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Single-stage technogy for granulated foam glass production based on the composition of tripoli and technogical microsilica

The possibility of foam glass production by means of one-stage technology based on the natural tripoli and technogenic silica is determined. 45 % sodium hydroxide solution is used for the synthesis of foam glass. The addition of microsilica as an extra component in an amount of 10 to 50 wt. % in batch component increases the strength of the obtained material to 4 MPa and reduces foaming temperature from 870 to 830 °C. The increased mechanical strength of the material is due to the residual quartz dissolution and cristobalite crystallization. The samples obtained from batch with 30 wt. % microsilica have maximum strength.


Introduction
Foam glass is considered to be one of the most environmentally friendly, durable and efficient thermal insulators. Its effectiveness is due to a unique combination of properties, such as low water absorption, frost resistance, resistance to aggressive media, relatively high strength. Currently, the field of foam glass application has been rapidly developing. Granulated foam glass is used for fill insulation, and as an effective filler of various composites, for example, in the production of warm plasters as light concrete and cement slurries. Glass cullet or finely crashed glass melted in a glass-melting tank is a raw material for foam glass production. In view of the limited resources of waste glass the raw material base development for foam glass production is relevant. Granulate melting is associated with high energy and capital intensity of glassmaking. European researches are mainly focused on the use of various types of glass cullet, for example, glass of electron -ray tubes and computer screens, etc. [1][2][3][4][5][6][7]. Some researchers consider not only glass cullet but also various types of natural and technogenic materials as raw material [8][9][10][11].
The objective of this study is to obtain foam glass material by means of one-stage technology based on natural silica component in the form of tripoli at a foaming temperature not exceeding 850 ° C.

Materials and Experimental method
Tripoli and microsilica were chosen as objects to study siliceous materials of natural and technogenic origin. Chemical and granulometric composition of silica raw materials are shown in Table 1. The chemical composition of tripili differed from the standard silica sand used in glass production, as it had a reduced content of glass-forming oxide and an increased content of SiO 2 Al 2 O 3 , Fe 2 O 3 . Silica fume, which is a byproduct of metallurgical production in melting of ferrosilicon and its alloys, corresponds to the sand grade used for the production of foam glass, cans and semiwhite glass bottles. Along with chemical composition the high dispersion with average particle size of less than 100 microns was the main requirement to the raw material. Initial tripoli contains coarse particles in the form of clay aggregated in an amount of 3 -7 wt. %. Therefore, tripoli was previously dried and then milled in a ball mill. The results of the grain-size composition of the prepared tripoli were presented in the following table. Microsilica was mainly presented in a form of particles of less than 0.5 microns without additional preparing. According to the results of X-ray analysis it was determined that microsilica refers to X-ray amorphous material. Crystal silicon (d = 0,313; 0,192 nm), carborundum (d = 0.251; 0.217; 0.154 nm), and graphite (d = 0,335) practically without a crystal phase were presented in the results. Small amount of amorphous halo (ring) reflection maxima were shown on the X-ray photograph of tripoli, which corresponded to different modifications of silica, ie, material was amorphous crystalline. A few of diffraction maxima specific for albite were also presented. In the study, granular sodium hydroxide, a white crystalline powder with the composition of NaOH -99.5 wt. %; Na 2 CO 3 -0.5 wt. %; NaCl, Na 2 SO 4 , Fe 2 O 3traces, was used as an alkali component.
The study of phase composition of the raw and obtained materials was carried out using a X-ray diffractometer DRON-3M in copper radiation. Quantitative X-ray analysis was performed using the Match program. Macro-and microstructure analysis of porous samples was performed on a scanning electron microscope (JCM -6000).

Experimental results
Preparation of foam glass was carried out according to the scheme shown in Figure 1. First, the source raw material was dried, then sieved, and, if necessary, milled. The prepared materials were mixed in a bowl of the mixer-granulator, then granulated with binding substance in an amount of 30 wt.  Use of only microsilica as a silica component showed that microsilica was completely dissolved in alkali due to structural features and high dispersion. In addition, the pellets foaming occurred at a temperature of 200 °C. However, the foam density exceeded 800 kg/m 3 , which didn't comply with the requirements of thermal insulation materials. Therefore, microsilica can be used only as an additional component. Its amount in the batch was varied from 10 to 50 % by weight in 10% increments. The batch compositions were presented in Table 2.
According to X-ray analysis it was determined that reflection peaks corresponding to different sodium silicate were presented on the X-ray photographs of the dried granules (up to 200 °C) of all compositions (Fig. 2). Diffraction peak intensity increased with an increase of microsilica in the batch, which indicated more intensive silication development. When the granulated material was heated up to 750 °C and above, a glass foaming process was observed at the foaming stage, and the obtained porous material had a vitrified surface. The process proceeded in stages. The first phase was obtaining of eutectic melt by ternary eutectic melting formed among Na 2 O•2SiO 2 and SiO 2 and albite, at a temperature of 740 °C and the binary eutectic among Na 2 O•2SiO 2 and SiO 2 at a temperature of 793°C. In the second stage, the residual silica was dissolved in the primary melt, then, the granules were cooled to form sodium-alumino-silicate glass at foaming temperatures (800-900 °C). Quantitative X-ray analysis of the synthesized foam glass showed that the amount of the glass phase in the obtained product depended on the content in the starting batch of microsilica. The glass phase content increased from 74 % to 78 % with microsilica increasing from 10 to 20 %, respectively. The amount of microsilica in the starting material of 30 % resulted in a decrease up to 64 % of glass phase, and when the content of microsilica was 40% and 50 %, the amount of the glass phase increased up to 69 %. Different content of the glass phase in the final product was due to the glass crystallization processes occurring at the stage of pellets foaming. The X-ray photographs of all samples presented reflection peaks corresponding to quartz, and new reflection peaks corresponding to cristobalite appeared (Fig. 3). With an increase of silica in the batch the intensity peaks of cristobalite increased.
On the electronic micrographs of the samples obtained with the addition of microsilica, it can be seen a new crystal phase in foam glass. The picture showed needle-like crystals, which were absent in the samples without microsilica (Fig. 3). According to the X-ray diffraction analysis and the needle shaped crystals one can determine the cristobalite crystallization. The granular material strength values specified by the standard techniques showed the following: with an increase of the content of microsilica in the batch up to 30 % the mechanical strength of the granules increased to 4 MPa, the strength reduced to 3.3 MPa with more content of microsilica, but the strength of the granulated foam glass without microsilica addition of 1.5 MPa became higher. The increased strength of the foam glass was due to crystallization processes that occurred at the foaming stage. At a density of 500 kg/m 3 the strength of foam glass was two times larger than the strength of the foam glass obtained by one-stage technology based on tripoli, and was 1.3 times larger than the strength of the foam glass obtained from the glass cullet.

Conclusions
The