Dry-mixed mortar is a combination of cementitious materials (cement, fly ash, slag powder, etc.), special graded fine aggregates (quartz sand, corundum, etc., and sometimes requires light Granules, expanded perlite, expanded vermiculite, etc.) and admixtures are uniformly mixed in a certain proportion, and then packed in bags, barrels or supplied in bulk in dry powder state as a building material.
According to the application, there are many kinds of commercial mortar, such as dry powder mortar for masonry, dry powder mortar for plastering, dry powder mortar for ground, special dry powder mortar for waterproofing, heat preservation and other purposes. To sum up, dry-mixed mortar can be divided into ordinary dry-mixed mortar (masonry, plastering and ground dry-mixed mortar) and special dry-mixed mortar. Special dry-mixed mortar includes: self-leveling floor mortar, wear-resistant floor material, non-fire wear-resistant floor, inorganic caulking agent, waterproof mortar, resin plastering mortar, concrete surface protection material, colored plastering mortar, etc.
So many dry-mixed mortars require admixtures of different varieties and different mechanisms of action to be formulated through a large number of tests. Compared with traditional concrete admixtures, dry-mixed mortar admixtures can only be used in powder form, and secondly, they are soluble in cold water, or gradually dissolve under the action of alkalinity to exert their due effect.
1. Thickener, water-retaining agent and stabilizer Ordinary mortar prepared by cement, inert or active mineral admixture, and fine aggregate, its main disadvantages are poor cohesion, poor stability, easy bleeding, segregation, Subsidence, difficult construction, after construction, the bonding strength is low, easy to crack, weak waterproof, poor durability, etc., must be modified with appropriate additives. In terms of improving the cohesion, water retention and stability of mortar, the additives that can be selected include cellulose ether, modified starch ether, polyvinyl alcohol, polyacrylamide and thickening powder.
Cellulose ether methyl cellulose (MC), hydroxypropyl methyl cellulose (PMC) and hydroxyethyl methyl cellulose (HEMC) are all made of natural polymer materials (such as cotton, etc.) Non-ionic cellulose ether produced by chemical treatment. They are characterized by cold water solubility, water retention, thickening, cohesiveness, film forming, lubricity, non-ionic and pH stability. The cold water solubility of this kind of product is greatly improved, and the water retention capacity is enhanced, the thickening property is obvious, the diameter of the air bubbles introduced is relatively small, and the effect of improving the bonding strength of the mortar is greatly enhanced.
Cellulose ether not only has a variety of varieties, but also has a wide range of average molecular weight and viscosity from 5mPa. s to 200,000 mPa. s, the impact on the performance of the mortar in the fresh stage and after hardening is also different. A large number of tests should be carried out when selecting the specific selection. Choose a cellulose variety with a suitable viscosity and molecular weight range, a small dosage, and no air-entraining property. Only in this way can it be obtained immediately. Ideal technical performance, but also has good economy.
Starch ether Starch ether is an ether formed by the reaction of hydroxyl groups on starch glucose molecules with chemical reagents, called starch ether or etherified starch. The main varieties of modified starch ethers are: sodium carboxymethyl starch (CMS), hydrocarbon alkyl starch (HES), hydrocarbon propyl ethyl starch (HPS), cyanoethyl starch, etc. They all have excellent functions of water solubility, bonding, swelling, flowing, covering, desizing, sizing, dispersion and stabilization, and are widely used in medicine, food, textile, papermaking, daily chemical and petroleum and other departments.
At present, the prospect of starch ether applied to dry powder mortar is also very promising. The main reasons are: ① The price of starch ether is relatively cheap, only 1/3 to 1/4 of cellulose ether; ② Starch ether mixed into mortar will also improve The viscosity, water retention, stability and bonding strength of the mortar; ③ Starch ether can be compounded with cellulose ether in any proportion, so as to better improve the anti-sagging effect of the mortar. In some mortar products, such as ceramic wall and floor tile adhesives, interface treatment agents, caulking agents and ordinary commercial mortars, starch ether is used as the main thickening and water-retaining agent and stabilizer. But looking at the starch ether manufacturers in my country, many of them only stay in the supply of primary products, and only a few manufacturers produce and supply modified starch ether to meet part of the demand of mortar manufacturers.
Thickened powder mortar Thickened powder is a new product developed to adapt to the production of ordinary dry powder (ready-mixed) mortar. It is mainly composed of inorganic minerals and organic polymer materials, and does not contain lime and air-entrained components. Its dosage is about 5% to 20% of the weight of cement. At present, in the production of ordinary commodity mortar in Shanghai, thickening powder is generally used as a thickening, water-retaining and stabilizing component, and the effect is remarkable.
Polyvinyl alcohol and polyacrylamide also have a wide viscosity range, but sometimes the air-entraining amount is large, or the water demand of the mortar increases too much after being mixed, so a large number of tests should be used to select.
2. The main function of the redispersible latex powder thickener is to improve the water retention and stability of the mortar. Although it can prevent the mortar from cracking (slow down the water evaporation rate) to a certain extent, it is generally not used to improve the toughness and crack resistance of the mortar. and waterproof means.
The practice of adding polymers to improve the impermeability, toughness, crack resistance and impact resistance of mortar and concrete has been recognized. Commonly used polymer emulsions for the modification of cement mortar and cement concrete include: neoprene rubber emulsion, styrene-butadiene rubber emulsion, polyacrylate latex, polyvinyl chloride, chlorine partial rubber emulsion, polyvinyl acetate, etc. With the development of scientific research, not only the modification effects of various polymers have been studied in depth, but also the modification mechanism, the interaction mechanism between polymers and cement, and cement hydration products have also been theoretically studied. More in-depth analysis and research, and a large number of scientific research results have appeared.
