What are the varieties of water-retaining and thickening material?
The water-retaining and thickening material is generally divided into two categories, organic and inorganic, and mainly has a water-retaining and thickening effect. It can adjust the consistency, water retention, cohesiveness and thixotropy of mortar. The commonly used organic water-retaining and thickening material includes methyl cellulose, hydroxypropyl methyl cellulose and hydroxyethyl methyl cellulose. The water-retaining and thickening material consisting mainly of inorganic material includes mortar thickening powder. The water-retaining and thickening material mainly used in premixed and ordinary dry mixed mortar is mortar thickening powder, while the special dry mixed mortar mainly uses cellulose ether as the water-retaining and thickening material.
The traditional water-retaining and thickening material is lime putty. Based on the principle of physically absorbing water of plane multiplayer mineral structure, it can make the water of mortar not easily precipitate from slurry before setting and hardening and make the mortar mixture form paste. Mortar can not only deform under external force, but also withstand a certain load by itself after the external force disappears. After hardening, the moisture maintained by the lime putty enables the cement hydration in the mortar to obtain plenty of moisture. Therefore, lime putty is a traditional water-retaining and thickening material, and the cement-plaster mixed mortar prepared by lime putty is widely used in construction projects. After adding lime putty into mortar, it will become soft, good in water retention, and easy for construction. Its disadvantages are poor water resistance, large shrinkage, low bond strength, and poor durability. It also needs the ash pond for on-site preparing of lime putty, sometimes may produce a series of quality problems due to the incomplete dissolution of lime and cause pollution to the environment. This shows that the use of lime putty has been increasingly restricted.
The non-lime water-retaining and thickening material is usually added into the premixed mortar, and the main varieties are mortar plasticizing agents, cellulose ethers, and mortar thickening powder.
(1). Mortar Plasticizing Agents
The main ingredients of mortar plasticizing agents are sulphonates of rosin or long carbon chain, and its principle is to make mortar fluffy and soft by introducing tiny air bubbles into the cement mortar. However, after adding air-entraining agents, the strength of mortar masonry will be reduced by 10% or more. And the addition amount of air-entraining agent is tiny. Once the measurement is inaccurate, it will significantly reduce the strength or workability of mortar. Meanwhile, the bubble stability problems also exist in air-entraining agent products. The air content is also closely related to stirring time, approaches, varieties of cement and water consumption.
The cellulose ether is a general term for a series of products generated by the reaction of alkali cellulose and etherifying agent under certain conditions, and is a chemically modified polysaccharide with water solubility and colloidal structure. The cellulose ether mainly has the following three functions:
① It can make the freshly mixed mortar thickened so as to prevent segregation and obtain a uniform plastic body;
② It has an entraining effect itself, and can also stabilize the uniform tiny air bubbles introduced into the mortar;
③ As the water-retaining agent, it helps to maintain the moisture in the thin layer of mortar (free water) so that cement can have more time for hydration after the construction of mortar.
The cellulose ether is a water-soluble polymer. In the fresh mortar, it will migrate to the surface of mortar exposed to air with the evaporation of water and form enrichment, thereby causing the crusting of cellulose ether on the surface of new mortar. The result of crusting is to form a layer of more dense film on the surface of mortar, which will shorten the opening hours of mortar so that the bond strength of later stage will decline. The crusting phenomenon of cellulose ether can be improved by such methods as regulating the formulation, selecting appropriate cellulose ethers and adding other additives.
It should be noted in using cellulose ether that when the dosage of cellulose ether is too high or the viscosity is too large, it will increase the water demand quantity of mortar, reduce workability and feel difficult in construction. Cellulose ether will delay the setting time of cement.
Especially at a higher dosage, the setting retarding effect is more obvious. In addition, cellulose ether will also affect the opening hours, anti-sagging properties and bond strength of mortar.
Cellulose ethers are often applicable to the dry mixed mortar products with a thickness of 5mm or less.
- Mortar Thickening Powder
Mortar thickening powder is a non-lime and non-air-entraining powdery material, with montmorillonite, organic polymer modifiers and other mineral additives as the main ingredients, and enables mortar to achieve water-retaining and thickening purposes by the physical adsorption for water. Since its water-retaining and thickening effect is obtained mainly by inorganic materials and supplemented by organic materials, it enables cement mortar not only to have a certain water-retaining and thickening effect, but also to avoid the crusting phenomenon of cellulose ether. It is well compatible with a variety of cement. The building mortar mixed with thickening mortar is waterproof. The strength develops stably for long-term immersion in water and also in the atmosphere. After the freeze-thaw cycles, the strength loss and quality loss are less. Under the condition of the same amount of cement, the mortar mixed with thickening powder will be increased by 25% in the bond strength, reduced by 35% in the shrinkage, and increased by 25% in the permeability resistance compared with cement-lime mixed mortar. Therefore, mortar thickening powder is mainly used as the water-retaining and thickening material used in wet-mixed mortar and ordinary dry-mixed mortar currently in Shanghai region.
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