T H E J O U R S - I L OF I N D U S T R I A L A N D E X G I S E E R I A - G C H E M I S T R Y
466
I
1-01.
By G. A. RANKIX
T h e question of t h e constitution of P o r t l a n d cement clinker has long been i n dispute a n d h a s called f o r t h a large n u m b e r of papers a n d m a n y theories; unf o r t u n a t e l y , however, t h e experimental bases of most of t h i s work h a v e been altogether insufficient t o decide t h e several questions a t issue. T h e r e h a s been i n general a failure t o realize t h e fact t h a t a system so complicated a s t h i s c a n be unravelled only b y proceeding systematically, using as a guide t h e principle known a s t h e phase rule a n d establishing definite criteria for t h e recognition of t h e several substances which occur. T h e work recounted i n t h e following pages forms p a r t of a complete investigation of t h e whole s y s t e m of mixtures of lime-alumina-silica, carried o u t i n t h i s systematic w a y ; i t is believed therefore t h a t t h e conclusions reached can b e regarded as established. I n t h e g r e a t b u l k of t h e published work o n t h e cons t i t u t i o n of P o r t l a n d cement t h e evidence offered i n s u p p o r t of t h e s t a t e m e n t s a s t o t h e reactions which t a k e place during t h e burning of cement a n d t h e several substances formed is largely unconvincing; accordingly, a n extended review of t h e l i t e r a t u r e would be futile. T h e most c o m m o n fault is t h a t t h e criteria employed t o define a compound h a v e been indefinite or insufficient; t h u s i n some cases i t is t a k e n for g r a n t e d t h a t a s u b s t a n c e is necessarily a compound, if, when mixed with water, it remains “ v o l u m e constant.” Again t h e refractive index is frequently spoken of a s “ h i g h ; ” b u t very m a n y substances h a v e a “ h i g h ” refractive index, so t h a t such a s t a t e m e n t helps us little i n identifying a compound a n d m a y actually be misleading. Such evidence, t o be of a n y use, m u s t b e capable of q u a n t i t a t i v e s t a t e m e n t . Cement ‘clinker is a mixture of substances of very similar properties, a n d is moreover exceedingly fine grained, a s a consequence of which it is a m a t t e r of some difficulty t o m a k e q u a n t i t a t i v e determinations of t h e optical characteristics of t h e c o n s t i t u e n t s ; b u t t h i s difficulty can be s u r m o u n t e d b y s t u d y i n g separately each of t h e presumable constituents of t h e clinker a n d determining definite values of certain properties which serve t o characterize i t a n d t o distinguish i t f r o m t h e other possible constituents. According t o Meade,2 t h e average of a large n u m ber. of analyses of commercial P o r t l a n d cement is:
. ... .,. ... ... .. ..
CaO.. . SiOz.. AIzOJ.,
, ,
6 2 . 0 per cent 2 2 . 0 per cent 7 . 5 per cent
FezOa.. .
. . ... .. . .
MgO.. , , , , . SOa.. . . . . . . .
2.5 per cent 2 . 5 per cent 1 . 5 per cent
T h a t is, more t h a n 90 per cent of a n average P o r t l a n d cement consists of t h e t h r e e oxides, C a O , A1203, SiOt; one would therefore expect t h a t i t s properties a r e d u e mainly t o t h e presence of t h e above 3 components a n d t h a t t h e relatively small a d m i x t u r e of t h e other oxides exerts a t most a wholly secondary 1 Paper presented a t the 50th Meeting of the American Chemical Society, New Orleans, M a r c h 31 t o April 3, 1915. 2 Richard K . Meade, “Portland Cement,” p. 30.
So. 6
1
ORIGINAL PAPERS
THE CONSTITUENTS OF PORTLAND CEMENT CLINKER’
;.
influence. Indeed Clifford Richardson’ h a s shown t h a t good P o r t l a n d cement c a n be m a d e b y using only t h e p u r e oxides lime, alumina a n d silica i n t h e proper proportions. Accordingly t h e first problem is t o isolate a n d determine all t h e possible CaO, Al2O3, Si02 compounds which we m a y expect t o find in P o r t l a n d cement clinker, t o establish their relations a t high t e m p e r a t u r e s , a n d t o ascertain their optical characteristics, which constitute t h e most convenient a n d satisfactory criterion of t h e i d e n t i t y of t h e several substances. These characteristic properties of t h e various solid substances containing CaO, Xl2O3, Si02 only, which a r e likely t o occur i n P o r t l a n d cement, we h a v e det e r m i n e d i n t h e course of a complete investigation of all compounds formed when a n y mixture of these t h r e e oxides is h e a t e d t o a high t e m p e r a t u r e . I n P o r t l a n d cement clinker t h e relative proportions of these oxides v a r y only between comparatively n a r row limits: S O 2 , 18. j t o 2 3 . 2 ; A1203,6 . I t o 11.9;C a O , 6 3 . I t o 6 8 . I ; 2 i n o t h e r words, i n considering t h i s special problem we h a v e t o deal with a very restricted portion of t h e equilibrium diagram for t h e whole s y s t e m , CaO-Al208-Si02. T h i s problem is therefore b u t a small portion of t h e larger problem which we undert o o k a n d i t s investigation mas incidental t o t h a t work. We believe, however, t h a t t h e results of our investigation, which were concerned with t h e behavior of s y s t e m s m a d e u p only of t h e t h r e e pure oxides, establish a working basis from which those more especially interested i n t h e s t u d y of commercial P o r t l a n d cement clinker m a y s t a r t with confidence; work of t h i s character has already been u n d e r t a k e n a t t h e P i t t s b u r g h l a b o r a t o r y of t h e B u r e a u of S t a n d a r d s 3 a n d has led t o results which, a s we shall see l a t e r , bear o u t t h e conclusions derived f r o m t h e work here. T h e experimental results utilized in t h e present p a p e r a r e t a k e n from t h e complete report4 of t h e investigation of t h e three-component system CaOA1203-Si02, a p a p e r t o which t h e reader desirous of fuller information with respect t o t h e application of t h e phase rule, t h e experimental procedure, t h e course of crystallization, etc., is referred. THE T E R N A R Y SYSTEM
