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Application of differential thermal analysis in cement research
Ramachandran, V. S.; Feldman, R. F.; Sereda, P. J.
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S e r
THl
N21r2 no. 249c .
2 BLDGNATIONAL RESEARCH COUNCIL CANADA
DIVISION O F BUILDING RESEARCH
Application of Differential Thermal
Analysis in Cement Research
by
V. S. RAMACHANDRAN, R. F. FELDMAN and P . J. SEREDA
Reprinted f r o m
Highway R e s e a r c h R e c o r d Number 62 (1964) Highway R e s e a r c h Board, Washington, D. C . , U. S. A.
p. 40 - 61 R e s e a r c h P a p e r No. 249 of the Division of Building R e s e a r c h P r i c e 50 c e n t s OTTAWA )- --- ...-..-
---
May 1965 b , (, ilk5;AHCH N A T I O N A L R € ~ E h r ( C k f C O U N C I L NRC 8515APPLICATION DE L'ANALYSE THERMIQUE D I F F E R E N T I E L L E A LA RECHERCHE SUR L E CIMENT
SO MMAIRE
L ' a n a l y s e t h e r m i q u e d i f f h r e n t i e l l e ( A T D ) a r h c e m m e n t p r i s une i m p o r t a n c e g r a n d i s s a n t e c o m m e i n s t r u m e n t de r e c h e r c h e e n c h i m i e d u c i m e n t . L ' A T D qui s e r t h a b i t u e l l e m e n t
\a
l ' i d e n - t i f i c a t i o n d e c e r t a i n s m a t h r i a u x p o u r r a i t Gtre k g a l e m e n t e m - ployhe pour o b s e r v e r l e s r h a c t i o n s en chauffant g r a d u e l l e m e n t l e s c o r p s e n r h a c t i o n jusqul\a d e s t e m p k r a t u r e s h l e v h e s . L ' e m - ploi d ' a p p a r e i l s s e n s i b l e s e t d ' u n e g r a d u a t i o n adhquate f a c i l i t e l ' e s ti m a t i o n quantitative d e s m a t h r i a u x . C e r t a i n s c o m p o s h s ma1 c r i s t a l l i s h s e t d i f f i c i l e m e n t i d e n t i f i a b l e s p a r d i f f r a c t i o n a u x r a y o n sX
a t t e i g n e n t de s s o m l n e t s t h e r m i q u e s c a r a c t 6 r i s-
t i q u e s . Cette htude p o r t e s u r l ' a p p l i c a t i o n de l l A T D a u c i m e n t P o r t l a n d e t5
d ' a u t r e s c i m e n t s . On y m o n t r e c o m m e n t 1'ATD p e r m e t de m i e u x c o m p r e n d r e l a c h i m i e d u c i m e n t e n g h n h r a l e t de m i e u x connal'tre l e s possibi1iti.s d e l a m6thode e l l e - m G m e .Application of Differential Thermal
Analysis in Cement Research
V. S. RAMACHANDRAN, R. F . FELDMAN, and P. J. SEREDA
Respectively, P o s t D o c t o r a t e Fellow, C e n t r a l Building R e s e a r c h Institute, R o o r k e e , India; and R e s e a r c h Officer and Head, Inorganic M a t e r i a l s Section, Division of Building R e s e a r c h , National R e s e a r c h Council of Canada
D i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) h a s r e c e n t l y attained i n - c r e a s i n g i m p o r t a n c e a s a r e s e a r c h t o o l i n c e m e n t c h e m i s t r y . DTA is n o r m a l l y u s e d i n identifying v a r i o u s m a t e r i a l s b u t could a l s o b e employed t o follow r e a c t i o n s by heating t h e r e a c t a n t s g r a d u a l l y t o e l e v a t e d t e m p e r a t u r e s . Use of s e n s i t i v e equipment and p r o p e r c a l i b r a t i o n f a c i l i t a t e quantitative e s t i m a t i o n of m a t e - r i a l s . C e r t a i n poorly c r y s t a l l i z e d compounds not e a s i l y identifi- a b l e by x - r a y diffraction g i v e c h a r a c t e r i s t i c t h e r m a l peaks.
T h i s r e v i e w p r e s e n t s t h e application of DTA to portland c e - m e n t . Other c e m e n t s a r e included to i l l u s t r a t e the i m p o r t a n c e of DTA to a n u n d e r s t a n d i n g of the c h e m i s t r y of c e m e n t i n g e n e r a l and a l s o of the potentialities of the method itself.
*THE TECHNIQUE of d i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) is widely applied i n clay m i n - e r a l o g y , but only r e c e n t l y h a s i t b e e n extended t o o t h e r f i e l d s . The method c o n s i s t s of m e a s u r i n g t h e h e a t changes a s s o c i a t e d with p h y s i c a l o r c h e m i c a l t r a n s f o r m a t i o n s o c c u r - r i n g d u r i n g the g r a d u a l heating of a s u b s t a n c e . T h e r m a l c h a n g e s , s u c h a s d e h y d r a t i o n , c r y s t a l l i n e t r a n s i t i o n , l a t t i c e d e s t r u c t i o n , oxidation and decomposition, a r e g e n e r a l l y accompanied by a n a p p r e c i a b l e r i s e o r f a l l i n t e m p e r a t u r e and a r e a m e n a b l e t o DTA investigation. The DTA technique h a s found application i n d i v e r s e f i e l d s s u c h a s c r i m i - nology, pyrotechnics, c a t a l y s i s , c o a l c h e m i s t r y , p o l y m e r s y s t e m s , radioactivity, s o a p and lubricating s y s t e m s and will continue t o b e introduced to v a r i o u s b r a n c h e s of study.
The DTA method had i t s o r i g i n i n 1887 when Le C h a t e l i e r applied t h e r m a l a n a l y s i s t o the study of t h e constitution of c l a y s (1 - 4). However, although o n e of the p i o n e e r s in the f i e l d of c e m e n t c h e m i s t r y , h e d i d i o t r e a l i z e the potentialities of the method t o the study of c e m e n t s . Kalousek e t al. (5) w e r e probably f i r s t t o i n t r o d u c e DTA i n c e - m e n t c h e m i s t r y . The u s e f u l n e s s of t h e m e t h o d w a s r e a l i z e d and d i s c u s s e d a t the T h i r d International Symposium on t h e C h e m i s t r y of C e m e n t s held in London i n 1952, and s i n c e then s e v e r a l p a p e r s have a p p e a r e d dealing with the DTA of c e m e n t i t i o u s m a t e r i a l s . T h e r e m a r k s m a d e by R. H. Bogue (6) - a t the London s y m p o s i u m a r e significant:
Our a b i l i t y t o d e t e c t and measure s m a l l temperature d i f f e r e ! l t i a l s has been g r e a t l y extended b y nmr s l e c t l - o n i c i n s t i - ~ u n e n i s c a p a b l e of r a p i d and automatic r e c o r d i n g of minute changes i n energy l e v e l s . With t h e s e i n s t r u m e n t s , d i f f e r e n t i a l t h e r m a l - a n a l y t i c a l methods
a r e r e p l a c i n g t h e o l d e r qu=.nching techniques f o r f o l l o w i n g phase r e l a t i o n s w i t h t h e a s s u r a n c e t h a t energy changes t o o s m a l l o r t o o r a p i d o t h e r w i s e t o be observed w i l l he i d e n t i f i e d .
Between 1952 and 1960, i n c r e a s i n g attention w a s paid t o t h e application of DTA i n c e m e n t c h e m i s t r y and a t the F o u r t h International Symposium on t h e C h e m i s t r y of C e m e n t , held in 1960, s e v e r a l i n v e s t i g a t o r s i n t e r p r e t e d t h e i r r e s u l t s through t h e r m o g r a p h i c a n a l y s i s .
Paper sponsored b y Cormittee on Basic Research Psi-taining t o P o r t l a n d Cement and Concrete.
DESCRIPTION O F THE METHOD
Essentially the DTA equipment consists of a furnace, a t e m p e r a t u r e regulator, a specimen block, thermocouples, and a temperature-recording s y s t e m . Several types of apparatus a r e in u s e . Refinements have been suggested f r o m time to time to give g r e a t e r accuracy in r e s u l t s and to widen the scope of the method.
The sample to b e studied is placed in one of the cavities of the specimen block (Fig. 1). In the second cavity i s placed calcined alumina, a-A12Q, which does not experience any t h e r m a l change within the t e m p e r a t u r e range usually used in the study. Two thermo- couples, constituting the differential thermocouple, a r e connected as shown. In the third cavity of the block a s e p a r a t e thermocouple is embedded to m e a s u r e the temperature of the specimen. The t e m p e r a t u r e can a l s o b e m e a s u r e d by connecting the thermocouple ends a - b f r o m the i n e r t m a t e r i a l a s shown in F i g u r e l b .
