A single admixture has limitations in improving the performance of gypsum slurry. If the performance of gypsum mortar is to achieve satisfactory results and meet different application requirements, chemical admixtures, admixtures, fillers, and various materials are required to be compounded and complemented in a scientific and reasonable manner.
01. Coagulation regulator
Coagulation regulators are mainly divided into retarders and accelerators. In gypsum dry-mixed mortar, retarders are used for products prepared with plaster of paris, and accelerators are required for products prepared with anhydrous gypsum or directly using dihydrate gypsum.
02. Retarder
Adding retarder to gypsum dry-mixed building materials inhibits the hydration process of hemihydrate gypsum and prolongs the setting time. There are many conditions for the hydration of plaster, including the phase composition of plaster, the temperature of plaster material when preparing products, particle fineness, setting time and pH value of prepared products, etc. Each factor has a certain influence on the retarding effect, so there is a big difference in the amount of retarder in different situations. At present, the better retarder for gypsum in China is the modified protein (high protein) retarder, which has the advantages of low cost, long retardation time, small strength loss, good product construction, and long open time. The amount used in the preparation of bottom-layer stucco plaster is generally 0.06% to 0.15%.
03. Coagulant
Accelerating the slurry stirring time and prolonging the slurry stirring speed are one of the methods of physical coagulation acceleration. Commonly used chemical coagulants in anhydrite powder building materials include potassium chloride, potassium silicate, sulfate and other acid substances. The dosage is generally 0.2% to 0.4%.
04. Water retaining agent
Gypsum dry-mix building materials are inseparable from water-retaining agents. Improving the water retention rate of gypsum product slurry is to ensure that water can exist in the gypsum slurry for a long time, so as to obtain a good hydration hardening effect. To improve the construction of gypsum powder building materials, reduce and prevent segregation and bleeding of gypsum slurry, improve the sagging of slurry, prolong the opening time, and solve engineering quality problems such as cracking and hollowing are all inseparable from water retaining agents. Whether the water retaining agent is ideal depends mainly on its dispersibility, instant solubility, moldability, thermal stability and thickening property, among which the most important index is water retention.
There are four types of water retaining agents:
①Cellulosic water retaining agent
At present, the most widely used in the market is hydroxypropyl methylcellulose, followed by methyl cellulose and carboxymethyl cellulose. The overall performance of hydroxypropyl methylcellulose is better than that of methylcellulose, and the water retention of the two is much higher than that of carboxymethylcellulose, but the thickening effect and bonding effect are worse than those of carboxymethylcellulose. In gypsum dry-mixed building materials, the amount of hydroxypropyl and methyl cellulose is generally 0.1% to 0.3%, and the amount of carboxymethyl cellulose is 0.5% to 1.0%. A large number of application examples prove that the combined use of the two is better .
② Starch water retaining agent
Starch water retaining agent is mainly used for gypsum putty and surface plaster plaster, and can replace part or all of cellulose water retaining agent. Adding starch-based water-retaining agent to gypsum dry powder building materials can improve the workability, workability, and consistency of slurry. Commonly used starch-based water-retaining agents include tapioca starch, pregelatinized starch, carboxymethyl starch, and carboxypropyl starch. The amount of starch-based water-retaining agent is generally 0.3% to 1%. If the amount is too large, it will cause mildew of gypsum products in a humid environment, which will directly affect the quality of the project.
③ Glue water retaining agent
Some instant adhesives can also play a better water retention role. For example, 17-88, 24-88 polyvinyl alcohol powder, Tianqing gum and guar gum are used in gypsum dry-mixed building materials such as gypsum, gypsum putty, and gypsum insulation glue. Can reduce the amount of cellulose water retaining agent. Especially in fast-bonding gypsum, it can completely replace the cellulose ether water-retaining agent in some cases.
④ Inorganic water retention materials
The application of compounding other water-retaining materials in gypsum dry-mixed building materials can reduce the amount of other water-retaining materials, reduce product costs, and also play a certain role in improving the workability and constructability of gypsum slurry. Commonly used inorganic water-retaining materials include bentonite, kaolin, diatomaceous earth, zeolite powder, perlite powder, attapulgite clay, etc.
05. Adhesive
The application of adhesives in gypsum dry-mixed building materials is second only to water-retaining agents and retarders. Gypsum self-leveling mortar, bonded gypsum, caulking gypsum, and thermal insulation gypsum glue are all inseparable from adhesives.
