Effect of Calcium Silicates on Immobilization of Fluorine in Aqueous Solution

ISIJ International, Vol. 41 (2001), No. 5, pp. 513–518

Hongye HE and Hideaki SUITO

Research Institute for Advanced Materials Processing, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577 Japan. (Received on December 18, 2000; accepted in ﬁnal form on February 20, 2001)

Immobilization of fluorine in aqueous solution with 2CaO · SiO2 (C2S) and 3CaO · SiO2 (C3S), and the effect of C2S and C3S on the fluorine immobilization with 3CaO · Al2O3 (C3A) and 12CaO · 7Al2O3 (C12A7) have been studied. The hydration products are identified by X-ray diffraction method and the fluorine-substituted com-

pounds are confirmed by electron microprobe analysis. It is found that C3S appears more effective to immobilization of fluorine than C2S and (OH ) site in calcium silicate hydrates is substituted with F ion. The behavior of fluorine immobilization by calcium aluminates changes with the addition of calcium silicates be-

cause calcium silicates affect the formation of Ca3Al2(OH)12xFx, which is the most effective calcium aluminate hydrate in view of the incorporation of F ion. The ﬂuorine immobilization with C12A7 can be promoted in the presence of appropriate amount of C2S or C3S. KEY WORDS: immobilization; ﬂuorine; calcium silicate; calcium aluminate; hydration.

2. Experimental 1. Introduction 2.1. Materials Calcium silicates, 3CaO·SiO2 and 2CaO·SiO2, exist in cementitious materials such as Portland cement, slag-based 2CaO·SiO2 (C2S) and 3CaO·SiO2 (C3S) were prepared cement and steelmaking slag. Calcium silicate hydrate by sintering the mixtures of analytical grade CaCO3 and formed by cementitious reaction of calcium silicates in the SiO2 in an appropriate proportion at 1 400 and 1 600°C, re- presence of water is a well known host for a variety of spectively, for 2 d. For the formation of 2CaO·SiO2, waste ions such as Co, Mo, and so on.1,2) Even though there 1 mass% P2O5 was added to prevent the dusting of C2S, are a great deal of coverage on the immobilization of which is caused by the transformation from b-C2S to g- harzadous elements by cement-base materials, the specific C2S. The methods of the preparation of 12CaO·7Al2O3 (C12A7) and 3CaO·Al2O3 (C3A) are described in detail else- attention has not been paid to the study on the fluorine re- 3) moval. where. All compounds were confirmed by X-ray diffrac- We have been working on a project to immobilize fluo- tion analysis (XRD). rine in hot metal dephosphorization slags by using sec- 2.2. Shaking Test ondary refining slags. The main constitutes of the latter are Shaking test was made at room temperature. The test calcium aluminates (12CaO·7Al2O3 and 3CaO·Al2O3) and procedure which is based on the Japanese standard test pro- calcium silicates (3CaO·SiO2 and 2CaO·SiO2), of which 3) cedure (codified as Environment Agency Notice 46) is al- 12CaO·7Al2O3 is the major phase. In our previous study ready explained in previous article.3) The particle size of 12CaO·7Al2O3 has been found less effective than 3CaO· calcium silicates and calcium aluminates is less than Al2O3 to the immobilization of fluorine in aqueous solution. 0.1 mm and the fluorine aqueous solution of 20 ppm was To stabilize fluorine-bearing wastes by using secondary re- made by diluting hydrofluoric acid (47%) with distilled fining slags, the knowledge on how calcium silicates affect water. The solid sample and fluorine aqueous solution were the hydration of calcium aluminate which is related to fluo- charged in 500 ml polyethylene bottle and were shaken hor- rine removel is of crucial importance. izontally at a speed of 200 times per min. For comparison, In this study, the immobilization of fluorine in aqueous special grade CaF2 (99.9% purity, grain size 2 mm) was solution with 3CaO·SiO2 and 2CaO·SiO2 is first studied, also used. The concentration of F in aqueous solution was and then the effects of calcium silicates on the F immobi- determined by the selective ion-electrode method (JIS-K- lization by calcium aluminates and their mechanisms are 0101), and those of Ca, Al and Si were determined by in- discussed based on the microscopic observation, X-ray dif- ductively coupled plasma (ICP) atomic emission spectrom- fraction analysis and electron microprobe analysis. etry.3) 2.3. X-ray Analysis and Microscopic Observation X-ray diffraction analysis, microscopic observation, and

Fig. 2. SEM images of hydrated C2S (a) and C3S (b) particle surface obtained at shaking time of 24 h.

