1 Introduction
Cellulose ether (MC) is widely used in the building materials industry and used in a large amount. It can be used as a retarder, water retention agent, thickener and adhesive. In ordinary dry-mixed mortar, external wall insulation mortar, self-leveling mortar, tile adhesive, high-performance building putty, crack-resistant interior and exterior wall putty, waterproof dry-mixed mortar, gypsum plaster, caulking agent and other materials, cellulose Ethers play an important role. Cellulose ether has an important influence on the water retention, water demand, cohesiveness, retardation and construction of the mortar system.
There are many different types and specifications of cellulose ethers. Commonly used cellulose ethers in the field of building materials include HEC, HPMC, CMC, PAC, MHEC, etc., which are used in different mortar systems according to their respective characteristics. Some people have done research on the influence of different types and different amounts of cellulose ether on the cement mortar system. This article focuses on this basis and explains how to choose different varieties and specifications of cellulose ethers in different mortar products.
2 Functional characteristics of cellulose ether in cement mortar
As an important admixture in dry powder mortar, cellulose ether has many functions in mortar. The most important role of cellulose ether in cement mortar is to retain water and thicken. In addition, due to its interaction with the cement system, it can also play an auxiliary role in entraining air, retarding setting, and improving tensile bond strength.
The most important performance of cellulose ether in mortar is water retention. Cellulose ether is used as an important admixture in almost all mortar products, mainly because of its water retention. Generally speaking, the water retention of cellulose ether is related to its viscosity, addition amount and particle size.
Cellulose ether is used as a thickener, and its thickening effect is related to the etherification degree, particle size, viscosity and modification degree of cellulose ether. Generally speaking, the higher the degree of etherification and viscosity of cellulose ether, the smaller the particles, the more obvious the thickening effect. By adjusting the above characteristics of MC, the mortar can achieve the appropriate anti-sagging performance and the best viscosity.
In the cellulose ether, the introduction of the alkyl group reduces the surface energy of the aqueous solution containing the cellulose ether, so that the cellulose ether has an air-entraining effect on the cement mortar. Introducing appropriate air bubbles into the mortar improves the construction performance of the mortar due to the “ball effect” of the air bubbles. At the same time, the introduction of air bubbles increases the output rate of the mortar. Of course, the amount of air-entrainment needs to be controlled. Too much air-entrainment will have a negative impact on the strength of the mortar, because harmful air bubbles may be introduced.
2.1 Cellulose ether will delay the hydration process of cement, thereby slowing down the setting and hardening process of cement, and prolonging the opening time of mortar accordingly, but this effect is not good for mortar in colder regions. When choosing cellulose ether, the appropriate product should be selected according to the specific situation. The retarding effect of cellulose ether is mainly extended with the increase of its etherification degree, modification degree and viscosity.
In addition, cellulose ether, as a long-chain polymer substance, can improve the bonding performance with the substrate after being added to the cement system under the premise of fully maintaining the moisture content of the slurry.
2.2 The properties of cellulose ether in mortar mainly include: water retention, thickening, prolonging setting time, entraining air and improving tensile bonding strength, etc. Corresponding to the above properties, it is reflected in the characteristics of MC itself, namely: viscosity, stability, content of active ingredients (addition amount), degree of etherification substitution and its uniformity, degree of modification, content of harmful substances, etc. Therefore, when choosing MC, the cellulose ether with its own characteristics that can provide suitable performance should be selected according to the specific requirements of the specific mortar product for a certain performance.
3 Characteristics of cellulose ether
Generally speaking, the product instructions provided by cellulose ether manufacturers will include the following indicators: appearance, viscosity, degree of group substitution, fineness, active substance content (purity), moisture content, recommended areas and dosage, etc. These performance indicators can reflect part of the role of cellulose ether, but when comparing and selecting cellulose ether, other aspects such as its chemical composition, modification degree, etherification degree, NaCl content, and DS value should also be examined.
3.1 Viscosity of cellulose ether
The viscosity of cellulose ether affects its water retention, thickening, retardation and other aspects. Therefore, it is an important indicator for examining and selecting cellulose ether.
Before discussing the viscosity of cellulose ether, it should be noted that there are four commonly used methods for testing the viscosity of cellulose ether: Brookfield, Hakke, Höppler, and rotational viscometer. The equipment, solution concentration and test environment used by the four methods are different, so the results of the same MC solution tested by the four methods are also very different. Even for the same solution, using the same method, testing under different environmental conditions, the viscosity
Results also vary. Therefore, when explaining the viscosity of a cellulose ether, it is necessary to indicate which method is used for testing, solution concentration, rotor, rotating speed, testing temperature and humidity and other environmental conditions. This viscosity value is valuable. It is meaningless to simply say “what is the viscosity of a certain MC”.
