Properties of Portland Cement Pastes Incorporating Nanometer-SizedFranklinite Particles Obtained from Electric-Arc-Furnace Dust

Antonio Balderas,† Hugo Navarro,‡ Luisa Maria Flores-Velez,§ and Octavio Dominguez†

Instituto de Metalurgia, Instituto de Investigacio´n en Ciencia Optica, and Facultad de Quı´mica,Universidad Auto´noma de San Luis Potosı´, 78210 San Luis Potosı´/SLP, Mexico

The present work presents preliminary results concerningordinary portland cement (OPC) blended with electric-arc-furnace dust (EAFD) obtained from steel-smelting plants. Thepowder obtained after acid treatment of the EAFD consistedbasically of nanometer-sized particles of ZnFe2O4. Incorpora-tion of the EAFD in the OPC produced retardation of thesetting process. Nevertheless, after 7 d, the compressivestrength of the OPC/EAFD pastes was superior to undopedOPC, and, after 28 d, the extent of hydration of the OPC/EAFD pastes was equivalent to undoped OPC. A compressivestrength of 72 MPa was attained after 42 d for OPC dopedwith 10 wt% EAFD.

I. Introduction

THE densified cement system in which silica fume or micro-silica (submicrometer SiO2 particles) is packed between ce-

ment particles has been widely investigated.1–4 This material isnow commonly referred to as densified with small particles ofcement. The pozzolanic activity of microsilica is considered tohave a significant influence on the microstructure of the system,although particle packing is quite important as well.5 Microsilica isa byproduct of the silicon and ferrosilicon smelting industries, andintensive research effort initiated during the early 1970s hasbecome the basis of microsilica technology.3

At present, one of the major waste products of the steelmakingindustry is electric-arc-furnace dust (EAFD), which is generated insignificant volumes. EAFD is considered a hazardous solid waste,because it contains small amounts of lead, arsenic, and chromiumoxides that are evaporated at high temperatures above the steelbath and condensed in the off-gas systems. Because of theextremely fine particles in the EAFD powders, one of theirpossible applications is as reinforcing particles in cement.6

The present work presents preliminary results concerning ordi-nary portland cement (OPC) blended with EAFD. This reportshows the influence of ZnFe2O4 nanometer-sized ceramic particleson the hydration reaction with OPC and the mechanical propertiesof OPC blends, presenting a new potential composite materialcapable of developing high compressive strength.

II. Experimental Procedure

Commercial type-I OPC, EAFD from a domestic steelmakingindustry, and distilled water (electrical resistivity of 500��m)were used in this study. Before the materials were mixed, the

EAFD was sieved to remove coarse particles (200 mesh size),treated for 24 h in a H2SO4 solution (pH 5), and then washed,dried, and milled for blending. All OPC/EAFD mixes were madeusing a water/cement ratio of 0.5 and aged at 30°C. The specimensin the same series were tested for compressive strength at ages of1, 3, 7, 14, 28, and 42 d. The mix proportions of the pastes weremade with 2, 5, 8, and 10 wt% of EAFD. In the present work, nowater-decreasing agent or other chemical compound was added tothe OPC/EAFD blends.

Setting time of the pastes was determined by monitoring thechanges in electrical conductivity of specimens made of eachcomposition. The cell used to contain the cement paste compriseda cylinder having a diameter of 25 mm and a length of 50 mm.Brass electrodes were attached to the two opposite faces of the cell.The conductance values were measured using alternating current at120 Hz and converted to resistivity by the introduction of a cellgeometry constant.

Compressive strengths were measured on samples cast incylindrical plastic molds having a diameter of 25 mm and a lengthof 50 mm. Compressive testing was performed under displacementcontrol, using a universal testing machine with a capacity of 5000kg. A crosshead speed of 0.76 mm�min�1 was used. Ten cylindersfrom each composition were tested for each measurement.