The block with the specimen to be studied and i n e r t m a t e r i a l , a-A1203, is placed in a furnace and the t e m p e r a t u r e i s r a i s e d a t a uniform r a t e . The differential couple r e - c o r d s z e r o potential difference when the specimen is not undergoing any thermal change. During a t h e r m a l transformation, the i n c r e a s e o r d e c r e a s e in the t e m p e r a t u r e of the specimen, relative to that of the i n e r t m a t e r i a l , will depend on the exo- o r endothermal c h a r a c t e r of the change. With the completion of the t h e r m a l change, the specimen u s u - ally r e a t t a i n s the t e m p e r a t u r e of the i n e r t m a t e r i a l . In some c a s e s , the z e r o line shows a d r i f t a f t e r a t h e r m a l transformation and
i s attributed to the differences in t h e r m a l c h a r a c t e r i s t i c s between the specimen and
the i n e r t m a t e r i a l . The differential t e m - THERMOCOUPLE p e r a t u r e is recorded a s a function of time FOR TEMPERATURE RECORD o r the t e m p e r a t u r e of the specimen block.
In the t h e r m o g r a m s , usually the differen- tial t e m p e r a t u r e , AT, and the block t e m - perature, T, a r e recorded o r plotted in such a manner that the endothermic peaks a r e shown downwards and the exothermic peaks, upwards with r e s p e c t to the b a s e - line, AT = 0. The details of the DTA technique, theory, and varied applications a r e discussed in various s o u r c e s (7
-
14). The discussion herein pertains m a i x y to the application of DTA in the field of portland cement. The inclusion of the r e - lated work was necessitated to indicate i t s importance to an understanding of the chemistry of cement and of the potential- ities of the method itself.D I F F E R E N T I A L COUPLE
PORTLAND CEMENT CLINKER Raw Materials
Portland cement i s formed by firing chalk, limestone, m a r l o r m a r i n e s h e l l s with shale o r clay o r other siliceous m a t e - r i a l s to a t e m p e r a t u r e of 1400 to 1500 C, mixing the resulting sintered clinker with 4 to 5 percent gypsum, and grinding to a
0
very fine powder of.average d i a m e t e r of b 10 p. In some cement plants gypsum i s t replaced by anhydrite o r a mixture of a n - THERMOCOUPLE
FOR TEMPERATURE RECORD
hydrite and gypsum.
In the formation of cement clinker, the ( b )
clay contributes SiO2, -41203, and, Pas- F i g w e 1. Specimen bloclr. with thermocouple sibly, iron, alkalies and alkaline e a r t h s , connections.
depending on the clay mineral composition. More 5 percent MgO in cement causes unsoundness of concrete. Consequently, clay minerals rich in Mg, such a s attapulgite, sepiolite and chlorite, do not make suitable raw materials for cement manufacture. Kaolinite comprises mainly S i G and AL03 and, hence, i s suitable a s a raw material, especially for the manufacture of white portland cement. Illite and montmorillonite clay minerals normally form suitable raw materials. Schweite (15) observed that kaolinite is much more suitable f o r wet manufacturing process b e c a u s e i t exhibits better burning characteristics and produces l e s s dusting during grinding. Caution should be exercised in choosing a suitable lime source. If the original limestone contains MgO in the form of dolomite, the resultant cement will be unsound.
The application of DTA to the examination of raw materials f o r cement production should prove useful in assessing their suitability. Figure 2 shows typical thermograms of some raw materials. The temperature of the peak, the endo- o r exothermal char- a c t e r , the intensity and other general characteristics a r e used to identify each material.
Nature of Cement Clinker
Portland cement clinker contains a
CHLORITE S E P I O L I T E ATTAPULGITE CALCITE IRON- B E A R I N G L I M E S T O N E OOLOMITE 0 2 0 0 4 0 0 6 0 0 BOO 1 0 0 0 T E M P E R A T U R E 'C
Figure 2 . DTA o f s~:rn- raw m a t e r i a l s f o r znanuf ac t u r e of se!nent .
T E M P E R A T U R E O C
Fi,wre 3 . ,Thnrmograns of a l i t n and p u r e t r i c a l c i u m s i l i c a t e .
mixture of four major phases, C3S, 8-C,S, C3A and a f e r r i t e phase extending in com- position f r o m C4AF to C B A F ~ (16). (Sym- bols used a r e defined a s follows: C = CaO, S = S i G , A = A L B , F = FezOj, M = MgO, K = G O , N = N a 2 0 a n d H =
H20. ) The C3S phase contains some Al and Mg in solid solution, whereas the C2S phase contains some K 2 0 and the C3A phase, some NkO. The clinker has a glassy phase ranging from 2 to 20 percent, and frequently the g r e a t e r proportion of C3A and f e r r i t e exists in this phase, Ac- cording to some workers there i s essen- tially no glassy phase in the clinker.
Le Chatelier designated the predomi- nant component of portland cement clinker as"aliteW with a probable formula, C3S. Nurse and Welch (17) c a r r i e d out DTA of pure C3S, alite anTa cement clinker r i c h in alite. In Figure 3 a r e shown the DTA of pure C3S and alite in the range 150 to 1500 C a s obtained by them. Six endo- thermal transformations a r e observed in pure C3S. The peak a t 464 C i s due to loss of water of hydration from the s u r - face of CaO grains and those a t 622 and 750 C a r e due to
6
-
a' and y-
a' t r a n s - formations of C2S, respectively. The peak a t 1465 C i s attributed to a '-
atransformation. The thermal effects a t 923 and 980 C for C3S a r e more specula- tive and a r e ascribed to either, triclinic 923 C monoclinic 980 C, trigonal o r t r i -
-
clinic
-
trigonal-
trigonal+
rotation of anions. Synthetic alite (C54Sla' M' A) ex- hibits two endothermic peaks a t 825 and 1427 C. The peak a t 825 C i s due to monoclinic-
trigonal transformation and the higher temperature endothermic peakis due to transformation i n C2S. A cement containing 60 percent C3S, however, indi- cated no t h e r m a l t r a n s f o r m a t i o n s that could b e a s c r i b e d to C3S. The m e t a s t a b l e i n v e r - s i o n s a t 923 and 980 C i n p u r e C3S and a t 850 C i n a l i t e r e p o r t e d by N u r s e and Welch w e r e confirmed by o t h e r s (18, - - - 99, 100).
T h e r e a r e four polymorphic f o r m s of CZS (y, B, a' and a ) s t a b l e i n the r a n g e 0 to 1600 C. Only the B and, occasionally, the y f o r m s a r e likely to o c c u r i n the cement clinker. Minute quantities of a' f o r m w e r e o b s e r v e d under exceptional conditions (19). It should a l s o b e noted that a, a ' , and
8
f o r m s a r e not s t a b l e a t r o o m t e m p e r a t u r e in a b s e n c e of s t a b i l i z e r s . DTA h a s successfully provided the a c c u r a t e inversion t e m p e r a - t u r e s of the f o u r f o r m s of C2S and h a s played a n i m p o r t a n t r o l e i n establishing the i n t e r - relationship of the v a r i o u s f o r m s (20). The quenching technique would not p r o v e useful in this investigation because n e i t h e r t h e a n o r the a' f o r m is stable a t r o o m t e m p e r a - t u r e . Newman and Wells and Vasenin (21, - - 22) studied the t r a n s f o r m a t i o n s i n C2S, and the work c a r r i e d out a t the B r i t i s h Building R e s e a r c h Station h a s provided a c c u r a t e in- v e r s i o n points of v a r i o u s f o r m s of CZS(%).
The r e s u l t s a r e p r e s e n t e d i n F i g u r e 4. C u r v e A shows two peaks obtained by heating y-CzS. At f i r s t sight t h e s e t r a n s f o r m a - tions could b e m i s t a k e n f o r y - p and y-
/3
t r a n s f o r m a t i o n s . A b e t t e r i n t e r p r e t a t i o n is based on the cooling c u r v e of the s a m e substance. C u r v e B shows t h r e e peaks c o r r e - sponding t o t h r e e i n v e r s i o n s .If
the cooling is stopped a t about 600 C and heating r e - s t a r t e d , c u r v e C r e s u l t s instead of c u r v e A. C u r v e A is reproducible if the m a t e r i a l is allowed to cool to r o o m t e m p e r a t u r e before heating is r e - s t a r t e d . The interpreta- tion of t h e s e c u r v e s is a s follows: on heating the y-CzS, the phase change is i n the s e - quence y-
a'-
a, the 8 - f o r m appearing only on the cooling cycle below 600 C ; the/3
-
yinversion t a k e s place on slow cooling o r 8
-
a'-
a inversion on heating. T h e s e r e s u l t s w e r e confirmed by d e K e y s e r (24).The stabilizing influence of b a r i u m o r t h o s i l i c a t e on CZS w a s studied by DTA. C2S t r e a t e d with v a r i o u s amounts of the stabilizing agent indicated a b s e n c e of y
-
a' i n v e r - sion on addition of m o r e than 1 0 p e r c e n t(25).
