Mechanism of action and influence factors of cellulose ether in cement mortar

Abstract: Cellulose ether can significantly improve the properties of cement mortar in terms of water retention, thickening, retarding, and air-entraining. By introducing the chemical structure and performance characteristics of cellulose ether, the action mechanism of cellulose ether in cement mortar is discussed, and some problems in practical application are analyzed.

Key words: Mechanism of cellulose ether, influencing factors

Cellulose ether is a commonly used polymer admixture in cement mortar for construction. It is a cellulose derivative obtained by using cellulose from wood and cotton, reacting with caustic soda, and then etherifying with an etherifying agent. Cellulose ether is fully dispersed in water to form a transparent viscous liquid. After adding cement in a small amount (less than 1% by mass), it can improve the flow and coagulation characteristics of cement mortar, so it is widely used in the field of building materials. There are many varieties and specifications of cellulose ether, and some articles have studied the effects of different types and amounts of cellulose ether on the properties of cement mortar. However, there are no good reference materials on how to choose cellulose ethers of different varieties and specifications.

1. The main role of cellulose ether in cement-based materials

1.1 As a water retaining agent and thickener

Since the oxygen atom on the hydroxyl group and the ether bond associates with the water molecule to form a hydrogen bond, the free water becomes bound water, so the cellulose ether can be used as a water retention agent. The water retention effect of cellulose ether can prevent the substrate from absorbing too much water too quickly and hinder the evaporation of water, so as to ensure that there is enough water when the cement is hydrated. Take the plastering operation as an example. When ordinary cement slurry is applied to the surface of the substrate, the dry and porous substrate will quickly absorb a large amount of water from the slurry, and the cement slurry layer close to the substrate will easily lose its hydration. Therefore, not only the cement gel with bonding strength cannot be formed on the surface of the substrate, but also it is easy to cause warping and water seepage, so that the surface cement slurry layer is easy to fall off. When the grout applied is thin, it is also easy to form cracks in the whole grout. Therefore, in the past plastering operation, the substrate was usually wetted with water first. But this operation is not only labor-intensive and time-consuming, but also the operation quality is difficult to control.

Generally speaking, the water retention of cement slurry increases with the increase of cellulose ether content. The greater the viscosity (molecular weight) of the added cellulose ether, the better the water retention. Cellulose ether can increase the viscosity of the liquid, so it can be used as a thickener. It has played an important role in paints, latex paints and cement grouts. In fact, cellulose ether acts as a protective colloid, which overcomes the flocculation of solid particles. The cement slurry added with cellulose ether has a higher viscosity, the slurry is not easy to stratify, and the total time and construction operations required for the mixing process are less, which improves the workability and construction performance of the mortar. In addition, the mortar with cellulose ether has a certain yield stress when it flows, which can improve the anti-sag ability of the grout, help reduce the slippage of the tiles bonded on the vertical wall, and reduce the loss of the tile adhesive. Dosage.

The water retention and thickening effect of cellulose ether mainly depends on the viscosity (ie molecular weight) and addition amount of cellulose ether itself added to the cement slurry. The viscosity of the cellulose ether itself is represented by the viscosity of a 20° C. cellulose ether aqueous solution with a mass fraction of 2% in a Brookfield viscometer at 20 revolutions per minute. The higher the degree of polymerization (or the greater the molecular weight), the greater the viscosity of the cellulose ether.

