Characteristics of HPMC in dry mixed mortar
1, HPMC in the characteristics of ordinary mortar
HPMC is mainly used as retarder and water retaining agent in cement ratio. In concrete components and mortar, it can improve viscosity and shrinkage rate, strengthen bonding force, control the setting time of cement, and improve initial strength and static flexural strength. Because it has the function of water retention, can reduce the loss of water on the surface of the coagulation, can avoid the occurrence of cracks at the edge, and can improve the adhesion and construction performance. Especially in the construction, can prolong and adjust the setting time, with the increase of HPMC dosage, mortar setting time has been prolonged; Improve machinability and pumpability, suitable for mechanized construction; It can improve the construction efficiency and prevent the weathering of water-soluble salts on the building surface.
2, HPMC in special mortar characteristics
HPMC is an efficient water retaining agent for dry mortar, which reduces the bleeding rate and stratification degree of mortar and improves the cohesiveness of mortar. HPMC can significantly improve the tensile strength and bonding strength of mortar, although the bending strength and compressive strength of mortar are slightly reduced by HPMC. In addition, HPMC can effectively inhibit the formation of plastic cracks in mortar, reduce the plastic cracking index of mortar, mortar water retention increases with the increase of the viscosity of HPMC, and when the viscosity exceeds 100000mPa•s, water retention is no longer significantly increased. HPMC fineness also has a certain effect on the water retention rate of mortar, when the particle is fine, the water retention rate of mortar has been improved, usually used for cement mortar HPMC particle size should be less than 180 microns (80 mesh screen). The suitable content of HPMC in dry mortar is 1‰ ~ 3‰.
2.1, mortar HPMC after dissolved in water, because the surface active role to ensure the gelled material effectively uniform distribution in the system, and the HPMC as a kind of protective colloid, “package” solid particles, and on its exterior surface to form a layer of lubrication film, make the slurry system is more stable, also raised the mortar in the mixing process of liquidity and the construction of the slip may just as well.
2.2 HPMC solution due to its own molecular structure characteristics, so that the water in mortar is not easy to lose, and gradually released in a long period of time, giving mortar good water retention and construction. Prevents water from moving too quickly from the mortar to the base, so that the retained water remains on the surface of the fresh material, which promotes the hydration of the cement and improves the final strength. In particular, if the interface in contact with cement mortar, plaster and binder loses water, this part has no strength and almost no bonding force. Generally speaking, the surface in contact with these materials are adsorption bodies, more or less to absorb some water from the surface, causing this part of the hydration is not complete, so that the cement mortar and ceramic tile substrate and ceramic tile or plaster and metope bond strength decline.
In the preparation of mortar, the water retention of HPMC is the main performance. It has been proved that water retention can be as high as 95%. The increase of HPMC molecular weight and cement dosage will improve the water retention and bond strength of mortar.
Example: because the tile binder must have a high bond strength between the base and tile, so the binder is affected by two aspects of adsorption water; Base (wall) surfaces and tiles. Special ceramic tile, quality difference is very big, some of the pores are very large, ceramic tile water absorption rate is high, so that the bond performance is destroyed, water retention agent is particularly important, and the addition of HPMC can well meet this requirement.
2.3 HPMC is stable to acids and bases, and its aqueous solution is very stable in the range of pH=2 ~ 12. Caustic soda and lime water do not have much effect on its properties, but alkali can accelerate its dissolution rate, and slightly improve the viscosity.
2.4, added HPMC mortar construction performance has been significantly improved, mortar seems to have “oily”, can make the wall joints full, smooth surface, so that tile or brick and base bonding firm, and can prolong the operation time, suitable for large area of construction.
2.5 HPMC is a kind of non-ionic and non-polymeric electrolyte. It is very stable in aqueous solution with metal salts and organic electrolytes, and can be added in building materials for a long time to ensure its durability improvement.
HPMC production process is mainly cotton fiber (domestic) after alkalization, etherification and the generation of a polysaccharide ether products. It has no charge itself, and does not react with charged ions in the gelled material, and its performance is stable. The price is lower than other types of cellulose ether, so it is widely used in dry mortar.