S y s t e m s With CaO, A124, SiOL and FezQ
The t h e r m a l r e a c t i o n s in the binary, t e r n a r y and q u a t e r n a r y s y s t e m s involving CaQ, S i a , Fe203, and A12Q a r e of g r e a t i m p o r t a n c e in the study of the c h e m i s t r y of c e m e n t clinker. Rapid heating of t h e s e m i x t u r e s i n a t h e r m a l a n a l y s e r r e s u l t s i n the a p p e a r - ance of t h e r m a l inflections caused by the formation of v a r i o u s products.
Budnikov and Sologubova (26) applied DTA to the study of the reaction between kaolin and calcium carbonate in t h e p r o d u c t i o n of white cement. The formation of CaO. A k a w a s indicated by an e x o t h e r m a l peak above 1000 C . The m o r t a r s c u r e d f o r 28 days ex- hibited an endothermic effect a t 320 to 340 C which w a s attributed to dehydration of 2 CaQ'Ak03'7 HzO. The formation of t r i c a l c i u m s i l i c a t e by the interaction of calcium carbonate and s i l i c a h a s been studied by DTA
(27).
De K e y s e r h a s done extensive workA. HEATING CURVE S T A R T I N G W I T H ')' B. C O O L I N G C U R V E
-377
C. H E A T ING C U R V E E I T H E R S T A R T I N G 1447 W I T i lP
OR R E - H E A T I N G B E F O R E T H E,8-;Y
I N V E R S I O N Figure 4. Inversions in C,Son various oxide s y s t e m s . He heated a mixture of SiQ-Ca0-A1203 and a l s o a CaO- kaolin m i x t u r e between 1000 and 1400 C and studied the r e a c t i o n by DTA (28). Between 1200 and 1300 C , the reaction was controlled by the diffusion of oxides. lnCaO-kaolin, gehlenite (C2AS) was found to f o r m between 900 and 1000 C , c o n t r a r y t o the r e s u l t s of Budnikov and Sologubova (26). At higher t e m p e r a t u r e s CA converted to C5A3. A mix- t u r e of C a ( 0 H k and SiQ, when heated rapidly in a t h e r m a l a n a l y s e r , exhibited s e v e r a l inflections i n the t h e r m o g r a m s . Samples w e r e withdrawn a t v a r i o u s t e m p e r a t u r e s c o r - responding to t h e r m a l inflections and examined by x - r a y diffraction. T h r e e endothermic peaks a t 148, 490 and 555 C and t h r e e exothermic peaks a t 800, 846 and 970 C w e r e ob- s e r v e d . The x-ray r e s u l t s showed a t about 480 C the p r e s e n c e of y-C2S which converted to 8-C2S a t 960 C.
F u r t h e r workby d e Keyser (29) o n t h e s y s t e m 2 CaO
,
F e z @ , 6 CaO+
A L @+
2 F e z @ , 4 CaO+
ALCh+
F e z @ and 6 CaO+
2 AL@+
F e z @ i n the range 500 to 1300 C indicated that a l l these m i x t u r e s f o r m 2 C a 0 . F e 2 0 3 f r o m 800 C and C a 0 . F e 2 0 3 a t higher t e m p e r a - t u r e s . At s t i l l higher t e m p e r a t u r e s , CaO' FezQ r e a c t e d , with CaO to f o r m 2 CaO' F e 2 0 3 with the maximum r a t e of formation a t 1200 C. F o r m i x t u r e s containing A1203, 3 A b Q ' 5 CaO was the product between 1000 t o 1250 C .A s y s t e m a t i c work h a s been c a r r i e d out by B a r t a and c o - w o r k e r s (101-104) to follow the reaction when different m i x t u r e s of CaO, AL03, SiQ and F e 2 0 3 a r e g r z a l l y heated in the DTA furnace. DTA gives valuable information on the t e m p e r a t u r e of t r a n s f o r m a - tions, though the values may be different f r o m those obtained f r o m equilibrium studies. Some of the r e s u l t s obtained a r e shown i n F i g u r e s 5 to 8.
Figure 5 shows the DTA behavior of
1000 I 2 0 0 1400 3 CaO
+
SiQ and a l s o the effect of addi-tions of ALO3 and m i x t u r e s of AL03
+
cao
A I ~ O ~ Fez% on the t e m p e r a t u r e of formation of O/o O/oC3S. The exothermic peak a t 1450 C in-
d i c a t e s the formation of C3S. The peak 19 81 t e m p e r a t u r e d e c r e a s e s t o 1380, 1350 and
1320 C with additions of 5 percent A12Q, 2 5 7 5
2 percent AL03
+
2 percent F e 2 0 3 and 5 percent ALQ+
5 p e r c e n t Fez03, r e s p e c -tively. 3 0 70
F i g u r e 5. Thermogra-s o f 3 CaO + SiO, : -4, T E M P E R A T U R E , O C
3 CaO + SiO, ; B, w i t h 5% A& O3 ; C, w i t h 2%
A& 0, + 2% F%O, ; D, w i t h 5% .4&03 + 5% F i g u r e 6 . Thermograms o f rnixLd;ures o f CaO
%O3. and A:& O3 .
A
B
C
D 3 6 64 4 2 5 8 48 5 2 5 5 4 5 6 2 3 8 68 3 2 1000 1200 1400 T E M P E R A T U R E , O C1000 1 2 0 0 1 4 0 0 T E M P E R A T U R E , OC
COO A I 2 O 3 F e 2 0 3 O/o O/o O/o 2 5 4 0 3 5
1000 1 2 0 0 1 4 0 0 T E M P E R A T U R E , OC
F i g u r e 8 . Thermograms of t e r n a r y m i x t u r z F i g u r e 7 . Tnermograms o f mixi;ures of CaO c o n t a . i n i n g CaO, A1,0, a ~ d F% 0,
.
and F% 0, .
In Figure 6, the DTA c u r v e s of different proportions of CaO in AhQ a r e shown. The exothermic effect a t 950 t o 1000 C is attributed to a simultaneous formation of CA and C12A7. The dip a t 1170 C r e p r e s e n t s the change of y-A1203 to (Y f o r m and the other
a t 1200 C corresponds to the beginning of formation of C3A. The endothermic peak a t 1300 C indicates melting in the s y s t e m . Other endothermic effects a t higher t e m p e r a - t u r e s a r e not discussed.
Reactions of different mixes of CaO and Fe203 a r e r e p r e s e n t e d by t h e r m o g r a m s i n Figure 7. The exothermic maximum a t 950 C is due to formation of C F and is indepen- dent of the relative proportions of CaO and Fez%. Melting effect is shown by a n endo- thermic effect a t 1150 C and is lower than the eutectic t e m p e r a t u r e of 1200 C in the phase diagram. The compound C 2 F is formed subsequently, the minimum a t 1300 C corresponding to an incongruent melting of the mixture.
The reactions in the t e r n a r y mixture containing CaO, Fe2Q and A12Q a r e shown i n Figure 8. An exothermic effect o c c u r s a t about 980 C owing to the formation of C4AF. At lower contents of ALQ, C 2 F is a l s o formed, indicated by an endothermic dip a t about 1160 C.
The effect of v a r i o u s m i n e r a l i z e r s , p r e s e n t in the m a s s o r in the oven atmosphere, on the reaction products in cement manufacture h a s a l s o been studied by DTA by B a r t a and co-workers.
HYDRATION REACTIONS Hydration of Individual P h a s e s in Cement Clinker
A study of the hydration of the individual components of the clinker compounds pro- vides a b e t t e r understanding of the complex reactions taking place in the cement. C3S r e a c t s quickly with water, producing Ca(OH)2 and an ill-crystallized compound with C/S r a t i o of 1 . 5 to 1 . 8 . The compound is nearly related to natural tobermorite, C 5 S a s , and is designated tobermorite gel.
