Self-leveling material is a kind of inorganic or organic cementitious material as the base material, adding appropriate amount of aggregate and water reducing agent, water retaining agent and other admixtures formed new building ground self-leveling material. Its biggest characteristic is self-leveling, can flow freely under the action of its own gravity to form a flat ground, can overcome the common problem of ordinary cement mortar construction flatness is difficult to control.
The main substrate of self leveling material composition can be divided into organic system and inorganic system. Organic self-leveling materials mainly refer to epoxy self-leveling materials, whose unique chemical structure and two-component organic compounds make organic self-leveling materials have good physical and mechanical properties and acid, grease and other chemical properties, and have become the most widely used self-leveling materials. Inorganic system self-leveling material can be divided into cement system self-leveling material with cement as the base material and gypsum system self-leveling material with gypsum as the base material. Cement self-leveling material has high strength, good mobility, and excellent water resistance, which is widely recognized in the market. Gypsum self-leveling material has the characteristics of good thermal insulation, sound insulation, fire prevention and durability. Compared with cement base, it is slightly elastic, light weight, good sense of foot and high comfort. The gypsum base self-leveling materials have high requirements for gypsum quality, but there are few high-quality natural gypsum in our country, so the chemical gypsum system self-leveling has great potential for development and economic effects.
1. Types of chemical gypsum
1.1 Phosphogypsum
Phosphogypsum is a solid waste produced by phosphate fertilizer plants and other chemical enterprises. Every 1 ton of wet process phosphoric acid produces about 5 tons of phosphogypsum. In recent years, the average annual discharge of phosphogypsum has reached more than 40 million tons. In addition to calcium sulfate dihydrate, phosphogypsum also contains undecomposed apatite, insoluble P2O5, eutectic P2O5, fluoride, phosphate and sulfate of fluorine, aluminum, magnesium, organic matter and other impurities, which are the main factors affecting the properties of phosphogypsum. At present, phosphogypsum is mainly used in the production of cement retarder, gypsum brick, subgrade filler, gypsum block, gypsum board, plaster gypsum, plaster gypsum and sulfuric acid co-production cement and soil improvement.
1.2 Desulfurized Gypsum
Desulphurized gypsum is an industrial waste obtained from flue gas desulphurization (FGD) purification treatment of flue gas produced after coal burning in thermal power plants. The state stipulates that all coal burning enterprises must adopt desulfurization technology to deal with sulfur dioxide. According to 1 ton to produce about 3 tons of desulfurized gypsum calculation, in 2011 by thermal power plants and factories desulfurized gypsum produced by the annual emissions of about 25 million tons, will become the most chemical gypsum emissions of industrial waste. The main component of desulfurized gypsum is calcium sulfate dihydrate, generally 88% ~ 95%, the main impurities are calcium carbonate, calcium sulfite and chloride, etc. The research and utilization of desulphurization gypsum in European and American developed countries began in the late 1970s, and now a complete research and application system has been formed. The utilization rate is as high as 80% to 90%. Our research on desulphurization gypsum starts late, and it has not yet formed large-scale development and utilization. Desulphurized gypsum can be used in the preparation of putty powder, high strength gypsum powder, cementing material, plaster gypsum, as cement retarder, and can also be applied in the field of fire retardant coating for steel structure.
1.3 Fluogypsum
Fluogypsum is an industrial waste discharged from fluoride plant by reaction of fluorite and concentrated sulfuric acid to produce hydrogen fluoride. In general, calcium sulfate dihydrate in calcium fluoride is more than 75%, and the content of calcium sulfate dihydrate in fluogypsum produced by dry process aluminum fluoride is more than 95%, which has reached the standard of high quality natural gypsum. Every year our country fluoropgypsum emission 100 tons or so. Fluogypsum can be used as cement retarding agent, mineralizing agent, ceramic gypsum mold, cementing material and production of gypsum blocks, gypsum board and other building materials.
1.4 Titanium gypsum
Because of the monopoly of titanium dioxide enterprises in Europe and America developed countries, the production of titanium white is still by sulfuric acid method in our country, and the production of titanium white by sulfuric acid method has produced a large amount of acid wastewater. Titanium gypsum is produced with the lime and wastewater treatment. 1 ton of titanium dioxide produces about 5 tons of titanium gypsum, the national annual output of titanium gypsum is about 1 million tons. Titanium gypsum dihydrate calcium sulfate is generally 70% ~ 85%, the main impurities are iron, aluminum, titanium, chromium. Our research, development and utilization of titanium gypsum started late, and at present, there are some experts from Chongqing University and Jinan University and other universities for researches on resource utilization of titanium gypsum, and they mainly focus on roadbed fillings, cement retarder and walling materials.
1.5 Lemon Plaster
Lemon gypsum is a byproduct of citric acid production from sweet potato and other raw materials. Every ton of citric acid produces about 2.4 tons of lemon plaster, and our country disposes about 500,000 tons of lemon plaster every year. Lemon gypsum is the main component of gypsum dihydrate content up to 95%, in addition, contains a small amount of impurities such as fierce, fluorine. At present, it is mainly used in the preparation of wall materials such as block and paper gypsum board.
1.6 Other chemical gypsum
In addition to the above chemical gypsum, there are boric acid plant emission of boron gypsum, seawater salt or desalination process produced salt gypsum and emissions less wine acid gypsum, monosodium glutamate gypsum, copper gypsum, additive gypsum, chromium gypsum, sodium nitrate gypsum, lactic acid gypsum, etc.
2 Composition of self-leveling material prepared by chemical gypsum
After pretreatment, the content and structure of main components of chemical gypsum are similar to that of natural gypsum. With a certain mix ratio, gypsum base self-leveling material with similar properties to the self-leveling material prepared by natural gypsum can be synthesized.
Japan is the first country to study gypsum based self-leveling material, the most representative is Japan Housing Corporation, then the United States, South Korea, Germany and other developed countries in Europe and the United States have developed gypsum based self-leveling material. Table 2 shows the typical formula of gypsum based self-leveling materials in Japan, and Table 3 shows the properties of typical gypsum based self-leveling materials.