The viscosity of the freshly mixed mortar with cellulose ether is not only related to the molecular weight (viscosity) and dosage of cellulose ether, but also related to the solubility characteristics of cellulose ether. Because the viscosity of the cellulose ether can be fully expressed only when the cellulose ether is fully dissolved, the viscosity of the cellulose ether solution gradually increases within a period of time after starting to stir, and the time to reach the final viscosity increases with the dissolution rate of the cellulose ether. And shortened. This is also the reason why many dry-mixed mortar instructions require the mortar to stand (cure) for a certain period of time after mixing before using it. Most of the cellulose ethers sold in the market are in the form of white powder, and the fineness of the powder will affect the development speed of the cement slurry viscosity. The smaller the powder particles, the better the solubility of the fully dispersed cellulose ether in water, and the shorter the time required to reach the final viscosity, thus shortening the standing curing time. In addition, temperature and pH value also have a certain influence on the function of cellulose ether. Cellulose ether is not easily soluble in hot water but easily soluble in cold water. In the process of rising water temperature, water molecules gradually break away from the shackles of hydroxyl groups; when the temperature reaches a certain value, water molecules become very active, and no longer stabilize with cellulose ether, so that the latter interact with each other A polymer entanglement coil is formed. The temperature at which this effect occurs is called the gel point. When the aqueous solution temperature of most cellulose ethers is below the gel point, the lower the solution temperature, the higher the viscosity and the more obvious the effect of water retention and viscosity increase. Some varieties do not have a gel point, such as carboxymethyl cellulose ether and hydroxyethyl cellulose ether, which dissolve in both cold and hot water. When the pH value is in the range of 3 to 11, due to its non-ionic properties, the viscosity of cellulose ether does not fluctuate much with the change of pH value; only in strong acid or strong alkali, the acid and alkali will dehydrate the cellulose ether, resulting in a decrease in viscosity .

1.2 Other functions

In addition to water retention and thickening, cellulose ether also affects other properties of cement mortar, such as retarding, entraining air, and increasing bond strength.

Cellulose ether slows down the setting and hardening process of cement, thus prolonging the working time. Therefore, it is sometimes used as a coagulant. The retardation effect has little to do with the molecular weight of the cellulose ether, but more to do with the degree of substitution of the alkyl group. The smaller the degree of alkyl substitution, the larger the hydroxyl content, and the more obvious the retarding effect. This rule exists in the three most commonly used cellulose ethers - HEC, MHEC and MHPC.

The effect of increasing the bonding strength is reflected in the improvement of the bonding ability of the cement slurry added with cellulose ether to non-absorbent substrates such as steel mesh and glass. This mainly comes from the bonding effect provided by the viscosity of cellulose ether, a long-chain polymer substance.

The introduction of alkyl groups reduces the surface energy of the aqueous solution containing cellulose ether, so cellulose ether has an air-entraining effect on cement mortar. After adding cellulose ether, the liquid is not only easy to introduce bubbles, but also the toughness of the bubble film is higher than that of pure water bubbles, and the drainage effect is reduced, so the introduced bubbles can not be broken, and the bubbles are not easy to be discharged. The air-entraining effect of cellulose ether is negative for mechanical strength, because it may introduce harmful air bubbles and reduce the compressive strength; but when used as a foam stabilizer to prepare foamed concrete, it can improve the stability of the foam. If you need to eliminate air bubbles, you can add a defoamer.

2. Influencing factors

Cellulose ether is used as an additive to cement mortar, and the dosage usually does not exceed 2% of the cement mass. The research on cellulose ether has a long history in foreign countries. For example, Dow Company successfully developed and introduced it to the market as early as 1938. However, the production life of several major production enterprises in the territory is relatively short, and the basic research is relatively weak. Whether it is a customer or a manufacturer, most of the understanding of cellulose ether comes from the experience summary in the actual construction process.

2.1 Variety selection

The performance indicators of cellulose ether provided by suppliers generally include appearance, viscosity, degree of group substitution, fineness, purity, moisture content and pH value, etc., and recommended application fields and dosage. Nevertheless, there are still large differences between different cellulose ethers used in the same field. Li Yuhai's impact strength test on different types of cellulose ether and two types of latex powder compounded modified mortar showed that the impact strength of the three different cellulose ether modified mortars with a nominal viscosity of 20000mPa s was very different. There are also some differences in the laws of different cellulose ethers and different latex powders mixed with modified mortar. However, the structure and related properties of these cellulose ethers are not reported in this paper, so the reasons for these differences cannot be analyzed.

Another phenomenon is that a company has a variety of cellulose ether products, and often one product is recommended for multiple purposes. Conversely, multiple products are often recommended for the same purpose, but the reasons for their recommendation are not. There is no introduction.

