As the most important admixture in building dry-mixed mortar products, cellulose ether plays a key role in the performance and cost of dry-mixed mortar. There are two types of cellulose ethers: one is ionic, such as sodium carboxymethyl cellulose (CMC), and the other is non-ionic, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC), Hydroxypropyl cellulose (HPMC), etc. With the increasing popularity of the application of dry-mixed mortar products, my country will become the world’s largest producer of dry-mixed mortar, the application of cellulose ether will further increase, and its manufacturers and product varieties will also increase. The product performance of cellulose ether in dry-mixed mortar has become the focus of attention of producers and users.
The most important property of cellulose ether is its water retention in building materials. Without the addition of cellulose ether, the thin layer of fresh mortar dries out so quickly that the cement cannot hydrate in the normal way and the mortar cannot harden and achieve good cohesion. At the same time, the addition of cellulose ether makes the mortar have good plasticity and flexibility, and improves the bonding strength of the mortar. Let’s talk about the impact on the application of dry-mixed mortar from the product performance of cellulose ether.
1. The fineness of cellulose
The fineness of cellulose ether affects its solubility. For example, the lower the fineness of cellulose ether, the faster it dissolves in water and the improvement of water retention performance. Therefore, the fineness of cellulose ether should be included as one of its investigation properties. Generally speaking, the sieve residue of cellulose ether fineness over 0.212mm should not be more than 8.0%.
2. Drying weight loss rate
The drying weight loss rate refers to the percentage of the mass of the lost material in the mass of the original sample when the cellulose ether is dried at a certain temperature. For a certain quality of cellulose ether, the drying weight loss rate is too high, which will reduce the content of active ingredients in cellulose ether, affect the application effect of downstream enterprises, and increase the purchase cost. Usually, the weight loss on drying of cellulose ether is not more than 6.0%.
3. Sulfate ash content of cellulose ether
For a certain quality of cellulose ether, the ash content is too high, which will reduce the content of active ingredients in cellulose ether and affect the application effect of downstream enterprises. The sulfate ash content of cellulose ether is an important measure of its own performance. Combined with the current production status of my country’s existing cellulose ether manufacturers, usually the ash content of MC, HPMC, HEMC should not exceed 2.5%, and the ash content of HEC cellulose ether should not exceed 10.0%.
4. Viscosity of cellulose ether
The water retention and thickening effect of cellulose ether mainly depends on the viscosity and dosage of cellulose ether itself added to the cement slurry.
5. The pH value of cellulose ether
The viscosity of cellulose ether products will gradually decrease after being stored at a higher temperature or for a long period of time, especially for high-viscosity products, so it is necessary to limit the pH. Generally, it is advisable to control the pH range of cellulose ether to 5-9.
6. Light transmittance of cellulose ether
The light transmittance of cellulose ether directly affects its application effect in building materials. The main factors affecting the light transmittance of cellulose ether are: (1) the quality of raw materials; (2) the effect of alkalization; (3) the process ratio; (4) solvent ratio; (5) neutralization effect. According to the use effect, the light transmittance of cellulose ether should not be less than 80%.
7. The gel temperature of cellulose ether
Cellulose ether is mainly used as viscosifier, plasticizer and water retention agent in cement products, so viscosity and gel temperature are important measures to characterize the quality of cellulose ether. The gel temperature is used to determine the type of cellulose ether, which is related to the degree of substitution of other cellulose ethers. In addition, salt and impurities can also affect the gel temperature. When the temperature of the solution rises, the cellulose polymer gradually loses water, and the viscosity of the solution decreases. When the gel point is reached, the polymer is completely dehydrated and forms a gel. Therefore, in cement products, the temperature is usually controlled below the initial gel temperature. Under this condition, the lower the temperature, the higher the viscosity, and the more obvious the effect of thickening and water retention.