Glass-Ceramics from Trachyte Rock- Limestone or Magnesite

Inexpensive sintered glass-ceramic glaze was prepared from a mixture of Egyptian trachyte with either limestone or magnesite. A represented trachyte rock was pulverized to powder (<0.083 mm) and also both limestone and magnesite. The well mixed batches were melted near 1450 °C/3 h temperature then the glass melt was quenched in water, dried, pulverized to powder and finally shaped in moulds. The sintering process of the pre-shaped glasses, within 1000-1100 °C range, gave augite and olivine in case of trachyte-magnesite whereas wollastonite and Ca-olivine in case of trachyte-limestone. However, cristobalite was developed in both cases. The microstructures in both cases showed glassy matrix scattered with submicron and nano-size crystals either in irregular shape in case of trachyte-limestone or clear crystals in case of trachyte-magnesite. The densities of the sintered samples were in the range of 2.36 and 2.57 g/cm3 in case of the trachyte-limestone and 2.36 and 2.64 g/cm3 in case of the trachyte-magnesite. The coefficient of thermal expansion (CTE) and the hardness of the sintered glass-ceramic were in the range of 6.2-8.5 × 10−6 °C−1 and 440-563 GPa respectively. However, the CTE values decreased in case of trachyte-limestone whereas the hardness values were high in case of trachyte-magnesite. The present glass-ceramic samples had porcelain nature and could be used in cladding of wall and floor.


Introduction
Ceramic glaze is impermeable glossy coating which serves in decorating or waterproof. Glaze based on cordierite glassceramic, has good resistance for abrasion and acidic medium, and was obtained from industrial borosilicate frits [1]. Within MgO-CaO-Al 2 O 3 -SiO 2 system, a glazed tile containing diopside as major phase and have good hardness was produced [2]. Different floor glazes have good hardness and glossiness in the CaO-MgO-SiO 2 -Al 2 O 3 -ZrO 2 system were prepared [3]. Many publications were done to form glaze based on low cost and solid waste materials. An ecological porcelaneous ceramic glaze was prepared from external soda-lime-silica glass with flyash of coal power thermal plant [4]. Low-cost glaze, has good chemical resistance in acid and alkaline medium, was prepared from the addition of cement kiln dust to ceramic glaze [5]. On testing on some raw crystalline frits, the authors test the addition in frits to induce the developed crystalline phases in glass-ceramic glaze [6]. In the SiO 2 -Al 2 O 3 -RO (R = Ca, Mg, Sr) glass-ceramic glaze, an increase of heating rate leads to a decrease of the sintering crystallization temperature [7]. For SiO 2 -Al 2 O 3 -CaO-MgO-Na 2 O-K 2 O glass system, the addition of nano-grained quartz improves the mechanical of the developed glass-ceramic glaze [8]. Others cleared that the glass-ceramic glaze enjoys better mechanical and chemical properties than the traditional ceramic glaze [9]. Addition of ZrO 2 in different ratios to SiO 2 -Al 2 O 3 -CaO-MgO-K 2 O-Na 2 O system produce a good glaze but with rough surfaces in case of higher ZrO 2 ratios [10].
Trachyte is a volcanic igneous rock of light color; containing mainly alkali feldspar with little mafic and fine texture. Trachyte is corresponding to the syenite plutonic rocks [13]. In Egypt, trachyte spread as several extrusive plugs and sheets in south eastern desert of Egypt (Hamrat Salma) [14]. The present trachyte is very hard, non-porphyritic and its color is reddish to buff with trachytoidal structure, formed of feldspar laths, aegirine and iron oxide granules. Chemically it contains 60 to 65% SiO 2 and high ratio of Na 2 O and K 2 O (> 10%, Table 1) [14,15].
Trachyte rock was used in many material science fields like, ceramic, composite, glass and zeolite. It can be used for the synthesis of economic zeolite (Alsubstituted 11 Ǻ), by activation of trachyte with 3.0 M NaOH will yield Al-substituted tobermorites [16]. Natural trachyte glass deposits were activated with deionized water or salty solution (CaCl 2 , NaCl and KCl) and the zeolitic minerals were developed between 100 and 200 °C [17]. Trachyte was used alone, after the removal of iron, or with albite and pegmatite in a commercial floor tile body [17]. Trachyte was used to alternate the flux K-feldspar in the ceramic industries [18,19].
The present research deals with the preparation and crystallization of glass from local trachyte rock with either limestone or magnesite. The thermal behavior, crystalline phases and microstructures were studied. Also some properties such as density, hardness, chemical durability and coefficient of thermal expansion were also investigated.

