Reaction product of lime and silica from rice husk ash

Reaction product of lime and silica from rice husk ash

CEMENT and CONCRETE RESEARCH. Vol. 16, pp. 67-73, 1986. Printed in the USA 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Press, Ltd. REACTION P ...

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CEMENT and CONCRETE RESEARCH. Vol. 16, pp. 67-73, 1986. Printed in the USA 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Press, Ltd.

REACTION P R O D U C T OF LIME A N D SILICA FROM RICE HUSK ASH

3ose 3ames and M.Subba Rao D e p a r t m e n t of Inorganic and Physical C h e m i s t r y Indian I n s t i t u t e of S c i e n c e Bangalore - 560 012 India

(Communicated by D.M. Roy) (Received Sept. 17, 1985) ABSTRACT

Rice husk ash (about 95% silica) with known physical and c h e m i c a l c h a r a c t e r i s t i c s has b e e n r e a c t e d with lime and w a t e r . The s e t t i n g process for a l i m e - e x c e s s and a l i m e - d e f i c i e n t m i x t u r e has been investigated. The p r o d u c t of the r e a c t i o n has been shown to be a c a l c i u m s i l i c a t e h y d r a t e , C-S-H(I) + by a c o m b i n a t i o n of t h e r m a l analysis, XRD and e l e c t r o n m i c r o s c o p y . F o r m a t i o n of C-S-H(I) a c c o u n t s for the s t r e n g t h of l i m e - r i c e husk ash c e m e n t .

Introduction Rice husk ash (RHA) is an i m p o r t a n t source of silica. By c o n t r o l l e d t h e r m a l d e c o m p o s i t i o n of rice husk (l), it is possible to produce an ash which c o n t a i n s r e a c t i v e silica. The ash is largely composed of silica with minor a m o u n t s of a l k a lis and other e l e m e n t s . Silica from rice husk is finding use as a c o n s t i t u e n t of low-cost c e m e n t s . Though the use of rice husk a s h - l i m e m i x t u r e as a c e m e n t i tious m a t e r i a l has been known for a long t i m e , the r e a c t i o n of silica in RHA with lime is l i t t l e studied (2). It has been r e p o r t e d t h a t the R H A - l i m e c e m e n t has v a r y i n g s t r e n g t h , depending on the r e a c t i v i t y of ash. Even with r e a c t i v e ash, the s t r e n g t h i n c r e a s e s upto 28 days and l a t e r a g r a d u a l d e c r e a s e is noTticed (3). The n a t u r e of the r e a c t i o n p r o d u c t b e t w e e n RHA and lime is not well u n d e r s t o o d . Here an a t t e m p t has been made to u n d e r s t a n d the c h e m i s t r y involved in the r e a c t i o n b e t w e e n RHA and l i m e in the p r e s e n c e of w a t e r and to suggest possible causes for the long t e r m d e c r e a s e in the s t r e n g t h of R H A - l i m e c e m e n t . Experimental Rice husk ash Rice husk used for the p r e p a r a t i o n of ash is from the E x t e n s i o n C e n t r e + Symbols used : C=CaO

S=SiO 2

H=H20 67

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of ASTRA (Centre for A p p l i c a t i o n of Science and Technology to Rural Areas). Husk is washed w i t h water to free it from adhering soil and clay and dried in air at ambient temperature. The dried husk is heated in a m u f f l e furnace at 500°C for 12 hours to obtain ash. Lime BDH l a b o r a t o r y r e a g e n t grade c a l c i u m hydroxide powder from fresh unopened bottles. RHA-lime mixtures Mixtures in two r a t i o s of a s h : l i m e were p r e p a r e d - l;3(lime excess) and 2:1 (lime d e f i c i e n t ) mole ratios. These m i x t u r e s were mixed with m i n i m u m q u a n t i t y of w a t e r to p r e p a r e p a s t e s . Blocks of 1.0 cm size were prepared from these pastes and the blocks were kept in d e s i c c a t o r s to avoid c o n t a c t with c a r b o n dioxide from air. The blocks were kept moist by wrapping wet c o t t o n wool around t h e m . A few blocks were kept i m m e r s e d in w a t e r and a few were kept moist and exposed to a t m o s p h e r i c c a r b o n dioxide. At known i n t e r v a l s ranging from one day to 2g days, the blocks were t a k e n out, powdered and i m m e r s e d in a c e t o n e , washed with e t h e r and thoroughly dried in a v a c u u m and were analysed for the products. C-S-H(1) A c o m p a r i s o n s a m p l e of C-S-H(I) was p r e p a r e d by a known p r o c e d u r e (4) from sodium s i l i c a t e and c a l c i u m hydroxide. X - r a y d i f f r a c t o g r a m s were recorded w i t h a Philips PWI050/70 diffractometer w i t h Cu K~ radiation at a scan rate of 2°/min. Thermal analyses were carried out on a co~nbined TG, DTG, D T A u n i t - U L V A C 1 5 0 0 . Infrared spectra in the 200-4000 cm region were recorded in alkali halide pellets on a Perkin Elmer 597 Spectrometer. Electron micrographs were recorded w i t h a Cambridge Stereoscan 150 scanning electron microscope. Transmission micrographs were taken w i t h a Philips EM 301 microscope. Results and Discussion The c h a r a c t e r i s t i c s of the rice husk ash are as follows: analysis-SiO2-95%; CaO -0.441%;

