New environmentally friendly substance formulation based on steel slag for the manufacture of stoneware

In this paper, stoneware was prepared using steel slag shale as the main raw material. A typical ceramic sintering procedure was used to evaluate the effects of a sintering temperature of 1050∼1130 °C on the linear shrinkage, water absorption, bulk density, and flexural strength parameters of stoneware samples. The stoneware samples were characterized by XRF, XRD, and SEM. The results showed that the performance of the steel slag content of 40% at sintering temperature of 1110 °C was better than the other samples. The main phases of the sample are composed of the quartz phase, the pyroxene phase, and the anorthite phase. Among them, the water absorption rate is 1.36%, the linear shrinkage rate is 11.36%, the bulk density is 2.46 g cm−3, and the flexural strength is 71.13 Mpa. Anorthite and pyroxene improve the bending strength of stoneware samples, which is realized by the prestress mechanism.


Introduction
Steel slag is a type of solid waste with a high discharge rate and a negative environmental impact, and its total volume is growing [1][2][3].The storage of steel slag can pose a great threat to the soil, water, atmosphere, and surrounding environment.It is an urgent problem to study the comprehensive utilization of steel slag and realize its effective large-scale industrial application so as to reduce the burden of emission enterprises, reduce environmental pressure and realize sustainable development.At present, steel slag is mainly used as a filler for subgrade engineering besides being used as a secondary resource to recover iron [4,5].These utilization methods have low added value and low utilization rates.Steel slag piles occupy a vast amount of land resources, pollute water and soil.Therefore, it has become an urgent problem to find an acceptable, cost-effective, ecofriendly and efficient utilization of steel slag [6][7][8][9].
From the perspective of chemical composition, steel slag is mainly composed of CaO, SiO 2 , Fe 2 O 3 and MgO.Therefore, steel slag can be used to develop ceramic materials.Previous researchers have done a great deal of work on the preparation of steel slag as a raw material in ceramics [10][11][12][13][14]. Zhang et al [15] prepared steel slag ceramics by mixing steel slag, clay, quartz, feldspar, and talc.The findings indicate that the MgO/Al 2 O 3 ratio has a significant impact on these properties.The transformation of the main crystalline phase from quartz and pyroxene to anorthite and quartz occurs once the MgO/Al 2 O 3 ratio is reduced.The formation of the pyroxene phase is assisted by a high MgO content.The formation of the anorthite phase is facilitated by a high Al 2 O 3 content.When the MgO/Al 2 O 3 ratio is 0.6, the sample has a very high bending strength of 62.20 MPa.Deng et al [16] effectively synthesized glass-ceramics in the CaO-MgO-Al 2 O 3 -SiO 2 systems.The results revealed that when the temperature and steel slag content increased, the viscosity of the glass solution reduced; with the increase of steel slag ratio, the number and coarsening degree of augite grains increased.Wu et al [17] utilized steel slag and kaolin as critical raw materials and the polyurethane sponge as the template to prepare the transparent and efficient porous ceramic structure.The bulk density of porous ceramics was 157.869 kg m −3 , the porosity was 94.05%, and the compressive strength was 0.2 MPa.Its crystal phases were mainly composed of anorthite, gehlenite, forsterite, and quartz.Tang et al [18] used 6% steel slag and 94% sand shale as raw materials for preparing foamed ceramics.The resulting sample had the total porosity of 67.22%, the bulk density of 0.821 g cm −3 , the thermal conductivity of 0.08 W/ (mK), and the compressive strength of 29.98 MPa.Xu et al [19] prepared foam ceramics using steel slag and found that the increase in slag content contributed to the formation of diopside.The size of SiC impacts intra-pore and surface pressures, resulting in a change in porosity.However, these still can not solve the problem of low utilization rate of steel slag, complex technology and high cost and price, there is an urgent need to find other ways to solve the problem of steel slag to promote large-scale, resource recycling of steel slag.In recent years, stoneware has been gradually favored by people in the construction industry because of its low water absorption, high bending strength and chemical resistance [20,21].All these provide the experimental basis for the preparation of ceramics from steel slag, but the preparation of stoneware from all-solid waste, mainly from steel slag, is rarely studied.
In this paper, high-strength stoneware was prepared from steel slag, shale, clay and feldspar.The influence of steel slag on stoneware performance was studied, and the phase composition and micro-morphology of stoneware were analyzed.The research results of this paper have important theoretical significance for the application of steel slag in stonewares.
The chemical composition of raw materials was determined by XRF, and the results were presented in table 1.According to table 1, it is known that these raw materials are mainly composed of Ca, Si and Al elements.The ternary phase diagram of CaO-SiO 2 -Al 2 O 3 is shown in figure 2. According to the Ca-Si-Al ternary phase  diagram, it is known that if the SiO 2 content is larger, the quartz phase is easily formed and the firing temperature is lower.However, during the cooling process, the quartz phase is prone to phase change, which causes the expansion of the material volume.Table 2 shows the SiO 2 crystal transformation and volume expansion rate.If the mixture, containing more CaO and Al 2 O 3 , the melting temperature is higher, which is not conducive to crystal precipitation.Therefore, adjust the ratio of raw material to make SiO 2 account for about 50%, CaO 20%, Al 2 O 3 30%, which is easy to form anorthite, which is conducive to the mechanical properties of the material and reduce energy consumption.According to table 3, accurately weigh the raw materials, control the ratio of raw materials to water at 10 : 1, and then put them in a cement mixer for mixing until they were evenly mixed.The evenly mixed materials were put into a 25 MPa tablet press to make green bodies, and the pressed green bodies were put into an oven and dried at 55 °C for 12 h and 75 °C for 12 h.The dried green body was placed in a muffle furnace and heated to 1050 °C, 1070 °C, 1090 °C, 1110 °C, and 1130 °Cat a heating rate of 7 °C min −1 for 30 min.After firing, the samples were removed and chilled to room temperature.The properties of sintered samples were tested.The manufacturing processes of stoneware were presented in figure 3. The chemical composition of was stonewares shown in table 3.

