Alkali-carbonate rock reaction

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Publisher’s version / Version de l'éditeur:

Highway Research Record, 45, pp. 21-40, 1965-05-01

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Alkali-carbonate rock reaction

Swenson, E. G.; Gillott, J. E.

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Reprinted fl.om Hlghway Research Record No. 45 (1964) Hlghway R e s e a r c h Board, Washington, D. C .

Alkali-Carbonate Rock Reaction

E

.

G. SWENSON and J . E

.

GILLOTT, Inorganic Materials Section, Division of Build- ing Research, National Research Council, Ottawa, Canada

This paper constitutes a review of eight y e a r s of r e s e a r c h on the alkali-carbonate rock reaction by the Division of Building Research, National Research Council, Ottawa, Canada. Most of the material has been, o r will be, published elsewhere. A summary i s given of the original Kingston c a s e which f i r s t r e - vealed excessive expansion in concrete resulting f r o m alkali- carbonate reactivity in a dolomitic limestone coarse aggregate. Further field studies a r e described, a s well a s investigations into the nature of the reaction, methods of test and identification, and s o m e hypotheses concerning the mechanism of reaction and expansion.

.THE expansive type of alkali-carbonate rock reaction in concrete has now been established a s a phenomenon quite distinct f r o m the better known alkali-silica reaction. It appears to be limited to certain fine-grained, argillaceous, dolomitic Limestones when these a r e used a s c o a r s e aggregate. The r a t e and extent of the expansion of the con- c r e t e increases with increase in alkali content of the cement, and the reaction requires the presence of moisture.

Since the f i r s t publications on the alkali-reactive carbonate rock a t Kingston, Ontario, by the present authors (1, 2), problems with limestone aggregates in other a r e a s have been r e - examined. some zf these have been identified with the alkali- carbonate rock reaction, the most extensive studies being those of Hadley (3) and Newlon and Sherwood (4). A l e s s clear-cut but apparently related type of r e a c t i o n o c c u r s with carbonate rocks in Iowa, a s reported by Bisque and Lemish (5, - - - 6, 7) and Lemish, Rush, and Hiltrop (8).

Although the actual mechanism of the expansive reaction involving carbonate rock i s s t i l l being debated, much has been learned about the compositional and environmental f a c t o r s involved. Methods of identification and testing have been developed, remedial measures have been determined (9), and s o m e progress has been made in applying these to specifications and practice in the Kingston a r e a .

Fortunately, the occurrence of expansive carbonate rock appears to be relatively r a r e , and confidence in the u s e of limestones and dolomites a s concrete aggregate is

not impaired. Nevertheless, the evidence now available indicates that in affected a r e a s the problem i s of much more than academic interest.

This paper represents a summary of the studies c a r r i e d out by the Division of Build- ing Research on this reaction. It deals with investigations concerning (a) the solution of the practical problem in Kingston, (b) the nature of the reaction, ( c ) methods of t e s t and identification, and (d) possible mechanisms. Most of the test r e s u l t s shown have been taken from r e p o r t s published elsewhere by the authors, the references being given in each c a s e .

STUDIES DIRECTED TO THE FIELD PROBLEM The Barriefield Case

In the fall of 1955 the Division was asked by Defence Construction (1951) Limited to investigate unusual cases of growth and cracking of recently placed concrete a t B a r r i e -

Papcr sponsored by Committee on Performance of Concrete-Chemical Aspects.

(5)

field, s i t e of a l a r g e a r m y establishment on t h e o u t s k i r t s of Kingston, Ontario. The most d r a m a t i c evidence of expansion was in sidewalks and c u r b s where joints closed up, concrete buckled, and map- cracking o c c u r r e d ( F i g . 1). The f a c t that this took place within t h e f r o s t - f r e e s e a - son suggested s o m e type of cement- aggregate reaction, and this line of investigation was followed f r o m t h e beginning.

F i r s t t e s t s consisted of making con- c r e t e p r i s m s using job m a t e r i a l s and mix designs, and exposing t h e s e to continuous fog-room conditions. Expansions of 0 . 1 ~ e r c e n t and cracking o c c u r r e d within

-

six

weeks. Similar t e s t s w e r e made using F i g u r e 1. Expanding c o n c r e t e r e s u l t e d i n

low alkali cement a s well a s the local c l o s i n g of j o i n t s , b u c k l i n g , and map-

high alkali cement normally supplied in c r a c k i n g .

this a r e a . The r a t e and d e g r e e of expansion increased with increasing alkali content of the cement ( F i g . 2). Extensive

t e s t s with various combinations of different c o a r s e and fine aggregates revealed that t h e dolomitic limestone c o a r s e aggregate was the expansive component. Thus, f r o m t h e s t a r t of the investigation, it was clearly evident that an alkali-aggregate reaction was involved.

Concurrent with t h e s e t e s t s , t h e aggregates w e r e subjected to standard ASTM acceptance t e s t s . On t h i s b a s i s t h e "reactive" c o a r s e aggregate could b e classified a s satisfactory f o r c o n c r e t e . Also, concurrently, t h e aggregates w e r e t e s t e d f o r r e a c - tivity according to ASTM methods. Although abnormal expansion o c c u r r e d in the mor- t a r b a r t e s t (ASTM Specification C227), i t could not b e classified a s excessive on the b a s i s of the then accepted limits ( 1 ) . Pozzolan and chemical inhibitors, which had proved effective f o r reducing expansion in concretes made with other alkali-reactive aggregates, w e r e not effective in this c a s e , a s shown by t e s t s over a 2-yr period (2). It was a l s o noted that affected concretes did not show evidence of t h e gel exudations normally associated with the alkali-silica reaction, and "rim formations" on aggregate p a r t i c l e s w e r e f e w e r and different in appearance.

F i g u r e 2. R e a c t i v e c o a r s e a g g r e g a t e i n c o n c r e t e produced e x p a n s i o n a c c o r d i n g t o c o n t e n t of cement a l k a l i . 2 0

-

c 16 W U a w

-

n I _I _I

,

1

_I

AVG OF TWO 3 x 3 1 1 0 INC4 CONCRETE PRISMS, i CONDITIONED I AT I 0 0 % R H , 7 3 4 "F

-A-q-

I I I I I

!<*

yk~qToiAl

I

i A L K A L I = o b 9 I 2 - 10 12 14 16 18 2 0 T I M E , MONTHS ( I M 0 = 2 8 DAYS I

(6)

At the outset, the Division sent samples to the Bureau of Reclamation and the U.S. Corps of Engineers for expert petrographic examination. It was concluded f r o m observations of Mielenz, K. Mather and B . Mather that s o m e s o r t of cement-aggregate

reaction was involved but that it was an unusual type not previously experienced. The

Division was fortunate in having continuing a c c e s s to supporting evidence f r o m s i m i l a r studies by the Portland Cement Association which initiated work on the Kingston rock only a few months after the investigations in this laboratory were begun, and whose f i r s t published r e s u l t s by Hadley appeared later ( 3 ) .

In early 1957, the Division made tentative recommendations f o r Kingston construc- tion agencies to u s e a cement of sufficiently low alkali content, o r obtain non-reactive aggregates f r o m other sources.

Field Concretes in the Kingston A r e a

The Division of Building Research made extensive field studies and observations a t the beginning of the investigation and selected c a s e s have been followed to the present date. As with other problems of this type, such observations w e r e not only fruitful in characterizing the phenomenon as a supplement to laboratory experiments, but were essential in clarifying the local situation which was inevitably fraught with conflicting interests and preconceived opinions. F r o m a m e r e superficial examination of field concrete, one might have concluded that the problem was not extensive, and that much of the evidence was contradictory. Detailed examination, however, and c a r e in obtaining reliable information as to kinds and sources of materials, methods and practices followed, and environmental history, invariably supported experimental results and showed clearly the wide occurrence of the phenomenon in the Kingston a r e a .

