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Studies about a sulphate resistant cement. Influence of admixtures
~
Cementand Concrete Research, Vol.24, No.6, pp. 1177-1184,1994 Pergamon
Copyright © 1994 Elsevier Science l.,td Printed in the USA. Allrights reserved
0008-8846/94 $6.00+.00
0008-8846(94)
STUDIES ABOUT ADMIXTURES
A
SULPHATE
00061-1
RESISTANT
CEMENT.
M.T. S l a n c o ( * ) , S. G a r c i a ( * * ) , S. G i m 6 n e z ( * * * ) , F. Puertas(*) and T. V~zquez(*)
INFLUENCE
A.
OF
Palomo(*),
(*) I n s t i t u t o de C i e n c i a s de la C o n s t r u c c i 6 n "Eduardo (C.S.I.C.) - A p a r t a d o 19002 - M a d r i d (Spain) (**) U n i v e r s i d a d Complutense. M a d r i d (***) Repsol Quimica. C e n t r o de Investigaci6n. M a d r i d
Torroja"
(Ref~eed) (R~eived Octo~r 14, 1993; in h a l f o ~ M ~ 10, 19~)
ABSTRACT
S t a r t i n g from an i n d u s t r i a l cement o b t a i n e d by m i l l i n g t o g e t h e r a new low e n e r g y c l i n k e r and gypsum, two b l e n d e d c e m e n t s containing 20% wt of fly ashes and 10% wt of l i m e s t o n e r e s p e c t i v e l y , have been prepared. The b e h a v i o u r of these three c e m e n t s in Na2SO 4 d i s s o l u t i o n and sea w a t e r has b e e n studied. The m e t h o d used for t e s t i n g the s p e c i m e n s has been the KochS t e i n n e g e r one. As a c o n s e q u e n c e of the study it can be said that the three c e m e n t s keep a good b e h a v i o u r in all the aggressive media although the strengths are lower ( i n d e p e n d e n t l y of the age of the specimens) in Na2SO 4 and sea w a t e r than in d i s t i l l e d water. Also, the c o m p o s i t i o n of the hydrated pastes conserved in the aggressive liquids for d i f f e r e n t p e r i o d s of time has b e e n studied. The e v o l u t i o n of the c o n t e n t of Ca(OH)2 , g y p s u m and Mg(OH)2 in the p a s t e s as well as the v a r i a t i o n s o b s e r v e d in the m o r p h o l o g y of C-S-H gel a l o n g the time and in f u n c t i o n of the a g g r e s s i v e medium, seem to keep a good c o r r e l a t i o n w i t h the s t r e n g t h s evolution. F i n a l l y it has b e e n c o n c l u d e d that those cements p r e p a r e d w i t h the new low e n e r g y cement have such a c o m p o s i t i o n that makes them to be r e s i s t a n t to the sulphate attack. Introduction About 1.000 tons of clinker, white P o r t l a n d type, have been o b t a i n e d t h r o u g h a new m a n u f a c t u r i n g process. P r e v i o u s w o r k s on this s u b j e c t have b e e n d e s c r i b e d in several p u b l i c a t i o n s (i), (2), (3). S u m m a r i l y speaking, the p r o d u c t was o b t a i n e d s t a r t i n g from gypsum, fluorspar, limestone, kaolin, and sand, by p r o p e r l y d d o s i f y i n g a raw mixture; the c h e m i c a l a n a l y s i s of w h i c h is shown on Table I. That Table also shows the m i n e r a l o g i c a l c o m p o s i t i o n of
1177
1178
M.T. Blanco et al.
Vol. 24, No. 6
the clinker. The m a t e r i a l was o b t a i n e d t h r o u g h a wet process, the m a x i m u m c l i n k e r i n g t e m p e r a t u r e n e a r e d 1.350~C.
and
It has been o b s e r v e d that the q u a l i t a t i v e c o m p o s i t i o n of this new c l i n k e r does not s i g n i f i c a n t l y d i f f e r from that of c o n v e n t i o n a l clinker, but in the p r e s e n c e of f l u o r e l l e s t e a d i t e and s u l f o - a n d fluoaluminates.
with
The t e c h n i c a l b a l a n c e - s h e e t of the i n d u s t r i a l the new p r o c e d u r e gave p o s i t i v e results (3).
process
tested
Gim~nez et al. have s t u d i e d the b e h a v i o u r shown by the new cement a l o n g time and faced to a g g r e s s i v e s o l u t i o n s (4). Perhaps, the most i m p o r t a n t c o n c l u s i o n to be e x t r a c t e d from this study is that the new cement b e h a v e s as a good sulphate r e s i s t a n t m a t e r i a l due to its low C3A content. A l t h o u g h special a t t e n t i o n was p a i d in o r d e r to c o r r e l a t e the m i c r o s t r u c t u r e of pastes w i t h the h a r d l y o b s e r v e d m e c h a n i s m s of e x p a n s i o n due to sulphate presence, more i n v e s t i g a t i o n must be c a r r i e d out. In that sense, data p u b l i s h e d by o t h e r a u t h o r s should be taken in c o n s i d e r a t i o n (5,6 and 7). Finnaly, the fact of k n o w i n g that a d m i x t u r e s can affect sulphate-resistant properties of concrete (8) justifies d u r a b i l i t y studies c o n d u c t e d w i t h the new cement, to w h i c h ashes (20%) and limestone (10%) were r e s p e c t i v e l y added; r e s u l t s from the study are p r e s e n t e d here.