Characterization
The sintering linear shrinkage of the specimens was obtained by measuring the dimensional changes of the specimens before and after sintering with vernier calipers.Water absorption and bulk weight were determined using Archimedes' principle ( Chinese Standard GB/ T3810.3-2016.).The bending strength of the samples was determined using an electronic universal material testing machine (Chinese Standard GB/T 3810.).An electronic universal material testing machine (Chinese Standard GB/T 3810.4-2016) was used to determine the bending strength of the samples.The element composition, microstructure and phase composition of samples were examined using x-ray fluorescence (XRF), scanning electron microscopy (SEM), and x-ray diffraction   (XRD) testing methods.The mass percentage of the phase in the sample was obtained by using Xpert High Plus software.

Results and discussion
3.1.Macro picture of a stoneware samples Figure 4 shows photograph of stoneware samples fired at 1110 °C.From figure 4, it can be seen that the stoneware sample has a brownish gray color that turns into a grayish black color, which may be related to the iron trioxide contained in the sample [21,22].The firing surface of A1-A3 sample is smooth and complete without cracks.The surface of samples A4 and A5 displayed the bulging phenomena.This is because as the amount of slag inserted develops, the content of CaO, MgO, and other minerals rises, reducing the sintering temperature and causing product deformation.

Macroscopic properties of stoneware samples
The relationship between physical properties and firing temperature is shown in figure 5.It can be seen from figure 5(a), with the increase of firing temperature, the linear shrinkage of A1 ∼ A5 samples first increases and then drops; when fired at 1110 °C, the linear shrinkage of A1 ∼ A5 samples is the highest.Among them, the linear shrinkage of the A3 sample reaches the maximum value of 11.36%.Water absorption is one of the influencing factors in the densification process.The water absorption of sintered samples at various  temperatures is depicted in figure 5(b).With the increase of firing temperature, the water absorption of all samples shows a downward trend.This is because the increase of firing temperature promotes the increase of the content of liquid phase, which can fill the pores of the sample [23].The trend of volume change with temperature is similar to linear shrinkage, as shown in figure 5(c).When all samples are fired at 1110 °C, their bulk density reaches the maximum, which is in line with the linear shrinkage trend.When the combustion temperature is in the range of 1050 ∼ 1110 °C, a large number of liquid phases is formed in the sample with the increase of temperature.Under the action of capillary force, the liquid phase migrates and fills the gap between crystal particles, cuts off the initially interconnected pores, and begins to form closed circular pores, resulting in the decrease of water absorption [23,24].Therefore, the water absorption rate decreases significantly with the increase in firing temperature.In contrast, the linear shrinkage rate and bulk density are opposite.However, The bulk density of all samples decrease as the firing temperature exceeds 1110 °C.After 1110 °C, the sample shows partial soft melting deformation, resulting in volume expansion.The relationship between the bending strength  .The more steel slag there is, the more alkaline earth oxide and the lower the melted viscosity during sintering.The densification of material is accelerated and its macroscopic properties are improved when the melt viscosity is reduced.According to the above analysis, the properties of the samples such as linear shrinkage, bulk density, water absorption, and flexural strength show an inflection point at the firing temperature of 1110 °C.Therefore, we chose the samples fired at this temperature for analysis.

Phase analysis of samples