will be discribed in next section. 3.1.2. Microscopic Observation The X-ray diffraction analysis failed to prove the forma-

Fig. 1. Immobilization of F by C2S and C3S with size less than tion of fluorine-containing calcium silicate hydrates be- 0.1 mm. cause the hydration products were not well crystallized, electron microprobe analysis with energy dispersive X-ray namely, almost amorphous. The Ca(OH)2 phase was identi- (EDX) and wave-length dispersive X-ray (WDX) spec- fied by XRD only in the case of C3S. troscopy are used for the identification of hydration prod- Calcium silicate hydrates are usually known as poorly ucts and fluorine-bearing compounds. crystallized phase or gel. In cement chemist’s notation, these phases are generally referred to as C–S–H, where C,

S and H represent CaO, SiO2 and H2O, respectively. The 3. Results and Discussion composition of these calcium silicate hydrate gels changes 3.1. Immobilization of Fluorine with Calcium Silicates during the reaction period and it also varies with water/solid ratio and temperature. 3.1.1. Time Dependence of Immobilization of Fluorine The SEM image of the surface of hydrated C S and C S The variations of the concentrations of F, Si and Ca in 2 3 particles (0.84–1.0 mm) obtained by shaking for 24 h are aqueous solution with shaking time for C S, C S and CaF 2 3 2 shown in Fig. 2. In the case of C S particles, the fluorine- are shown in Fig. 1. It can be seen that when finely ground 2 containing calcium silicate hydrates, determined by EDX C S particles (0.1 mm) are shaken in the fluorine aqueous 3 and WDX, have the CaO/SiO molar ratio of 1.5 to 2, with solution, high Ca and extremely low Si contents are ob- 2 the F uptake of 1.5 to 2 mass%. In the case of C S particles, served along with a rapid decrease of the F content in the 3 the fluorine-containing hydrates have the CaO/SiO molar initial period of 30 min. This immobilization mechanism 2 ratio of 1.5 to 2.5 with the F uptake of 2.6 to 4.3 mass%. A will be explained in Sec. 3.1.3. In the case of C S only a 2 small amount of CaF was also observed on the surface of slight decrease of the F content is observed, characterized 2 C S particles treated in F aqueous solution for 2 h. The C S by relatively high Si and low Ca contents, compared with 3 3 particles are almost completely covered by the C–S–H hy- the case of C S. The pH value in aqueous solution was 3 dration products, whereas the C S particles remain almost about 12 for C S and 13 for C S. 2 2 3 unattacked even after 24 h. This observation coincides with The results for CaF were obtained by shaking CaF 2 2 the degree of fluorine-immobilization by C S and C S powders (2 mm) in distilled water. The initial pH was ad- 3 2 shown in Fig. 1. justed to 12.0 by the addition of a small amount of 20 w/v%

NaOH solution. The dissolution of CaF2 sustains the F con- 3.1.3. Mechanism of Fluorine Immobilization tent in aqueous solution constant around 10 mass ppm, During the hydration of C3S, calcium hydroxide precipi- which is almost the same level as that obtained by C2S. The tates from aqueous solution and calcium silicate hydrate Ca content in aqueous solution is around 2.7 ppm. The sol- gels, C–S–H, are formed on the particle surface, of which 2 ubility product of CaF2, [Ca][F] , calculated from these F the C–S–H gel with the CaO/SiO2 molar ratio of 1.5 and and Ca contents is 1010.49. This value is in good agreement 2.5 has high capacity of F uptake. The structure of C–S–H with that in previous report (1010.41).4) is similar to that of tobermorite5,6) and jennite6) in many as-

It can be concluded, therefore, that C3S is more effective pects. than C2S to the immobilization of ﬂuorine in aqueous solu- The dissolution of C3S by reaction (1) continuously rais- tion. This is explained by the fact that the hydration of C3S es the concentration of Ca in aqueous solution, which leads proceeds much faster than C2S and thus a large amount of to the formation of CaF2 and calcium hydroxide in aqueous ﬂuorine-containing calcium silicate hydrates are formed, as solution by reactions (2) and (3), respectively.

2 3CaO·SiO2 3H2O 3Ca HSiO3 5OH .....(1) 2 Ca 2F CaF2 ...... (2) 2 Ca 2OH Ca(OH)2 ...... (3) Since the Ca content in aqueous solution is considerably high, it is expected that ﬂuorine in aqueous solution is immobilized mainly through the formation of CaF2 in the early stage, as shown in Fig. 1. In the meanwhile, with the rapid formation of the calcium silicate hydrates on the C3S particle surface by reaction (4), ﬂuorine in aqueous solution is gradually incoporated into C–S–H according to reaction (5), in which (OH) in C–S–H can be substituted with F.

