What is the role of water retaining agent mixed into gypsum powder material?
Answer: plastering gypsum, bonded gypsum, caulking gypsum, gypsum putty and other construction powder materials are used. In order to facilitate construction, gypsum retarders are added during production to prolong the construction time of gypsum slurry. A retarder is added to inhibit the hydration process of the hemihydrate gypsum. This type of gypsum slurry needs to be kept on the wall for 1 to 2 hours before it condenses, and most of the walls have water absorption properties, especially brick walls, plus Air-concrete walls, porous insulation boards and other light-weight new wall materials, so the gypsum slurry should be water-retained to prevent part of the water in the slurry from transferring to the wall, resulting in water shortage when the gypsum slurry hardens and insufficient hydration. Completely, causing the separation and shelling of the joint between the plaster and the wall surface. The addition of water-retaining agent is to maintain the moisture contained in the gypsum slurry, to ensure the hydration reaction of the gypsum slurry at the interface, so as to ensure the bonding strength. Commonly used water-retaining agents are cellulose ethers, such as: methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), etc. In addition, polyvinyl alcohol, sodium alginate, modified starch, diatomaceous earth, rare earth powder, etc. can also be used to improve water retention performance.

No matter what kind of water-retaining agent can delay the hydration rate of gypsum to varying degrees, when the amount of retarder remains unchanged, the water-retaining agent can generally retard setting for 15-30 minutes. Therefore, the amount of retarder can be appropriately reduced.

What is the proper dosage of water retaining agent in gypsum powder material?
Answer: Water-retaining agents are often used in construction powder materials such as plastering gypsum, bonding gypsum, caulking gypsum, and gypsum putty. Because this type of gypsum is mixed with a retarder, which inhibits the hydration process of hemihydrate gypsum, it is necessary to Carry out water retention treatment on the gypsum slurry to prevent part of the water in the slurry from transferring to the wall, resulting in water shortage and incomplete hydration when the gypsum slurry is hardened. The addition of water-retaining agent is to maintain the moisture contained in the gypsum slurry, to ensure the hydration reaction of the gypsum slurry at the interface, so as to ensure the bonding strength.

Its dosage is generally 0.1% to 0.2% (accounting for gypsum), when gypsum slurry is used on walls with strong water absorption (such as aerated concrete, perlite insulation boards, gypsum blocks, brick walls, etc.), and When preparing bonding gypsum, caulking gypsum, surface plastering gypsum or surface thin putty, the amount of water-retaining agent needs to be larger (generally 0.2% to 0.5%).

Water-retaining agents such as methyl cellulose (MC) and hydroxypropyl methylcellulose (HPMC) are cold-soluble, but they will form lumps in the initial stage when they are directly dissolved in water. The water-retaining agent needs to be pre-mixed with gypsum powder to disperse. Prepare into dry powder; add water and stir, let stand for 5 minutes, stir again, the effect is better. However, there are currently cellulose ether products that can be directly dissolved in water, but they have little effect on the production of dry powder mortar.

How does the waterproofing agent play a waterproof function in the gypsum hardened body?
Answer: Different types of waterproofing agents exert their waterproof function in the gypsum hardened body according to different modes of action. Basically can be summarized into the following four ways:

(1) Reduce the solubility of the gypsum hardened body, increase the softening coefficient, and partially convert the calcium sulfate dihydrate with high solubility in the hardened body into calcium salt with low solubility. For example, saponified synthetic fatty acid containing C7-C9 is added, and appropriate amount of quicklime and ammonium borate are added at the same time.

(2) Generate a waterproof film layer to block the fine capillary pores in the hardened body. For example, blending paraffin emulsion, asphalt emulsion, rosin emulsion and paraffin-rosin composite emulsion, improved asphalt composite emulsion, etc.

(3) Change the surface energy of the hardened body, so that the water molecules are in a cohesive state and cannot penetrate into the capillary channels. For example, various silicone water repellents are incorporated, including various emulsified silicone oils.

(4) Through external coating or dipping to isolate water from immersing in the capillary channels of the hardened body, a variety of silicone waterproofing agents can be used. Solvent-based silicones are better than water-based silicones, but the former makes the gas permeability of the gypsum hardened body has declined.

Although different waterproofing agents can be used to improve the waterproofness of gypsum building materials in different ways, gypsum is still an air-hardening gelling material, which is not suitable for outdoor or long-term humid environments, and is only suitable for environments with alternating wet and dry conditions.

What is the modification of building gypsum by waterproofing agent?
Answer: There are two main ways of action of gypsum waterproofing agent: one is to increase the softening coefficient by reducing the solubility, and the other is to reduce the water absorption rate of gypsum materials. And reducing water absorption can be done from two aspects. One is to increase the compactness of the hardened gypsum, that is, to reduce the water absorption of the gypsum by reducing the porosity and structural cracks, so as to improve the water resistance of the gypsum. The other is to increase the surface energy of the gypsum hardened body, that is, to reduce the water absorption of gypsum by making the pore surface form a hydrophobic film.

Waterproofing agents that reduce porosity play a role by blocking the fine pores of gypsum and increasing the compactness of the gypsum body. There are many admixtures for reducing porosity, such as: paraffin emulsion, asphalt emulsion, rosin emulsion and paraffin asphalt composite emulsion. These waterproofing agents are effective in reducing the porosity of gypsum under proper configuration methods, but at the same time, they also have adverse effects on gypsum products.

The most typical water repellent that changes the surface energy is silicone. It can infiltrate the port of each pore, change the surface energy within a certain length range, and thus change the contact angle with water, make the water molecules condense together to form droplets, block the infiltration of water, achieve the purpose of waterproofing, and at the same time maintain Air permeability of plaster. The varieties of this type of waterproofing agent mainly include: sodium methyl siliconate, silicone resin, emulsified silicone oil, etc. Of course, this waterproofing agent requires that the diameter of the pores cannot be too large, and at the same time it cannot resist the infiltration of pressure water, and cannot fundamentally solve the long-term waterproof and moisture-proof problems of gypsum products.

Domestic researchers use the method of combining organic materials and inorganic materials, that is, based on the organic emulsion waterproofing agent obtained by co-emulsification of polyvinyl alcohol and stearic acid, and adding alum stone, naphthalenesulfonate aldehyde condensate A new type of gypsum composite waterproofing agent is made by compounding the salt waterproofing agent. The gypsum composite waterproofing agent can be directly mixed with gypsum and water, participate in the crystallization process of gypsum, and obtain better waterproofing effect.

What is the inhibitory effect of silane waterproofing agent on efflorescence in gypsum mortar?
Answer: (1) The addition of silane waterproofing agent can significantly reduce the degree of efflorescence of gypsum mortar, and the degree of efflorescence inhibition of gypsum mortar increases with the increase of silane addition within a certain range. The inhibitory effect of silane on 0.4% silane is ideal, and its inhibitory effect tends to be stable when the amount exceeds this amount.

(2) The addition of silane not only forms a hydrophobic layer on the surface of the mortar to prevent the intrusion of external water, but also reduces the migration of internal lye to form efflorescence, which significantly improves the inhibitory effect of efflorescence.

(3) While the addition of silane significantly inhibits efflorescence, it has no adverse effect on the mechanical properties of industrial by-product gypsum mortar, and does not affect the formation of the internal structure and final bearing capacity of industrial by-product gypsum dry-mix building materials .