XRD analysis was used to investigate the effect of steel slag content on the phase composition of the samples, as shown in figure 6. Figure 6 shows the XRD spectra of stonewares with different amounts of steel slag.It can be seen from the figure that the crystalline phases of all samples are composed of anorthite (CaAl 2 Si 2 O 8 ), pyroxene (Ca(Mg, Al, Fe)(Si, Al) 2 O 6 ), and quartz (SiO 2 ) phase.This is caused by the reaction of CaO in the steel slag with silica and alumina in the shale clay to form anorthite and pyroxene.Fe 3+ and Mg 2+ in the steel slag material may dissolve in the silicate structure of anorthite and occupy structural vacancies, resulting in the transformation of the anorthite phase to the pyroxene phase.It is found that kaolinite in clay is dehydrated into metakaolinite, crystal lattice is destroyed into amorphous state and carbonate is decomposed under low temperature sintering.With the increase of temperature, the fluxing components such as alkali metal oxides and alkaline earth metal oxides Na 2 O, K 2 O and Fe 2 O 3 in the batch react with SiO 2 and Al 2 O 3 in the raw materials to form liquid phase.With the melting of liquid phase and solid phase, the amount of molten liquid phase increases continuously, at the same time, the green body becomes uniform, the dense pores decrease and the crystal phase content increases.In the end, the existence of a large number of crystalline phases is beneficial to the improvement of the mechanical strength of stoneware materials.Possible chemical changes at this stage are as follows: Using Xpert High Plus software were obtained as a percentage by mass of the physical phases in the samples and are shown in table 4. From the table, the glass phase increases, the content of the anorthite phase first increases and then decreases, and the pyroxene phase increases continuously, while the quartz phase gradually decreases.This is mainly analyzed in the following two aspects.
In general, the presence of low melting points of Na 2 O and K 2 O tends to reduce the melt viscosity and the increase in the glass phase.However, in combination with table 3, the content of Na 2 O and K 2 O in the A1-A5 component analysis gradually decreases, and the contents are low.Therefore, this is not the main cause of the glass phase.Figure 7 shows the phase diagram of FeO-SiO 2 -CaAl 2 S i2 O 8 .According to the FeO-SiO 2 -CaAl 2 S i2 O 8 phase diagram, as the iron oxide content increases, the melting temperature of the system decreases significantly, which tends to form the glass phase.Therefore, in this experiment, as the amount of steel slag increases, the amount of Fe 2 O 3 in the mix gradually increases and the melting temperature decreases, which may be a reason for the increase in the glass phase.
The existence of anorthite and pyroxene phases can improve the mechanical properties of the samples.Anorthite and pyroxene have improved the flexural strength of ceramic samples in earlier studie [26].The enhancement of flexural strength is obtained via a prestressing mechanism, according to Tai et al [26].When ceramics are at room temperature, the coefficient of thermal expansion (CTE) of the surrounding glass phase is greater than the material of the crystalline phase, leading to unproductive prestressing, according to [27].According to Bao et al [28], flexural prestress arises in the lower CTE phase in ceramic systems.In contrast, prestress tension develops in the higher CTE phase.The findings revealed CTE values of 3.0 × 10 −6 °C−1 , 15.0 × 10 −6 °C−1 , and 7.0 × 1010 −6 °C−1 for the glass phase, anorthite, and pyroxene, respectively.Because of the low CTE of the glass phase during cooling, the glass phase is surrounded by the anorthite and pyroxene crystalline phases.This kind of flexural prestress strengthens the glass phase, which in turn increases the bending strength of the stonewares [26].From table 3, it can be found that samples A3 ∼ A5 contain more glass phases, which will generate stronger compressive pre-stress, resulting in an increase in sample bending strength.However, the excessive formation of liquid phase in samples A4 and A5 will affect the sample bending strength.Therefore, sample A3 has the maximum bending strength.