Ca0-A1203-Si02
T h e working o u t of t h i s system necessitate& t h e investigation of a b o u t I ooo different concentrations a n d fully 7000 h e a t t r e a t m e n t s a n d microscopical examinations. E a c h concentration was m a d e u p of especially p u r e C a O , A1203 a n d Si02 a n d was fused repeatedly i n a p l a t i n u m crucible with fine grinding bet w e e n fusions, i n order t o o b t a i n a p r o d u c t chemically homogeneous. E a c h of these products was subjected t o a careful optical s t u d y t o determine t h e crystalline Clifford Richardson, Ce?nc?il,5 , 31-1. Ibid.,6 , 20. a “ T h e Constitution of Portland Cement,” by P. H Bates. Cr’ll cvcle-cement Age (Cement Mi11 Section), 2 (1913). 3 . 4 “The Ternary System, Ca0-.4120~-Si0?,” G. A . Rankin. “CV~tic:il Study.” F r e d . E . Wright. . A v i e r . Jouu. S r i . . 141 39 11915). 1b79 1
pryhelit a t temperatures ranging from t h a t a t which melting Legins t o t h a t a t TT-hich t h e charge is complerelL- melted. The d a t a required are obtained most readily by nieans o i t h e “ q u e n c h i n g ” method n-hich was found t o iJe invaluai3le. In this method a small charge of t h e desired comp:)sition is placed in a platinum resistance furnace m c l held a t a constant ternperaturc until equilihriuin is a t t a i n e d , \Then t h e charge is quenched ’‘ (suddenly cooled by dropping i n t o a b a t h of mercury or w:itt.ri. In this way one is able t o o b t a i n for microscopic:il e x a x i n a t i o n t h e material as i t existed a t t h e temperature in t h c furnace. T h u s i l t h e charge were completely meltecl a t t h e temperature of the furnace, it n.ould on Iquenching appear ns B glass; h u t if t h e charge h a d been only partially melted t h e chilled product would contain unmelted or crystalline material embedded in glass, n-hereas if no melting hac1 t a k e n place, the chilled product would contain no glass. These differences are very easily recognized b y microscopical exaniination; hence by holding charges of t h e same composition a t a series of constant t e m peratures. one is enabled t o fix, within ‘ 3 ’ C., tlie temperature a t which melting begins a n d is complete for t h e particular mixture One is able t o obtain b y t h i s method not only melting temperatures u p t o 1 6 2 j ’ C. (the limit of t h e platinum furnace) b u t also t o grow crystals, t h e optical properties of which can be readily measured. since in t h e quenched product t h e y are embedded in clear glass. B y proceeding systematically in this %\ray t h e optical properties of all compounds TTliich occur in melts of C a O , A1203 a n d SiOz were definitely determined. This general description of t h e mode of a t t a c k in n-orking o u t the ternary system will suffice for t h e present purpose; we shall n o t t a k e u p t h e o t h e r methods employed or a detailed discussion of t h e results obtained. a s all this has already been published in t h e full report already referred t o . T h a t report presents t h e following d a t a n h i c h . it is t o he remembered, a p ply strictly only t o conditions of heating or cooling such t h a t equilibrium was continuously a t t a i n e d : I The compo sit i o ns, melting or dissociation t e niperatures, a n d optical properties. o l t h e coniponents and of a11 t h e conipounds both binary a n d ternary which are found in d r y rnelts of CaO. AlZOS. SiO?. ,.-The temperatures a n d regions within which t h e several components and compounds, pure or in mixtures, can exist in equilibrium in contact with melt (liquid). 3-The order in which t h e sel-ern1 solid phases crystallize from a slo~vlycooling melt in-hich is t h e reverse of the order of disappearance of t h e several crystals when a solid mixture is slonly heated). p - T h e final products obtained \Then any melt composed only of these three oxides ha.; completely crystallized. T h e final products of crystallization are t h e only sssential d a t a required for the elucidation of most of t h e problems x i t h which we are concerned. If equilibrium has been continuously a t t a i n e d , t h e final Froduct n-ill,. in accordance with thc requirements “
.
of t h e phase rule, consist aln-a>-s of a group of three solid phases (except t h e composition is exactly t h a t of a pure compound or a binary mixture). Tlie manner in which t h e constitution of these groups varies with variations in t h e composition of t h e original mixture can be determined only b y experiment. t h e results of which can be most readily understood if t h e y are plotted on a n equilaterai triangular concentration diagram such as Fig. I . In this diagram t h e pure components C a O , A1203, Si02 are represented by t h e apices of t h e triangle; t h e binary mixtures CaO--U203, -ll,03-Si0, and Si0,CaO, respectively, b y points on t h e three sides of the triangle a n d t h e ternary mixtures by points within t h e triangle. Each side of this equilateral triangle is divided i n t o I O O p a r t s ; this enables one t o represent t h e s u m of t h e percentage a m o u n t s of t h e components (in either binary or ternary mixtures) by n side of t h e triangle. Tlie perci.ntag6.s may be given either as molecular 0:’ n-eight percentages; in
/
c.0
x
x,
FIG. ~ - D I A G R A Y TO
\ SCiO.lr,O,
SC.O.Jh.D,
.