The most dramatic and clear-cut c a s e s of excessive expansion were found in exposed slabs on ground where differential movement had occurred due t o difference in mois- t u r e environment between the top and bottom surfaces. This has produced the now

familiar pattern cracking in sidewalks, floors, exposed footings, and loading platforms

(Fig. 3 ) .

Where moisture conditions have been relativelv uniform, o r where the concrete elements were thidk o r of low surface a r e a to volume ratio, excessive expansion is evidenced by closing of joints and ex- trusion of joint f i l l e r (sidewalks), buck- ling (curbs, floors), and cracking in ren- dering o r brick superstructure.

An interesting feature of affected, exposed concretes that have resisted other environmental factors over many y e a r s , is that the concrete sections within the crack boundaries a r e essentially intact and the edges at the cracks haveremained sharp

(1,

2 ) .

Special study was made of many c a s e s where no evidence of excessive expansion had occurred. Almost without exception, this was found to be due to the absence of c o a r s e aggregate, as in some r e p a i r top- pings on sidewalks; the presence of a low proportion of c o a r s e aggregate of s m a l l maximum size; o r the u s e of non-reactive stone. In some instances, expansion had

been retarded by early drying, as in

slender bridge railings.

Basementand garage floors w e r e a t F i g u r e 3. T y p i c a l p a t t e r n c r a c k i n g i n s i d e -

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inasmuch as c u r s o r y observation revealed no cracking. When t h e s e w e r e wiped with a damp cloth o r flushed with w a t e r , t h e f a m i l i a r p a t t e r n cracking became visible. I t w a s e s t i m a t e d by t h e Division that a t l e a s t 70 p e r c e n t of t h e concrete i n the Kingston a r e a was visibly affected to a g r e a t e r o r l e s s e r extent by t h i s reaction.

Kingston Limestones

T h r e e limestone q u a r r i e s had been t h e main s o u r c e s of c o a r s e aggregate f o r con- c r e t e in t h e a r e a f o r many y e a r s p r i o r to t h e s t a r t of t h e studies by this Division. Field evidence showed that rock f r o m a l l t h r e e had been responsible f o r e x c e s s i v e expansion and cracking of local field c o n c r e t e s . One of t h e two l a r g e q u a r r i e s , h e r e - a f t e r designated as q u a r r y A, was studied m o s t exhaustively in t h i s laboratory. It had been the s o u r c e of c o a r s e aggregate in t h e B a r r i e f i e l d c a s e . Quarry B was a l s o studied in considerable detail. The reactivity of t h e rock in q u a r r y C, a s m a l l e r q u a r r y , mined by t h e penitentiary in Kingston, was established largely through observations of field c o n c r e t e s .

The v a r i a b l e reactivity of the r o c k s in q u a r r i e s A and B is given in Table 1 in t e r m s of expansion of concrete b e a m s . The most expansive rock w a s t h e 0- t o 24-ft section in q u a r r y A . A s c o a r s e aggregate, i t had t o be c l a s s e d as physically satisfac- t o r y f o r concrete when evaluated by conventional acceptance t e s t s and specifications accepted a t that t i m e . Despite t h e lower expansive tendencies produced by the r o c k s i n q u a r r y B, laboratory and field evidence showed that f a i l u r e of c o n c r e t e o c c u r r e d when a high alkali cement was u s e d . It i s noted that a v e r y low cement a l k a l i is r e - quired in the e x t r e m e c a s e s t o produce a n essentially non-expansive c o n c r e t e . T h e distribution of r o c k beds of high, moderate, and low expansivity, makes it virtually

TABLE 1

EXPANSION OF CONCRETE' MADE WITH ROCK FROM DIFFERENT LEVELS IN QUARRIES A AND B

Cement

Q u a r r y _Alkali, Linear Expansion ($)

as Na20 6 Mo. 12 Mo. 18 Mo. 24 Mo.

( f t )

_(S)

Q u a r r y A: 0-24

Q u a r r y B: 0- 12

'prisms, 3 x 4 x 16 in. IvIix = 1:2:3l/,

Condition: 100% R.H. M a x . size rock =

3/,

in.

(8)

impossible to develop selective quarrying in these two particular q u a r r i e s . In a l a t e r section concerned with the nature of the reaction, f u r t h e r consideration is given t o variability in reactivity.

Early geological investigations in the vicinity of Kingston were made by the Ontario Bureau of Mines (10) and by the Department of Geological Sciences, Queen's University, Kingston. These indicated that the limestones belong to the Black River Group of the Ordovician System. Following the early studies of the Division of Building R e s e a r c h which revealed the problem of expansion in concrete, m o r e detailed geological work was c a r r i e d out by Maycock (Department of Geological Sciences, Queen's University, M. S . t h e s i s , unpublished). The petrological r e s u l t s of this study w e r e essentially in accord with the conclusions reached by the Division.

Maycock a l s o investigated the problem of alkali reactivity of the r o c k s . His r e s u l t s , based on "limited expansion data, " disagreed with those obtained in this laboratory only in the magnitude of expansion a s recorded on concrete beams.

Recommended Field P r a c t i c e

The preliminary experiments c a r r i e d out at the Division of Building R e s e a r c h provided sufficient information to p r e s c r i b e m e a s u r e s which would permit the continuing u s e of the local aggregate in the Kingston a r e a

(9).

Subsequent laboratory studies and field performance have supported these e a r l y recommendations.

Laboratory t e s t s showed that expansion of concrete with highly reactive carbonate r o c k could be reduced to "safe" values only if the alkali content in the cement is below 0 . 4 5 o r 0.40 percent (total alkali calculated a s NazO). Thus, the normally accepted maximum of 0.60 percent total alkali in low alkali cement is not adequate f o r the highly expansive carbonate r o c k . A s p e c i a l low alkali cement was produced by a cement plant in Ontario f o r t r i a l u s e a t Kingston. T h r e e c a s e s where concrete was made with this cement and the reactive aggregate f r o m q u a r r y A have been under observation by the Division up to the present (6 y e a r s ) . In each c a s e the concrete has not shown visible signs of d i s t r e s s due to excessive expansion and cracking.

In laboratory experiments, concrete p r i s m s made with the highly expansive limestone and a cement alkali content a s low a s 0.36 percent (total as NazO) have gradually expanded to an excessive extent a f t e r two to s i x y e a r s when exposed continuously in a fog room. P a r t i a l drying of the concrete a f t e r curing has, however, been shown to r e t a r d expansion even with the most reactive combinations ( F i g . 4). Thus normalfield concrete made with cement of sufficiently low alkali content should perform satisfac- torily f o r a full lifetime, except in those s p e c i a l c a s e s where the concrete remains continuously wet.

The durability of concretes made with special low alkali cement was investigated thoroughly, using stone f r o m various beds in q u a r r i e s A and B . Freeze-thaw t e s t s consisted of 6 hr _{freezing in a i r a t 18 F and 6 h r thawing in w a t e r . After about 3,650}

cycles (over a 6-yr period), air-entrained concrete made with the most reactive aggregate and the s p e c i a l low alkali cement showed no measurable deterioration, and compared favorably with reference concrete made with aggregate of good durability history. With high alkali cement the concretes made with reactive aggregate had expanded excessively a t about 150 cycles, and showed a s h a r p reduction in dynamic modulus a f t e r about 300 cycles.