X-ray diffractometry (Model DMAX, Rigaku Co., Tokyo,Japan) with CuK� radiation and a nickel monochromator was usedat a scanning speed of 0.01°�min�1 over the range 2� from 5° to90°. Microstructures were examined using transmission electronmicroscopy (TEM; JEOL, Tokyo, Japan) and scanning electronmicroscopy (SEM; Philips, Eindhoven, The Netherlands) withenergy-dispersive X-ray spectrometry (EDS; EDAX International,Mahwah, NJ). In all cases, gold was evaporated on the specimensto avoid electrostatic charge accumulation and, thus, improve thesignal/noise ratio of the EDS spectra. Fourier-transform infraredspectroscopy (FTIR) spectra were recorded (Model IFS, BruckerInstruments, Billercia, MA) using a spectrometer coupled with aphotoacoustic detector (MTEC-Photoacoustics, Ames, IA). TheFTIR measurements were recorded in the range 6000–400 cm�1

using 150 scans, and 8 cm�1 resolution was obtained.

III. Results and Discussion

(1) Starting MaterialsXRD and EDS were used to determine the oxide and mineral-

ogical compositions of the OPC and EAFD. Figure 1(b) showsthat, before acid treatment, the EAFD was basically franklinite(ZnFe2O4) with some amounts of zincite (ZnO) and hematite(Fe2O3). EDS analysis conducted on the as-received EAFD (Fig.1(c)) indicated the presence of small amounts of lead, copper,manganese, arsenic, and chromium. The main effect observed afteracid treatment was the decrease of ZnO present in the as-receivedEAFD, evidenced by a decrease in the intensities of the corre-sponding ZnO diffraction peaks (Fig. 1(e)). Moreover, EDSanalysis of EAFD conducted after acid treatment (Fig. 1(f))indicated the presence of lead, copper, manganese, and chromiumin the particles, but showed an important decrease of zinc, lead,and arsenic in the chemical composition of the powders. The size,

C. M. Jantzen—contributing editor

Manuscript No. 188116. Received November 29, 2000; approved July 20, 2001.†Instituto de Metalurgia.‡Instituto de Investigacio´n en Ciencia Optica.§Facultad de Quı´mica.

J. Am. Ceram. Soc., 84 [12] 2909–13 (2001)

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distribution and shape of the EAFD particles were evaluated usingTEM images (Fig. 1(a) and (d)). In both cases, the EAFD wasmainly composed of ultrafine, spherically shaped particles, gener-ally �180 nm in size. Moreover, pycnometric measurements

conducted on the as-received EAFD indicated a mean apparentdensity of 4.3 g�cm–3. XRD patterns were used to obtain approx-imate estimates of the amounts of unreacted clinker phases andcrystallized products. OPC main components were alite (Ca3SiO5,

Fig. 1. (a) Bright-field TEM micrograph illustrating particle size and morphology, (b) XRD pattern, and (c) EDS spectra showing chemical compositionof as-received EAFD. (d) bright-field TEM micrograph illustrating particle size and morphology, (e) XRD pattern, and (f) EDS spectra showing chemicalcomposition of EAFD after acid treatment.

2910 Journal of the American Ceramic Society—Balderas et al. Vol. 84, No. 12

C3S) and belite (Ca2SiO4, C2S) with small amounts of ferrite(Ca2FexAl2–xO5) and gypsum (CaSO4�2H2O).

(2) OPC/EAFD PastesIt is generally accepted that poorly crystallized calcium hydro-

silicate phases (CSH) of variable stoichiometries and morpholo-gies and Ca(OH)2 are the two most important chemicals formedduring the hydration of C3S and C2S.3,7 It is recognized that somemetals in the solid wastes can react with the hydrating cementphases and, in some instance, cause complete failure of thehydraulic reactions.8 Zinc in different forms (ZnO and ZnSO4) hasbeen understood for many years to delay hydration of cement.9

Although ZnO severely retards cement hydration during an initialperiod, it has been found to increase the strength after aging.10 Itis believed that, during the retardation period, a protective cover ofamorphous Zn(OH)2 is formed on the grain surfaces. After theretarding effect, high concentrations of Ca2� and OH� trans-form Zn(OH)2 to crystalline calcium zinc hydroxideCa(Zn(OH)3)2�2H2O.8

