Alkali - aggregate reaction in Nova Scotia. IV. Character of the reaction

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Alkali - aggregate reaction in Nova Scotia. IV. Character of the reaction

Gillott, J. E.; Duncan, M. A. G.; Swenson, E. G.

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CEMENT and CONCRETE RESEARCH. Vol

.

3,

pp.

521-535, 1973. Pergamon Press, Inc.

Printed in the United States.

ALKALI-AGGREGATE REACTION IN NOVA SCOTLA IV. CHARACTER O F T H E REACTION

J. E. Gillott, Division of Building R e s e a r c h , Nat. R e s . Council of Canada, now with Univ. of C a l g a r y , C a l g a r y , A l b e r t a . M. A. G. Duncan, Atlantic I n d u s t r i a l R e s e a r c h Institute, Halifax, now

with M i n i s t r y of Roads, S a l i s b u r y , Rhodesia, and

E.G. Swenson, Div. of Building R e s e a r c h , Nat. R e s . Council of Canada.

(Communicated

by

G.

M. Idorn)

ABSTRACT

Studies of a n a l k a l i - a g g r e g a t e r e a c t i o n i n Nova Scotia have shown t h a t it d i f f e r s in i m p o r t a n t c h a r a c t e r i s t i c s f r o m the well-known a l k a l i - s i l i c a r e a c t i o n . It i s d i s c u s s e d in

r e l a t i o n t o p r e v i o u s l y published c o n c e p t s of m e c h a n i s m and an explanation i s p r o p o s e d f o r t h e a l k a l i - e x p a n s i v i t y of s o m e Appalachian r o c k s .

Des htudes d'une & a c t i o n a l c a l i n e d e s agrCgats en Nouvelle- E c o s s e

-

v o i r l e s a r t i c l e s dhj'a publiCs

-

m o n t r e n t qu'elle diffkre s u r d e s points i m p o r t a n t s d e l a r h a c t i o n a l c a l i n e d e s s i l i c e s , bien connue. E l l e e s t e x a m i n h e 3 _{l a lumi'ere d e notions} m h c a n i q u e s dhj'a publiCes et l y o n p r o p o s e une explication de l y e x p a n s i o n en a l c a l i n s d e c e r t a i n e s r o c h e s appalachiennes.

A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M

V o l . 3, N o . 5

T h r e e p r e v i o u s p a p e r s ( 1 , 2, 3) d e s c r i b e a n a l k a l i - a g g r e g a t e r e - action o c c u r r i n g in Nova Scotia that d i f f e r s i n i m p o r t a n t r e s p e c t s f r o m the well-known a l k a l i - s i l i c a r e a c t i o n . In t h i s fourth p a p e r of t h e s e r i e s a

c o m p a r i s o n i s m a d e between the two and a m e c h a n i s m i s suggested t o account for t h e Nova Scotia r e a c t i o n . T h e scanning e l e c t r o n m i c r o s c o p e and an x - r a y diffraction method w e r e used to identify m i n e r a l s and t o follow t h e changes that o c c u r r e d in the r e a c t i o n .

T h e alkali-expansive r o c k t y p e s in Nova Scotia a r e mainly g r e y - wackes, a r g i l l i t e s , ~ h y l l i t e s , and s o m e of the q u a r t z i t e s and s c h i s t s . Two p r e v i o u s l y r e p o r t e d c a s e s , a l s o in t h e Appalachian r e g i o n and both apparently involving p h y l l i t e s , would not appear t o be r e l a t e d ( 4 , 5 ) , a s m a y well be a r e p o r t e d i n s t a n c e in t h e Canadian Shield (6). C e r t a i n _, phyllites i n Norway and Sweden a r e believed r e s p o n s i b l e for alkali-expan- sivity of c o n c r e t e ( 7 , 8 ) .

P r e s e n t Hypotheses

T h e l i t e r a t u r e i n d i c a t e s gener a1 a g r e e m e n t that e x c e s s i v e expansion of c o n c r e t e r e s u l t i n g f r o m a l k a l i - s i l i c a r e a c t i o n i s caused by the swelling of a gel, produced by r e a c t i o n between the a l k a l i s defiving f r o m t h e c e m e n t and c e r t a i n active s i l i c e o u s m i n e r a l s and r o c k t y p e s , e. g.

,

opal, opaline c h e r t s and r o c k s containing an amorphous o r low c r y s t a l l i n e p h a s e of s i l i c a (7 to 14). Swelling p r e s s u r e s a r e believed due to t h e imbibition of w a t e r by the g e l and t h i s c a n exceed t h e t e n s i l e s t r e n g t h of m o r t a r o r c o n c r e t e (15, 16).

Differences of opinion exist on s o m e d e t a i l s of t h e m e c h a n i s m : t h e r o l e played by l i m e and t h e r e l a t i v e swelling p r e s s u r e of the a l k a l i - s i l i c a gel and t h e l i m e - a l k a l i - s i l i c a gel; the e x i s t e n c e of a s e m i - p e r m e a b l e

m e m b r a n e and i t s r o l e in dev eloping an o s m o t i c f o r c e ; and, t h e i m p o r t a n c e of r a t e of diffusion and t h e f a c t o r s governing i t .

