1、Preparation and properties of glassceramics derived from blast-furnace slag by a ceramic-sintering process Abstract Glassceramics were synthesized using ground blast-furnace slag and potash feldspar additives by a conventional ceramic-sintering route. The results show 5 wt% potash feldspar can enhan
2、ce the sintering properties of blast-furnace slag glass and the results glassceramics have desirable mechanical properties. The main crystalline phase of the obtained glassceramic is gehlenite (2CaO_Al2O3_SiO2). A high microhardness of 5.2 GPa and a bending strength higher than 85 MPa as well as a w
3、ater absorption lower than 0.14% were obtained. 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction Glassceramics are fine-grained polycrystalline materials formed when glasses of suitable compositions are heat-treated and thus undergo controlled crystallization to reach a
4、 lower energy crystalline state 1. Since the early 1960s, using waste to prepare glassceramics has been developed in Russia, by employing slag of ferrous and non-ferrous metallurgy, ashes and wastes from mining and chemical industries 2. Lately, the waste of coal combustion ash, fly ash and filter d
5、usts from waste incinerators, mud from metal hydrometallurgy, pass cement dust, different types of sludge and glass cullet or mixtures of them have been considered for the production of glass ceramics 37. Using waste to prepare glassceramics is significant for industrial applications as well as for
6、environment protection 8 The conventional approaches to sinter glassceramics usually include two steps: first vitrifying raw materials at a high temperature (13001500 8C) and then following a nucleation and crystal growth step. The disadvantage of the conventional route is that it is difficult to vi
7、trify the raw materials and the high energy consumption in this step. An alternative manufacturing method to produce sintered glass. ceramics, in which sintering and crystallization of fine glass powders take place simultaneously, has recently been reported9. In such route fine glass powders are pre
8、ssed and sintered, and the crystallization occurs with densification. However, this route also needs a short time to vitrify raw materials at high temperature for preliminary glass-making. The blast-furnace slag is formed in the processes of pig iron manufacture from iron ore, contains combustion re
9、sidue of coke, fluxes of limestone or serpentine, and other materials. If the molten slag was cooled quickly by high-pressure water, fine grain glass of vitreous CaAlMg silicate can be formed 10. This suggests the blast-furnace slag can be used as a glass source to make sintered glassceramics and vi
10、trifying raw materials at high-temperature step can be omitted. The free glass surfaces are preferable sites for devitrification and thus crystallization may occur without any nucleating agent. Therefore, the finely ground slag powder can used as the main component of parent glass. Comparing with th
11、e two sintered methods mentioned above, a remarkable advantage of the present study is absent of vitrification step because of the using of blast-furnace slag. Thus a low energy cost and manufacture simplicity can be expected. However, in our previous studies glassceramics prepared with pure blastfu
12、rnace slag show poor properties 11. Therefore, some sintering additives are needed. In this study, we show when using blast-furnace slag to prepare glassceramics by a conventional ceramics route, if suitable amount of potashfeldspar is added. Glassceramics with high microhardness andbending strength
13、 as well as lower water absorption can be obtained. 2. Experimental procedure The slag (provided by Anyang iron Corporation of China) was pulverized by ball milling for about 24 h (size in the range of 1020 mm), and then blended with 510 wt% potash feldspar powder. The mixtures were ball milling for
14、 2 h. We use K5, K8 and K10 to denote the weight percent of potash feldspar in the samples. The process used in our study is illustrated in Fig. 1. The blended powders were uniaxially pressed in a steel die at room temperature, using a hydraulic pressure of 4060 MP a without any binder. The obtained
15、 green bodies were sintered in air at nucleation temperature of 720760 and crystallization temperature of 800900 for different times (from 20 to 60 min), with heating rates of 25 /min, followed by a hightemperature treatment at 1200 The blended powders were examined by differential scanning calorime
16、try (DSC) (Labsys, Setaram, France) in air with a heating rate of 10 /min from room temperature to 1100 . The phase of the blast-furnace slag and obtained glassceramics were examined by X-ray diffraction (XRD) (Model D/MAX-3B, RIGAKU, Japan). The samples surfaces were polished and corroded in HF (5
17、vol%) for 20 s and then observed by scanning electron microscopy (SEM) (Model JSM-5610LV, JEOL, Japan). The density of the samples was measured by the Archimedes method. The microhardness was measured by a microhardness-tester (Model HX- 1000TM, Taiming, China) with a measuring force of 9.807 N and
18、a load time 20 s. Samples for bending strength tests with dimensions of 3 mm _ 4 mm_ 30 mm were carefully polished and tested by a universal testing machine (Zwick/Roell Z030, Germany). The chemical resistance of glassceramics was tested by a chemical etch method. The samples were corroded in the HC
19、l (0.5 vol%) and NaOH (0.5 vol%) solution for 95 h and then the residual rate was calculated. 3. Results and discussion Table 1 shows the chemical composition of blast-furnace slag obtained by X-ray fluorescence. The nominal composition of the blended mixture is given in Table.2. Fig.2 shows the XRD
20、 patterns of the blast-furnace slag. It can be seen that the chemical composition of the slag involves SiO2, CaO, Al2O3, and MgO as its major components. Fe2O3 and TiO2 that act as nucleating agents can greatly enhance the crystallization. Due the presence of Fe2O3 and TiO2, other nucleating agents
21、are not needed. A small amount of gehlenite exists in the amorphous glass( Fig.2). The primary-precipitated phase can act as heterogeneous nucleating centers. In fact, crystallization is favored at the surface of glass particles and primary-precipitated phase, since nuclei may be formed without the
22、impedance of the surrounding materials, taking into account the volume variation from glass to crystal. Fig.3shows the DSC curves of the blended mixture. The small endothermic peak (737741 ) indicates the molecular rearrangement in this temperature. The exothermic peak (800900 ) corresponds to the c
23、rystallization reaction of the glass. With 5 wt% potash feldspar additive, the glass powders have an intense exothermic effect at 840 , indicating the formation and growth crystals. With more potash feldspar additive, the exothermic peak increases slightly. Therefore, a nucleation temperature in the
24、 range of 720760 and a crystallization temperature in the range of 800900 were employed, respectively. Fig.4shows the SEM images of the samples. The results show that crystallization is mainly induced by surface nucleation in samples K8 and K10, but by both surface and bulk nucleation in sample K5 s
25、ince crystallization take place throughout the entire volume in the sample. Uniform, ultrafine crystalline grains with sizes of 25 mm exist in sample K5. In contrast, there are only small amounts of crystalline phases in K8 and K10 as shown by XRD patterns (Fig.5). Alkali feldspar crystals are known
26、 to give excellent glasses but they are unable to be crystallized in practical periods of time. They have been successfully developed by exploiting the tendency of powdered glass to devitrify during appropriate heat treatments in recent works . Our results show glass powder with 5 wt% potash feldspa
27、r can be successfully devitrified during the heat treatment because potash feldspar can act as fluxing agents during sintering process thus decrease the sintering temperature and reduce the viscosity of glass. The value of viscosity during sintering and crystallization is very important. If the valu
28、e of viscosity is too low, crystallization will be too fast, which can hinder sintering and creates a large amount of porosities. On the other hand, if the value of viscosity is too high, crystallization is difficult. Therefore, controlling the glass viscosity is very important. With more potash feldspar additive, the crystallization rate of the glass will decrease. According to our experimental data add 5 wt% potash feldspar will be an appropriate amount.