Polymer emulsion can be used in the production of ready-mixed mortar, but it is obviously impossible to directly use it in the production of dry powder mortar, so redispersible latex powder was born. At present, the redispersible latex powder used in dry powder mortar mainly includes: ① vinyl acetate-ethylene copolymer (VAC/E); ② vinyl acetate-tert-carbonate copolymer (VAC/VeoVa); ③ acrylate homopolymer ( Acrylate); ④ vinyl acetate homopolymer (VAC); 4) styrene-acrylate copolymer (SA), etc. Among them, vinyl acetate-ethylene copolymer has the largest usage ratio.
Practice has proved that the performance of redispersible latex powder is stable, and it has incomparable effects on improving the bonding strength of mortar, improving its toughness, deformation, crack resistance and impermeability, etc. Adding hydrophobic latex powder copolymerized by polyvinyl acetate, vinyl chloride, ethylene, vinyl laurate, etc. can also greatly reduce the water absorption of the mortar (because of its hydrophobicity), making the mortar air-permeable and impermeable, enhancing the It is weather resistant and has improved durability.
Compared with improving the flexural strength and bonding strength of mortar and reducing its brittleness, the effect of redispersible latex powder on improving the water retention of mortar and enhancing its cohesion is limited. Since the addition of redispersible latex powder can disperse and cause a large amount of air-entrainment in the mortar mixture, its water-reducing effect is very obvious. Of course, due to the poor structure of the introduced air bubbles, the water reduction effect did not improve the strength. On the contrary, the strength of the mortar will gradually decrease with the increase of the redispersible latex powder content. Therefore, in the development of some mortars that need to consider the compressive and flexural strength, it is often necessary to add a defoamer at the same time in order to reduce the negative impact of latex powder on the compressive strength and flexural strength of the mortar.
3. Due to the addition of cellulose, starch ether and polymer materials, the defoamer undoubtedly increases the air-entraining property of the mortar. On the one hand, it affects the compressive strength, flexural strength and bonding strength of the mortar, and reduces its elastic modulus. On the other hand, it also has a great impact on the appearance of the mortar, so it is very necessary to eliminate the air bubbles introduced in the mortar. At present, imported dry powder defoamers are mainly used in China to solve this problem, but it must be noted that due to the high viscosity of commodity mortar, the elimination of air bubbles is not a very easy task.
4. Anti-sagging agent When pasting ceramic tiles, foamed polystyrene boards, and applying rubber powder polystyrene particle insulation mortar, the biggest problem faced is falling. Practice has proved that adding starch ether, sodium bentonite, metakaolin and montmorillonite is an effective measure to solve the problem of mortar falling after construction. The main solution to the problem of sagging is to increase the initial shear stress of the mortar, that is, to increase its thixotropy. In practical applications, it is not easy to choose a good anti-sagging agent, because it needs to solve the relationship between thixotropy, workability, viscosity and water demand.
5. Water-repellent agent The waterproof or water-repellent function of the plastering mortar, tile caulking agent, decorative color mortar and dry-mixed mortar used for the exterior wall of the thin plastering insulation system is indispensable, which requires the addition of Powdered water repellent, but it should have the following characteristics: ① make the mortar hydrophobic as a whole and maintain long-term effect; ② have no negative impact on the bonding strength of the surface; ③ some water repellents commonly used in the market, such as Calcium fatty acid is not a suitable hydrophobic additive for dry-mixed mortar, especially for plastering materials for mechanical construction, because it is difficult to mix quickly and uniformly with cement mortar.
A silane-based powder water-repellent agent has recently been developed, which is a powdered silane-based product obtained by spray-drying silane-coated water-soluble protective colloids and anti-caking agents. When the mortar is mixed with water, the protective colloid shell of the water-repellent agent dissolves rapidly in water, and releases the encapsulated silane to redisperse it into the mixing water. In the highly alkaline environment after cement hydration, the hydrophilic organic functional groups in silane are hydrolyzed to form highly reactive silanol groups, and the silanol groups continue to irreversibly react with the hydroxyl groups in the cement hydration products to form chemical bonds , so that the silane connected together by cross-linking is firmly fixed on the surface of the pore wall of the cement mortar. As the hydrophobic organic functional groups face the outside of the pore wall, the surface of the pores acquires hydrophobicity, thereby bringing the overall hydrophobic effect to the mortar.
6. Pantherine Inhibitor Pantherine will affect the aesthetics of cement-based decorative mortar, which is a common problem that needs to be solved. According to reports, a resin-based anti-pantherine additive has been successfully developed recently, which is a redispersible powder with good stirring performance. This product is especially suitable for use in relief coatings, putties, caulks or finishing mortar formulations and has good compatibility with other additives.
7. Fiber Adding appropriate amount of fiber in the mortar can increase the tensile strength, enhance the toughness and improve the crack resistance. At present, chemical synthetic fibers and wood fibers are commonly used in dry-mixed mortar. Chemical synthetic fibers, such as polypropylene staple fiber, polypropylene staple fiber, etc. After surface modification, these fibers not only have good dispersibility, but also have low content, which can effectively improve the plastic resistance and cracking performance of mortar. The mechanical properties are not significantly affected. The diameter of wood fiber is smaller, and the addition of wood fiber should pay attention to the increase of water demand for mortar.