Evidence h a s been p r e s e n t e d ( 8 , 17, 18) t h a t swelling o c c u r s in t h e outer s h e l l of a g g r e g a t e p a r t i c l e s a n d t h a t t h i s i s r e s p o n s i b l e for t h e u l t i m a t e breakdown of c o n c r e t e .

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T h e a l k a l i - c a r b o n a t e r o c k r e a c t i o n i s a m o r e r e c e n t d i s c o v e r y . Although it s h a r e s s o m e m a j o r c h a r a c t e r i s t i c s of t h e a l k a l i - s i l i c a r e a c t i o n , and s e v e r a l a t t e m p t s w e r e m a d e t o put i t into t h e s a m e c l a s s of r e a c t i o n , t h e d i f f e r e n c e s w e r e s o m a r k e d t h a t t h e s a m e m e c h a n i s m of r e a c t i o n was c l e a r l y not applicable. In t h i s c a s e c e r t a i n dolomitic l i m e s t o n e s used a s c o a r s e a g g r e g a t e have been shown to r e a c t with c e m e n t a l k a l i s t o p r o d u c e e x c e s s i v e expansion of c o n c r e t e . T h e expansion w a s e a r l y a s c r i b e d t o t h e dolomite c r y s t a l s t h e m s e l v e s , t o an o s m o t i c m e c h a n i s m , and to c o m p l e x s a l t f o r m a t i o n (14). M o r e r e c e n t s t u d i e s (19) offer evidence t h a t expansion r e s u l t s f r o m m o i s t u r e uptake by t h e c l a y o r c l a y - l i k e component following a dedolomitization r e a c t i o n .

E x p e r i m e n t a l Methods

A s u i t e of 4 g r e y w a c k e s , 3 p h y l l i t e s , 2 a r g i l l i t e s , 4 q u a r t z i t e s and 1 rhyolite w e r e s e l e c t e d f r o m t h e m a n y r o c k s t e s t e d i n t h i s study for m o r e detailed investigation. Included w e r e both expansive and non-expansive Nova Scotia r o c k s . Length change d a t a , x - r a y diffraction a n a l y s e s , o p t i c a l m i c r o g r a p h s , and scanning e l e c t r o n m i c r o g r a p h s w e r e obtained. T h e length change c h a r a c t e r i s t i c s of t h e r o c k s i m m e r s e d i n a l k a l i w e r e d e t e r

-

mined i n c y l i n d e r s by a t e s t modified f r o m that d e s c r i b e d i n ASTM C-586 and a l s o by m e a n s of T u c k e r m a n e x t e n s o m e t e r s . T h e x - r a y r e s u l t s w e r e r e c o r d e d f r o m r o c k s l i c e s and s i z e - f r a c t i o n a t e d p o w d e r s . C o m p a r i s o n s l i c e s of r o c k w e r e analysed at i n c r e a s i n g t i m e i n t e r v a l s of t r e a t m e n t i n w a t e r and 2M NaOH; powders w e r e t r e a t e d in MgC1 NaC1, NaOH and KOH.

2'

F r a c t u r e and saw-cut s u r f a c e s w e r e examined in t h e e l e c t r o n m i c r o s c o p e . The s a m e a r e a s of s u r f a c e w e r e o b s e r v e d in t h e u n t r e a t e d condition and a f t e r i n c r e a s i n g t i m e i n t e r v a l s in a l k a l i ( 2 0 ) .

The d a t a obtained i n t h i s s e r i e s of e x p e r i m e n t s on t h e Nova S c o t i a r o c k s , and t h e r e s u l t s of o t h e r s , cannot be r e c o r d e d h e r e b e c a u s e of s p a c e l i m i t a t i o n but a r e d r a w n upon i n t h i s p a p e r .

C h a r a c t e r i s t i c s of t h e Nova Scotia Reaction

All a l k a l i - a g g r e g a t e r e a c t i o n s a p p e a r to h a v e t h e c o m m o n r e s p o n s e to a l k a l i s , m o i s t u r e , and t e m p e r a t u r e , and i n s e v e r e c a s e s lead t o e x -

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A L K A L I AGGREGATE R E A C T I O N , MECHANISM

pansion and c r a c k i n g of c o n c r e t e . T h e Nova Scotia r e a c t i o n r e s p o n d s i n l i k e m a n n e r and, f u r t h e r m o r e , i s s u p p r e s s e d by t h e p r e s e n c e of a pozzo- l a n i c m 2 t e r i a l a s in t h e c a s e of t h e a l k a l i - s i l i c a r e a c t i o n .

