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Strength of C3A paste containing gypsum and CaCI2
Traetteberg, A.; Sereda, P. J.
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681
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I
I
NATIONAL RESEARCH COUNCIL OF CANADA
I
OONSElL NATIONAL DE RECHERCHES
DU
CANADA
I
I
STRENGTH OF C3A PASTE CONTAINING
I
GYPSUM AND CaC12
by
A. Traetteberg and P. J. Sereda '
0
Reprinted from
CEMEM AND CONCRETE RESEARCH
Vol. 6, No. 4, July 197614 p. -
BUILDING
RESEARCH
-
LIBRARY
-
AUG
10
1976
NATMHAL REKARCH C W C I L DBR Paper No. 681Division of Building Research
6009
:CEMENT
and CONCRETE
RESEARCH.Vol
.
6, pp.461
-474, 1976.Pergamon
Press, Inc. Printedin
theUnited
States.1
STRENGTH OF C ~ A PASTE CONTAINING GYPSUM AND CaC12
A N A ~ ~ ~ E D
IAud Traetteberg
Cement and Concrete Research Institute The Norwegian Institute of Technology
7234 Trondheim, Norway P. J. Sereda
Head, Building Materials Section Division of Building Research
National Research Council of Canada, Ottawa, Canada
(Communicated
by
F.D. Tamas)
(Received March
25,
1976)ABSTRACT
A study has been made of the strength of 3Ca0.AR203 and 3Ca0.AR203 + gypsum pastes as well as of the influence of the admixture CaCR2 on these pastes. The methods employed were microhardness indentation and cone penetration. With 16 per cent CaCR2 added to the 3Ca0.AR203 paste a significant gain in strength was obtained after 3 months, while an 8 per cent CaCR2 addition to the 3Ca0.AR203 + gypsum system gave the highest strength at the same age. Morphological changes in the pastes were found to be related to changes in hardness. From a comparison of the two methods it was concluded that cone penetration measurements are useful up to a certain hardness level.
On a fait une gtude de la rgsistance des p8tes de 3Ca0.AL203 et de 3Ca0.AR203 plus gypse ainsi que de l'effet de l'addition de CaCR2 2
ces mcmes p8tes. On a utilisg les mgthodes de la microduretg par indentation et de la pgngtration d'un c8ne. Lorsqu'on ajoute 16 pour cent de CaCR2 2 la pBte de 3CaO.AR2O3, la rgsistance augmente
considgrablement en trois mois, tandis que l'addition de 8 pour cent de CaCR2 au systsme 3Ca0.AR203 plus gypse produit la rgsistance la plus 6levGe pour cette msme pgriode. Les changements morphologiques de la pBte sont ligs 2 des changements de duretg. Une comparaison des deux mgthodes indique que les mesures de la pgngtration d'un c8ne sont utiles jusqu's un certain degr6 de duretg.
Presented at the VIth International Congress on the Chemistry of Cement, Moscow, September 1974.
461
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No.4
A .
Traetteberg,
P. J .Sereda
I n t r o d u c t i o n
Information about s t r e n g t h developing d u r i n g h y d r a t i o n of t h e p u r e cement components seems t o be l i m i t e d . An a t t e m p t t o i d e n t i f y morphology and m i c r o s t r u c t u r e w i t h s t r e n g t h was t h e o b j e c t of t h i s examination. I n v e s t i g a - t i o n s c a r r i e d o u t by S o l o v ' e v a and Segalova (1) and Segalova, S o l o v ' e v a and Rebinder ( 2 ) i n c l u d e examination of p l a s t i c s t r e n g t h developing i n s u s p e n s i o n s of C3A* and f i n e l y ground q u a r t z sand (C3A/quartz = 5/95) u s i n g a w / s r a t i o i n t h e range 0 . 3 t o 0 . 7 . A c o n i c a l p l a s t o m e t e r was u s e d . The p ' l a s t i c s t r e n g t h was found t o d e c r e a s e w i t h an i n c r e a s e i n t e m p e r a t u r e . T h i s s t r e n g t h d e c r e a s e was r e l a t e d t o a d e c r e a s e i n t h e p a r t i c l e s i z e of t h e hydroaluminates. The formation o f t h e c u b i c C3AH6 caused a breakdown of t h e c r y s t a l l i z a t i o n s t r u c t u r e and t h u s a d e c r e a s e i n s t r e n g t h . The s t r e n g t h o f t h e h y d r a t i o n p r o d u c t s was found t o d e c r e a s e w i t h i n c r e a s i n g s p e c i f i c s u r f a c e a r e a of C3A.