Companion s a m p l e s to these w e r e subjected to wetting-drying cycling s i m i l a r t o the Scholer t e s t (11). Again, the concretes made with expansive aggregate and low alkali cement d i d y o t expand excessively a f t e r about 500 cycles, whereas the concrete made with the high alkali cements expanded a t about the s a m e r a t e a s those exposed in the fog room (2).

Early in txe investigation, t e s t s w e r e made t o determine the effectiveness of inhibitors normally found effective in reducing expansion due to alkali-silica reaction. Typical r e s u l t s w e r e published in a previous paper

(2).

Ten pozzolanic materials w e r e selected, one being a well-known California calcined s h a l e . The usual c o n c r e t e p r i s m s w e r e made with the reactive Kingston carbonate rock as coarse aggregate. The pozzolans w e r e used a s a 25 percent replacement of the high alkali cement. Compared

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T I M E , MONTHS ( I M O z 2 8 DAYS1 2 4 2 0

-

C 6 16 U CT W 0.

-

12 Z F i g u r e

4.

Expansion of t e s t c o n c r e t e s f o l l o w i n g i n i t i a l d r y i n g p e r i o d . I I I I

A _{AVG OF TWO 3 x 4 1 16 INCH PRISMS} 1 CURED 7 DAYS 7 0 DAYS 5 0 %

I R H AND 73'F THEN IMMERSED IN WATER I -- I I I 1 1 / I

with the reference specimens the concrete beams containing pozzolan showed varying degrees of reduced expansion for the f i r s t 16 months of continuous moist room storage. At this age the most highly active pozzolans could be considered a s effective. At 2 years, however, all expansions had attained the s a m e magnitude. These disquieting results were confirmed by a test using a known chemical inhibitor of alkali-silica reaction, lithium chloride, in which no reduction in expansion occurred at any time up to 2 y e a r s .

Mortar b a r test results (1) showed that the same calcined shale effected a considerably reduced expansion up t o 3 9 months. Despite this, and because the concrete prism test was considered more direct and realistic, it was concluded that inhibitors normally found effective f o r alkali-silica reaction could not be relied on to control expansion in the alkali-carbonate rock reaction. It appears that this laboratory i s the only one that has reported test results on the effectiveness of pozzolans. Further investigation is obviously needed.

On the basis of these test results, consisting of tests on more than 500 concrete prisms, with observations extending to more than 6 years, the Division recommended that, where the reactive Kingston dolomitic limestone aggregates were to be used in concrete, the alkali content of the cement (calculated a s NazO) should not exceed 0.40 o r 0.45 percent (2, - - 9). Because laboratory tests had shown that the rate and ultimate extent of expansion decreased with decreasing maximum size of coarse aggregate (2), it was also recommended that maximum stone size be kept to a minimum consistent with good quality concrete. Minimum cement content and minimum coarse aggregate content, also consistent with quality concrete, were recommended. Blending o r dilution of reactive aggregate with non-reactive aggregate had been shown to yield expected reduction in expansion. Where possible this was suggested a s an additional precautionary measure.

It was further recommended that special attention be given to expansion joints and that provision be made f o r permitting the concrete to dry out after curing. When carbonate rock aggregate derived from sources other than the quarries investigated, it was urged that it be tested f o r reactivity before u s e .

Apart from the t r i a l u s e of low alkali cement, the extent to which the foregoing recommendations have been practiced in Kingston has not been followed closely. To a very considerable extent, outside aggregates of non-carbonate type a r e now beingused

0

Y)

z

:

A REACTIVE LIMESTONE t HlGH ALK CEMENT

4

dB "

0. 0 8 t LOW

.a ,c

X

w I C REFERENCE LIMESTONE t HIGH ALK CEMENT

CT CRACKS VISIBLE I " + L O W cm 4 W 0 4 1 - A I n I 1 B

;_c__-

I 0 c 8 -- - -- -. I 0 3 6 9 12 15 I 8 21

(10)

in place of the limestones f r o m the existing q u a r r i e s . In a recent shopping center development, however, the reactive limestone was used with a high alkali cement, with predictable r e s u l t s : sidewalks show extensive map-cracking.

STUDIES ON THE NATURE OF THE REACTION E x ~ a n s i o n of Concretes

The i n c r e a s e in r a t e and degree of expansion with i n c r e a s e in alkali content in laboratory concrete made with reactive carbonate rock a s c o a r s e aggregate, was found to hold whether a l l the alkali derived f r o m the cement o r whether p a r t of t h e alkali was added ( 2 ) . This effect is shown in Figure 5, which may be compared with values in ~ i g u r e 3 . F i r s t visual evidence of cracking of 3- by 4- by 16-in. concrete p r i s m s o c c u r r e d a t expa~lsions f r o m 0 . 0 4 to about 0 . 0 8 percent. It is also s e e n that the sodium f o r m of the alkali gives a m o r e s e v e r e reaction than the potassium f o r m a t the higher alkali content.

In making comparisons between the expansion values of the various concrete t e s t specimens described herein, one must b e a r in mind that the length changes which a r e measured a f t e r cracking has begun, may b e greatly influenced by mechanical and other f a c t o r s not directly associated with the reaction p r o p e r . It is therefore desirable to rely mainly on expansion data up to about 0 . 0 5 percent. This applies equally to length change information on r o c k p r i s m s described in a l a t e r section.

T e s t s with 15 cements of varying composition showed that cement components other than alkali appeared t o have no significant influence on the expansive properties of t h e reactive c o a r s e aggregate.

In one s e r i e s of comparative t e s t s with concrete p r i s m s , the r a t e and degree of expansion was highest under conditions of wetting-drying cycling ( l l ) , slightly l e s s in fog-room conditioning a t 73 F , considerably l e s s in freeze-thawcycling, and lowest in

continuous outside exposure. In a l l c a s e s , however, expansion was excessive and cracking o c c u r r e d

(1).

The reaction was obviously affected by variations in moisture and t e m p e r a t u r e .

In another experiment concrete p r i s m s w e r e cured f o r 7 days, d r i e d f o r 63 days a t 50 percent relative humidity and 73 F , and then placed in water a t 73 F ( 2 ) . No expansion o c c u r r e d during the drying period, but rapid expansion followed i m m e r s i o n in water ( F i g . 4). These r e s u l t s show the d i r e c t dependence of the reaction on m o i s t u r e .

AVG OF TWO 3 1 4 x 1 6 INCH PRISMS, CONDITIONED A T /

I

/

i p L u ~ K o H ~ ~ ~ l l ~

&.--~-

'

A P L U S N o O H TO 0 . 7 1 % A i i I I 0 3 6 9 12 15 18

TIME, MONTHS ( I M 0 = 2 8 DAYS 1

F i e

5 .

C o n c r e t e e x p a n s i o n w i t h r e a c t i v e c a r b o n a t e r o c k and low a l k a l i cement r j i t h added a l k a l i s .

(11)

28

AVG OF TWO 3 ~ 4 x 1 6 INCH PRISMS

1 0 0 % R H 1 1 1 I I

T I M E , MONTHS ll M 0 = 2 8 D A Y S I

F i g u r e 6. E f f e c t o f t e m p e r a t u r e on e x p a n s i o n o f c o n c r e t e made w i t h r e a c t i v e c a t . b o n a t e r o c k .

Similar concrete p r i s m s were conditioned a t 73, 100 and 130 F a t 100 percent R . H.

The highest expansion occurred at 100 F (Fig. 6), a t which temperature even low alkali cements produce excessive expansion. These results, discussed in more detail in a previous paper (2), show a c6rtain dependence of the reaction on temperature. These results suggest that the reaction is essentially chemical in nature.

