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Fly Ash and Sodium Carbonate as AdditivesTo Soil-Cement MixturesC O L E M A N A . O ' F L A H E R T Y , M A N U E L M A T E O S , and D O N A L D T . D A V I D S O N ,R e s p e c t i v e l y , A s s i s t a n t P r o f e s s o r of E n g i n e e r i n g G r a p h i c s , R e s e a r c h A s s o c i a t e , andP r o f e s s o r of C i v i l E n g i n e e r i n g , Iowa E n g i n e e r i n g E x p e r i m e n t S t a t i o n , I o w a StateU n i v e r s i t y of Science and T e c h n o l o g y , A m e sA l t h o u g h the use of f l y a s h i n m a s s c o n c r e t e and i n l i m es t a b i l i z a t i o n has been e x t e n s i v e l y i n v e s t i g a t e d , t h e r e hasbeen r e l a t i v e l y l i t t l e w o r k done o n i t s use i n s o i l - c e m e n ts t a b i l i z a t i o n . A p p a r e n t l y t h e r e i s no p u b l i s h e d m a t e r i a la v a i l a b l e on the use of s o d i u m c a r b o n a t e i n s o i l - c e m e n t f l y ash m i x t u r e s .In t h i s i n v e s t i g a t i o n , t h r e e s o i l s (a dune sand, a f r i a b l el o e s s , and a n a r t i f i c i a l s a n d - l o e s s m i x t u r e ) w e r e s t u d i e d .T h r e e f l y ashes f r o m t h r e e d i f f e r e n t s o u r c e s w e r e u s e d .T h e c e m e n t w a s T y p e I and the s o d i u m c a r b o n a t e w a sreagent grade.It was f o u n d t h a t f l y ash c o u l d be u s e d as a n a d d i t i v et o , o r as a r e p l a c e m e n t f o r , c e m e n t i n f r i a b l e s o i l - c e m e n tm i x t u r e s . The s m a l l e r the l o s s on i g n i t i o n and the f i n e rthe p a r t i c l e s i z e o f the f l y a s h , the m o r e u s e f u l i t i s asan a d d i t i v e o r r e p l a c e m e n t ; h o w e v e r t h e s e c r i t e r i a a r enot i n t h e m s e l v e s s u f f i c i e n t t o f u l l y d i f f e r e n t i a t e betweenthe v a r y i n g q u a l i t i e s o f the f l y a s h e s . F o r e a c h f l y a s ht h e r e a p p e a r s t o be a n o p t i m u m r a t i o of c e m e n t to f l ya s h . The advantages o f f l y a s h a r e m a i n l y r e f l e c t e d i nl o n g - t e r m s t r e n g t h s . The a d d i t i o n of f l y a s h t e n d s t or e t a r d the s e t t i n g - u p o f s o i l - c e m e n t m i x t u r e s , thusa l l o w i n g m o r e t i m e f o r m i x i n g and c o m p a c t i n g . Theb e n e f i c i a l e f f e c t s o f the a d d i t i o n o f s o d i u m c a r b o n a t eare most noticeable a f t e r short c u r i n g periods. Sodiumc a r b o n a t e c a n be d e t r i m e n t a l o v e r a l o n g p e r i o d o f t i m eto soil-cement and s o i l - c e m e n t - f l y ash m i x t u r e s containing low cement contents. S O I L - C E M E N T , w h e r e p r o p e r l y designed and c o n s t r u c t e d , has a n e n v i a b l e r e c o r d asa r e l i a b l e base c o u r s e m a t e r i a l . N e v e r t h e l e s s , as w i t h a l l m a t e r i a l s o f c o n s t r u c t i o n ,t h e r e i s s t i l l some r o o m f o r i m p r o v e m e n t i n i t s p r o p e r t i e s a n d i n i t s economy of u s e .A r e d u c t i o n i n the c e m e n t r e q u i r e m e n t , w i t h o u t s a c r i f i c i n g needed s t r e n g t h and d u r a b i l i t y , i s desirable f r o m many view points.T h e m a i n o b j e c t i v e , t h e r e f o r e , w a s t o e x p l o r e the p o s s i b i l i t y o f b e n e f i t i n g s o i l - c e m e n ts t a b i l i z a t i o n by u s i n g f l y ash as a n a d d i t i v e t o , o r as a r e p l a c e m e n t f o r , the c e m e n t .In addition, selected mixes were treated w i t h a trace chemical (sodium carbonate) tod e t e r m i n e i t s e f f e c t o n the s t r e n g t h p r o p e r t i e s o f s o i l - c e m e n t and s o i l - c e m e n t - f l y a s h .BACKGROUNDF l y ash i s the f i n e , gray, d u s t - l i k e ash collected i n power plants that b u r n powderedc o a l . A s the f i n e l y d i v i d e d p u l v e r i z e d c o a l i s b u r n e d , p a r t i c l e s o f the a s h a r e suspendedi n the gas s t r e a m and c o l l e c t e d o n the p l a t e s o f e l e c t r o s t a t i c o r o n m e c h a n i c a l p r e c i p i 108
