the solidification of cement and dry mortar hardening. The decrease of water retention rate leads to the obvious feeling that both constructability and cracking resistance are affected. In this condition, reducing the influence of temperature factors becomes particularly critical. Although the additive of methyl hydroxyethyl cellulose ether is considered to be at the forefront of technological development, its dependence on temperature will still lead to the weakening of the properties of dry mortar. Even with the increase of methyl hydroxyethyl cellulose dosage (summer formula), the construction and cracking resistance still cannot meet the needs of use. Through some special treatment of MC, such as increasing the degree of etherification, the water retention effect of MC can maintain a better effect under high temperature, so that it can provide better performance under harsh conditions.

In addition, cellulose ether thickening and thixotropy: cellulose ether second action – thickening depends on: cellulose ether polymerization degree, solution concentration, shear rate, temperature and other conditions. The gelation property of solution is unique to alkyl cellulose and its modified derivatives. Gelation characteristics are related to degree of substitution, solution concentration and additives. For hydroxyl alkyl modified derivatives, gel properties are also related to the degree of hydroxyl alkyl modification. For the solution concentration of low viscosity MC and HPMC can be prepared 10%-15% concentration solution, medium viscosity MC and HPMC can be prepared 5%-10% solution, and high viscosity MC and HPMC can only be prepared 2%-3% solution, and usually cellulose ether viscosity grading is also to 1%-2% solution to grade. High molecular weight cellulose ether thickener efficiency, the same concentration of solution, different molecular weight polymers have different viscosity, viscosity and molecular weight can be expressed as follows, [η]=2.92×10-2 (DPn) 0.905, DPn is the average polymerization degree of high. Low molecular weight cellulose ether to add more to achieve the target viscosity. Its viscosity is less dependent on shear rate, high viscosity to achieve the target viscosity, the amount needed to add less, viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of solution) and solution viscosity must be guaranteed. The gelation temperature of the solution decreased linearly with the increase of the concentration of the solution, and gelation occurred at room temperature after reaching a certain concentration. HPMC has a high gelation concentration at room temperature.

The consistency can also be adjusted by selecting particle size and cellulose ethers with different degrees of modification. The so-called modification is the introduction of hydroxyl alkyl group in a certain degree of substitution on the skeleton structure of MC. By changing the relative substitution values of the two substituents, that is, the DS and MS relative substitution values of methoxy and hydroxyl groups. Various properties of cellulose ether are required by changing the relative substitution values of two kinds of substituents.

The relationship between consistency and modification: the addition of cellulose ether affects the water consumption of mortar, and changes the water-binder ratio of water and cement, which is the thickening effect. The higher the dosage, the more water consumption.

Cellulose ethers used in powdery building materials must dissolve quickly in cold water and provide the right consistency to the system. If a given shear rate is still flocculent and colloidal it is a substandard or poor quality product.

There is also a good linear relationship between cement slurry consistency and the dosage of cellulose ether, cellulose ether can greatly increase the viscosity of mortar, the greater the dosage, the more obvious the effect. Cellulose ether aqueous solution with high viscosity has high thixotropy, which is one of the characteristics of cellulose ether. Aqueous solutions of MC type polymers usually have pseudoplastic, non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with the increase of molecular weight or concentration of cellulose ether and is independent of substituent type and degree. 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 increases, structural gel is formed and high thixotropic flow occurs. Cellulose ethers with high concentration and low viscosity exhibit thixotropy even below the gel temperature. This property is of great benefit to the construction of building mortar to adjust its flow and flow hanging property. It needs to be explained here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, the corresponding reduction of its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect of mortar, but it is not a complete proportional relationship. Some low viscosity, but modified cellulose ether in improving the structural strength of wet mortar has a more excellent performance, with the increase of viscosity, cellulose ether water retention improved.

Cellulose ether retardation: cellulose ether third role is to delay the hydration process of cement. Cellulose ether endows mortar with various beneficial properties, but also reduces the early hydration heat release of cement, delaying the hydration dynamic process of cement. This is unfavorable to mortar use in cold areas. This kind of retarding effect is cellulose ether molecule adsorption on C-S-H and Ca (OH) 2 hydration products caused by, due to the increase of pore solution viscosity, cellulose ether reduces the activity of ions in the solution, thus delaying the hydration process. The higher the concentration of cellulose ether in mineral gel material, the more obvious the effect of hydration delay. Cellulose ether not only delays the setting, but also 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 HEMC methylation, the better the retarding effect of cellulose ether. The retarding effect of hydrophilic replacement is stronger than that of water-increasing replacement. But the viscosity of cellulose ether has little effect on the hydration kinetics of cement.

With the increase of cellulose ether content, the setting time of mortar increases significantly. The initial setting time of mortar has a good linear correlation with the content of cellulose ether, and the final setting time has a good linear correlation with the content of cellulose ether. We can control the operational time of mortar by changing the dosage of cellulose ether.

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