Characteristics of HPMC in dry mortar
1. Characteristics of HPMC in ordinary mortar
HPMC is mainly used as a retarder and water retaining agent in cement ratio. In concrete components and mortar, it can improve viscosity and shrinkage, strengthen adhesion, control cement setting time, and increase initial strength and static bending strength.
Because it has the function of retaining moisture, it can reduce the loss of moisture on the surface of the coagulation and avoid cracks on the edges. It can improve adhesion and construction performance.
Especially during construction, the setting time can be extend and adjust. With the increase of HPMC content, the setting time of mortar is prolonged; the processability and pumpability are improved, which is suitable for mechanized construction; the construction efficiency is improve and the weathering of water-soluble salts is prevent on the building surface.
2. Characteristics of HPMC in special mortar
HPMC is a high-efficiency water-retaining agent for dry mortar, which reduces the bleeding rate and stratification of the mortar and improves the adhesion of the mortar. Although HPMC makes the mortar’s flexural strength and compressive strength slightly reduced, it can significantly improve 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 the mortar increases with the viscosity of the HPMC, and when the viscosity exceeds 100,000 mPa•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 fine, the water retention rate of the mortar is improve, and the HPMC particle size usually use for cement mortar should be less than 180 microns (80 mesh screen). The suitable dosage of HPMC in dry mortar is 1‰～3‰.
2.1
After the HPMC in the mortar is dissolve in water, the surface active action ensures that the cementitious material is effectively and uniformly distributed in the system. As a protective colloid, HPMC “wrapps” solid particles and forms a lubricating film on the outer surface. The mortar system is more stable, and the fluidity of the mortar during the mixing process and the slipperiness of the construction are also improve.
2.2
Due to its molecular structure, the HPMC solution makes it difficult to lose moisture in the mortar. And gradually released over a long period of time, giving the mortar good water retention and construction. It prevents water from flowing too quickly from the mortar to the substrate. So that the retained moisture remains on the surface of the fresh material, which promotes hydration of the cement and increases the final strength.
Especially if the interface with cement mortar, plaster and binder is lost. This part has no strength and almost no adhesion. In general, the surfaces that come into contact with these materials are all adsorb, and more or less water is adsorb from the surface. This part of the hydration is incomplete, which reduces the bond strength between the cement mortar and the tile substrate and the tile or plaster and the wall.
In the preparation of mortar, the water retention of HPMC is a major property. It has been confirm that the water retention can be as high as 95%, and the increase in the molecular weight of HPMC and the increase in the amount of cement increase the water retention performance and bond strength of the mortar.
Example: Since the tile adhesive must have a high bond strength between the base layer and the tile, the binder is affect by the adsorption of water in two aspects. Base (wall) surface and tile.
Special ceramic tiles, the quality varies greatly, some pores are large, the water absorption rate of the tiles is high, the bonding performance is destroy, the water retaining 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 their properties. But alkali accelerates its dissolution rate and slightly increases the viscosity.
2.4
The mortar construction performance with the addition of HPMC has been significantly improved, and the mortar seems to have “oiliness”. It can make the wall joints full and smooth, make the tile or brick and the base layer firmly bond. And can extend the operation time, suitable for large-area construction.
2.5
HPMC is a non-ionic and non-polymeric electrolyte that is very stable in aqueous solutions with metal salts and organic electrolytes. It can be add to building materials for a long time to ensure its durability.