109t a t o r s . F l y ash i s a v a i l a b l e i n v a s t q u a n t i t i e s a t some i n d u s t r i a l c e n t e r s , and becausei t i s a waste product, i t i s low i n cost.Because o f i t s l o w cost and i t s k n o w n p o z z o l a n i c b e h a v i o r , f l y ash has been w i d e l yused i n c o n c r e t e c o n s t r u c t i o n as a p a r t i a l r e p l a c e m e n t f o r the m o r e e x p e n s i v e c e m e n t(1).A p o z z o l a n i c m a t e r i a l i s one t h a t , i n the p r e s e n c e of m o i s t u r e , w i l l r e a c t w i t hl i m e to f o r m a c e m e n t i t i o u s c o m p o u n d (2). C a l c i u m h y d r o x i d e ( l i m e ) i s l i b e r a t e d byh y d r a t i n g p o r t l a n d c e m e n t . T h i s c o m p o u n d i s b e l i e v e d t o c o n t r i b u t e l i t t l e to the s t r e n g t hof the c e m e n t i n g a c t i o n i n c o n c r e t e and i t m a y i n f a c t be l e a c h e d out by p e r c o l a t i n g w a t e r s .I t has been suggested, t h e r e f o r e , t h a t i f f l y ash i s p r e s e n t w h e n c e m e n t i s h y d r a t i n g ,t h e n the f l y ash w i l l r e a c t w i t h the l i b e r a t e d l i m e to p r o d u c e s u p p l e m e n t a l c e m e n t i t i o u scompounds (1).A l t h o u g h the use of f l y ash i n m a s s c o n c r e t e {1, 3, 4 , 5) and i n l i m e s t a b i l i z a t i o n(6, 7) has been e x t e n s i v e l y i n v e s t i g a t e d , t h e r e i s l i t t l e p u b l i s h e d m a t e r i a l a v a i l a b l e o ni t s use i n s o i l - c e m e n t s t a b i l i z a t i o n . B r e n n a n (8), w o r k i n g w i t h a f i n e - g r a i n e d s o i l andone f l y a s h , r e p o r t e d t h a t a m i x t u r e c o n t a i n i n g 3 p e r c e n t c e m e n t and 3 p e r c e n t f l y a s hgave good s t r e n g t h i m p r o v e m e n t . L i l l e y (9) r e p o r t e d t h a t a f l y a s h d i d n o t c o n t r i b u t eto the s t r e n g t h s o f c e m e n t s t a b i l i z e d cohesive s o i l s . D a v i d s o n et a l . ( 0) c o n c l u d e dt h a t the a d d i t i o n o f a f l y ash a p p e a r e d t o be m o s t b e n e f i c i a l w i t h c o a r s e - g r a i n e d , p o o r l yg r a d e d s o i l s w h e r e s t r e n g t h i m p r o v e m e n t s w e r e due to i m p r o v e d s o i l g r a d a t i o n — r e s u l t i n gi n m o r e s u r f a c e c o n t a c t areas—and t o c o m p l e m e n t a r y c e m e n t a t i o n f r o m p o z z o l a n i cr e a c t i o n products. The f l y ash was observed to reduce shrinkage c r a c k i n g d u r i n g c u r i n gof a c e m e n t s t a b i l i z e d c l a y s o i l . M a t e o s and D a v i d s o n (11) f o u n d t h a t the a d d i t i o n o f af l y ash to m i x t u r e s of s i l t and c e m e n t i n c r e a s e d s t r e n g t h s to a m i n o r e x t e n t . W r i g h tand Ray ( 2) added a f l y ash t o m i x t u r e s o f t h r e e d i f f e r e n t s o i l s and c e m e n t and c o n c l u d e d t h a t f l y a s h c a n r e p l a c e some o f the c e m e n t .S o d i u m c a r b o n a t e has been f o u n d to a c c e l e r a t e the h a r d e n i n g o f s o i l - l i m e - f l y a s hm i x t u r e s (6, 7, 11, 13, U) and s o i l - c e m e n t m i x t u r e s ( 1 1 , 15). Hence, i t i s notu n r e a s o n a b l e t o b e l i e v e t h a t i t should t h e r e f o r e b e n e f i t the s t r e n g t h s o f s o i l - c e m e n t - f l yash m i x t u r e s .M A T E R I A L S A N D PROCEDURESSoilsT w o s o i l s w e r e e x t e n s i v e l y u s e d i n t h i s i n v e s t i g a t i o n . One w a s a w e l l - g r a d e d , h i g hd e n s i t y s a n d - l o e s s m i x w h i c h w a s s a m p l e d f r o m the blended m a t e r i a l u s e d i n the s o i l c e m e n t base o f Iowa 117, n o