The building gypsum is the main raw material that guarantees the bond strength of adhesive plaster, and should meet the requirements of national standard GB / T 9776-2008.
As for the setting time of building gypsum, the standard requires that the initial setting be greater than 6min and that the final setting be less than 30min. But it is unable to conduct construction operations by relying solely on the setting time. According to the differences in the materials and parts to be bonded, adhesive plaster is divided into fast-setting and slow-setting types. Therefore, appropriate retarders need to be added during the preparation. Different from plastering gypsum and plaster putty, adhesive plaster requires not very long setting time. Generally, the requirements of fast-setting type are the initial setting time not less than 5min and the final setting time not more than 20min; while the requirements of common type are the initial setting time not less than 25min and the final setting time not more than 120min. Therefore, there are many varieties of retarders optional. Currently the most widely used are still citric acid and sodium citrate at home, especially the sodium citrate, due to the small dosage and few effects on the strength of gypsum.
The same as other pgypsum building materials, adhesive plaster has poor water retention. The moisture in the slurry will be quickly adsorbed by the substrate material, not only increasing the difficulty of construction operation, but also reducing the adhesive power due to the loss of moisture required for hydration, and in severe cases, losing all the bond strength. Therefore, the water-retaining agent is an important admixture in the preparation of adhesive plaster. For the preparation of adhesive plaster, the high viscosity methyl cellulose can be selected, or its compound with high-viscosity carboxymethyl cellulose can also be selected. This not only improves the water retention of adhesive plaster, but also makes the adhesive mortar thickened, thereby increasing its adhesion, easy for bonding operations.
The binder is a raw material served to enhance the adhesive force of adhesive plaster, and generally used for special bonding inside the adhesive plaster (such as pasting the polystyrene insulation board on brick or concrete walls). The commonly used are the binary copolymer of polyvinyl alcohol, ethylene-vinyl acetate, and the ternary copolymer of vinyl chloride, ethylene, vinyl laurate and other redispersible polymer powder. Although the bond strength of polyvinyl alcohol will decline with the increase of time, as the adhesive plaster for indoor applications, however, most of its bond strength is from gypsum cementing materials. Therefore, as the bond reinforcing, polyvinyl alcohol is still the preferred binder. From the long-term and special application effects, the use of redispersible latex powder is still the most ideal, with not only high bond strength, but also certain water retention and waterproof effects.
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