the the fly the
Experimental Procedure In the present study three d i f f e r e n t c e m e n t s were used: CB (cement o b t a i n e d by a p p l y i n g the p r o d u c t i o n m e t h o d for the lowe n e r g y clinker, with a fineness of 436.5 m2/Kg); C B C (80% of CB plus an a d d i t i o n of 20% fly ash type F-ASTM C 618-85-); and CBCa (90% of CB plus 10% c a l c a r e o u s filler). The d u r a b i l i t y of cements t o w a r d s a g g r e s s i v e s o l u t i o n s was e v a l u a t e d by the K o c h - S t e i n e g g e r method. The s o l u t i o n s used were: d e i o n i z e d water, Na2SO 4 4.4% wt s o l u t i o n and a r t i f i c i a l sea w a t e r A S T M D 1141-76. The c o r r e s p o n d i n g specimens were g r a d u a l l y e x t r a c t e d from the a g g r e s s i v e solutions, at p r e - d e t e r m i n e d ages and t e s t e d at flexural
TABLE
I
Raw Mix Chemical Analyses and Clinker Mineralogical Data through XRD) RAW MIX (% wt)
SiO 2 14.7
AI203 1.5
CLINKER (% wt)
C3S 69.5
C2S 6.8
* Fluorelleastadite ** CIIA7. CaF2 *** CaSO 4. I/2H20
Fe203 0.2
CaO 45.4
MgO 0.4
SO 3 2.3
FL* 14.0
CaO 3.0
C3A 3.0
C4A3S 2.0
(Obtained
CaF 2 0.8
I.L. 34.5
CAF** 0.5
H*** 0.7
Vol. 24, No. 6
SULPHATE-RESISTA CEMENT, NT ADMIXTURES,BLENDS
and c o m p r e s s i o n . The e v o l u t i o n of the m i n e r a l o g i c a l w i t h time w a s m a i n l y s t u d i e d by X - R a y d i f f r a c t i o n .
Results New white cement
and
1179
composition
Discussion
(CB)
Table II shows the flexural and compressive strengths d i s c l o s e d by CB c e m e n t m o r t a r s s u b m e r g e d in the t h r e e m e d i a d e s c r i b e d above until 545 days.
TABLE Flexural
AGES days
1 7 28 56 90 180 270 365 545
and Compressive
WATER FLEX. MPA 5.60 5.58 6.17 6.47 6.34 6.57 6.31 5.95 6.32
II
Strengths Media
of CB Mortars
4.4% Na2SO 4 SOL.
in Agressive
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
37.24 32.36 49.35 52.92 63.81 58.23 63.31 54.31 61.27
6.25 6.06 6.61 5.49 5.42 5.27 5.33 4.43 4.65
39.99 33.25 38.61 41.25 52.67 40.96 30.37 31.28 32.70
5.73 5.19 5.44 4.56 5.04 4.61 4.57 4.61 4.53
45.99 26.99 39.50 44.11 48.23 53.13 48.33 53.41 55.25
In o r d e r to e s t i m a t e the e v o l u t i o n in time of c r y s t a l l i n e c o m p o u n d s in the c e m e n t p a s t e (CB), w h e n m o r t a r s p e c i m e n s of that c e m e n t w e r e s u b m e r g e d in d i f f e r e n t solutions, the i n t e n s i t y of the X R D r e f l e c t i o n s d e s c r i b e d in (4) was m e a s u r e d and the v a l u e was d i v i d e d into the f l u o r e l l e s t e a d i t e r e f l e c t i o n v a l u e (28 = 25.64). It m u s t be p o i n t e d out that b o t h IEtt/IFL and Icalcite/IFL q u o t S e n t s r e m a i n c o n s t a n t b o t h in time and in all media. In F i g u r e 1 the v a l u e s of the f o l l o w i n g q u o t i e n t s : ICH/IFL , Igyp.sum/IFL and IMg(OH)2/IFL are s h o w n v e r s u s time in the d i f f e r e n t meala. The use of a f l u o r e l l e s t e a d i t e r e f l e c t i o n as i n t e r n a l s t a n d a r d is j u s t i f i e d by the fact that this c o m p o u n d is a c o n s t i t u e n t of the clinker, b e c a u s e of its high i n s o l u b i l i t y and b e c a u s e it does not u r d e r g o any m o d i f i c a t i o n w i t h time. From Figure a)
1 it is i m p o r t a n t
to emphasize:
The c o n t e n t in Ca(OH)2 is at 7 days s i m i l a r in CB c e m e n t p a s t e s in c o n t a c t w i t h the t h r e e media, a l t h o u g h t h o s e from Na2SO 4 s o l u t i o n and sea w a t e r are s l i g h t l y lower. D u r i n g the t r e a t m e n t time, the c o n t e n t in Ca(OH)2 d e c r e a s e s in the p a s t e s e x p o s e d to a g g r e s s i v e m e d i a c o m p a r e d to those p r e s e r v e d in water.
1180
M.T. Blanco et al.
b)
Vol. 24, No. 6
In the case of m o r t a r s o b t a i n e d using the new cement and p r e s e r v e d in a Na2SO 4 s o l u t i o n it should be o b s e r v e d that simultaneously to the d e c r e a s e in the q u a n t i t y of Ca(OH)2 there is also an important increase of the content of g y p s u m w h i c h is the m a j o r c r y s t a l l i n e phase at the 180 day stage. :
1
7
-
1
~
NEWC£MENT(CB) ~
- - H = O ~
CH
------No=SO~. . . . . .
..--m O
/
~,o. Ii ~~ ~
~
\..
\
i
"~
/
"---~
i'~
//
-.
~
/
.......
~
~.~'~
~
=
--
Z ~ <
/#
~
/
#/
/
,,~.. ~
~ -,
..~. ~ '
~."
C
GYPSUM FL
GYPSUM~
SEA
[~" / * " ' "
-- ' ~
..
./
~ .,
~ ~"
~
\../
Z
~
NozSO,
CH
• --
~' FL ~ ~
-~-,_~ i " .~ •
~'~...~
~
/
5-
X
~2
/
/
2
xi
ill #
x/
x/ Ii
~x--x--x--x--~__ ~-~.~.
x--x-- x--x --x --x /
~
i
~
~
~
~
DAYS
FIGURE
1
The v a l u e s of x - r a y i n t e n s i t y ratios: . IC-./IFL, I~.-sum/IFL and ~ ~ ~ P . IMg(OH)2/IFL, for new cement CB versus t~me ~n d l f f e r e n t m e d ~ a The f o l l o w i n g d e s c r i b e d above:
conclusions
could
be
drawn
from
the
results
a)
Mortars between days.
