Cellulose ether is generated from natural cellulose as the basic raw material through alkalization and etherification reactions. Cellulose has a highly crystalline structure that only swells and does not dissolve in water, so it does not react with etherification agents. However, through the swelling agent, the hydrogen bonds between cellulose molecules are broken, and the activity of hydroxyl groups is stimulated, resulting in etherification reaction – OH to – OR, thus obtaining cellulose ether. The methyl cellulose ether (MC) we commonly use is a monoether, and hydroxypropyl methylcellulose ether (HPMC) is a mixed ether.
The molecular formula of MC is: [C6H7O2 (OH) 3-h (OCH3) h] x
The molecular formula of HPMC is: [C6H7O2 (OH) 3-mn (OCH3) m (OCH2CH (CH3) OH) n] x
The amount of cellulose ether added to mortar is very small, but it can increase the viscosity of the solution and improve the workability of freshly mixed mortar. Its functions in mortar include thickening, water retention, and construction. Generally speaking, as the molecular weight of cellulose ether increases, its viscosity increases and its water retention effect improves. However, as the molecular weight increases, the solubility of cellulose ether decreases, which is more unfavorable for the construction performance of mortar.
1. Adsorption of cellulose ether
The cellulose ether in mortar will expand when it absorbs water, forming a gel like membrane structure with adsorption effect. This substance adsorbs on the surface of cement particles and plays a network connection role. Some cellulose ether particles aggregate together to form larger clusters, dispersed on the surface and pores of mortar particles. The adsorption of cellulose ether depends on factors such as the type of substituent R, pH value, and mineral composition in cement.
2. The delaying effect of cellulose ether on early hydration of clinker minerals
The fastest hydration of clinker minerals is C3A, and the highest content is C3S. Cellulose ether, after absorbing water, coats the surface of cement particles, slowing down the hydration rate of C3A and C3S, delaying the growth of hydration products, and reducing their nucleation rate.
3. The influence of cellulose ether on the pore structure of freshly mixed mortar
Cellulose ether affects the pore structure of mortar, thereby changing the way water is transported. The addition of cellulose ether increases the number of large pores in the freshly mixed mortar, and the membrane structure formed by cellulose ether can adsorb at the bubble interface, stabilizing the bubbles and allowing more small bubbles to successfully fuse, reducing the transmission rate of water from flow to flow and diffusion. In addition, the water absorption capacity of cellulose ether itself plays a role in water retention. Therefore, the addition of cellulose ether can cause an increase in the porosity of mortar.