Hydration characteristics of monocalcium aluminate at a low water-solid ration

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Cement and Concrete Research, 3, November 6, pp. 729-750, 1973-11-01

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Hydration characteristics of monocalcium aluminate at a low

water-solid ration

Ramachandran, V. S.; Feldman, R. F.

https://publications-cnrc.canada.ca/fra/droits

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CEMENT and CONCRETE RESEARCH. Vol

.

3, pp. 729-750, 1973. Pergamon Press, I n c . P r i n t e d i n t h e U n i t e d States.

HYDRATION CHARACTERISTICS O F MONOCALCIUM ALUMMATE AT A LOW WATER/SOLID RATIO

by

V. S. Ramachandran and R. F. Feldman

R e s e a r c h Officers, Materials Section, Division of Building Research National R e s e a r c h Council of Canada

(Communicated by F. D. ~ a m s s )

ABSTRACT: The hydration c h a r a c t e r i s t i c s of calcium monoaluminate w e r e studied using an effective water/aluminate r a t i o of 0. 15 a t 20" o r 80°C, f r o m a few minutes to two months. T h e m a t e - r i a l hydrated a t 80°C shows a l a r g e shrinkage while a t the lower t e m p e r a t u r e a continuous expansion occurs. The p r o - duct a t 80°C shows a much higher strength than that hydrated at 20°C. The main initial hydration products a r e 2Ca0, AA203, 8H20 and alumina gel. Microcracks a r e developed in t h e products hydrated a t 20°C while at the higher t e m p e r a t u r e a v e r y compact m a s s r e s u l t s . The data indicate that i t i s possible t o obtain a durable high alumina cement by using a low water/cement r a t i o and hydrating a t higher temperatures, and under t h e s e conditions C3AH6

-

C3AHB bond i s favoured. L e s c a r a c t e r i s t i q u e s d'hydratation de lgaluminate monocalcique ont BtB Btudiees 'a l'aide d'un rapport eau-aluminate effectif d e 0. 15 'a 20" ou

3

80°C s u r une periode variant d e quelques minutes

'a

deux mois. Le m a t e r i a u hydrate 'a 80°C subit un r e t r a i t considerable tandis qug'a une t e m p e r a t u r e plus b a s s e il s e produit une dilatation continue. La r e s i s t a n c e du produit hydrate 'a 80°C e s t beaucoup plus grande qul'a 20°C. Les principaux produits dlhydratation initiaux sont 2Ca0, AA20,,

8H20 et l e gel d'alumine. Des m i c r o f i s s u r e s apparaissent dans l e s produits h y d r a t e s

3

20°C, tandis qul'a l a t e m p e r a t u r e plus &levee l e r e s u l t a t e s t une m a s s e tr'es compacte. Les donnees indiquent qu'on peut obtenir un ciment alumineux durable en utilisant un faible rapport eau-ciment et en effectu- ant lthydratation d e s t e m p e r a t u r e s plus elevkes, et que dans de t e l l e s conditions une liaison CaAH,

-

C3AH6 e s t facilitee.

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7 3 0 V o l . 3 , N o . 6

CALCIUM ALUMINATE, WATER CEMENT R A T I O , HYDRATION

Introduction

High a l u m i n a c e m e n t , a l s o known a s aluminous c e m e n t o r calcium a l u m i

-

n a t e c e m e n t develops a v e r y high s t r e n g t h within 24 h r of c u r i n g in w a t e r . It h a s a good r e s i s t a n c e to s u l f a t e a t t a c k and c a n b e used in r e f r a c t o r y c o n c r e t e . T h i s cement i s a l s o used a s a n expanding o r s t r e s s i n g c e m e n t in combination with gypsum and o t h e r constituents

.

T h e high a l u m i n a c e m e n t c o m p r i s e s s e v e r a l p h a s e s v i z . , CA, CA,, Cl,A7, CaAS, C G , C,F and C,AF*, Much attention h a s been d i r e c t e d t o a study of t h e hydration c h a r a c t e r i s t i c s of CA. T h e hydration p r o d u c t s of CA c o n s i s t of CAH,,, CaAH,, C3AHB and AH, ( g e l o r c r y s t a l l i n e ) , t h e r e l a t i v e p r o p o r t i o n s of which depend on t h e h y d r a t i o n conditions and c u r i n g p e r i o d . At lower t e m - p e r a t u r e s CAH,,, CaAHs and AH, g e l a r e p r e f e r e n t i a l l y formed and t h e s e a r e converted to C3AH6 and gibbsite a t h i g h e r t e m p e r a t u r e s , a c c o r d i n g t o t h e following s c h e m e (1).

CAH,,

1 2 1

-

35OC

C A _C2AH3

₊

_AH,

T h e 'conversion' o r ' i n v e r s i o n ' t h a t o c c u r s a s a consequence of t h e t r a n s f o r - m a t i o n of t h e hexagonal p h a s e s , CAHlo o r C2AH, into t h e cubic p h a s e (C,AH6) i s known t o b e accompanied by a l o s s in s t r e n g t h of t h e h a r d e n e d a l u m i n a c em ent

.

T h e c o n v e r s i o n r e a c t i o n s m a y b e d e s c r i b e d a c c o r d i n g to t h e following equations.

3CAHlo C3AH6 -F 2AH3

+

18H 3CaAHe+2C3AHe+ AH,

+

9H

T h e c o n v e r s i o n of CAHlo t o C3AHe r e s u l t s in a volume d e c r e a s e t o about 50

p e r c e n t w h e r e a s t h a t of CaAHe r e s u l t s in a d e c r e a s e t o about 65 p e r cent of t h e o r i g i n a l volume of t h e r e a c t a n t s .

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V o l . 3, N o . 6 73 1 CALCIUM ALUMINATE, WATER CEMENT R A T I O , HYDRATION

S e v e r a l t h e o r i e s have been proposed to explain the l o s s in strength in high alumina cement. They a r e s u m m a r i z e d below.

