Construction and Building Materials 24 (2010) 811–817

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Construction and Building Materials

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Direct utilization of liquid slag from phosphorus-smelting furnace to preparecast stone as decorative building material

Jun Zhou a, Zhu Shu b, Xiaohua Hu b, Yanxin Wang b,*

a Faculty of Material Science and Chemical Engineering, China University of Geosciences, 430074 Wuhan, People’s Republic of Chinab MOE Key Laboratory of Biogeology and Environmental Geology and School of Environmental Studies, China University of Geosciences, 430074 Wuhan, People’s Republic of China

a r t i c l e i n f o

Article history:Received 16 July 2009Received in revised form 13 September 2009Accepted 15 October 2009Available online 7 November 2009

Keywords:Phosphorus slagCast stoneDecorative building materialWaste recycling

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.10.026

* Corresponding author. Tel.: +86 27 67883998; faxE-mail address: yx.wang@cug.edu.cn (Y. Wang).

a b s t r a c t

Discharge of huge amount of liquid slag from phosphorus-smelting electric furnace has posed seriousthreat to the environment. In this study, a novel approach of directly recycling the liquid slag to preparecast stone as decorative building material was proposed and experimentally tested. In our lab experi-ments, 76 wt.% water-quenched slag was re-melted at 1400 �C for 0.5 h (thus simulating liquid slag fromphosphorus-smelting electric furnace), and then mixed with 18 wt.% quartz powder and 6 wt.% calcinedkaolinitic clay, and subsequently melted together at 1450 �C for 1 h into modified liquid slag which wascast, heat-treated, annealed and transformed into cast stone. The optimal temperature for heat-treatingcast stone ranges from 850 to 900 �C. And as-prepared samples present excellent properties of bendingstrength, acid resistance and alkali resistance. ESEM images show that their microstructures are com-posed of spherical granules with particle sizes of about 0.2–0.4 lm, which are non-crystalline as indi-cated by the results of XRD analysis. EDS results show that the contents of major elements in thegranule are different from those in its background area. Results of TCLP test show that heavy metals fromraw slag have been solidified in the cast stone. The practical feasibility of the new technology was exam-ined further by on-site experiments, in which fresh liquid slag from phosphorus-smelting electric furnacewas directly mixed with quartz and kaolinitic clay to produce cast stone. The results were quite stableand consistent with those of lab experiments, proving that the proposed approach of direct utilizationof both energy and material of liquid phosphorus slag to produce cast stone as decorative building mate-rial is feasible, cost-effective and environment-friendly.

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1. Introduction

Elemental phosphorus is generally produced by the process ofphosphorus-smelting electric furnace in China. After the extractionof elemental phosphorus, liquid slag with the temperature of about1300–1500 �C is discharged from the furnace as well. It is esti-mated that about 8–10 tons of liquid slag is generated to produce1 ton of elemental phosphorus, and there were about 640–790 mil-lion tons of the slag generated in 2006 for the production of 79 mil-lion tons of elemental phosphorus in China [1]. The commonlyadopted way of disposing the liquid slag discharged from the fur-nace is to introduce it into water first and then to dump thewater-quenched solid grains in the field. Such an approach of dis-posal not only occupies the land, but also pollutes the environmentdue to leaching of hazardous substances such as phosphate, fluo-ride and heavy metals in the slag [2].

To solve the environmental problem related to water-quenchedphosphorus slag disposal, a lot of efforts have been made in recent

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years to utilize it to produce cements, bricks and fertilizers [2–5].However, all of the recycling technologies have only reused thematerial of slag itself. The enormous heat energy contained inthe hot liquid slag has not been used. Therefore, some researcherstried to reuse the material and heat energy at the same time, bypreparing glass–ceramics directly from the liquid slag [6]. Never-theless, the production of glass–ceramics requires various addi-tives such as nucleating agents and modifying materials. Owingto the great difficulty of online mixing of the liquid slag with theadditives, the high cost of the additives, and the relatively low dos-age proportion of the liquid slag (generally less than 55 wt.%),industrial manufacturing of glass–ceramics using liquid slag is stillnot feasible.