i lCQ.51&
CiO.h,O,
FINALP R O D U C T S
bo,
CRYSTALLIZATION OF SOLUTIOKS OF C A O , A1202 AID Si02 Within each small triangle symbols, C = C a O ; A=AhCh; S=SiOr, a r e used to simplify t h e designation of t h e compounds. T h u s : 3CaO.SiOz=CaS: jCa0,3AlrO3=CsA3; etc. SHOW
OF
Fig. I all compositions are given as percentage weights of t h e components. b s t h e location of points which represent compositions in a triangular diagram may be puzzling t o those n o t familiar with its use i t m a y be well t o illustrate with examples. T h e apices of t h e triangle, of course, represent tlie components, each I O O per cent pure, so t h a t it is a n easy m a t t e r t o determine t h e composition of a binary mixture represented b y a point on a side of t h e triangle: t h u s consider t h e point A1203.SiO2 on t h e side !&o3-SiO2 as representing a n unknown composition, t h e n , since t h e length of t h e side AlzOa-SiO? is t a k e n a s 100,t h e portion SiosX1203.Si02will give t h e percentage weight of AI203 a n d t h e portion A 1 03 . Si 0*-A 1 0 t he percent age weight S i 0 2 in t h e composition represented by t h e point A1203.Si02. As a n illustration of points within t h e triangle, consider point Q a s representing a ternary mixture of C a O , ;2I2O3. S i 0 2 of unknown composition. If? n o w , one line is dran-n through Q parallel t o t h e
468
T H E J O C R N A L OF I Y D C S T R I A L A N D ENGIiVEERISG C H E M I S T R Y
side Si0241203 a n d another through Q parallel t o t h e side CaO-SiO,, we find t h a t t h e side CaO-Al2O3 h a s been divided i n t o t h r e e parts, CaO - X = I O . o per cent, X - X 1 = I O per cent, a n d X1-AI2O3 = 80 per cent, which give t h e weight percentage of A1203, Si02 a n d CaO, respectively, i n t h e composition represented b y point Q. If, o n t h e other h a n d , t h e composition of a mixture is known a n d one wishes t o locate t h e corresponding position i n t h e triangle, i t is only necessary t o reverse t h e order of procedure. A very useful property of t h e equilateral triangle should be noted here. P o i n t s o n a straight line, joining a n apex of t h e triangle with t h e opposite side will represent compositions, i n which t h e relative proportions of t w o of t h e components remain constant. T h u s i n Fig. I for all compositions represented b y points on t h e line joining ! d 2 0 3 a n d zCaO.SiO2, t h e relative proportion of CaO a n d Si02 remain constant being CaO 65 per c e n t , SiOy 3 j per cent. Having shown how compositions of CaO, A1203, Si02 are represented b y points within t h e triangular diagram, Fig. I , we shall proceed with t h e discussion of t h e final products of crystallization obtained from melts of these t h r e e oxides as given in t h i s diagram. .is has been said, t h e final product of crystallization, under conditions of equilibrium. from a melt of CaO, X120a, Si02 will consist of three solid phases. I t is also t r u e t h a t t h e same three final solid phases will be f o u n d together a t all compositions within a region which has definite boundaries. These boundaries are given i n Fig. I ; it will be seen t h a t t h e y divide t h e large triangle i n t o a large number of small triangular areas. E a c h of these triangles represents all possible mixtures of those t h r e e compounds whose compositions are represented b y t h e apices of t h a t triangle. Though a given group of t h r e e compounds c a n exist only i n o n e triangle, yet as can be seen i n t h e diagram, a n y one of these compounds m a y be f o u n d i n a n u m ber of adjacent triangles. I n applying t h e d a t a presented in Fig. I t o a n y problem involving a p a r t or all of t h i s diagram, i t must be remembered t h a t t h e diagram represents equilibrium conditions; i. e . , t h i s diagram represents t h e compounds a n d components as t h e y would occur together in mixt u r e s if t i m e mere allowed for all reactions t o go t o completion, Hence, it is not permissible t o apply directly t h e d a t a in Fig. I t o problems involving mixtures containing only t h e three oxides CaO, &\1?03, SiO,, unless one is sure t h a t equilibrium conditions have been substantially a t t a i n e d . I n t h e problem t o which we wish t o apply this diagram, we know t h a t t h e reactions involved are not always completed, t h a t equilibrium conditions are not a t t a i n e d . I t does n o t follow, however, t h a t t h i s diagram is worthless in i t s application t o such problems. On t h e cont r a r y , i t simply means t h a t t h i s diagram must be studied i n order t o determine t h e possible conditions n-hich m a y exist if equilibrium is n o t reached. , For example, if a n y mixture of these three oxides is heated. t h e compounds present i n t h e product would be t o a large extent those given i n t h e small triangle i n which t h e point representing t h e composition of t h e
Vol. 7 , K O . 6
original mixture occurs, a n d if a n y other compounds did occur (owing t o t h e n o n - a t t a i n m e n t of equilibrium) one would expect t h a t t h e y would be members of t h e groups belonging t o t h e adjacent triangles. A P P L I C A T I O S TO PORTLAND C E M E N T C L I N K E R
