In the composition of dry powder mortar, methyl cellulose is a relatively low addition amount, but it has an important additive that 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 etherifying it with an etherifying agent.
Types of Methyl Cellulose Ether
A． Hydroxypropyl methylcellulose (HPMC) is mainly made of highly pure refined cotton as raw material, which is specially etherified under alkaline conditions.
B． Hydroxyethyl methyl cellulose (HEMC), a non-ionic cellulose ether, is a white powder, odorless and tasteless.
C． Hydroxyethylcellulose (HEC) is a non-ionic surfactant, white in appearance, odorless and 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, but because this product is prone to mildew and is not water-resistant, 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 too quickly 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 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 cannot form a cement gel with adhesive strength on the surface of the substrate, but also easily 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 surface plastering operation, the base material was usually wetted with water first, but this operation was labor-intensive and time-consuming, and the operation quality was 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, thereby prolonging the working time, so it is sometimes used as a set regulator.
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.
1. The working characteristics of modified mortar are closely related to the viscosity development of cellulose ether. Although products with high nominal viscosity have relatively high final viscosity, due to slow dissolution, it takes a long time to obtain the final viscosity; in addition , The cellulose ether with coarser particles takes longer to obtain the final viscosity, so the product with higher viscosity does not necessarily 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.