Study on the preparation of self leveling material with chemical gypsum
Quan Liuquan 1, Li Dongxu 1, Zhang Liang 2
(1. School of materials science and engineering, Nanjing University of technology, Nanjing 210009, Jiangsu; 2. National Starch Chemistry (Shanghai) Co., Ltd., Shanghai 201613)
(editor's note: This paper is supported by the national "973" key basic research and development program. It was completed in February 2007 and published in IM & P chemical minerals and processing, Issue 9, 2007)
Abstract: chemical gypsum was treated by light burning and autoclave. The effects of treatment temperature and additives on the properties of gypsum were studied. The effect of gypsum, cement, lignin fiber, defoamer and redispersible rubber powder on the properties of gypsum based self leveling materials was studied by 2 * 6 factorial experiment. In addition, the effect of different crystal transformation agents on the properties of self leveling materials was also studied.
Key words: desulfurized gypsum; phosphogypsum; gypsum based self leveling material; 2 * 6 factorial experiment
Phosphogypsum is the solid waste discharged in the production of phosphoric acid based products. Usually 4.5-5.5t phosphogypsum will be produced for every 1tp2o5 produced. At present, most of it is discharged as waste. FGD gypsum is the waste produced by FGD in power plant. In order to reduce the pollution of chemical gypsum such as phosphogypsum and desulfurized gypsum to the environment and the occupation of land, its application field must be widened to turn waste into treasure.
Therefore, the performance of desulfurized gypsum and phosphogypsum treated with different methods and the performance of gypsum based self leveling material prepared with these treated chemical gypsum are studied in this project.
1 raw materials and experimental methods
1.1 raw materials
Phosphogypsum, desulfurized gypsum, pii52.5 cement, sulfonated trimethylamine superplasticizer, calcium lignosulfonate, redispersible rubber powder, anhydrous citric acid, sodium citrate, calcium oxide, aluminum sulfate (chemical purity), sodium alkylsulfonate (chemical purity), ultrafine sand and defoamer, etc.
The chemical gypsum after treatment was analyzed by X-ray diffractometer produced by Japanese science company.
1.3 treatment of chemical gypsum
In this experiment, combustion method and autoclave method were used to treat chemical gypsum. The change of setting time was studied by burning desulfurized gypsum at different temperatures. Setting time is one of the important properties of gypsum. A reasonable treatment temperature can be determined by studying the setting time of gypsum. Then the self leveling material was prepared with the treated chemical gypsum, and its performance change was studied.
The autoclave method is to place the chemical gypsum mixed with a certain amount of crystal transformation agent in the autoclave. Under the condition that the autoclave temperature is 125? 130x3 and the pressure is higher than 101.325kpa, the gypsum powder is obtained by drying and grinding under constant temperature and pressure for 10h.
1.4 fluidity of self leveling materials
The fluidity test mold is a hollow metal cylinder with an inner diameter of ± 30mm ± 0.1mm and a height of 50mm ± 0.1mm. The test plate is a flat glass with an area of 500mmx500mm. When testing the fluidity, place the fluidity test mold vertically in the center of the test plate, and the surface of the test plate shall be flat and smooth without water drop. After filling the mixed sample with the fluidity test mold, lift the fluidity test mold 5-10cm in the vertical direction of about 2S, and keep it for 10-15s to make the sample flow freely. After 4 minutes, measure the diameter of the two vertical directions, and take the average value of the two diameters, which is the fluidity value of the sample. Put the same batch of samples in the mixing pot for 20min, and test according to the above method, that is, the fluidity value of 20min.
1.5 mechanical experiment
The test piece shall be shaped according to the provisions of GB / t17671, without vibration, placed under the ambient temperature of 23 ± 2c, and demoulded after 24 hours according to GB/
T17671 was used to test 24 hours strength.
1.6 setting time
The setting time of gypsum is tested according to GB / t17669.4-1999.