In China, the term cast stone generally refers to a type of inor-ganic engineering materials, which is prepared from hot moltenmass through four-step procedures: raw material melting, castmolding, heat treating and annealing. Cast stone usually possessesexcellent abrasive resistance and chemical resistance, and there-fore has been widely applied in different industry sectors, suchas metallurgy, chemical engineering and electrical engineering.The main raw materials for cast stone production comprise two

812 J. Zhou et al. / Construction and Building Materials 24 (2010) 811–817

types of materials: natural minerals and industrial wastes such asblast furnace slag, steel slag, copper slag and so on. In the past,these cast stone products were generally not used as decorativebuilding material because of their disagreeable dark color and highmelting cost.

However, with high contents of SiO2 and CaO and low contentsof coloring components such as Fe2O3 and TiO2, liquid phosphorusslag should be suitable for preparing light-colored cast stone asdecorative building material. Moreover, the enormous thermal en-ergy contained in the liquid phosphorus slag can be reused to savethe energy consumption at raw material melting stage of caststone production. Thus, a new approach of preparing decorativebuilding cast stone directly from liquid phosphorus slag was pro-posed by the authors and envisaged as an effective way of recyclingboth the material and the heat energy simultaneously.

Currently, the most widely used decorative building materialsare ceramic tiles and natural stone panels. Manufacturing ofceramic tiles not only consumes large amount of mineral andenergy resources, but also might pose serious threat to the atmo-spheric environment, if the way of resource utilization is not sus-tainable and environment-friendly [7–9]. Although there havebeen numerous efforts to produce ceramic tiles from solid wastes,the proposed processes inevitably consume large proportion of tra-ditional raw materials, which can not avoid entirely the problemsof mineral resource consumption and environment pollution [10–15]. Similarly, the exploitation of natural stone panels causes seri-ous damages to ecological environment [7,16]. Therefore, the pro-duction and application of cast stone as decorative buildingmaterial, prepared directly from the liquid phosphorus slag, shouldbe an alternative approach for the sustainable development of dec-orative building material industry, not only by partly replacing theconsumption of ceramic tiles and natural stone panels, but also bydecreasing the exploitation of non-renewable mineral resources aswell as reducing environmental pollution.

This paper reports the results of the experimental work on a no-vel process of preparing cast stone as decorative building materialdirectly from liquid phosphorus slag. In this process, the slag ismodified by adding only a small quantity of quartz powder and cal-cined kaolinitic clay, without any nucleating agents or fluxingagents; so the online mixing of the liquid slag and modifying mate-rials is easy to operate and the manufacturing cost quite low.

Fig. 1. Flowchart of preparing cast stone as decorative building material directlyfrom liquid slag.

Table 1Chemical composition of raw materials (wt.%).

Samples SiO2 Al2O3 CaO MgO

Phosphorus slag grain 41.12 5.05 46.53 1.75Quartz powder 98.50 0.15 0.31 0.06Calcined kaolinitic clay 52.89 44.97 0.02 0.07

2. Experimental

The flowchart of preparing cast stone directly from the liquid phosphorus slag isdesigned and shown in Fig. 1. The liquid slag is mixed with pre-heated modifyingmaterials, and then melted to get modified liquid slag which is cast into hot parentpanels for subsequent heat-treatment to obtain cast stone after annealing.

In our lab experiments, water-quenched phosphorus slag grains, provided by anelemental phosphorus production company in Guizhou province, China, were re-melted into liquid which was used as the liquid slag shown in Fig. 1. The chemicalcomposition and the XRD pattern of phosphorus slag grain sample are, respectively,shown in Table 1 and Fig. 2. It can be seen that the slag sample is composed ofamorphous glass and its major components are SiO2 and CaO.

Before the experiments, it is necessary to characterize the inherent crystalliza-tion behavior of the liquid slag. The water-quenched phosphorus slag grain samplewas re-melted in an electrical furnace at 1400 �C. The obtained liquid slag was thencast into the panel with the size of 60 � 20 � 10 mm3, and subsequently heat-trea-ted at 900 �C for 1 h and naturally annealed, and turned into the cast stone frompure liquid slag (abbreviated as PLS-CS). The major crystal phase of PLS-CS is CaSiO3

(PDF 02–0506) (Fig. 2b), and its macroscopic appearance in the cross-sectionresembles that of sandstones. The bending strength of PLS-CS is only 8.2 MPa anddoes not meet the requirements of decorative building materials (the Chinese stan-dard is more than 35 MPa). The low value of bending strength obtained can be ex-plained as follows: the chemistry of pure liquid slag falls into the range betweenCaO�SiO2 and 3CaO�2SiO2 in the CaO–Al2O3–SiO2 phase diagram (Fig. 3), and theproportion of CaO in the liquid slag is too high (44.40%), and consequently the par-ent panel from pure liquid slag has a strong tendency to form CaSiO3 crystals withgreat granular size. The interfaces among these great crystal grains cause a lowmechanical strength of PLS-CS.