Midgley studied the hydration product obtained by curing C3S in water f o r 2 y r in the paste f o r m (30). T h e r m o g r a m s indicated t h e p r e s e n c e of Ca(OH)2, C a C G and calcium silicate hydrate with the f o r m u l a 1 . 7 7 CaO. SiOL. 3 Hz0 (CaO/SiG = 1 . 7 7 ) representing
a poorly crystalline tobermorite, CSH(I1). The g e l gave a c h a r a c t e r i s t i c endothermic peak a t about 120 C. Gaze and Robertson (31) studied by DTA the calcium s i l i c a t e hy- d r a t e in c o m m e r c i a l products. The exothermic reaction between 800 and 850 C was
attributed t o the transformation of CSH(1) to 8-wollastonite. An endothermic peak a t 760 C r e p r e s e n t e d the p r e s e n c e of CaC03. Van B e m s t (32) a l s o studied the DTA of hydrates obtained by hydration of d i - and t r i c a l c i u m s i l i c a t e s . 8-CzS r e a c t s a t a slower r a t e and f o r m s compounds s i m i l a r to those resulting f r o m C3S. C3A r e a c t s v e r y rapidly to f o r m C3AHe, but i t s formation i n cement is much debated. The DTA r e s u l t s of Kalousek e t a1 (5) did not indicate the p r e s e n c e of C3AHe in m o s t of the cement pastes.
The DTA of &A a f t e r t r e a t m e n t with water shows a prominent bulge a t 315 to 330C (5, 105). Young (33) and J o n e s (64), however, found two endothermic effects c o r r e - sponding to the stepwise d e h y d r a r o n of CSAHB. Gohlert (106) r e p o r t e d t h r e e peaks f o r C3A p a s t e s a f t e r 3 and 28 days of hydration. Budnikov e t x (107) and Sauman (108) a l s o obtained t h r e e peaks. Govorov (log), however, r e p o r t e d f o u r peaks. The differ- ences in behavior of the hydrated C s ~ X ~ o r t e d by different w o r k e r s could be attributed to the d e g r e e of purity of the s a m p l e , the t e m p e r a t u r e and quantity of water used, and the period of hydration. The sensitivity of the DTA apparatus is another f a c t o r which might have caused the nonregistration of minute effects.
The hydration r e a c t i o n s i n the f e r r i t e phase, C4AF, proceed a t a s l o w e r r a t e , con- t r a r y to e a r l i e r belief, and a r e much m o r e complicated. J o n e s (34) h a s suggested t h a t the f e r r i t e phase f o r m s a solid solution of CeA(aq)-C4F(aq) a s a f i r s t s t e p in the t r a n s - formation to C3A(aq)-CsF(aq) solid solution.
Kalousek and Adams (3 5) followed the hydration of C4AF by DTA. T h e r m a l c u r v e s f o r hydrates of C4AF w i t h a n d without addition of gypsum a r e shown i n F i g u r e 9. The 7-day s a m p l e s without gypsum showed the p r e s e n c e of the hydrogarnets and a s m a l l amount of hexagonal plates of C4A' 13 HzO. With i n c r e a s i n g age, hydrogarnets d e c r e a s e d and C4A. 13 HzO o r a r e l a t e d phase i n c r e a s e d , with a peak t e m p e r a t u r e a t about 250 C. Hydrogarnets w e r e r e p r e s e n t e d by a n endothermic effect a t 300 to 400 C.
In cements, C3A r e a c t s with the r e t a r d e r , gypsum, to f o r m sulfoaluminates of com- positions, C3A. 3 CaS04. 32 H 2 0 o r 3 C3A- CaS04. 12 HzO. It is not established which f o r m s f i r s t in cement. In cements the formation of analogous sulfoferrites is a l s o reported. Hydration of Cement
DTA is considered one of the m o s t powerful tools in the investigation of the hydration of cement p a s t e s . T h e r m o g r a m s have yielded useful r e s u l t s on the cement hydrated f o r various lengths of t i m e under different conditions
(2,
30, 36,
5,
38, 39). T y p i c a l t h e r -
m o g r a m s of portland c e m e n t hydrated f o r various periods a s obtained by Greene (36) a r e shown in F i g u r e 10. The unhydrated cement exhibits two endothermic peaks a t 1 4 0and 170 C due t o stepwise dehydration of gypsum. The endothermal dent below
WITHOUT GYPSUM WITH GYPSUM 500 C is attributed to Ca(OH)2 formed
during exposure t o a i r . The b r o a d endo-
7 t h e r m i c effect i n the r a n g e 700 to 800 C
2 8 is caused by the decomposition of CaC03,
a l s o f o r m e d by exposure t o air. Five
6 0 "7 > 4 CJ UNHYDRATED 9 0 5 M I N U T E S I HOUR I 8 0 4 HOURS 2 4 HOURS 7 DAYS 2 0 0 4 0 0 6 0 0 8 3 0 2 0 0 4 0 0 6 0 0 8 0 0 TEMPERATURE "C 2 0 0 4 0 0 6 0 0 8 0 0
F i g u r e 9 . Thermal curves o f C,AF w i t h o u t TEMPERATURE "C
and w i t h g y p s m and hydrated f o r v a r i o u s F i g u r e 1 0 . Thermograms o f p o r t l a n d cement p e r i o d s o f t i m e . hydrated f o r v a r i o u s l e n g t h s o f t h e .
m i n u t e s a f t e r hydration, a n e n d o t h e r m i c peak a p p e a r s a t 130 C due to the f o r m a t i o n of high-sulfate c a l c i u m sulfoaluminate, 3 CaO. A k a . 3 CaS04. 31 HzO. The d e c r e a s e in gypsum content is evident f r o m the reduction of the gypsum peaks. An hour a f t e r hy- d r a t i o n , the i n t e n s i t i e s of t h e gypsum p e a k s d e c r e a s e f u r t h e r and the peak d u e to sulfo- aluminate b e c o m e s m o r e pronounced. At 4 h r , a n additional endothermic peak above 500 C is observed. The peak below 500 C is believed to b e due t o chemisorbed w a t e r on the s u r f a c e of f r e e l i m e p a r t i c l e s , and that above 500 C t o the m o r e c o a r s e l y c r y s - talline Ca(OH), f o r m e d by c r y s t a l l i z a t i o n through solution. A f t e r 24 h r of hydration, the double peak d u e t o gypsum d i s a p p e a r s and a s m a l l e n d o t h e r m i c shoulder a p p e a r s on the low t e m p e r a t u r e flank of t h e sulfoaluminate peak d u e t o c a l c i u m s i l i c a t e hydrate. After 7 d a y s t h i s e n d o t h e r m i c peak i n c r e a s e s . T h e a p p e a r a n c e of a s m a l l e n d o t h e r m i c peak a t 200 C may b e due to e i t h e r the low-sulfate c a l c i u m sulfoaluminate, 3 CaO. A k Q - CaS04. 12 HzO, o r to a solid solution of t h i s compound with t e t r a c a l c i u m aluminate hy-
0 2 0 0 4 0 0 T E M P E R A T U R E OC C O ( O H 1 2 F R E E WATER T O B E R M O R I T E G E L , C S H III) S E T P O R T L A N D C E M E N T 14 DAYS O L D 0 2 0 0 4 0 0 6 0 0 T E M P E R A T U R E OC C 3 A . CO SO,. 13 H 2 0 S E T P O R T L A N D C E M E N T I Y E A R O L D
drate, 4Ca0.AL03. 13 HzO. The other important feature of the 24-hr and 7-day curves i s the gradual loss of water in the range 200 to 500 C and increase in Ca(OH)2 and C a C 4 contents.
The thermal effects below 200 C in hydrated cements a r e of great interest in the in- terpretation of various reactions. Midgley (30) has discussed the endothermic peak effects obtained in a 14-day and 1-yr s e t cement. The three endothermic effects occur- ring very close to one another could be identified by comparing the standard thermo- g r a m s of purer systems, such a s Ca(OH),, f r e e water, tobermorite gel CSH(II), ettring- ite and low sulfoaluminate (Fig. 11). The 14-day hydrated compound gives three peaks at 114, 127 and 147 C due to f r e e water, CSH gel and ettringite, respectively. The s e t cement after 1 y r shows two endothermic peaks below 200 C and a peak above 200 C. The peak above 200 C corresponds to one of the peaks of the low sulfoaluminate. A quantitative estimation of tobermorite, ettringite and Ca(OH)z contents was attempted by Midgley by calibrating the peak a r e a with different amounts of the p u r e r substances. The estimation of Ca(OH), content i s relatively accurate. The figures given f o r tober- morite and ettringite a r e open to question, however, because there i s no way of sepa- rating the individual peaks from the triple peak effect in the range 100 to 200 C.