T h e m a i n alkali-expansive r o c k t y p e s of Nova Scotia a r e of s e d i m e n - t a r y origin: greywackes, a r g i l l i t e s , and phyllites. T h e s e r o c k s contain l i t t l e o r no opal, v i t r e o u s s i l i c a o r volcanic g l a s s . F i n e q u a r t z , which i s a common constituent, f r e q u e n t l y showed s i g n s of s t r a i n , inclusions, i n t e r growth and s u t u r e d b o u n d a r i e s . T h e r e s u l t i n g defect m o l e c u l a r p a c k - ings on t h e s u r f a c e probably constitute a significant p r o p o r t i o n of t h e t o t a l q u a r t z volume. Although such m a t e r i a l m a y be a c a u s a t i v e agent of e x - p a n s i v e a l k a l i - s i l i c a r e a c t i o n , t h i s h a s yet to be d e m o n s t r a t e d through e x p e r i m e n t . T h i s f i n e q u a r t z i s t h e only m i n e r a l p r e s e n t i n significant q u a n t i t y , however, that c a n be r e l a t e d t o t h e r e a c t i v e component of the c l a s s i c a l a l k a l i - s i l i c a a g g r e g a t e s .

In the alkali-silica r e a c t i o n a "pessimum" proportion of active m i n e r a l o r r o c k type, i n t h e o r d e r of about 5 p e r cent of t h e t o t a l a g g r e g a t e , h a s been r e p o r t e d t o give m a x i m u m expansion (21). T h e p e r c e n t a g e c o m - position at which t h e p e s s i m u m i s found, however, i s thought t o b e dependent upon p a r t i c l e s i z e , alkali contents and type of r e a c t i v e constituents. F o r e x a m p l e , d e n s e Danish flints m a y show a p e s s i m u m a t 100 p e r c e n t under s o m e conditions w h e r e a s i t m a y o c c u r a t 2 p e r c e n t f o r opaline Danish flint (G. Idorn p r i v a t e communication). F o r the Nova Scotia rock types that exhibited e x c e s s i v e alkali-expansivity t h e r e a p p e a r e d to b e a maximum e x - pansion when the p e r c e n t of expansive r o c k type ranged f r o m 35 to 100 p e r - c e n t . The h e t e r o g e n e o u s n a t u r e of t h e s e r o c k s would, h o w e v e r , p r e c l u d e any quantitative i n t e r p r e t a t i o n of such r e s u l t s .

A t e m p e r a t u r e i n v e r s i o n at about 1 3 0 ° F , noted i n m o r t a r b a r o r

c o n c r e t e p r i s m t e s t s i n both t h e a l k a l i - s i l i c a and i n t h e a l k a l i - c a r b o n a t e r o c k r e a c t i o n s , was not o b s e r v e d i n t h e p r e s e n t c a s e . "Rims" s u r r o u n d - ing r e a c t i v e a g g r e g a t e p a r t i c l e s , t o s o m e d e g r e e e m p h a s i z e d i n t h e a l k a l i - s i l i c a r e a c t i o n c o n c r e t e s , w e r e o b s e r v e d infrequently i n t h e affected Nova Scotia c o n c r e t e s , and t h e n in both affected and unaffected c o n c r e t e s .

V o l . 3, N o . 5

A L K A L I AGGREGATE REACTION, MECHANISM

T h i s h a s a l s o b e e n o b s e r v e d b y I d o r n (8).

E x c e s s i v e expansion of Nova S c o t i a c o n c r e t e s , i n l a b o r a t o r y and field, w a s accompanied by c o n s i d e r a b l e g e l f o r m a t i o n i n s o m e c a s e s and p r a c t i c a l l y none i n o t h e r s . Unaffected c o n c r e t e s s o m e t i m e s contained l i t t l e gel and o t h e r s l a r g e amounts. In s o m e field c o n c r e t e s t h a t had

expanded and c r a c k e d , whitish d e p o s i t s i n c r a c k s and c a v i t i e s w e r e c l e a r l y t h e p r o d u c t s of leaching. A n a l y s e s of r a n d o m s a m p l e s of t h e s e g e l s gave c o m p o s i t i o n s t h a t w e r e c o m p a r a b l e with t h o s e produced by t h e a l k a l i - s i l i c a r e a c t i o n .

T h e ASTM Quick C h e m i c a l T e s t , C289, w a s c a r r i e d out on 1 4 s e l e c t e d Nova S c o t i a r o c k types: 6 g r e y w a c k e s , 1 r h y o l i t e , 3 q u a r t z i t e s , 1 a r g i l l i t e , 2 ~ h ~ l l i t e s and 1 s c h i s t . T h i s method i s b a s e d on r e a c t i o n between a l k a l i and a c t i v e s i l i c a . By e x p a n s i o n t e s t s , 8 w e r e highly a l k a l i - expansive, 2 m o d e r a t e l y s o , 2 s l i g h t l y so, and 1 non-expansive. Another l a b o r a t o r y c a r r i e d out t h e quick c h e m i c a l t e s t s on a s i m i l a r s e t of Nova S c o t i a r o c k t y p e s . T h e 25 t e s t r e s u l t s obtained a g r e e d with the a l k a l i - expansivity b a s e d on length change i n 13 c a s e s and d i s a g r e e d i n 12 c a s e s , tantamount t o a g u e s s . A g r e e m e n t w a s obtained i n t h e c a s e of t h e r h y o l i t e and two of t h e q u a r t z i t e s and t h e a r g i l l i t e .