It is a well known f a c t t h a t P o r t l a n d cement can be made from t h e pure oxides CaO, A1203 a n d SiO2. According t o Richardson’s d a t a l cement clinker can be made from mixtures of these three oxides whose compositions a r e represented b y a group of points near t h e letter P in Fig. I . T h e points lie entirely within t h e triangle whose apices are 3Ca0.Si02-3Ca0.X1203aCaO.SiO2. Therefore a cement clinker made from p u r e C a O , A1203 a n d Sios, burned a t a sufficiently high t e m p e r a t u r e for a sufficiently long time, would consist of t h e three compounds zCaO.SiO2, g C a 0 . S i 0 2 a n d 3Ca0.-41203. If, however, equlibrium were a p proached b u t not reached we would expect t o find, i n addition t o these t h r e e compounds, either free lime or t h e c.ompound jCaO.3A1203 or b o t h , CaO a n d t h e compound jCa0.3A1203 being t h e only other constituents present i n t h e adjacent triangles. T o illustrate with a concrete example, let us consider t h e point P ,which represents a mixture whose composition is CaO 6 8 . 4 , &03 8.0, Si02 2 3 . 6. Richardson2 found t h a t a perfectly burned clinker of this composition, when ground a n d made i n t o a m o r t a r , possessed all t h e properties of a desirable P o r t l a n d cement. We ha\-e found t h a t a perfectly burned clinker of this composition will consist of t h e compounds gCaO.SiO2. 3CaO.Al2O3 a n d z C a 0 . S i 0 2 . Therefore i t m a y be definitely s t a t e d t h a t a clinker consisting only of these three compounds can be prepared which will possess t h e properties of a desirable Portland cement. Furthermore, subsequent work b y t h e Bureau of S t a n d a r d s has shown t h a t these three compounds go t o make u p t h e major portion of clinkers of commercial P o r t l a n d cement. However. before t a k i n g u p a discussion of t h e constitution of commercial P o r t l a n d cement clinker, d a t a a r e presented on t h e properties a n d behavior of t h e substances. composed solely of pure CaO, A l s o s , S i 0 2 . which are found t o occur i n P o r t l a n d cement. T H E C a O , A1203,Si02 C O M P O U S D S IX P O R T L A S D C E M E S T CLIKKER
T h e sulcistances composed only of CaO. & 0 3 and SiO, which have been obsers-ed i n Portland cement clinker are fi1-e i n n u m b e r : t h e component CaO a n d t h e compounds aCaO.SiO2, g C a 0 . S i 0 2 . jCa0.3-A1.203 a n d 3 c a O . M 2 o 3 . E a c h of these has received caref u l s t u d y . t h e results of lvhich we shall now consider, taking i n order ( I ) t h e methods of formation a n d ( 2 ) their optical characteristics. I n t h e course of this discussion frequent reference will be made t o t h e x o r k done a t t h e P i t t s b u r g laboratory of t h e Bureau of S t a n d ards b y P . H. Bates3 a n d *\. &\. Klein a n d , A . J . Phillip~.~ 1 “The Constitution of Portland Cement,” Clifford Richardson, Coneprl, 5 (1904), 316. 9 CliBord Richardson, C e m c n f . 5, 315. 3 “ T h e Constitution of Portland Cement,” by P. H. Bates, Cur>rrefec e m e n t A g e (Cement Mill Section), 2 (1913), 3. 4 O v i ~ .C u m m . 8th I n t e r n . Congr. .Ipp. Chem., 5 , 81.
J u n e , 1915
T H E J O C R L V d L O F I S D C S T R I A L .LLVD E S G I S E E R I i T G C H E M I S l R Y METHODS O F FORMATIOX
THE C O U P O N E K T
C a O is formed b y burning C a C 0 3
to drive off Con; i t is stable a t its melting point which is 2 j 7 o o C.l THE COMPOUXD zCaO.SiOs is easily formed b y crystallizing a melt of t h e composition 2 C a 0 . 1 S i 0 2 ( b y weight CaO 6 j , S i 0 2 3 j ) , t h o u g h i t will also form a t temperatures much below i t s melting point which is 2130' C. I t occurs in four forms: a , which is stable between 2130-1420'; @, which is stable between 1420-67j0; y, which is stable belon- 6 7 j " ; a n d p ' , which is unstable or nionotropic a n d is found occasionally in charges rapidly cooled from a temperature of a b o u t 1400'. T h e change @ t o y a t a temperature of 6 7 j 0 is accompanied b y a volume change of a b o u t I O per cent, which shatters t h e crystals; this is t h e phenomenon commonly known as "dusting." Of these four forms of sCaO.SiOs, t h e high temperature forms a a n d p are t h e more common in Portland cement. Bates finds t h a t aCaO.SiO2 is found in well burned cornniercial clinkers largely as t h e @ form. T h e y form is not found t o a n y extent except in underburned clinkers which "dusted " on cooling. THE COMPOUXD jCa0.3&O3 ( b y weight CaO 47. 78, A1203 5 2 . 2 2 ) is t h e most easily fusible; it melts a t 1455' a n d crystallizes again readily from t h e melt. It is also found in a second form which is monotropic a n d generally occurs in melts t h a t have cooled rather r a p i dl y , though t 1ie p r e c ise conditions under n- hi ch i t does form have n o t been ascertained. T h e stable form of this compound is, according t o Bates, t h e more common in commercial cement clinker. THE C O M P O U x D 3 C a 0 . & 0 3 ( b y weight CaO 6 2 . 2 2 , A1203 37. 78) has n o stable melting p o i n t ; a t a t e m perature of 1 j . 3 5 ~C . i t dissociates i n t o CaO a n d liquid wherefore i t is exceedingly difficult t o obtain free from admixture of free lime a n d jCa0.3L%1203. If a well-mixed charge of t h e oxides in t h e proportion 3 C a O (as CaC03) : - 4 1 2 0 3 is fused a n d examined under t h e microscope, it is seen t o consist largely of 3Ca0.Ai1203,b u t t o contain also some free C a O a n d jCa0.3A1203. If: however, t h e mixed oxides are not fused b u t are held for several hours a t a t e m perature somewhat below I j 3 j ' ( i . e . . t h e dissociation point of this compound)-say 1400'-the charge becomes homogeneous a n d contains only 3 CaO.AI2Os. This compound has been found in b u t one form. THE COXPOUND 3 C a 0 . S i 0 2 also has no melting point, dissociating a t a b o u t 1 9 0 0 ~i n t o free C a O and a C a 0 . S i 0 2 , b u t this dissociation differs in one respect from t h a t of 3Ca0.A1203 in t h a t t h e charge remains solid because goo' is still a b o u t 100' below t h e t e m perature a t which melt (liquid) appears in t h i s case. It can be obtained in t h e pure s t a t e b y a procedure analogous t o t h a t for \;Ca0.A1203, namely b y prolonged heating of a mixture of t h e oxides in t h e proper proportions at a t e m p e r a t u r e somewhat below 1900'. T h e formation of this compound pure is very difficult, owing t o t h e high t e m p e r a t u r e required. T h e compound gCaO.SiO2 occurs in b u t one f o r m ; on fusion i t is transformed completely into CaO a n d aCaO.SiO2. 1
Kanolt. J . Wash. Acod. Sci., S (1913) 315.