r t h of C o l f a x . Because t h i s s o i l was q u i t e w e l l g r a d e d , i tw a s f e l t t h a t any a d d i t i o n a l f l y ash w o u l d not be c o n s i d e r e d as a f i l l e r m a t e r i a l , butw o u l d i n s t e a d d i s p l a c e s o i l p a r t i c l e s . Hence, any s t r e n g t h i n c r e a s e s t h a t m i g h t o c c u rc o u l d p o s s i b l y be a t t r i b u t e d t o the supposed p o z z o l a n i c r e a c t i o n . The o t h e r e x t e n s i v e l yi n v e s t i g a t e d s o i l w a s a m o r e p o o r l y g r a d e d f r i a b l e l o e s s w h i c h i s t y p i c a l of t h a t f o u n din eastern Iowa.I n a d d i t i o n , a t y p i c a l e a s t e r n I o w a dune sand w a s u s e d i n a p r e l i m i n a r y e v a l u a t i o nof the e f f e c t s o f f l y a s h and s o d i u m c a r b o n a t e i n s o i l - c e m e n t s t a b i l i z a t i o n . A f i e l dd e s c r i p t i o n and some p h y s i c a l and c h e m i c a l p r o p e r t i e s of each s o i l a r e g i v e n i n T a b l e 1.C e m e n t and F l y A s h e sT y p e I P o r t l a n d c e m e n t , w h i c h i s the t y p e c o m m o n l y used i n s o i l - c e m e n t c o n s t r u c t i o n , was used i n t h i s i n v e s t i g a t i o n . A c c o r d i n g to D a v i s (3), a l a r g e r r e p l a c e m e n t o fc e m e n t by a p o z z o l a n can be made w h e n T y p e I o r T y p e I I c e m e n t i s u s e d i n s t e a d o fType I V . The t r i c a l c i u m s i l i c a t e content o f a c e m e n t i s b e l i e v e d t o be d i r e c t l y r e l a t e dto the a m o u n t o f l i m e l i b e r a t e d d u r i n g the c e m e n t h y d r a t i o n , and T y p e s I and H c e m e n t sc o n t a i n m o r e t r i c a l c i u m s i l i c a t e t h a n T y p e I V . The a n a l y s i s of the p o r t l a n d c e m e n t i sg i v e n i n T a b l e 2.Because fly ashes d i f f e r g r e a t l y w i t h r e s p e c t to t h e i r p h y s i c a l and p o z z o l a n i c p r o p e r t i e s , t h r e e d i f f e r e n t types o f f l y ash f r o m t h r e e d i f f e r e n t s o u r c e s w e r e s e l e c t e d f o rt h i s s t u d y . T h e i r p r o p e r t i e s and d e s c r i p t i o n s a r e g i v e n i n T a b l e 3. The choice o f thef l y ashes was made on the b a s i s o f a p r e v i o u s study o n the r e a c t i v i t y of d i f f e r e n t f l y
110TABLE 1P H Y S I C A L A N D C H E M I C A L P R O P E R T I E S O F T E S T SOILSM i x ofGeologicaldescriptionS o u r c e (Iowa)Soil s e r i e sHorizonSamplingd e p t h (ft)Texturalc o m p o s i t i o n (%):G r a v e l ( 2mm)S a n d ( 2 . 0 - 0 . 074mm)S i l t (0. 0 7 4 - 0 . 0 0 5mm)approximately82% w a s t e s a n d f r o mhydraulic graveldredging operationsa n d 18% W i s c o n s i n age l o e s s , o x i d i z e d ,calcareous, mediumplasticJasper Co.T a m a (loess)C (loess)Stock pile (sand),b o r r o w pit (loess)Dune S a n d(S-6-2)Friable Loess(100-8)Sand-Loess(Colfax mix)PropertyWisconsin-age loessoxidized, c a l c a r e ous,friableScott C o .FayetteC2 5 - 2 5 1/2Wisconsin-ageeolian sand, finegrained, oxidizeBenton Co.CarringtonC6-110. 00. 00.070. 72. 894, 422. 385. 212. 01. 64. 08.93. 5C l a y ( 0. 0 0 5 m m ) 7. 0Colloids6. 0( 0. 0001 m m )PredominantM o n t m o r i l l oni te,clay m i n e r a hemical properties:Cation exchangecap. *( m e q / 1 0 0 g)C a r b o n a t e s (%)11. 03. 811. 68. 020. 0matter0.2(%)Physical properties:L i q u i d l i m i t (%)19P l a s t i c l i m i t (%) 163P l a s t i c i t y indexClassification:Sandy l o a mTextural---06. 5OrganicEngineering(AASHO)A-2-40. 2270. 1207N. P .N. P .N. P .Silty loamSandA.4(8)A-3(0) F o r f r a c t i o n p a s s i n g N o . 40 s i e v e ,b p o r f r a c t i o n p a s s i n g N o . 10 s i e v e .c T e x t u r a l c l a s s i f i c a t i o n i s f r o m t r i a n g u l a r c h a r t d e v e l o p e d b y U . S. B u r e a u of P u b l i cRoads.S a n d a n d s i l t s i z e s a r e s e p a r a t e d on N o . 2 0 0 s i e v e (0. 074 m m ) .