There i s some controversy regarding the type of sulfoaluminate that forms initially in cements, although i t i s widely held that the high sulfoaluminate f o r m s first. Figure 12 obtained by Greene gives the DTA curves of cement clinker hydrated for various periods of time without the addition of gypsum. At 5 min, the curve shows a peak at 200 C due to either 4 C a 0 . A 1 2 4 ' 13 H 2 0 o r a related solid solution. The curve does not exhibit a low-temperature endothermic peak a t 130 C, indicating that pure clinker com- pounds by themselves a r e not responsible f o r the peak. Addition of 2 percent gypsum produces a fairly large amount of high-sulfate form and prevents the formation of 4 C a O . A L 4 ' 13 H20, indicated by the absence of a 200 C peak. The results do not s e e m to be conclusive and much more systematic work i s needed to establish the products in the initial stages of hydration, F o r example, the endothermic peak at 200 C f o r sample containing 1 percent gypsum may be partly due to the presence of low-sulfoaluminate f o r m which may convert to high form a t higher gypsum concentrations. The low sulfo- aluminate i s reported to be the preferred product in presence of Ca(OH), and CaS04 (40). The DTA work of Greene, in which the cement was hydrated for 2 min in distilled water a s well a s in saturated Ca(OH)2-CaS04 solutions, showed only the endothermic peak corresponding to high-sulfoaluminate form in distilled water and the absence of m y sulfoaluminate in Ca(OH)2-CaS04. Turriziani and Schippa (41, - - 42) believed that DTA can be used to estimate semiquantitatively the sulfoaluminate phases in cement.
In the DTA of hydrated cement, a single peak a t about 130 C i s attributed to the high f o r m of calcium sulfoaluminate hydrate, whereas the peak a t about 200 C i s attributed to the low-sulfate form. The DTA of purer sulfoaluminate, however, shows t h r e e peaks. The thermograms of the high- and low-sulfate forms obtained by Nemecekand Barta (110) a r e shown in Figure 13. The higher form containing 31 molecules of Hz0 loses 2 r m o l e c u l e s (zeolitically held) a t 180 C, 3 molecules a t 280 C, and the r e s t at 370 C. The monosulfate containing 12 mole- cules of H 2 0 loses 8 molecules (zeolitically held) a t 160 C, 2 molecules a t 230 C, and the
UNHYDRATED most strongly held a t 300 C.
5 M I N U T E S 6 HOURS 16 HOURS 100 2 0 0 3 0 0 4 0 0 5 0 0 100 2 0 0 3 0 0 4 0 0 2 4 HOURS I 1 8 0 ' 2801 3701 I
'
160 ' 2 3 0I
-
I 7 DAYS 2 0 0 4 0 0 6 0 0 8 0 0 TEMPERATURE O C HIGH S U L F O 4 L U M I N 4 T E LOW S U L F 0 4 L U M l N 4 T E F i g u r e 1 2 . DTA curves ofp o r t land cement h y d r a t e d F i g u r e 1 3 . DTA o f h i g h and low calcium without gypsum. s u l f o a l u m i n a t e h y d r a t e s
.
Figure 12 shows no indication of formation of the hexahydrate of C3A i n cement hy- d r a t e d without gypsum. Kalousek e t al. (5), however, r e p o r t e d the evidence of f o r m a - tion of C3AHa in s o m e c l i n k e r s by a n endothermic effect a t about 315 t o 330 C . The other clinkers t r e a t e d with gypsum did not exhibit the endothermic peak a t 315 t o 3 3 0 C . Only s m a l l amounts of SO, ions w e r e r e q u i r e d t o s u p p r e s s the formation of the hexa- hydrate.
Kalousek e t al. believe that a solid solution is f o r m e d between 3 C a O A 1 2 4 3 CaS04 (aq) and 3 CaO' AE03' 3 Ca(OH)2 (aq) and a l s o between the low sulfoaluminate 3 CaO. AEo3' CaS04 (aq) and 3 CaO. AI203- Ca(OH)z (aq). Midgley and Rosaman (43) c a r r i e d out DTA of t h r e e cements and found the f i r s t phase was pure calcium sulfoaluminate, with the solid solution containing calcium hydroxy aluminate hydrate a f t e r longer periods. T h e r m o g r a m s of the likely complexes of C3A and C3F a r e shown i n F i g u r e 14.
The r o l e of f e r r i t e phases in c e m e n t h a s been a subject of much controversy and i s much m o r e obscure. Malquori and C i r i l l i (44) and Schippa (45) presented evidence t o show that calcium s u l f o f e r r i t e s analogous t o h i g h sulfate andTow sulfate a r e formed. Watanabe and Iwai (46), by using x - r a y and DTA, observed no compound corresponding to the composition 3 C a 0 . F e z 0 3 . 3 CaS04- 3 1 Hz0 in the hydration of C2F-CaS04. Budnikov and Gorshkov (47) d i s c u s s e d the relative stabilities of calcium hydro sulfoaluminate and calcium hydro z l f o f e r r i t e by DTA. Kalousek e t al. (5, - - 35) c a r r i e d out DTA t o a s c e r - tain whether C&F produced hydrogarnet o r a r e l a t e d hexagonal crystalline compound. The 7-day s a m p l e without gypsum (Fig. 9) shows a n endothermic peak a t about 350 C , indicating the p r e s e n c e of hydrogarnet, and one a t 220C, indicating the p r e s e n c e of a s m a l l amount of 4 CaO. A h a . 13 HzO. At longer periods of hydration, hydrogarnet d e - c r e a s e s and C4AH13 o r a r e l a t e d phase i n c r e a s e s . The reaction is modified by gypsum. The 7-day s a m p l e a f t e r t r e a t m e n t with gypsum i s essentially the sulfoaluminate and sulfoferrite of calcium. At 90 d a y s , the product h a s a maximum of solid solution of C 3 A A E 4 . CaS04. 12 Hz0-3 C a O . A E 4 - Ca(OH)ze 12 HzO and, presumably, 3 CaO- Fez03- CaS0,' 12 HzO-3 CaO' FezO3' Ca(OH)z' 12 HzO. In p r e s e n c e of gypsum, the hydration of 4 CaO' AEo3' F e z 0 3 proceeds rapidly without the formation of hydrogarnets. Attempts w e r e m a d e to follow the effect of Ca(OH), and gypsum on the hydration of C 2 F
+
C&F. S e v e r a l endothermic effects w e r e observed, and the interpretation of the effects was incomplete (5).The e f f e c t o f humid a i r on the products obtained during the s t o r a g e of c e m e n t s h a s been followed by DTA (48). Water combines f i r s t with CaS04 and then with CaO and calcium silicate. The exother- m i c peak a t 600 t o 700 C found in the
t h e r m o g r a m s of c e m e n t s was t r a c e d t o
Fq
C3F. 3Co SO4 o q the p r e s e n c e of sulfides.
Systems Containing CaO, Sick, AE03, Fe2O3, E t c . , in Hz0
Studies on s y s t e m s containing CaO, C , A . ca SC, g q Sick, A E 4 , F e z @ , alkalies and CaS04 in
HzO a r e of g r e a t importance in the c h e m - i s t r y and morphology of hydrous c a l c i u m -
complex compounds. Kalousek (49 - - - 53)
:.:.:.-
did extensive work on the Ca0-Sick-HzO , A . Cs ;CHI, a q s y s t e m . The s y s t e m s containing CaO-1
(i4h. a q )SiQ-HzO and C a O - A h a - H z O w e r e studied a t room t e m p e r a t u r e by Kalousek e t al.
(5)
and J a m b o r (54). The l i m e - s i l i c a g e l s 100 200 300 400 show i r r e g u l a r exothermal bulges s t a r t i n g 1 E M P E H A i U R E " Cf r o m 350 t o 400 C and extending up to
500 C and a n exothermic bulge a t 800 t o Figure ILL. Thermograms of C,A m d C,A-
g e l s shows, a t 7, 14, and 28 days of aging, a n endothermic bulge a t 200 t o 210 C and 470 C. The bulge a t 200 t o 210 C indicates the p r e s e n c e of 3 CaO*AL03*Ca(OH)2. 12 H20. Newman (55) examined 1 5 m i x t u r e s containing Ca(OH)2-SiQ-H20 in different m o l a r r a t i o s by
DTA.