2 → Ca HSiO3 OH C–S–H ...... (4) C–S–HF → C–S–H–FOH...... (5)

The notation of C–S–H–F is used to indicate the F-substituted calcium silicate hydrate gels, which appear to have lower solubility of F than that of CaF2 and control the F content in aqueous solution. Due to the continuous formation of C–S–H–F, the precipitated CaF2 begins to decom- pose by the reverse reaction of reaction (2) with a decrease Fig. 3. Effect of calcium silicates on immobilization of F by in the ﬂuorine content in aqueous solution. As a result of C3A; the C2S (C3S)/C3A mass ratio 2 and particle size is this, the immobilization of F is improved and the F content less than 0.1 mm. decreases to a signiﬁcant degree after 10 h, as shown in Fig. 1.

The immobilization reaction of C2S in F aqueous solution is considerably slower than that of C3S because of the much lower hydration rate of C2S. Therefore, calcium hydroxide produced could not be observed by XRD. The fluorine content in aqueous solution decreases slightly at the beginning and remains constant within 24 h, as shown in Fig. 1. From a similar trend with respect to fluorine concentration level for both C2S and CaF2, it is considered that the immobilization of fluorine occurs by the formation of a small amount of CaF2 on C2S surface through reaction (2) although CaF2 was not identified by XRD. This is because the Ca content on the solid/liquid reaction interface is high- er than that in the bulk aqueous solution. The substantial decrease in the F content did not occur in short period of reaction time, suggesting the slow formation of C–S–H–F.

3.2. Effect of C2S and C3S on F Removal by Calcium Aluminates 3.2.1. Time Dependence

The effects of C2S and C3S on the immobilization of F with C3A and C12A7 are shown in Figs. 3 and 4, respectively, where the contents of F, Ca and Al are plotted against Fig. 4. Effect of calcium silicates on immobilization of F by shaking time. The calcium silicate to calcium aluminate C12A7; the C2S (C3S)/C12A7 mass ratio 2 and particle size is less than 0.1 mm. mass ratio, R, is chosen as 2. In the case of C3A shown in Fig. 3, the addition of C2S does not affect the immobilization of fluorine to a significant degree, while the addition of see from Fig. 5 that in the case of C3A C2S, the fluorine C3S has a negative effect. In the case of C12A7 shown in content remains almost unchanged with increasing R ratio, Fig. 4, however, both C3S and C2S have a positive effect on indicating that the addition of C2S does not affect the im- the immobilization of fluorine. It is clear that the immobi- mobilization of fluorine. A similar trend is observed in the lization of fluorine is improved in the order of C12A7 case of C3A C3S in the range of R 1. The Al contents in C12A7 C3S C12A7 C2S. both cases decrease continuously with R ratio. Variations of the contents of F, Ca, and Al with the R As shown in Fig. 6, the fluorine content for C12A7 C2S ratio are shown in Figs. 5 and 6 for C3A and C12A7, respec- decreases gradually with an increase of R ratio, suggesting tively. The data are obtained at shaking time of 6 h. One can that the addition of C2S improves the immobilization of flu-

Table 1. Identiﬁcation of hydrates obtained at shaking time of 6 h for C3A C2S(C3S) mixture by XRD.

Table 2. Identiﬁcation of hydrates obtained at shaking time of 6 h for C12A7 C2S(C3S) mixture by XRD. Fig. 5. Effect of calcium silicate(C2S, C3S)/C3A weight ratio on immobilization of F.

and increases with R ratio. This is different from C3A C3S, where this compound is identiﬁed only at R0.5. As will be discussed later, no F uptake is observed in this compound. It can be concluded from the XRD analysis that the pres-

