Reactivity of SiO2 fume from ferrosilicon production with Ca(OH)2 under hydrothermal conditions

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 134-140, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

REACTIVITY OF SiO 2 FUME FROM FERROSILICON PRODUCTIONWITH Ca(OH) 2 UNDER HYDROTHERMALCONDITIONS B. Kurbus, F. Bakula and R. Gabrov~ek Boris Kidri~ I n s t i t u t e of Chemistry, P.O. Box 30 61115 L j u b l j a n a , Yugoslavia ¥

(Communicated by Z. Sauman) (Received June I ; in f i n a l form Nov. 5, 1984) ABSTRACT R e a c t i v i t y has been studied of SiO 2 fume from f e r r o s i l i c o n production with Ca(OH)2 in suspension, with s t i r r i n g , under hydrothermal conditions at 190°C and 200°C from 1 to 12 hours. Reaction product a f t e r a 4-hour synthesis at 200°C is calcium s i l i c a t e hydrate x o n o t l i t e C6S6H*, with some C-S-H(1) phase. When glass f i b e r s were added to the s l u r r y , which was subsequently f i l t e r e d , pressed, and d r i e d , the product thus obtained was l i g h t w e i g h t thermal i n s u l a t i o n m a t e r i a l , temperaturer e s i s t a n t up to 800°C and usable up to IO00°C. The product has excell e n t i n s u l a t i o n properties with thermal c o n d u c t i v i t y 0.042 - 0.081W/mK at bulk d e n s i t i e s 150 - 500 kg/m3. Introduction The review of the relevant s c i e n t i f i c and technical l i t e r a t u r e has shown that the integrated i n d u s t r i a l - o r i e n t e d use of SiO 2 fume does not e x i s t as yet. The research of i t s range of a p p l i c a t i o n s follows mainly the f o l l o w i n g three directions: -

-

SiO 2 fume as pozzolanic a d d i t i v e to Portland cement and concrete, SiO 2 fume as an a d d i t i v e to mixtures for the production of sintered i n sulating materials,

-

SiO 2 fume as s i l i c e o u s source in mixtures with Ca(OH)2 for hydrothermal syntheses. The behavior of s i l i c a fume in mixtures with Portland cement at temperatures up to 80°C has been studied e x t e n s i v e l y ( I - 6 ) . However, data on the autoclaving of mixtures of SiO 2 fume and Ca(OH)2 are scarce and o r i g i n a t e mainly from patent l i t e r a t u r e (7-12). The basic synthesis of calcium s i l i c a t e hydrates, p a r t i c u l a r l y t h a t of x o n o t l i t e , described in the l i t e r a t u r e (7-11), is carried out as follows: suspension of SiO 2 dust, lime, asbestos and water (a few cases also l i s t other cons t i t u e n t s ) is heated at temperatures under lO0°C. Gelled s l u r r y is e i t h e r f u r ther autoclaved in suspension or molded before autoclaving. *Abbreviations: C=CaO, S=SiO2, H=H20 134