A lowering o f t h e w / s r a t i o caused more i n t e r n a l s t r e s s e s i n t h e hardened s t r u c t u r e .
The s t r u c t u r e formation of C3A suspension and t h e e f f e c t of gypsum and CaCR2 on t h e h y d r a t i o n s t r u c t u r e of C3A has been s t u d i e d by Andreeva and Segalova ( 3 ) . The a d d i t i o n o f gypsum r e s u l t e d i n an i n c r e a s e i n t h e s t r e n g t h of t h e s t r u c t u r e formed, w h i l e t h e a d d i t i o n of CaCR2 t o t h e C3A + gypsum system caused expansion g e n e r a t i n g i n n e r s t r e s s e s and hence a reduced s t r e n g t h of t h e s t r u c t u r e .
The c o n i c a l p l a s t o m e t e r t h a t t h e y used h a s been found convenient f o r s t r e n g t h measurements o f p l a s t i c - v i s c o u s d i s p e r s e systems. I t has been
e v a l u a t e d t h e o r e t i c a l l y i n a p a p e r by Gorazdovskii and Rebinder ( 4 ) .
The t e c h n i q u e of microhardness i n d e n t a t i o n s has been found u s e f u l a s a t o o l f o r s t r e n g t h measurements w i t h t h e added advantage t h a t , because t h e samples a r e n o t broken, a s m a l l e r p i e c e i s r e q u i r e d and t h e same specimen can be used f o r s e v e r a l examinations. In e a r l i e r r e p o r t s from t h e D i v i s i o n of Building Research ( 5 , 6 , 7 ) , t h e r e s u l t s o b t a i n e d by t h i s method on gypsum and cement systems have been r e l a t e d t o o t h e r p h y s i c a l p r o p e r t i e s such a s modulus of e l a s t i c i t y , p o r o s i t y , and f r a c t u r e s t r e n g t h . The s i g n i f i c a n c e of micro- h a r d n e s s o f porous i n o r g a n i c m a t e r i a l s was d i s c u s s e d i n a r e c e n t paper by Sereda (8) and i t was concluded t h a t a r e l a t i o n e x i s t s between microhardness and f l e x u r a l s t r e n g t h f o r cement p a s t e .
I n an e a r l i e r r e p o r t (9) t h e phase r e l a t i o n s , m i c r o s t r u c t u r e and h y d r a t i o n mechanisms i n t h e systems C 3 A and C 3 A + gypsum, w i t h and without CaCR2 added, were s t u d i e d and d i s c u s s e d . The aim of t h e p r e s e n t work i s t o r e l a t e m i c r o s t r u c t u r e w i t h mechanical s t r e n g t h a s measured by t h e two methods, microhardness i n d e n t a t i o n and cone p e n e t r a t i o n , and t o compare t h e two methods
o f measuring s t r e n g t h p r o p e r t i e s .
Exverimental Work
The c h a r a c t e r i s t i c s of t h e C3A ( s u p p l i e d by T e t r a t e c h I n t e r n a t i o n a l , San Diego) a r e t h e f o l l o w i n g : 61.6 p e r c e n t CaO, 37.8 p e r c e n t AR203, 0 . 5 p e r c e n t MgO, and 0.1 p e r c e n t Na20. The C3A had a s u r f a c e a z e a o f 0.58 m2/g. The gypsum and calcium c h l o r i d e , CaCR2.6H20, were b o t h F i s h e r c e r t i f i e d r e a g e n t s . The water used was decarbonated and d e - i o n i z e d . S i x d i f f e r e n t
*
Standard cement nomenclature i s u s e d : C = CaO, A = AL203, H = H20 and w/s = w a t e r : s o l i d r a t i o by weight. G = gypsum1,
Vol.
6,
No. 4
C3A
PASTE, STRENGTH, GYPSUM,CaC12
p a s t e s w i t h a w a t e r / s o l i d r a t i o of 1 . 0 were p r e p a r e d : C 3 A , C3A + 8 p e r c e n t CaCL2, C3A + 16 p e r c e n t CaCR2, C3A + 25 p e r c e n t gypsum, C3A + 25 p e r c e n t gypsum + 8 p e r c e n t CaC9.2, and C 3 A + 25 p e r c e n t gypsum + 16 p e r c e n t CaCR2. The w / s r a t i o of 1 . 0 was s e l e c t e d i n an a t t e m p t t o s i m u l a t e t h e c o n d i t i o n s f o r C3A i n t h e e a r l y h y d r a t i o n p e r i o d of cement p a s t e .
