Preparation of porous materials from hydrothermally hot pressed glass compacts

Abstract

The importance of the recycling of waste materials has been recognized for sustainable development of our human life. Recently, the authors [1] proposed the recycling of used glasses for building materials using the hydrothermal hot-pressing technique (hereafter referred as HHP), which is useful for preparation of solid compacts from various inorganic powders at low temperatures below 300 ◦C [2–4]. In this paper, we focused on the preparation of porous glass compacts to develop another alternative route for the recycling of used glasses. Porous materials are very attractive because of their controlled porosity and insulating properties for numerous applications such as thermal insulators, substrates for catalyst, filter systems and building materials. A wide variety of porous materials have been produced in an industrial scale using several raw materials such ceramics, glasses, cements and organic materials [5]. The porous ceramics are traditionally produced by a combination of solid state sintering and decomposition of the organic compound or calcium carbonate during the firing process, which results in the formation of a great number of pores in the ceramic bodies. Another routes to prepare porous materials are a phase transformation and a chemical reaction [6, 7]. For the preparation of porous glasses, the phase separation and sol-gel process are commonly employed [8]. The porous glasses have a very wide range of applications such as separation membranes, catalyst precursors, packing materials for chromatography, molecular sieves, and thermal insulators [8, 9]. In general, all the methods employed to produce porous glass materials involve very high cost of fabrication, because of the high sintering temperatures and a special route for the porosity control. In the present work, we prepared porous glass compacts with closed pores for insulating materials by simply heating the glass compacts densified by HHP. A powder of the used glass of blue color was employed to prepare glass compacts by HHP. The chemical analysis [1] of the glass powder showed that it consisted of SiO2 (69.58 wt%), Na2O (14.61 wt%), CaO (10.9 wt%) and other oxides. The glass powder (10 g) with particle size between 46–53 μm was well ground in a mortar with water (5–20 wt%). The mixture was placed in the cylindrical chamber of an autoclave for HHP, with inner diameter of 20 mm, uniaxially compressed at 20 MPa, and then heated to 200 ◦C at a rate of 5 ◦C/min. The temperature and pressure were kept constant for 2 h. The glass compacts prepared by HHP were heated in air for 1 h at a temperature in the range 50–750 ◦C. All the heat treatments were conducted at a constant heating rate of 5 ◦C/min. The weight of the compacts was measured before and after the heat treatment to determine the weight loss. The bulk density of the heated specimens was also determined using the cubic samples (10 × 10 × 10 mm3) shaped from the heated specimens. The weight loss of the compacts prepared with different contents of water is shown in Fig. 1. Large weight loss was observed by heating the compacts for 1 h at temperatures up to 750 ◦C. By hydrothermal hot pressing of the glass powder, a new amorphous phase was formed surrounding the original glass particles [1]. Energy dispersive X-ray analysis conducted on both the residual original glass particles and new phase showed that both of them consisted of the same elements, though infrared spectra of the compacts demonstrated that a significant amount of water was incorporated in the compacts during the hydrothermal hot-pressing treatment [10]. The new phase may be formed by the hydrothermal reaction of the glass powder with water. The weight loss observed during heating the compacts must be caused by the release of water included in the compacts prepared by HHP. Even after the heat treatment at 850 ◦C, no crystalline phases were detected by X-ray diffraction analyses.