Microstructure analysis of samples
As shown in figure 8, It can be understood that the stoneware samples have a good crystallization process.A coarsening of the crystals is observed as the steel slag percentage increases.There are some fine crystals in samples  A1 and A2, which are attached to the molten glass phase.The A3 crystal becomes larger, and the liquid phase formed inside the sample increases and becomes denser.At this time, the liquid phase closely surrounds the crystal, thus improving the mechanical properties of the stoneware sample.The bending strength of the sample is the highest and the water absorption is the lowest.The formation of glass phase plays a crucial role in the mechanical properties and densification of samples.Pores on the surfaces of samples A4 and A5 lead to a decrease in bending strength, which is related to the pores created by the decomposition of Fe 2 O 3 .
Figure 9 shows the SEM and EDS patterns of sample A3 at 1110 °C, and EDS analysis results of chosen sites, as illustrated in table 5.As shown in figure 9, the rod-shaped crystals may be pyroxene, the granular crystals are anorthite, and the irregular shape is quartz, which is the same as the results of EDS analysis.The amount of liquid phase in the green body gradually increases, and the surface tension of the liquid phase makes the solid particles in the green body, such as anorthite, pyroxene and quartz, close together, which makes the green body compact.At the same time, the amorphous liquid phase can also dissolve some particles and other crystal materials in the body.Quartz, anorthite and pyroxene form a dense structure with the help of the bonding effect of the amorphous glass liquid phase, improving the mechanical strength of stoneware materials.

Conclusion
Steel slag resource utilization is essential for sustainable preservation.Stoneware was effectively prepared using steel slag and shale as primary ingredients in this paper.The steel slag level of 40% at 1110 °C was discovered to have the best characteristics, including water absorption rate of 1.36%, the linear shrinkage rate of 11.36%, bulk density of 2.46 g cm −3 , and flexural strength of 71.13 MPa.Anorthite, pyroxene and quartz are the phases that makeup stoneware in this period.This study reveals the potential usage steel slag as a raw resources for stoneware production.

Figure 4 .
Figure 4. Macro pictures of stoneware samples fired at 1110 °C.

Figure 9 .
Figure 9. EDS patterns of the given spots.

Table 2 .
The SiO 2 crystal transformation and volume expansion rate.

Table 3 .
Proportions of raw materials and chemical composition of the prepared stoneware samples (wt%).

Table 5 .
The findings of the elemental composition analysis of the selected points in figure6(at%).