With t h e exception of r h y o l i t e and a l k a l i - e x p a n s i v e Nova S c o t i a r o c k t y p e s expand when i m m e r s e d i n NaOH. In t h e c a s e of t h e r h y o l i t e and o t h e r s e l e c t e d r o c k s known t o be a l k a l i - s i l i c a r e a c t i v e , t h e r e w a s a n a c t u a l c o n - t r a c t i o n when i m m e r s e d f o r extended p e r i o d s i n NaOH, e. g., chalcedony,

a g a t e , flint ( s e e T a b l e I). T h e l a t t e r behaviour would a p p e a r t o b e due to t h e l o s s of solid s i l i c a by d i s s o l u t i o n i n t h e alkali.

It w a s found t h a t Nova S c o t i a a l k a l i - e x p a n s i v e r o c k s showed a slight weight l o s s of about 2 p e r cent when i m m e r s e d i n 1. OM NaOH f o r 120 weeks. By c o m p a r i s o n a c h e r t l o s t 2 4 p e r cent and a flint 37 p e r c e n t in 120 weeks; a n a g a t e l o s t 35 p e r cent of i t s weight i n 6 1 w e e k s .

In one e x p e r i m e n t r o c k c y l i n d e r s of a l k a l i - e x p a n s i v e Nova S c o t i a r o c k s w e r e i m m e r s e d f o r 72 weeks i n v a r i o u s c o n c e n t r a t i o n s of NaOH a t

V o l . 3, N o . 5 A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M T A B L E I T y p i c a l V o l u m e t r i c E x p a n s i o n s w i t h T i m e of C y l i n d e r s of Nova S c o t i a R o c k T y p e s I m m e r s e d i n 1 . OM NaOH a t 7 3 ° F C o m p a r e d w i t h V o l u m e C h a n g e s of R o c k T y p e s P r e v i o u s l y R e c o g n i z e d a s P o t e n t i a l l y A l k a l i - S i l i c a R e a c t i v e . Vol. exp.

,

p e r c e n t , a t w e e k s s h o w n R o c k T y p e 4 12 4 8 7 2 9 6 120 N. S. Q u a r t z i t i c

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200 q u a r t z i t e C h a l c e d o n y ( R i v e r s d a l e )

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0 . 3 2 LL U L 0 . 1 6 C LL E 3 0 2 0 > 0 . 1 6 4 0 1 6 3 2 4 8 6 4 8 0 9 6 1 1 2 1 2 8 LL Z T E S T D U R A T I O N , W E E K S F i g u r e 1 T i m e - m e a n v o l u m e t r i c e x p a n s i o n r e l a t i o n s h i p s f o r g r e y w a c k e A P L ~ / S ~ s h o w i n g c o n t i n u e d e x p a n s i o n i n d i s t i l l e d w a t e r a t 100" F a f t e r i n i t i a l c u r i n g i n v a r i o u s NaOH s o l u t i o n s a t 100"

F

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ALKALI AGGREGATE REACTION, MECHANISM

i n c r e a s e with i n c r e a s i n g alkali content. After 72 weeks the c y l i n d e r s w e r e removed f r o m alkali and placed in w a t e r f o r 56 weeks. During t h i s t i m e

t h e r e was a slight d r o p in r a t e of expansion but i t did continue. E i t h e r t h e alkali had effected an i r r e v e r s i b l e change which allowed continuing uptake of w a t e r with consequent expansion, o r the alkali cannot be r e a d i l y d i s - lodged f r o m t h e r o c k element with which i t i s r e a c t i n g . Hence r e - generation of the alkali by the calcium ion, c o n s i d e r e d by s o m e to b e n e c e s s a r y in the a l k a l i - s i l i c a r e a c t i o n ( l o ) , a p p e a r s not t o be r e q u i r e d i n the Nova Scotia reaction.

T h e above d i f f e r e n c e s in behaviour of the Nova Scotia a l k a l i -

expansive r o c k s f r o m t h o s e v a r i o u s l y c l a i m e d of t h e c l a s s i c a l a l k a l i - s i l i c a r e a c t i o n s t r o n g l y suggested that another m e c h a n i s m m u s t be o p e r a t i v e , at

l e a s t in p a r t . It was ~ a r t i c u l a r l y i m p o r t a n t to account f o r continuing

expansion in highly w a t e r -leachable conditions, such a s obtained in h y d r a u l i c s t r u c t u r e s i n Nova Scotia w h e r e expansion was shown, by a c t u a l m e a s u r e - ment on t h e s t r u c t u r e s , to b e continuing a f t e r 30 y e a r s ( 2 2 ) . T h i s slow but

continuing expansion over m a n y y e a r s h a s s i n c e been noted i n at l e a s t one c a s e in the Canadian shield ( p r i v a t e communication f r o m the H y d r o - E l e c t r i c P o w e r C o m m i s s i o n of Ontario).