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135 SILICA FUME, HYDROTHERMAL, REACTIVITY, Ca(OH)2, CONDUCTIVITY

Japanese patent (12) reports on the synthesis of x o n o t l i t e from suspension of SiO 2 fume and lime with s t i r r i n g speeds ranging from 3000 to 8000 r.p.m. Autoclaving conditions are not quoted. Molded products have bulk density 150 kg/m3, thermal c o n d u c t i v i t y 0.053 W/mK, compressive strength 1.37 MPa, and bending strength 0.78 MPa. Our paper presents a study of the reactions as well as of the r e a c t i v i t y of SiO 2 fume with Ca(OH)2 in suspensions, with or w i t h o u t s t i r r i n g , under saturated steam conditions at 190 ° and 200°C. Main reaction product, whose rate of format i o n depends on the time of reaction and on the i n t e n s i t y of s t i r r i n g , was x o n o t l i t e with small amounts of C-S-H(1). The r e a c t i v i t y of SiO 2 fume with Ca(OH) 2 at 55 ° and 90°C was also studied. Lightweight thermal i n s u l a t i n g material was obtained by adding glass f i b e r s to hydrothermally treated s l u r r y with subsequent f i l t e r i n g , molding and drying. Experimental SiO 2 fume from Tovarna du~ika Ru~e (Nitrogen-producing f a c t o r y ) , Yugoslavia had the f o l l o w i n g chemical composition: SiO 2 92.6%, AI203 0.43%, CaO 0.53%, MgO 1.7%, Na20 0.50%, K20 0.78%, Fe203 0.80%, SO3 0.61%, C 0.51%, and P205 0.01%. S i l i c a fume had a s p e c i f i c surface area of 29.9 m2/g (BET, N2) and consisted of submicron p a r t i c l e s amorphous to X-rays. Calcium hydroxide used was of t e c h n i cal grade with 69.2% CaO. The r e a c t i v i t y of amorphous SiO 2 with Ca(OH)2 at 55° and 90°C was studied in paste (w/s = 0.50). Paste hydration was performed in closed polyethylene b o t t l e s under saturated steam conditions from 2.5 to 24 hours. Hydrothermal syntheses were conducted in a 2 L autoclave e i t h e r w i t h o u t or with i n t e n s i v e s t i r r i n g (600 r.p.m.) as well as in a 50 L autoclave equipped with magnetic s t i r r e r (up-and-down motion, 30 cycles per minute). C/S molar r a t i o of s t a r t i n g materi a l s was l.O,w/s r a t i o 10-15, reaction temperature 190 ° and 200°C, and reaction time from 1 to 12 hours. Saturated steam conditions were maintained throughout. In regular time i n t e r v a l s suspension samples were taken, f i l t e r e d , dried and f u r t h e r analyzed. A l l reaction products were analyzed for free CaO content; X-ray d i f f r a c t i o n phase analysis was used only for the examination of hydrothermally synthesized products. The product of 4-hour synthesis at 200°C was used f o r the preparation of a t e s t specimen of i n s u l a t i n g m a t e r i a l . Glass f i b e r s were added to the s l u r r y of hydrothermally synthesized product, suspension was f i l t e r e d , molded, pressed, and dried. The products with bulk d e n s i t i e s from 150 to 500 kg/m3 were obtained by combining f i l t e r i n g and pressing parameters. Thermal c o n d u c t i v i t y at 20°C and l i n e a r thermal expansion from 20° to 800°C were measured for a l l specimens. Volume changes were followed from 800°C to I050°C at the heating rate of lO°/min. For compressive strength measurements c y l i n d r i c a l specimens of equal height and diameter (23 mm) were used, while the diameter of the specimens for bending strength measurements equaled 150 mm with a thickness from 15 to 25 mm. Measurements were done on INSTRON 1195 at a speed of 1 mm/min. Thermal c o n d u c t i v i t y was measured on specimens with the same dimensions as those for bending strength measurements. Standard method of thermal c o n d u c t i v i t y measurement could not be used because of too small dimensions of specimens, therefore measurements were done according to the comparative method with an accuracy of + 0.005 W/mK.

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Vol. 15, No. 1 B. Kurbus, et a l .

I001\ 90 ~ 80~',

/.CaO added • 10 %CaO

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FIG. 1

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Kinetics of reaction between SiO 2 fume and Ca(OH)2 with regard to temperature and time of hydration