Petrographic examination of concrete made with aggregate from the top 24 ft of quarry A showed that "reaction r i m s " did not surround a l l the coarse aggregate particles. At that time it was suspected that r i m formation was related in some way to the expansive reactivity. It was therefore concluded that the s t r a t a represented in the top 24 f t of quarry A contained beds which differed in reactivity.

Concrete beams were made in which the aggregate was taken f r o m selected beds within the 24-ft section. Test results verified the supposition of varying reactivity a s the beams showed different r a t e s of expansion (Fig. 7 ) . This does not necessarily support the postulated relationship, however, between expansive reactivity and r i m formation. The nature of the r i m s has not been established for the Kingston c a s e and further investigation would be required to determine whether a connection exists between r i m formation and expansion. Some non-expansive rim-forming dolomitic limestone aggregates in Iowa have been described (7). -

Expansion of Carbonate Rock in Alkaline Solutions

Extensive experiments showed that the reactive dolomitic limestones expanded excessively when immersed directly in 2-molar alkaline solutions; the r a t e and degree of expansion were dependent on concentration of alkali ( 2 ) . Companion p r i s m s were immersed in water and similarly measured for comparative purposes. These p r i s m s showed no significant expansion. Non-reactive rocks did not expand excessively in strong alkali.

One method consisted of saw-cutting rock p r i s m s of 1% by 1% by 5% in., planing the ends, and measuring in a mortar b a r comparator with adaptor plates f o r the ends of the p r i s m s . Expansions were of the s a m e o r d e r a s for concretes containing reactive coarse aggregate (Fig. 8). Similar o r d e r s of expansion were obtained by vacuum saturating coarse aggregate with alkaline solutions and measuring the total volume change of aggregate plus solution by means of capillary tubes.

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. 2 0 . I I I I I I Apparatus was developed f o r

CEMENT measuring expansion of the reactive HA = HIGH ALKALI

. I 8 - L A = LOW ALKALI - rock in powdered form (12). This consisted of a cell in w h x h the pow- L E V E L dered rock was contained in a com- 16 - A . 6 , - 7 , - pression chamber. The sample was

B = 10'1; - 12' confined under a p r e s s u r e of 5 lb be-

c = 2 0 ' - 21' _{tween a porous plate of stainless} s t e e l and a rubber membrane. Vol- ume' change was recorded in a 1-mm bore precision capillary tube. The compression chamber communicated via the porous plate with a r e s e r v o i r . This was filled with water f o r the initial portion of the experiment until equilibrium was established, and

-

then the water was replaced by 2M alkali. Lf _{100 m l of 2M NaOH and}

about 30 gm of Kingston rock were employed, the reaction went to com- pletion. A total expansion of more than 7 percent of the initial solid volume was recorded in some instances. Rate and total expansion varied with the particle s i z e of the sample.

1 Petrography

0 2 8 5 6 8 4 112 140 168 196

In the spring and summer of 1958

F i g w e 7. D i f f e r e n t e x p a n s i o n s i n c o n c r e t e

c a u s e d by K i n g s t o n a g g r e g a t e from s p e c i f i c the authors began a detailed investi-

s t r a t a i n q u a r r y A. gation of the petrology of the rocks

near Kingston (13). Rocks were obtained a t various localities in the a r e a , but sampling was most detailed in the quarries from which the reactive aggregates had been obtained. The rocks a r e well stratified and t h e r e a r e some beds of the o r d e r of 1 f t o r s o in thickness. The rocks a r e very fine-grained dolomitic limestones which weather pale to dark grey. Bulk specific gravity was 2 . 7 and absorption 0 . 7 percent. There a r e also certain beds that reach a thickness of about 6 ft, and weather pale green. The dolomite, clay and quartz content is higher in this type of rock. It is also more porous and has a high absorption of 3 percent. Some of the quartz crystals a r e visible to the naked eye a s rounded grains about 1 mm i n diameter. The content of CaC03 (calcite)

is low. This greenish rock, which produced only very slight expansion when used a s

aggregate in concrete, is properly classified a s an argillaceous dolomite. The highly

reactive rocks, on the other hand, contained more calcite and a r e correctly described a s dolomitic limestones. In these rocks the porosity was about 0 . 5 percent and the acid-insoluble content was lower than in the greenish rock.

In one quarry specific beds were sampled and the rock used a s aggregate f o r p a i r s of concrete beams, In each case one pair of concrete beams was made with high alkali cement and an additional pair was made with low alkali cement. The concrete beams were stored a t a relative humidity close to 100 percent a t 73 F . Rock f r o m different beds, which was quite s i m i l a r petrographically, caused different r a t e s of expansion in the beams made with high alkali cement (Fig. 8). The reactive rocks, however, were a l l of a s i m i l a r petrographic type.

Microscopic thin sections were prepared f r o m the rock in the natural condition, f r o m the rock after soaking in alkali, and f r o m concrete on which large expansions hadbeen recorded.

(13)

F i g u r e 8. 0-0 v ,, , 0 4 4 81 A - V ., , 0 0 6 8, 16-*-• V , WATER SEE NOTE I 2 0 8 .

H = BEDDING PLANES PERPENDICULAR TO LENGTH 0 4 " PARALLEL "

.

-

TIME, MONTHS ( I M 0 = ? 8 DAYS1

Expansion of r e a c t i v e c a r b o n a t e r o c k prisrns i n a l k a l i n e s o l u t i o n s of d i f f e r e n t c o n c e n t r a t i o n s (6- t o 7 - f t bed, q u a r r y A ) .

In the reactive Kingston rocks ( F i g . 9) dolomite occurs a s well-formed crystals which a r e generally of rhombic shape and attain a s i z e , a t most, of 0.05-mm (50- microns) cross-section. The crystals often contain inclusions of dusky material, which i s probably clay, and this i s sometimes regularly arranged with respect to prominent crystallographic planes (Fig. 10).

Only a s m a l l amount of iron and manganese was found on chemical analysis of dolomite concentrates (13). This makes it unlikely that the included material represents carbonate such a s ankerite, ferrodolomite o r kutnahorite formed, either by zonal growth upon, o r by exsolution f r o m within, the parent lattice. Differential t h e r m a l analysis also gave no indication of the presence of ankerite.

The dolomite i s of fairly even distribution when the rock i s considered on the scale of hand-size specimens, but t h e r e i s some lack of uniformity on the microscale. Ob- servational evidence shows that the dolo-

mite i s not sedimentary and i t s crystal-

F i g u r e 1 0 . Photomicrograph of r e a c t i v e r o c k from Kingston, Ont., showing dolomite c r y s - F i g u r e 9 . Photomicrograph of a l k a l i - r e a c - tal c o n t a i n i n g i n c l u s i o n s arranged w i t h

(14)

lization postdated the deposition of the rock. This dolomitization process was to some extent selective and accounts for the slight lack of regularity in dolomite distribution. During deposition, wave o r current action broke up partly consolidated sedi- ment which was then redeposited in quieter water. Thin sheets accumulated of irreg- ular particles, 0.2 to 0.1 mm across, composed of very fine calcite. Space between the particles is often filled with material that contains some larger crystals of clear calcite which petrographic criteria indicate to have formed a s a void and pore filling cement. These layers a r e interbedded with more homogeneous material consisting of micron-sized calcite which appears dusky brown in section under the microscope. It shows little o r no sign of recrystallization into larger crystals. This fine calcite is evenly mixed with clay, fine quartz and other tiny crystals of detrital minerals. This calcite-mud mixed with clay and fine detrital minerals makes up the larger proportion of the rock.

The relatively unreactive greenish rock differs microscopically from the reactive rocks. There i s much less calcite and more quartz and clay. The dolomite is present a s crystals which a r e well formed, a s in the reactive rocks.