The p a s t e h y d r a t i o n s were performed i n s p e c i a l l y designed p l e x i g l a s s c y l i n d e r s w i t h a d i a m e t e r o f 2.5 cm and h e i g h t of 1 . 3 cm. While c u r i n g under vapour-sealed c o n d i t i o n s , t h e m i x t u r e s were r o t a t e d u n t i l t h e samples had completely s e t t o p r e v e n t s e g r e g a t i o n (10). The same p a s t e was t e s t e d r e p e a t e d l y f o r microhardness a t d i f f e r e n t i n t e r v a l s d u r i n g t h e e n t i r e hydra- t i o n t i m e . The s p e c i a l shape was chosen t o g i v e a s u f f i c i e n t l y l a r g e s u r f a c e a r e a t o permit microhardness and cone p e n e t r a t i o n t e s t s on t h e same sample. From a p r e v i o u s i n v e s t i g a t i o n (9) i t was found t h a t t h e shape of t h e sample can i n f l u e n c e t h e thermal behaviour of t h e p a s t e . To make u s e of d a t a o b t a i n e d p r e v i o u s l y it was ensured t h a t t h e experimental c o n d i t i o n s i n t h e p r e s e n t c a s e d i d n o t change t h e sequences and r a t e s of formation of t h e
d i f f e r e n t h y d r a t i o n p r o d u c t s . T h i s was v e r i f i e d by monitoring t h e t e m p e r a t u r e e f f e c t i n t h e C3A p a s t e .
I
The s t r e n g t h of t h e m a t e r i a l s was examined a f t e r h y d r a t i n g f o r
1
d i f f e r e n t p e r i o d s up t o 6 months.I
i
MicrohardnessA L e i t z Miniload Hardness T e s t e r was used. The t e s t method employed t h e Vickers pyramid i n d e n t e r . The h a r d n e s s was found by t h e formula
where P = load i n grams
I
d = mean v a l u e of t h e i n d e n t a t i o n d i a g o n a l . Cone P e n e t r a t i o nIn t h i s method a cone i s p r e s s e d i n t o t h e p a s t e w i t h d i f f e r e n t p r e s s u r e s and t h e d e p t h of p e n e t r a t i o n i s measured. T h i s method h a s been found p a r t i c u l a r l y u s e f u l f o r s t r e n g t h i n v e s t i g a t i o n s o f cement p a s t e i n t h e e a r l y s e t t i n g p e r i o d . The s i g n i f i c a n c e of t h i s method on d i f f e r e n t k i n d s of m a t e r i a l s h a s been i n v e s t i g a t e d by Sereda (11).
I
I t i s an open q u e s t i o n whether t h e a p p l i e d l o a d p e r u n i t a r e a should be c a l c u l a t e d with an a r e a corresponding t o t h e b a s e of t h e p a r t of t h e cone t h a t h a s p e n e t r a t e d i n t o t h e m a t e r i a l o r t h e c i r c u m f e r e n t i a l a r e a of t h i s p a r t . The c h o i c e between t h e two a r e a s might be c o n s i d e r e d t o be dependent on t h e f r i c t i o n between t h e m a t e r i a l and t h e cone d u r i n g i n d e n t a t i o n . This l e a d s t o two p o s s i b l e formulae f o r c a l c u l a t i n g t h e h a r d n e s s of t h e m a t e r i a l :
H = l o a d (kg) and H = load (kg) r e s p e c t i v e l y 1.05
.
h2 C 2 . 1 h2f o r a 60 O cone, where h = depth of p e n e t r a t i o n .
The l o a d , which was produced by a i r p r e s s u r e i n a p i s t o n - c y l i n d e r arrangement, was s u p p l i e d i n s t e p s of 3 l b and t h e p e n e t r a t i o n d e p t h was measured a t each l e v e l a f t e r r e a c h i n g t h e e q u i l i b r i u m v a l u e . The p l o t of
Vol. 6, No.
4
A.
Traetteberg,
P.J .
Sereda
load v e r s u s t h e d e p t h of p e n e t r a t i o n squared g e n e r a l l y g i v e s two s t r a i g h t l i n e s with t h e s l o p e g r e a t e r f o r t h e l i n e a t low p e n e t r a t i o n . The f i n a l s l o p e of t h e p l o t r e p r e s e n t i n g t h e d e e p e r p e n e t r a t i o n s i n c l u d e s t h e major number o f measurements and i s b e l i e v e d t o r e p r e s e n t t h e h a r d n e s s of t h e t h r e e -
dimensional s t r u c t u r e .