469
As was t o be expected, it was found b y Bates t h a t t h e relative a m o u n t s of this compound in commercial clinker increased with t h e t e q p e r a t u r e of hurning. Having considered t h e formation of each pure conipound separately, let us now t a k e up t h e mode, ;inti order of formation of t h e four compounds in t h e burning of cement clinker made u p from t h e pure oxides CaO ( a s C a C 0 3 ) , L % 1 2 0 3 , SiO? in t h e proper proportions. When such a mixture is heated, t h e first change is t h e evolution of t h e C O ? ; t h e lime t h e n t!nites n-ith t h e other components t o form t h e compounds 5CaO. 3A1203 a n d zCaO.SiO? (both of which form readily. as we have shown) probably in this order. since the former melts a t a lower temperature t h a n t h e l a t i e r : subsequently the,se t w o compounds unite in part with more lime a n d the compounds 3CaO.SiO2 a n d 3Ca0.A1203 appear. This formqtion of the last t w o compounds-a process which goes on very slon-l y in mixtures of their own composition-is materially facilitated b y t h e circumstance t h a t in t h e ternary mixtures a portion of t h e charge has already 'melted a n d promotes reaction b y acting as a flux or solvent. T h e temperature a t which this flux first appears is 1335' C.,I t h e eutectic temperature for the three compounds z C a 0.Si 0?, j C a 0.3AIy0 3 C a 0. A l ? Oa. .Is t h e temperature of burning gradually rises ahove 1,335 ' t h e relative a m o u n t of flus increases a n d t h e rate of formation of 3CaO.&03 a n d gCaO.SiO: increases correspondingly. A t a temperature someTThnt above 1335' (the exact temperature depending on t h e original composition) t h e compound, jCa0.3A1203 mill have completely melted in t h e flux a n d t h e formation of t h e compound 3CaO.AI2O1 will be completed. T h e substances present a s crystals a t this stage are 3CnO.S O 2 , 2CaO.SiO2, 3Ca0.A120a a n d free CaO Of these t h e g C a 0 . S i 0 2 is rapidly increasing in a m o u n t , due t o combination of zCaO.Si01 x i t h CaO, while t h e a m o u n t of solid 2Ca0.SiOz, C a O a n d 3Ca0.A1203 are all decreasing, t h e zCaO.SiO? partially hy conibination with C a O a n d partially b y dissolving d o n g m-ith 3Ca0.A1203 in t h e flux. ,St a temperature of about 147 j " C . , t h e 3Ca0.LiZ1203will all h a r e dissolved in t h e flus a n d most of t h e C a O will have combined with 2 C a 0 . S i 0 2 t o form 3CaO.SiO2, so t h a t t h e crystalline substances now present in t h e flux a r e ,3CaO,S O r , 2 C a 0 . S i 0 2 a n d CaO. t h e temperatitrct is raised still further each of these crystalline compounds will gradually disappear, b u t it is possible i o s t a t e which of these three crystalline phases will lie t h e l a s t t o dissolre only if t h e original composition oi t h e charge is known. I n a n y case the temperature of complete melting will be a b o u t I C / O O ' a n d if such a charge is cooled t h e final product of crystallization will consist largely of jCaO.SiO?. zCaO.SiO, :tnd ~ c ~ o . A ~ ~ o ~ . I n order t o show t h e temperature requireti anti tlie percentage composition of t h e final product oht:iined in a well burned clinker! let us consider. for example, a mixture (represented in t h e diagram) hy t h e point P 1 T h e temperatures, presented here, are taken from d a t a which are given in a diagram in t h e complete report of the ternary s y s t e m CaO, AhOa, SOP. T h e explanation of this diagram requires a discusyiun of i o i n e length a n d is n o t presented here.
