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Vol. 19 No. 3 Journal of Wuh an Universityof Technology Mater.Sci. Ed. Sep. 2004

C o n c r e t e w i t h H i g h l y A c t i v e R i c e H u s k A s h *

F E N G Q i n g - g e 1 '2 ) L I N Q i n g - y u2) Y U Q i - j u n 3) Z H A O S a n - y i n g 3)Y A N G L u - f e n g 2/ Sh uich i Su gita 1)

1)H a c h i n o h e I n s t i t u t e of Tech nology( Japan ) ;2) Gu angxi Un iversi ty 3 ) Sou th China Univers i ty o f Technology( R e c e i v e d : A p r i l 10, 2003; Accep ted: Ma y 13,2004)Abstract: The overal l aim was to inves t igate the e ffec t of high ly ac t ive r ice husk ash ( RHA ) produced byan indust rial furna ce on som e propert ies of concre te . The s t rength, pore volume an d pore di s t r ibut ion o f concre teand the C a( OH )2 content in concre te were inves tigated by J IS A 1108 ( Method of t es t for compress ive s t rength ofconcrete ), a mercury instrument porosimeter , and the thermo gravimetric an alys is, respectively. The resalts showtha t , wi th RHA replacement o f cem ent , the compress ive s t rength of concre te is increased ev ident ly ; the average poreradius of concrete is grea tly decreased, especially the portion o f the pores greater than 20nm in radius is decreasedwhi le the amount o f smal ler pores i s increased, and the more the RHA replacemeat , the less the amount of Ca( OH)~ in concre te . The lat ter two result s are the main reasona or the s t rength er thancement of concre te .

Key words: rice husk ash ; pore structure; poz.zolanic activi ty ; con crete

1 I n t r o d u c t i o nFor engin eering , economic and ecological benefits,

the use of pozzolanic and cementitious m aterials, such asgranula ted blast - furnace slag, f ly ash, s i l ica fume andnatural pozzolan, in the cement and concrete industry hasr isen great ly f rom the middle of the last cen tury and con-t inues to be fur ther increased to meet the r i s ing dem and ofcement in the world. Except for the materials mentionedabove, the use of r ice husk ash (R HA ) in concre te i s un-der deve lopm ent. More than 120 million tons of rice huskare available for disposal in the world every year. As a re-newable source of ene rgy, i t is quite attractive as a substi-tute for fossil fuels for heat an d electricity generation (T hecalor if ic va lue of r ice husk i s about 50 % of co al ) . In ad-dit io n, und er a controlled combustion cond ition, the burn-ing of r ice husk can yie ld about 20% ash, conta ining89 % -9 7% amorphous SiP2 and wi th very high surface ar-ea. Experimental resu lts showed that with the substi tutionof RH A for normal por t land cement in concre te , not onlythe strength of concrete was en han ced but also the du ra-bil i ty of concrete was improved IvT] . He nce , the produc-tion of RHA with high pozzolanic activity and the use ofRH A as an effective pozzolanic material in concrete havereceived mu ch attention since 198 0s. Maeta Corporationof Japan designed and bui l t a r ice hu sk combustion fur-nace ( Fig . 1 ) , wi th which RH A w ith very high pozzolanicactivity is produced accompanied by the recovery of the

FENG Qing-ge ( 7-~ ~ ~ ) : Born in 1% 7; Asso c.Prof. ; Col-lege of Chemistry and Chem ical Engineering, G uangxi University,Nanning 53000 4, China* Funded by the National "863" Research Project of China(2002AA3350)and the High-level UniversityConstructionProject ofSouth C hina U niversityof China(D6580)

heat energy from the combustion of r ice husk. In thiss tudy , t he i n f luence o f RHA produced by t he fum ace onconcrete strength a nd the requirement of air-entrainingagent and superplasticizer of RH A blen ded concrete w ereinvest iga ted.2 E x p e r i m e n t a l

Raw mater ia ls used in the exper iment were :cement :ordinary portland c em en t;fin e aggregate: river sand withdens it y o f 2 .60g /e ra 3 a n d F M . o f 2 . 8 4 g / c m3 ; and coarseaggregate : c rushed sand stone wi th densi ty of 2 . 64 g/c m 3and maximum size of 20m m. The physical properties andchemical composit ions of the cement are shown in Table 1and particle size distribution is shown in Fig . 2.