o b t a i n e d from CB cement and p r e s e r v e d in w a t e r reach, 28 and 56 days, almost the s ~ e s t r e n g t h than at 545
b)
CB cement m o r t a r s s u b m e r g e d in Na2SO 4 s o l u t i o n show up to the 28 day stage g r e a t e r e n d u r a n c e than those m o r t a r s kept u n d e r H20 , due to ~ p s u m f o r m a t i o n (at the expense of Ca(OH)2 ) that m a k e s the system become more dense. The m a s s i v e ~ps~ formation with detriment b e c a u s e of its e x p a n s i v e nature, brings along d e c r e a s e in a l o n g - t e r m basis. In
addition
it
seems
probable
that
the
C-S-H
to Ca(OH)2 , an e n d u r a n c e
gel
evolution
Vol. 24, No. 6
SULPHATE-RESISTA CEMENT, NT ADMIXTURES,BLENDS
1181
process, in r e f e r e n c e to its m o r p h o l o g y and q u a l i t i e s , c o u l d b e c o m e a l t e r e d b e c a u s e of the e x i s t e n c e of SO42- and Na + ions and that the e n d u r a n c e d e c r e a s e c o u l d not o n l y be due to the g y p s u m e x p a n s i v e f o r m a t i o n p r o c e s s (4). c)
The e n d u r a n c e d e c r e a s e of CB m o r t a r s p r e s e r v e d in sea w a t e r in r e l a t i o n to those p r e s e r v e d in w a t e r c o u l d be p a r t l y due to Mg(OH) 2 f o r m a t i o n at the e x p e n s e of Ca(OH)2 w h i c h w o u l d be d i s s o l v e d (the Mg(OH)2 m o l a r v o l u m e is = 2/3 lower t h a n the Ca(OH)2 one).
It seems that in some w a y the C-S-H gel m i c r o s t r u c t u r e b e i n g m o d i f i e d by the a g g r e s s i v e s o l u t i o n (4).
New w h i t e
cement
with
20%
fly ashes
is
(CBC)
Table III shows the flexural and c o m p r e s s i v e strengths d i s p l a y e d at d i f f e r e n t ages by the m o r t a r s p e c i m e n s made w i t h C B C c e m e n t and s u b m e r g e d in the t h r e e m e d i a m e n t i o n e d before.
TABLE Flexural
and C o m p r e s s i v e
AGES days
WATER FLEX. MPA
1 7 28 56 90 180 270 365 545
5.01 4.81 5.72 6.97 7.02 7.49 8.29 7.69 8.33
III
Strengths Media
of CBC M o r t a r s
4.4% NaqSO 4 SOL.
in A g r e s s i v e
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
38.17 33.87 32.80 54.79 48.47 62.06 66.83 71.88 70.58
4.95 5.31 5.54 5.32 7.07 5.80 6.02 5.63 4.92
27.04 28.27 35.54 47.16 50.67 49.71 52.26 43.97 49.14
5.15 4.66 4.90 6.05 5.54 6.34 5.29 6.05 5.68
28.30 22.24 28.44 34.73 37.19 48.28 35.36 44.59 -
The m i n e r a l o g i c a l c o n s t i t u e n t s of the C B C c e m e n t p a s t e s are n a t u r a l l y the same as those of the CB s t a n d a r d cement, w h i c h e v e r the c o n s e r v a t i o n m e d i a used. However, a c l e a r v a r i a t i o n in the p r o p o r t i o n s and e v o l u t i o n of the p h a s e s is found. Figure
2 shows the v a l u e s of the ratios:
ICa(OH) 2 IFell
versus
time
ICaSO4.2H2 O IFell
in the d i f f e r e n t
IMg(OH)2 IFell
media.
It m a y be o b s e r v e d in such a F i g u r e that the a m o u n t of Ca(OH)2
1182
M.T.Blancoetal.
,7-
Vol.24,No. 6
C.B.C.
15~I0~-
CH FL
~
H20
~
CH N°zS04 F L
....
NozSO,~GYP sum
FL
CH .......
= ~
TC
Gyp~m p ~ FL |<
~.
u~o.~=l ~
~ ~
FL J
// \
I1~ ~,-
,i
~
~
""
, ~
~
~ ~%
~.~.
~
/ "~.~
~
~
-"
-.... ",~.. ~..
!
,,...
~
~
"~'~ • - ~.~.~.
~ !
~
/
~
~...
_ _ ~
"~'~'~
~
~
~ ' - ~ - ~ ' ~
~
~
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. ....~..~,"
•
~
'
,~0
d~
~
DAYS FIGURE
2
The v a l u e s of x - r a y i n t e n s i t y ratios:. ICH/IFL,, ~I~gypsum/IFL and IMg(OH)2/IFL,. for new cement CBC versus time in Q i i i e r e n t m e d i a d e c r e a s e s in w a t e r - c u r e d s p e c i m e n s w i t h respect to the c o n t e n t s in the s a m p l e s w i t h o u t any a d d i t i o n (Fig. I), due to the p o z z o l a n i c r e a c t i o n of ashes w i t h Ca(OH) 2. The i n c r e a s e in the CBC cement s t r e n g t h s w i t h r e s p e c t to those shown by CB cement is due to the m e n t i o n e d p o z z o l a n i c r e a c t i o n w i t h the p r o d u c t i o n of CSH gel, a p r o d u c t w h i c h d e n s i f i e s the system. The e v o l u t i o n w i t h time of the specimens kept in Na2SO 4 s o l u t i o n is s p e c i m e n s kept in water.
Ca(OH)2 similar
contents to that
in of
the the
There is a gradual increase of the g y p s u m c o n t e n t s in the s p e c i m e n s s u b m e r g e d in Na2SO 4 solution, but to a lesser d e g r e e than those c o r r e s p o n d i n g to the CB cement. This is due to the lesser amount of Ca(OH) 2 a v a i l a b l e and to the h i g h e r d e n s i t y of the s y s t e m w h i c h h i n d e r s access to sulfate ions. In these s p e c i m e n s there are two c o n c u r r i n g reactions: (i) the p u z z o l a n i c r e a c t i o n of the fly ashes and (ii) the f o r m a t i o n of g y p s u m by r e a c t i o n of sulfates w i t h Ca(OH) 2 • In spite of the fact that the p o z z o l a n i c r e a c t i o n takes place in these m o r t a r s s u b m e r g e d in Na2S04, their l o n g - t e r m s t r e n g t h s did not i n c r e a s e as could be expected, as it h a p p e n s in s p e c i m e n s kept
Vol. 2~, No. 6
SULPHATE-RESISTANT CEMENT, ADMIXTURF~, BLENDS
i 183
in water. This is due to the e x p a n s i v e p r o c e s s of the f o r m a t i o n of gypsum. A d d i t i o n a l l y , s t u d i e s c o n d u c t e d t h r o u g h SEM have d i s c l o s e d that the p r e s e n c e of s u l f a t e ions does a l t e r to a c e r t a i n d e g r e e the m o r p h o l o g y and p r o p e r t i e s of the CSH gel, by d e l a y i n g and e v e n p a r t i a l l y i n h i b i t i n g the d e n s i f y i n g p r o c e s s e x p e r i e n c e d by the CSH w i t h time. Finally, a g r a d u a l and m o r e i n t e n s e d e c r e a s e of the Ca(OH)2 c o n t e n t s is o b s e r v e d in the s a m p l e s kept in sea water, as well as an i n c r e a s i n g a u g m e n t a t i o n in the g y p s u m and b r u c i t e c o n t e n t s a l o n g time. The s t r e n g t h b e h a v i o u r of C B C c e m e n t m o r t a r s kept in sea w a t e r is q u i t e s i m i l a r to that s h o w n by the same m o r t a r s c o n s e r v e d in Na2SO 4 solution. The m i c r o s t r u c t u r a l e x p l a n a t i o n of this b e h a v i o u r c o u l d be f o u n d in the f o l l o w i n g r e a c t i o n s : Ca(OH)2
+ MgSO 4
CaSO4.2H20 + Mg(OH)2
M g S O 4 + C - S - H gel + H20
CaSO4.2H20 + Mg(OH)2
The f o r m e r r e a c t i o n i m p l i e s an l a t t e r a d e s t r u c t i o n of the CSH gel.