Therefore, it is evident that the pure phosphorus liquid slag can not be directlycast and heat-treated to produce qualified cast stone. In order to obtain cast stonewith mechanical property meeting the requirements of decorative building mate-rial, modification of the raw liquid slag is needed. In our lab experiments, quartzpowder and calcined kaolinitic clay with chemical compositions shown in Table 1were added into the pure liquid slag to modify its contents of SiO2 and Al2O3 thatare glass-forming and network-forming oxides and control the tendency of crystaldeposition in molten mass. Such a modification decreases the proportion of CaO andincreases the proportions of SiO2 and Al2O3 (Fig. 3), and it’s easier to experimentallycontrol the crystallization behavior of the modified liquid slag.

The optimal dosage of raw materials, selected according to the results of ninescreening experiments, is listed in Table 2. The lab experiments were then done fol-lowing the flowchart shown in Fig. 1: 18 wt.% quartz powder and 6 wt.% calcinedkaolinitic clay were weighted, blended and then pre-heated at 1000 �C as the mod-ifying raw materials. At the same time, 76 wt.% phosphorus slag grains charged intohigh-aluminum crucible were re-melted at 1400 �C for 0.5 h and transformed backinto liquid slag. The pre-heated modifying materials were charged into the liquidslag which was then stirred and mixed thoroughly and melted for 1 h at 1450 �Cto obtain modified liquid slag. The as-obtained melt was then cast into60 � 60 � 10 mm3 and 60 � 20 � 10 mm3 moulds pre-heated at 800 �C to formhot parent panels which were subsequently heat-treated for 1 h at 750 �C, 800 �C,850 �C, 900 �C, 950 �C and 1000 �C, respectively. After annealing, the cast stonesamples from modified liquid slag (abbreviated as MLS-CS) were obtained.

Cross-section photographs of MLS-CS samples were taken by a scannerequipped with computer. The crystal phases of the samples were identified usingX-ray powder diffractometer (XRD; D/Max-3B, Rigaku) with Cu Ka radiation, at35 kV and 40 mA with 10 s scanning time. Environmental scanning electron micros-copy (ESEM; Quanta-200FEG, FEI) equipped with energy dispersive X-ray spectros-copy (EDS) was used to observe the microstructure and analyze the micro-regionalelemental composition of the samples that had been polished and etched for 60 s in5% HF solution beforehand.

Cast stone samples (60 � 20 � 10 mm3) were used for measuring the three-point bending strengths with 50 mm span at a crosshead speed of 1 mm/min. Somesamples with the same size were cut into three pieces of equal sizes which wereimmersed either in 1 wt.% H2SO4 or in 1 wt.% NaOH at room temperature for650 h, and then washed with distilled water, and subsequently dried at 110 �C.Their weights before and after the immersion were measured to get the mass lossrate and thus to estimate their chemical resistance. Another cast stone sampleswere boiled for 2 h in distilled water and then immersed continually for 4 h, andtheir weights before and after the treatment were measured to get the mass in-crease rate characterized as water adsorption. Total porosity of the cast stonewas measured by the Archimedes method using water as the medium. The same

K2O Na2O TFe2O3 TiO2 P2O5 Others

0.68 0.29 0.17 0.22 2.93 1.260.06 0.12 0.35 0.16 0.06 0.230.13 0.13 0.28 0.22 0.04 1.25

Fig. 2. XRD pattern of water-quenched phosphorus slag grain sample used in labexperiments (a) and that of cast stone prepared from pure liquid slag (PLS-CS) (b).

Fig. 3. CaO–Al2O3–SiO2 phase diagram. PLS-CS stands for cast stone from pureliquid slag and MLS-CS for cast stone prepared from modified liquid slag by addingquartz and kaolinitic clay.

Table 2Dosage of raw materials (wt.%).

Raw material Liquid slag Quartz powder Calcined kaolinitic clay

Proportion 76.00 18.00 6.00

Fig. 4. Cross-section photographs of MLS-CS samples heat-treated for 1 h at: (a)750, (b) 800, (c) 850, (d) 900, (e) 950 and (f) 1000 �C.

