The Fabrication Of CaO-Al₂O₃-SiO₂ Lightweight Refractory Materials Based On Gelcasting

Lightweight refractory materials are indispensable material used in high temperature industries for insulation.Their technological advancements will meet the requirement of our country for energy conservation.In this paper,Ca O-Al₂O₃-SiO₂system was selected as the research object,two types of gelcasting were developed to fabricate lightweight refractories.The two gels are based on sodium alginate（Na-alg）and tetramethyl ammonium hydroxide（TMAH）.Solid content of slurry and the gelation behaviour were investigated by the change of slurry viscosity.The mineralogical phase of the solidified body and sintered sample were detected by X-ray diffraction（XRD）.The gelation mechanism and the chemical structure were studied by infrared spectroscopy（FTIR）.The microstructure of the sintered samples were observed by scanning electron microscopy（SEM）.Finally,the loss rate,shrinkage rate,volume density,porosity and thermal conductivity were tested.The following conclusions were obtained:（1）The anorthite lightweight refractories were developed based on Na-alg gelation.The proper solid content is 50 wt.%determined by slurry viscosity;The additions of Na-alg and glucolactone（GDL）in the gelation system are 0.6 wt.%and 0.015 g/m L,respectively.The p H of slurry is gradually decreased by hydrolysis of GDL and the combination of Na-alg and Ca²⁺is enhanced,which lead to the forrnation of green body with good strength.After freeze-dried,the anorthite refractories sintered at 1300°C have better physical characterics,such as volume density of 0.745 g/cm³,porosity of 68.02%,and thermal conductivity of 0.1604 W/（m·K）.（2）The anorthite lightweight refractories were developed based on TMAH geopolymer gelation.The proper solid content is 55 wt.%determined by slurry viscosity and the analysis of various parameters of the green/sintered body;The additions of TMAH and foaming agent（sodium dodecyl benzene sulfonate）in the gelation system are 8.0 wt.%and 6.0 wt.%,respectively.Due to the TMAH doesn’t contain metal cations,only the Ca²⁺with weak polarity in the raw material,so the slurry can only produced C-A-S-H gel with a low number of units in the mixing stage,and the slurry viscosity is relatively low,when the foaming agent（sodium dodecyl benzene sulfonate）carries sodium ion into the slurry,most C-A-S-H gels begin to convert into N-A-S-H gels which has higher polymerized,the slurry viscosity increases sharply and the body with high strength can be obtained after 40°C drying treatment.The anorthite refractories sintered at 1300°C have better physical characterics,such as volume density of 0.697 g/cm³,porosity of 71.55%,and thermal conductivity of 0.1273 W/（m·K）.（3）Based on the related parameters of the geopolymer gelation,the effect of Mg O and blast furnace slag on the preparation of lightweight refractories were developed.The Mg O content and replacement amount of blast furnace slag are 3 wt.%and 37.11 wt.%respectively determined by slurry viscosity and the analysis of various parameters of the green/sintered body;Due to the presence of the Mg O,the Mg（OH）₂crystal and M-S-H gel that has characteristics of expansion will be generated in the slurry,which will greatly reduce the shrinkage rate of the green body and the roasted body with a roasting temperature of 1200°C and below.The anorthite-magnesium refractories was obtained at the blast furnace slag replacement amount of 37.11 wt.%and the roasting temperature of1200°C,such as volume density of 0.602 g/cm³,porosity of 75.43%,and thermal conductivity of 0.1206 W/（m·K）.（4）Based on the related parameters of the geopolymer gelation,The preparation of anorthite-mullite refractories by increasing the proportion of mullite phase design content was developed.Slurry viscosity and the analysis of XRD/FTIR of the green/sintered body showed:When the mullite phase design content doesn’t exceed 60 wt.%,Ca O content was saturated relative to C-A-S-H,increase the mullite phase design content was beneficial for curing,the main crystal phase of refractories are anorthite at 1400°C;When the mullite phase design content reached 80 wt.%,due to the low content of Ca O,the slurry viscosity decreases sharply,even when the mullite phase design content was 100 wt.%,the slurry cannot be cured,at this time,the main crystal phase of refractories are mullite at 1400°C.