I l lTABLEPROPERTIES OFTYPE2IPORTLAND CEMENTPropertyValueC h e m i c a l c o m p o s i t i o n (% b y w t . ) :Silicon dioxideA l u m i n u m oxideF e r r i c oxideC a l c i u m oxideM a g n e s i u m oxideSulfur trioxideL o s s on i g n i t i o nInsoluble residuePhysical properties:Specific surface (Wagner) (sq c m / g )TABLE5, 6 2 53F L Y ASHESFly AshSt,Clair,Mich,L o s s on i g n i t i o n (%)Specific surface, Blaine(sq c m / g )Specific gravityF i n e n e s s (% p a s s i n g N o . 3 2 5 s i e v e )C h e m i c a l c o m p o s i t i o n (% b y w t . ) :S i l i c o n d i o x i d e (SiOz)M a g n e s i u m oxide (MgO)C a l c i u m oxide ( C a O )A l u m i n u m oxide (AI2O3)I r o n oxide ( F e 2 0 3 )S u l f u r t r i o x i d e (SO3)902658162,2693 , 7211Source2.2.0.0.3. 155. 150. 120C o m p r e s s i v e s t r e n g t h ( 1 : 2 . 7 5 G . O . S. ) ( p s i ) :At 3 daysAt 7 daysA t 28 d a y sProperty620597051, 8 5 53, 395A i r Permeability (Blaine)(sq cm/g)Setting t i m e ( G i l l m o r e test) (hr):InitialFinalA u t o c l a v e e x p a n s i o n (%)ANALYSIS OF21.5.2.64.3.92, 8202. 5891. 843. 50. 22.923. 224. 80. 8Fly Ash3Fly Ash4Louisville,Ky.2. 6C. Rapids,Iowa18. 63, 2 2 62.6086. 142.0.5.23.20.587402, 34, 5502. 3754. 936. 20. 98. 315. 816.71. 5a s h e s w i t h l i m e ( 6 ) . I n t h a t s t u d y f l y a s h 3 p r o v e d t o be o f v e r y good q u a l i t y , f l y a s h1 o f g o o d , and f l y a s h 4 o f p o o r q u a l i t y w h e n u s e d i n l i m e a n d f l y ash m i x t u r e s .O f t e n u s e d c r i t e r i a f o r j u d g i n g the q u a l i t y o f a f l y a s h a r e the l o s s o n i g n i t i o n ( w h i c hi s a p p r o x i m a t e l y e q u a l t o the c a r b o n c o n t e n t ) and the f i n e n e s s o f t h e f l y a s h as m e a s u r e db y the a m o u n t p a s s i n g the N o . 325 s i e v e . U s i n g t h e s e c r i t e r i a , f l y a s h e s 3 and 1 c o u l dbe c o n s i d e r e d t o be o f good q u a l i t y and 4 t o be of r a t h e r p o o r q u a l i t y .