J. Zhou et al. / Construction and Building Materials 24 (2010) 811–817 813

fresh samples were heat-treated from room temperature to 100 �C at a speed of5 �C/min, and their lengths before and after the heat-treatment were measured tocalculate linear thermal expansion coefficient. Leaching behavior of heavy metalsin MLS-CSs was evaluated following the toxicity characteristic leaching procedure(TCLP) of the US Environmental Protection Agency (EPA Test Method 1311-TCLP).The MLS-CSs and phosphorus slag grain samples for the TCLP tests were milled intopowder with the particle size of less than 50 lm. The powder was immersed intoCH3COOH water solution (pH 4.93) at 22 ± 3 �C for 18 h. The heavy metal concentra-tion in the leachate was then tested by inductively coupled plasma-atomic emissionspectroscopy (ICP-AES; IRIS Intrepid II XSP). Five specimens for each sample weretested and the data were averaged.

To examine the practical feasibility of the new technology, on-site experimentswere carried out beside one phosphorus-smelting electric furnace of the companythat supplied phosphorus slag grains for the above lab experiments. The on-siteexperiments were done also following the flowchart shown in Fig. 1: 18 wt.% quartzpowder and 6 wt.% calcined kaolinitic clay were weighted, blended and then pre-heated. When the furnace was working and discharging liquid slag, 76 wt.% fresh

Table 3Chemical composition of MLS-CS (wt.%).

Chemical component SiO2 Al2O3 CaO MgO

Composition 53.48 6.65 34.53 0.81

liquid slag was directly bailed out and charged into a pre-heated high-aluminumcrucible together with the pre-heated modifying materials, then stirred and mixedand melted for 1 h at 1450 �C to obtain modified liquid slag. The as-obtained meltwas then cast into 60 � 60 � 10 mm3 pre-heated moulds to form hot parent panelswhich were heat-treated for 1 h at 900 �C. After annealing, on-site MLS-CS sampleswere obtained. The mechanical property and chemical resistance of the MLS-CSwere measured using the same methods as in lab experiments.

3. Results and discussion

3.1. Chemical composition

The chemical composition of the MLS-CS obtained in lab exper-iments is listed in Table 3. It can be seen that, the major compo-nents are SiO2, CaO and Al2O3, and fall into the CaO�SiO2 area inthe CaO–Al2O3–SiO2 phase diagram (Fig. 3).

3.2. Macroscopic appearance

From the photographs of the cross-sections of various MLS-CSsamples (Fig. 4), it can be seen that the MLS-CS heat-treated at750 �C remained as amorphous glass. When the heat-treatmenttemperature increased to 800 �C, the MLS-CS was slightly emul-sive. With the further increase of the heat-treatment temperature,i.e., 850 �C and 900 �C, the MLS-CSs became highly emulsive andpresented conchoidal texture. When the heat-treatment tempera-ture reached 950 �C, there appeared a layer of heterogeneous

K2O Na2O TFe2O3 TiO2 P2O5

0.59 1.00 0.19 0.21 2.11

Fig. 5. ESEM images of MLS-CSs heat-treated for 1 h at: (a) 750, (b) 800, (c) 850 and (d) 900 �C, and ESEM images in central layers of MLS-CSs heat-treated for 1 h at (e) 950and (f) 1000 �C and in surface layer of MLS-CS heat-treated for 1 h at (g) 1000 �C.

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materials in the surface of the MLS-CS, although the central part re-mained highly emulsive. Moreover, the thickness of the heteroge-neous layer increased further and that of the emulsive layerdecreased while the temperature increased to 1000 �C.

3.3. Microstructure and emulsification

The ESEM images of MLS-CSs are shown in Fig. 5. It can be seenthat the MLS-CS obtained at 750 �C was uniformly vitreous(Fig. 5a). When heat-treated at 800 �C, the sample presented agreat amount of granules with particle sizes generally between0.2 and 0.3 lm (Fig. 5b). With the increase of heat-treatment tem-perature, the particle size increased but the amount of granules de-creased (Fig. 5c–e). When the temperature reached above 950 �C,the MLS-CSs possessed a sandwich-like structure (Fig. 4e and f):the central layers remained as emulsified glass and consisted ofgranules with sizes up to about 0.6 lm (Fig. 5e and f), and the sur-face layers presented crystalline structure (Fig. 5g).