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Linear thermal expansion c o e f f i c i e n t and volume changes were measured with a modified v e r t i c a l dilatometer on c y l i n d r i c a l specimens with the height and diameter of I0 mm. Results and Discussion R e a c t i v i t y of SiO 2 fume with Ca(OH)2 at 55 ° and 90°C is shown in Figure I . The purpose of studying the reaction between SiO2 fume and Ca(OH)2 was to establ i s h the k i n e t i c s of reaction at the temperatures under IO0°C in case p r e t r e a t ment were required f o r a more successful synthesis of x o n o t l i t e in the autoclave. From the diagram shown in Figure I , i t can be seen that the r e a c t i v i t y of SiO 2 at the beginning of hydration s t r o n g l y depends on temperature: at 90°C a f t e r only 2.5 hours as much as 68-95% of added CaO (depending on the s t a r t i n g q u a n t i t i e s ) has reacted. However, at 55°C only 25-55% has reacted. A f t e r 24 hours of hydration the q u a n t i t y of unreacted CaO is p r a c t i c a l l y independent of both temperature and the s t a r t i n g q u a n t i t y of Ca(OH)2, and equals 3-8%. The k i n e t i c s of hydrothermal synthesis of x o n o t l i t e from amorphous SiO 2 and Ca(OH)2 was monitored at 190 ° and 200°C in suspensions, with or without stirring. Experiments have shown great dependency of reaction rate on s t i r r i n g i n t e n s i t y : at 200°C in suspension w i t h o u t s t i r r i n g , x o n o t l i t e appears in X-ray d i f f r a c t i o n patterns only a f t e r 12 hours (Fig. 2a). At slow s t i r r i n g speeds generated by means of magnetic s t i r r e r , x o n o t l i t e appears already a f t e r 4 hours (Fig. 2b), while at i n t e n s i v e s t i r r i n g (600 r.p.m.) i t appears already a f t e r 1 hour. The rate of formation of x o n o t l i t e at 190°C at slow s t i r r i n g speeds is twice slower than the rate of formation at 200°C. I t is i n t e r e s t i n g to note that in the products at 190°C no 1.13 nm tobermorite, as an intermediate product, could be detected. I t did appear, however, at the syntheses at 200°C without stirring. This f u r t h e r confirms a marked influence of s t i r r i n g on the reaction rate of hydrothermal reactions of SiO2 fume in the system CaO-SiO2-H20. For successful synthesis of x o n o t l i t e the reaction should not proceed through 1.13 nm tobermorite stage according to the evidence of tobermoritex o n o t l i t e conversion which is the rate-determining step in the formation of

Vol. 15, No. 1

137 SILICA FUME, HYDROTHERMAL, REACTIVITY, Ca(OH)2, CONDUCTIVITY

a

X~-S-H(1)

f

zo"

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FIG. 2 Kinetics of x o n o t l i t e formation from SiO2 fume and Ca(OH)2 at 200°C from 1 to 12 hours; (a) w i t h o u t s t i r r i n g , (b) with s t i r r i n g Xo = x o n o t l i t e , T = I , 13 nm tobermorite, C-S-H(1) phase x o n o t l i t e (13). With regard to the slowness of formation of tobermorite from amorphous s i l i c a , the fact t h a t has been noted by many researchers, our r e s u l t s are in accordance with t h a t . Great influence of s t i r r i n g can be explained with hypothesis of EI-Hemaly et a l . (14) which was r e c e n t l y supported by Mitsuda, et a l . (13): great r e a c t i v i t y and higher s o l u b i l i t y of amorphous s i l i c a favors the formation of C-S-H phase, which is s t r u c t u r a l l y less t o b e r m o r i t e - l i k e and does not e a s i l y convert i n t o i t . The free CaO content of a l l products is low on account of the great react i v i t y of Si02 fume: from 1.48% CaO in product synthesized in 1 hour without s t i r r i n g to 0.15% CaO in 12-hour product with s t i r r i n g . Because of low CaO cont e n t , the glass f i b e r s used were a l k a l i - n o n r e s i s t a n t . Unfortunately, glass f i b e r s a v a i l a b l e softened already at temperatures from 800°-840oc, which is why the data on strengths a f t e r heating at 800 ° and IO00°C are somewhat d i s t o r t e d . From X-ray d i f f r a c t i o n patterns we ascertained t h a t the main differences in phase compositions of products synthesized at 200°C were d i f f e r e n t content of

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Vol. 15, No. 1 B. Kurbus, et a l .