ence of C2S or C3S has a significant effect on the formation Fig. 6. Effect of calcium silicate(C S, C S)/C A weight ratio 2 3 12 7 of Ca3Al2(OH)12, which plays a key role in the immobiliza- on immobilization of F. tion of F. The SEM observation for the cross section and the sur- orine. On the other hand, the immobilization of fluorine face of hydrated C A particles in the presence of C S par- changes more drastically with the addition of C S and sig- 12 7 2 3 ticles are shown in Fig. 7. The samples were made by treat- nificant improvement is obtained in the range of R between ing a mixture of C A and C S particles (R1, particle 0.5 and 1.0. 12 7 2 size: 0.84–1.0 mm) in F aqueous solution (20 ppm) for 6 h 3.2.2. X-ray Analysis and Microscopic Observation without shaking. The photographs of a and b in Fig. 7 rep- The results obtained from XRD are summarized in resent two different parts of a hydrated particle. As shown Tables 1 and 2 for the C3A C2S(C3S) and C12A7 C2S(C3S) in Fig. 7(a), Ca3Al2(OH)12xFx(C), which contains fluorine systems shown in Figs. 5 and 6, respectively. It can be seen up to 3.2 mass% and SiO2 less than 5 mass%, is identified. from Table 1 that Ca3Al2(OH)12 into which fluorine is inco- A mixture of hexagonal crystals of C2A·8H2O and C3A· 3) porated is the major phase for C3A C2S and C3A C3S Ca(OH)2·18H2O is also observed, as shown in Fig. 7(b). (R 1), but Ca3Al2(OH)12 becomes the minor phase for The electron microprobe analysis with EDX reveals that C3A C3S only at R 2. The behavior of fluorine shown in calcium silicate hydrates formed on the C2S particles for Fig. 5 can be interpreted by the formation of a large amount C12A7 C2S contains Al up to 14 mass%. A similar result of Ca3Al2(OH)12. was also obtained on the surface of C3S particles for In the case of C12A7 C3S (C2S), the variation of the C12A7 C3S, where the Al uptake in calcium silicate hy- Ca3Al2(OH)12 phase with R ratio given in Table 2 is also re- drates is up to 17 mass%. No significant amount of fluorine sponsible to the behavior of fluorine shown in Fig. 6. The was observed in these Al-containing C–S–H gels, indicat-

C3A·Ca(OH)2·18H2O phase which is also the fluorine re- ing that most of fluorine was immobilized into calcium alu- 3) ceptor decreases with increasing R ratio. However, the ef- minate hydrate in the form of Ca3Al2(OH)12xFx. In the fect of this phase on the immobilization of fluorine is small- case of a mixture of C12A7 C3S, the Ca2Al2SiO7·8H2O er than that of Ca3Al2(OH)12 because the F uptake in the phase without F uptake was identified mainly on the surface 3) former is much lower than that in the latter. In the case of of C12A7 particles. C12A7 C3S, the Ca2Al2SiO7·8H2O phase appears at R 0.5

Fig. 7. SEM images of the cross section and surface of a hydrated C12A7 particle with addition of C2S (6 h, without shaking); a and b are two different parts of this hydrated particle: A: C2A·8H2O (no F), B: C3A·Ca(OH, F)2·18H2O (F 0–0.2%), and C: Ca3Al2(OH)12xFx (x 0–0.63, SiO2 0–5%).

3.3. Fluorine Immobilization Mechanism of Calcium Aluminate–Calcium Silicate Mixtures It follows from the aforementioned results that most of the ﬂuorine in aqueous solution is incorporated into the calcium aluminate hydrate, Ca3Al2(OH)12xFx, despite of the presence of calcium silicates, and the F uptake in C–S–H gels is not signiﬁcant. Furthermore, the hydration of calcium silicates yields the effect on the formation of

Ca3Al2(OH)12, and eventually affects the immobilization of ﬂuorine by calcium aluminates. The variations of Al and Ca contents in aqueous solution with the C3S/C3A (C12A7) ratio (R) given in Tables 1 and 2 are shown in Fig. 8, where the solubilty lines7) of