2 experiments and results
2.1 XRD analysis of chemical gypsum
See Fig. 1 and Fig. 2 for XRD pattern of explosive chemical gypsum.
Fig. 1 is the XRD spectrum (from bottom to top) of desulfurized gypsum treated at 70 ℃, 110 ℃ and 150 ℃. It can be seen from Figure 1 that the composition of desulfurized gypsum treated at 110 ℃ and 150 ℃ is not very different, and there are diffraction peaks of hemihydrate gypsum with 0.5 and 0.662 water, while the XRD pattern of desulfurized gypsum treated at 70 ℃ has a strong diffraction peak of dihydrate gypsum, and the diffraction peak of hemihydrate gypsum is weak, indicating that the main composition of desulfurized gypsum treated at 70 ℃ is dihydrate gypsum.
Fig. 2 is the XRD pattern (from bottom to top) of phosphogypsum treated at 70 ℃, 130 ℃ and 170 ℃. It can be seen from Fig. 2 that the phosphogypsum treated at 130 ℃ and 170 ℃ also has diffraction peaks of hemihydrate gypsum with 0.5 and 0.662 water, and there is no obvious diffraction peak of dihydrate gypsum and anhydrous gypsum.
The XRD patterns of autoclaved chemical gypsum are shown in Figure 3 and Figure 4
In Fig. 3, from top to bottom, the XRD patterns of phosphogypsum treated by adding aluminum sulfate and calcium lignosulfonate, without crystal conversion agent, and adding aluminum sulfate and sodium citrate autoclave are shown respectively. In Fig. 4, from top to bottom, the XRD patterns of desulfurized gypsum added with aluminum sulfate and calcium lignosulfonate, aluminum sulfate and sodium citrate respectively, without autoclaved treatment of crystal transformation agent. For the same kind of chemical gypsum with different crystal transformation agents, after autoclave treatment, its XRD diffraction peak position is basically the same, with 0.5 and 0.662 water diffraction peaks, but its intensity is different. Compared with the XRD pattern of phosphogypsum and desulfurized gypsum, the XRD pattern of phosphogypsum shows a strong diffraction peak when the diffraction angle is 2? 26.6875 °, which is the diffraction ray of silica. It shows that phosphogypsum contains more silica compared with desulfurized gypsum, which is consistent with the result of chemical analysis.
2.2 effect of treatment temperature and retarder on setting time of gypsum
The effect of burn out temperature on setting time of gypsum was studied. 0.1% anhydrous citric acid was added into gypsum when setting time was tested. At the same time, the effect of the content of citric acid and tartaric acid on the setting time of FGD gypsum after 110 ℃ treatment was also studied. The results are shown in Table 1.
With the increase of treatment temperature of FGD gypsum, the setting time of gypsum has a maximum value. When the treatment temperature is 130 ℃, the setting time of gypsum is the maximum. From the XRD of the treated FGD gypsum, it can be seen that there are 0.5 and 0.662 water diffraction peaks in the treated FGD gypsum at 110 ℃ and 150 ℃, and there are no obvious diffraction peaks of dihydrate gypsum and anhydrous gypsum at these two temperature points, indicating that the FGD gypsum treated at 110 ℃ and 150 ℃ has no dihydrate gypsum and anhydrous gypsum or its content is very small. In the range of 110? 150 ℃, the specific content proportion of hemihydrate gypsum of desulfurized gypsum after different temperature treatment is different, thus affecting its setting time. It can be seen from table 1 that the setting time of both retarders increases with the increase of the dosage. The retarding effect of anhydrous citric acid is better than that of tartaric acid. The retarder can achieve its retarding effect by strongly inhibiting the growth of gypsum crystal in the long axis direction and changing the relative growth rate of each crystal surface .
