The important role of cellulose ether in ready mixed mortar

The addition of cellulose ether in premixed mortar is very low, but it can significantly improve the performance of wet mortar. It is a major additive which influences the workability of mortar. Reasonable choice of different varieties, different viscosity, different particle size, different degree of viscosity and amount of cellulose ether, the performance of dry mortar improved and produced a positive impact. At present, many masonry and plastering mortar water retention performance is not good, a little rest for a few minutes there will be water separation.
Water retention is an important performance of methyl cellulose ether. It is also the concern of many dry mortar manufacturers in China, especially in the areas with higher temperature in South china. The factors influencing the water retention effect of dry mortar include MC addition, MC viscosity, particle fineness and the temperature of the use environment.
Cellulose ether is a kind of synthetic macromolecule polymer made from natural cellulose as raw material and chemically modified. Cellulose ether is a derivative of natural cellulose. Cellulose ether is different from synthetic polymer. Its most basic material is cellulose, a natural polymer compound. Because of the special structure of natural cellulose, cellulose itself has no ability to react with etherification agent. But after the treatment of the swelling agent, the strong hydrogen bonds between the molecular chains and the chains are destroyed,
The active release of hydroxyl groups becomes reactive alkali cellulose, where cellulose ethers are obtained by conversion of etherification agent OH to OR. The properties of cellulose ethers depend on the type, number, and distribution of substituents. The classification of cellulose ethers is also classified according to the type of substituent, degree of etherification, solubility and related applied energy. According to the substituent types on the molecular chain, we can be divided into single ethers and mixed ethers. We usually use MC as a single ether and HPMC as a mixed ether. Methyl cellulose ether MC is the product structure of natural cellulose glucose unit is hydroxy methoxy substituted after [C O H7O2 (OH) 3-h (OCH3) H]x, hydroxypropyl methyl cellulose ether HPMC is part of a unit on the hydroxyl group of methoxy substituted, another part is the product of Hydroxypropyl Substitution the structure of [C6H7O2 (OH) 3-m-n (OCH3) m[OCH2CH (OH) CH3] n]x and hydroxyethyl methyl cellulose HEMC, which is currently on the market for several main varieties of application and sales.
Solubility can be divided into ionic and non-ionic. The water soluble non-ionic cellulose ether is mainly composed of two series of alkyl ethers and alkyl ethers. Ionic CMC is mainly used in synthetic detergents, textiles, printing and dyeing, food and oil production. But non ionic MC, HPMC, HEMC and so on are mainly used in building materials, latex coatings, medicine, daily chemical and so on. Used as thickener, water retaining agent, stabilizer, dispersant and film-forming agent.
Water retention: cellulose ether in building materials especially in the production of dry mortar, cellulose ether plays an irreplaceable role, especially in the special mortar (modified mortar) production, is an important part of an indispensable. The important role of water soluble cellulose ether in mortar mainly has three aspects, one is good water holding capacity, two is the influence on mortar consistency and thixotropy, and three is interaction with cement. The water retention function of cellulose ether depends on the water absorption at the base, the composition of the mortar, the thickness of the mortar, the water demand of the mortar and the setting time of the setting material. The water retention of the cellulose ether itself comes from the solubility and de hydration of the cellulose ether itself. As we all know, cellulose chains contain large amounts of highly hydrated OH groups, but they are not soluble in water because of the high crystallinity of cellulose structures. The hydration of hydroxyl alone is not enough to pay for the strong hydrogen bonds between molecules and the Fan Dehua force. So the water swelling is not dissolved, when the introduction of substituents in the molecular chain, not only destroyed the hydrogen substituent chain, but also because of the adjacent chain substituents and the destruction of hydrogen bonds between wedge chain substituents, the greater the distance between the molecules is opened. The larger the hydrogen bond effect is, the more the cellulose lattice expands, the solution enters, and the cellulose ether becomes water-soluble, forming a high viscosity solution. When the temperature rises, the hydration of polymer weakens, while the water between chains is expelled. When the water is removed, the molecules begin to aggregate to form a three-dimensional network of gel folds.

