Cellulose ether (such as HPMC, Hydroxypropyl Methylcellulose) and MHEC (Methyl Hydroxyethyl Cellulose) are common building admixtures and are widely used in building mortars. They play an important role in improving the bonding strength of mortars, improving construction performance and extending the operability time of mortars.

1. Basic properties of HPMC and MHEC
HPMC is a polymer compound obtained by chemical modification of natural cellulose. Its molecules contain hydroxypropyl and methyl groups, which make it have good water solubility, thickening and stability. MHEC is similar to HPMC, but it has more hydroxyethyl groups in its molecular structure, so the water solubility and performance stability of MHEC are different. They can both form a network structure in the mortar and enhance the physical properties of the mortar.

2. Mechanism of action of cellulose ether in mortar
After adding HPMC or MHEC to the mortar, the cellulose ether molecules form a stable colloidal system through the interaction with water, other chemical components and mineral particles. This system can significantly improve the bonding properties of mortar.

Thickening effect: HPMC and MHEC can increase the consistency of mortar, making it easier to operate during construction. This thickening effect also helps reduce the fluidity of cement paste, improve the adhesion of mortar, and thus enhance the bonding strength of mortar.

Water retention effect: The molecular structure of HPMC and MHEC contains hydrophilic groups, which can absorb a large amount of water and release it slowly, thereby extending the open time of mortar and avoiding surface cracking or poor bonding due to rapid evaporation of water.

Improve fluidity and construction performance: Cellulose ether helps to improve the fluidity of mortar, allowing it to be applied more evenly on the surface of the base, which is conducive to the uniform distribution of bonding force.

3. Effect of cellulose ether on mortar bonding strength
The addition of cellulose ether to mortar usually directly affects the bonding strength of mortar. Specifically, the effects of HPMC and MHEC on mortar bonding strength are reflected in the following aspects:

3.1 Influence on the initial bonding strength of mortar
HPMC and MHEC can improve the bonding performance between mortar and base surface. When the construction is just completed, the bonding strength between the mortar surface and the substrate is significantly improved because cellulose ether can retain water and reduce the premature drying of cement paste. This is because the cement hydration reaction can proceed smoothly, which promotes the early hardening of the mortar.

3.2 Influence on the long-term bonding strength of mortar
As time goes by, the cement component of the mortar undergoes a continuous hydration process, and the strength of the mortar will continue to increase. The water retention performance of cellulose ether still plays a key role in this process, avoiding the rapid volatilization of water in the mortar, thereby reducing the strength reduction caused by insufficient water.

3.3 Improve the crack resistance of mortar
HPMC and MHEC can also improve the crack resistance of mortar. Its mechanism of action is mainly to enhance the internal structural stability of the mortar and slow down the evaporation rate of water on the surface of the mortar, thereby reducing the crack problem caused by rapid evaporation of water. In addition, the colloidal structure formed by cellulose ether in the mortar can improve the overall toughness of the mortar, making it less likely to crack when subjected to external forces.

3.4 Effects on the strength improvement of mortar
Studies have shown that adding an appropriate amount of HPMC or MHEC can improve the bonding strength of mortar without significantly increasing the weight of mortar. Generally, the optimal dosage of cellulose ether is 0.5%-1.5%. Excessive addition may cause the mortar to have excessive fluidity, which in turn affects its bonding properties. Therefore, a reasonable amount of cellulose ether added is crucial to enhancing the bonding strength of mortar.

4. Comparison of different types of cellulose ethers
Although HPMC and MHEC are similar in their mechanism of action, their effects on the bonding strength of mortar are different in actual applications. MHEC is more hydrophilic than HPMC, so in a humid environment, MHEC may have a more significant effect on improving bonding strength. HPMC, on the other hand, is more stable under normal temperature and humidity conditions, and is particularly suitable for some traditional mortar preparations.

Cellulose ethers (HPMC and MHEC) are commonly used additives for mortars, which significantly improve the bonding strength of mortars through thickening, water retention, and improved fluidity. Reasonable use of cellulose ether can not only enhance the adhesion between mortar and substrate, but also improve the crack resistance and durability of mortar, and extend the service life of mortar. Different types of cellulose ether have different applicability, and choosing the right product and dosage is crucial to improving mortar performance.