▲ Redispersible latex powder
Redispersible latex powder is widely used in gypsum self-leveling mortar, gypsum insulation compound, gypsum caulking putty, etc. Especially in gypsum self-leveling mortar, it can improve the viscosity and fluidity of the slurry, and also play a great role in reducing delamination, avoiding bleeding, and improving crack resistance. The dosage is generally 1.2% to 2.5%.
▲ Instant polyvinyl alcohol
At present, the instant polyvinyl alcohol used in a large amount in the market is 24-88 and 17-88. It is often used in products such as bonding gypsum, gypsum putty, gypsum composite thermal insulation compound, and plastering plaster. 0.4% to 1.2%.
Guar gum, Tianqing gum, carboxymethyl cellulose, starch ether, etc. are all adhesives with different bonding functions in gypsum dry-mixed building materials.
06. Thickener
Thickening is mainly to improve the workability and sagging of gypsum slurry, which is similar to adhesives and water-retaining agents, but not completely. Some thickener products are effective in thickening, but not ideal in terms of cohesive force and water retention. When formulating gypsum dry powder building materials, the main role of admixtures should be fully considered in order to apply admixtures better and more reasonably. Commonly used thickener products include polyacrylamide, Tianqing gum, guar gum, carboxymethyl cellulose, etc.
07. Air-entraining agent
Air-entraining agent, also known as foaming agent, is mainly used in gypsum dry-mixed building materials such as gypsum insulation compound and plaster plaster. Air-entraining agent (foaming agent) helps to improve construction, crack resistance, frost resistance, reduce bleeding and segregation, and the dosage is generally 0.01% to 0.02%.
08. Defoamer
Defoamer is often used in gypsum self-leveling mortar and gypsum caulking putty, which can improve the density, strength, water resistance and cohesiveness of the slurry, and the dosage is generally 0.02% to 0.04%.
09. Water reducing agent
Water reducing agent can improve the fluidity of gypsum slurry and the strength of gypsum hardened body, and is usually used in gypsum self-leveling mortar and plaster plaster. At present, domestically produced water reducers are ranked according to their fluidity and strength effects: polycarboxylate retarded water reducers, melamine high-efficiency water reducers, tea-based high-efficiency retarded water reducers, and lignosulfonate water reducers. When using water reducing agents in gypsum dry-mix building materials, in addition to considering water consumption and strength, attention should also be paid to the setting time and fluidity loss of gypsum building materials over time.
10. Waterproofing agent
The biggest defect of gypsum products is poor water resistance. Areas with high air humidity have higher requirements for water resistance of gypsum dry-mixed mortar. Generally, the water resistance of hardened gypsum is improved by adding hydraulic admixtures. In the case of wet or saturated water, the external addition of hydraulic admixtures can make the softening coefficient of the gypsum hardened body reach more than 0.7, so as to meet the product strength requirements. Chemical admixtures can also be used to reduce the solubility of gypsum (that is, increase the softening coefficient), reduce the adsorption of gypsum to water (that is, reduce the water absorption rate) and reduce the erosion of gypsum hardened body (that is, water isolation). Gypsum waterproofing agents include ammonium borate, sodium methyl siliconate, silicone resin, emulsified paraffin wax, and silicone emulsion waterproofing agent with better effect.
11. Active stimulator
Activation of natural and chemical anhydrites imparts adhesiveness and strength for the production of gypsum dry-mix building materials. The acid activator can accelerate the early hydration rate of anhydrous gypsum, shorten the setting time, and improve the early strength of gypsum hardened body. The basic activator has little effect on the early hydration rate of anhydrous gypsum, but it can significantly improve the later strength of the gypsum hardened body, and can form part of the hydraulic gelling material in the gypsum hardened body, effectively improving the water resistance of the gypsum hardened body sex. The use effect of the acid-base compound activator is better than that of a single acidic or basic activator. Acid stimulants include potassium alum, sodium sulfate, potassium sulfate, etc. Alkaline activators include quicklime, cement, cement clinker, calcined dolomite, etc.
12. Thixotropic lubricant
Thixotropic lubricants are used in self-leveling gypsum or plastering gypsum, which can reduce the flow resistance of gypsum mortar, prolong the open time, prevent the layering and settlement of slurry, so that the slurry can obtain good lubricity and workability. At the same time, the body structure is uniform, and its surface strength is increased.
Post time: Apr-20-2023