The XRD patterns of MLS-CSs are shown in Fig. 6. The MLS-CSsamples heat-treated at 750 �C, 800 �C, 850 �C and 900 �C areamorphous, since no obvious diffraction peaks occurred (Fig. 6a–d). In contrast, the surface layers of the MLS-CSs heat-treated at

950 �C and 1000 �C are comprised of b-wollastonite (PDF 10–0487) crystals (Fig. 6e and g), although the central layers remainamorphous (Fig. 6f and h). It is evident that the surface crystalliza-tion of MLS-CSs heat-treated at 950 �C and 1000 �C can be attrib-uted to the strong inherent surface crystallization tendency ofthe parent melt, the composition of which falling into the CaO�SiO2

area in the CaO–Al2O3–SiO2 phase diagram [17–19].On the other hand, there were spherical granular substances as

shown in the ESEM images (Fig. 5) which became bigger and moreregular with the increase of heat-treatment temperature from 800to 1000 �C in the whole bodies of MLS-CS samples heat-treated at800 �C, 850 �C and 900 �C (Fig. 5b–d) and in the central layers ofMLS-CS samples heat-treated at 950 �C and 1000 �C (Fig. 5e andf). However, the results of XRD analysis (Fig. 6) prove that no crys-tal peaks occur in both MLS-CSs heat-treated at 800 �C, 850 �C and900 �C (Fig. 6b–d) and the central layers of MLS-CSs heat-treated at950 �C and 1000 �C (Fig. 6f and h). Thus, it is obvious that thespherical granular substances in the ESEM images are still non-crystalline.

To further investigate the emulsification phenomenon, EDSanalysis was made on a spherical granule and its background areaof a MLS-CS sample heat-treated at 900 �C, and the results are

Fig. 6. XRD patterns of MLS-CSs heat-treated for 1 h at: (a) 750, (b) 800, (c) 850 and(d) 900 �C, respectively, and those of the samples from the surface layers of MLS-CSsheat-treated for 1 h at (e) 950 and (g) 1000 �C and from the central layers of MLS-CSs heat-treated for 1 h at (f) 950 and (h) 1000 �C.

J. Zhou et al. / Construction and Building Materials 24 (2010) 811–817 815

shown in Fig. 7. It can be seen that the chemical components of thegranule (point A) and its background area (point B) are different inthe contents of major elements, of which the content of elementcalcium (Ca) in the former is much higher than that in the latter.It can therefore be postulated that there exist mass transport andphase separation in the course of heat-treatment so as to formthe non-crystalline granules observed in the ESEM images.

Fig. 7. (a) ESEM image of MLS-CS sample heat-treated at 900 �C for 1 h. (b) EDSspectrum of a spherical granule marked as point A in (a). (c) EDS spectrum of thebackground area marked as point B in (a).

Fig. 8. Changes in bending strengths of MLS-CS samples heat-treated at varioustemperatures for 1 h.

3.4. Mechanical property

Since MLS-CS is expected to be used as decorative buildingmaterial, mechanical property is the most significant factor affect-ing its potential application. The results of bending strength mea-surements are shown in Fig. 8 and Table 4. Comprehensivelyconsidering cross-section photographs (Fig. 4), ESEM images(Fig. 5) and bending strength data (Fig. 8), it can be concluded thatMLS-CS heat-treated at low temperature (i.e., 800 �C) presentedslight emulsification and relatively low bending strength whichis only comparable with that of ordinary window glass. In contrast,MLS-CSs heat-treated at high temperatures (i.e., 950 �C and1000 �C) formed sandwich-like structure and possessed extraordi-narily low bending strength, due to the inner stress caused by thedifference in thermal expansion coefficients between the surfaceand the central layers. Only when heat-treated at 850 �C and900 �C, the MLS-CSs presented deep emulsification and isotropicstructure, and exhibited excellent bending strength. The reasonfor the change in bending strength can be further explained bythe changes in microstructures. As shown in Fig. 5, there wereplentiful non-crystalline granules in MLS-CSs heat-treated at850 �C and 900 �C. Because the granules help to prevent thespreading of the micro-cracks in the residual glass phase of thesamples under loading [20] and thus enhance their bendingstrengths. Therefore, the bending strengths of deeply emulsifiedMLS-CSs heat-treated at 850 �C and 900 �C with plentiful granulesare higher than those of both non-emulsified MLS-CS heat-treatedat 750 �C and slightly emulsified MLS-CS heat-treated at 800 �C,and also far higher than those of natural marble and granite, and

more than 35 MPa required in the Chinese standard, so as to meetthe strength requirements of decorative building materials(Table 4). In conclusion, the optimal heat-treatment temperature

816 J. Zhou et al. / Construction and Building Materials 24 (2010) 811–817

for preparing MLS-CS in terms of mechanical property require-ments ranges between 850 and 900 �C.