As a l r e a d y stated the conversion of t h e hexagonal t o t h e cubic phase r e s u l t s in a substantial d e c r e a s e in the solid volume. In t h e hardened c o n c r e t e this m a y b e accompanied by t h e formation of p o r e s and hence lower s t r e n g t h . This i s essentially t h e explanation f o r s t r e n g t h l o s s advanced by Robson ( 2 ) , Neville ( 3 ) , Lafuma ( 4 ) , Cottin ( 5 ) , Midgley (6), and Mehta ( 7 ) . T h e porosity m a y a l s o i n c r e a s e due to t h e evaporation of water that i s formed in t h e con- v e r s i o n reaction.

The e a r l i e r t h e o r y that t h e oxidation of f e r r o u s i r o n in t h e high alumina cement i s responsible for t h e r e t r o g r e s s i o n in s t r e n g t h s h a s been conclusive- l y disproved by Lea and o t h e r s (8, 9, 10).

Lehman and L e e r s (1 1) a s c r i b e the higher s t r e n g t h s in t h e alumina cement to CAH,, having a high s u r f a c e a r e a . Conversion, producing t h e cubic f o r m with a lower s u r f a c e a r e a i s thought t o b e r e s p o n s i b l e for low s t r e n g t h s . In a r e c e n t paper Mehta (12) h a s affirmed that a poor adhesive capacity of t h e cubic p h a s e and low specific a r e a a r e additional f a c t o r s contributing to a l o s s in strength. It i s a l s o possible that the conversion of the flat plates with o v e r - lapping o r interlocking s y s t e m to i c o s i t e t r a h e d r o n o r cubic morphology dis

-

locates t h e bonds.

Wells and C a r l s o n (13a) failed to o b s e r v e substantial d e c r e a s e in t h e s u r - f a c e a r e a during conversion and hence attribute t h e f a l l in s t r e n g t h to t h e for

-

mation of m a c r o c r y s t a l l i n e alumina h y d r a t e f r o m t h e m i c r o c r y s t a l l i n e a l u - mina hydrate. This is not in accord with t h e opinion of Lehman and L e e r s (11) who believe that c r y s t a l l i n e alumina h y d r a t e i n c r e a s e s the strength in high alumina c e m e n t s .

R6vay found that during conversion reaction t h e s u r f a c e a r e a i s actually i n - c r e a s e d due to the formation of alumina gel (13b).

Ueda (14a) did not find a d i r e c t relationship between t h e p o r o s i t y and s t r e n g t h in high alumina cement. He h a s attributed t h e l o s s in strength to a d e c r e a s e in the combined w a t e r in t h e converted product and a l s o to the i n - c r e a s e in t h e quantity of water filling the s m a l l p o r e s .

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C A L C I U M A L U M I N A T E , W A T E R C E M E N T R A T I O , H Y D R A T I O N

Vol.

3,

No.

6

T h e d e t e r i o r a t i n g influence of CO,, t h r o u g h t h e f o r m a t i o n of CaCO,

+

AH,

+

H f r o m C3AH6 h a s b e e n p r o p o s e d b y T a l a b e r f o r a l u m i n a c e m e n t s (14b).

It h a s now b e e n d i s c o v e r e d t h a t c o n v e r t e d a l u m i n a c e m e n t c a n b e s t r o n g and of low p e r m e a b i l i t y if p r e p a r e d a t a low w a t e r / c e m e n t r a t i o . Robson and o t h e r s (1 5, 16) m a i n t a i n that a t a low w a t e r / c e m ent r a t i o t h e w a t e r r e l e a s e d d u r i n g t h e ' c o n v e r s i o n ' r e a c t s with t h e a n h y d r o u s c e m e n t k e r n e l not utilized i n t h e i n i t i a l h y d r a t i o n r e a c t i o n , and f i l l s t h e p o r e s and thus p r e v e n t s s t r e n g t h l o s s e s . S t i g l i t z (17) explains t h a t t h e s t r e n g t h l o s s i s m a i n l y c a u s e d by i n - c r e a s e d p o r o s i t y d u e t o t h e e v a p o r a t i o n of w a t e r r e l e a s e d d u r i n g c o n v e r s i o n . At low w a t e r / c e m e n t r a t i o s t h i s w a t e r c a n bind with t h e unhydrated CA. A c c o r d i n g t o Midgley (18) t h e m a j o r f a c t o r t h a t p r e v e n t s l o s s i n s t r e n g t h i s t h e g r a i n s i z e of t h e c o n v e r t e d m i n e r a l s v i z . , C,AH, and AH,. At low w a t e r / c e m e n t r a t i o s t h e low p o r o s i t y i s m a i n t a i n e d b y t h e packing of t h e s m a l l c r y s - t a l l i t e s w h i l e a t h i g h e r w a t e r / c e m e n t r a t i o s t h e p o r o s i t y i s i n c r e a s e d b y t h e c r y s t a l s i z e . F r o m t h e f o r e g o i n g s u r v e y i t c a n b e concluded that t h e s t r e n g t h l o s s in a n o r m a l l y c u r e d high a l u m i n a c e m e n t i s a s c r i b e d t o t h e c o n v e r s i o n r e a c t i o n ; t h e exact m e c h a n i s m h o w e v e r , i s not c l e a r . It i s a l s o i m p l i c i t in t h e l i t e r a - t u r e t h a t t h e cubic h y d r a t e (C3AH6) d o e s not h a v e a binding c a p a c i t y . F o r

example, t h e s t r e n g t h s of t h e a l u m i n a t e h y d r a t e s a r e thought t o b e in t h e d e - c r e a s i n g o r d e r , CAH,,>C2AHe>C,AH6(19). But t h i s i s not in c o n s o n a n c e with t h e s t r e n g t h r e s u l t s obtained a t low w a t e r / c e m e n t r a t i o s even a f t e r c o n v e r s i o n

(16, 20).

E a r l i e r w o r k (21) on t h e h y d r a t i o n of C3A i n d i c a t e d t h a t t h e c o n v e r s i o n of C,A t o C3AHB u n d e r c e r t a i n conditions, in f a c t , e n h a n c e s t h e s t r e n g t h . It a p p e a r s t h a t t h e f o r m a t i o n of C3AH6 p e r s e should not b e c o n s t r u e d a s d e t r i - m e n t a l t o s t r e n g t h . In o r d e r t o e x a m i n e t h e a p p l i c a b i l i t y of t h e above findings t o t h e h y d r a t i o n and s t r e n g t h development in high a l u m i n a c e m e n t s , CA w a s h y d r a t e d a t a low w a t e r / c e m e n t r a t i o a t 2 0 ° C and 80°C f o r d i f f e r e n t p e r i o d s . T h e w a t e r / s o l i d r a t i o w a s c h o s e n a t 0. 1 5 t o e m p h a s i z e i t s s i g n i f i c a n t effect on t h e m e c h a n i s m of h y d r a t i o n and s t r e n g t h development. T h e r e s u l t s m a y b e a d v a n t a g e o u s l y extended t o p r a c t i c a l m i x e s h a v i n g low w a t e r / s o l i d r a t i o s .