112Trace ChemicalA l t h o u g h a n u m b e r o f c h e m i c a l s a r e k n o w n to a c t as a c c e l e r a t o r s f o r p o z z o l a n i cr e a c t i o n s , s o d i u m c a r b o n a t e i s b e l i e v e d to be one of the b e t t e r , s a f e r and m o r e e c o n o m i c a l to use ( l i , 15). S o d i u m c a r b o n a t e , r e a g e n t g r a d e , w a s used i n t h i s i n v e s t i g a t i o n .Mixture ProportionsI n t h i s i n v e s t i g a t i o n the p e r c e n t a g e s of c e m e n t , f l y a s h , and s o i l a r e e x p r e s s e d asp e r c e n t a g e s o f the t o t a l d r y w e i g h t of the s o i l , c e m e n t , and f l y ash i n the m i x . Thea m o u n t of s o d i u m c a r b o n a t e i s e x p r e s s e d as a p e r c e n t a g e o f the t o t a l d r y w e i g h t ofs o i l , c e m e n t , and f l y ash i n the m i x t u r e u n d e r c o n s i d e r a t i o n .I n a p r e l i m i n a r y s t u d y , s p e c i m e n s w e r e f i r s t p r e p a r e d w h i c h c o n t a i n e d dune sand,11 p e r c e n t c e m e n t , and 0, 3, 6 and 9 p e r c e n t f l y ash 1. S p e c i m e n s w e r e a l s o p r e p a r e dt h a t c o n t a i n e d v a r y i n g a m o u n t s o f c e m e n t and f l y ash b u t i n a l l o f these a d d i t i o n a l s p e c i m e n s , the t o t a l c e m e n t and f l y ash content w a s 11 p e r c e n t . A l l s p e c i m e n s w e r e t h e nd u p l i c a t e d except that 72 p e r c e n t s o d i u m c a r b o n a t e was added.T h r e e c e m e n t contents (5, 8, and 11 p e r c e n t ) w e r e used i n the m a i n s t u d i e s . I t w a sf e l t t h a t these p e r c e n t a g e s w o u l d w e l l enclose the c e m e n t r e q u i r e m e n t t o m e e t the c r i t e r i a f o r s o i l - c e m e n t . S o i l - c e m e n t m i x t u r e s w e r e f i r s t p r e p a r e d w i t h o u t f l y ash andt h e n w i t h f l y a s h contents o f 3, 6, and 9 p e r c e n t o f each o f the d i f f e r e n t f l y a s h e s . E x t r am i x t u r e s w e r e t h e n p r e p a r e d i n w h i c h Va p e r c e n t s o d i u m c a r b o n a t e w a s added i n p o w d e rf o r m to a l l s o i l , c e m e n t and f l y ash c o m b i n a t i o n s c o n t a i n i n g 5 p e r c e n t c e m e n t .M i x i n g and M o l d i n gA l l s o i l s w e r e a i r d r i e d , and g r o u n d w i t h m o r t a r and p e s t l e t o pass t h r o u g h the N o . 10s i e v e b e f o r e any m i x i n g t o o k p l a c e . M i x i n g of batches was done i n a H o b a r t k i t c h e nm i x e r , m o d e l C - 1 0 0 , a t l o w speed i n the f o l l o w i n g sequence o f o p e r a t i o n : the d r yi n g r e d i e n t s w e r e m i x e d f o r 30 sec, the r e q u i r e d a m o u n t o f d i s t i l l e d w a t e r w a s t h e nadded, and the b a t c h w a s t h e n m i x e d f o r 1 m i n . The m i x t u r e was t h e n hand m i x e d f o rabout 30 sec so as t o c l e a n the side o f the m i x i n g b o w l and the paddle, a f t e r w h i c h them i x t u r e was machine m i x e d f o r another m i n u t e . A f t e r m i x i n g was completed, a dampc l o t h w a s p l a c e d o v e r the b o w l so as t o p r e v e n t d r y i n g of the m i x t u r e d u r i n g m o l d i n g .M o l d i n g o f t e s t s p e c i m e n s w a s s t a r t e d as soon as p o s s i b l e a f t e r c o m p l e t i o n of m i x i n g .T e s t s p e c i m e n s , 2 i n . i n d i a m e t e r by 2 0. 05 i n . h i g h , w e r e c o m p a c t e d w i t h the I o w aState c o m p a c t i o n a p p a r a t u s (6, 7, 10, JJ , 13, 1 , J ) . M a x i m u m d e n s i t i e s o b t a i n e du s i n g t h i s a p p a r a t u s have been v e r y c l o s e l y c o r r e l a t e d w i t h those m a x i m u m d e n s i t i e so b t a i n e d u s i n g the s t a n d a r d P r o c t o r a p p a r a t u s (16).N u m b e r s o f SpecimensT h e a v e r a g e of t h r e e s p e c i m e n s w a s used t o o b t a i n each p o i n t o n the m o i s t u r e - d e n s i t yand m o i s t u r e - s t r e n g t h r e l a t i o n s h i p c u r v e s . W h e n the o p t i m u m m o i s t u r e content f o rm a x i m u m d e n s i t y was o b t a i n e d f o r each s o i l - c e m e n t - f l y ash c o m b i n a t i o n , f o u r s p e c i m e n