It i s c e r t a i n that r e l a t i v e l y low a v e r a g e annual t e m p e r a t u r e s p l a y a r o l e in the r a t e of r e a c t i o n , but l a b o r a t o r y s t u d i e s d e m o n s t r a t e t h a t e x - pansion i s v e r y slow and g e n e r a l l y does not l e v e l off even a f t e r t h r e e y e a r s

u n d e r a c c e l e r a t e d conditions.

M i c r o s c o p y and X-Ray Analysis

P e t r o g r a p h i c d e s c r i p t i o n s of the four m a i n r o c k types

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greywackes, q u a r t z i t e s , phyllites and a r g i l l i t e s have been given in a n e a r l i e r p a p e r (2).

T h e chief m i n e r a l s p r e s e n t a r e f r a m e w o r k s i l i c a t e s ( q u a r t z and f e l d s p a r s ) and s h e e t s i l i c a t e s (10A

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m i c a , c h l o r i t e , v e r m i c u l i t e o r v e r m i c u l i t e -type mixed l a y e r m i n e r a l and a s m e c t i t e that i s probably m o n t m o r i l l o n i t e o r a

c l o s e l y r e l a t e d m i n e r a l ) . A c c e s s o r i e s a r e v a r i o u s i r o n m i n e r a l s , graphite and t o u r m a l i n e . T h e proportioning of t h e s e constituents v a r i e s with t h e n a t u r e of the r o c k and s o m e of t h e m i n e r a l s a r e not always p r e s e n t .

V o l . 3 ,

No.

5 A L K A L I AGGREGATE REACTION, MECHANISM

Rock w a f e r s w e r e i m m e r s e d i n 2M NaOH a t 1 0 0 ° F and x - r a y d i f - f r a c t i o n p a t t e r n s r e c o r d e d b e f o r e and a f t e r t r e a t m e n t with a l k a l i . Dif- f r a c t o g r a m s of m a n y of t h e r o c k s showed a s h i f t i n p e a k p o s i t i o n f r o m about 0 0 10A t o about 12. 6A ( F i g u r e 2, a t o d). U N T R E A T E D 1

399

DAYS 1 2 M N a O H 1 0 0 ° F X - R A Y : C u K a , 4 5 K V , 2 5 m A , 1 ° 2 8 / M I N . F i g u r e 2 X-Ray d i f f r a c t o g r a m s of p h y l l i t e and g r e y w a c k e w a f e r s b e f o r e and a f t e r s o a k i n g i n 2M NaOH at 1 0 0 ° F

V o l . 3, N o . 5 A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M E x a m i n a t i o n was m a d e on t h e scanning e l e c t r o n m i c r o s c o p e of t h e s a m e a r e a of r o c k s u r f a c e b e f o r e and a f t e r t r e a t m e n t with a l k a l i (20). In s o m e s a m p l e s exfoliation of l a y e r - s t r u c t u r e s i l i c a t e s w a s o b s e r v e d ( F i g u r e 3, c and d ) , followed a t l a t e r a g e s by c o m p l e t e r e m o v a l of t h e s e m i n e r a l s . New r h o m b i c o r cubic c r y s t a l s within t h e l a m i n a e w e r e a l s o s o m e t i m e s p r e s e n t . T h e l a y e r - s t r u c t u r e s i l i c a t e which showed exfoliation w a s c o r -

r e l a t e d b y s i z e t o t h e brown p l e o c h r o i c c r y s t a l s o b s e r v e d o p t i c a l l y and believed t o b e m a i n l y a v e r m i c u l i t e - t y p e m i n e r a l . S i m i l a r l y t h i s m i n e r a l w a s believed r e s p o n s i b l e f o r t h e shift f r o m about 10A t o 1 Z . 6A in p e a k

p o s i t i o n o b s e r v e d on x - r a y d i f f r a c t o g r a m s of a l k a l i - t r e a t e d r o c k s . In s o m e c a s e s b r a n c h i n g s t r u c t u r e s w e r e v i s i b l e on t h e a l k a l i - e t c h e d s u r f a c e , t h e

m a t e r i a l e i t h e r having u n d e r l a i n t h e p h y l l o s i l i c a t e s o r f o r m e d f r o m t h e m due t o t h e a l k a l i n e a t t a c k ( F i g u r e 3, e and f ) . T h e f o u r m a i n r o c k t y p e s a l s o contained l a y e r - s t r u c t u r e s i l i c a t e s which w e r e a p p a r e n t l y not affected by a l k a l i while t h e r o c k s p e c i m e n s w e r e i m m e r s e d ( F i g u r e 3, a and b).

D i s c u s s i o n and C o n c l u s i o n s

Much of t h e d a t a on t h e expansion of r o c k s , c o n c r e t e s and m o r t a r s d e s c r i b e d i n t h e f i r s t t h r e e p a p e r s in t h i s s e r i e s (1, 2, 3) w e r e d e r i v e d by conventional t e s t m e t h o d s a c c e l e r a t e d v a r i o u s l y b y e l e v a t e d t e m p e r a t u r e s and i n c r e a s e d a l k a l i c o n t e n t s . E a r l y w o r k i n a c c e l e r a t i o n of slow r e a c t i o n s w a s r e p o r t e d by C a r l s o n (13) who a l s o t e s t e d a g r e y w a c k e and a phyllite. It i s e m p h a s i z e d again t h a t in t h e Nova S c o t i a s t u d i e s c o n c l u s i o n s w e r e b a s e d on s u c h r e s u l t s only i f c o r r e s p o n d i n g evidence had b e e n obtained by n o n - a c c e l e r a t e d e x p e r i m e n t s and field evidence.

