Elsevier

Ceramics International

Volume 39, Issue 5, July 2013, Pages 5427-5435
Ceramics International

Technological properties of glass-ceramic tiles obtained using rice husk ash as silica precursor

https://doi.org/10.1016/j.ceramint.2012.12.050Get rights and content

Abstract

This paper reports the results of a study focused on the obtainment of glass-ceramic by using rice husk ash (RHA) as silica precursor. RHA is a by-product generated in biomass plants using rice husk as fuel for kilns or in the rice mills to generate steam for the parboiling process. Worldwide, it is annually produced about 132 Mt of rice husk, which gives rise to a production of 33 Mt/year of RHA. Glass-ceramic tiles were produced by a sinter-crystallization process using a glassy frit formulated in the MgO–Al2O3-SiO2 composition system. The realized glass-ceramics were studied according to ISO rules for sintering and technological properties (water absorption, apparent density, bending strength, Young's modulus, deep abrasion, Mohs hardness). To complete the investigation crystalline phase formation and microstructural characterization of the glass-ceramic materials was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Finally, chemical durability tests on parent glass and derived glass-ceramics were performed. The results obtained showed that it is possible to use RHA to produce glass-ceramic tiles by a sinter-crystallization process, obtaining nepheline (Na2O*Al2O3*SiO2) as main crystalline phase and forsterite (2MgO*SiO2) at 900 °C. Regarding technological features, the sintered materials showed bending strength values and Mohs hardness higher with respect to commercial glass-ceramics like NeopariesR. Other properties as water absorption (0.5%) allowed to classify these materials into the Group BIa characteristic of high sintered ceramic tiles according to European Standard rule.

Introduction

Rice cultivation is the principal activity and source of income for millions of households around the globe, and several countries of Asia and Africa are highly dependent on rice as a source of foreign exchange earnings and government revenue. Rice is the second largest produced cereal in the world. At the beginning of the 1990s, annual production was around 350 million tons and in 2008 it has reached 661 million tons [1]. The first step of the production process, after the rice harvest, is the milling process that generates a by-product known as husk (a low-density residue of the process) that surrounds the paddy grain. During milling of paddy about 80% of weight is received as rice, broken rice and bran while the remaining 20% is received as husk. Consequently, in 2008 the world rice production has also generated 132 million tons of rice husks. Disposal of rice husk is, therefore, an important issue in those Countries that cultivate large quantities of rice (i.e. East and South-East Asia). Even if there are some uses for rice husk, it is still often considered a waste product and therefore often either burned outdoor or dumped on wasteland. Rice husk occupies large areas, has a very low nutritional value and, as it takes very long time to decompose, it is not appropriate for composting or manure. One effective method used today to reduce the rice husk accumulation is to use it as fuel for kilns to produce bricks and other clay products or in the rice mills to generate steam for the parboiling process. Rice husk contains about 75% organic volatile matter, thus 25% of the weight of this husk is converted into ash during the firing process. The latter, known as rice husk ash (RHA), is an ash product with an unburned carbon percentage (∼10%), which contains around 85%–90% of amorphous silica. This means that every 1000 kg of milled paddy, about 220 kg (22%) of husk is produced, and when this husk is burnt, about 55 kg (25%) of RHA is generated. Using this data, the total global rice husk ash production could be as high as 33 million tons per year [2], [3].

The presence of silica in RHA was known since 1938 [4] and an extensive search of the literature highlighted many uses of RHA. Two main industrial uses were identified: as an insulator in the steel industry and as a pozzolanic material in the cement industry [5], [6]. RHA is used by the steel industry in the production of high quality flat steel. In fact, RHA is an excellent insulator, having low thermal conductivity, high melting point, low bulk density and high porosity. It is this insulating property that makes it an excellent ‘tundish powder’ that prevents rapid cooling of the steel and ensures uniform solidification in the continuous casting process. In addition, substantial research was carried out on its use in the manufacture of concrete. In particular, there are two areas in which RHA is used: in the manufacture of low cost building blocks, and in the production of high quality cement. The addition of RHA to cement enhances the cement properties. In particular its addition to Portland cement not only improves the early strength of concrete, but also leads to the formation of a calcium silicate hydrate gel around the cement particles, which becomes highly dense and less porous. This may increase the strength of concrete against cracking. In general, concrete made with Portland cement containing RHA has a higher compressive strength.