FIG. 3 Weight loss a f t e r heating versus autoclaving time.

2 6 6 8 10 12 - A u t o c l a v i n g t i m e [hours)

-

C-S-H(1) and various c ~ y s t a l l i n i t y of products. The same conclusion was reached from weight loss measurements at various temperatures, the r e s u l t s of which are shown in Figure 3. For technological reasons we decided to make a more detailed i d e n t i f i c a t i o n , and subsequently to mold a t e s t specimen from a 4-hour product which had the highest C-S-H(1) content and the highest weight loss of a l l x o n o t l i t e - c o n t a i n i n g products. X o n o t l i t e product synthesized in 4 hours at 200°C from amorphous SiO 2 a~pears as porous material with a c i c u l a r c r y s t a l s of about I0 ~m in length (Fig. ~)

With a d d i t i o n of 4% of glass f i b e r s to the slurry~molded products were prepared. Their bulk density varied from 150 to 500 kg/mJ, depending on pressing. Thermal c o n d u c t i v i t y , d i l a t a t i o n up to lO00°C, and mechanical properties with regard to bulk density were measured. Thermal c o n d u c t i v i t i e s of x o n o t l i t e molded products are low and increase with increased bulk density (Fig. 5). Products are stable up to 800°C according to dilatometer data: average thermal expansion c o e f f i c i e n t from 20 ° to 800°C is 1.7 - 2.4 x 10-6 °C- I .

FIG. 4 SEM photo of x o n o t l i t e product.

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15,

No.

139

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SILICA FUME, HYDROTHERMAL, REACTIVITY, Ca(OH)2, CONDUCTIVITY

4

FIG. 5 Shrinkage of the samples at heating from 800 ° to I050°C and thermal conductivity at 20°C versus bulk density.

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Bulk densi~y(kg/ms)

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FIG. 7 Bending strength versus bulk density.

Shrinkage begins at 800°C, i t s value from 800 ° to I050°C equals 1.8 - 3.6 vol.% (Fig. 5). Compressive strengths of molded products, depending on bulk density, range from 0.05 to 3.6 MPa (Fig. 6), corresponding bending strengths being 0.08 to 2.5 MPa (Fig. 7). Strengths were measured also on molded products heated at 600 ° , 800 ° , and IO00°C (Figs. 6 and 7). I t can be ascertained that mechanical properties of molded products heated over 800°C deteriorate markedly, which however, is p a r t l y due to thermal instab i l i t y of glass fibers used.

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Vol. 15, No. 1 B. Kurbus, et al. Conclusions

Results have shown great r e a c t i v i t y of SiO2 fume from f e r r o s i l i c o n production with Ca(OH)2 under hydrothermal conditions at 190° and 200°C. Main reaction products in the syntheses with s t i r r i n g were x o n o t l i t e and C-S-H(1). The greatest influence on reaction rate is that of s t i r r i n g . Molded products, prepared from material, obtained in 4 hours at 200°C with s t i r r i n g contained mainly xQnotlite together with some C-S-H(I~ At bulk dens i t i e s from 150 to 500 kg/mj t h e i r thermal conductivity was 0.v42 - 0.081W/mK, compressive strength from 0.05 to 3.6 MPa, and bending strength from 0.08 to 2.5 MPa. The shrinkage of the above products was 1.8 - 3.6 vol. %. Acknowledgment The authors thank Tovarna du~ika Ru~e for the sample and for the analysis of SiO2 fume. This work was funded by Resaerch Community of Slovenia and by Building Materials Industry SALONIT ANHOVO. The authors thank the l a t t e r for having permitted publication of the results. References I. 2. 3. 4. 5. 6. 7. 8. 9. I0. II. 12. 13. 14.

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