A s u r v e y of the l i t e r a t u r e i n d i c a t e s t h a t m o s t of t h e o b s e r v a t i o n s u s e d in developing h y p o t h e s e s f o r m e c h a n i s m of a l k a l i - s i l i c a r e a c t i o n w e r e

founded on s t u d i e s of a l k a l i r e a c t i o n with o p a l - b e a r i n g r o c k s ( e . g . , ( 1 6 ) ) . T h e t h e o r i e s p r o p o s e d a p p e a r t o account s a t i s f a c t o r i l y f o r expansion of t h i s and r e l a t e d m i n e r a l s o r r o c k t y p e s . In t h e study of t h e Nova S c o t i a a l k a l i - a g g r e g a t e r e a c t i o n , and i n s o m e p r e v i o u s l y r e p o r t e d s t u d i e s involving p h y l l i t e s , a r g i l l i t e s and r e l a t e d n o n - c a r b o n a t e s e d i m e n t a r y r o c k s u s e d a s c o n c r e t e a g g r e g a t e , t h e r e i s s t r o n g evidence t h a t t h e m e c h a n i s m s p r e v i - o u s l y p r o p o s e d f o r t h e a l k a l i - s i l i c a r e a c t i o n a r e not e n t i r e l y applicable.

A L K A L I AGGREGATE R E A C T I O N , MECHANISM

V a l . 3, N o . 5

A R G l L L l T E

E

U N T R E A T E D I N YoOH

F

5 \ ~ F E K ~ 1 1 4 , , y -

-

- - r

F i g u r e 3

Scanning e l e c t r o n m i c r o g r a p h s of greywacke (no. 9 ) , phyllite (no. 3 ) , and a r g i l l i t e (no. 4) b e f o r e and a f t e r t r e a t m e n t with a l k a l i

V o l . 3 , N o . 5 5 3 1 A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M

Many of t h e Nova S c o t i a r o c k s used a s c o n c r e t e a g g r e g a t e s contain a s m e c t i t e r e l a t e d t o m o n t m o r i l l o n i t e and p o r p h y r o b l a s t s of a brown p l e o c h r o i c m i n e r a l that looks like biotite under t h e optical m i c r o s c o p e but i s identified by x - r a y a n a l y s i s t o b e v e r m i c u l i t e o r a c l o s e l y r e l a t e d m i x e d - l a y e r m i n e r a l . Scanning e l e c t r o n m i c r o g r a p h s show t h a t s o m e of t h e

p h y l l o s i l i c a t e s exfoliate when t r e a t e d with c a u s t i c soda solution.

T h e shift in p e a k p o s i t i o n on x - r a y d i f f r a c t o g r a m s f r o m 10A t o 12. 6A t h a t r e s u l t e d f r o m t r e a t m e n t of the r o c k with NaOH d e m o n s t r a t e s expansion of t h e m i n e r a l . The s p a c i n g c o n f o r m s f a i r l y c l o s e l y t o that r e p o r t e d by Brown (24) f o r a i r - d r y N a - v e r m i c u l i t e a t r o o m t e m p e r a t u r e . In w a t e r , h o w e v e r , l a r g e r s p a c i n g s of about 14.8A a r e quoted. T h e s p a c i n g of t h e m i n e r a l i n t h e Nova S c o t i a r o c k w a f e r s f a i l e d t o i n c r e a s e t o a value g r e a t e r than 12. 6A even a f t e r prolonged soaking i n LM NaOH a t 100" I?, and even though t h e w a f e r s w e r e s a t u r a t e d and x - r a y e d under high r e l a t i v e humidity conditions. A s t r o n g peak a t 1 4 - 15A w a s , h o w e v e r , r e g i s t e r e d on d i f f r a c - t o g r a m s r e c o r d e d f r o m oriented p o w d e r s of m a n y of t h e ground, d i s p e r s e d and s i z e d r o c k s which had been t r e a t e d with solutions of NaOH. P o s s i b l y t h e acid, H 0 and t h e NaOH used during d i s p e r s i o n and sizing of t h e

2 L

powdered r o c k affected t h e m i n e r a l s o t h a t g r e a t e r expansion was p o s s i b l e . T h e m i n e r a l s in o r i e n t e d powder mounts a r e a l s o f r e e f r o m r e s t r a i n t s t h a t m a y n o r m a l l y r e s t r i c t t h e expansion of m i n e r a l s i n r o c k s .