Considering the ceramic sector, in these last years, different studies were performed in order to valorize this type of waste as silica precursor; in particular RHA was used for whiteware [7] and ceramic production [8], [9], synthesis of pigments [10], [11] and glaze production [12], [13]. In fact, in this field the use of this waste as silica precursor can be seen as a prevention of waste produced during the extraction of raw materials according to the environmental sustainability criteria.

Regarding glass-ceramics, Naskar et al. used this agricultural waste material to successfully synthesize lithium aluminum silicate (LAS) powders in the form of β-eucryptite (LA2S) and β-spodumene (LA4S) [14].

Among the different glass ceramic system, the nepheline-based glass-ceramics are characterized by high mechanical strength and impact resistance and thus they are very attractive for the potential use such as in microwave ovens [15] and dental applications [16], [17].

Nepheline glass-ceramics are usually prepared from high purity proportions of pure chemicals and by the addition of different nucleating agents, such as TiO2, Cr2O3, ZrO2 or LiF to promote crystallization [18], [19]. They can be also prepared from cheaper raw materials such as wastes because the glass-ceramic process was established as a suitable way to valorize mining and industrial wastes [20], [21], including fly ash from incineration [22], [23] and thermal power plants [24], [25], wastes from hydrometallurgical processing plants [26], residual glass fibers from polyester matrix composites [27], among others wastes.

Forsterite has a sintering temperature of about 1500 °C, which is too high and imposes limits to its application. Many efforts were made to improve the workability of forsterite, including the reduction of the firing temperature by alumina addition [28] or using low melting point glasses [29]. Another way to produce forsterite materials at lower temperatures could be through the glass-ceramic process. Indeed, several investigations report the devitrification of forsterite as secondary crystalline phase in glass-ceramics belonging to different composition systems. In particular, Demirci and Günay [30] studied the crystallization behavior of cordierite glass-ceramics containing B2O3 boron oxide. They concluded that crystallization started at 950 °C after 1 h of heat treatment thus obtaining indialite (Mg2Al4Si5O18) as primary phase, followed by forsterite (Mg2SiO4) as secondary one with contents of around 89.2% and 10.8% respectively.

In a recent work [31], the authors demonstrated the feasibility of producing nepheline-forsterite-based glasses with the use of rice husk ash (RHA) as the main raw material (∼46 wt%). The glass-ceramics were produced by a sintering process of a glassy frit formulated in the MgO–Al2O3–SiO2 based system with the addition of B2O3 and Na2O to facilitate the melting and pouring processes and Al2O3 and MgO to favor the fosterite crystallization. These glasses lead to glass-ceramics that combine the beneficial properties associated with both the forsterite and nepheline crystalline phases.

Starting from these researches, the aim of this work is to study the effect of sintering temperature on the densification process and the microstructure and mineralogical composition of the nepheline–fosterite glass-ceramic tiles obtained from RHA in order to determine the correlation with their physical, chemical and technological properties.

Section snippets

Materials and methods

The rice husk ash (RHA) used in this work comes from a plant producing parboiled rice (Garibaldi 1889, Colussi S.p.A., Milan, Italia). The as received RHA contained about 8% of unburned organic carbon. Prior to its use, the ash was treated at 500 °C and sieved to a particle size <250 μm. A glass (hereafter designed RHA glass) belonging to the SiO2–Al2O3–MgO base system were formulated with the addition of B2O3 and Na2O to facilitate the melting process and Al2O3 and MgO to favor the fosterite

Results and discussion

The chemical composition of both the rice husk ash (RHA) after the thermal treatment at 500 °C and the obtained RHA glass are reported in Table 1. The major component of the ash is very pure SiO2 (∼91 wt%) and among the minor components P2O5 shows the highest concentration (3.62 wt%). Other oxides as alkalines and alkaline earths are in percentages lower than 1.60 wt%. The chromophore oxides as Fe2O3 and TiO2, that can influence the color developed in a ceramic matrix, are present only as traces.

Conclusions

The results obtained in this study confirm the possibility to use rice husk ash as silica precursor for the development of nepheline–fosterite glass ceramics. The materials obtained by a thermal treatment at 900 °C for 40 min showed density values similar to the nepheline density reported in literature. The presence of high amount of nepheline causes medium chemical resistance of glass-ceramic, so a correction in the formulation of the materials can be useful. Rietveld R.I.R analysis shows that

Acknowledgments

Dr. M.I. Martín expresses her gratitude to the Spanish National Research Council (CSIC) for her contract through the JAE Program (JAEDoc-08-00032), co-financed by the European Social Fund.

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