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V o l . 3, No. 6

733

CALCIUM ALUMINATE, WATER CEMENT RATIO, HYDRATION

c h a r a c t e r i s t i c s and other properties such a s microhardness, porosity, m o r - phology and dimensional changes.

Experimental

Materials

The calcium monoaluminate used for this investigation was supplied by Tetratech International, San Diego. The sample contained 35. 2 p e r cent, CaO, 64. 3 p e r cent A& 0, with f r e e lime l e s s than 0. 2 per cent. The x - r a y analysis of this m a t e r i a l showed all the lines typical of CA. The surface a r e a was 0. 62m2/g. No t h e r m a l effects w e r e observed in the differential scanning calorimeter (DSC) or thermogravimetric analysis (TGA).

Calcium monoaluminate powder was compacted at a load of 75, 000 lb into discs 1. 25 in. in nominal diameter and 0. 05 in. thick. The effective water: aluminate ratio for hydration was calculated to be 0. 15. TBe compacts were hydrated in duplicates at a t e m p e r a t u r e of 20°C or at 80°C for 10 min, 30 min,

1 h r , 2 h r , 5 h r , 10 h r , 1 day, 2 days, 5 days, 10 days, 21 days and 60 days. At the end of each period the sample was washed with alcohol and evacuated f o r 24 h r using liquid a i r t r a p . The sample was ground to approximately 100 m e s h s i z e for t h e r m a l and x - r a y analysis, fractured for microscopic examination and cut t o a rectangular shape for length change m e a s u r e m e n t s .

Methods

Differential thermograms of the samples w e r e obtained by a Differential Scanning Calorimeter (DSC) supplied a s a module to Du Pont 900 t h e r m a l analyser. This unit utilizes chromal-constantan f o r differential t e m p e r a t u r e measurement. The reference m a t e r i a l was ignited

a

-

AJ2O, and the heating r a t e was maintained at 1 o 0 c / m i n . The differential temperature was regis

-

tered at a sensitivity of 0. 02 mv/in. Thermograms w e r e obtained in a i r . Both the sample and the reference m a t e r i a l w e r e contained in aluminum foil cups. In each experiment 40 m g of the sample was weighed into the aluminum cup.

Thermogravimetric analysis (TGA) of the samples was c a r r i e d out using a sensitive Cahn Balance at a heating r a t e of 1 o 0 c / m i n . All the runs were c a r r i e d out in a continuous vacuum.

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7 3 4 V o l . 3 , No. 6 CALCIUM ALUMINATE, WATER CEMENT RATIO, HYDRATION

T h e x - r a y powder photographs w e r e obtained with P h i l i p s C a m e r a u s i n g Cu K s o u r c e . T h e r e l a t i v e i n t e n s i t i e s of t h e l i n e s w e r e obtained b y d e n s i -

a

t o m e t e r t r a c e s of t h e powder p h o t o g r a p h s . T h e f r a c t u r e d s u r f a c e s of t h e s p e c i m e n s w e r e examined b y a s c a n n i n g e l e c t r o n m i c r o s c o p e supplied b y C a m b r i d g e Co. S a m p l e s cut t o a r e c t a n g u a l a r s h a p e a p p r o x i m a t e l y 1 i n . b y 0. 25 in. w e r e exposed t o w a t e r a t 2 0 ° C o r 8 0 ° C continuously f o r p e r i o d s of up t o 7 days in a c e l l and t h e length change w a s m e a s u r e d p e r i o d i c a l l y by a modified

T u c k e r m a n gauge e x t e n s o m e t e r . T h e d e t a i l s of t h i s method a r e d e s c r i b e d e l s e w h e r e (22).

T h e V i c k e r s h a r d n e s s v a l u e of t h e unhydrated and t h e h y d r a t e d c o m p a c t s was d e t e r m i n e d with a L e i t z m i n i l o a d h a r d n e s s t e s t e r . E a c h value r e p r e - s ents an a v e r a g e of 10 d e t e r m i n a t i o n s c a r r i e d out on both s i d e s of t h e c o m

-

p a c t .

T h e Aminco-Winslaw p o r o s i m e t e r w a s u s e d . t o d e t e r m i n e t h e p o r e s i z e d i s t r i b u t i o n and p o r o s i t y of both t h e u n h y d r a t e d and h y d r a t e d s a m p l e s . T h e i n s t r u m e n t m e a s u r e s p o r e s i z e d i a m e t e r i n t h e r a n g e 0 . 1 m m t o 0 . 012 p

.

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

A conduction c a l o r i m e t e r with s i x c h a m b e r s w a s u s e d t o follow t h e r a t e of h e a t development d u r i n g t h e h y d r a t i o n of CA. T h e s e n s i t i v i t y of t h e c a l o r i - m e t e r w a s 20

~ v / w .

S u r f a c e a r e a w a s obtained with

N2

a s t h e a d s o r b a t e by a N u m i n c o - O r r s u r f a c e a r e a

-

p o r e volume a n a l y s e r . R e s u l t s and D i s c u s s i o n D i f f e r e n t i a l Scanning C a l o r i m e t e r R e p r e s e n t a t i v e d i f f e r e n t i a l s c a n n i n g c u r v e s of CA h y d r a t e d a t 20" o r 80°C f o r d i f f e r e n t p e r i o d s of up t o 60 days a r e shown in F i g u r e s 1 and 2. At 20°C h y d r a t i o n p r o c e e d s slowly i n t h e f i r s t few h o u r s . At 10 h r h y d r a t i o n i s i n d i - c a t e d b y a n e n d o t h e r m a l v a l l e y with a p e a k a t about 1 0 0 ° C . A s m a l l endo- t h e r m a l doublet in t h e r a n g e of 175" t o 2 2 5 ° C i s a l s o r e g i s t e r e d . At 1 day a