Thin sections were prepared from reactive rocks which had been soaked in 2MNaOH. Under the microscope they appeared quite similar to untreated rock. Dolomite rhombs were still intact and qualitative study did not reveal that dedolomitization had occurred

(2, -

3).

Micro-cracks were present in some sections.

Examination of polished surfaces of affected concrete showed thin dark r i m s s u r - rounding some of the aggregate. Such r i m s were less apparent when thin sections were examined. In reacted concrete, cracks were visible which intersected both aggregate and cement paste.

The most significant features of the petrography of the reactive rocks a r e a s follows: 1. There is a considerable amount of both dolomite and calcite in the carbonate fraction.

2 . The dolomite crystals a r e well-formed rhombohedra often reaching sizes of about 0.05 mm across o r larger. Other minerals in the rocks a r e commonly finer and below the limits of resolution of the optical microscope. This imparts a fine-grained texture to the rocks.

3 . There a r e clay minerals distributed throughout the carbonates sometimes giving

the rock a murky appearance to microscopic observation. 4 . They often have a low porosity.

Mineralogy

The minerals of which the Kingston rocks a r e composed have been described in detail (13). Microscopic, X-ray diffraction, and differential thermal analyses were performed on concentrates and residues.

Separations. -Su£ficient dolomite was concentrated from the reactive rocks for a partial chemical analysis and f o r analysis by powder X-ray diffraction. The mineral i s less soluble in acid and has a higher specific gravity than calcite; it also occurs a s larger crystals in the Kingston rock. These differences were exploited in the separa- tion procedure adopted. Sink-float methods in heavy liquids, a Frantz isodynamic magnetic separator, and sedimentation techniques were a l l employed. As clay occurs a s inclusions within the crystals it was not possible to obtain a completely pure concentrate of dolomite.

Small concentrates of other minerals separated at various stages in the separations were also examined.

The acid-insoluble fraction of the rock was also separated. During this work the possibility of dissolutior. by acid treatment of swelling clay minerals was considered. Some of these, notably hectorite, a r e known to be acid soluble. A technique in which solution of the carbonate was effected by ion exchange resin was adopted. This was an adaptation of a procedure described by Ray, Gault and Dodd (14). Investigation showed that it was possible to detect hectorite, added to the sample7in a concentration of only 1 percent. In later phases of the work, dilute acetic acid and dilute hydrochloric acid were used. Prior to analysis, the clay minerals were flocculated with CaC12 to replace hydrogen with calcium ions on the exchange s i t e s .

(15)

G- 10- 14' 18' 72' 2d 30' 3 3 38' 4 2 ' 46' 50' The Carbonates. -In ideal dolomite t h e r e a r e a n equal num- b e r of calcium and magnesium ions p e r formula unit. It is known, however, that dolomite may in- corporate other ions in the lattice and also that t h e r e may be a n ex- c e s s of calcium above the ideal 50 mol percent (15). Such dolomite is known a s ~ r o t o d o l o m i t e and is metastable although it may p e r s i s t f o r periods of t i m e meas- u r a b l e on the geological s c a l e . It may be detected by X-ray diffraction a s t h e r e a r e c h a r a c t e r i s - t i c effects on i t s powder pattern by which it may be distinguished f r o m ideal dolomite.

Substitution of iron o r manganese in place of magnesium may

6' 10' 14' 18' 22' 26" 30' 34' 38' 42' 46- 50- produce s i m i l a r effects on the

~ I ~ ~ I I I I I I _{powder X-ray diffraction pattern}I *

HEATEV 5 5 0 " ~ I HR

O q e

t o those produced by substitution

of calcium f o r magnesium

(15).

='<

I ,

, The p a r t i a l chemical analyses

'!'

-3 showed the dolomite concentrated

f r o m the Kingston rocks contained only about 1 percent i r o n and even s m a l l e r amounts of manganese.

1 1 I t I I I I I

The quantities found w e r e insuf- ficient to account f o r the effects

S E T T I N G OF GENERAL E L E C T R I C CO. D I F F R A C T O M E T E R _{observed on the X-ray diffraction}

X R D - 5 _{p a t t e r n s . It was concluded that}

E X I T S L I T : 3' T I M E C O N S T A N T : 2

x - R A D I A T I O N : c r t h e dolomite in a l l the Kingston

S O L L E R S L I T S : M E D I U M R E S O L U T I O N

R E C E I V I N G S L I T : 0 . 2 ' F I L T E R : V rocks examined was of the meta- R E S P O N S E : L I N E A R K V : so s t a b l e type in which t h e r e is an

F U L L S C A L E D E F L E C T I O N : 2 0 0 c.p.5 m n : 1 6 e x c e s s of calcium ions incorpo- S C A N N I N G R A T E : I ~ M I H . r a t e d in the lattice. A s i m i l a r

study of calcite in Kingston rocks

Figure11.X-raydiffractometerpatternsof a c i d - ~ h o w e d t h a t s m a l l a m o u n t s o f i o n s

i n s o l u b l e f r a c t i o n of composite K i n g s t o n r o c k which _as_magnesium_with_a_smaller

had b e e n soaked i n 2M a l k a l i f o r s e v e r a l months. radius than calcium were held in

solid solution in the m i n e r a l . Clay Minerals. -The mineralogy of the clays in the acid- insoluble fraction of the Kingston rocks was investigated in detail. It was considered possible that swelling clay minerals w e r e p r e s e n t in the rock o r developed f r o m non- swelling clays a s a r e s u l t of the action of alkali. F o r this r e a s o n t h e methods of sep- aration which w e r e found to be least destructive to clay minerals w e r e employed, in c a s e acid-soluble expansive clay m i n e r a l s w e r e p r e s e n t . No expanding lattice clay minerals w e r e found, however, e i t h e r in the original rock o r in the rock which had been soaked in 2M NaOH. Clay m i n e r a l s separated f r o m rock which had been soaked in alkali w e r e not detectably different f r o m those in the untreated rock ( F i g . 11). The clay minerals identified w e r e illite with l e s s e r amounts of chlorite. These identifica- tions w e r e made by X-ray diffraction and differential t h e r m a l methods of analysis.

The illite was found a l s o to b e intergrown with variable amounts of organic m a t e r i a l

(g)

.

Residues. -Small concentrates of d e t r i t a l minerals w e r e obtained a t various s t a g e s of the separations. Tourmaline, limonite, pyrite, and ceylonite w e r e among minerals

(16)

identified by microscopic methods. In s o m e instances identification was confirmed by powder X-ray diffraction photographs.

An attempt was made to concentrate active s i l i c a , s u c h a s opal, which may have been p r e s e n t in the r o c k . This m i n e r a l has been held responsible f o r s o m e of t h e w o r s t c a s e s of alkali-silica reaction (16), and although t h e r e was very little microscopic evidence f o r gel formation, i t s presence was suggested by adsorption studies (17). The acid-insoluble fraction of the r e a c t i v e Kingston rocks was studied by the s i z - f l o a t technique using a liquid of specific gravity 2 . 4 . Very little m a t e r i a l was light in this

liquid and no active s i l i c a was detected in any s a m p l e . Chemistry

Chemical analyses w e r e obtained on the r o c k s as total solid and on the acid-insoluble fraction. P a r t i a l chemical analyses w e r e a l s o obtained on t h e dolomite concentrated f r o m s o m e of t h e r o c k s . The reactive r o c k s have a n acid-insoluble content of 6 to 17 percent and the non-reactive greenish r o c k contains 40 to 45 percent acid insoluble. A

computation was performed in which oxides w e r e recombined into m i n e r a l s . This supported the X-ray and petrographic finding that the r o c k s a r e of intermediate composition between limestone and dolomite. The analyses showed that the acid-insoluble f r a c - tion of the reactive r o c k s is quite s i m i l a r t o a n "average shale1' (18). P y r i t e is known t o occur in the rocks but computation showed that sulfur is l i k e l y 5 be combined with carbon a s well a s iron.

