1. The characteristics of HPMC in ordinary mortar
HPMC is mainly used as retarder and water retention agent in cement proportioning. In concrete components and mortar, it can improve viscosity and shrinkage rate, strengthen cohesive force, control cement setting time, and improve initial strength and static bending strength. Because it has the function of retaining water, it can reduce the loss of water on the concrete surface, avoid cracks at the edge, and improve adhesion and construction performance. Especially in construction, the setting time can be extended and adjusted. With the increase of HPMC content, the setting time of mortar will be extended successively; improve the machinability and pumpability, suitable for mechanized construction; improve construction efficiency and benefit the building surface Protects against weathering of water-soluble salts.
2. Characteristics of HPMC in special mortar
HPMC is a high-efficiency water-retaining agent for dry powder mortar, which reduces the bleeding rate and delamination of the mortar and improves the cohesion of the mortar. Although HPMC slightly reduces the flexural and compressive strength of the mortar, it can significantly increase the tensile strength and bond strength of the mortar. In addition, HPMC can effectively inhibit the formation of plastic cracks in mortar and reduce the plastic cracking index of mortar. The water retention of mortar increases with the increase of HPMC viscosity, and when the viscosity exceeds 100000mPa·s, the water retention does not increase significantly. The fineness of HPMC also has a certain influence on the water retention rate of the mortar. When the particles are finer, the water retention rate of the mortar is improved. The HPMC particle size usually used for cement mortar should be less than 180 microns (80 mesh screen). The suitable dosage of HPMC in dry powder mortar is 1‰～3‰.
2.1. After the HPMC in the mortar is dissolved in water, the effective and uniform distribution of the cementitious material in the system is ensured due to the surface activity. As a protective colloid, HPMC “wraps” the solid particles and forms a layer on its outer surface. A layer of lubricating film makes the mortar system more stable, and also improves the fluidity of the mortar during the mixing process and the smoothness of construction.
2.2. Due to its own molecular structure, the HPMC solution makes the water in the mortar not easy to lose, and releases it gradually over a long period of time, endowing the mortar with good water retention and constructability. It can prevent the water from flowing too quickly from the mortar to the base, so that the retained water stays on the surface of the fresh material, which can promote the hydration of the cement and improve the final strength. Especially if the interface in contact with cement mortar, plaster, and adhesive loses water, this part will have no strength and almost no cohesive force. Generally speaking, the surfaces in contact with these materials are all adsorbents, more or less absorb some water from the surface, resulting in incomplete hydration of this part, making cement mortar and ceramic tile substrates and ceramic tiles or plaster and walls The bonding strength between the surfaces decreases.
In the preparation of mortar, the water retention of HPMC is a main performance. It has been proved that the water retention can be as high as 95%. The increase in the molecular weight of HPMC and the increase in the amount of cement will improve the water retention and bond strength of the mortar.
Example: Since tile adhesives must have a high bond strength both between the substrate and the tiles, the adhesive is affected by the adsorption of water from two sources; the substrate (wall) surface and the tiles. Especially for tiles, the quality varies greatly, some have large pores, and the tiles have high water absorption rate, which destroys the bonding performance. The water-retaining agent is particularly important, and adding HPMC can well meet this requirement.
2.3. HPMC is stable to acid and alkali, and its aqueous solution is very stable in the range of pH=2~12. Caustic soda and lime water have little effect on its performance, but alkali can speed up its dissolution and slightly increase its viscosity.
2.4. The construction performance of the mortar added with HPMC has been significantly improved. The mortar seems to be “oily”, which can make the wall joints full, smooth the surface, make the tile or brick and the base layer bond firmly, and can prolong the operation time, suitable for large Area construction.
2.5. HPMC is a non-ionic and non-polymeric electrolyte, which is very stable in aqueous solutions with metal salts and organic electrolytes, and can be added to building materials for a long time to ensure its durability is improved.