New white cement with 10% limestone
expansive
+ SiO 2
process
and
the
(CBCa)
T a b l e IV shows f l e x u r a l and c o m p r e s s i v e s t r e n g t h s s p e c i m e n s m a d e w i t h CBCa c e m e n t and s u b m e r g e d in the m e d i a d e s c r i b e d and at d i f f e r e n t ages.
of m o r t a r different
The n a t u r e and e v o l u t i o n of the m i n e r a l o g i c a l c o n s t i t u e n t s is v e r y s i m i l a r to t h o s e a l r e a d y d e s c r i b e d for CB c e m e n t w i t h o u t any addition. The Ca(OH)2 c o n t e n t s d e c r e a s e s s i g n i f i c a n t l y and p r o g r e s s i v e l y in the m o r t a r s kept in s e a - w a t e r and Na2SO 4 solution. T h e s e m o r t a r s do a l r e a d y c o n t a i n g y p s u m at one d a y ' s age, the p r o p o r t i o n of w h i c h i n c r e a s e s w i t h time.
TABLE Flexural Media/
and Compressive
AGES days
WATER
1 7 28 56 90 180 270 365 545
FLEX. MPA 4.93 4.88 5.97 6.22 5.74 5.57 6.35 5.95 6.29
Strengths
IV of CBCa mortars
4.4% NaoSO 4 SOL.
in A g r e s s i v e
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
34.82 34.63 37.48 47.68 46.55 47.14 49.95 54.53 57.06
5.98 5.95 4.56 3.74 4.69 4.23 4.09 3.47 3.40
41.67 31.81 36.41 37.53 36.79 30.76 30.29 31.52 31.09
4.86 4.26 4.69 4.34 4.22 4.22 3.90 4.28 4.34
32.53 18.48 26.11 36.60 40.38 45.03 29.48 45.96 -
1184
M.T. Blanco ct al.
Vol. 24, No. 6
S p e c i m e n s kept in sea w a t e r show small g y p s u m c o n t e n t s (but in this case at up to 28 days' age) and i n c r e a s i n g Mg(OH)2 contents. Therefore, the d e g r a d a t i o n m e c h a n i s m s of the m a t e r i a l s e e m to be the same in CBCa c e m e n t m o r t a r s as w i t h CB cement, w h a t w o u l d j u s t i f y the s i m i l a r m e c h a n i c a l b e h a v i o u r , at least in the case w h e r e the a g g r e s s i v e m e d i u m used is sea water. This b e h a v i o u r seems to be s o m e w h a t different when the a g g r e s s i v e m e d i u m is Na2SO 4 solution, since f l e x u r a l strength d e c r e a s e s a p p e a r to be about 20% w i t h r e s p e c t to the f l e x u r a l s t r e n g t h s s h o w n by CB cement m o r t a r specimens. This d i m i n u t i o n c a n n o t be due to a c h e m i c a l i n t e r a c t i o n of the l i m e s t o n e a d d e d to the system. However, such a d m i x t u r e m o d i f i e s the m i c r o s t r u c t u r e of the paste, since its g r a i n s can b e h a v e like small inert a g g r e g a t e s , so that a t o p o c h e m i c a l g r o w t h of Ca(OH)2 upon CaCO 3 c r y s t a l s m i g h t occur; it w o u l d f a c i l i t a t e the a c c e s s of the s u l f a t e ions and the e x p a n s i v e r e a c t i o n of the f o r m a t i o n of gypsum. As a g e n e r a l c o n c l u s i o n , it may be stated that the n e w c e m e n t produced, as well as its d e r i v a t i v e s , display a satisfactory b e h a v i o u r to the a g g r e s s i v e m e d i a studied. Acknowledgement The a u t h o r s w i s h to t h a n k to the S p a n i s h D i r e c c i 6 n G e n e r a l de I n v e s t i g a c i 6 n C i e n t i f i c a y T ~ c n i c a for the s p o n s o r s h i p of the p r o j e c t PB87-0293. References i.2.3.4.5.6.7.8.-
M.T. B l a n c o - V a r e l a , T. V ~ z q u e z and A. Palomo. S p a n i s h P a t e n t n~ 542691 (1985) M.T. B l a n c o - V a r e l a , T. V ~ z q u e z and A. Palomo. Cem. Concr. Res. Vol. 16 p. 97 (1986) S. Gim~nez, M.T. B l a n c o - V a r e l a , A. P a l o m o and F. Puertas. Zement K a l k Gips I p. 12 (1991) S. Gim~nez, S. Garc~a, M.T. Blanco, A. Palomo. Cem. and Concr. Res., Vol. 22, p. 793 (1992) R.S. G o l l o p and H.F.W. Taylor. Cem. and Concr. Res., Vol 22, p. 1027 (1992) H.F.W. Taylor. Adv. Cem. Res., Vol 2, p. 73 (1989) K. P e t t i f e r and P.J. Nixon. Cem. Concr. Res.; Vol I0, p. 173 (1980) P.S. M a n g a t and J.M. Ei-Khatib. Cem. Concr. Res., Vol 22, p. 1089 (1992)
Cementand Concrete Research, Vol.24, No.6, pp. 1177-1184,1994 Pergamon
Copyright © 1994 Elsevier Science l.,td Printed in the USA. Allrights reserved
0008-8846/94 $6.00+.00
0008-8846(94)
STUDIES ABOUT ADMIXTURES
A
SULPHATE
00061-1
RESISTANT
CEMENT.