To understand the effects of EAFD on the setting time, theelectrical properties were used as an indirect method to monitor thesetting of the blends.11–13 Figure 2 shows the changes in theelectrical resistivity of OPC and OPC paste containing 10 wt% ofEAFD. ZnO reference samples were chosen to match the corre-sponding ZnO coming from the ZnFe2O4 compound incorporatedduring the addition of the EAFD. There were discernible differ-ences in the setting processes, with OPC giving the shortest settingtime (Fig. 2(a)), whereas EAFD-doped specimens showed varioussetting times. Concerning the OPC selected in this case, the settingprocess started after 4 h (t1), followed by a lack of activity withinthe paste, and subsequently increased resistivity at 8 h (t2), whichassociated this phenomenon to the final setting of the OPC. On theother hand, it was observed that the setting time of OPC pastesremained almost the same (t1 � 5 h and t2 � 8 h) when doped withsmall amounts of EAFD, which indicated that the addition of smallamounts of EAFD in the OPC pastes did not affect the OPChydration. However, once the EAFD content in the OPC pastereached 10 wt%, the setting times attained values of t1 � 32 h andt2 � 44 h (Fig. 2(b)). Nevertheless, despite the retardationobserved, the hydration process took place contrary to the behaviordetected with the OPC/ZnO reference specimens, where there wassevere failure of the hydration reactions for an equivalent amountof ZnO.

XRD patterns were used to obtain approximate estimates of theamounts of ZnFe2O4, unreacted C3S phase, and Ca(OH)2 orCa(Zn(OH)3)2�2H2O crystallized products. Ca(OH)2 productionand C3S consumption could be detected by XRD, and theyprovided a useful means for assessing the extent of hydration.14,15

In the present case, there were observable differences in the rate ofC3S consumption as the EAFD content was increased. Neverthe-less, it was observed (Fig. 3) that OPC and OPC/EAFD specimensafter 28 d presented equivalent amounts of C3S and Ca(OH)2.Because the addition of EAFD did not unduly affect Ca(OH)2

production, despite the retardation reactions observed, it wasimplied that CSH production should be similarly unaffected.Therefore, the present knowledge7,8 suggested that retardation ofhydration was associated with amorphous Zn(OH)2 formed onthe grain surfaces, leading to crystalline Ca(Zn(OH)3)2�2H2O. Inthe present case, XRD patterns obtained after 28 d showed onlythe presence of Ca(OH)2, ZnFe2O4, and unreacted C3S;

Fig. 2. Electrical resistivity as a function of time obtained from (a) OPC paste and (b) OPC paste containing 10 wt% of EAFD. Water/cement ratio in allcases was 0.5, and measurements were conducted at 30°C.

Fig. 3. Progress of hydration of Ca3S obtained from OPC paste and OPCpastes containing 10 wt% of EAFD.

December 2001 Properties of Portland Cement Pastes Incorporating Nanometer-Sized Franklinite Particles 2911

Ca(Zn(OH)3)2�2H2O was never detected in OPC/EAFD speci-mens. The lack of peaks associated with crystallizedCa(Zn(OH)3)2�2H2O could be a consequence of the intrinsicresolution limit of the XRD techniques. To surmount this possibleeffect, FTIR spectroscopy was used to identify the presence ofCa(Zn(OH)3)2�2H2O. Figure 4(a) shows the corresponding FTIRspectrograms obtained from OPC doped with 10 wt% of EAFDafter 1 and 28 d. FTIR spectra of hydrated OPC doped with EAFDpresents the Si–O asymmetric stretching band (designated by b)shifted to higher frequency (960 cm�1) because of the polymer-ization of the silicates as well as a strong absorption band(designated by f) observed for EAFD at 2900 cm�1. It has beenreported in the literature8 that a strong absorption band occurs inthe 3650 cm�1 zone for the Ca(Zn(OH)3)2�2H2O compound. Inthis case, there was no evidence of the strong absorption band at