2.3 effect of composition of self leveling material on its performance
The effects of gypsum, cement, lignin fiber, defoamer and redispersible rubber powder on the properties of gypsum based self leveling materials were studied. In addition, a proper amount of sulfonated trimethylamine superplasticizer, anhydrous citric acid and superfine sand are added in the preparation of gypsum based gravity flow. The amount of mixing water is 35% (accounting for the dry quality of the material). Gypsum is treated desulfurization gypsum. The experiment is designed by 2 * 6 factorial experiment method, and the results are shown in Table 2
Note: the solution made of anhydrous citric acid is added to the self leveling material.
See Table 3 for the contrast analysis of the fluidity of self leveling materials.
It can be seen from table 3 that the main factors influencing the fluidity of self leveling materials are gypsum and cement. Increasing the content of gypsum will reduce the fluidity of self leveling material, while increasing the content of cement will increase the fluidity of self leveling material. This is because the setting time of gypsum is very short, hydration occurs in a very short time, and there is overlapping between gypsum crystals, so as to increase the content of gypsum and make the fluidity of self leveling materials worse. The fluidity test of self leveling material is completed in a short time after adding water. In this short period of time, for gypsum, part of the hemihydrate gypsum has hydration reaction. However, at this time, cement is only equivalent to fine aggregate, playing the role of "ball", reducing the resistance of relative slip between the components of self leveling materials, and increasing the fluidity of self leveling materials.
The variance analysis of flexural strength and compressive strength of self leveling materials is shown in Table 4 and table 5.
It can be seen from table 4 and table 5 that the main factors influencing the flexural strength of self leveling materials are gypsum, cement, defoamer and redispersible rubber powder. Gypsum and redispersible rubber powder are the main factors influencing the compressive strength of self leveling materials. Gypsum and redispersible rubber powder are used as inorganic and organic cementitious materials respectively. By increasing their content, more hydration products can be produced in the hydration process of self leveling materials, and the bonding strength between materials can be enhanced, so as to improve the strength of self leveling materials. Defoamer is the main influence factor on the flexural strength of self leveling materials, but not on the compressive strength. This is mainly because the influence of the hole in the material on the flexural strength is more obvious than that on the compressive strength. By increasing the content of defoamer, the porosity in the self leveling material is reduced, and the flexural strength of the material is increased significantly.
2.4 effect of crystal conversion agent on properties of self leveling materials
See Table 6 for the influence of crystal conversion agent on the properties of self leveling materials.
The addition of ions has an important effect on the growth of gypsum crystal surface (2). For example, sodium citrate, aluminum sulfate and alkyl are added to desulfurization gypsum
Sodium benzoate introduces not only the anion rcoo -, but also the cation Na +, Al3 +, which are adsorbed on the (111) surface. One end of rcoo group is adsorbed with Ca2 + on (111) crystal surface, and the other end is connected with Al3 +, forming a network structure adsorbed on (111) surface, forming a network like "buffer film" composed of organic macromolecular adsorbed metal ions. According to the theory of periodic bond chain, it is difficult to combine the crystal growth elements with the crystal surface adsorbed with the above-mentioned network like film, because it is difficult to combine with the crystal surface only after destroying the existing adsorption bond of the network like film on the crystal surface. Therefore, the mesh film hinders the combination of crystalline elements on the crystal surface, thus slowing down the growth rate of gypsum crystal in the c-axis direction, making the growth rate in all directions close to equilibrium, and the product is hexagonal short column.
It can be seen from table 6 that the properties of self leveling materials are greatly changed when the chemical gypsum mixed with different crystal transformation agents is autoclaved. When a suitable crystal transformation agent is added into the chemical gypsum, the fluidity and strength of the gypsum based self leveling material can be greatly improved due to the change of the crystal habit of the gypsum. However, the properties of self leveling materials will not be improved, but will be reduced when the improper crystal conversion agent is added into the chemical gypsum. For example, magnesium acetate, aluminum sulfate and sodium alkylbenzoate are added to phosphogypsum, and sodium citrate is added to desulfurization gypsum. This is due to the decrease of supersaturation after the incorporation of these substances, which reduces the number of crystal embryos, slows down the growth of crystals, and allows sufficient time for dihydrate gypsum crystals