M.T. S l a n c o ( * ) , S. G a r c i a ( * * ) , S. G i m 6 n e z ( * * * ) , F. Puertas(*) and T. V~zquez(*)
INFLUENCE
A.
OF
Palomo(*),
(*) I n s t i t u t o de C i e n c i a s de la C o n s t r u c c i 6 n "Eduardo (C.S.I.C.) - A p a r t a d o 19002 - M a d r i d (Spain) (**) U n i v e r s i d a d Complutense. M a d r i d (***) Repsol Quimica. C e n t r o de Investigaci6n. M a d r i d
Torroja"
(Ref~eed) (R~eived Octo~r 14, 1993; in h a l f o ~ M ~ 10, 19~)
ABSTRACT
S t a r t i n g from an i n d u s t r i a l cement o b t a i n e d by m i l l i n g t o g e t h e r a new low e n e r g y c l i n k e r and gypsum, two b l e n d e d c e m e n t s containing 20% wt of fly ashes and 10% wt of l i m e s t o n e r e s p e c t i v e l y , have been prepared. The b e h a v i o u r of these three c e m e n t s in Na2SO 4 d i s s o l u t i o n and sea w a t e r has b e e n studied. The m e t h o d used for t e s t i n g the s p e c i m e n s has been the KochS t e i n n e g e r one. As a c o n s e q u e n c e of the study it can be said that the three c e m e n t s keep a good b e h a v i o u r in all the aggressive media although the strengths are lower ( i n d e p e n d e n t l y of the age of the specimens) in Na2SO 4 and sea w a t e r than in d i s t i l l e d water. Also, the c o m p o s i t i o n of the hydrated pastes conserved in the aggressive liquids for d i f f e r e n t p e r i o d s of time has b e e n studied. The e v o l u t i o n of the c o n t e n t of Ca(OH)2 , g y p s u m and Mg(OH)2 in the p a s t e s as well as the v a r i a t i o n s o b s e r v e d in the m o r p h o l o g y of C-S-H gel a l o n g the time and in f u n c t i o n of the a g g r e s s i v e medium, seem to keep a good c o r r e l a t i o n w i t h the s t r e n g t h s evolution. F i n a l l y it has b e e n c o n c l u d e d that those cements p r e p a r e d w i t h the new low e n e r g y cement have such a c o m p o s i t i o n that makes them to be r e s i s t a n t to the sulphate attack. Introduction About 1.000 tons of clinker, white P o r t l a n d type, have been o b t a i n e d t h r o u g h a new m a n u f a c t u r i n g process. P r e v i o u s w o r k s on this s u b j e c t have b e e n d e s c r i b e d in several p u b l i c a t i o n s (i), (2), (3). S u m m a r i l y speaking, the p r o d u c t was o b t a i n e d s t a r t i n g from gypsum, fluorspar, limestone, kaolin, and sand, by p r o p e r l y d d o s i f y i n g a raw mixture; the c h e m i c a l a n a l y s i s of w h i c h is shown on Table I. That Table also shows the m i n e r a l o g i c a l c o m p o s i t i o n of
1177
1178
M.T. Blanco et al.
Vol. 24, No. 6
the clinker. The m a t e r i a l was o b t a i n e d t h r o u g h a wet process, the m a x i m u m c l i n k e r i n g t e m p e r a t u r e n e a r e d 1.350~C.
and
It has been o b s e r v e d that the q u a l i t a t i v e c o m p o s i t i o n of this new c l i n k e r does not s i g n i f i c a n t l y d i f f e r from that of c o n v e n t i o n a l clinker, but in the p r e s e n c e of f l u o r e l l e s t e a d i t e and s u l f o - a n d fluoaluminates.
with
The t e c h n i c a l b a l a n c e - s h e e t of the i n d u s t r i a l the new p r o c e d u r e gave p o s i t i v e results (3).
process
tested
Gim~nez et al. have s t u d i e d the b e h a v i o u r shown by the new cement a l o n g time and faced to a g g r e s s i v e s o l u t i o n s (4). Perhaps, the most i m p o r t a n t c o n c l u s i o n to be e x t r a c t e d from this study is that the new cement b e h a v e s as a good sulphate r e s i s t a n t m a t e r i a l due to its low C3A content. A l t h o u g h special a t t e n t i o n was p a i d in o r d e r to c o r r e l a t e the m i c r o s t r u c t u r e of pastes w i t h the h a r d l y o b s e r v e d m e c h a n i s m s of e x p a n s i o n due to sulphate presence, more i n v e s t i g a t i o n must be c a r r i e d out. In that sense, data p u b l i s h e d by o t h e r a u t h o r s should be taken in c o n s i d e r a t i o n (5,6 and 7). Finnaly, the fact of k n o w i n g that a d m i x t u r e s can affect sulphate-resistant properties of concrete (8) justifies d u r a b i l i t y studies c o n d u c t e d w i t h the new cement, to w h i c h ashes (20%) and limestone (10%) were r e s p e c t i v e l y added; r e s u l t s from the study are p r e s e n t e d here.
the the fly the
Experimental Procedure In the present study three d i f f e r e n t c e m e n t s were used: CB (cement o b t a i n e d by a p p l y i n g the p r o d u c t i o n m e t h o d for the lowe n e r g y clinker, with a fineness of 436.5 m2/Kg); C B C (80% of CB plus an a d d i t i o n of 20% fly ash type F-ASTM C 618-85-); and CBCa (90% of CB plus 10% c a l c a r e o u s filler). The d u r a b i l i t y of cements t o w a r d s a g g r e s s i v e s o l u t i o n s was e v a l u a t e d by the K o c h - S t e i n e g g e r method. The s o l u t i o n s used were: d e i o n i z e d water, Na2SO 4 4.4% wt s o l u t i o n and a r t i f i c i a l sea w a t e r A S T M D 1141-76. The c o r r e s p o n d i n g specimens were g r a d u a l l y e x t r a c t e d from the a g g r e s s i v e solutions, at p r e - d e t e r m i n e d ages and t e s t e d at flexural
TABLE
I
Raw Mix Chemical Analyses and Clinker Mineralogical Data through XRD) RAW MIX (% wt)
SiO 2 14.7
AI203 1.5
CLINKER (% wt)
C3S 69.5
C2S 6.8
* Fluorelleastadite ** CIIA7. CaF2 *** CaSO 4. I/2H20
Fe203 0.2
CaO 45.4
MgO 0.4
SO 3 2.3
FL* 14.0
CaO 3.0
C3A 3.0
C4A3S 2.0
(Obtained
CaF 2 0.8
I.L. 34.5
CAF** 0.5
H*** 0.7
Vol. 24, No. 6
SULPHATE-RESISTA CEMENT, NT ADMIXTURES,BLENDS
and c o m p r e s s i o n . The e v o l u t i o n of the m i n e r a l o g i c a l w i t h time w a s m a i n l y s t u d i e d by X - R a y d i f f r a c t i o n .