3650 cm–1 associated with the Ca(Zn(OH)3)2�2H2O. Moreover, theabsorption band at 2900 cm�1, corresponding to ZnFe2O4, wasobserved, even after 28 d. The present results suggested that nodiscernable reaction took place between OPC and ZnFe2O4 andthat, if weak bands between 3600 and 3700 cm–1 were ob-served in the FTIR spectra, they should be associated toCa(Zn(OH)3)2�2H2O coming from the reaction between Ca2� andthe Zn2� leaching from the small amount of ZnO present in theEAFD. On the other hand, SEM images (Fig. 4(b)) obtained fromthe 90OPC�10EAFD specimen after 42 d indicated the presence ofnanometer-sized particles of EAFD, even at this stage of hydra-tion, which suggested that the hydration process was delayed bythe ZnO present in the EAFD, but probably there was no reactionbetween the Zn2� forming the ZnFe2O4 and the Ca2� and OH�

ions, or it should have occurred at an extremely low rate on the

Fig. 4. (a) FTIR spectra obtained with 90OPC�10EAFD pastes at two hydration times. (b) SEM photograph of the microstructure of 90OPC�10EAFD after42 d (arrows point to ZnFe2O4 particles).

Fig. 5. Compressive strength results for various mixtures of cement and EAFD. Water/cement ratio in all cases was 0.5, and measurements were conductedat 30°C.

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surface of the particles. Nevertheless, further studies have to beconducted to improve the acid treatment to produce a suitablecompound, with the use of pure ZnFe2O4, to properly identify thechemical processes involved in these composite pastes, especiallythe leaching rate of Zn2� from the ZnFe2O4 crystal lattice.

Concerning the effect of the ZnFe2O4 nanometer-sized particleson the mechanical properties of cement, the strength of thehardened doped cement pastes was determined. Figure 5 shows thecurves of compressive strength versus hydration time obtainedfrom OPC and OPC/EAFD pastes. It was observed that cementdoped with 2 and 5 wt% EAFD had, since the beginning of thehydration process, greater strength than the control cement. On theother hand, the addition of �8 wt% of EAFD to cement decreasedthe strength before the third day, but increased it drastically afterseven days. This behavior was a consequence of the delay in thehydration process observed when OPC was doped with EAFD(Fig. 2). Despite the retardation of strength, cement doped withEAFD presented an interesting time-dependent mechanical behav-ior. It has been established that ZnO severely retards cementhydration during an initial period, but it also has been learned fromother studies that it has increased the strength at longer ages (�28d).3,7,10 After retardation, the strength accelerated and finallyachieved a higher compressive strength than OPC pastes. Figure 5indicates that EAFD can improve the compressive strength from40 MPa for OPC to 72 MPa for the 90OPC�10EAFD paste.Considering that concrete containing 5%–15% silica fume attainedhigh compressive strength, up to 100 MPa,1–4 the present resultssuggested the potential capability of the OPC/EAFD pastes tobecome a new cement particle composite formulation.

IV. Conclusions

A relevant aspect of EAFD concerns the particle size of thispowder, usually formed by particles in the nanometer-sized do-main. XRD confirms that, after acid treatment, there continues toexist a small amount of ZnO, whereas ZnFe2O4 is the predominantcompound forming EAFD. The setting process of cement isdelayed as the EAFD content is increased; nevertheless, thehydration reactions seem to take place, as in OPC, after 3 d. Acompressive strength of 72 MPa is attained after 42 d for OPCdoped with 10 wt% EAFD. This value is quite similar to the limitstrength usually reported for equivalent OPC pastes doped with

silica fume. Further research has to be done, but the presentpreliminary results suggest that OPC/EAFD pastes have thepotential to become a new cement particle composite formulationif suitable chemical treatments for EAFD are developed.

References

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December 2001 Properties of Portland Cement Pastes Incorporating Nanometer-Sized Franklinite Particles 2913