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Vol. 3, No. 6

7 3 5

CALCIUM ALUMINATE, WATER CEMENT RATIO, HYDRATION

' I ' I ' I ' I '

*

1-

a::

\

'

+

T E M P E R A T U R E , O C FIG. 1 T h e r m o g r a m s of C A Hydrated t o Different P e r i o d s a t 20°C a t a w a t e r / s o l i d Ratio of 0. 15 5 M I N 1 0 M I N 3 0 M I N 1 HR 2 HRS 5 HRS 1 0 HRS 1 D A Y 2 D A Y S 5 D A Y S 1 0 D A Y S

a

2 0 D A Y S 6 0 D A Y S 0 1 0 0 200 300 400 500 600 T E M P E R A T U R E , "C FIG. 2 T h e r m o g r a m s of C A Hydrated a t 80°C t o Different P e r i o d s a t a w a t e r / s o l i d Ratio of 0. 15

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736 V o l . 3, N o . 6

s h a r p endothermal p e a k a p p e a r s a t about 175°C and i n c r e a s e s in intensity a s the hydration p r o g r e s s e s . Also an endotherm of s m a l l magnitude appearing

a t about 260°C a t 1 day grows in intensity. An additional endothermal p e a k a p p e a r s a t about 300°C a t 5 days and t h i s a l s o i n c r e a s e s in intensity up t o 60 days. T h e l a r g e endotherm appearing f r o m t h e v e r y e a r l y period with a peak around 100°C m a y be attributed t o r e m o v a l of w a t e r f r o m t h e alumina gel. This endotherm i s followed by another a t about 125°C (not a p p a r e n t in s o m e i n s t a n c e s ) and m a y b e a s c r i b e d t o t h e p r e s e n c e of C,AH8. An unmistakable endotherm appearing at about 175°C i s due t o t h e p r e s e n c e of CAH,,. T h e dual peaks o c c u r r i n g in the t e m p e r a t u r e r a n g e 200" to 325" C r e p r e s e n t dehydration r e a c t i o n s in gibbsit e and C3AH6. Gibbsite i s f o r m e d f r o m t h e c r y s t a l l i z a t i o n of alumina gel and C3AH6 i s t h e conversion product of CAH,, and C,AH,.

T h e t h e r m a l behaviour of CA hydrated a t 80°C i s significantly different f r o m t h a t hydrated a t 20°C ( F i g u r e 2). A detectable amount of hydration i s evident even at 10 m i n . By 30 m i n l a r g e endothermal effects a p p e a r a t about 100°, 145", 210°, and 280°C. T h e f i r s t effect i s caused by alumina gel and i s p r a c t i c a l l y absent a f t e r 2 days of hydration. T h e endotherm effect a t about 150°C i s p r e s e n t in a l l s a m p l e s up t o 1 day and i s a t t r i b u t a b l e t o t h e p r e - s e n c e of C,AH,. T h e endothermal effect a t 280°C which i n c r e a s e s in i n t e n s i - ty with hydration i s mainly caused by C3AHB and gibbsite. T h e i n i t i a l down- w a r d slope of t h i s c u r v e s u g g e s t s the possibility of an additional endotherm.

In f a c t a t 2 days a c l e a r endotherrn e m e r g e s a t about 225°C which m a y b e due to CAH1,; a maximum r a t e of f o r m a t i o n of this compound s e e m s t o occur between 2 and 5 days. P o s s i b l y gibbsite a l s o contributes t o t h i s endothermal effect. T h e resolution of t h e l a r g e endothermal effect into two effects a t about 300" and 340°C a t 5 days c o n f i r m s the f o r m a t i o n of gibbsite and C3AH6. The endothermal p e a k a t about 500°C r e p r e s e n t s t h e typical stepwise dehydration effect of C3AH,.

T h e r m o g r a m s suggest that c o m p a r e d with hydration a t 20°C that a t 8 0 ° C p r o c e e d s a t a much f a s t e r r a t e . T h e d e g r e e and r a t e of conversion of t h e hexagonal p h a s e s and alumina gel t o t h e cubic and gibbsite p h a s e s r e s p e c t i v e - ly a r e a l s o enhanced a t higher t e m p e r a t u r e . In addition it i s p o s s i b l e that c o n v e r s i o n t o C,AH, and gibbsite p h a s e s o c c u r s d i r e c t l y on the s u r f a c e of CA

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p a r t i c l e s . After an initial f a s t r e - action, f u r t h e r hydration, e s p e c i a l - l y a f t e r about 5 days, tends t o b e slow a t 8 0 ° C . T h e r e i s s t i l l a l a r g e amount of unhydrated CA even a t 60 days. T h e r a t e of formation and the conversion effect a r e not only depen- dent on the t e m p e r a t u r e of h y d r a - tion but a l s o on the initial w a t e r / solid r a t i o . At room t e m p e r a t u r e a higher water/solid r a t i o of 0. 5

p r o m o t e s f a s t e r conversion and c r y s t a l l i z a t i o n r e a c t i o n s c o m p a r e d with CA hydrated a t a lower/solid r a t i o ( F i g u r e 3). X - r a y diffraction R a t e s of hydration w e r e c o m - p a r e d using d e n s i t o m e t e r t r a c i n g s of the x - r a y f i l m s f o r CA compacts hydrated t o different p e r i o d s . T h e T E M P E R A T U R E , "C t r a c i n g s w e r e not intended f o r e v a l - FIG. 3

uating, quantitatively, the h y d r a - T h e r m o g r a m s of CA Hydrated a t 20°C a t a ~ a t e r / S o l i d Ratio of 0. 5 tion reaction but w e r e a guide t o

t h e new products that a r e f o r m e d . As t h e peaks w e r e l e s s distinct a t higher d values, the f i l m s w e r e used f o r c o m p a r i s o n p u r p o s e s . T a b l e I shows s o m e of the m o r e prominent l i n e s by which v a r i o s s p h a s e s w e r e recognized.

