(Information**: China Mortar Network).
Building gypsum powder is the main raw material of gypsum wall scraping agent, and is the basic raw material to ensure the bonding strength and impact strength, so its quality requirements are strict.
a) Physical properties.
The fineness should all pass through the 120 mesh sieve, the initial setting time is 6min, the final setting time is <30min, and the flexural strength is 2h>21mpa
Compressive strength 2h >49MPa, whiteness directly as a decorative layer agent requires >85, and a base agent for coating or pasting wallpaper" 75
b) Chemical composition.
1 Cas04·2H2O content in gypsum stone for the production of building gypsum powder" 75
2 Harmful impurities Na2O<0 03 , K2O 003, Cl ion 10ppm
3 Cas04·2H2O L gypsum in building gypsum powder
Talcum powder is mainly used in gypsum agent to improve the workability of the slurry, easy to scrape, and increase the surface smoothness. The fineness should all pass through a 325 mesh sieve, and the Na2O content should be <010, K2O content 030％
The scraping performance of the guarantee slurry is mainly guaranteed by the water retention agent. Ensure the workability of the agent slurry, and make the water in the agent layer will not be absorbed by the wall too quickly, resulting in insufficient water required for gypsum hydration, and the phenomenon of powder falling off appears. The water-retaining agent is mainly derived from cellulose, such as: methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC) and carboxymethyl cellulose (CMC).
Although CMC has a certain viscosity in the ingredients of gypsum wall scraper, it will have different degrees of damage to the strength of gypsum, especially the surface strength, so it is necessary to add a small amount of binder to migrate to the surface during the drying process of gypsum wall scraping agent and increase the surface strength of gypsum wall scraper, otherwise the gypsum wall scraper scraper scraping agent on the wall will have surface powder loss due to not spraying paint for a long time. However, the use of MC, HPMC or HEC can be unadulterated with binders, which are different from CMC in that they can be used as powdered binders with little or no reduction in the strength of gypsum.
The commonly used binders for gypsum are: gelatinized starch, starch, oxidized starch, water-soluble polyvinyl alcohol at room temperature, and redispersible polymer powder.
Although some cellulose ethers and binders have a retarding effect on gypsum, the retarding effect cannot meet the service time requirements of gypsum wall scraping agent, so a certain amount of retarder must be added. For details, see the introduction of retarders in Chapters 1 and 3 of this article.
In order to make the gypsum wall scraper better combined with the substrate, a very small amount of penetrant is mixed with the gypsum wall scraper. Commonly used penetrants are anionic and non-ionic.
The gypsum hardener itself is soft and brittle, and once the wall scraping agent layer is too thick, it is easy to peel off from the two agent layers, so adding a certain amount of softening agent and penetrant can improve the flexibility of the agent and further improve the operation performance of the agent slurry. Commonly used softeners are various sulfonates, lignin fibers, etc.