3.2 Product Stability of Cellulose Ether
Cellulose ethers are known to be susceptible to attack by cellulosic molds. When the fungus erodes the cellulose ether, it first attacks the unetherified glucose unit in the cellulose ether. As a linear compound, once the glucose unit is destroyed, the entire molecular chain is broken, and the product viscosity will drop sharply. After the glucose unit is etherified, the mold will not easily corrode the molecular chain. Therefore, the higher the degree of etherification substitution (DS value) of cellulose ether, the higher its stability will be.
3.3 Active ingredient content of cellulose ether
The higher the content of active ingredients in cellulose ether, the higher the cost performance of the product, so that better results can be achieved with the same dosage. The effective ingredient in cellulose ether is cellulose ether molecule, which is an organic substance. Therefore, when examining the effective substance content of cellulose ether, it can be indirectly reflected by the ash value after calcination.
3.4 NaCl content in cellulose ether
NaCl is an inevitable by-product in the production of cellulose ether, which generally needs to be removed by multiple washings, and the more washing times, the less NaCl remains. NaCl is a well-known hazard to the corrosion of steel bars and steel wire mesh. Therefore, although the sewage treatment of washing NaCl for many times may increase the cost, when choosing MC products, we should try our best to choose products with lower NaCl content.
4 Principles of choosing cellulose ether for different mortar products
When selecting cellulose ether for mortar products, first of all, according to the description of the product manual, select its own performance indicators (such as viscosity, degree of etherification substitution, effective substance content, NaCl content, etc.) Functional characteristics and selection principles
4.1 Thin plaster system
Taking the plastering mortar of the thin plastering system as an example, since the plastering mortar directly contacts the external environment, the surface loses water quickly, so a higher water retention rate is required. Especially during construction in summer, it is required that the mortar can better retain moisture at high temperature. It is required to select MC with a high water retention rate, which can be considered comprehensively through three aspects: viscosity, particle size, and addition amount. Generally speaking, under the same conditions, choose MC with higher viscosity, and considering the requirements of workability, the viscosity should not be too high. Therefore, the selected MC should have a high water retention rate and a low viscosity. Among MC products, MH60001P6 etc. can be recommended for adhesive plastering system of thin plastering.
4.2 Cement-based plastering mortar
Plastering mortar requires good uniformity of mortar, and it is easier to apply evenly when plastering. At the same time, it requires good anti-sagging performance, high pumping capacity, fluidity and workability. Therefore, MC with lower viscosity, faster dispersion and consistency development (smaller particles) in cement mortar is selected.
In the construction of tile adhesive, in order to ensure safety and high efficiency, it is especially required that the mortar has a longer opening time and better anti-slide performance, and at the same time requires a good bond between the substrate and the tile. Therefore, tile adhesives have relatively high requirements for MC. However, MC generally has a relatively high content in tile adhesives. When choosing MC, to meet the requirement of longer opening time, MC itself needs to have a higher water retention rate, and the water retention rate requires appropriate viscosity, addition amount and particle size. In order to meet the good anti-sliding performance, the thickening effect of MC is good, so that the mortar has strong vertical flow resistance, and the thickening performance has certain requirements on viscosity, etherification degree and particle size.
4.4 Self-leveling ground mortar
The self-leveling mortar has higher requirements on the leveling performance of the mortar, so it is suitable to choose low-viscosity cellulose ether products. Since self-leveling requires that the evenly stirred mortar can be automatically leveled on the ground, fluidity and pumpability are required, so the ratio of water to material is large. In order to prevent bleeding, MC is required to control the water retention of the surface and provide viscosity to prevent sedimentation.
4.5 Masonry mortar
Because the masonry mortar directly contacts the surface of the masonry, it is generally a thick-layer construction. The mortar is required to have high workability and water retention, and it can also ensure the bonding force with the masonry, improve the workability, and increase the efficiency. Therefore, the selected MC should be able to help the mortar to improve the above performance, and the viscosity of the cellulose ether should not be too high.
4.6 Insulation slurry
Since the thermal insulation slurry is mainly applied by hand, it is required that the selected MC can give the mortar good workability, good workability and excellent water retention. MC should also have the characteristics of high viscosity and high air-entrainment.
5 Conclusion
The functions of cellulose ether in cement mortar are water retention, thickening, air entrainment, retardation and improvement of tensile bond strength, etc.
Post time: Jan-30-2023