Collapsed s m e c t i t e s a l s o h a v e a b a s a l s p a c i n g of about 10A that under c e r t a i n conditions i n c r e a s e s t o about 18A o r m o r e . It i s p o s s i b l e that the shift in p e a k position on t h e x - r a y d i f f r a c t o g r a m s r e s u l t e d f r o m expansion of the s m e c t i t e a s well a s t h e v e r m i c u l i t e , but t h e s p a c i n g s r e c o r d e d a r e c l o s e r t o t h o s e n o r m a l l y a t t r i b u t e d t o v e r m i c u l i t e than m o n t m o r i l l o n i t e . T h e s i z e of the exfoliated m i n e r a l on t h e scanning e l e c

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t r o n m i c r o g r a p h s a l s o c o r r e l a t e s with t h e v e r m i c u l i t e o b s e r v e d by o p t i c a l m i c r o s c o p y .

Complexes of h y d r a t e d aluminum and i r o n h a v e been r e p o r t e d to o c c u r i n t h e i n t e r l a y e r s p a c e of m i n e r a l s t h a t n o r m a l l y show v a r i a b l e b a s a l spacing (25), (26). T h i s i n t e r l a y e r p r e c i p i t a t e s o m e t i m e s i n t e r f e r e s with t h e expansion and c o n t r a c t i o n c h a r a c t e r i s t i c s of t h e m i n e r a l . Its p r o p e r t i e s

532 Vol. 3 , No. 5 A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M a r e s a i d t o be i n t e r g r a d i e n t between t h o s e of v e r m i c u l i t e , c h l o r i t e and m o n t m o r i l l o n i t e . It s e e m s p o s s i b l e that s i m i l a r i n t e r l a y e r p r e c i p i t a t e s of p o o r l y o r g a n i z e d s i l i c a m a y e x i s t . I n t e r l a y e r p r e c i p i t a t e s h a v e been e x t r a c t e d by a v a r i e t y of r e a g e n t s that have included a m i x t u r e of KOH p l u s KC1 (17) and boiling NaOH ( 1 5 ) . If s u c h p r e c i p i t a t e s e x i s t between t h e s h e e t s of v e r m i c u l i t e and m o n t - m o r i l l o n i t e i n t h e Nova Scotia r o c k s , i t i s p r o b a b l e t h a t t h e y would be r e m o v e d t o a g r e a t e r o r l e s s e r extent by a n alkaline solution a s obtains in a n o r m a l c o n c r e t e m i x . Once t h i s p r e c i p i t a t e w a s r e m o v e d t h e m i n e r a l would be c a p a b l e of expansion through w a t e r a d s o r p t i o n and t h e f o r c e would be t r a n s m i t t e d t o t h e r o c k . Hence t h e volume of t h e r o c k should i n c r e a s e . E x t r a c t i o n by c e m e n t a l k a l i of s u c h i n t e r l a y e r c o m p l e x e s m a y a l s o l e a d t o p r e c i p i t a t i o n e l s e w h e r e i n t h e c o n c r e t e of gel-like s e c o n d a r y p r o d u c t s . S o m e g e l m a y a l s o be d e r i v e d by d i s s o l u t i o n of fine l a t t i c e defect q u a r t z that m a y o r m a y not c o n t r i b u t e t o r o c k expansion a s postulated i n e a r l i e r h y p o t h e s e s . In any c a s e t h e p r e s e n c e of such g e l s would account f o r t h e a s sumption of l l c l a s s i c a l " a l k a l i - s i l i c a r e a c t i o n c o m m o n l y m a d e in routine evaluations of d e t e r i o r a t e d c o n c r e t e s containing t h i s type of r e - a c t i v e a g g r e g a t e .

D i f f r a c t o g r a m s r e c o r d e d f r o m r o c k w a f e r s soaked i n hot w a t e r f o r p e r i o d s of a y e a r o r m o r e showed no shift i n p e a k position. T h i s m a y indicate that t h e i n t e r l a y e r p r e c i p i t a t e s a r e i n s o l u b l e i n hot w a t e r and a r e e x t r a c t e d only by t h e much m o r e a g g r e s s i v e 2M NaOH.

T h e e n e r g y r e l e a s e d through c h e m i - s o r p t i o n of w a t e r m o l e c u l e s on an a c t i v e s u r f a c e , p a r t i c u l a r l y i f newly f o r m e d , h a s been shown i n t h e c l a y and c e m e n t l i t e r a t u r e t o be q u i t e high and a n a l y s e s and equations have been p r o p o s e d f o r t h e p r e s s u r e s e x e r t e d by a d s o r b e d w a t e r i n swelling m a t e r i a l s (28, pp. 107

-

113).

In t h e c a s e of t h e a l k a l i - e x p a n s i v e r o c k s i n t h e p r e s e n t s t u d y t h e l a y e r e d - s t r u c t u r e s w e r e o b s e r v e d , on a m i c r o - s c a l e , i n an a p p a r e n t l y "collapsed" condition. Soaking i n a l k a l i p r o d u c e d a n i n c r e a s i n g exfoliation with t i m e . S i n c e t h i s w a s a c t u a l l y o b s e r v e d i n a r o c k w h e r e r e s t r a i n t s a r e high, i t follows that t h e p r e s s u r e s e x e r t e d by s u c h exfoliation m u s t h a v e

V o l . 3 , No. 5 5 3 3 A L K A L I AGGREGATE R E A C T I O N , MECHANISM

b e e n g r e a t enough t o p r o d u c e e x p a n s i o n of t h e r o c k . T h e r e l a t i v e l y s m a l l s e p a r a t i o n of l a y e r s and t h e t y p e of bonding n o r m a l t o a n a c t i v e s u r f a c e would m a i n t a i n t h e p r e s s u r e i n t h e s y s t e m . T h i s d i r e c t evidence, though not q u a n t i t a t i v e i s of m i n i m u m m a g n i t u d e and t h e r e f o r e a d e q u a t e i n t h e p r e s e n t a r g u m e n t .