s w e r e m o l d e d at t h i s m o i s t u r e content f o r each c u r i n g c o n d i t i o n . The a v e r a g e o fthese f o u r s t r e n g t h v a l u e s i s r e p o r t e d .CuringS p e c i m e n s f o r the m a i n study w e r e c u r e d f o r 7, 28, and 90 days and f o r 7, 28, and120 days f o r the p r e l i m i n a r y i n v e s t i g a t i o n . The c u r i n g r o o m w a s m a i n t a i n e d a t at e m p e r a t u r e o f 71 3 F and a r e l a t i v e h u m i d i t y o f g r e a t e r t h a n 90 p e r c e n t . T o m a i n t a i nthe m o i s t u r e i n the s p e c i m e n s b e t t e r and t o r e d u c e a b s o r p t i o n o f c a r b o n d i o x i d e f r o mthe a i r , they w e r e w r a p p e d i n w a x paper and sealed w i t h cellophane tape b e f o r e b e i n gp l a c e d i n the h u m i d r o o m .Strength TestingA t the end of a s p e c i f i e d c u r i n g p e r i o d , the s p e c i m e n s w e r e u n w r a p p e d , i m m e r s e di n d i s t i l l e d w a t e r f o r 24 3 h r and t h e n t e s t e d f o r u n c o n f i n e d c o m p r e s s i v e s t r e n g t h
113r120Unconfinedcompressive .gostrength,psi\l20daydoy120 dayZSdoy 2Sdoy2«diiyiTdoy11%RC. i%N C i»a— 1 1 %0»No,CO,(a)X Fly ash no. IF i g u r e 1. (b)% Cement plus f l y ash no. I( a ) F l y a s h 1 w i t h o u t cement; ( b ) f l y a s h 1 w i t h cement.Colfax soilLoes* toil00 /% Moisture contentfor maximumdry density//lint- 45* Inn«»/y.Fly oth n a 4mixturttIA l l o t h t r mlxturatj -S,/10II121314o Fly o i h no 4fflixtwn All o l l x r mmhirM19IlUW171III*toitl tl5% Moisture content for maximum 7 doy strengthFigure 2.Comparison o f optimum m o i s t u r e c o n t e n t s f o r maximum d e n s i t i e s w i t h optimumm o i s t u r e c o n t e n t s f o r maximum s t r e n g t h s .
114u s i n g a load t r a v e l rate o f 0 . 1 i n . per m i n . Strength values w e r e evaluated f o r r e l i a b i l i t y i n a s t a t i s t i c a l m a n n e r (J7) and a n y v a l u e s w h i c h w e r e t h e n d i s q u a l i f i e d w e r enot c o n s i d e r e d i n d e t e r m i n i n g a v e r a g e c o m p r e s s i v e s t r e n g t h s .PRELIMINARY INVESTIGATIONB e f o r e the m a i n s t u d y , t h e e f f e c t o f f l y ash and s o d i u m c a r b o n a t e o n p o o r l y g r a d e ddune s a n d - c e m e n t m i x t u r e s w a s i n v e s t i g a t e d . I n t e s t i n g f o r the e f f e c t o f f l y ash as a na d d i t i v e , o n l y one c e m e n t content, 11 p e r c e n t , and f l y a s h 1 w e r e u s e d . M i x t u r e s w e r ea l s o p r e p a r e d a t t w o l o w e r c e m e n t contents and f l y a s h added u n t i l the t o t a l c e m e n t andf l y a s h c o n t e n t t o t a l e d 11 p e r c e n t . A l l o f these m i x t u r e s w e r e t h e n d u p l i c a t e d e x c e p tt h a t 0 . 5 p e r c e n t s o d i u m c a r b o n a t e w a s added t o e a c h b e f o r e d r y m i x i i .Specimensw e r e c u r e d f o r 7, 28, and 120 days b e f o r e t e s t i n g f o r u n c o n f i n e d c o m p r e s s i v e s t r e n g t h .The r e s u l t s o b t a i n e d a r e shown i n F i g u r e 1 . The a d d i t i o n o f f l y ash t o 11 p e r c e n tc e m e n t causes d e c r e a s i n g s t r e n g t h s a t 7 days ( F i g l a ) . A t 28 d a y s , t h e r e i s p e r h a p sa s l i g h t s t r e n g t h i n c r e a s e . A f t e r 120 days the a d d i t i o n o f f l y a s h i s seen t o be m o s tb e n e f i c i a l , the a d d i t i o n of 4 p e r c e n t f l y a s h c a u s i n g a 25 p e r c e n t i n c r e a s e i n s t r e n g t h .W h e n 0. 5 p e r c e n t s o d i u m c a r b o n a t e w a s added t o t h e s e m i x t u r e s , g r e a t i n c r e a s e s i ns t r e n g t h w e r e n o t e d . F o r the s o i l - c e m e n t m i x e s , i n c r e a s e s a m o u n t e d t o 16 p e r c e n ta t 7 and 28 days and 33 p e r c e n t a t 120 d a y s . B e s t r e s u l t s w e r e o b t a i n e d w i t h 11 p e r cent c e m e n t , 2 p e r c e n t f l y ash, a n d 0. 