Refractory thermal insulation materials are generally used in industrial furnaces, metallurgy, pipelines and thermal equipment to slow the loss of heat. At present, thermal insulation materials mainly include rock wool, glass wool, foam plastic, expanded perlite, aluminum silicate fiber and other five categories. In addition to foam plastics used in packaging, building thermal insulation and other fields, the other four types of thermal insulation materials Both can be used in industrial kilns, but based on the microstructure characteristics of these materials generally having interconnected pores, the thermal insulation performance under high temperature conditions is not very satisfactory. The maximum use temperature of rock wool is 500 ℃, the maximum use temperature of glass wool is 200 ℃, and the maximum use temperature of expanded perlite is 800 ℃. Aluminum silicate refractory fiber is currently the fastest growing and the most widely used refractory fiber in high-temperature industrial furnaces. It has the advantages of high-temperature heat insulation, heat preservation, fire resistance, noise reduction, insulation and light weight. Its maximum use temperature is 1260 ℃, The long-term use temperature is 950～1050 ℃, but the compressive and flexural strength is relatively poor, it is easy to generate dust, the waterproof performance is not very good, the water absorption rate is large, and the connected pores are not conducive to heat preservation.
In view of the above problems, this research uses aluminum silicate active powder as the cementing material, adds other mineral raw materials, and forms porous foam parisons through physical foaming. After high-temperature calcination, a high-temperature light-resistant material with certain strength, dry density and low water absorption is obtained. Quality thermal insulation materials. Compared with other material systems, the advantages of the method involved in this test are: ①The heat-resistant insulation layer of the kiln can be integrally formed to minimize the heat dissipation of the splicing joint; ②Compared with aluminum silicate fiber, the internal pores It is a closed stomata, showing low water absorption.
1 Test materials and methods
1.1 Raw materials
Aluminum silicate active powder, No. 800 high alumina refractory cement; Silica powder, particle size 1 μm; Physical foaming agent, self-made.
1.2 Test method
1.2.1 Material preparation
This experiment uses the most basic ratio. The specific production steps are as follows: ① Weigh 200 g of high-alumina refractory cement and 60 g of silica powder, place them in a 2000 mL stainless steel container, add 100 mL of water, and stir to form a slurry for later use; ②Use compressed air to blow the physical foaming agent into fine foam for use; ③Add foam to the cement slurry one by one, while stirring evenly, until the volume reaches 1 L; ④Pour the stirred foam slurry into the model , Wrapped with plastic film, put it in a curing box for curing, the temperature is set to 37 ℃, curing time is 28 days; ⑤After taking out the parison and let it dry naturally for 3 days, dry it at 90 ℃ for 24 h, and then heat it up to 120 ℃ Drying for 24 h; ⑥After drying, some samples of the parison are left to be tested, and the remaining samples are calcined at 1180 ℃ for 2 h, the heating rate is 2 ℃/min, and the temperature is cooled with the furnace.
1.2.2 Test method
The 28-day age of the test block and the compressive strength and dry density after calcination were measured respectively. The test method refers to GB/T 11969-2008 "Test Method for Performance of Autoclaved Aerated Concrete". The thermal conductivity test method refers to GB/T 10294-2008 "Measurement of Steady Thermal Resistance and Related Characteristics of Thermal Insulation Material Protective Hot Plate Method", and the equipment used is the Dre-2C thermal conductivity tester.
The specific test method of water absorption refers to GB/T 11969-2008 "Test Method for Performance of Autoclaved Aerated Concrete", and the volumetric water absorption is calculated by formula (1), with an accuracy of 0.1%.
W = (M－M0) / (ρV)
(1) W is the volumetric water absorption of the test piece, %; M is the mass of the test piece after water absorption, M0 is the mass of the test piece before water absorption, kg; ρ is the density of water, ρ = 1000 kg / m3; V is The volume of the test block, m3.
The XL-30EMSE environmental electron scanning microscope was used to observe the microscopic morphology of the parison and the sintered body. The DX-2700 X-ray diffractometer was used to analyze the composition of the parison and the sintered body.
2 Results and discussion
After testing, the dry density, compressive strength, thermal conductivity, and water absorption test results of aluminum silicate lightweight high-temperature thermal insulation brick are listed in Table 1.
2.1 Dry density
There are two factors that affect the dry density: the mass of the solid phase material and the volume of the constant volume. This is also a method to obtain the approximate design dry density, that is, when the cement and mineral materials are fixed to 1 L, theoretically, the dry density of the parison or sintered body is numerically equal to the sum of the mass of the cement and mineral materials.
The test results show that the dry density of the parison is 285 kg/m3, and the dry density of the sintered body is 268 kg/m3, which are both higher than the theoretical dry density of 260 kg/m3. The reason is that water occurs in the cementation process. Chemical reaction, part of the water becomes the composition of the solid phase, making the dry density of the parison higher than the theoretical dry density. After high-temperature sintering, due to the limitation of the purity of the raw materials, the moisture and volatile matter in the raw materials and the structural water formed during the gelation process are all burned out. The dry density of the sintered body should be lower than the theoretical dry density, but a certain amount of damage occurs during the sintering process. The volume shrinkage of the sintered body increases the dry density, and the dry density of the sintered body will depend on the combined result of the mass loss and volume shrinkage during the sintering process.
