The dosage of hydroxypropyl methylcellulose (HPMC) varies in different applications, primarily determined by the water retention needs. HPMC is suitable for various mortar applications.
Water-retaining agents are commonly used in construction powder materials such as plaster, bonding gypsum, joint-filling gypsum, and gypsum putty. The dosage typically ranges from 0.1% to 0.2% (based on the amount of gypsum). When gypsum slurry is used on walls with strong water absorption or in the preparation of bonding gypsum, joint-filling gypsum, surface plaster, or thin-layer putty, a higher dosage of water-retaining agent is required (generally 0.2% to 0.5%).
Hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), and other water-retaining agents are cold-soluble. While they tend to clump when directly dissolved in water, it is recommended to pre-disperse the water-retaining agent in gypsum powder, creating a dry powder mixture. After adding water, stir the mixture, let it stand for 5 minutes, and then stir again for optimal results.
The belief that the higher the water retention in mortar, the better, is incorrect. The water retention of mortar should be controlled within an appropriate range.
Reasons: Excessive water retention can lead to issues such as mortar bulging, foaming, and hollowing when applied to non-absorbent shear walls. If the water retention is too high, the mortar layer remains wet, and hurried plastering work may lead to premature cracking as the mortar sags. Excessive water retention can also cause the mortar to become sticky, resulting in poor workability. When plasterers repeatedly rub the mortar layer, it may lead to hollow spots.
[Note: This translation may be subject to further refinement based on specific context and terminology preferences.]
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