Cellulose ether mainly has the following three functions:
1) It can thicken the fresh mortar to prevent segregation and obtain a uniform plastic body;
2) It has an air-entraining effect, and can also stabilize the uniform and fine air bubbles introduced into the mortar;
3) As a water-retaining agent, it helps to maintain the water (free water) in the thin-layer mortar, so that the cement can have more time to hydrate after the mortar is constructed.
In dry-mixed mortar, methyl cellulose ether plays the role of water retention, thickening and improving construction performance. Good water retention performance ensures that the mortar will not cause sanding, powdering and strength reduction due to water shortage and incomplete cement hydration; the thickening effect greatly enhances the structural strength of the wet mortar, and the good anti-sagging ability of tile adhesive is an example; The addition of base cellulose ether can significantly improve the wet viscosity of wet mortar, and has good viscosity to various substrates, thereby improving the wall performance of wet mortar and reducing waste.
When using cellulose ether, it should be noted that if the dosage is too high or the viscosity is too high, the water demand will increase, and the construction will feel laborious (sticky trowel) and workability will decrease. Cellulose ether will delay the setting time of cement, especially when the content is higher, the retarding effect is more significant. In addition, cellulose ether will also affect the open time, sag resistance and bond strength of mortar.
The appropriate cellulose ether should be selected in different products, and its functions are also different. For example, it is advisable to choose MC with higher viscosity in tile adhesive, which can prolong the opening time and adjustable time, and improve the anti-slip performance; in self-leveling mortar, it is advisable to choose MC with lower viscosity to maintain the fluidity of the mortar, and at the same time It also acts to prevent stratification and water retention. Suitable cellulose ethers should be determined according to the manufacturer’s recommendations and corresponding test results.
In addition, cellulose ether has a foam stabilizing effect, and due to early film formation, it will cause skinning in the mortar. These cellulose ether films may have formed during or immediately after stirring, before the redispersible rubber powder started to form a film. The essence behind this phenomenon is the surface activity of cellulose ethers. Since the air bubbles are physically brought in by the agitator, the cellulose ether quickly occupies the interface between the air bubbles and the cement slurry to form a film. The membranes were still wet and thus very flexible and compressible, but the polarization effect clearly confirmed the orderly arrangement of their molecules.
Since cellulose ether is a water-soluble polymer, it will migrate to the surface of the mortar contacting the air with the evaporation of water in the fresh mortar to form enrichment, thus causing the skinning of cellulose ether on the surface of the new mortar. As a result of the skinning, a denser film is formed on the surface of the mortar, which shortens the open time of the mortar. If the tiles are pasted on the surface of the mortar at this time, this layer of film will also be distributed to the interior of the mortar and the interface between the tiles and the mortar, thereby reducing the later bonding strength. The skinning of cellulose ether can be reduced by adjusting the formula, selecting the appropriate cellulose ether and adding other additives.