Results New white cement
and
1179
composition
Discussion
(CB)
Table II shows the flexural and compressive strengths d i s c l o s e d by CB c e m e n t m o r t a r s s u b m e r g e d in the t h r e e m e d i a d e s c r i b e d above until 545 days.
TABLE Flexural
AGES days
1 7 28 56 90 180 270 365 545
and Compressive
WATER FLEX. MPA 5.60 5.58 6.17 6.47 6.34 6.57 6.31 5.95 6.32
II
Strengths Media
of CB Mortars
4.4% Na2SO 4 SOL.
in Agressive
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
37.24 32.36 49.35 52.92 63.81 58.23 63.31 54.31 61.27
6.25 6.06 6.61 5.49 5.42 5.27 5.33 4.43 4.65
39.99 33.25 38.61 41.25 52.67 40.96 30.37 31.28 32.70
5.73 5.19 5.44 4.56 5.04 4.61 4.57 4.61 4.53
45.99 26.99 39.50 44.11 48.23 53.13 48.33 53.41 55.25
In o r d e r to e s t i m a t e the e v o l u t i o n in time of c r y s t a l l i n e c o m p o u n d s in the c e m e n t p a s t e (CB), w h e n m o r t a r s p e c i m e n s of that c e m e n t w e r e s u b m e r g e d in d i f f e r e n t solutions, the i n t e n s i t y of the X R D r e f l e c t i o n s d e s c r i b e d in (4) was m e a s u r e d and the v a l u e was d i v i d e d into the f l u o r e l l e s t e a d i t e r e f l e c t i o n v a l u e (28 = 25.64). It m u s t be p o i n t e d out that b o t h IEtt/IFL and Icalcite/IFL q u o t S e n t s r e m a i n c o n s t a n t b o t h in time and in all media. In F i g u r e 1 the v a l u e s of the f o l l o w i n g q u o t i e n t s : ICH/IFL , Igyp.sum/IFL and IMg(OH)2/IFL are s h o w n v e r s u s time in the d i f f e r e n t meala. The use of a f l u o r e l l e s t e a d i t e r e f l e c t i o n as i n t e r n a l s t a n d a r d is j u s t i f i e d by the fact that this c o m p o u n d is a c o n s t i t u e n t of the clinker, b e c a u s e of its high i n s o l u b i l i t y and b e c a u s e it does not u r d e r g o any m o d i f i c a t i o n w i t h time. From Figure a)
1 it is i m p o r t a n t
to emphasize:
The c o n t e n t in Ca(OH)2 is at 7 days s i m i l a r in CB c e m e n t p a s t e s in c o n t a c t w i t h the t h r e e media, a l t h o u g h t h o s e from Na2SO 4 s o l u t i o n and sea w a t e r are s l i g h t l y lower. D u r i n g the t r e a t m e n t time, the c o n t e n t in Ca(OH)2 d e c r e a s e s in the p a s t e s e x p o s e d to a g g r e s s i v e m e d i a c o m p a r e d to those p r e s e r v e d in water.
1180
M.T. Blanco et al.
b)
Vol. 24, No. 6
In the case of m o r t a r s o b t a i n e d using the new cement and p r e s e r v e d in a Na2SO 4 s o l u t i o n it should be o b s e r v e d that simultaneously to the d e c r e a s e in the q u a n t i t y of Ca(OH)2 there is also an important increase of the content of g y p s u m w h i c h is the m a j o r c r y s t a l l i n e phase at the 180 day stage. :
1
7
-
1
~
NEWC£MENT(CB) ~
- - H = O ~
CH
------No=SO~. . . . . .
..--m O
/
~,o. Ii ~~ ~
~
\..
\
i
"~
/
"---~
i'~
//
-.
~
/
.......
~
~.~'~
~
=
--
Z ~ <
/#
~
/
#/
/
,,~.. ~
~ -,
..~. ~ '
~."
C
GYPSUM FL
GYPSUM~
SEA
[~" / * " ' "
-- ' ~
..
./
~ .,
~ ~"
~
\../
Z
~
NozSO,
CH
• --
~' FL ~ ~
-~-,_~ i " .~ •
~'~...~
~
/
5-
X
~2
/
/
2
xi
ill #
x/
x/ Ii
~x--x--x--x--~__ ~-~.~.
x--x-- x--x --x --x /
~
i
~
~
~
~
DAYS
FIGURE
1
The v a l u e s of x - r a y i n t e n s i t y ratios: . IC-./IFL, I~.-sum/IFL and ~ ~ ~ P . IMg(OH)2/IFL, for new cement CB versus t~me ~n d l f f e r e n t m e d ~ a The f o l l o w i n g d e s c r i b e d above:
conclusions
could
be
drawn
from
the
results
a)
Mortars between days.