Ca3Al2(OH)12, Ca(OH)2, and Al(OH)3 are also included. The precipitation regions for Ca3Al2(OH)12 and C3A· Ca(OH)2·18H2O which are the major phases for C3A and Fig. 8. Variations of Al and Ca contents in solution with the C12A7, respectively, are drawn by the shaded areas based on C S/C A (C A ) ratio (R) given in Tables 1 and 2. the present results shown in Figs. 5 and 6. The dash-dotted 3 3 12 7 and broken lines represent the composition changes with different R ratios for the aqueous solutions of C3A C3S silicate hydrates. This has been confirmed by the results ob- and C12A7 C3S, respectively. It can be seen that the precip- tained by electron microprobe analysis, as described in Sec. 8) 3 itation area of Ca3Al2(OH)12 is located at low Al content re- 3.2.2. According to Richardson and Groves, the Al ion gion and spreads over wide range of Ca content along the is substituted for Si4 tetrahedral site in C–S–H gels. In the 7) solubility line of Ca3Al2(OH)12 which is relatively flat present study, the Al content in aqueous solution decreases against the variation of Ca concentration. This indicates with this substitution of Al3 for Si4 in C–S–H gels and that the formation of Ca3Al2(OH)12 is much more sensitive when it reaches the precipitation area of fluorine-bearing to the change of Al content compared with that of Ca con- Ca3Al2(OH)12, the immobilization of F is improved. It can tent. In the case of C12A7 C3S where the Al concentration be expected that there must be a range of Al concentration is high at low R ratio, the hexagonal hydrate, C3A· favorable to the formation of Ca3Al2(OH)12, and beyond Ca(OH)2·18H2O is formed as a dominant phase. With an this region, the formation of this compound becomes re- increase in R ratio, the Al content decreases and the forma- tarded. On the basis of the results shown in Figs. 5 and 6, it tion of the C3A·Ca(OH)2·18H2O phase is prevented when can be said that the Al concentration in the range of 10 to the boundary between Ca3Al2(OH)12 and C3A·Ca(OH)2· 100 mass ppm seems to be favorable to the formation of 18H2O phases is reached. Thereafter, the Ca3Al2(OH)12 Ca3Al2(OH)12. phase starts to form as a major phase. However, further in- It is noted from the results in Tables 1 and 2 that in the crease in R ratio finally leads to the extreme depletion of presence of C3S at high R ratio, the Al content in aqueous 3 Al ions in aqueous solution. As a result, the formation of solution is too low to form hexagnal hydrates, C2A·8H2O Ca3Al2(OH)12 is retarded. In the case of C3A C3S a similar and C3A·Ca(OH)2·18H2O. Therefore, Ca3Al2(OH)12 is behavior is observed. The decrease of Al concentration in formed directly on the surface of calcium aluminate parti- aqueous solution is related to the uptake of Al into calcium cles other than through the conversion from the intermedi-

ate hexagonal hydrates, as observed in the hydration of C2S. 3) C3A. (2) For the fluorine removal by C3A or C12A7 in the 2 In the range of low Ca content, the solubility of presence of C2S or C3S, the cubic calcium aluminate hy- 2 Al(OH)3 increases with increasing the Ca content. This drate, Ca3Al2(OH)12, is the major F-containing phase and can be interpreted as follows: in the CaO–Al2O3–H2O sys- the F uptake in calcium silicate hydrates does not occur sig- 2 tem, only Ca , AlO2 and OH ions exist in the aqueous nificantly. solution. Since the solution containing Ca2 is prepared by (3) The incorporation of Al into calcium silicate hy- dissolving Ca(OH)2 with H2O, the OH ion is introduced drates reduces the Al content in aqueous solution, which in- through the following reaction: fluences the formation of Ca3Al2(OH)12. 2 (4) In the presence of an appropriate amount of C2S or Ca(OH)2 Ca 2OH ...... (6) C3S, the role of C12A7 in the F removal can be improved by 2 Therefore, with an increase in the Ca content, the concen- promoting the formation of Ca3Al2(OH)12. tration of OH increases, which results in the acceleration (5) The Ca3Al2(OH)12 phase can be formed directly on of the dissolution of Al(OH)3 by reaction (7): the surface of C3A or C12A7 particles, with the addition of an appropriate amount of C2S or C3S. Al(OH)3 OH AlO2 2H2O ...... (7) As a result, the AlO2 content increases with an increase of REFERENCES 2 the Ca concentration. 1) D. Bonen and S. L. Sarkar: Advances Cem. & Concr., Proc. of an Engineering Foundation Conf., ed. by Michael W. Grutzeck and Shondeep L. Sarkar, American Society of Civil Engineers, New 4. Conclusions York, (1994), 481. The immobilization of fluorine in aqueous solution with 2) A. Kindness, E. E. Lachowski, A. K. Minocha, and F. P. Glasser: Waste Management, 14 (1994), 97. C2S and C3S and the effects of calcium silicates on the im- 3) H. Hongye and H. Suito: ISIJ Int., 41 (2001), 506. mobilization of fluorine with calcium aluminates (C3A and 4) M. A. Elrashidi and W. L. Lindsay: Soil Sci. Soc. Amer. J., 49 C12A7) have been studied, and the following summary is (1985), 1133. given as conclusions: 5) J. G. M. de Jong, H. N. Stein and J. M. Stevels: J. Appl. Chem., 17 (1) The immobilization of fluorine in aqueous solution (1967), 246. 6) H. F. W. Taylor: Adv. Cem. Basic. Mat., 1 (1993), 38. with C2S or C3S proceeds through the formation of F-sub- 7) F. M. Lea: The Chemistry of Cement and Concrete, 3rd ed., Edward stituted calcium silicate hydrate gels, C–S–H. In this case, Arnold Ltd., Glassgow, (1973), 214. C3S is more effective to the immobilization of fluorine than 8) I. G. Richardson and G. W. Groves: Cem. Concr. Res., 23 (1993),