T h e above c o n c e p t s explain t h e l o n g - t e r m expansion of field c o n - c r e t e s i n w a t e r - l e a c h a b l e e n v i r o n m e n t s and t h e continued e x p a n s i o n of r o c k c y l i n d e r s i n w a t e r a f t e r p r i o r t r e a t m e n t i n a l k a l i ( F i g u r e 1). The r a t e of d i s s o l u t i o n of t h e s e " c e m e n t i n g p r e c i p i t a t e s " m a y i n l a r g e p a r t d e t e r m i n e t h e slow r a t e s of r e a c t i o n o b s e r v e d i n t h e field and l a b o r a t o r y . Once a l k a l i h a s f r e e d - u p t h e l a y e r s , w a t e r i m b i b i t i o n and e x p a n s i o n should continue f o r a t i m e p e r i o d t h a t w i l l depend on s u c h f a c t o r s a s m o i s t u r e

availability, p e r m e a b i l i t y , i n t e r n a l r e s t r a i n t , and t h e q u a n t i t y and d i s t r i - bution of p o t e n t i a l l y e x p a n s i v e m i n e r a l .

It i s concluded t h a t t h e r e a r e e x t r a c t a b l e i n t e r l a y e r p r e c i p i t a t e s between t h e b a s a l p l a n e s of t h e v e r m i c u l i t e -like m i n e r a l in a l k a l i

-

expansive Nova S c o t i a r o c k s and t h a t o n c e t h i s p r e c i p i t a t e i s r e m o v e d by NaOH t h e m i n e r a l expands and c a u s e s a n i n c r e a s e i n t h e volume of t h e r o c k . T h e r e a r e s m a l l a m o u n t s of s m e c t i t e p r e s e n t which a l s o m a y c o n t r i b u t e t o expansion.

R e f e r e n c e s

1. M. A. G. Duncan, E. G. Swenson, J. E. Gillott, and M. R. F o r a n . A l k a l i - a g g r e g a t e r e a c t i o n i n Nova Scotia. I. S u m m a r y of a f i v e - y e a r study. C e m . and C o n c r . R e s . , Vol.

S

NO. 1, p. 16, (1973). 2. M . A . G . Duncan, J. E. Gillott,and E. G. Swenson. A l k a l i - a g g r e g a t e r e a c t i o n i n Nova S c o t i a . 11. F i e l d and p e t r o g r a p h i c s t u d i e s . C e m . and C o n c r . R e s .

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Vol. 2, No., 2, p. 1 1 9 (1973).

3. M. A. G. Duncan, E. G. Swenson,and J. E. Gillott. A l k a l i - a g g r e g a t e r e a c t i o n i n Nova Scotia. 111. L a b o r a t o r y s t u d i e s of v o l u m e change. C e m . and C o n c r . R e s . , Vol. L N o . 3 ; p.233 (1973).

4. H. A. K a m m e r , and R. W. C a r l s o n . I n v e s t i g a t i o n of c a u s e s of delayed e x p a n s i o n of c o n c r e t e i n Buch H y d r o e l e c t r i c P l a n t . J o u r n a l A. C. I . , Vol.

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12, No. 6, p. 665,(1941).

V o l . 3, N o . 5 A L K A L I AGGREGATE R E A C T I O N , M E C H A N I S M E. G. Swenson. C e m e n t - a g g r e g a t e r e a c t i o n of c o n c r e t e of a C a n a d i a n b r i d g e . P r o c . A . S . T . M . , Val.

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p. 1043, (1957). L. Dolar-Mantuani. A l k a l i - s i l i c a r e a c t i v e r o c k s i n t h e C a n a d i a n Shield. Highway R e s . R e c o r d 268, (1969). G. M. Idorn. S t u d i e s of d i s i n t e g r a t e d c o n c r e t e , P a r t 111, P r o g r e s s R e p o r t , S e r i e s N, No. 4, Copenhagen, (1964). G. M. Idorn. D u r a b i l i t y of c o n c r e t e s t r u c t u r e s i n D e n m a r k . T e c h n i c a l U n i v e r s i t y of D e n m a r k , Copenhagen, p. 208, (1967). T . C . P o w e r s , and H. H. S t e i n o u r . An i n t e r p r e t a t i o n of published r e s e a r c h e s in a l k a l i - a g g r e g a t e r e a c t i o n , P a r t s I and 11. P r o c . A. C. I . , Vol. 51, pp. 497 and 785, (1955).

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