S u r f a c e a r e a was o b t a i n e d by means o f a Numinco-Orr s u r f a c e a r e a - pore volume a n a l y z e r withNg a s t h e a d s o r b a t e .
Scannine E l e c t r o n M i c r o s c o ~ v
The morphology of p a s t e s was s t u d i e d by means of a Cambridge S t e r e o s c a n Mark 2A.
R e s u l t s and D i s c u s s i o n Microhardness
The i n d e n t a t i o n w i t h t h e Vickers i n d e n t e r was made on f i v e s p o t s on t h e s u r f a c e o f t h e p a s t e sample and t h e t e n d i a g o n a l s measured were averaged t o g i v e a h a r d n e s s v a l u e t h a t was assumed t o be r e p r e s e n t a t i v e f o r t h e sample i n v o l v e d . The v a r i a t i o n i n t h e measured v a l u e s was sometimes l a r g e and sometimes i n s i g n i f i c a n t , depending on t h e s u r f a c e f e a t u r e s of t h e p a r t i c u l a r sample. A t y p i c a l s e t o f measurements i s given i n Table I . No a p p r e c i a b l e d r y i n g took p l a c e w h i l e measuring.
TABLE I
Measurements of Microhardness f o r P a s t e o f C3A + G + 8 p e r c e n t CaCR2 a f t e r 3 Days of Hydration ( l o a d 500g)
S e t o f Diagonals Microhardness (kg/mrn2)
Average d i a g o n a l : 160.5
Microhardness : 2 . 2 3 kg/mm2
I n connection with t h e formation of l a r g e amounts o f C3AH6
accompanied by c o n s i d e r a b l e s h r i n k a g e , t h e p a s t e of C3A became a t h i n s l u r r y d u r i n g t h e t e m p e r a t u r e e f f e c t a f t e r 5 t o 6 minutes' h y d r a t i o n . T h i s p a s t e was n o t h a r d enough f o r any s t r e n g t h t e s t s up t o one week, and t h e s u r f a c e of t h e specimen was t o o i r r e g u l a r f o r microhardness i n v e s t i g a t i o n .
V o l . 6, No. 4 465 C3A PASTE, STRENGTH, GYPSUM, CaC12
s t r u c t u r e , F i g u r e l a , o s t e n s i b l y w i t h v e r y weak bonding between t h e c r y s t a l s . The shape of t h e s e c r y s t a l s (rhombic dodekahedrons), however, i s such t h a t a q u i t e dense packing i s p o s s i b l e i f t h e h y d r a t i o n t a k e s p l a c e i n compacts where t h e d i f f e r e n t p a r t i c l e s a r e f o r c e d t o g e t h e r by an o u t e r p r e s s u r e . Under such c o n d i t i o n s t h e hydrogarnet phase y i e l d s s t r o n g e r i n t e r - p a r t i c l e bonding and, hence, a s t r o n g e r m a t e r i a l s t r u c t u r e (12). FIG. l a P a s t e of C3A h y d r a t e d 28 days. FIG. l b P a s t e of C 3 A + 8% CaCR2 h y d r a t e d 28 days. The a d d i t i o n of 8 p e r c e n t CaC!L7 t o t h e C3A p a s t e reduced t h e s e t t i n g t i m e s o t h a t a f t e r 2 hours t h e micro- hardness was measured a t i n t e r v a l s up t o 3 months. Table I1 g i v e s t h e o b t a i n e d v a l u e s which show a s l i g h t i n c r e a s e by h y d r a t i n g t o 30 days and a g r e a t i n c r e a s e a f t e r 3 months. The minor r e d u c t i o n i n s t r e n g t h observed
i n t h e p e r i o d between 3 and 30 days might be due t o a more a c t i v e
formation of t h e c u b i c C3AH6, an e f f e c t t h a t has been confirmed i n a p r e v i o u s work ( 9 ) . The micrograph, F i g u r e l b , shows a dominant p l a t y s t r u c t u r e t h a t i s expected t o r e s u l t i n improved i n t e r - p a r t i c l e bonding compared w i t h t h e c u b i c phase. An ageing e f f e c t might b e t h e r e a s o n f o r t h e s t r e n g t h d e v e l o ~ e d a f t e r 3 months. F I G . l c T h i s i s s i p p o r t e d b; t h e morphological P a s t e o f C3A + 16% CaCR2 h y d r a t e d change a s demonstrated i n F i g u r e 2b
and by t h e d e c r e a s e d s u r f a c e a r e a a t 28 days. t h i s s t a g e , Table I11 and Table I V .
Vol.
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No.