T H E J O U R N A L O F I , V D C S T R I A L A N D E.VGILVEERI~YG C H E M I S T R Y
470
whose composition is CaO 6 8 . 4 per cent, A1203 8 . 0 per cent, Si02 23.6 per cent-.a mixture which Richardson found on burning produced a clinker which when ground a n d t r e a t e d with water possessed t h e properties of a desirable Portland cement. During t h e burning of t h i s mixture t h e reactions which t a k e place will proceed, u p t o a temperature of 14;j O C. i n t h e manner already described; t h a t is, a t 147 j ' we shall have t h e three crystalline phases 3CaO.Si02, a C a 0 . S i 0 2 , CaO a n d liquid. But a t a temperature somewhat above 1 4 ; j " t h e free CaO will also h a v e entirely disappeared as a crystalline e n t i t y , so t h a t t h e clinker will t h e n consist of 3 C a 0 . S i 0 2 , 2Ca0.SiOz a s crystalline material in contact with a liquid (flux) s a t u r a t e d with respect t o those compounds. As t h e temperature is raised still further t h e gCaO.SiO2 a n d 2CaO.SiOz will gradually dissolve i n t h e flux. B u t i t is n o t necessary t o raise t h e temperature u n til t h e charge is completely melted as normal cement clinker is obtained a t temperatures much below complete melting; i n other words, t h e necessary re-
v01. j
,
KO. 6
I n t h e foregoing discussion we have followed t o completion t h e course of t h e reactions which t a k e place when cement clinker composed of pure CaO, AI2O3, SiOz is b u r n e d ; i n other words, we have shown t h e order of formation of t h e compounds during t h e burning of mixtures of these three oxides in t h e proper proportions for cement clinker a n d s t a t e d which compounds will be present in t h e final product if t h e burning is continued long enough a n d a t sufficiently high temperatures t o obtain a condition of equilibrium. If, however, equilibrium is not attained, t h e two constituents CaO a n d jCa0.3A1203 which are present during t h e earlier stages of t h e burning will not disappear completely a n d so will be found i n t h e final product. OPTICAL CHARACTERISTICS
T h e earlier investigators did n o t use sufficiently definite criteria in their endeavor t o identify t h e sex-era1 substances present i n Portland cement clinker; hence, their evidence is not conclusive. Definite criteria can be established only b y isolating each sub-
TABLEI-OPTICAL-CRYSTALLOGRAPHICAL PROPERTIES OF THE COMPOUNDS PRESEKT I N PORTLAND CEMENTCLINKER^ Composition CaO y-2CaO.SiOz
Crystal system isometric probably monoclinic
Crystal habit unmodified cubes prismatic
&ZCaO.SiO? orthorhombic or monoclinic
irreg. grains prisms
a-ZCaO.Si02 monoclinic or triclinic
irregular grains
8'-2CaO.Si02 3 C a 0 Si02
...
.., ,
monoclinic())
Cleavaae perfect (100) g (001) g (100) poor
H a r d - Elon- Optical ness gation orientation 3-4 .. ... .
Optical character
.. . .
S .., .
1.83
..
1.642
1.64j
a
b = 6 c : y = 30(?)
1.654
-
'-6
' '
,.....
1.71i
5-6
.,
.
,
,.
...
y
.
c : a' = 18O
rr
1.715
l.i20
y
Optic axial angle ,
,
.,
2 E = 52O
large
l.i3i
equant gr
......
I.il5
+
2:;:;
c = y
1.715
-
jCa0.3.41203 (unstable)
probably 1orthorhombic
equant grains fibers prisms
5 p , prismatic
5
,. a
1.608
small or uniaxial
1687
1.692
,
( , ~
1 tical anomalies.
'Obtainable only a s fine powder. Plane of optic axes normal t o cleavage direction. Plane of optic axes parallel t o elongation. I n t q c a f e poJysynthetic twinnlng is character( istic of this form.
'
{
1
urgyag, Birefringence very weak.
1.i10 5 C a 0 . 3 8 1 ~ 0 31 isometric (stable) I
REMARKS
1 Crystals often show op-
..
,
large
very weak. 1\ Birefringence Twinning lamellae n o t uncommon.
gray inter;( Occasionally ference colors appear a s result of strain. Stable form. . ( P l a n e of optic axes parallel with fibei
7
I elongation.
1 This table is a portion of t h a t giving optical properties o fall compounds of CaO, AhOa a n d Si02 as determined b y F. E. Wright and published in .4m. J o u r . Sei,, 141 39 (19151, 7 5 - 7 6 ,
actions will go t o completion a n d t h e final product obtained will in timc reach t h e s t a t e of equilibrium even below t h e t e m p e r a t u r e required for complete melting. T h e rapidity with which t h e reactions go t o completion will of course depend upon t h e t e m perature a n d upon t h e a m o u n t of t h e flux a t t h a t t e m perature, t h i s a m o u n t increasing of course when small quantities of admixtures such as FeO, hIgO, etc., are present. I n normal commercial cement t h e t e m p e r a t u r e required for burning has been found t o be about 142 j C. Clinker of t h e above composition, which is composed of pure CaO, A1203 a n d S O r , would require a much higher temperature, owing t o t h e high CaO content. At a temperature of 1650' C. t h i s clinker would be a b o u t 30 per cent melted a n d 7 0 per cent solid crystalline material, a proportion of flux which would be sufficient t o a d m i t of t h e necessary reactions going t o completion i n a reasonable time. The charge will always crystallize completely on cooling; t h e percentage composition (based on actual d a t a ) of t h e clinker t h u s obtained would be a p proximately sCaO.SiO2 45 per cent, 2Ca0.SiOr 3 j per cent, 3Ca0.A120320 per cent.