5 p e r c e n t s o d i u m c a r b o n a t e . A t 7 days t h e r ewas a n e g l i g i b l e gain i n s t r e n g t h over those m i x t u r e s containing cement alone. Howe v e r , a f t e r 28 and 120 days these m i x t u r e s gave s t r e n g t h s t h a t w e r e 82 p e r c e n t and 95p e r c e n t , r e s p e c t i v e l y , g r e a t e r t h a n the s o i l - c e m e n t s t r e n g t h s .F i g u r e l b shows q u i t e c l e a r l y t h a t the s u b s t i t u t i o n of f l y a s h 1 f o r a n e q u a l a m o u n tof c e m e n t i s not b e n e f i c i a l . H o w e v e r , t h i s l i m i t e d study i n d i c a t e d t h a t t h e r e m i g h t besome r a t i o of cement to f l y ash, w i t h o r without sodium carbonate, that m i g h t allowsome economical substitution.M A I N STUDY RESULTSA m o i s t u r e - d e n s i t y r e l a t i o n s h i p w a s r u n f o r each c o m b i n a t i o n of s o i l , c e m e n t , andf l y a s h . F i v e p o i n t s w e r e o b t a i n e d o n each c u r v e , each p o i n t b e i n g the a v e r a g e o f t h r e es p e c i m e n s . A f t e r t a b u l a t i o n o f the m o i s t u r e - d e n s i t y data, the s p e c i m e n s w e r e p l a c e di n the h u m i d r o o m , c u r e d f o r 7 d a y s a n d t h e n t e s t e d u n d e r u n c o n f i n e d c o m p r e s s i o n . M o i s t u r e s t r e n g t h c u r v e s w e r e t h e n d r a w n f o r each of the s o i l - c e m e n t - f l y a s h c o m b i n a t i o n s .The o p t i m u m m o i s t u r e contents f o r b o t h d e n s i t y and s t r e n g t h i n c r e a s e d as the a m o u n to f f l y a s h i n a p a r t i c u l a r m i x i n c r e a s e d . The e f f e c t o f the d i f f e r e n t t y p e s o f f l y ash o nt h e s e o p t i m u m m o i s t u r e c o n t e n t s i s n o t e w o r t h y . F o r a p a r t i c u l a r s o U , the m i x e sc o n t a i n i n g f l y a s h e s 1 and 3 had t h e i r o p t i m u m m o i s t u r e contents a l l c o n t a i n e d w i t h i n1,3 p e r c e n t a g e p o i n t s and t h i s s p r e a d c o n t a i n e d t h e s e r e q u i r e d f o r the c e m e n t and s o i la l o n e ( F i g . 2 ) . On the o t h e r hand, the m i x t u r e s c o n t a i n i n g the c o a r s e s t and h i g h e s tc a r b o n c o n t e n t f l y a s h ( N o . 4) had a s p r e a d o f 4 . 5 p e r c e n t a g e p o i n t s .The o p t i m u m m o i s t u r e contents f o r m a x i m u m 7-day s t r e n g t h s w e r e c o n s i s t e n t l y o nthe d r y s i d e o f the o p t i m u m m o i s t u r e content f o r m a x i m u m d e n s i t y f o r b o t h the C o l f a xand l o e s s s o i l s . T h e r e a s o n f o r t h i s i s not e x a c t l y k n o w n . A n i n t e r e s t i n g c o n j e c t u r er e a s o n s t h a t the d i f f e r e n c e i s r e l a t e d t o s o i l t e x t u r e . T h e s u r f a c e s of s u c h sands ands i l t s a r e r e l a t i v e l y i n a c t i v e and hence l i t t l e o f the l u b r i c a t i n g w a t e r i s a d s o r b e d ontothe s o U p a r t i c l e s . A s a r e s u l t m o s t o f the w a t e r t h a t has been added to e n s u r e m a x i m u mdensity is also available f o r cement hydration purposes. T h i s amount of f r e e water maybe o v e r and above the a m o u n t r e q u i r e d f o r c e m e n t h y d r a t i o n . I n c o n c r e t e w o r k , thel o w e r the w a t e r t o c e m e n t r a t i o , t h e h i g h e r the s t r e n g t h . T h e r e f o r e , i t m a y be t h a tthe o p t i m u m m o i s t u r e content f o r o b t a i n i n g m a x i m u m d e n s i t y f o r a s o i l - c e m e n t - f l y a s ho r s o i l - c e m e n t m i x t u r e i s g r e a t e r t h a n the w a t e r r e q u i r e m e n t s t o s a t i s f y the c e m e n t i t i o u s r e a c t i o n . A s a r e s u l t , the o p t i m u m m o i s t u r e content f o r m a x i m u m s t r e n g t h w i l lbe o n the d r y side of the o p t i m u m m o i s t u r e content f o r m a x i m u m d e n s i t y .In g e n e r a l , d e n s i t i e s d e c r e a s e d as the a m o u n t of f l y a s h i n a m i x t u r e i n c r e a s e d . A g a i n thed i f f e r e n t f l y ashes had d i f f e r e n t e f f e c t s on d e n s i t i e s . F o r f l y ashes 1 and 3, d e n s i t y d e c r e a s e sr a n g e d f r o m 1 t o 4 p c f depending on the a m o u n t of f l y a s h u s e d . W i t h f l y a s h 4 d e n s i t y d e c r e a s e sw e r e as m u c h as 12 p c f . T h i s i s due t o the l o w u n i t w e i g h t of t h i s f l y a s h .