The unhydrated CA shows a l l t h e s t a n d a r d d values. At 20°C up to 10 h r t h e r e i s no indication of any hydrated product though DSC shows formation of s o m e hydration product. At 1 day faint lines a p p e a r a t 10. 5 and 5 . 2 0 due t o C,AH,. At 5 days the existence of CAH,, i s indicated. T h e CAH,, lines b e - c o m e m o r e intense a t 60 days and t h e r e i s a l s o a s m a l l amount of C,AHG. Gibbsite could not be detected with c e r t a i n t y . At 60 days c o n s i d e r a b l e amounts of unhydrated CA a r e s t i l l p r e s e n t .

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T A B L E I P h a s e 0 d v a l u e s (A) G i b b s i t e 4. 85, 4. 32-4. 37, 2. 42-2. 45, 2. 39. T h e c h a r a c t e r i s t i c s of h y d r a t i o n of CA a t 8 0 ° C a r e d i f f e r e n t f r o m t h o s e of CA h y d r a t e d a t 2 0 ° C . CA i s c o n s u m e d m o r e r a p i d l y a t e a r l y t i m e s . At 60 m i n u t e s t h e r e i s a n indication of t h e f o r m a t i o n of C2AHB but no evidence of CAH,,. C3AH, i s p o s s i b l y f o r m e d a t t h i s s t a g e . At 5 days l i n e s due t o C3AH, b e c o m e c l e a r e r and CAHlobegins t o f o r m . Gibbsite i s a l s o detected a t t h i s p e r i o d . At 10 days i n c r e a s e d a m o u n t s of CAH,, a r e f o r m e d . At 21 days and 60 days t h e p h a s e s p r e s e n t a r e CA, CAH,,, C2AH8, C3AH6 and gibbsite. T h e r e s u l t s a r e g e n e r a l l y in a c c o r d with t h o s e of DSC. C o m p a r e d with x - r a y DSC a p p e a r s t o b e a m o r e s e n s i t i v e method f o r identification and e s t i m a t i o n of h y d r a t i o n p r o d u c t s of CA.

T h e r m o g r a v i m e t r i c a n a l y s i s

TGA was c a r r i e d out i n a continuous vacuum and t h e t e m p e r a t u r e w a s m e a - s u r e d b y a t h e r m o c o u p l e p l a c e d n e a r t h e s a m p l e . TGA showed inflections c o r r e s p o n d i n g t o t h e d i f f e r e n t i a l c u r v e s obtained b y DSC but t h e t e m p e r a t u r e s w e r e d i s p l a c e d .

CA h y d r a t e d f o r 5 m i n a t 8 0 ° C showed a t o t a l weight l o s s of about 1 . 8 p e r c e n t a t 500°C and i s a s s o c i a t e d with a b r o a d e n d o t h e r m a l v a l l e y in t h e DSC c u r v e of t h i s s a m p l e ( F i g u r e 2). At 10 m i n t h e hydration s e e m s t o h a v e p r o - c e e d e d t o a g r e a t e r extent a s indicated b y a weight l o s s of 9 . 98 p e r c e n t . At 30 m i n TGA c l e a r l y shows two inflections and DSC a l s o exhibits two endo- t h e r m s a t t h i s p e r i o d . T h e f i r s t inflection c o r r e s p o n d s t o a weight l o s s of

6 . 97 p e r cent and t h e s e c o n d , t o about 6. 6 p e r c e n t . At 5 h r s TGA exhibits f o u r s t e p s in weight l o s s e s c o r r e s p o n d i n g t o f o u r e n d o t h e r m a l e f f e c t s . At t h i s p e r i o d t h e t o t a l weight l o s s i s 20. 76 p e r c e n t . At 1 day t h e t o t a l weight

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l o s s i s 18. 8 4 p e r c e n t . At 2 d a y s only v e r y l i t t l e l o s s i n weight o c c u r s up t o 200°C and DSC a l s o shows a m u c h r e d u c e d p e a k below 2 0 0 ° C . At 5 d a y s t h e t o t a l weight l o s s i s 28. 22 p e r c e n t , a n i n c r e a s e of 7. 86 p e r c e n t f r o m 2

d a y s . Most of t h i s i n c r e a s e a r i s e s f r o m t h e f i r s t inflection c o r r e s p o n d i n g t o CAH,,, Between 2 and 5 days t h e weight l o s s c u r v e i n d i c a t e s t h a t t h e amount of C,AHG h a s not i n c r e a s e d .

TGA c u r v e s f o r CA h y d r a t e d a t 2 0 ° C show inflections t h a t could b e c o r - r e l a t e d with t h e e n d o t h e r m a l p e a k s i n DSC. T h e ignition l o s s e s f o r s a m p l e s h y d r a t e d f o r 1 h r , 10 h r , 24 h r , and 5 days a r e r e s p e c t i v e l y 1 . 34, 2. 49,

18. 96 and 22. 83 p e r cent and t h e c o r r e s p o n d i n g v a l u e s f o r s a m p l e s h y d r a t e d a t 8 0 ° C a r e 17. 38, 21. 13, 18. 8 4 and 28. 22 p e r c e n t . T h e m a g n i t u d e of t h e s e v a l u e s does not r e p r e s e n t t h e d e g r e e of h y d r a t i o n but i t d o e s show t h a t t h e h y d r a t i o n s e q u e n c e a t 20°C i s d i f f e r e n t f r o m t h a t a t 8 0 ° C . Conduction c a l o r i m e t r y T h e r a t e s of h e a t development of t h e c o m p a c t s exposed t o w a t e r w e r e d e t e r m i n e d a t 2 5 ° C o r 7 2 ° C by t h e conduction c a l o r i m e t r i c c u r v e s ( F i g u r e 4). Though t h e c u r v e s i n d i c a t e t h e r e l a t i v e r a t e s of h e a t development f o r h y d r a - tion t h e y a r e not d i r e c t l y c o m p a r a b l e t o t h e d i f f e r e n t i a l t h e r m o g r a m s . At TIME, HR FIG. 4 Conduction C a l o r i m e t r i c C u r v e s of CA H y d r a t e d a t 2 5 ° C o r 7 2 ° C

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25" C an initial peak r e s u l t s immediately on contact with water due to heat of wetting and possibly some s u r f a c e hydration. At about 1 day hydration seems to commence at a f a s t r a t e with maximum intensity in the interval between 28 and 31 h r . The r a t e of heat evolution d e c r e a s e s steadily thereafter and hydrati& proceeds slowly after 2 days. DSC curves show that considerable hydration takes place at 1 day, further hydration and conversion progressing

slowly ( F i g u r e 1). The hydration at 72°C s e e m s to proceed f a s t e r , a s evi- denced by an intense heat effect with a peak appearing e a r l i e r than 10 min. The major heat development occurs within 30 min. In the differential t h e r m o - g r a m s considerable reaction s e e m s to occur between 10 and 30 min ( F i g u r e 2 ) .