3.5. Chemical and physical properties

The chemical and physical properties of MLS-CSs heat-treated,respectively at 850 �C and 900 �C are listed in Table 4. It can beseen that the MLS-CS samples exhibit better chemical resistancesand physical properties than marble and granite, and thereforeare qualified as decorative building materials.

Table 4Properties of MLS-CS samples as compared with rocks used as decorative buildingmaterials.

Parameters MLS-CS at850 �C for 1 h

MLS-CS at900 �C for 1 h

Marblea Granitea

Bending strength (MPa) 43.30 45.60 10.00 15.00Acid resistance (wt.%) 0.33 0.35 10.30 0.91Alkali resistance (wt.%) 0.07 0.06 0.28 0.08Water absorption (wt.%) 0.05 0.07 0.61 0.26Total porosity (vol.%) 0.15 0.18 2.17 0.79Linear thermal expansion

coefficient (10–6/�C)1.27 1.15 5.90 2.41

a The specimens were the decorative marble and granite panels in a decorativebuilding material market.

Table 5Results of TCLP tests (lg/g).

Samples Heavy metals

Cu Zn Cr Cd Pb As

Phosphorus slag grain 11.50 19.23 1.54 0.24 24.17 2.61MLS-CS at 850 �C for 1 h 0.20 1.15 0.18 NDa 0.09 0.14MLS-CS at 900 �C for 1 h 0.21 1.09 0.11 ND 0.11 0.18Leaching concentration

limitsb100.00 100.00 5.00 1.00 5.00 5.00

a ND-not detected.b The maximum leaching concentration limits of heavy metals from environ-

mentally friendly materials (40CFR261.24).

Fig. 9. The photograph of MLS-CS sample prepared in on-site experiments.

3.6. Toxicity characteristics

The American EPA’s toxicity characteristic leaching procedure(TCLP) was used to evaluate the leachability of heavy metals inthe as-received phosphorus slag grain and the MLS-CSs (Table 5).The amount of heavy metals leached from as-received phosphorusslag grain was relatively high. When the phosphorus slag wastransformed to MLS-CS, the leachability of heavy metal ions wassubstantially reduced, and the heavy metal concentrations in theleachate were less than the maximum leaching concentration lim-its of environmentally friendly materials according to the Ameri-can Code of Federal Regulations (40CFR261.24). In other words,the heavy metals from phosphorus slag can well be solidified inMLS-CS and consequently there is no risk of environment pollu-tion. Therefore, the results of TCLP and chemical resistance testsindicate that the MLS-CS products are environmentally friendlymaterials.

3.7. Results of on-site experiments

The photograph of MLS-CS obtained in on-site experiments isshown in Fig. 9. The results of bending strength, acid resistanceand alkali resistance of the MLS-CS samples were, respectively,45.26 Mpa, 0.32% and 0.07%, which were consistent with those ofthe MLS-CS samples prepared in lab experiments. The results fur-ther prove the stability and practical feasibility of the proposed no-vel process to prepare cast stone as decorative building materialdirectly from liquid slag discharged from phosphorus-smeltingelectric furnace.

4. Conclusions

(1) Cast stone can be prepared using 76 wt.% liquid phosphorusslag modified with 18 wt.% quartz powder and 6 wt.% cal-cined kaolinitic clay. The modified mixture is meltedtogether at 1450 �C for 1 h into modified liquid phosphorusslag, and then cast, and subsequently heat-treated for 1 hat 850–900 �C, and transformed into cast stone product.

(2) The MLS-CS samples obtained at 850 �C and 900 �C exhibitedexcellent mechanical property and chemical resistance tomeet the requirements for decorative building materials.Moreover, the results of TCLP tests indicate that the amountof heavy metals leached from MLS-CS is low, and thereforethe MLS-CS product is a new type of the environment-friendly decorative building material.

(3) The results of on-site experiments further prove that theproposed novel approach of simultaneously recycling heatenergy and material of liquid slag from phosphorus-smelt-ing electric furnace to produce cast stone as decorativebuilding material is practically applicable and cost-effective.

Acknowledgements

The research work was financially supported by National Natu-ral Science Foundation of China (Grant No. 40830748) and Depart-ment of Science and Technology of Guiyang city, Guizhou province,China.

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