In the ready-mixed mortar, the addition amount of HPMC is very low, but it can significantly improve the performance of the wet mortar and is a major additive that affects the construction performance of the mortar. Reasonable selection of HPMC with different varieties, viscosities, particle sizes and added amounts will have a positive impact on the improvement of the performance of dry mortar. If the water retention performance of masonry and plaster mortar is not good, the water slurry will separate if it is left for a few minutes. Water retention is an important performance of HPMC, and it is also a performance that many domestic dry mortar manufacturers, especially those in areas with higher temperatures in the south, pay attention to. Factors that affect the water retention effect of dry mortar include the amount of HPMC added, viscosity, particle fineness and the temperature of the use environment.
The important role of water-soluble cellulose ether in mortar is mainly in three aspects. One is excellent water retention capacity, the other is the influence on the consistency and thixotropy of mortar, and the third is the interaction with cement. The water retention effect of cellulose ether depends on the water absorption of the base layer, the composition of the mortar, the layer thickness of the mortar, the water demand of the mortar, and the setting time of the coagulating material. The water retention of cellulose ether itself comes from its solubility and dehydration. It is well known that although the cellulose molecular chain contains a large number of highly hydratable OH groups, it is not soluble in water itself. This is because the cellulose structure has a high degree of crystallinity.
The hydration capacity of the hydroxyl group alone is not enough to cover the strong hydrogen bonds and van der Waals forces between molecules. Therefore, it only swells and does not dissolve in water. When a substituent is introduced into the molecular chain, not only the substituent destroys the hydrogen chain, but also the hydrogen bond between the chains is broken due to the wedging of the substituents between adjacent chains. The greater the distance. The greater the effect of breaking the hydrogen bond, after the cellulose lattice expands, the solution enters and the cellulose ether becomes water-soluble, forming a high-viscosity solution. When the temperature rises, the hydration of the polymer weakens, and the water between the chains is expelled. When the water removal effect is sufficient, the molecules begin to aggregate, forming a three-dimensional network structure gel and folding out.
The factors that affect the water retention of mortar include the viscosity of HPMC, the amount of addition, the fineness of particles and the temperature of use.
The greater the viscosity of the cellulose ether, the better the water retention performance. Viscosity is an important parameter of HPMC performance. At present, different cellulose manufacturers use different methods and instruments to determine viscosity. The main methods are Haake Rotovisko, Hoppler, Ubbelohde and Brookfield. For the same product, the viscosity results measured by different methods are very different, and some are even doubled. Therefore, when comparing the viscosity, it must be carried out between the same test methods, including temperature, rotor, etc. Generally speaking, the higher the viscosity, the better the water retention effect. But the higher the viscosity, the higher the molecular weight, and its solubility will decrease accordingly. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the more viscous the wet mortar will be, which is manifested by the sticky scraper and high adhesion to the substrate during construction.