What is the effect of hydroxypropyl methylcellulose in mortar?

The functions of hydroxypropyl methylcellulose in mortar are: water retention, increased adhesion, thickening, affecting setting time, and air entrainment. Because of these characteristics, it has a wide range of applications in building materials mortars.
1. The water retention of hydroxypropyl methylcellulose is the most important characteristic in mortar applications.
The main factors affecting the water retention of hydroxypropyl methylcellulose: viscosity, particle size, dosage, active ingredient, dissolution rate, water retention mechanism: the water retention of hydroxypropyl methylcellulose itself comes from hydroxypropyl methyl The solubility and dehydration of cellulose itself. Although the cellulose molecular chain contains a large number of hydroxyl groups with strong hydration properties, it itself is not soluble in water. This is because the cellulose structure is highly crystalline, and the hydration ability of hydroxyl groups alone is not enough to destroy the strong intermolecular bonds. Hydrogen bonds and van der Waals forces, so it only swells but does not dissolve in water. When a substituent is introduced into a molecular chain, not only does the substituent destroy the hydrogen bond, but also the inter-chain hydrogen bond is destroyed due to the wedge of substituents between adjacent chains. The larger the substituent, the greater the distance between molecules, destroying the hydrogen bonding effect. The larger the cellulose lattice is, the solution will enter and the cellulose ether will become water-soluble, forming a high-viscosity solution. As the temperature increases, the hydration of the polymer weakens and the interchain water is driven out. When dehydration is sufficient, molecules begin to aggregate and form a three-dimensional network structure to precipitate.
(1) Effect of particle size and mixing time of hydroxypropyl methylcellulose on water retention
As the viscosity of the same dosage of hydroxypropyl methylcellulose increases, the water retention of the mortar increases; as the dosage and viscosity of hydroxypropyl methylcellulose increase, the water retention of the mortar increases. When the content of hydroxypropyl methylcellulose exceeds 0.3%, the changes in the water retention rate of the mortar tend to be balanced. The water retention capacity of mortar is largely controlled by the dissolution time. The finer hydroxypropyl methylcellulose dissolves faster and the water retention capacity develops faster.
(2) Effects of etherification degree of hydroxypropyl methylcellulose and temperature on water retention
The water retention decreases as the temperature increases. The higher the degree of etherification of hydroxypropyl methylcellulose, the better the high temperature water retention of hydroxypropyl methylcellulose. During use, the temperature of fresh mortar is usually lower than 35°C. Under special climatic conditions, the temperature may reach or even exceed 40°C. In this case, the formula must be adjusted and a product with a higher degree of etherification must be selected. That is, consider selecting the appropriate cellulose ether.
2. Effect of hydroxypropyl methylcellulose on the air content of mortar
In dry mixed mortar products, due to the addition of hydroxypropyl methylcellulose, a certain amount of tiny, evenly distributed, and stable bubbles are introduced into the fresh mortar. Due to the rolling effect of the bubbles, the mortar has good workability. Reduce mortar cracking and shrinkage and increase mortar yield.
3. Effect of hydroxypropyl methylcellulose on cement hydration
Hydroxypropyl methylcellulose has a delaying effect on the hydration of cement-based mortar, and the delaying effect increases with the increase in the amount of hydroxypropyl methylcellulose. The influencing factors of hydroxypropyl methylcellulose on cement hydration are: dosage, degree of etherification, and type of cement.