| Ultra-light material of calcium silicate has been widely used in industry for its low thermal conductivity, wide applying temperature range and low density. However, it was limited due to the disadvantage of mechanical properties. Xonotlite whiskers, which are required in the development of modern composite materials, have been attracting attention as a kind of strengthening and toughening materials. The xonotlite whiskers and spherical particles were synthesized using the hydrothermal synthesis method, and the ultra-light material of calcium silicate had been strengthened with the methods of"adding reinforcing agent"and"whisker in–situ growth"respectively.In this paper, xonotlite crystal was firstly prepared by the hydrothermal synthesis method, and the effect of raw materials and hydrothermal synthetic technologies on the morphology of xonotlite was also studied. The results indicated that the morphologies of hydrothermal synthesis products were determined by the precursor states of kiesel materials. Base on the different morphology characteristics of the products, the growth mechanism and reaction mechanism of xonotlite whiskers and spherical particles were explored, and the growth of xonotlite whiskers and spherical particles were accorded with the dissolution-precipitation mechanism and in situ transformation mechanism. In the CaO-SiO2-H2O system, the reaction mechanism of xonotlite crystal was described as follows while having CaO:SiO2(mol)=1.0: amorphous C-S-H gel→calcium-rich C-S-H(II)→silicon-rich C-S-H(I)→xonotlite. The kinetics formulas of xonotlite whiskers and spherical particles were found based on the reaction mechanism of xonotlite crystal: ? dc A /dt = kcA4/5 is for xonotlite whiskers and ? dc A /dt = kcA0.9 is for xonotlite spherical particles. At last, the apparent activation energy of xonotlite whiskers and spherical particles are 75-77 kJ·mol-1 and 77-84 kJ·mol-1 respectively through those kinetics formulas and Kondo model. The anlysis results of kinetics indicated that the growth and reaction mechanisms of xonotlite crystal are correct.The microstructures of xonotlite whiskers prepared at different process parameters were analyzed comprehensively. The results showed that the xonotlite whiskers, which are of well crystallized, excellent thermal stability and aspect ratio of 50-100, were synthesized by hydrothermal processing with Ca(OH)2 suspension which was prepared by using CaCl2 and 100g·L-1KOH solution as calcareous material and K2SiO3 solution as kiesel materials, controlling the system of pH=12.6 and reactant concentration of 0.05mol·L-1 at 225℃for 15 h. The effect of impurities and its content on the morphologies of hydrothermal synthesis products was studied mainly in this paper, and adding Cl- ions when using CaCl2 as calcareous material, controlling the Na2O content less than 5% and Al2O3 content less than 1% are beneficial to the preparation of xonotlite whiskers.The xonotlite crystal was prepared for the first time using the carbide slag as raw material. The requirements of raw materials are different attributed to the different morphologies of hydrothermal synthesis products, and then the xonotlite whiskers were synthesized from the carbide slag which was pretreated by the way of pickling under the conditions of pH=8.0 when pickling, at 225℃for 15 h, which are the best process parameters in this paper. The xonotlite spherical particles were prepared from the carbide slag which was pretreated by the methods of calcination and ultrasonic technologies respectively, under the conditions of using Ca(OH)2 suspension which was prepared from the carbide slag calcined at 800℃for 2 h and digested by the deionized water of 12 times and 80℃for 30 min as calcareous material, or using the carbide slag slurry at 160 W ultrasonic for 3 h as calcareous material, CaO/SiO2 mol ratio 1.0, water/solid weight ratio of 28:1, stirring speed of 400 rpm and 150 rpm when heating and reaction stage respectively, at 215℃for 10 h.Base on the growth characteristics of xonotlite crystal, the ultra-light material of calcium silicate was strengthened with the methods of"adding reinforcing agent"and"whisker in–situ growth"respectively. The product of 109 Kg·m-3 was prepared under the conditions of aspect ratio of 50-100, whiskers mixing amount of 5%, forming pressure of 0.5 MPa, demould and drying at 75℃for 12 h, and its flexural and compressive strength are 0.37 MPa and 0.59 MPa respectively when having the method of"adding reinforcing agent". The product of 118 Kg·m-3 was prepared under the conditions of K2SiO3/H2SiO3 mol ratio 2.0, hydrothermal treatment proceeded at 225℃for 15 h, forming pressure of 0.5 MPa, demould and drying at 75℃for 12 h, and its flexural and compressive strength are 0.42 MPa and 0.66 MPa respectively when having the method of"whisker in–situ growth". The specific strengthF and specific strengthC of the products prepared by adding glass fibers of 3% are 2.22×103 cm and 3.65×103 cm respectively, and the specific strengthF and specific strengthC of the products prepared by adding xonotlite whiskers of 5% are 3.40×103 cm and 5.41×103 cm respectively. The results indicated that the product prepared by adding xonotlite whiskers is of lower density, higher specific strength and well thermal insulation performance. |
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