As already shown hydration in compacts i s c a r r i e d out at an effective water/solid r a t i o of about 0 . 1 5 and it i s expected that hydration and conver-

sion reactions should occur at a m o r e rapid r a t e at a higher water/solid ratio. This can be demonstrated by comparing the c a l o r i m e t r i c curves of CA at a water/solid r a t i o of 0. 15 or 0. 5 ( F i g u r e 4). The maximum r a t e of hydration i s found t o proceed e a r l i e r (12 and 20 h r ) and the total heat developed i s a l s o of g r e a t e r magnitude at a higher water/solid ratio in the f i r s t few days. The total heat developed i s also m o r e than that registered f o r the compact hydrated at 72" C ( F i g u r e 4). The total amount of heat evolved for compacts hydrated at 72°C (for the f i r s t 30 min), 25" C (in about 2 days) and at 25°C a t a water/solid ratio of 0. 5 (1 day) a r e , respectively, 56. 0, 39. 0 and 72.6 ~ a l / ~ m .

Microstructure

The unhydrated CA consists of p a r t i c l e s of indefinite morphology of v a r i - ous s i z e s ( F i g u r e 5A). P r a c t i c a l l y no hydration products a r e detected up t o 10 h r ( F i g u r e 5B). At 1 day t h e r e i s evidence of hydration on the s u r f a c e of the p a r t i c l e s ( F i g u r e 5C). The hydrated sample i s a l s o characterized by extensive microcracking. The c r a c k s m a y have either developed during hydration o r during the preparation for the microscopic examination. It i s m o r e likely that they would have developed during hydration a s the c r a c k s seem to pass through the unhydrated CA p a r t i c l e s ; the micrograph of unhydrated CA does not show any c r a c k s . In some instances hydrated products

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V o l . 3, N o . 6 7 4 1 C A L C I U M ALUMINATE, WATER CEMENT R A T I O , HYDRATION

FIG. 5A: Unhydrated CA x3. 5K

FIG. 5B: CA h y d r a t e d f o r 10 h r a t 2 0 ° C x6. OK

FIG. 5C: C A h y d r a t e d f o r 1 day FIG. 5D: CA h y d r a t e d f o r 2 1 a t 2 0 ° C x 6 . 8 K d a y s a t 20°C x3. 1K

tinuous expansion a s t h e hydration p r o g r e s s e s and t h i s m a y h a v e caused c r a c k development. A l a r g e amount of f i b r o u s m a t e r i a l i s s e e n in t h e s a m p l e hydrated f o r 21 days ( F i g u r e 5D) indicating t h e e x i s t e n c e of CAH,,

.

At 60 d a y s t h e s a m p l e shows s o m e unhydrated a m o r p h o u s , p l a t y and f i b r o u s m a t e - r i a l with no evidence of cubic C3AH6 a s typified by t h e two m i c r o g r a p h s ( F i g u r e s 5 E and F ) .

T h e m i c r o s t r u c t u r e f e a t u r e s of t h e h y d r a t e d p r o d u c t a t 8 0 ° C a r e d i f f e r - ent f r o m t h a t f o r m e d a t 20°C ( F i g u r e 6). Even a t 1 h r t h e r e i s a n indication of t h e hydration p r o d u c t s c o v e r i n g t h e CA p a r t i c l e s ( F i g u r e s 6A and B). By

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FIG. 5E: CA hydrated f o r 60 days a t 20°C x3. 2K

FIG. 5F: CA hydrated for 60 L

days a t 20°C x3. 2K

FIG. 6A: CA hydrated for 1 h r FIG. 6B: CA hydrated for 1 h r a t 8 0 ° C x6 0 a t 8 0 ° C x6. 5K

2 days l a r g e amounts of hydration p r o d u c t s a r e evident a s c u r l e d s h e e t s and p l a t e s of C,AH, and CAH,, ( F i g u r e s 6C and D). By 5 days t h e s h e e t s and p l a t e s h a v e d i s a p p e a r e d and a d e n s e s t r u c t u r e i s f o r m e d (Figu're 6E). At 60 days t h e s u r f a c e p r e s e n t s a ' v i t r i f i e d ' a p p e a r a n c e with occasional m a s s e s , p r o b a b l y of C,AHG ( F i g u r e s 6 F and G). T h e r e was no indication of c r a c k development in a n y of t h e s a m p l e s h y d r a t e d a t 8 0 ° C .

Leneth changes

Length change m e a s u r e m e n t s in compacted bodies h a v e been shown t o p r o - vide i m p o r t a n t data on t h e m e c h a n i s m of hydration (21 t o 26). In this study

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F I G . 6C: C A hydrated for 2 days F I G . 6D: C A hydrated for 2 days at 8 0 ° C x l . 4K at 8 0 ° C x 3 . 5 K

F I G . 6E: C A hydrated for 5 days F I G . 6 F : C A hydrated for 6 0 days at 8 0 ° C x 3 . 3K at 8 0 ° C x 0 . 7 K

F I G . C A hydrated for 60 days at 8 0 ° C x l . 5K

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a n a t t e m p t h a s b e e n m a d e t o u n d e r s t a n d t h e h y d r a t i o n c h a r a c t e r i s t i c s of CA b a s e d on t h e length changes o c c u r r i n g a t 2 0 ° C o r 8 0 ° C , up t o a p e r i o d of 5 d a y s . T h e r e s u l t s a r e plotted in F i g u r e s 7 and 8. At 2 0 ° C c o n s i d e r a b l e e x - p a n s i o n o c c u r s within a n h o u r and i t continues a t a s l o w e r r a t e t h e r e a f t e r . At 4hr, d u r i n g which about 50 p e r c e n t of t h e t o t a l expansion o c c u r s , h y