Samples with different Ca(0H)JSiQ r a t i o s and s t o r a g e t i m e s and t e m p e r a t u r e s show c h a r a c t e r i s t i c t h e r m a l behavior. Some s a m p l e s showed the p r e s e n c e of C3&H2. The behaviors of hydrous calcium s i l i c a t e s f o r m e d in the s y s t e m C a O - S i a - H 2 0 with various C/S r a t i o s w e r e studied by DTA (56, - - 57), x - r a y and elec- t r o n microscopy. T o b e r m o r i t e exhibited a n endothermic effect a t 260 C. The phase with C/S of 0 . 8 t o 1.33 showed exothermic peaks a t 830 and 900 C, and those with C/S of 1. 5 t o 2 . 0 exhibited peaks a t variable t e m p e r a t u r e s . The DTA i n conjunction with x - r a y could be applied to differentiate 0 . 8 t o 1.33 and 1. 5 t o 2 . 0 C/S hydrates.The r e a c t i o n s in the s y s t e m Ca(OH),-quartz ( o r s i l i c a ) between 125 and 175 C in the m o l a r composition range, 0 . 8 CaO/SiQ to 1 . 2 5 CaO/Si&, w e r e studied by DTA (58). X - r a y s failed to differentiate between different compositions, w h e r e a s t h e r m o g r a m s exhibited c h a r a c t e r i s t i c peaks. F i g u r e 15 r e p r e s e n t s the DTA of products m a d e with C a O - S i 4 with 0 . 8 C/S (S = q u a r t z ) and autoclaved a t 175 C f o r different periods (58). The 1 - h r product shows the p r e s e n c e of Ca(OH), by an endothermic effect b e t w e e n 3 0 0 and 600 C. At 1. 5 h r , about two-thirds of Ca(OH), i s consumed t o f o r m poorly c r y s - tallized a-type hydrate. At 3. 5 h r , t h e r e is almost complete absence of Ca(OH)2 with an appearance of a-hydrate of 1 . 7 5 C/S. At longer p e r i o d s reaction o c c u r s between the l i m e - r i c h phase and the r e s i d u a l quartz. The f o r m u l a f o r the 1 . 7 5 C/S product is C7S4Hn. The various s t e p s i n the reaction could b e given a s follows on the b a s i s of t h e r m o g r a m s :
7 C
+
4 S+
a q-
C7S4Hn C5S4Hn+
S-
5 CSHn 4 CSHn+
S-
C4S5HnC4S5Hn converts t o t o b e r m o r i t e , indicated by the 8-hr curve. P r o d u c t s of Ca(OH)2- s i l i c i c acid mixture a l s o followed a s i m i l a r sequence, but the reaction was compara- tively f a s t e r .
Midgley and Chopra (59) studied the l i m e - r i c h p a r t i n the s y s t e m C a O - S i a - H 2 0 . T h e r m o g r a m s w e r e obtained f o r m i x e s containing CaO/SiOz = 2:l and autoclaved a t
150 C . The m i x t u r e autoclaved f o r 30 days indicated a t h e r m a l peak a t 460 C f o r the p r e s e n c e of a-C2S hydrate. After
I HR 14 days of autoclaving a t 180 C , DTA
again showed the p r e s e n c e of a-C2S hy- d r a t e . At 90 days, the p r e s e n c e of a-
and 8-C2S hydrate w a s indicated. The peak a t about 500 C was attributed t o a -
hydrate and that a t 550 C to the 8-hydrate.
1 % HR The mixture with CaO/SiQ r a t i o of 1
f o r m e d c r y s t a l l i z e d t o b e r m o r i t e . In another s e r i e s of experiments, the auto-
3 % HR claved product of CSH(1) and l i m e r e -
sulted i n an initial formation of F l i n t ' s CSH(A) and C2S-a hydrate. With the s t a r t i n g m a t e r i a l a s 8-CzS, the product 5 % HR was C2S-y hydrate. p-C2S and l i m e gave
7 HR C2S a-hydrate. With C3S a s the s t a r t i n g
8 HR m a t e r i a l , the product w a s C3Sa 1% HzO.
The r e p r e s e n t a t i v e t h e r m o g r a m s of v a r i -
200 400 600 800 ous h y d r a t e s f o r m e d in an autoclaved-
TEMPERATURE "C
t r e a t e d s y s t e m a r e given in F i g u r e 16.
F i g u r e 1 5 . DTA of p r o d u c t s o b t a i n e d by Bozhenov e t al. (60) studied by DTA the
a t m by adding various quantities of CaO and SiQ t o p-C2S. Lime-sand autoclaved products showed many endothermic ef-
f e c t s . The effect i n the r a n g e 120 to 400 c , ~ , I l r D R A T i
C was due t o dehydration of zeolitic w a t e r f r o m the g e l s t r u c t u r a l component, a t 450 C i t was due t o ~ V I ~ ( O H ) ~ decomposi- tion, a t 573 C t o the a
-
/3 conversion i n quartz, a t 680 t o 750 C i t correspondedto the dehydration of crystalline calcium 0 2 0 0 4 0 0 0 0 0 8 0 0 1 0 0 0 TEMPERATURE 'C
hydrosilicate. Recrystallization of dehy-
d r a t e d s i l i c a t e s was manifested by a Ylgure 16. D l f r = r z n t l a l i ! ~ e r m o ~ r a n s ~ f
s h a r p exothermic r e a c t i o n in the r a n g e v a r l o u s calclum s l h c a i e l ~ y d r a t z s .
800 t o 850 C. F u r t h e r information on CaO-SiQ-H20 under hydrothermal condi- tions h a s been published recently (61).
The DTA of products f o r m e d b y h y d r o t h e r m a l interaction of lime, with o r without addition of cement, with s i l i c e o u s a g g r e g a t e s such as s i l i c a , pumice, expanded s h a l e o r s l a g showed c e r t a i n c h a r a c t e r i s t i c p r o p e r t i e s of the products (51, - - 62). Autoclaved products with cement-lime and s i l i c a ( o r q u a r t z ) c o n s i s t of a s e r i e s of hydrates having
a composition f r o m about 0 . 9 t o 1 . 3 C/S. The m i x e s different i n s i l i c a f i n e s f o r m e d
a poorly c r y s t a l l i n e l i m e - r i c h phase. With pumice, c e m e n t - l i m e f o r m e d hydrates s t r u c t u r a l l y s i m i l a r t o those made f r o m quartz but with C/S r a t i o higher than 1 . 3 . DTA successfully differentiated between p h a s e s of different compositions. The DTA w a s applied by Midgley and Chopra (63) t o identify the compounds f o r m e d by hydro- t h e r m a l r e a c t i o n of lime with aggregates such a s fuel a s h , expanded colliery shale, ground quartz, granulated and foamed b l a s t f u r n a c e slag. Tobermorite and a hydro- g a r n e t w e r e f o r m e d with pulverized fuel a s h o r s h a l e a s aggregates, t o b e r m o r i t e and zonotlite with quartz, poorly c r y s t a l l i n e t o b e r m o r i t e , dicalcium s i l i c a t e a-hydrate and
a hydrogarnet with slag.
Attempts have been made t o study the DTA of quaternary s y s t e m , C a 0 - A 1 2 4 - C a S 0 4 - H 2 0 (41). DTA shows the p r e s e n c e of new p h a s e s , C+4' 13 HzO and C3A. CaS04- 12 H20. ~ a j u c d a r and Roy (64) used DTA to study the s y s t e m C a 0 - A 1 2 4 - H 2 0 between 100 and 1000 C under w a t e r p r e s s u r e s up t o 3000 a t m . DTA showed only endothermic peaks f o r the p r e s e n c e of Ca(OH)2, C3AHe and C4A3H3.
Autoclaved Portland Cement P r o d u c t s
Setting and hardening of c o n c r e t e products can be a c c e l e r a t e d by curing them i n s t e a m a t 180 C and a t a p r e s s u r e of 100 t o 200 psi. In this type of curing, the cement is mixed with about 60 percent of i t s weight
with fine quartz o r other r e a c t i v e s i l i c a and aggregate. Crystalline t o b e r m o r i t e i s the main product f o r m e d by autoclave
treatment. In absence of r e a c t i v e s i l i c a , 0 % s 1 0 2
21% S1 o 2
a-C2S is the product responsible f o r low 2 4 9~ S , o 2
s t r e n g t h s . No f r e e Ca(OH)2 is detected. 3 % S , O ~ 3 0 % s l o Z
The reaction of C3A and the f e r r i t e phase
6 % SnOz
is not well known. 3 6 % s , o Z
Kalousek and Adams (35) c a r r i e d out a 9 % 51 o 2
s y s t e m a t i c investigation o f t h e cement- 1 2 % s , ~ ~
s i l i c a (finely ground quartz) m i x t u r e s by 4 4 % s 1 0 2
autoclaving a t 175 C. The DTA of the 15% 58 O 2
1 8 % 5 1 O2 6 0 % S # 0 2
products containing SiQ up to 60 percent
is given in Figure 17. The peak due to ZOO 4 0 0 6 0 0 8 0 0 ZOO 1 0 0 6 0 0 8 0 0 l E M P E R A T U R E 'C
decomposition of Ca(OH), a t 560 C is ob-
s e r v e d only i n the products containing F i t a r e 1 7 . Thermal a n a l y s i s of c e m e n t - s i l - l e s s than 9 p e r c e n t S i G . The absence of i c a m i x t ~ u r e s p r o c e s s e d a t 175 C f o r 24 hr.