The combustion temperature of rice husk in the fur-nace was controlled at around 5dO~ "The p roduc t ,RH A, is with 112 m2/g of specific surface area (N2 a d-sorption) and th e sil ica in i t is mainly in amorphous form(Fi g. 3 ). Its chemical composition and phy sical proper-ties are given in Table 1. The variation in electrical con-ductivity of saturated Ca (O H )2 solution af ter adding2 . 5% (wt /vo l ) o f t he RHA i s 4 .7 5 mS/cm , showing theRHA has very high ac t ivi tyES] .

S Sugita and Q Yu observed that for a given work-abi l i ty more mixing water was need ed for RHA blendedconcrete I3'43 . The relationships between RH A replacem entand requirement of superplasticizer (S P , a newly devel-oped polycarboxylic acid-based matter) and air-entrainingagent in F ig. 4 are fo r concretes with a ir volume of 5 %and slump o f 80m m. No segregation and b leeding hasbeen observed for all the fresh cone1~etes with R HA .

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76 Journal of Wuh an Universityof Technology M ater.Sci. Ed. S e p . 2004Table 4 Pore structure of RHA concretes

RH A Total pore volume Average poreW /B r e p l ac e m e n t o f / ( m m 3 / g ) r a d i u s/ r a n

cem en t / % 2 8 d ay s 9 1 d ay s 2 8 d ay s 9 1 d ay s

0.30

0.35

0 6 4 . 6 2 - - 1 5 . 6 8 - -1 0 5 6 . 3 9 - - 1 0 . 7 3 - -2 0 6 0 . 2 6 - - 4 . 9 7 - -0 6 7 . 3 3 4 8 . 1 2 1 5 . 7 9 1 2 . 9 31 0 5 6 . 7 6 4 2 . 7 3 1 2 . 9 3 7 . 3 12 0 4 6 . 4 6 4 2 . 0 6 8 . 8 4 4 . 9 83 0 3 9 . 4 2 3 0 . 2 8 4 . 9 8 4 .1 1

3 . 3 E f f e c t o f R H A a d d i t io n o n t h e a m o u n t o f C a( O H ) 2 i n c o n c r et e

0.2 600.1 ~ 50

i 4 0~,~,m ~ 30

~ "" 0.1 "_~ 200.0 ~ 100.0 00 10 20 30RHA content(%)Fig .4 Relat ionship between requi rem ent ofsu p e rp l a s t i c i t e r (SP) ,a i r -en tra i n i n g (A E)and RHA content (B:b inder)--I---W/B=O .35(AE) r W/B=O.30(AE)

Ak W/B=O.35(SP) ~ W/B=O .30(SP)6 05 04 o

.~ 2 0:~ 10

Control* RHA 10%- 1 - - R H A 20 %

o '10 100 1000 10000 100000P o r e r a d i u s / n m

Fig .7 Relat ionsh ip between re la t ive pore volumea n d p o r e r a d i u s o f c e m e n t m o r t a r(W/B=0.30,28 d a y s )

1 2 0

~ 10 0so6o

200

Mortar por tions (cub es by about 5m m) were takenfrom concre tes cured for 7 , 28 an d 91d ays, a f ter washedwith ace tone they were D-dr ied for one week. TheCa(OH)2 content in them was determined by TG analyses(hea t i ng r a t e : 1 0~2 /min , t e s t r ange : 30 -1050~C) . Theresults show that in the investigated range of RH A replace-ment , the more the added RHA, the less the amount ofCa (O H) 2 in concre te , an d for the concretes wi th 30%RHA replacement of cement cured for 28 days and 91days, amount of Ca( OH )2 in them fa l ls be low the de tec-t ion l imi t of the inst rument . Fig . 9 gives the Ca (O H )2eontent in the concretes at W~ B of 35 % . Curing age doesnot significantly affect the Ca(OH)2 content in RHA con-cre te .