4A. T r a e t t e b e r g , P. J. Sereda
TABLE I 1
Microhardness (kg/mm2) for Pastes of C3A With Addition of 25 Per Cent Gypsum and CaCt2
Hydrated for Different Periods
Hydration C3A + 8% C 3 A + 1 6 % C 3 A + C 3 A + G + C 3 A + G +
Time C3A CaCi2 CaCt2 gypsum 8% CaCk2 16% CaCk2
2 hours 0.46
-
-
0.98-
4 hours 0.60 0.75 1.54-
8 hours 0.98 1.28-
2.42 1 day 1.00 1.77 0.58 2.60 2.84 2 days 1.08 2.26 0.94 3.14 3.16 3 days 1.77 2.39 1.11 2.26 3.05 7 days 1.38 3.52 1.32 2.63 2.91 10 days 1.37 3.32 1.45 2.67 3.10 14 days (15) 1.30 4.29 1.73 2.90 3.54 30 days 1.85 10.20 1.65 3.41 3.65 3 months 8.74 14.07 6.24 12.64 9.52 F I G . 2a P a s t e o f C3A h y d r a t e d 3 months. FIG. 2b P a s t e o f C3A + 8% CaCL2 h y d r a t e d 3 months.V o l . 6, No. 4
C3A PASTE, STRENGTH, GYPSUM, CaC12 The paste of CqA + 16 Der
cent CaCR2 showed a simiiar trenh in strength development as the previous mixture. While higher values in microhardness were obtained in this series, the formation of C3AH6 was further retarded by this addition. This corresponds to the delay in the small decrease in strength that, in this case, occurred after about 10 days. The ageing effect seemed to take place at an earlier stage for this mixture. This might be suggested from the micrographs, Figures lb and c, that demonstrate a denser packing and a smaller crystal size of the hydro- alwninates in the C3A + 16 per cent CaCR2 paste. The increased addition of CaCR2 to paste of C3A improves the strength of the structure of the hydration products up to 1 month, an
effect which might be related to FIG. 2c
morphological features (Figure 2c). Paste of C3A + 16% CaCL2 hydrated
3 months.
TABLE I 1 1
Characteristics of Pastes of C3A and C3A + Gypsum With and Without CaC!L2 Hydrated for 30 Days
Paste Phases Present Surface Micro- Cone
Area hardness Penetration
(m2/g) (kg/mm2) (kg/mm2)
C3&, minor amounts 4.2
-
0.28of C2AH8 and C4AH13
C3A + 8% CaCE2 C3&, C 2 M 8 and 8.2 1.85 0.96
(SS) C4AH13
-
C3A.CaCE2.H10C3A + 16% CaCi2 C 3 w 6 , C 2 M 8 and 9.9 10.20 7.53
(SS) C4M13 -, C3A.CaCE2.H10
C3A + gypsum (ss) C3A.CaS04.H13
-
11.0 1.65 1.00I C 4 M 1 3, C 2 M 8 and traces of C 3 M 6 C3A + G + 8% (SS) C3A.CaS04.H13 - 13.3 3.41 3.60 CaC!L2 C3A.CaCL2.Hl0
-
I C 4 M 1 3 and C2Aii8 C3A + G + 16% (ss) C3A.CaS04.H13-
12.6 3.65 5.31 CaC!L2 C3A.CaCL2.H10-
C 4 m l 3 and C 2 M 8I
(ss) = solid solutionV o l .
6 ,
No. 4A.
Traetteberg,
P. J .Sereda
TABLE I V
C h a r a c t e r i s t i c s o f P a s t e s of C 3 A and C3A + Gypsum With and Without CaCR2 Hydrated f o r 3 Months P a s t e S u r f a c e Micro- Cone
Area hardness P e n e t r a t i o n (m2/g) (kg/mm2 (kg/mm2)
C 3 A + 25% gypsum 3.7 6.24 4.94
The microhardness of t h e C 3 A + 25 p e r c e n t gypsum p a s t e appeared t o be c l o s e t o t h a t f o r t h e C3A + 8 p e r c e n t CaCR2 system. Traces of C3AH6 have been d e t e c t e d a t l a t e s t a g e s i n t h e h y d r a t i o n of t h e C3A + gypsum p a s t e ( 9 ) , and t h i s e f f e c t might a l s o be t h e reason f o r t h e s l i g h t d e c r e a s e i n micro- hardness around 30 days. The m i c r o s t r u c t u r e of t h e s e two m i x t u r e s i s a l s o q u i t e s i m i l a r , both a t 30 days (compare F i g u r e s l b and 3a) and a t 3 months
( F i g u r e s 2b and 4 a ) . The i n c r e a s e i n s t r e n g t h w i t h t h e a d d i t i o n of gypsum t o C3A i s i n agreement with t h e r e s u l t of Andreeva and Segalova ( 3 ) .