MgO -0.425%; Fe20 3 -0.713%;

K20

-1.048%; N a 2 0 -0.415%; and o

carbon less than 1%. The average c r ~ s t ~ l l i t e size of silica was around 25 A and the surface area of the ash was 150 m g Thermograms of R H A : l i m e mixtures before and a f t e r setting are shown in figure I. The w e i g h t loss step in the temperature region 490-550°C (figure la) represents the decomposition of calcium hydroxide to calcium oxide. This step is absent in lb, i n d i c a t i n g the absence of free lime in the f i n a l product. Even in the lime excess m i x t u r e , the mass of free lime and calcium carbonate present in the final p r o d u c t is much less than the i n i t i a l mass of lime. The set lime.d e f i c i e n t m i x t u r e e x h i b i t s an e x o t h e r m i c peak around g40°C in DTA. The X-ray d i f f r a c t o g r a m s show i n i t i a l l y p r o m i n e n t r e f l e c t i o n s of c a l c i u m hydroxide, while the final p r o d u c t s show only broad r e f l e c t i o n s around 29.4 ° and 32 ° two t h e t a for a l i m e - d e f i c i e n t m i x t u r e . Even in l i m e - e x c e s s m i x t u r e s in addition to the r e f l e c t i o n s from u n r e a c t e d lime, these a d d i t i o n a l r e f l e c t i o n s are n o t i c e d . Howe v e r , the X-ray r e f l e c t i o n s of u n r e a c t e d lime and c a l c i u m c a r b o n a t e (formed

I

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69 RICE HUSK ASH, C-S-H(1), XRD, SEM, TG

by reaction with atmospheric carbondioxide) overlap the C-S-H(I) phase. (Figs. 2-4) The absence of unreacted lime is further corroborated by the i n f r a r e d l s p e c t r a of product OH-absorption around 3600 cm Ls absent absence of free lime (5) after setting.

the principal reflections of in a lime deficient mixture the samples. In the final (figure 5b), confirming the

0

(a)

Figure 1 TG of RHA:lime mixtures (a) 2:1 mixture before setting (b) 2:1 mixture after setting (c) TG ol synthetic C-S-H(I) (d) TG of 1:3 mixture alter of setting. (e) DTA of 2:1 mixture before setting. (f) DTA of 2:1 mixture a!ter setting. (g) DTA of syntheticC-S-H(I) (h) DTA of 1:3 mixture a l t e r 28 days of setting.

(b)

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(¢)

28 days

o

~oo

~;o

~o

~

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Temperature, oC

The thermal behaviour (6), XRD of the set product and that of the residue obtained by heating to 900°C, are identical to those ol the synthetic sample of C-S-H(I). Hence there is conclusive evidence for the formation of calcium silicate hydrate in the setting of rice husk ash-lime pastes. The reaction appears to be complete in less than four days (figure 2) for a lime-deficient mixture. Alkalis act as catalysts in the formation of calcium silicate hydrate from lime and silica. This might be due to the formation of sodium silicate which further reacts with calcium hydroxide to form C-S-H(I). Presumably the NaOH released

(c)

Figure 2 XRD of (a) 2:1 RHA:lime mixture before setting. (b) 2:1 RHA:lime mixture after setting. (c) synthetic C-S-H.

,o----- 2 B

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J. James and M. Subba Rao £:

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Figure 3 X R D of (a) h3 mixture before setting (b) 1:3 mixture after setting for 2g days exposed to air. (c) 1:3 mixture after setting exposed to air. (d) 1:3 mixture after setting immersed in water. (e) 1"3 mixture after setting in a desiccator free from atmospheric carbondioxide.