2.2 Viscosity

First of all, the higher the viscosity of cellulose ether, the lower the mass concentration required for its aqueous solution to achieve the same viscosity. For example, if the viscosity of the aqueous solution is 10000 mPa·s, the dosage of cellulose ether with a viscosity of 20000 is only about 1.6%; when the viscosity is 12000, the dosage is about 1.8%; About 5%. That is to say, using high viscosity cellulose ether, the amount needed to achieve the same final viscosity is less than that of low viscosity cellulose ether. However, if the viscosity does not increase much, then its dosage decreases not much. For example, the viscosity increases from 12,000 to 20,000, and the viscosity increases by 80%. The amount to reach 10,000 viscosity is reduced from 1.8% to 1.6%, which is only 11%.

On the other hand, the cellulose ether with higher viscosity (or molecular weight) will take longer for its aqueous solution to reach the final viscosity due to the slower dissolution. In this way, the mortar prepared with cellulose ether with higher viscosity takes longer to reach the final viscosity, that is, it takes longer to stand (or mature) after the mortar is stirred. For example, for an aqueous solution of methyl hydroxypropyl cellulose ether (MHPC) with a viscosity of 20,000 mPa·s, it takes 40 minutes to reach the final viscosity. However, there is no experimental data on whether the viscosity develops faster in cement mortar.

Finally, in the case of unmodified cellulose ethers, the maximum viscosity depends on the degree of polymerization of the cellulose pulp used to prepare the cellulose ether. For example, the degree of polymerization of the cellulose of spruce is 600-1700, and that of beech is 800-1100. Among all cellulose pulps, the degree of polymerization of cotton pulp is the largest, generally between 450 and 2650. Based on the maximum degree of polymerization of 2650, based on the existing data on the viscosity and degree of polymerization of methyl cellulose ether, the corresponding maximum possible viscosity of methyl cellulose ether should not exceed 150,000. Therefore, the author is skeptical about the claim in some corporate promotional materials that cellulose ether with a viscosity higher than 180,000 can be produced according to user requirements.

2.3 Quality fluctuation

In the test, the properties of different batches of the same brand of cellulose ether are quite different. In the mortar consistency test with 0.6% cellulose ether in cement, different batches of cellulose ether can change the consistency of the mortar with the same proportion by more than 12mm. On the one hand, this kind of quality fluctuation may be due to the manufacturer's intentional adjustment of the formula, but did not notify the user; on the other hand, it may be a problem in production quality control. For the majority of cellulose ether users, no matter what the reason is, pre-testing before and after purchase is still essential. Among the quality indicators of cellulose ether, the most commonly used ones are viscosity and fineness. Measuring the viscosity of cellulose ether is not an easy task. First, because most of the cellulose ether used in cement-based materials is instant-soluble, it is easy to agglomerate when preparing an aqueous solution, resulting in slow dissolution; secondly, a viscometer and a viscometer are required. A constant temperature device can accurately determine its standard viscosity; therefore, many small and medium-sized enterprises that use cellulose ether have no way to carry out incoming inspection of cellulose ether. In addition, the fineness will affect the dissolution rate of cellulose ether, which will also affect the viscosity development of the solution. The smaller the fineness, the faster the cellulose ether dissolves, and the aforementioned differences between different batches of cellulose ether include the difference in fineness. It is relatively easy to monitor fineness, and it is recommended that all application companies should conduct testing.

3. Summary

With the development of dry-mixed mortar, cellulose ether has become an important cement mortar admixture. However, there are many varieties and specifications of cellulose ether, and the quality between batches still fluctuates. When using it, it is necessary to pay attention to: (1) The performance characteristics of modified mortar are closely related to the viscosity development of cellulose ether, although the nominal For products with high viscosity, the final viscosity is relatively large, but due to the slow dissolution, it takes a long time to obtain the final viscosity; in addition, the cellulose ether with coarse particles takes a long time to obtain the final viscosity, so it is not Products with higher viscosity are bound to have better working characteristics. (2) Due to the limitation of the degree of polymerization of cellulose ether raw materials, the maximum viscosity of cellulose ether is also limited. (3) It is necessary to check the purchase, production process and factory inspection to avoid quality fluctuations.