Ca(OH), decomposition peak is due to the reaction between SiOz and Ca(OH),. The endo- t h e r m i c effect a t 470 t o 480 C w a s identified a s plate-like dicalcium s i l i c a t e hydrate and was p r e s e n t in l a r g e r amounts up t o 9 p e r c e n t S i a , a f t e r which i t d e c r e a s e d p r o - g r e s s i v e l y . Menzel (65) had r e p o r t e d e a r l i e r that 8 to 10 p e r c e n t Si02 product c o r r e - sponded t o minimum s t r e n g t h and a t this composition, a maximum amount of plate-like dicalcium hydrate w a s observed. At higher SiOz concentrations, a n exothermic peak a p p e a r s a t 840 C , with a maximum inflection a t a composition of 40 to 45 p e r c e n t SiOz; this a l s o c o r r e s p o n d s t o g r e a t e s t strength. Hence, the high strength w a s a s c r i b e d to the phase responsible f o r the exothermic peak and w a s designated monocalcium hydrate. Subsequent s t u d i e s showed that high s t r e n g t h s in the autoclaved products w e r e due to the p r e s e n c e of t o b e r m o r i t e . The formation of a-C2S always r e s u l t e d i n a r t i c l e s of poor s t r e n g t h . Bozhenov e t al. (60 -9 - 66) presented r e s u l t s of DTA on autoclaved cement
p a s t e s obtained a t 8 to 100 a t m . T h e r m o g r a m s of products f o r m e d f r o m C2S i n admix- t u r e with CaO and SiOz w e r e examined. Evidence was found f o r a n interaction between 8-C2S and SiOz with the formation of b a s i c calcium hydrosilicates (67). DTA w a s ap- plied by Kalousek (51) to examine the autoclaved products o b t a i n e d v i t h different raw m a t e r i a l s made of l i m e - c e m e n t and a g g r e g a t e s s u c h as silica, pumice, expanded s h a l e o r expanded slag. The s a l i e n t r e s u l t s obtained by him have a l r e a d y been discussed.
E F F E C T O F EXTRANEOUS COMPOUNDS ON CEMENT HYDRATION Reactions of C a C a and COz With Cement
The possibility of formation of complex compounds s u c h a s carboaluminates in c e m e n t s in p r e s e n c e of COz, CaC03 o r alkali carbonates h a s received s o m e attention i n r e c e n t y e a r s . B e s s e y (68) p r e p a r e d the high- and low-carbonate f o r m s , 3 CaO. Al2* CaC03. x H 2 0 and 3 C a O Al.03.3 CaC03. x H 2 0 . T u r r i z i a n i and Schippa (69) identified the low- carbonate f o r m i n alumina cement p a s t e s c u r e d f o r 2 y r . ~ a r l s o n and B e r m a n (70) obtained a product of the probable f o r m u l a , 3 C a O A1203. C a C W 11 H20, in m i x t u r e s of C a 0 - A L G - H 2 0 exposed t o COz. Greene (36) found a difference in the t h e r m o g r a m s of cement hydrated with w a t e r and that hydrated with 2 p e r c e n t Na2C03 f o r 24 h r . The difference was attributed to the formation of carboaluminate. T h e s e t h e r m o g r a m s f o r carboaluminates a r e shown i n F i g u r e 1 8 f o r comparison. Although t h e s e may r e p r e - s e n t the carboaluminates, i t is r a t h e r difficult to believe they a r e identical. No attempt h a s been m a d e by any of these w o r k e r s t o d i s c u s s v a r i o u s peaks, and much m o r e s y s - tematic work s e e m s t o be needed t o e s t a b l i s h definitely the t h e r m a l behavior of c a r b o - aluminates.
Manabe e t al. (71) c a r r i e d out TGA of the s y s t e m C3A-CaC03-H20 and r e p o r t e d the formation of 3 cao7~l203. CaC03.10.6 H20. The carboaluminate w a s f o r m e d a f t e r 3 d a y s i n a m i x t u r e of cement and CaC03.
Cole and Kroone (72) c a r r i e d out DTA t o investigate how COz r e a c t e d with portland cement m o r t a r and calcium s i l i c a t e hydrate. C o n t r a r y t o e a r l i e r views that only c a l - c i t e is f o r m e d , poorly c r y s t a l l i z e d v a t e r i t e and aragonite in addition t o s o m e calcite w e r e found.
The possibility of the o c c u r r e n c e of calcium silicoaluminates analogous t o sulfoalu- minates h a s been r e p o r t e d by Flint and Wells (73). The compound 3 C a O AEQ.
3 C a S i a - 3 0 - 3 2 H 2 0 r e p o r t e d by Flint w a s re-examined by DTA by C a r l s o n and B e r m a n (70). The compound which had been exposed to COz indicated a n endothermic effect f o r t h e expulsion of w a t e r a t the s a m e t e m p e r a t u r e found f o r 3 CaO- AE03- 3 CaC03.32 HzO. The endothermal peak found a t 815 C w a s due t o COz evolution. The exothermic effect a t 850 C was due t o l i m e - s i l i c a reaction. The significance of o t h e r i r r e g u l a r i t i e s i n the t h e r m a l c u r v e s w a s not d i s c u s s e d . The conclusion, however, was that the s i l i c o - aluminate in p r e s e n c e of COz f o r m s a mixture of aluminate s i l i c a t e and carboaluminate.
Work on the effect of v a r i o u s amounts of C a C a on the hydration of C3A h a s been c a r r i e d out a t the Division of Building R e s e a r c h by the a u t h o r s ( r e s u l t s unpublished). The r e s u l t s of dimensional changes i n the compacts f o r m e d f r o m C3A and C3A-CaC03 and hydrated i n w a t e r w e r e c o r r e l a t e d with those obtained by DTA. T h e r m o g r a m s showed that hydrated C3A exhibits a n intense endothermal effect with a peak a t 310 C and additions of CaC03 s u p p r e s s this peak. A carboaluminate complex formed on the s u r f a c e of the C3A g r a i n s a l s o appeared t o inhibit f u r t h e r hydration.
400 600 800 1000 1200
TEMPERATURE O C
v
TEMPERATURE "CTEMPERATURE "C
F i ~ q r e 18. DTA of' caiiboaluminates: ( a ) C a r l s o n and Berman; ( b ) T u r r i z i a n i and S c h i p p a ; and ( c ) Greene.
Surface Active Agents
Addition of s m a l l quantities of s a l t s of lignosulfonic acid to concrete lowers the water requirement and r e t a r d s the setting of the concrete mix. DTA was applied by Young (33) to study the r o l e of lignosulfonate on the hydration behavior of C3A alone and in
of gypsum and lime. No attempts w e r e made to explain s o m e of the t h e r m a l peaks. It was concluded that in p a s t e s of C3A, lignosulfonates favor the formation of CzAH8 and C4AH13 and modify t h e i r c r y s t a l habit. In presence of gypsum and lime, for- mation of low sulfoaluminate f o r m is favored. In Figure 19 a r e represented the DTA of the typical m i x t u r e s obtained by Young. The C3A hydrated product shows two endo- t h e r m i c peaks; the one a t 300 to 400 C was a l s o reported by o t h e r s a s being due to C3AHe. In p r e s e n c e of lignosulfonate, t h r e e endothermic peaks a r e obtained below 400 C and a r e attributed to a mixture of C4AH13 and C2AH8. The exothermic peak a t 520 C was not satisfactorily explained. C3A paste with gypsum and lime shows endothermal effects a t 100, 210, 320, 520 and 820 C. These a r e due to low-sulfoaluminate form, CzAH8 and CAH13. The s a m e mixture in p r e s e n c e of lignosulfonate gave l e s s intense endothermal effects due to retardation of the hydration reaction.
F a l s e Set
Some cements exhibit the phenomenon of "false set" o r p r e m a t u r e stiffening. V a r i - ous theories of f a l s e s e t have been reviewed by Blanks and Gilliland (74). - According
HYDRATED C 3 A - 6 M I N U T E S
to g e n e r a l views, f a l s e s e t is due to de- hydrated gypsum i n the f o r m of hemi- hydrate o r soluble anhydrite o r both, rapidly precipitating a s gypsum without H Y D R A T E D C , A - 7 D A Y S the evolution of much heat.