Experimental
In the present work, glass was prepared from Egyptian trachyte rock with local limestone or magnesite. The later raw materials were quantitatively analyzed by wavelength dispersive x-ray fluorescence (AXIOS, WD-XRF Sequential Spectrometer -Panalytical, 2005) in XRF device, National Research Centre in Egypt). The chemical analysis of the starting materials was declared in Table 1. The glass batches composed of successive additions of limestone or magnesite (10, 20 and 30%) to the trachyte rock powder. Table 2 displayed the compositions of the glass batches. Every glass batch was well mixed and homogenized in an agate ball mill for 2 hours. In a globar (SiC-heating element) furnace, glass batches were melted in sintered alumina crucibles. The melting temperature was between 1400 and 1450 °C. The glass melt was poured into normal water. After water quenching of the glass melt, each glass batch was dried and pulverized in an agate ball mill to reach grain size less than 0.083 mm.
However, the prepared glass powder was shaped into discs under uniaxial pressure (20 KN) using hexagon molds and a binder of PVA (Poly vinyl alcohol, 7%). Discs were sintered at 1000, 1050 and 1100 °C for 2 hours with rate of heating 10 °C/min to give glass-ceramic glaze as a final product (Fig. 8).
The mineralogical constituents of the as received represented trachyte powder and the developed crystalline phases, also, the powder of the sintered glass samples were identified using X-ray diffraction analysis (XRD, BRUKER, D8 ADVANCED CuO target, Germany). The microstructure was examined on the freshly fractured surface glass-ceramics using scanning electron microscopy (SEM, Model FEJ quanta 250 Fei-Holland) equipped with energy dispersive x-ray microanalysis (EDX). Some properties for instance: density, hardness, chemical durability and coeff icient of t her mal expansion were measured. Density was measured through Archimedes' method using distilled water. Hardness was measured using Vickers Micro Hardness Tester (Shimadzu, Type -M, Japan) the registered value was the average of ten readings. The coefficient of thermal expansion (CTE) was measured using dilatometer (NETZSCH DIL 402 PC, Germany) up to 600 °C with rate of 5 °C/min. The chemical durability was done on the calculated surface area of the sintered samples (at 1050 °C /2 h) after soaking the sample in water and acidic medium (1 M) at 90 ± 2 for 1 h). The weight loss% of the sintered sample was measured after washing and drying. To acquire the accuracy, two samples were measured in chemical durability.

Characterization of Trachyte Rock and Glass
The bulk and powder of the trachyte rock were examined optically and by XRD (Figs. 1 and 2). Optically polarizing microscopy showed trachytic texture (Fig. 1a) besides, calcite, iron oxides and chlorite were variably recorded as alteration products (Fig. 1b). XRD of the as received represented trachyte powder revealed the existence of albite, sanidine and aegirine as presented in Fig. 2.
The thermal behavior of T0 and TM30 glass batches did not give any indication of crystallization up to 1000 °C ( Fig. 3). However, practically the glass crystallization is either bulk or surface, therefore, the present work depend on sintering of glass powder (0.083 mm) between 1000 and 1100 °C/2 h. The X-ray diffraction analysis of the sintered glass discs at 1000 °C/2 h and 1050 °C exhibited the crystal-  Glass-ceramic glaze based on wollastonite and diopside was prepared by many authors [20][21][22][23]. Through CaO-MgO-Al 2 O 3 -SiO 2 glass system both later phases were developed and their crystalline phase's constituents based on the ratios of CaO and MgO. However, in case of using raw materials such as trachyte, oxides like Fe 2 O 3 and alkali can incorporate in the structure of both phases. In present study alkali in the sintered sample (Na 2 O-K 2 O = ~ 7 -11%) cannot form crystalline phases because their energy of formation are higher than wollastonite and augite [24].
The SEM micrographs of the sintered glass-ceramic samples presented little scattered pores in the samples matrix (Figs. 6 and 7). The matrix was mainly glassy combined with scattered crystals in euhedral, subhedral and anhedral shapes. However, the parent T0 sample indicated irregular nano-particles spread into the sample surface. The glassy matrix in TL10, TL20 and TL30 glass-ceramic revealed mainly irregular crystals while marking of quadrant, hexagon and rod like crystals in submicron and nano-scale were performed in TM10, TM20 and TM30 glass-ceramic (Figs. 6  and 7).
The EDX microanalysis of T0, TL30 and TM30 glasses at 1050 °C were distinguished in Fig. 8 and Table 3. Sample bodies were presented in Fig. 8. The vitreous state was clear in the samples. The chemical analysis of T0 sample illustrated relatively the chemical constituents of trachyte (compare Tables 1 and 3) whereas; the chemical composition in TL30 and TM30 demonstrated the major wollastonite and augite respectively. One can ask about the difference in ratios and the incorporation of other elements in each phase. Because the developed microstructure was in micro and nano scales, the EDX beam usually added the analysis outside the grains. Indeed, the substitution in the augite and wollastonite were recognized in both crystalline phases.

Properties of Sintered Glass-Ceramic Samples
The density of sintered samples disclosed an increasing in values upon increasing the incorporation of either limestone  micrographs of T0,  TM10, TM20 and TM30 glasses  sintered at 1050 °C/2 h   T0  TM10   TM20  TM30 or magnesite. In case of trachyte-limestone or magnesite; the density values were between 2.36 and 2.57 g/cm 3 or 2.64 g/ cm 3 respectively ( Table 4). The pre-mentioned densities of augite and wollastonite hinge on the substituted elements, therefore the density of augite was between 2.93 -3.49 g/ cm 3 and 2.86 -3.09 g/cm 3 respectively [25]. It was obvious that the increase of sintering temperature means relative decrease of densities in trachyte-limestone and trachytemagnesite samples as signified in Fig. 9. However, increase of the residual glass after sintering resulted in decrease of densities. The Vicker'micro hardness values were between 485 and 475 kg/mm 2 or 563 kg/mm 2 for the trachyte-limestone or magnesite respectively. The pre-mentioned hardness of wollastonite and augite values according to Vicker'scale were ~ 507-586 kg/mm 2 and ~ 616-698 kg/mm 2 respectively, these values rely on their compositions [25,26].

Conclusions
Sintered glass-ceramic glaze was prepared from trachyte-limestone or magnesite. Augite, olivine, Ca-olivine, wollastonite and cristobalite were crystallized in the sintered glasses. The microcrystalline structure of the samples was comprised from sub micro-to nano-scale cr yst alline par ticles scattered in the glassy matr ix. The densities of the sintered samples were between 2.39 g/cm 3 and 2.64 g/cm 3 whereas the hardness was between 440 and 563 Kg/mm 2 . The CTE was between 61.69 and 85.15 × 10 −7 °C −1 (25-500 °C).

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Conflict of Interest
The authors declare that they have no conflict of interest regarding the publication of this paper.

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Research Involving Human Participants and/or Animals Not Applicable.
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