FIG. 3a P a s t e of C 3 A + 2 5 % gypsum h y d r a t e d 28 days. FIG. 3b P a s t e of C3A + g + 8% CaCR2 h y d r a t e d 28 days.
V o l . 6, No. 4 469 C3A PASTE, STRENGTH, GYPSUM, CaC12
FIG. 3c P a s t e o f C 3 A + G + 16% CaCE2 h y d r a t e d 28 days. FIG. 4a P a s t e o f C3A + 25% gypsum h y d r a t e d 3 months. F I G . 4b P a s t e of C3A + G + 8% CaCR2 h y d r a t e d 3 months. FIG. 4c P a s t e of C3A + G + 16% CaCE2 h y d r a t e d 3 months.
The C 3 A + gypsum + 8 p e r c e n t CaCR2 mixture s e t w i t h i n t h e f i r s t 10 minutes of h y d r a t i o n . The hardness i n c r e a s e d s t e a d i l y up t o 2 days, when a small d e c r e a s e took p l a c e . The e f f e c t i n t h i s c a s e cannot be e x p l a i n e d by C3AH6 formation which i s prevented w i t h t h e a d d i t i o n of CaCR2 t o t h e C3A +
gypsum p a s t e ( 9 ) . The reason f o r t h e small r e d u c t i o n i n s t r e n g t h i s n o t known, but it might be due t o some r e o r g a n i z i n g of t h e bonding between t h e c r y s t a l l i t e s . The ageing e f f e c t a f t e r 3 months i s e s p e c i a l l y e v i d e n t i n t h i s c a s e . The i n c r e a s e d s t r e n g t h i n t h i s p a s t e might a r i s e from an improved
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bonding o r b e t t e r packing o f t h e s m a l l e r c r y s t a l s , a s i s i l l u s t r a t e d i n t h e micrographs, F i g u r e s 3b and 4b and p o s s i b l y due t o some carbonation. T h i s improved s t r e n g t h w i t h t h e a d d i t i o n of CaCL2 i s i n c o n t r a d i c t i o n w i t h t h e r e s u l t o f Andreeva and Segalova ( 3 ) . The r e a s o n f o r t h e d i f f e r e n t r e s u l t might a r i s e from t h e i r u s e o f s u s p e n s i o n s o f C3A and q u a r t z compared w i t h a p u r e p a s t e h y d r a t i o n of C3A i n o u r work.
An i n c r e a s e d a d d i t i o n o f CaCI2 does n o t seem t o g i v e any s i g n i f i c a n t improvement t o t h e h a r d n e s s o f t h e p a s t e . The s l i g h t d e c r e a s e i n s t r e n g t h a f t e r 2 days i s a l s o d e t e c t e d h e r e . F i g u r e s 3b and 3c a l s o demonstrate a s i m i l a r morphology. The microhardness a f t e r 3 months i s lower t h a n w i t h t h e
8 p e r c e n t a d d i t i o n and t h e micrograph, F i g u r e 4 c , i n d i c a t e s a d e n s e r packing o f t h e s t r u c t u r e b u t more c r a c k s and p o r e s t h a n i n t h e p r e v i o u s c a s e . The lower s t r e n g t h might be due t o some s t r e s s developed w i t h t h e a d d i t i o n a l amount o f CaCI2.
I t i s known t h a t carbon d i o x i d e may c a u s e s u r f a c e hardening of cement. A doubt t h e r e f o r e a r o s e t h a t t h e i n c r e a s e i n s t r e n g t h a t l a t e r h y d r a t i o n p e r i o d s may n o t o n l y be due t o an ageing e f f e c t b u t a l s o p a r t l y t o c a r b o n a t i o n o f t h e s u r f a c e . A 2-mm-thick p i e c e from t h e s u r f a c e of t h e samples was c u t o f f a f t e r 6 months1 h y d r a t i o n and analyzed f o r C02 c o n t e n t . Microhardness was measured on t h e o l d and new s u r f a c e s of t h e samples and t h e measurements a r e recorded i n Table V. The r e s u l t s r e v e a l t h a t some carbona- t i o n had t a k e n p l a c e . The e f f e c t does n o t seem t o have had a s i g n i f i c a n t e f f e c t on t h e microhardness, however, e x c e p t i n t h e c a s e s o f samples w i t h 8 p e r c e n t CaCL2 added t o t h e C3A and C3A + gypsum p a s t e s where a lower microhardness was o b t a i n e d on t h e i n n e r s u r f a c e . Experience h a s shown t h a t t h e e f f e c t of c a r b o n a t i o n i s f i r s t observed on t h e s u r f a c e and it i s
r e a s o n a b l e t o c o n s i d e r t h e v a l u e of microhardness f o r t h e new ( c u t ) s u r f a c e a s r e p r e s e n t i n g t h e b u l k p r o p e r t y .