The mineral auxiliary material used in this experiment is silica, which can be replaced or supplemented by graphite, kaolin, glass beads, etc. When replaced by graphite or glass beads, the theoretical dry density can reach less than 200 kg/m3.
2.2 Compressive strength
As a high-temperature thermal insulation material, the requirements for compressive strength are not as high as those for structural load-bearing parts, but it is necessary to have a certain degree of compressive strength.
The compressive strength of the parison is mainly derived from the cementation strength produced by the hydration reaction of the cement and mainly formed by molecular bonds. For a compact block, its 28-day final setting strength can reach 43 MPa. After foaming, due to a large number of pores If the molecular connection of the cement is separated, its compressive strength is greatly reduced. In other words, the compressive strength of the parison is positively correlated with the dry density in a theoretical sense.
Figure 1 shows the photomicrographs of the fractures and sintered bodies of aluminum silicate lightweight high-temperature thermal insulation materials for industrial kilns, and Figure 2 shows the preforms and sintered bodies of lightweight aluminum silicate thermal insulation materials for industrial kilns. X-ray diffraction results of the sintered body. It can be seen from Figure 1 (b) that, compared with the parison, the sintered body's pore wall is smooth and smooth, showing obvious vitrification, and the size of the micropores is reduced, and the number is reduced. The diffraction results showed that the diffraction peaks of silica in the sintered body disappeared. It is inferred that the silica was converted from crystal to glass phase, but the main phase structure did not change.
Fig 1 Frature morphologies of the parison( a) and sintered body( b)
During the sintering process of the parison, a certain degree of volume shrinkage occurs. At the same time, silica reacts with part of the cement to form a glass phase. From the perspective of chemical bonds, part of the molecular bonds and hydrogen bonds between the material components are transformed into ceramic bodies. Ionic bond, compared with parison, sintered body has higher compressive strength.
It should be noted that the existence of such fine pores has a passivation effect on crack propagation. Therefore, this type of material has good mechanical processability.
2.3 Water absorption and thermal conductivity
The test shows that the water absorption rate of the parison is 50% and the water absorption rate of the sintered body is 3%. From the measurement of the water absorption rate, it can be seen that the internal pores of the sintered body are basically closed pores, or extremely thin channels between the pores. As a result, the capillary force is greater than the pressure of the water entering from the outside, causing the test sample to always float on the surface of the water when immersed in the water, and external water cannot enter the inside of the sample. It may be better to call this type of stomata "quasi-closed stomata". It is obvious that the "closed pores" form has a lower thermal conductivity than the connected pore form (aluminum silicate fiber) (the thermal conductivity of aluminum silicate fiber is 0.159 W/ (m · K) at 800 ℃).
The thermal conductivity of the parison and the sintered body measured by the test are 0.08 and 0.06 W/ (m · K), respectively. Combined with the water absorption, the decrease of the thermal conductivity of the sintered body is mainly due to the change of the pore shape from the open pores of the parison to Due to the closed pores of the sintered body.
The size of the pores is determined by the size of the gaseous glue foam, and the quantity is determined by the amount of gaseous glue added. The amount of gaseous glue is directly negatively related to the dry density and compressive strength, that is, the greater the amount of gaseous glue, the lower the dry density and the compression The lower the strength, when the amount of gaseous glue added is too large, the cementitious material will not even be able to condense into a solid.
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There are two main factors affecting the thermal conductivity of foam materials: ① the thermal conductivity of the solid substance (frame); ② the characteristics of the pores (shape, quantity, size and distribution). In order to reduce the thermal conductivity, mineral additives should also be reasonably selected as the components of the solid phase substance (skeleton). In addition, under the premise of satisfying the dry density and thermal conductivity, in order to improve the compressive strength, adding some fiber-reinforced materials can also be considered.
3 Conclusion
1) Prepare a kind of aluminum silicate lightweight high temperature thermal insulation material by physical foaming method. The process operation is relatively simple, the theoretical dry density is very close to the actual test value, the process is easy to control, the production repeatability is good, and the cost is low. Used in industrial kilns, it can be cast as a whole, which reduces the impact of the stacking joints that must exist in the stacking of thermal insulation bricks on thermal insulation performance.
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2) The basic material ratio and process are 200 g high alumina refractory cement, 60 g silica powder, 100 mL water, constant volume 1 L, sintering temperature 1180 ℃, sintering time 2 h, design dry density 260 kg / m3.
3) The prepared material sintered body has a dry density of 268 kg/m3, a compressive strength of 4.2 MPa, a thermal conductivity of 0.06 W/ (m ·K), and a water absorption rate of 3%. It can be used as an industrial kiln thermal insulation material excellent.
