The action of hydroxypropyl methyl cellulose in dry powder mortar

Ⅰ,Overview of cellulose
Cellulose ether is a kind of synthetic macromolecule polymer made from natural cellulose as raw material and chemically modified. Cellulose ether
A derivative of natural cellulose. Cellulose ethers are produced differently from synthetic polymers. The 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 swelling agent, strong in the molecular chain and the intra chain hydrogen bond was destroyed, the activity of hydroxyl release into a reaction capability of alkali cellulose, after etherification agent OH reaction medium into OR medium to obtain cellulose ether.
1. kinds of cellulose, MC cellulose, HPMC cellulose, HEMC cellulose difference
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. In addition, there are hydroxyethyl methyl cellulose ether HEMC, which are widely used and sold in the market.
2. classification of cellulose types
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.

Ⅱ, the water retention of cellulose ether
Cellulose fiber plays an irreplaceable role in the production of building materials, especially dry mortar, especially in the production of special mortar.
1. the function of cellulose in mortar
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. It is well known that cellulose chains contain a large amount of highly hydrated OH
It is insoluble in water because the structure of cellulose is highly crystalline. 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 destroy 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
Aggregation, formation of three-dimensional mesh structures, gel folding. 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 performance, and the viscosity of polymer solution. The molecular weight (polymerization degree) of polymers depends on the molecular structure, chain length and chain morphology. The type, number and distribution of substituents also affect their viscosity range directly.
The viscosity of the polymer solution depends on the molecular weight of the polymer. The viscosity and concentration of cellulose ether solutions are related to the application in various fields. Therefore, each cellulose ether has many different viscosity specifications, and the regulation of viscosity is mainly achieved by the degradation of alkali cellulose, that is, the breaking of cellulose molecular chain. 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 the solubility is chosen as cellulose ether One of the elements.

Ⅲ, thickening and thixotropy of cellulose ethers
The second role of cellulose ether thickening depends on conditions such as the degree of polymerization of cellulose ether, the concentration of the solution, the rate of shear, and the 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.
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 addition of cellulose ether influences the water consumption of mortar, changes the water to cement ratio of water and cement, which is thickening effect, and makes the amount of admixture increase, and the water consumption increases.
Cellulose ethers used in powdered building materials must be rapidly dissolved in cold water and can provide the proper consistency of the system.
If given a shear rate is still flocculent and colloidal lump, this is substandard or poor quality products.
The amount of cement slurry consistency and cellulose ether has a good linear relationship, cellulose ether can greatly increase the viscosity of the mortar, content is greater, the more obvious effect, high viscosity cellulose ether water solution has high thixotropy, which is a major characteristic of cellulose ether. Therefore, cellulose ethers of the same viscosity grade, whether MC, HPmc, HEmc, remain constant as long as the concentration and temperature are constant, exhibiting the same rheological properties. 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 needs to be explained that the viscosity of cellulose ether is high and the water retention is better, but the higher the viscosity is, the higher the relative molecular weight of cellulose ether is, and the solubility of the cellulose ether decreases accordingly
Slurry concentration and construction performance have a negative impact. 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.

Ⅳ. The retarding property of cellulose ether
The third function of cellulose ether is to retard the hydration process of cement. Cellulose ether imparts a variety of beneficial properties to mortar. It also reduces the amount of heat released by early hydration of cement and delays the hydration process of cement. This is bad for the use of mortar in cold areas. As the pore solution viscosity increases, the 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 hydration of cellulose ether
The more obvious the delay is. 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 of 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.