Water retention is an important property of cellulose ether. The factors affecting the water retention effects of dry-mixed mortar include the addition amount, viscosity, fineness, the temperature of use environment of mortar.

 

• Effects of the Addition Amount of Cellulose Ether on Water Retention

When the addition amount of cellulose ether is within the range of 0.05%-0.4%, the water retention will increase with the increase of addition amount. When the addition amount is further increased, the tendency to increase water retention will begin to slow down.

For different varieties of mortar, its addition amount of cellulose ether is also not the same. In the practical application, the addition amount of cellulose ether should be determined according to the purpose of mortar, and by test verification, comply with the appropriate technical indicators of mortar.

 

• Effects of the Viscosity of Cellulose Ether on Water Retention

The viscosity of cellulose ether also has a similar relation with water retention. When the viscosity of cellulose ether is increased, the water retention is also improved. When the viscosity reaches a certain level, the increase rate of water retention has leveled off. In general, the higher the viscosity is, the better the water retention effects will be. However, the higher the viscosity is, the higher the molecular weight of cellulose ether will also be, and its solubility property will be reduced accordingly, which can have a negative effect on the strength and construction performance of mortar. The higher the viscosity is, the more obvious the thickening effect on the mortar will be, but it is not a proportional relationship. The higher the viscosity is, the greater the degree of stickiness of wet mortar will be, which in construction is manifested as sticking to the scraper and high adhesion to the substrate, but helps little in increasing the structural strength of wet mortar itself, and is not obvious in improving the sag resistance. In contrast, some methyl cellulose ethers of medium or low viscosity but after modification has a superior performance in improving the structural strength of wet mortar.

 

• Effects of the Fineness of Cellulose Ether on Water Retention

Fineness has a certain impact on the solubility of cellulose ether. The coarser cellulose ether is usually granular, easily dispersed or dissolved in water without caking, but the dissolution rate is very slow, not suitable for dry-mixed mortar. In the dry-mix mortar, cellulose ether is dispersed among the cementitious materials such as aggregate, fine aggregate and cement. Only the powder fine enough can avoid the caking of cellulose ether during stirring with water. If caking occurs when cellulose ether is dissolved in water, then it will be very difficult for re-dispersion and dissolution. The cellulose ether of coarser fineness can reduce the local strength of mortar. In large-scale construction, mortar of this kind will show a significant decrease in the curing speed of local strength and the occurrence of cracking resulting from the differences in curing time. For spraying mortar, since the stirring time is short, its requirement on fineness is higher. Therefore, the cellulose ether applied in dry-mix mortar should be powdery, low in water content, with fineness requirement of 20%-60% and the particle size of less than 63μm.

In terms of the effects of fineness of cellulose ether on the water retention, generally speaking, for the cellulose ether of the same viscosity but different fineness degrees, under the condition of the same addition amount, the finer the fineness is, the better the water retention effects will be.

 

• Effects of Operating Temperature on Water Retention

The water retention of cellulose ether also has a relationship with the operating temperature, and it will be reduced with the increase of operating temperature. In practical engineering, the construction of mortar is often carried out in a high temperature environment, such as the plastering work of external walls in the sun in summer, which is bound to accelerate the setting and hardening of cement mortar. The decrease in the water retention will lead to the decrease in the workability and crack resistance. In this situation, reducing the effect of temperature factor becomes particularly crucial. Testes have shown that improving the degree of etherification of cellulose ether enables its water retention effects to remain good even in the case of higher temperatures.