12 010 0

! 4020

. . , ~ 010 1O0 1000 10000 1-00000Pore radius/nmFig .5 T he re la t ionship hetween re la t ive porevolume andpo re radius of cement mortar(w/b=0.35,28 days)Ak Control r RHA 10%

R H A 2 0 % + R H A 3 0 %Control ] ~ 10 tt RH A IO % /

9 " i t - RH A 20% ~ 7 . 5 I! ' /2. 5

. O l0 10 100 1000 10000 100000P o r e rad i u s / n m

Fig .8 Pore s ize d is t r ibu t ion of cem ent m ortar(WIB=0.30,28 days)Being wi th very high ac t ivi ty , the sin2 in RHA can

quickly reac t wi th the Ca2+ and OH ions in the l iquidphase of hydra t ing cement to form C-S-H gel . M ore C-S-H gel and less portlandite, therefore, is formed in RHAblended concrete compared to plain concrete, also the hy-dration of ceme nt is promoted. As a result , the averagepore radius of concrete is decre ased ; especially the por-tion of the pores greater than 20 nm in radius is evidentlydecreased. Al l of them sure ly make the st ructure of con-crete more compact and the strength of concrete increase .With higher s t rength, less por t landi te , more C-S-H geland smal ler pore size , the durabi l i ty of RHA blendedconcrete is ce rtain ly superior to normal concrete I7"9"~ .

. . . . . . . i . . . . . . . . , . . . . . . . w m10 100 1000 10000 100000Pore radius/nmFig .6 Pore s ize d is t r ibut ion of cement

mo r t a r (W/B=0.35,28 days).t. Control ; RHA 10%

-- ' 1 " - - RH A 20 % - - RH A 3 0 %

0

- ~ - 7 d a y s\ t 2 8 d a y s

~91day

10 20 ~'0 40RH A c o n t e n t ~

Fig.9 Amount Ca(OH)a n c o n c r e t e(W/B=0.35)

4 C o n c l u s i o n s

a) For a given air volume and slump concretes withRHA addition have a higher requirement of air-entrainingagent (AE ) and superplast ic izer (S P ) .

b) Th e more the replacement of RHA , the higherthe compressive strength gain of concrete in spite of theW/B and cur ing age , a t 7-day age RHA concretes ge t themaximum strength increment ratio.

c) With RH A replacement of cement in concre te ,the average pore radius of concrete is dec rease d; especial-ly, the portion of the pores greater than 20n m in radiu s isdecreased while the amount of pores sm aller than 20n m is

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increased.d ) T h e a m o u n t o f C a ( O H ) 2 i n c o n c r et e i s r e d u c e d

e v i d e n t l y w i t h t h e i n c r e a s e R H A c o n t e n t i n c o n c r e t e .

AcknowledgementsT h e a u t h o r s a r e g r a te f u l to D r M S h o y a , D r Y T s u k i -

n a g a a n d D r M A b a , P r o f e s s o r s o f H a c h i n o h e I n s t i t u t e o fT e c h n o l o g y , f o r t h e i r c o n s t r u c t i v e d i s c u s s i o n s a n d g r e a th e l p t o t h i s s t u d y . T h e f i r s t a n d t h e t h i r d a u t h o r s a p p r e c i -a t e t h e f in a n c i al s u p p o r t f r o m N a t i o n a l " 8 6 3 " R e s e a r c hP r o j ec t o f C h i n a ( 2 0 0 2 A A 3 3 5 0 5 0 ) a n d fr o m t h e H i g h -l e v e l U n i v e r s i t y C o n s t r u c t i o n P r o j e c t o f S o u t h C h i n a U n i -v e r s i t y o f T e c h n o l o g y ( 1 3 6 5 0 8 0 ) .

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