o b t a i n e d from CB cement and p r e s e r v e d in w a t e r reach, 28 and 56 days, almost the s ~ e s t r e n g t h than at 545
b)
CB cement m o r t a r s s u b m e r g e d in Na2SO 4 s o l u t i o n show up to the 28 day stage g r e a t e r e n d u r a n c e than those m o r t a r s kept u n d e r H20 , due to ~ p s u m f o r m a t i o n (at the expense of Ca(OH)2 ) that m a k e s the system become more dense. The m a s s i v e ~ps~ formation with detriment b e c a u s e of its e x p a n s i v e nature, brings along d e c r e a s e in a l o n g - t e r m basis. In
addition
it
seems
probable
that
the
C-S-H
to Ca(OH)2 , an e n d u r a n c e
gel
evolution
Vol. 24, No. 6
SULPHATE-RESISTA CEMENT, NT ADMIXTURES,BLENDS
1181
process, in r e f e r e n c e to its m o r p h o l o g y and q u a l i t i e s , c o u l d b e c o m e a l t e r e d b e c a u s e of the e x i s t e n c e of SO42- and Na + ions and that the e n d u r a n c e d e c r e a s e c o u l d not o n l y be due to the g y p s u m e x p a n s i v e f o r m a t i o n p r o c e s s (4). c)
The e n d u r a n c e d e c r e a s e of CB m o r t a r s p r e s e r v e d in sea w a t e r in r e l a t i o n to those p r e s e r v e d in w a t e r c o u l d be p a r t l y due to Mg(OH) 2 f o r m a t i o n at the e x p e n s e of Ca(OH)2 w h i c h w o u l d be d i s s o l v e d (the Mg(OH)2 m o l a r v o l u m e is = 2/3 lower t h a n the Ca(OH)2 one).
It seems that in some w a y the C-S-H gel m i c r o s t r u c t u r e b e i n g m o d i f i e d by the a g g r e s s i v e s o l u t i o n (4).
New w h i t e
cement
with
20%
fly ashes
is
(CBC)
Table III shows the flexural and c o m p r e s s i v e strengths d i s p l a y e d at d i f f e r e n t ages by the m o r t a r s p e c i m e n s made w i t h C B C c e m e n t and s u b m e r g e d in the t h r e e m e d i a m e n t i o n e d before.
TABLE Flexural
and C o m p r e s s i v e
AGES days
WATER FLEX. MPA
1 7 28 56 90 180 270 365 545
5.01 4.81 5.72 6.97 7.02 7.49 8.29 7.69 8.33
III
Strengths Media
of CBC M o r t a r s
4.4% NaqSO 4 SOL.
in A g r e s s i v e
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
38.17 33.87 32.80 54.79 48.47 62.06 66.83 71.88 70.58
4.95 5.31 5.54 5.32 7.07 5.80 6.02 5.63 4.92
27.04 28.27 35.54 47.16 50.67 49.71 52.26 43.97 49.14
5.15 4.66 4.90 6.05 5.54 6.34 5.29 6.05 5.68
28.30 22.24 28.44 34.73 37.19 48.28 35.36 44.59 -
The m i n e r a l o g i c a l c o n s t i t u e n t s of the C B C c e m e n t p a s t e s are n a t u r a l l y the same as those of the CB s t a n d a r d cement, w h i c h e v e r the c o n s e r v a t i o n m e d i a used. However, a c l e a r v a r i a t i o n in the p r o p o r t i o n s and e v o l u t i o n of the p h a s e s is found. Figure
2 shows the v a l u e s of the ratios:
ICa(OH) 2 IFell
versus
time
ICaSO4.2H2 O IFell
in the d i f f e r e n t
IMg(OH)2 IFell
media.
It m a y be o b s e r v e d in such a F i g u r e that the a m o u n t of Ca(OH)2
1182
M.T.Blancoetal.
,7-
Vol.24,No. 6
C.B.C.
15~I0~-
CH FL
~
H20
~
CH N°zS04 F L
....
NozSO,~GYP sum
FL
CH .......
= ~
TC
Gyp~m p ~ FL |<
~.
u~o.~=l ~
~ ~
FL J
// \
I1~ ~,-
,i
~
~
""
, ~
~
~ ~%
~.~.
~
/ "~.~
~
~
-"
-.... ",~.. ~..
!
,,...
~
~
"~'~ • - ~.~.~.
~ !
~
/
~
~...
_ _ ~
"~'~'~
~
~
~ ' - ~ - ~ ' ~
~
~
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. ....~..~,"
•
~
'
,~0
d~
~
DAYS FIGURE
2
The v a l u e s of x - r a y i n t e n s i t y ratios:. ICH/IFL,, ~I~gypsum/IFL and IMg(OH)2/IFL,. for new cement CBC versus time in Q i i i e r e n t m e d i a d e c r e a s e s in w a t e r - c u r e d s p e c i m e n s w i t h respect to the c o n t e n t s in the s a m p l e s w i t h o u t any a d d i t i o n (Fig. I), due to the p o z z o l a n i c r e a c t i o n of ashes w i t h Ca(OH) 2. The i n c r e a s e in the CBC cement s t r e n g t h s w i t h r e s p e c t to those shown by CB cement is due to the m e n t i o n e d p o z z o l a n i c r e a c t i o n w i t h the p r o d u c t i o n of CSH gel, a p r o d u c t w h i c h d e n s i f i e s the system. The e v o l u t i o n w i t h time of the specimens kept in Na2SO 4 s o l u t i o n is s p e c i m e n s kept in water.
Ca(OH)2 similar
contents to that
in of
the the
There is a gradual increase of the g y p s u m c o n t e n t s in the s p e c i m e n s s u b m e r g e d in Na2SO 4 solution, but to a lesser d e g r e e than those c o r r e s p o n d i n g to the CB cement. This is due to the lesser amount of Ca(OH) 2 a v a i l a b l e and to the h i g h e r d e n s i t y of the s y s t e m w h i c h h i n d e r s access to sulfate ions. In these s p e c i m e n s there are two c o n c u r r i n g reactions: (i) the p u z z o l a n i c r e a c t i o n of the fly ashes and (ii) the f o r m a t i o n of g y p s u m by r e a c t i o n of sulfates w i t h Ca(OH) 2 • In spite of the fact that the p o z z o l a n i c r e a c t i o n takes place in these m o r t a r s s u b m e r g e d in Na2S04, their l o n g - t e r m s t r e n g t h s did not i n c r e a s e as could be expected, as it h a p p e n s in s p e c i m e n s kept
Vol. 2~, No. 6
SULPHATE-RESISTANT CEMENT, ADMIXTURF~, BLENDS
i 183
in water. This is due to the e x p a n s i v e p r o c e s s of the f o r m a t i o n of gypsum. A d d i t i o n a l l y , s t u d i e s c o n d u c t e d t h r o u g h SEM have d i s c l o s e d that the p r e s e n c e of s u l f a t e ions does a l t e r to a c e r t a i n d e g r e e the m o r p h o l o g y and p r o p e r t i e s of the CSH gel, by d e l a y i n g and e v e n p a r t i a l l y i n h i b i t i n g the d e n s i f y i n g p r o c e s s e x p e r i e n c e d by the CSH w i t h time. Finally, a g r a d u a l and m o r e i n t e n s e d e c r e a s e of the Ca(OH)2 c o n t e n t s is o b s e r v e d in the s a m p l e s kept in sea water, as well as an i n c r e a s i n g a u g m e n t a t i o n in the g y p s u m and b r u c i t e c o n t e n t s a l o n g time. The s t r e n g t h b e h a v i o u r of C B C c e m e n t m o r t a r s kept in sea w a t e r is q u i t e s i m i l a r to that s h o w n by the same m o r t a r s c o n s e r v e d in Na2SO 4 solution. The m i c r o s t r u c t u r a l e x p l a n a t i o n of this b e h a v i o u r c o u l d be f o u n d in the f o l l o w i n g r e a c t i o n s : Ca(OH)2
+ MgSO 4
CaSO4.2H20 + Mg(OH)2
M g S O 4 + C - S - H gel + H20
CaSO4.2H20 + Mg(OH)2
The f o r m e r r e a c t i o n i m p l i e s an l a t t e r a d e s t r u c t i o n of the CSH gel.