T h e r e was no apparent relationship between reactivity and oxide content o r computed m i n e r a l composition.

Studies of Other Carbonate Rocks

It has been recognized for many y e a r s that rocks which a r e of intermediate composition between limestone and dolomite a r e l e s s common than a r e the end m e m b e r s of the s e r i e s (19). The reactive r o c k s f a l l within t h i s relatively uncommon range of composition.

TKS

suggested that other r o c k s of this general composition might display s i m i l a r reactivity t o those f r o m Kingston. It was decided t o a s s e m b l e a s u i t e of carbonate r o c k s covering the composition range f r o m limestone t o dolomite. The dolomite was apparently the only mineral in the Kingston rock attacked by s t r o n g alkali. Insofar a s possible the rocks obtained w e r e low in acid-insoluble content. It was hoped by t h i s means to verify the tentative conclusion that the acid-insoluble minerals played no p a r t in causing expansioi~. At the t i m e , this a l s o s e e m e d to b e supported by the composition of the Kingston r o c k s , s o m e of the most reactive of which had an acid-insoluble content of only about 5 percent.

During the autumn and winter of 1959-60 a s u i t e of m o r e than 50 rocks was a s s e m - bled. The rocks covered the composition range f r o m limestone to dolomite and w e r e generally low in acid-insoluble minerals

.

Considerable difficulty was experienced in obtaining rocks in the intermediate range of composition which w e r e a l s o low in acid i ~ i s o l u b l e . Samples w e r e only obtained through the cooperation of many individuals in public and private c o n c e r ~ i s in Canada, the United States and the United Kingdom. Additional s a m p l e s , known t o be reactive, w e r e supplied t o the authors f r o m Virginia and Indiana by H . Newlon, D. Hadley, and W. L . Dolch.

The rocks w e r e crushed and s i z e d and used a s aggregate for concrete b e a m s . Each s a m p l e was used f o r two p a i r s of beams; one p a i r was made with high alkali cement and oiie p a i r with low alkali cement. T h e s e beams w e r e conditioned in t h e fog room at 73 F and length change r e c o r d e d . Rock p r i s m s w e r e a l s o cut and i m m e r s e d in 2M alkali. Length measurements w e r e recorded a t weekly intervals.

The rocks w e r e analyzed for Ca, Mg, F e and acid-insoluble content. More than 60 microscopic thin sections of these rocks and m o r e than 100 thin sections of the concrete w e r e p r e p a r e d and examined. As anticipated, the rocks differ widely in petrographic c h a r a c t e r . Degree of recrystallization, s i z e of c r y s t a l s , distribution of dolomite, grain shape, and porosity a l l v a r y . The majority of the r o c k s , however, caused no abnormal expansion in the concrete beam o r rock p r i s m t e s t s . None of the rocks with a very low acid-insoluble content caused expansion but considerable expansion was

(17)

recorded f r o m rock containing about 5 percent acid insoluble. Reactive r o c k s had in common the invariable p r e s e n c e of dolomite, calcite, and acid insoluble although the proportions varied. They a l s o s h a r e d s o m e o r a l 1 of the petrographic f e a t u r e s of the Kingston rocks, s o m e of t h e most reactive being closely s i m i l a r in composition and petrography to those f r o m Kingston.

METHODS O F TEST AND IDENTIFICATION ASTM Tentative Test Methods

This laboratory had previously concluded that the ASTM methods of t e s t f o r cement- aggregate reaction failed to classify the reactive Kingston rock a s excessively expan- s i v e in concrete (1, - - 2). In the m o r t a r b a r t e s t (ASTM C227) the reactive Kingston limestone showed "more than t h e n o r m a l expansion with high alkali cements, " but this expansion did not exceed t h e then accepted limits of 0 . 0 5 percent at 3 months and 0 . 1 0 percent a t 6 months f o r alkali-reactive a g g r e g a t e s . E a r l y uncertainties as to t h e nature of the reaction led this laboratory to c a r r y out a n extensive s e r i e s of C227 t e s t s , involving s o m e 100 m o r t a r b a r s . The main variables studied w e r e ( a ) different reactive and non-reactive m a t e r i a l s , (b) cement and cement-alkali contents, ( c ) water- cement r a t i o s , (d) aggregate gradings, ( e ) possible i n h i b i t o x , and ( f ) period of t e s t . None of the modifications attempted made this method m o r e effective f o r this p a r t i c u l a r c a s e . Necessary reduction of the expansive rock to sand s i z e , a s required by this t e s t , reduced the extent of expansion considerably a s compared with the expansion of con- c r e t e p r i s m s made with the s a m e rock a s c o a r s e aggregate (2').

F r o m these studies, of which typical r e s u l t s have been published (1, 2), i t was concluded that the m o r t a r b a r t e s t could not b e made applicable to the Kixgston m a t e r i a l u n l e s s a new s e t of lower limits w e r e established to distinguish excessive f r o m non- excessive expansion. When subsequent studies showed that t h e Kingston phenomenon was something different f r o m the alkali-silica reaction, f o r which t h e ASTM t e s t s had been developed, the "apparent weakness" of t h e s e t e s t s became understandable. On t h e b a s i s of t h e work done in t h i s laboratory, t h e concrete p r i s m t e s t , discussed i n a l a t e r section, is considered s u p e r i o r to the m o r t a r b a r t e s t .

The quick chemical t e s t (ASTM Method C289) did not r e v e a l the reactive Kingston rock a s deleterious. The inadequacy of this t e s t f o r dolomitic limestones has been long recognized (20). When applied to the insoluble r e s i d u e portion of the rock, t h e r e s u l t s w e r e i n c o ~ c l u s i v e , which is consistent with a non-siliceous reactivity ( 2 ) .

The Conrow t e s t (ASTM Method C342) also failed to show t h e deleterious e G a n s i o n i n t h e reactive Kingston r o c k . This could be expected as this t e s t is highly specific f o r c e r t a i n siliceous aggregates which a r e not necessarily alkali r e a c t i v e . This lab- o r a t o r y c a r r i e d out this t e s t on m o r e than 100 m o r t a r b a r s made with various combinations of m a t e r i a l s .

Expansion of Concrete Beams

The most reliable t e s t f o r determining t h e expansive properties of reactive carbonate rock is the measurement of length change of concrete p r i s m s ( 9 ) . Direct comparison can be made between aggregates and between low and high alkali cements. Use can be made of standard laboratory facilities s u c h a s molds, i n s e r t s , comparator, and fog r o o m . The one disadvantage is that s e v e r a l months may be required f o r c a s e s of moderate to low reactivity. The t e s t may be accelerated, however, by conditioning a t 100 F and 100 percent R . H . , r a t h e r than in a curing room. Conlparison s a m p l e s of non-expansive combinations a r e n e c e s s a r y in this t e s t .

Establishing limits f o r the expansion that can b e tolerated i n field concrete would depend on the function of the concrete element in question. It is suggested that, f o r ordinary c a s e s , a limit of 0.02 percent in 6 months be placed on p r i s m s of 3 by 4 by 16 in. conditioned a t 73 F and 100 percent R . H .

Laboratories having apparatus f o r the Scholer wetting-drying t e s t can obtain v e r y s i m i l a r r e s u l t s to those obtained by t h i s t e s t (1, - - 2 ) .