stance i n a substantially pure s t a t e , a n d determining t h e characteristic properties, whether chemical or physical, of such material; constants ascertained i n t h i s way m a y t h e n be applied with confidence in t h e detection of a crystalline substance in a complex mixt u r e . I n t h e present work t h e optical determinations were made on pure material largely b y Dr. F. E. Wright a n d i n p a r t b y D r . H. E. Merwin; t o these gentlemen t h e writer is especially indebted for their valuable collaboration. Each of t h e compounds found i n cement clinker made only of CaO, A1203 a n d Si02 has optical properties peculiar t o itself; certain of these properties m a y appear common t o several b u t each has a t least one or more characteristic properties which serve t o distinguish i t from t h e rest. T h e several opticalcrystallographical properties of each compound in i t s various forms are given i n Table I , t h e d a t a i n which were obtained b y a s t u d y of each compound b y itself. These values are constants for t h e individual compounds i n all mixtures which are composed only of compounds of CaO, A1203a n d Si02; i. e . , i n such mixtures t h e compounds are present as individuals of
J u n e , 1915
T H E J O L 7 R S A L OF I S D C S T R I A L A S D EAVGISEERISG CHE4fISY’RY
constant optical .properties a n d n o t as solid solutions. Consequcntly t h e d a t a of Table I serve as certain criteria for t h e identification of t h e several substances present in such mixtures. K e do n o t discuss a t length t h e distinguishing optical properties of all t h e forms a s given in Table I , b u t merely t a k e up those which characterize t h e several constituents of Portland cement clinker; namely, t h e t w o minor constituents CaO a n d j C a 0 . 3 X i 2 0 3 istable f o r m ) a n d t h e three major constituents P - z C a 0 . S i 0 2 , \jCaO.SiO2 a n d L3Ca0.L11203,which, as Bates has shown. go t o make u p t h e major portion of commercial clinker. CaO is easily recognized as it occurs in colorless isotropic rounded grains; its refractive index is I . 83. which is much higher t h a n t h e refractive indices of a n y of t h e C s O , A1?03, Si02 compounds. T h e compound jCa0.3A1203 (stable form) is also isotropic b u t is easily recognized b y its brown color a n d low refractive index, I . 608. T h e three major constituents /3-zCa0.Si02. 3CaO.Si02 a n d 3CaO.f&O3, however. are n o t so easily differentiated, owing t o a similarity of certain optical characteristics. T h i s similarity probably accounts for t h e fact t h a t certain investigators who have studied these compounds only in mixtures or viho have a t least not given accurate quantitative values t o t h e optical characteristics of each, have concluded t h a t Portland cement clinker is in t h e main either a solid solution or a single definite t e r n a r y compound. T h a t t h e conclusions of these investigators are erroneous is evident t o one who makes a careful s t u d y of these three compounds even a s t h e y occur in fine-grained mixtures. I t will be noticed b y reference t o Table I t h a t t h e refractive indices of these three compounds are more or less similar, especially those for 3CaO.SiOz a n d 3CaO.ALO3 which are respectively I . 7 1 j a n d I . ;IO. B u t 3CaO.SiOn is birefracting, though i t s birefringence is weak, often shows twinning lamellae, a n d is optically negative, while 3Ca0.X1203, on t h e other h a n d , is isotropic. Both appear as colorless grains. T h e @-2CaO.SiO2 is more easily identified b y its somewhat higher refractive indices 011. 717, 7 1 . 73 j, its stronger birefringence a n d optical character, which is positive. I t m a y be noted t h a t these conclusions relative t o t h e formation a n d occurrence of t h e above compounds in cement clinker made from pure materials a n d t o t h e optical properties of these compounds h a v e been corroborated b y subsequent work under t h e auspices of t h e Bureau of Standards. Hitherto we have considered t h e constituents of clinkers composed of t h e three pure oxides only; let us now inquire i n t o t h e effect of t h e presence of F e 2 0 3 a n d l I g 0 , small quantities of which are always present in commercial clinker, on t h e optical characteristics of Ca0-XI203-Si02 compounds. K e have done little direct work on t h i s question here, so we shall rely mainly 011 t h e results obtained b y Bates, Klein and Phillips. T h e y found t h a t b o t h t h e compounds P-zCaO.SiO? a n d 5CaO.3AI2O3 are colored by t h e iron oxide b u t
4;1
t h a t othern-ise the optical properties are not matcrially affected. T h e iron oxide apparently does not color either gCaO.SiO2 or 3Ca0.A1203. I n so far as wc have studied t h e effect of iron oxides on t h e optical properties of these compounds our results corroborate t h e statements just made. T h e question as t o t h e effect of SIgO on t h e optical properties of compounds in Portland cement clinker is t h e subject of a special investigation by Klein a n d Phillips.’ T h e y found in certain Portland cement clinkers of high N g O content t h a t t h e /3-.2Ca0.Si02 a n d 3CaO.&O3 agree very closely with those descrilied in t h e published work of this laboratory! in all 011tainable properties except t h e index of refraction which v a s lower in both instances and furthermore not constant. This led t h e m t o a n investigation of the possibility of t h e replacement of CaO by M g O in these compounds, from m-hich t h e y concluded, t h a t in all compositions in which hIgO replaces u p t o I O per thc cent of t h e CaO in t h e compound ,jCaO.BI~O:>, product obtained on burning is homogeneous. with a refracti7-e index n7hich ranges from I . 7 1 ( t h a t of pure 3 C a 0 . & 0 3 ) t o 1 . 