TABLE V
Carbon Dioxide Content and Microhardness for Pastes of C3A With and Without Admixtures, Hydrated for 6 Months
Paste C02 content (%) Microhardness (kg/mm2) original surface cut surface*
C3A 0.95 C3A + 8% CaCQ2 2.24 C3A + 16% CaCQ2 4.24 C3A + 25% gypsum 3.42 C3A + G + 8% CaCL2 3.33 C3A + G + 16% CaCL2 2.21
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Cone P e n e t r a t i o n
F i g u r e 5 shows a t y p i c a l r e s u l t o f t h e cone p e n e t r a t i o n measurement t h a t i n c l u d e s a number of l o a d v e r s u s d e p t h d e t e r m i n a t i o n s . High humidity was maintained d u r i n g measurement t o p r e v e n t d r y i n g . In a few c a s e s t h e r e were t h r e e d e t e r m i n a t i o n s c a r r i e d o u t t o e s t a b l i s h t h e r e l i a b i l i t y of t h e measurements. I t was found t h a t i n t h i s range of h a r d n e s s o n l y a s l i g h t s c a t t e r i n g i n t h e v a l u e s took p l a c e and one d e t e r m i n a t i o n was, t h e r e f o r e , c o n s i d e r e d s u f f i c i e n t t o g i v e a measure of t h e s t r e n g t h of t h e m a t e r i a l . The h a r d n e s s , c a l c u l a t e d p e r u n i t c i r c u m f e r e n t i a l a r e a of t h e cone, i s found t o be t h e most s u i t a b l e f o r t h e s e samples. A f t e r 7 days t h e 8 I I i n i t i a l l y weak s t r u c t u r e of t h e C 3 A p a s t e reached a 7 - h a r d n e s s s u f f i c i e n t l y l a r g e /* t o p e r m i t cone p e n e t r a t i o n measurements t o be done, 6
-
-
I I /*Table VI. The micrograph of t h e 30 d a y s t s a m p l e , F i g u r e la,
i
shows a l o o s e morphology3r
-
i
mainly of t h e c u b i c phasern C3AHs. The ageing e f f e c t
x
./
was v e r y pronounced i n t h i s s4--
u 0 p a s t e a s t h e h a r d n e s s A i n c r e a s e d t o 2 3 kg/mm2 a f t e r 3-
3 monthst h y d r a t i o n . The micrograph, F i g u r e 2a dem- H A Y D N E S S = == 1 . 1 1 b g v m 2 ~ . l h ' o n s t r a t e s a d e n s e r s t r u c t u r e o f t h e c r y s t a l s and t h e d e c r e a s e d s u r f a c e a r e a o f I/:
,
I
t h i s e v i d e n t l y c o a l e s c e n c e sample corresponds of (Table t h e i n d i v i d u a l IV) t o ao p a r t i c l e s .
0.5 1.0 1 . 5
P E N E T R A T I O N D E P T H S Q U A R E D , m m 2 The h a r d n e s s v a l u e s
o b t a i n e d by cone p e n e t r a t i o n FIG. 5 i n t h e C3A + 8 p e r c e n t
Cone p e n e t r a t i o n measurements f o r CaCR2 p a s t e a s a f u n c t i o n o f p a s t e o f C 3 A + gypsum + 8% CaCa, h y d r a t i o n time show a s i m i l a r
h y d r a t e d f o r 3 d a y s . p a t t e r n a s t h o s e o b t a i n e d by microhardness measurements. The observed d e c r e a s e i n s t r e n g t h , c o n s i d e r e d t o be due t o more a c t i v e formation o f C3AH6, i s a l s o d e t e c t e d i n t h e p a s t e o f C 3 A + 16 p e r c e n t CaCR2, but was s l i g h t l y d e l a y e d i n t h i s c a s e . Both methods show a marked s t r e n g t h i n c r e a s e i n t h e 3 month samples, but t h e a b s o l u t e v a l u e s d i f f e r . I t i s b e l i e v e d t h a t a f t e r a c e r t a i n h a r d n e s s l e v e l t h e cone p e n e t r a t i o n method g i v e s v a l u e s t h a t a r e t o o h i g h .