New white cement with 10% limestone
expansive
+ SiO 2
process
and
the
(CBCa)
T a b l e IV shows f l e x u r a l and c o m p r e s s i v e s t r e n g t h s s p e c i m e n s m a d e w i t h CBCa c e m e n t and s u b m e r g e d in the m e d i a d e s c r i b e d and at d i f f e r e n t ages.
of m o r t a r different
The n a t u r e and e v o l u t i o n of the m i n e r a l o g i c a l c o n s t i t u e n t s is v e r y s i m i l a r to t h o s e a l r e a d y d e s c r i b e d for CB c e m e n t w i t h o u t any addition. The Ca(OH)2 c o n t e n t s d e c r e a s e s s i g n i f i c a n t l y and p r o g r e s s i v e l y in the m o r t a r s kept in s e a - w a t e r and Na2SO 4 solution. T h e s e m o r t a r s do a l r e a d y c o n t a i n g y p s u m at one d a y ' s age, the p r o p o r t i o n of w h i c h i n c r e a s e s w i t h time.
TABLE Flexural Media/
and Compressive
AGES days
WATER
1 7 28 56 90 180 270 365 545
FLEX. MPA 4.93 4.88 5.97 6.22 5.74 5.57 6.35 5.95 6.29
Strengths
IV of CBCa mortars
4.4% NaoSO 4 SOL.
in A g r e s s i v e
SEA W A T E R
COMPR. MPA
FLEX. MPA
COMPR. MPA
FLEX. MPA
COMPR. MPA
34.82 34.63 37.48 47.68 46.55 47.14 49.95 54.53 57.06
5.98 5.95 4.56 3.74 4.69 4.23 4.09 3.47 3.40
41.67 31.81 36.41 37.53 36.79 30.76 30.29 31.52 31.09
4.86 4.26 4.69 4.34 4.22 4.22 3.90 4.28 4.34
32.53 18.48 26.11 36.60 40.38 45.03 29.48 45.96 -
1184
M.T. Blanco ct al.
Vol. 24, No. 6
S p e c i m e n s kept in sea w a t e r show small g y p s u m c o n t e n t s (but in this case at up to 28 days' age) and i n c r e a s i n g Mg(OH)2 contents. Therefore, the d e g r a d a t i o n m e c h a n i s m s of the m a t e r i a l s e e m to be the same in CBCa c e m e n t m o r t a r s as w i t h CB cement, w h a t w o u l d j u s t i f y the s i m i l a r m e c h a n i c a l b e h a v i o u r , at least in the case w h e r e the a g g r e s s i v e m e d i u m used is sea water. This b e h a v i o u r seems to be s o m e w h a t different when the a g g r e s s i v e m e d i u m is Na2SO 4 solution, since f l e x u r a l strength d e c r e a s e s a p p e a r to be about 20% w i t h r e s p e c t to the f l e x u r a l s t r e n g t h s s h o w n by CB cement m o r t a r specimens. This d i m i n u t i o n c a n n o t be due to a c h e m i c a l i n t e r a c t i o n of the l i m e s t o n e a d d e d to the system. However, such a d m i x t u r e m o d i f i e s the m i c r o s t r u c t u r e of the paste, since its g r a i n s can b e h a v e like small inert a g g r e g a t e s , so that a t o p o c h e m i c a l g r o w t h of Ca(OH)2 upon CaCO 3 c r y s t a l s m i g h t occur; it w o u l d f a c i l i t a t e the a c c e s s of the s u l f a t e ions and the e x p a n s i v e r e a c t i o n of the f o r m a t i o n of gypsum. As a g e n e r a l c o n c l u s i o n , it may be stated that the n e w c e m e n t produced, as well as its d e r i v a t i v e s , display a satisfactory b e h a v i o u r to the a g g r e s s i v e m e d i a studied. Acknowledgement The a u t h o r s w i s h to t h a n k to the S p a n i s h D i r e c c i 6 n G e n e r a l de I n v e s t i g a c i 6 n C i e n t i f i c a y T ~ c n i c a for the s p o n s o r s h i p of the p r o j e c t PB87-0293. References i.2.3.4.5.6.7.8.-
M.T. B l a n c o - V a r e l a , T. V ~ z q u e z and A. Palomo. S p a n i s h P a t e n t n~ 542691 (1985) M.T. B l a n c o - V a r e l a , T. V ~ z q u e z and A. Palomo. Cem. Concr. Res. Vol. 16 p. 97 (1986) S. Gim~nez, M.T. B l a n c o - V a r e l a , A. P a l o m o and F. Puertas. Zement K a l k Gips I p. 12 (1991) S. Gim~nez, S. Garc~a, M.T. Blanco, A. Palomo. Cem. and Concr. Res., Vol. 22, p. 793 (1992) R.S. G o l l o p and H.F.W. Taylor. Cem. and Concr. Res., Vol 22, p. 1027 (1992) H.F.W. Taylor. Adv. Cem. Res., Vol 2, p. 73 (1989) K. P e t t i f e r and P.J. Nixon. Cem. Concr. Res.; Vol I0, p. 173 (1980) P.S. M a n g a t and J.M. Ei-Khatib. Cem. Concr. Res., Vol 22, p. 1089 (1992)