6 ; when I O per cent of t h e CaO h a s been replaced b y hIgO. HoxTever, on repeating their experiments vie found t h a t in no case was t h e final product honiogeneous; on t h e contrary, i t is a mixture of free LIgO. t h e compound j C a 0 . 3 d 1 2 0 3 and t h e compound 3CaO.AI2O3. Furthermore, t h e refractive index of t h e compound 3Ca0.L\1203in these mixtures is constant, being I . ; I : indeed, t h e refractive index of this compound r e q a i n s constant for all compositions of CaO, 1112011 51gO in which it occurs as we have found in t h e course of a n extended investigation of t h e ternary system of these three oxides.? There are, t h e n , t w o similar series of experiments performed with t h e same oxides in t h e same proportions resulting in apparently widely different products : b u t this discrepancy c a n , we belicvc, be explained readily. T h e products we obtained are composed of U g O , 3Ca0.L11203a n d j C a 0 . 3 X 1 2 0 3 all of which are isotropic with refracti\-e indices I . 7\34] I . I O a n d I . 6 0 8 . respectively. T a k e n as individuals, t h e refractive index of each of these compounds is easily determined b u t in fine-grained mixtures of all of t h e m . t h e product might appear homogeneous with n refractive index equal t o t h a t of t h e aggregnte unless great care is exercised in t h e microscopical analysis. T h a t Klein a n d Phillips m a y have mistaken this a g gregate effect for homogeneity appears probalile if we consider t h e value of t h e average r e f r a c t i w index of lxgo, 3 C a O . i ~ 1 ~a0n~d jcao.3x1203 in t h c proportions such t h a t t h e t o t a l composition of the mixture can be represente8 as 3Ca0.X1203 in which a portion of t h e CaO is rcplaced b y illgo. From o u r s t u d y of t h e percentage of each of these compounds present in such a case. m-e know t h a t as t h e MgO, replacing t h e CaO. increases f r o m o t o I O per cent. t h e 3(>a0.L4i203 decreases from IOO t o a b o u t j o per cent while j C n 0 . Comm. 8th I n i e v n a l i o n a i Congv. d p p . C h e m . , 5, 81. T h e results of an investigation b y G . A. R a n k i n and €1. E.Irferwin of this system will be pitblished in t h e near future. 1 Ovig,
T H E J O U R N A L OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
472
-
3A12O3 increases from o t o a b o u t 4 0 per c e n t ; t h a t is, t h e gCa0.3A1203 of low refractive index increases i n a m o u n t relatively faster t h a n t h e MgO of high refractive index, while 3Ca0.A1203 decreases in a m o u n t ; consequently, t h e average refractive index will decrease with increase of MgO. If now we calculate t h e average refractive index for t h e aggregate containing I O per cent MgO, jo per cent 3Ca0.A1203, 40 per cent jCa0.3A1203 we obtain t h e value I . 672-a value which corresponds very closely with I . 6 7 obtained b y Klein a n d Phillips as t h e refractive index of their socalled homogeneous compound i n which I O per cent of t h e C a O i n t h e 3-1 aluminate is replaced b y AIgO. F u r t h e r increase in t h e percentage of hlgO does not materially change t h e aggregate refractive index; moreover, as Klein a n d Phillips found, t h e identification of free MgO is t h e n more readily made, though t h e y were apparently n o t able t o distinguish 3CaO.hl2o3a n d 5Ca0.3X1203 i n these mixtures. . I n t h e foregoing discussion we have presented a n explanation for t h e apparent partial replacement of CaO b y MgO i n t h e compound 3 C a 0 . A 1 2 0 3 ,which Klein a n d Phillips believe t o t a k e place b u t which our inlrestigation would t e n d t o disprove. We, have found, moreover, t h a t pure MgO does not form solid solution with pure 3CaO.A12O3. It m a y be, however,
Vol. 7 , No. 6
I-Well-burned cement clinker made u p from t h e pure oxides CaO, A1203, SiOz, in t h e proper proportions, will consist largely of t h e compounds 3CaO.Si02, 2Ca0.A1203, aCaO.SiO2, with small quantities of free lime a n d of t h e compound jCa0.3Al203. These are present as individuals of constant optical properties a n d n o t as solid solutions. 11-The work a t t h e Bureau of S t a n d a r d s has shown t h a t t h e constituents of commercial Portland cement clinker are essentially t h e same as those which we found i n clinker made u p only of CaO, &o3, SiOa. This fact is best illustrated with d a t a such as are presented i n Table 11. T h e examples given in Table I 1 t o prove t h i s point are based on t h e averages for a large n u m b e r of analyses of each of three t y p e s of Portland cement clinker, zliz., pure cement, made only of CaO, A1203, SiO,; commercial white cement; a n d t h e more common gray variety of commercial Portland cement. If t h e raw material for pure cement is perfectly burned a t a temperature of 1650°, t h e clinker obtained will consist of t h e three compounds-orthosilicate of lime, tricalcic silicate, a n d tricalcic aluminate. T h e example of a pure cement, given i n T a b l e 11, has t h e chemical composition 6 8 . 4 per ceni lime, 8 . o per cent alumina. 23.6 per cent silica.
TABLB IT-COMPOSITIONSA N D BURNINGTEMPERATURES OF VARIOUSPORTLAND CEMBNTS~ PERCENTAGE COMPOSITION OF CLINKER Relative t o content of CaO-AIzO&iOz CaO g.01 100.0 AlzOa 23.6) Si01 23.6
Pon'i I. A F D CEMENTS
ACTUAL CONSTITUENTS CaO 68.4
PURE
(PI
1
Burning temperature OC 1650
CONSTITUENTS OF
RESULTING CEMENTS \ 2 CaOSiOz i3 CaOSiOn / 3 CaO.AIz08
' i
63.2) CaO 66.7 Cab 2 CaO.Si0z Small amounts of A1203 7.i r 93.3 AlzO3 9.0 1425 3 CaOSiOz 5 Ca0.3AI~Oa. Si02 24.3 Si02 22.4) 3 CaO.Ah0s CaO and ferrites I MeO, FezOa. SazO, KzO and SOa 6.7 T h e analyses of commercial clinkers are from publications from 1 T h e data g i v e n in this table are,,based largely on the work of this laboratory. the Bureau of Standards and from Portland Cement" by R. K. Meade. T h e temperatures of burning and the constituents given are based both on our work and t h a t of the Bureau of Standards. KAY
(B)
1