(18)

Expansion of Rock P r i s m

Measurement of length change of saw-cut rock p r i s m s i m m e r s e d in 1 to 2M solutions of alkali will produce expansions of alkali-reactive carbonate rock of the s a m e o r d e r a s f o r concrete p r i s m s

(2).

C a r e must be taken to avoid c r a c k s , joints, cleavages o r other planes of weakness that may produce spurious r e s u l t s . This may b e largely over- come by the u s e of s m a l l e r specimens a s in a comparable t e s t developed by the P o r t -

land Cement Association

(3).

The t i m e f o r this t e s t to yield significant r e s u l t s is f r o m two to s i x weeks, depending on s i z e of specimen, concentration of alkali, and reactivity of aggregate. Compar- ison specimens of non-reactive carbonate r o c k a r e n e c e s s a r y .

Powder- Cell T e s t

The procedure and apparatus a r e r e f e r r e d to i n an e a r l i e r section, and in g r e a t e r

detail elsewhere (12). The powdered rock is treated with alkali in a compression

chamber fitted w i t h a rubber membrane. Expansion in the powder is measured by

liquid in a capillary tube.

This method has an advantage o v e r the r o c k p r i s m t e s t in that samples can be made m o r e representative, variations due to directional properties a r e eliminated, and the fine particle s i z e gives a rapid r e s p o n s e . The length of time to complete a t e s t is about one month. P a r a l l e l runs with non-reactive rock a r e necessary for comparison. Petrographic and Other C r i t e r i a

It is not possible to say with certainty by petrographic c r i t e r i a whether a particular rock will cause excessive expansion if used a s aggregate with high alkali cement. Ag- gregates s o f a r known which cause trouble a r e , however, apparently r e s t r i c t e d a s to r o c k type. The c r i t e r i a f o r recognition have been discussed in a previous section and in e a r l i e r publications (3, - - 13). Although t h e r e is no direct relation between chemical

composition and reactivity, s o m e d e g r e e of elimination of non-reactive rock is possible by analysis. The rocks which expand excessively contain both calcite and dolomite ranging f r o m about 40 to about 60 percent of dolomite, and an acid-insoluble content ranging f r o m about 5 to about 20 percent. They a r e invariably fine grained, and generally of relatively low porosity o r absorpition.

Exceptions to these composition c r i t e r i a have been observed. It is possible that s o m e of the apparent discrepancies may b e due to analytical f a c t o r s . The quantitative

determination of magnesia by chemical methods is subject to considerable variability

i n r e s u l t s depending on the nature of the parent m a t e r i a l and on the methods u s e d . Such differences may b e f u r t h e r magnified when the magnesia values a r e converted into percent dolomite. Determination of acid-insoluble residue is even m o r e subject to variability a s a study of the method would show.

Identification of the expansive type of alkali-carbonate reaction in affected concrete by petrographic examination is also f a r f r o m certain. It is not established that r i m formation on aggregate p a r t i c l e s is a necessary consequence of the reaction. The oc-

c u r r e n c e of f r a c t u r i n g in the m o r t a r and aggregate can r e s u l t f r o m other c a u s e s . The only known end product of the reaction is brucite; however, this may also be present in non-expansive dolomitic r o c k .

SOME POSSIBLE MECHANISM O F EXPANSION

At present t h e r e is what may almost b e described a s a plethora of suggested mech- a n i s m s which have been proposed to account f o r the expansion in alkali of reactive carbonate r o c k s . This r e s u l t s f r o m the lack of conclusive experimental evidence in support of any one hypothesis.

The extensive t e s t s on the s u i t e of carbonate rocks ranging in composition f r o m

limestone to dolomite and of varying petrography showed that expansive reactivity is

r a t h e r uncommon. It i s a l s o not likely to b e a coincidence that many of the expansive rocks f r o m different a r e a s r e s e m b l e one another petrographically. In particular t h e s e

(19)

r o c k s contain well-formed dolomite euhedra which a r e of f a i r l y even distribution in the r o c k . The groundmass consists largely of fine-grained calcite which is in the micron s i z e r a n g e and often has a dusky brown appearance in thin section. T h e r e is a n acid- insoluble content of not l e s s than 5 percent.

Mineralogical s t u d i e s showed that the clay m i n e r a l s w e r e of the non-swelling type. The s i m p l e hypothesis that expansion was due to t h e p r e s e n c e o r formation of swelling clay milierals had t h e r e f o r e to be abandoned.

An interesting approach is provided by the work of A. A. deGast (Department of Mining Engineering 1962, Queen's University, M. S. t h e s i s , unpublished). He postulated that expansion of the rock resulted f r o m r e l e a s e of r e s i d u a l e l a s t i c s t r a i n due to the removal of r e s t r a i n t . The s t o r e d energy was inherited f r o m e a r l i e r epochs and resulted f r o m geological p r o c e s s e s acting on the calcite and dolomite which have dif- f e r e n t rheological p r o p e r t i e s . In support of t h i s , r e f e r e n c e is made to work by Handin (21) and Griggs (22). Handin showed that dolomite i s ten t i m e s m o r e r e s i s t a n t to de-

formation than calcite and Griggs demonstrated that, unlike calcite, dolomite is not

susceptible to recrystallization. It is concluded that e l a s t i c s t r a i n , if p r e s e n t , is

s t o r e d in t h e composite g r a i n s of the m a t e r i a l . The supposition of s t o r e d e l a s t i c energy within the r o c k s of t h e Kingston a r e a is supported by s e v e r a l field examples

A laboratory investigation was made in which s t r a i n was measured by t h e photo- e l a s t i c technique. This is of high sensitivity and p e r m i t s the detection of intergranular s t r a i n . Measurements w e r e made on limestone wafers and cubes and a n evaluation was made of t h e s t r a i n distribution in c o n c r e t e .

It was shown that s h e a r s t r a i n s w e r e p r e s e n t around aggregate of defective con- c r e t e . Studies of the limestone p r i s m s confirmed t h e r e s u l t s obtained by o t h e r w o r k e r s that the alkaline solution is the m a j o r f a c t o r contributing to deterioration. Expansion due to e l a s t i c readjustment of inherent e l a s t i c energy was concluded to b e of minor importance.

T h e r e is additional evidence to show that r e l e a s e of r e s i d u a l e l a s t i c s t r a i n is not the underlying cause f o r expansion of the Kingston rock in alkali. In p a r t i c u l a r , little o r no r e s i d u a l intergranular s t r a i n i s likely to r e m a i n in rock which has been powdered to a c r y s t a l l i t e s i z e of the s a m e o r d e r a s the component m i n e r a l s . Large expansions have, however, been r e g i s t e r e d by Kingston m a t e r i a l in the powder-cell t e s t . T h e r e i s a l s o evidence to show that the clay fraction is involved in t h e expansive mechanism. It i s very unlikely that clay m i n e r a l s would play a s p e c i a l r o l e in retaining e l a s t i c energy in the manner postulated.

The dedolomitization reaction (2, - - 3) i s the only significant chemical change known which takes place when these r o c k s a r e placed in a n alkaline environment. Dolomite i s attacked by t h e alkali hydroxide with formation of alkali carbonate, calcium carbonate (calcite), and magnesium hydroxide (brucite) ( F i g . 12). The reaction was considered a t a n e a r l y s t a g e a s the underlying cause f o r expansion. Calculations of t h e unit c e l l s i z e s , however, showed that t h e combined volume of t h e known solid products was l e s s than t h e initial volume of t h e dolomite. The lattice p a r a m e t e r s of the b r u c i t e and cal-

cite formed in t h e reaction a l s o appear n o r m a l .