The factors influencing the water retention of mortar include cellulose ether viscosity, adding amount, particle fineness and service temperature. The greater the viscosity of cellulose ether, the better the water retention property. Viscosity is an important parameter of MC performance. At present, different MC manufacturers adopt different methods and instruments to measure the viscosity of MC. The main methods are Haake, Rotovisko, Hoppler, Ubbelohde and Brookfield. For the same kind of products, the viscosity results obtained by different methods vary greatly, and some even vary exponentially. 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, the higher the molecular weight of MC, the solubility will be reduced accordingly, 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 more sticky the wet mortar, when the construction, the performance of sticky scraper and high adhesion to the substrate. But the increase in the structural strength of the wet mortar itself is of little help. When construction, the performance of anti sag is not obvious. In contrast, some low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar. The greater the amount of cellulose ether in the mortar, the better the water retention performance, the higher the viscosity, the better the water retention performance. For particle size, the finer the particle, the better the water retention. Cellulose ether and water contact of large particles, the surface immediately dissolved and forming a gel material wrapped to prevent water molecules to penetrate, sometimes stirred for a long time have not dispersed uniformly dissolved, forming a turbid flocculent solution or caking. The water retention function of cellulose ether is greatly affected, and solubility is one of the factors of choosing cellulose ether. Fineness is also an important performance index of methyl cellulose ether. MC for dry mortar requires powder, water content is low, and fineness also requires 20%~60% particle size is less than 63um. Fineness affects the solubility of methyl cellulose ether. The coarser MC is usually granular and is readily soluble and not agglomerated in water, but the dissolution rate is very slow and should not be used in dry mortar. In dry mortar, MC is dispersed between cement material such as aggregate, fine packing and cement, and only fine enough powder to avoid methyl cellulose ether agglomeration when mixing with water. When MC is dissolved in water, it is very difficult to disperse and dissolve it. The fineness of coarse MC is not only a waste, and will reduce the local strength of mortar, the mortar construction of large area, on the performance of local dry mortar curing rate was significantly reduced, due to cracking caused by the different curing time. For the use of mechanical construction of jet mortar, due to the shorter mixing time, higher fineness requirements. The fineness of MC also has some influence on its water retention. Generally speaking, the methyl cellulose ether with different viscosity and different fineness has better water retention effect under the condition of the same amount of addition. The water retention of MC is also related to the temperature used. The water holding capacity of methyl cellulose ether decreases with the increase of temperature. But in the actual application of the material in many circumstances, often dry mortar at high temperature (higher than 40) under the condition of construction on the hot substrate, such as a case of the putty plastering the sun in summer, which accelerates the curing and hardening of cement mortar. The decrease of water retention rate has obviously affected the workability and cracking resistance. In this case, the influence of temperature factors has become particularly critical. Although methyl hydroxyethyl cellulose ether additives are now considered to be at the forefront of technological development, their dependence on temperature will still lead to a decrease in the performance of dry mortar. In spite of increasing the amount of methyl cellulose (summer formula), workability and cracking resistance are still not met. With some special treatment, such as increasing etherification degree, MC can keep its water retention effect better under the condition of high temperature, and make it provide better performance under bad conditions.