115A f t e r the o p t i m u m m o i s t u r e content f o rm a x i m u m density was obtainedfor a p a r t i c u l a r m i x , 12 s p e c i m e n s w e r e m o l d e d at t h a tm o i s t u r e c o n t e n t . The s t r e n g t h r e s u l t s o b t a i n e d f r o m these s p e c i m e n s a r e d i s c u s s e dl a t e r . H o w e v e r , the e f f e c t of t i m e on thespecimen densities w i t h i n a p a r t i c u l a r batch( F i g . 3) i s n o t e w o r t h y . The t i m e f r o m w h e nm o i s t u r e w a s f i r s t added t o the d r y m i x u n t i lthe end of the c o m p a c t i n g of the l a s t s p e c i men v a r i e d f r o m 17 t o 24 m i n . The t i m ef r o m the b e g i n n i n g of c o m p a c t i o n of the f i r s ts p e c i m e n u n t i l the end of the l a s t v a r i e df r o m 14 t o 2 1 m i n . S o i l m i x t u r e s c o n taining only cement and sodium carbonateor c e m e n t alone showed d e f i n i t e d e c r e a s i n gd e n s i t y t r e n d s as c o m p a c t i o n t i m e p r o g r e s s e d .On the o t h e r hand, the a d d i t i o n of f l y ashes3 a n d 4 a p p a r e n t l y had the e f f e c t of r e t a r d i n gthe s e t t i n g - u p of the s o i l - c e m e n t . Them e d i u m t y p e f l y ash ( N o . 1) had l e s s of aretarding effect.IIXPCdensity.I I X P C 311 FA no 3dantlty.IIXPC 6%FA na3I I X P C 9 % F A no 3I I X P C -f 3 X F A no Idtntlty.UNCONFINED COMPRESSIVESTRENGTH RESULTSF l y A s h as A d d i t i v e t o S o i l C e m e n tA s m e n t i o n e d e a r l i e r , 12 s p e c i m e n sw e r e m o l d e d a t the o p t i m u m m o i s t u r econtent f o r m a x i m u m d e n s i t y f o r eachc o m b i n a t i o n o f c e m e n t , s o i l , and fly a s h .A f t e r b e i n g c u r e d i n the h u m i d r o o m , 4specimens w e r e tested a f t e r 7 days, 4 a f t e r28 d a y s , and the r e m a i n i n g 4 a f t e r 90 d a y s .The 4 specimens tested at a p a r t i c i i l a r t i m ew e r e selected s t a t i s t i c a l l y to m i n i m i z ed i f f e r e n c e s i n the t i m e and s p a c e . Theaverage strength r e s u l t s obtained are showni n F i g u r e s 4 and 5.I I X P C 3 X F A no 4density.l i X c i m a n t e X F A no 4I I X P C . 9 X F A no 4A s e x p e c t e d , b o t h the s o i l - c e m e n t andthe s o i l - c e m e n t - f l y a s h m i x t u r e s i n c r e a s e dS p w i m a n numbari n s t r e n g t h as c u r i n g t i m e p r o g r e s s e d .F i g u r e 3. E f f e c t o f f l y a s h a s a r e t a r d H o w e v e r , the m a n n e r i n w h i c h the d i f f e r e n tingagent on s e t t i n g up o f C o l f a x s o i l fly ashes a f f e c t e d these s t r e n g t h i n c r e a s e scement m i x t u r e s — t y p i c a l d a t a .was v e r y s i g n i f i c
Louisville, Ky. 2 . 6 3 226 2.60 86. 1 42. 5 0. 8 5. 7 23. 4 20. 0 2, 3 C. Rapids, Iowa 18 6 4, 550 2. 37 54. 9 36. 2 0. 9 8. 3 15. 8 16.7 1. 5 ashes with lime (6). In that study fly ash 3 proved to be of very good quality, fly ash 1 of good, and fly ash 4 of poor quality when used in lime and fly ash mixtures.