Solubility data suggest that the NazC03 f o r m e d in the reaction should p a s s into solution. T h e r e a r e hydrated double s a l t s of l a r g e c e l l volume such a s gaylussite

(Na2C03

-

CaC03. 5H20) and pirssonite (NaZCO3. CaC03 2H20). When a mixture of calcite

and dolomite was covered with 2M NaOH and left to evaporate slowly f o r s e v e r a l w e e k s , X-ray examination of the product showed portlandite and pirssonite to b e t h e dominant p h a s e s , together with brucite and calcite.

The rock-alkali mixture f o r m s a multi-component s y s t e m the phase relations of which have not been worked out. The s y s t e m sodium carbonate-calcium carbonate- water ( F i g . 13), however, although l e s s complex, includes s o m e of the most important components in t h e rock-alkali s y s t e m . Therefore, p h a s e s in the rock-alkali s y s t e m a r e likely to develop only a t quite s i m i l a r t e m p e r a t u r e s and concentrations to t h o s e shown in Figure 13. In p a r t i c u l a r it i s v e r y probable that a t 22 C a considerable concentration of Na2C03 in solution will b e required f o r the crystallization of gaylussite. The inevitable conclusion, based on any sensible assumption a s to concentration, i s that neither hydrates n o r hydrated double s a l t s a r e likely to f o r m in the rock when it i s i m m e r s e d in 1 o r 2M NaOH.

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0 ' - 2 4 ' S E R I E S C O M P O S I T E

B

0'- 24' S E R I E S A F T E R S O A K I N G I N A L K A L I F i g w e 1 2 . U e d o l o m i t i z a t i o t i OI? t r e a t m e n t w i t h a l k a l i .

C

I 0 $I- 12' B E D

10h'-

1 2 ~ B E D A F T E R S O A K I N G

I N

A L K A L I

Also t h e r e is no evidence froin X-ray diffraction studies f o r the crystallization of minerals of this s o r t in the reactive rock which expanded under conditions that p r e - vented evaporation of alkali.

Differential t h e r m a l studies of unwashed alkali-treated powders gave complex c u r v e s . Although t h e r e was s o m e correspondence to t h e r m o g r a m s of gaylussite, the evidence was f a r f r o m convincing because alkali-treated powders which failed to produce expansion in the powder-cell t e s t also gave thermograms which w e r e closely s i m i l a r to those f r o m the reactive rock ( F i g . 1 4 ) . The situation i s also complicated by the presence of Na,C03 and possibly by hydrates formed on drying the m a t e r i a l a f t e r its removal f r o m the alkali.

It was concluded that expansion did not r e s u l t f r o m formation of hydrated double s a l t s o r alkali-carbonate hydrate for the following reasons:

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C A L C I U M C A R B O N A T E

4

I I I I

LVAPOUR

- -

P

E - -

/"

LO 0 - - - - - - - - - ( I C E I

-

10

a

Na, CO, , %

F i g u r e 13. The s y s t e ~ ; ~ sodiwn carbonate-calciurri carbonate-water. ( a f t e r . Bury and Redd ( 2 3 ) ) .

-

P O W D E R E D KINGSTON ROCK

C O M P O S I T I O N A L L Y S I M I L A R POWDERED M I X T U R E

0 .

0 0 .

F i g u r e 14. Thermograrns of e x p m s i v e K i n g s t o n r o c k arid c o r n p o s i t i o r l a l l y s i r n i l a r , non- e ~ p a r i s i v e rnixtut.e a f t e r treatmerit, i n 2M NaOH f o r 14 days. Samples d r i e d i n a i r f o r 1 hour.

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1. Expansion begins quite soon a f t e r the reactive rock i s placed in 2M alkali when the concentration of alkali carbonate i s very low.

2 . Alkali carbonate formed, due to dedolomitization, i s highly soluble and i s unlikely to f o r m a solid phase in a 2M solution of alkali.

3 . P h a s e data suggest that the concentration of alkali carbonate is much too low f o r gaylussite o r alkali-carbonate hydrate to f o r m ( F i g . 13).

4 . Calculations made in this laboratory show that a concentration of gaylussite in excess of 1 percent would be required to account f o r the observed expansion.

5. An addition of 1 percent gaylussite was detectable by X-ray diffraction. 6 . No hydrated double s a l t was detected by X-ray diffraction analysis unless the concentration of solution was allowed to r i s e by evaporation.

7. Concrete beams held at a relative humidity of 50 percent o r lower for m o r e than 2 months showed no expansion ( F i g . 4 ) . Under such conditions the concentration of alkali s a l t s would be expected to be a t a maximum. These beams expanded rapidly a s soon a s moisture was made available.

8 . Powdered mixtures of the s a m e composition a s reactive rock showed neither expansion nor water imbibition when tested in the powder cell even though dedolomitization o c c u r r e d .

The crystallities of which the matrix i s composed a r e quite s i m i l a r to those in lithographic limestone. The very fine crystallite s i z e i s likely to r e s u l t in high chemical reactivity which it s e e m e d possible was connected with the expansion. Samples of Solenhofen lithographic stone (obtained f r o m Wards Mineral Suppliers) showed no r e - activity. F o r this and other r e a s o n s it is most likely that the significant textural prop- e r t y is the distribution of the clay m i n e r a l s .

It was proposed by Feldman and Sereda (17) that m a t e r i a l with gel-like properties formed in the r o c k a s a r e s u l t of the a l k a l i n e a t t a c k . Water imbibition is a property of s o m e gels. Materials develop t h e s e p r o p e r t i e s when in the colloidal s i z e r a n g e . The suggestion was therefore made that the products of dedolornitization w e r e trapped within the rocks' very fine p o r e s y s t e m a s c r y s t a l s of colloidal s i z e s . This was not confirmed by an investigation of the half-peak breadths of the X-ray powder lines of the brucite and calcite (24). The t r e a t e d rock, however, does show an increased s u r - f a c e a r e a and especially>n increased capacity f o r holding water a f t e r saturation (17).

Lf t h e s e c h a r a c t e r i s t i c s a r e not due to the products of the dedolomitization r e a c t i o r then they must be due to the parent m a t e r i a l , such a s the clay fraction, which becomes uncovered by the reaction. The dolomite euhedra contain inclusions which a r e apparently of clay (Fig. l o ) , and s u c h c r y s t a l s of clay will become exposed as a r e s u l t of the dedolornitization reaction. Cracks and channels will open up and allow a c c e s s of alkali to clay in the matrix. It is postulated that such clay i s in an "active" s t a t e quite different f r o m that found a f t e r extraction with acid and flocculation with CaC12. It i s this "active" clay, r e l e a s e d during dedolornitization, which contributes to the expansion. The steps i n the hypothesis now proposed a r e ( a ) dedolornitization exposes llactive" clay minerals ; (b) exchange s i t e s on the clay s u r f a c e adsorb s o m e sodium ions; and ( c ) water uptake by this "new" clay r e s u l t s in swelling.

This hypothesis with evidence based on the powder-cell t e s t i s described in detail

in another papaer

(24).

It is not thought that the ability to expand i s the s o l e prerogative of rocks which r e s e m b l e t h o s e f r o m Kingstonor that deleterious expansion will necessarily b e caused by s u c h r o c k s . The fact that rocks of this type f r o m s e v e r a l different and widely skparated localities do cause excessive expansion in concrete made with high alkali cement indicates, however, that s u c h rock should b e regarded with suspicion a s po- tential aggregate. Rock of this type can b e readily recognized by petrographic examination and should be subjected t o careful t e s t s , such a s t h e concrete beam t e s t , p r i o r to u s e a s aggregate with high alkali cement.

REFERENCES

1. Swenson, E . G . , "A Canadian Reactive Aggregate Undetected by ASTM T e s t s . ' I