In the composition of dry powder mortar, cellulose ether is an important additive with a relatively low addition amount, but it can significantly improve the mixing and construction performance of mortar. To put it simply, almost all the wet mixing properties of mortar that can be seen with the naked eye are provided by cellulose ether. It is a cellulose derivative obtained by using cellulose from wood and cotton, reacting with caustic soda, and then etherifying with an etherifying agent.
Types of cellulose ethers
A. Hydroxypropyl methyl cellulose (HPMC), which is mainly made of high-purity refined cotton as raw material, is specially etherified under alkaline conditions.
B. Hydroxyethyl methylcellulose (HEMC), a non-ionic cellulose ether, is white powder in appearance, odorless and tasteless.
C. Hydroxyethyl cellulose (HEC), a non-ionic surfactant, white in appearance, odorless, tasteless and easy-flowing powder.
The above are non-ionic cellulose ethers, and ionic cellulose ethers (such as carboxymethyl cellulose CMC).
During the use of dry powder mortar, because ionic cellulose (CMC) is unstable in the presence of calcium ions, it is rarely used in inorganic gelling systems with cement and slaked lime as cementing materials. In some places in China, Some interior wall putties processed with modified starch as the main cementing material and Shuangfei powder as the filler use CMC as the thickener. However, because this product is prone to mildew and is not resistant to water, it is gradually eliminated by the market. At present, the cellulose ether mainly used in China is HPMC.
Cellulose ether is mainly used as water retention agent and thickener in cement-based materials
Its water retention function can prevent the substrate from absorbing too much water and hinder the evaporation of water, so as to ensure that the cement has enough water when it is hydrated. Take the plastering operation as an example. When ordinary cement slurry is applied to the base surface, the dry and porous substrate will quickly absorb a large amount of water from the slurry, and the cement slurry layer close to the base layer will easily lose the water required for hydration. , so not only can not form a cement gel with bonding strength on the surface of the substrate, but it is also prone to 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 surface plastering operation, water is usually used to wet the substrate 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 the content of cellulose ether. The greater the viscosity of the added cellulose ether, the better the water retention.
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.
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.
Post time: Apr-25-2024