18

2e

in this reaction would react again with silica and thus continue the cycle. presence of alkalis in RHA could explain its rapid reaction with lime.

The

Morphologies of C-S-H(I) produced during the setting of R H A - l i m e cements are depicted in figure 6. Figure 6(a) shows dense fibrilar structure radiating in a 'porcupine' fashion from silica grains, while 6(b) shows gel coatings around the silica deposits. Closer scrutiny reveals fine surface protuberances on the gel coatings. Figures 6(c) and (d) show the transmission electron micrographs. Bundles of interlocking acicular fibres can be seen. A l t e r n a t e light and dark patches along the length of the fibre might indicate, hollow tubular crosssections /or these fibres. Similar features have been reported in the electron micrographs of set portland cement, where C-S-H is known to be one of the major products of hydration (7).

I J

~ 2 e

Figure 4 XRD of (a) set cement and (b) synthetic C-S-H, after heating at 900°C for 12 hours.

Vol. 16, No. l

71 RICE HUSK ASH, C-S-H(1),

Figure 5 IR spectra of (a) 1:3 m i x t u r e after setting exposed to air. (b) 2:1 mixture before setting. (c) 2:1 mixture after setting.

XRD, SEM, TG

i

3~

36o0 ~0o

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3000 , ~ Wavenumber

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It is reported (2) that the strength of portland cement increases when mixed with 10-20% of rice husk ash. In the setting of portland cement C-S-H gel and CH comprise over 75% of the hydrated cement paste and it has been postulated (g) that CH represents the weaker o5 the two phases. Primarily the strength of cement is attributable to the C-S-H component (9). The conversion of liberated lime also into C-S-H, confers enhanced strength to the mixture of portland

Figure 6a Scanning electron micrograph of 2:1 RHA:lime paste after setting.

Figure 6b Scanning electron micrograph of 2:1 RHA:lime paste after setting.

O~O

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J. James and M. Subba Rao

Figure 6c Transmission electron micrograph of 2:1 RHA:lime paste a f t e r setting.

0.1 um

Figure 6d Transmission electron micrograph of 2:1 RHA:lime paste a f t e r setting.

c e m e n t and RHA. The f o r m a t i o n of C-S-H(I) during the setting of a paste lime and rice husk ash accounts for the strength of the set mass.

of

The following r e a c t i o n sequence might explain the setting process leading to e i t h e r d e v e l o p m e n t of strength or its e n h a n c e m e n t . SiO2(RHA) + H20

> C-S-H + u n r e a c t e d silica

C S (portland cement) + H20

c32s

Portland c e m e n t + RHA + H20

> >

C-S-H + Ca(OH) 2

(1) (2)

C-S-H + u n r e a c t e d silica

(3)

The long-term decrease in strength of R H A - l i m e cement might be due to changes in m o r p h o l o g y / c r y s t a l l i n i t y of C-S-H and unreacted silica,

Vol. 16, No. l

73 RICE HUSK ASH, C-S-H(1),

XRD, SEM, TG

Acknowledgements We wish to thank our colleague, Professor A.K.N. Reddy, for his keen interest and e n c o u r a g e m e n t during this investigation. References 1. 3ose 3ames and Mo Subba Rao, Thermochim. Acta (in press).

UNIDO/ESCAP/RCTT,

2.

P.K. Mehta, Proc. Workshop on Rice Husk Ash Cement, pp. 113-122, Peshawar, Pakistan (1979).

3.

M.R. Yogananda, K.S. 3agadish and R. Kumar, Alternative Building Series 9, Studies on Surki and Rice Husk Ash Pozzolana, pp. g5-90 (1983).

4.

H. Funk, Silikatechnik

5.

D.S. Snell, J. Amer. Ceram. Soc. 5_gg, 292 (1975).

l__l_l,375 (1960).

6. S.A. Greenberg, 3. Phys. Chem. 58, 362 (1954). 7.

F.M. Lea, The Chemistry of C e m e n t and Concrete, 3rd Ed., Ch.9, pp. 177-249, Edward Arnold Ltd., London (1970).

8.

R.W. Williams, Electron Microscopy and Structure of Materials, (eds.) G. Thomas, R.M. Fulrath and R.M. Fisher, Univ. California Press, Berkeley,

(1972). 9.

S.O. Oyefesobi and D.M. Roy, Cem. Concr. Res., 7, 95 (1977).