-

d r a t i o n h a s not p r o g r e s s e d t o a n y significant extent ( F i g u r e 1). S i n c e t h e p a r t i c l e s a r e in i n t i m a t e c o n t a c t with e a c h o t h e r t h e i n i t i a l p r o d u c t s f o r m e d a t t h e i n t e r p a r t i c l e s u r f a c e s tend t o p u s h e a c h o t h e r . T h e expansion m a y a l s o b e c o n t r i b u t e d t o i n i t i a l l y b y a d e c r e a s e in t h e s u r f a c e e n e r g y . A s t h e h y d r a t i o n p r o g r e s s e s t h e hexagonal a l u m i n a t e h y d r a t e s f o r m e d c r e a t e s o m e

expansion and t h e a l u m i n a g e l i s a c c o m m o d a t e d in t h e p o r e s . At 5 days v e r y a

s l i g h t c o n v e r s i o n h a s t a k e n p l a c e ( F i g u r e 1). At 8 0 ° C a n a l m o s t i m m e d i a t e expansion i s followed b y l a r g e s h r i n k a g e ( F i g u r e s 7 and 8). T h i s t y p e of b e h a v i o u r h a s not b e e n o b s e r v e d b e f o r e d u r i n g h y d r a t i o n of c o m p a c t e d b o d i e s . T h e s m a l l i n i t i a l expansion i s s i m i l a r t o t h a t o b s e r v e d f o r t h e s a m p l e c u r e d a t 2 0 ° C . T h e l a r g e amount of s h r i n k - a g e o c c u r r i n g within t h e f i r s t 30 m i n i s a l s o r e f l e c t e d in t h e c o n s i d e r a b l e h y - TIME. H R FIG. 7

Length Changes i n C o m p a c t s of Monocalcium A l u m i n a t e H y d r a t e d a t 2 0 ° C o r 8 0 ° C i n t h e F i r s t F e w H o u r s

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d r a t i o n and i n t e r c o n v e r s i o n t h a t h a s o c c u r r e d a t t h i s p e r i o d ( F i g u r e s 2, 4, 6). T h e c o n t r a c t i o n m a y b e a t t r i b u t e d t o t h e f o r m a t i o n of hexagonal p h a s e s t h a t b r i d g e between p a r t i c l e s ; t h e i r v e r y r a p i d c o n v e r s i o n t o t h e m o r e d e n s e C,AH, p h a s e a t t h e o r i g i n a l s i t e s p r o v i d e s m o t i v e f o r c e f o r t h i s e f f e c t . It i s p o s s i b l e , h o w e v e r , t h a t a d i r e c t f o r m a t i o n of C3AH6 and AH, o c c u r r i n g a t

e a r l y p e r i o d s on t h e s u r f a c e of CA a l s o involves a b r i d g i n g effect of t h e i n t e r - f a c e s . T h e c o n t r a c t i n g f o r c e m a y b e caused by t r a n s p o r t a t i o n of AH, into t h e p o r e s . A f t e r a d a y t h e amount of C3AH6 f o r m e d d o e s not i n c r e a s e v e r y m u c h but t h e r e i s a g r a d u a l i n c r e a s e in t h e f o r m a t i o n of CAH,,; t h i s i s followed by a s l o w continuous expansion.

P o r o s i t y and m i c r o h a r d n e s s

A t t e m p t s w e r e m a d e t o s t u d y t h e effect of h y d r a t i o n on t h e p h y s i c a l and m e c h a n i c a l behaviour of CA. T h e p o r e s i z e distribution m a y b e evaluated f o r t h e unhydrated and h y d r a t e d CA f r o m F i g u r e 9. T h e unhydrated CA h a s a t o t a l p o r o s i t y of about 28 p e r c e n t of which t h e m a j o r p o r t i o n h a s a d i a m e t e r in t h e r a n g e of 0. 35 t o 0. 1 p . A f t e r 5 d a y s of h y d r a t i o n p o r o s i t y d e c r e a s e s TIME. HR FIG. 8 Length C h a n g e s i n C o m p a c t s of Monocalcium A l u m i n a t e H y d r a t e d a t 2 0 ° C o r 8 0 ° C up t o 5 Days

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4 0 _I _I _I ₁ _I _{I I} _{I I I} _I _I _{I I I I I 1}

--

- U N H Y D R A T E D C A -

-

H Y D R A T E D C A A T 2 0 ° C O R 8 0 ° C F O R 5 D A Y S I L-+,+-: ; ; 21 .O 10.5 7 . 0 4 . 0 3 . 0 2 . 0 1 .5 1 .O 0.5 0 . 4 0.3 0 . 2 0.1 0.05 0.04 0.02 0.01 D I A M E T E R O F P O R E S , p FIG. 9 P o r e S i z e Distribution C u r v e s of CA Hydrated a t 20°C o r 80°C

t o about 1 to 2 p e r cent whether CA i s hydrated a t 20°C o r 8 0 ° C . T h e d e - c r e a s e in p o r o s i t y cannot b e e a s i l y explained just in t e r m s of the absolute volume changes in the hydrated p r o d u c t s . T h e hydration of CA t o CAHloin- volves an enormous i n c r e a s e in t h e volume, w h e r e a s a d e c r e a s e in volume i s expected in t h e conversion of CAHl0 and C&H, t o t h e cubic p h a s e . In addition, the d e g r e e of hydration and i n t e r c o n v e r s i o n , t h e morphology, the possibility of clogging of m i c r o p o r e s by alumina gel and other products, a r e other f a c t o r s that cannot be e a s i l y subjected t o quantitative evaluation. It i s , however, obvious that t h i s i s m a i n l y due t o t h e p o r e s being filled up with the hydration p r o d u c t s . T h e t o t a l p o r o s i t y was a l s o determined b y vacuum s a t u - r a t i o n using methanol t o check if m e r c u r y had p e n e t r a t e d a l l the p o r e s . The methanol method gave a t o t a l p o r o s i t y of 29. 5 p e r cent f o r unhydrated CA and 2. 0 p e r cent f o r products obtained at 5 d a y s . T h i s indicated that m e r c u r y p e n e t r a t e s p r a c t i c a l l y a l l the p o r e s intruded b y methanol.