Hydroxypropyl methyl cellulose HPMC function in dry mixed mortar:
HPMC can make the new mix mortar thickened so as to have a certain wet viscosity, to prevent segregation. Water retention (thickening) is also the most important property, helping to maintain the amount of free water in the mortar, thus giving the cementitious material more time to hydrate after the mortar is applied. (water retention) its own air, can introduce uniform small bubbles, improve the construction of mortar.
Hydroxypropyl methyl cellulose ether viscosity greater water retention performance is better. Viscosity is an important parameter of HPMC performance. At present, different HPMC manufacturers use different methods and instruments to determine the viscosity of HPMC. The main methods are HaakeRotovisko, Hoppler, Ubbelohde and Brookfield, etc.
For the same product, the results of viscosity measured by different methods are very different, some are even multiple differences. Therefore, when comparing viscosity, it must be carried out between the same test method, including temperature, rotor, etc.
For particle size, the finer the particle, the better the water retention. Large particles of cellulose ether contact with water, the surface immediately dissolve and form a gel to wrap up the material to prevent water molecules from continuing to penetrate, sometimes long time stirring can not be evenly dispersed dissolved, the formation of a muddy flocculent solution or agglomerate. The solubility of cellulose ether is one of the factors to choose cellulose ether. Fineness is also an important performance index of methyl cellulose ether. MC for dry mortar requires powder, low water content, and fineness of 20%~60% particle size less than 63um. Fineness affects the solubility of hydroxypropyl methyl cellulose ether. Coarse MC is usually granular and can be easily dissolved in water without agglomerating, but the dissolution speed is very slow, so it is not suitable for use in dry mortar. In dry mortar, MC is dispersed between aggregate, fine fillers and cementing materials such as cement, and only powder that is fine enough can avoid clumping of methyl cellulose ether when mixing with water. When MC adds water to dissolve agglomerate, it is very difficult to disperse and dissolve it. MC with coarse fineness not only wastes, but also reduces the local strength of mortar. When such dry mortar is constructed in a large area, the curing speed of local dry mortar is significantly reduced, resulting in cracking caused by different curing time. For mechanical spraying mortar, because of the short mixing time, the fineness is higher.
Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity is, the higher the molecular weight of MC is, and the dissolution performance will decrease correspondingly, which has a negative impact on the strength and construction performance of mortar. The higher the viscosity, the more obvious the thickening effect of mortar, but it is not proportional to the relationship. The higher the viscosity, the wet mortar will be more sticky, both construction, the performance of the sticky scraper and high adhesion to the base material. But it is not helpful to increase the structural strength of wet mortar. In other words, the anti-sag performance is not obvious during construction. On the contrary, some low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
The water retention of HPMC is also related to the temperature of use, and the water retention of methyl cellulose ether decreases with the rise of temperature. But in the actual material application, many environments of dry mortar often will be in high temperature (higher than 40 degrees) under the condition of construction in hot substrate, such as summer insolation of the exterior wall putty plastering, which often accelerated the solidification of cement and dry mortar hardening. The decrease of water retention rate leads to the obvious feeling that both constructability and cracking resistance are affected. In this condition, reducing the influence of temperature factors becomes particularly critical. In this regard, the methyl hydroxyethyl cellulose ether additive is currently considered to be at the forefront of technological development. Even with the increase of methyl hydroxyethyl cellulose dosage (summer formula), the construction and cracking resistance still cannot meet the needs of use. Through some special treatment of MC, such as increasing the degree of etherification, the water retention effect of MC can maintain a better effect under high temperature, so that it can provide better performance under harsh conditions.
General HPMC has gel temperature, can be roughly divided into 60, 65, 75 type. For ordinary ready-mixed mortar using river sand enterprises had better choose high gel temperature 75 HPMC. HPMC dosage should not be too high, too high will increase the water demand of mortar, will stick to the plaster, condensation time is too long, affect the construction. Different mortar products choose different viscosity of HPMC, do not casually use high viscosity HPMC. Therefore, although hydroxypropyl methyl cellulose products are good, but it is good to choose the right HPMC is the primary responsibility of enterprise laboratory personnel. At present, there are a lot of illegal dealers in the compound with HPMC, the quality is quite poor, the laboratory should be in the selection of some cellulose, do a good experiment, ensure the stability of mortar products, do not covet cheap, causing unnecessary losses.