In addition, the thickening and thixotropy of cellulose ethers: second effects of cellulose ethers - thickening depends on conditions such as polymerization degree of cellulose ether, concentration of solution, shear rate and temperature. The gelling properties of the solution are unique to alkyl cellulose and its modified derivatives. Gelation characteristics were related to degree of substitution, concentration of solution and additives. For alkyl modified derivatives, the gel properties are also related to the degree of modification of the alkyl groups. For MC and HPMC solution concentration and low viscosity can be prepared 10%-15% solution concentration, medium viscosity MC and HPMC can be prepared 5%-10% solution, and the high viscosity of MC and HPMC can only be prepared 2%-3% solution, but the viscosity of cellulose ether is usually graded by 1%-2% solution to the classification. High molecular weight cellulose ether thickening efficiency is high, the same concentration of different molecular weight polymers with different viscosity, viscosity and molecular weight between available following said [77]=2.92 * 10-2 (DPn) DPn is 0.905, the average degree of polymerization of high. Low molecular weight cellulose ethers can be added to achieve target viscosity. The viscosity depends little on shear rate, and the high viscosity achieves the target viscosity. The amount of viscosity required is small, and the viscosity depends on thickening efficiency. Therefore, to achieve a certain consistency, the cellulose ether must be guaranteed a certain amount of added (solution concentration) and solution viscosity. The gel temperature of the solution increases the concentration of the solution decreases linearly, to reach a certain concentration at room temperature after gelation. The gelation concentration of HPMC at room temperature is higher. Consistency can also be adjusted by choice of particle size and selection of cellulose ethers of varying degree of modification. The so-called modification is the introduction of a certain degree of substitution of hydroxyl alkyl on the skeleton structure of MC. By changing the relative substitution values of the two substituents, that is, the relative substitution values of DS and MS for the methoxy group and the alkyl group are often referred to. By changing the relative substitution values of the two substituents, a variety of properties of cellulose ethers are obtained. 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, that is thickening effect, the higher the amount of admixture, the more water consumption. Cellulose ethers used in powdered building materials must be rapidly dissolved in cold water and provide a suitable consistency for the system. If given a shear rate is still flocculent and colloidal lump, this is substandard or poor quality products. There is also a good linear relationship between the consistency of cement paste and the amount of cellulose ether. The cellulose ether can greatly increase the viscosity of the mortar, and the larger the dosage, the more obvious the effect is. High viscosity aqueous solution of cellulose ether exhibits high thixotropy, which is a major characteristic of cellulose ethers. Aqueous solutions of MC polymers usually exhibit pseudoplastic and non thixotropic flow below their gel temperature, but exhibit Newtonian flow at low shear rates. The pseudo plasticity increases with the increase of the molecular weight or concentration of cellulose ether, irrespective of the type of substituent and degree of substitution. Therefore, cellulose ethers of the same viscosity grade, whether MC, HPMC, and HEMC, always exhibit the same rheological properties as long as the concentration and temperature remain constant. When the temperature is increased, the structure gel is formed, and there is a high thixotropy flow. High concentration, low viscosity cellulose ethers exhibit thixotropy, even below the gel temperature. This property is beneficial to the construction of mortar, to adjust its leveling and flow. It should be noted that, the higher the viscosity of cellulose ether, better water retention, but the viscosity is high, the higher the molecular weight of cellulose ether, its solubility decreased, which caused negative influence on mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect of mortar, but not completely proportional to the relationship. Some low viscosity, but modified cellulose ether has better performance in improving the structural strength of wet mortar. With the increase of viscosity, the water retention of cellulose ether is improved. Retarding of cellulose ether: the third function of cellulose ether is to retard the hydration of cement. Cellulose ethers give mortar a variety of beneficial properties, but also reduce the early hydration of cement, heat release, and retard the hydration process of cement. This is bad for the use of mortar in cold areas. The retarding effect is the adsorption of cellulose ether molecules in C-S-H and Ca (OH) caused by the 2 hydration products, due to the increase of pore solution viscosity, cellulose ether reduces the activity of ions in 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 retard coagulation, but also delay the hardening process of cement mortar system. Cellulose ether retarding effect not only depends on its mineral concentration in the gel system, and depends on the chemical structure, the higher the degree of HEMC methylation, cellulose ether retarding effect better, stronger retarding effect than water substituted hydrophilic substitution. But the viscosity of cellulose ether has little influence on the hydration kinetics of cement.
With the increase of cellulose ether content, the setting time of mortar increases remarkably. There is a good nonlinear correlation between the initial setting time of the mortar and the amount of cellulose ether. The final setting time has a good linear correlation with the amount of cellulose ether. We can control the operation time of mortar by changing the amount of cellulose ether.
To sum up, in the premixed mortar, cellulose ether plays a role in water retention, thickening, delaying the hydration power of cement, improving construction performance and so on. Good water retention capacity, cement hydration more complete, can improve wet mortar wet sticky, improve mortar bond strength, can adjust the time. The mechanical spraying mortar can be improved by adding cellulose ether