F o r a p a r t i c u l a r m a t e r i a l , if a l l t h e conditions a r e t h e s a m e , then s t r e n g t h i s known to be a function of p o r o s i t y . It follows that the reduced porosity due t o hydration of CA m u s t r e s u l t in an i n c r e a s e in t h e h a r d n e s s values for CA

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a t 20" C o r 8 0 ° C . C o m p a r e d with a value of 19. 9 k g / m m 2 for the unhydrated compact of CA s a m p l e , those hydrated at 20" o r 8 0 ° C show h a r d n e s s of 109. 5 and 160. 5 k g / m m 2 r e s p e c t i v e l y . It i s g e n e r a l l y believed that higher t e m p e r a t u r e s a r e d e t r i m e n t a l t o s t r e n g t h development in high alumina c e m e n t , a s t h e s e conditions favour t h e formation of t h e cubic p h a s e . T h e s e r e s u l t s d e m o n s t r a t e that n e i t h e r t h e high t e m p e r a t u r e n o r t h e formation of t h e cubic phase i s d e t r i m e n t a l t o s t r e n g t h development. On t h e other hand, although t h e s a m p l e hydrated a t 20°C h a s about t h e s a m e p o r o s i t y a s that hydrated a t 8OoC, t h e l a t t e r h a s a much higher h a r d n e s s value. At 80°C t h e r e a c t i o n p r o c e e d s v e r y f a s t and enhances hydration of CA t o t h e C3AH6 and AH, p h a s e s . As the p a r t i c l e s of CA in t h e compact l i e v e r y c l o s e t o each o t h e r , d i r e c t bond formation between C&H6 p r o d u c t s i s enhanced. At 20°C however, d i - r e c t bond formation due t o C3AH6 m a y not b e favoured a s C3AH6 and AH, products a r e t r a n s p o r t e d and r e c r y s t a l l i z e d in the p o r e s . T h i s i s evident f r o m t h e m i c r o s c o p i c examination in which only d e n s e s t r u c t u r a t i o n i s evident a t 8 0 ° C ( F i g u r e 6).

At 60 days t h e h a r d n e s s of t h e hydrated product f o r m e d a t 20" and 80°C a r e 73. 9 and 152. 4 k g / m m 2 respectively. T h e f a l l i n s t r e n g t h f o r the s a m p l e hydrated a t 20°C i s m a i n l y due t o t h e conversion of CAH,, and C&H, t o

alumina gel and C&H6. T h i s p r o c e s s involves a s l o w e r conversion of CA t o C+H6 and hence C3AH6

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C3AHB bonds a r e not promoted t o the s a m e extent. It i s v e r y likely that a t ambient conditions and a t a w a t e r / c e m e n t r a t i o higher than that used i n this investigation a s t i l l p o o r e r s y s t e m would r e s u l t . The s a m p l e s continuously exposed a t 80°C up t o 60 days do not significantly change in s t r e n g t h due t o s t r o n g bonds developed by C+H6 a t e a r l y s t a g e s . It i s expected that for higher s t r e n g t h s , CA m a y b e hydrated a t higher t e m p e r a t u r e s a t a sufficiently low w a t e r / s o l i d r a t i o f o r a few h o u r s only and then exposed t o n o r m a l t e m p e r a t u r e s .

T h e s e r e s u l t s s u g g e s t t h a t t h e p r e s e n t t h e o r i e s on s t r e n g t h d e t e r i o r a t i o n in high alumina c e m e n t s should t a k e into account t h e i m p o r t a n t contribution of t h e s t r e n g t h promoting p r o p e r t i e s of t h e cubic and gibbsite p h a s e s , e s p e - c i a l l y when f o r m e d a t low water/solid r a t i o s . It h a s a l s o been observed that t h e i n c r e a s e in the s u r f a c e a r e a p e r s e need not n e c e s s a r i l y m e a n that s t r e n g t h s

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i n c r e a s e . F o r example, the unhydrated CA has a surface a r e a of 0. 6 ~ m ' / ~ , the samples hydrated for 5 days at 20°C o r 80°C yield a surface a r e a of

1. 88m2/g and 1. 5 ~ m ' / ~ respectively but -the strength of the product formed a t 80°C i s much higher than other samples.

Conclusions

This study h a s thrown new light on the hydration c h a r a c t e r i s t i c s of CA and suggests r e a s s e s s m e n t of the popular concepts in the field of high alumina cements. At a low water/solid r a t i o some of the c h a r a c t e r i s t i c s of the hydrated product formed at 80°C a r e different from that prepared at 20°C. At 80°C a direct bond formation of C,AH, m a y occur on the surface of CA even within 30 min due to rapid conversion of the hexagonal phase; curing at 20°C involves a continuous expansion that at 80°C results in a substantial shrinkage. Strength development occurring a t 80°C in the f i r s t few days does not change during further hydration over s e v e r a l months. A deterioration in strength occurs in the sample hydrated a t 20°C a s the hydration and interconversions occur over longer periods.

Both a t 20°C and 80°C, C a H B and alumina gel and not CAH,, seem to be the main initial hydration products. At 80" C the hydration products do not show any definite morphological c h a r a c t e r i s t i c s of CAH,,, C a H , or C&He Contrary to general opinion enhanced t e m p e r a t u r e actually r e s u l t s in the de

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velopment of strength provided the water/solid r a t i o i s low. Also the for

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mation of C&H6 by itself should not n e c e s s a r i l y be construed a s detrimental to strength development; in fact C,AH6 enhances strength. An i n c r e a s e in the surface a r e a need not n e c e s s a r i l y r e s u l t in an i n c r e a s e in strength. S i m i - l a r l y in hydrated CA, identical strengths need not result for products of equal porosity. The present theories seeking to explain strength retentions in high alumina cement produced a t low water/solid r a t i o should a l s o take into account the possible contribution of C&H, phase for strength development.

It appears possible that a durable high alumina concrete can be produced within minutes of curing using a low water/cement r a t i o and higher t e m p e r a - t u r es

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Acknowledgment

T h e a u t h o r s acknowledge with thanks t h e m a j o r e x p e r i m e n t a l contributions of G. M. P o l o m a r k and S . E. Dods and t h e valuable a s s i s t a n c e of P . J.

Lefebvre, E. G. Quinn and R. M y e r s . T h i s p a p e r i s a contribution f r o m t h e Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, and i s published with t h e a p p r o v a l of t h e D i r e c t o r of t h e Division.

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