Takemoto e t al. (75) studied by DTA the extent to which had dehydrated
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 T E M P E R A T U R E 'C
H Y D R A T E D C j A IN PRESENCE OF LIGNOSULFONATE - 6 M I N U T E S
HYDRATED C3A IN PRESENCE OF LIGNOSULFONATE - 7 DAYS
HYORATED C 3 A IN PRESENCE OF GYPSUM AND L I M E - 3 DAYS
during grinding of a mixture of clinker and gypsum in an experimental mill. The DTA c u r v e s showed that the hemihydrate was formed a t 85 C when ground f o r 40 min. The soluble anhydrite was formed a t 130 C when the grinding time was 80 min. Gilliland (76) studied the DTA be- haviors of hemihydrate, insol- uble anhydrite, and a cement a s received and when heated to 300 C. Gypsum showed two endothermic peaks a t 190 and 210 C due to l o s s of 1% molecules and mole- cule of w a t e r , respectively. The exo-
F i ~ u r e 19. DTA. o f CC,A h y d r a t e d i n p r e s e n c e
sl' l i ~ n o s u l f i l n a t e and gypsw?! and i i l n e . t h e r m i c peak a t 380 C initiated t h e con-
version of soluble to insoluble anhydrite. The hemihydrate exhibited
an
intense endothermic peak above 200 C owing to the l o s s of1/2
molecule of w a t e r . No t h e r m a l effect was observed f o r CaS04 (insoluble). A commercially ground cement showed two peaks a t 140 and 160 C; that the l a t t e r was l a r g e r was attributed t o the formation of hemihydrate f r o m the gypsum present i n the cement. The soluble anhydrite was formed by heating the cement a t 300 C and then cooling. Soluble anhydrite partially converted to hemihydrate on exposure to air, show- ing a peak a t 130 to 13 5 C. F i s c h e r (111) - evaluated quantitatively the d e g r e e of dehy- dration of gypsum in cement by DTA.OTHER CEMENTS Alumina Cement
The s p e c i a l type of cement known a s "aluminous cement,
"
c h a r a c t e r i z e d by a high e a r l y strength is formed by firing limestone o r chalk with bauxite to a temperature of 1550 to 1600 C.Alumina cement contains compounds such as unstable C5A3, CA and C2AS. CA, the most important constituent, r e a c t s with water a t r o o m t e m p e r a t u r e to f o r m CAHID, the principal cementing agent. Under hot humid conditions, CAHlo t r a n s f o r m s to C3AHa with a l o s s i n strength. Pole and Moore (77) and Schneider (112) obtained DTA of alu- mina cement paste. The endothermic effect between 250 a n d 3 5 0 C showed the p r e s - ence of C3AHe. An exothermic peak extending over a wide range of 400 t o 1000 C was perhaps due to the burning of carbon p r e s e n t i n the cement slag. Nagai and Harada (78 -9 - 79) studied t h r e e alumina cements, CA and C3A5 by DTA. Rey (38) showed by DTA
that the aluminous cement hydrated f o r 7 days exhibits peaks corresponding to t e t r a c a l - cium aluminate hydrate and a t 13 days t o dicalcium aluminate hydrate. The various inflections i n the t h e r m o g r a m s w e r e not accounted f o r .
Pozzolanas and Pozzolanic Cements
Pozzolanas a r e m a t e r i a l s which combine with lime a t ordinary t e m p e r a t u r e s i n p r e s - ence of water to f o r m s t a b l e insoluble compounds possessing cementing properties. Pozzolanic cements a r e obtained by grinding together portland cement clinker and poz- zolana o r mixing together a hydrated l i m e and a pozzolana (80). T h e r e is every reason to believe that pozzolanic m a t e r i a l s f o r m cementitious compounds s i m i l a r to those ob- tained by the hydration of portland cement clinker.
T u r r i z i a n i (81, - - 82) has applied the DTA technique extensively t o a study of the chem- i s t r y of the pozzolanic activity. He r e p o r t e d that a pozzolana t r e a t e d with s a t u r a t e d lime solution gave a low t e m p e r a t u r e endothermic peak a t 140 C , indicating the p r e s - ence of T a y l o r ' s s i l i c a t e , CSH(1). The second effect a t 200 C was a s c r i b e d t o the p r e s - e n c e of C4AH13. Continued work (83) on the products with different proportions of Ca(OH)2 and pozzolana and on m o r t a r s confirmed the p r e s e n c e of calcium s i l i c a t e of t o b e r m o r i t e type. In m o r t a r s , DTA showed the p r e s e n c e of C4AH13. The reaction a t 45 C gave the product with a peak corresponding t o C3AHa (cubic). T h i s a g r e e s with r e s u l t s obtained by Malquori and C i r i l l i (84). The formation of s m a l l amounts of C2SAHx was a l s o considered. The deterznation of f r e e Ca(OH)2 in pozzolanic cements by DTA showed that a f t e r long curing the quantity is much s m a l l e r than that ordinarily f o r m e d in portland cement p a s t e s (85). The products of portland cement-pozzolanas examined up t o a period of 1 y r s h o v e d the p r e s e n c e of t o b e r m o r i t e type of calcium s i l i c a t e hydrate, t e t r a c a l c i u m aluminate hydrate, high-sulfate sulfoaluminate and low Ca(OH)2 content. A m i x t u r e of pozzolana, hydrated l i m e and gypsum in w a t e r shows a n endothermal peak corresponding to the formation of 3 CaO- A 1 2 4 C a S 0 4 32 H 2 0 and under c e r t a i n conditions, 3CaO.A1203-CaSO4.12H2O (86, 87). Rey (38) a l s o presented a typical c u r v e f o r Ca(OH)2-pozzolana mixture. S u r o z i n y n d Krylov(88) studied the interaction of lime and calcined clay. DTA showed exothermic peaks a t 3 1 0 and 890 C and an endothermic peak a t 540 C for the reaction between calcined clay and Ca(OH)2. Benton (113) c a r r i e d out a s y s t e m a t i c investigation of the lime-pozzolan and cement- p o z z o l a n ~ a c t i o n s by DTA and x - r a y . The pozzolans used f o r the study included pum- i c i t e , diatomite, kaolinite, bentonite, i l l i t e , gibbsite, quartz and fly a s h . T h e DTA of a l l lime-pozzolan m i x t u r e s except quartz showed endothermic peaks a t about 200 C f o r the p r e s e n c e of C4AH13. All m i x t u r e s exhibited endothermic peaks between 500 and 600 C, corresponding t o Ca(OH), decomposition, and the intensity of the peak was u s e d to d e t e r m i n e the reactivity of the pozzolans in lime-pozzolan and pozzolan-cement mix- t u r e s . In cement-pozzolan m i x t u r e s , high-temperature exothermic peaks i n c r e a s e d in intensity f o r m o r e active cements; t h i s w a s c o r r e l a t e d with dehydration of products into such m i n e r a l s a s wollastonite, 8-C2S and melilite. A much m o r e s y s t e m a t i c work should be undertaken t o study the interaction between clay m i n e r a l s f i r e d to different t e m p e r a t u r e s and t r e a t e d with l i m e s of varying reactivity.
Slag C e m e n t s
Slag c e m e n t s a r e obtained by grinding granulated b l a s t f u r n a c e slag (by quenching the slag issuing f r o m the blast f u r n a c e a t 1400 to 1500 C ) with a c t i v a t o r s such a s p o r t - land cement clinker o r hydrated l i m e and a m i x t u r e of one o r other of these with gyp- s u m o r anhydrite.
The c h e m i s t r y of slag c e m e n t s is l e s s c l e a r t h a n t h a t of portland cement. Lommatzsch (89) t r i e d to obtain an understanding of binding and setting p r o c e s s e s of granulated b l a s t f u r n a c e s l a g s under sulfate activation. The t h e r m a l changes in the range 800 t o 900 C w e r e thought t o be due to modifications and formations typical of the hydraulic b a s i c s l a g s . The formation of g l a s s i n the slag w a s studied by Kondo (90). Williams and Chopra (91) found that Indian slags containing 25 percent CaS04 a;;;i 5 percent cement clinker f o r m e d a good-quality supersulfated cement. T h e DTA of this mix indicated the p r e s e n c e of ettringite a t curing periods extending f r o m 3 to 90 days. The intensity of the endothermic effect a t 160 C gave an indication of the amount of ettringite formed. DTA h a s been applied by s o m e w o r k e r s to c h a r a c t e r i z e the b l a s t f u r n a c e slags (92, 93). Mchedlov-Petrosyan e t al. (94) - recently found that DTA is a rapid test method f o r t h e detection of g l a s s in slags.
The mechanism of activation of s l a g s w a s studied by Samaddar and L a h i r i (114). Synthetic s l a g s with a constant Si0~/A1,03 r a t i o and varying CaO contents w e r e y e p a r e d with lime-gypsum mixture. DTA r e s u l t s showed the p r e s e n c e of C4AH13 a s a n initial product on the s u r f a c e . During hydration the hydrate l i b e r a t e s lime which r e a c t s f u r - t h e r with m a s s of the slag.