For t h e C3A + gypsum system, cone p e n e t r a t i o n gave h a r d n e s s v a l u e s c l o s e t o t h o s e from t h e microhardness t e s t s . T h i s was a l s o t h e c a s e f o r t h e
samples w i t h t h e a d d i t i o n o f 8 F e r c e n t CaCR2 t o t h e system. However, t h e cone p e n e t r a t i o n method r e s u l t e d i n h i g h e r h a r d n e s s v a l u e s w i t h t h e 16 p e r c e n t a d d i t i o n of CaCR2.
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TABLE V I
H a r d n e s s Values ( k g / m m 2 ) Obtained by Cone P e n e t r a t i o n M e a s u r e m e n t s f o r P a s t e s of C j A With Addition of 25 P e r Cent Gypsum and C a C i z Hydrated
f o r Different P e r i o d s
Hydration C3A C3A
+
8% C3A+
16% C3A+
C , A + G + C 3 A + G +T i m e C aC 12 CaC t a gypsum 8% CaCfi2 16% C a C 4
$ hour 1 h o u r 2 h o u r s 4 h o u r s 8 h o u r s 24 h o u r s 2 d a y s 3 d a y s 7 d a y s 10 d a y s 14 d a y s 30 d a y s 3 m o n t h s
Table I11 d e m o n s t r a t e s t h e composition of t h e p a s t e s a t 30 d a y s ' h y d r a t i o n a s o b t a i n e d i n a n o t h e r r e p o r t (9), t o g e t h e r w i t h c o r r e s p o n d i n g d a t a o f s u r f a c e a r e a and h a r d n e s s .
A comparison between t h e d a t a a t 30 days and 3 months (Tables I11
and I V ) c l e a r l y d e m o n s t r a t e s t h e a g e i n g e f f e c t f o r a l l samples, i . e . , an
i n c r e a s e i n s t r e n g t h accompanied by a d e c r e a s e i n s u r f a c e a r e a . The a g e i n g might be due p a r t l y t o t h e h i g h humidity c o n d i t i o n s o f s t o r a g e .
Conclusions
The i n v e s t i g a t i o n o f s t r e n g t h i n d i f f e r e n t C 3 A systems by means of microhardness and cone p e n e t r a t i o n has shown t h a t changes i n t h e s e p r o p e r t i e s can be i d e n t i f i e d w i t h morphological changes. The d e n s e r morphology f o r a l l s i x p a s t e s a f t e r 3 months' h y d r a t i o n i s b e l i e v e d t o be t h e r e s u l t of an ageing e f f e c t i n v o l v i n g c o a l e s c e n c e of t h e i n d i v i d u a l c r y s t a l s by i n c r e a s i n g t h e a r e a of c o n t a c t due t o d i s s o l u t i o n and
r e c r y s t a l l i z a t i o n . The reduced s u r f a c e a r e a o b t a i n e d f o r a l l t h e p a s t e s a t t h i s s t a g e and evidence from micrographs s u p p o r t t h i s assumption.
The i n f l u e n c e o f t h e admixture CaCR2 on t h e s t r e n g t h of C3A and C3A + gypsum p a s t e s i s s i g n i f i c a n t up t o 1 month h y d r a t i o n where an i n c r e a s e of CaCI2 means an i n c r e a s e i n s t r e n g t h . However, i t s i n f l u e n c e i n connection w i t h t h e ageing e f f e c t i s l e s s c l e a r . The 16 p e r c e n t a d d i t i o n t o t h e C3A
V o l . 6, No. 4 473 C3A PASTE, STRENGTH, GYPSUM, CaC12
paste gave the highest hardness value but it is of the same magnitude,as the
one for the C3A paste alone. For the C3A + gypsum system the 8 per cent
CaCR2 addition gave the hardest structure.
A comparison between the two testing methods, microhardness indentation and cone penetration, shows good agreement in some mixtures and only fair agreement in others. It might be concluded that cone penetration is a convenient method for softer materials, but has its limitation on materials with hardness above a certain level.
Acknowledgements
The authors would like to thank E.G. Quinn for his valuable
contribution in the experimental work and in discussions. The assistance of R.E. Myers and G.M. Polomark is also greatly appreciated.
One of the authors, Aud Traetteberg, would like to thank the Royal Norwegian Council for Scientific and Industrial Research for financial
I support and the National Research Council of Canada for the opportunity to
work in the Division of Building Research. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.
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