J O U R N A L
of the
AMERICAN CONCRETE INSTITUTE
(ACI PROCEEDINGS Vol. 43)
-
V o l. 18 October 1946
No.
2CO N TEN TS
Pape s and Reports... 101-200
Studies of the Physical Properties of Hardened Portland Cement Paste...
.T. C . PO W ER S and T. L. B R O W N Y A R D 101
Minimum Standard Requirements for Precast Concrete Floor Units ( A C I 711 -46)
Report of A C I Committee 711, F. N . M en efee, Ch airm an ... 133 Recommended Practice for the Construction of Concrete Farm Silos ( A C I
714-46)
Report of A C I Committee 714, W illiam W . G urney, Chairm an ... 149 The Durability of Concrete in Service... F. H . J A C K S O N 165 W ear Resistance Tests on Concrete Floors and Methods of Dust Prevention.
. G E O R G W A S T L U N D and A N D E R S E R IK S S O N 181
Current Review s... 2 0 1 -2 0 8
New s Letter... 1 -2 0 R E P O R T O F T H E 1946 N O M IN A T IN G C O M M IT T E E • New
Members 0 W ho's W ho 9 Honor Roll
to pro vid e a com radeship in finding the best ways to do concrete work o f all kinds and in sp rea ding f ia t know ledge
A D D R E S S ; N E W C E N T E R B U I L D I N G , D E T R O I T 2, MI C H.
D IS C U S S IO N
Discussion closes March 1, 1947Sept. J l. '46 Reinforced Concrete Columns under Com bined Compression and Bending— H a ro ld E-
Wessman
Effect of Moisture on Thermal Conductivity of Lim erock Concrete— M a ck Tyner
Cement Investigations for Boulder Dam — Results of Tests on Mortars up to A g e of 10 Years
— Raymond E. Davis, W ilson C . H a n n a and Elw oo d H . Brown
A n a ly s is and Design of Elementary Prestressed Concrete Members— Herm an Schorer O c t. J I . J 4 6 Minimum Standard Requirements for Precast Concrete Floor Units ( A C I 71 1 -4 6 )
— Report of A C I Committee 7 11, F. N . M e n efe e, Chairm an
Recommended Practice for the Construction of Concrete Farm Silos ( A C I 7 1 4 -4 6 )
— Report of A C I Committee 7 14, W illiam W . G urn ey, Chairm an The Durability of Concrete in Service— F. H . Jackson
W ear Resistance Tests on Concrete Floors and M ethods of Dust Prevention—
— G e o rg W astlund and A n d e rs Eriksson
Discussion closes Ju ly 1, 1947
Studies of the Physical Properties of H ardened Portland Cement Paste
— T. C . Pow ers and T. L. Brow nyard
Resuming, with this volume year, the former
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instead of 6 for the year, the Supplement,
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Vol. 18—No. 2 October 1946 (Proceedings Vol. 43)
J O U R N A L
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Title 43 -5 a — a part o f P R O C E E D IN G S , A M E R IC A N C O N C R E T E IN STIT U TE V o l. 43
J O U R N A L o f the
A M E R I C A N C O N C R E T E I N S T I T U T E (copyrighted)
V o l. 18 N o. 2 7400 S E C O N D B O U L E V A R D , D ETRO IT 2, M IC H IG A N O ctober 1946
Studies of the P h ysical Properties of H a rd e n e d Portlan d Cem ent Paste*
By T. C. POWERSt
Member Am erican Concrete Institute
and T. L. BROWNYARDt
IN N IN E P A R T S
A Review of Methods That H ave Been Used for Studying the Physical Properties of Hardened Portland Cement Paste
Studies of W afer Fixation ,
A pp en dix to Part 2 V A . I x 3» W oV • S - A v j - 3 3 G Theoretical Interpretation of Adsorption Data - ~ “K G ' 4 S , M The Thermodynamics of Adsorption
A p p e n d ix to Parts 3 and 4 V u L* 4 8 n r' ° 15 AlVr\.tu*.rV - &C?2. - .Studies of the Hardened Paste by Means”of Specific-Volum e Measurement» - 11 — anti Part 6. ("Relation of Physical Characteristics of the Paste to Compressive Strength / S . b Part 7. ^Perm eability and Absorptivity - M — 'w o -J- £* £ 4 ^ - & S 0 ‘----—
Part 8) The Freezing of Water in Hardened Portland Cement Paste
Part 9/ General Summary of Findings on the Properties of Hardened Portland Cement Paste — it — rw o S £ . 't jA A - 'i .
S Y N . O P S I S
This paper deals m ainly w ith d ata on w ater fixation in hardened Port
land cement paste, th e properties of evaporable water, the density of the solid substance, and the porosity of the paste as a whole. The studies of the evaporable w ater include w ater-vapor-adsorption char
acteristics and the therm odynam ics of adsorption. The discussions in
clude th e following topics:
1. Theoretical interpretation of adsorption d ata
2. The specific surface of hardened portland cement paste 3. M inim um porosity of hardened paste
4. Relative am ounts of gel-water and capillary w ater 5. The therm odynam ics of adsorption .
6. The energy of binding of w ater in hardened paste 7. Swelling pressure
♦R eceiv ed b y th e I n s tit u te J u ly 8, 1946— sch ed u led for p u b lic a tio n in sev en in s ta llm e n ts ; O cto b e r 1946 to A p ril, 1947.
f M a n a g e r of B asic R esea rc h , P o r tla n d C e m e n t A ssn. R esea rc h L a b o ra to ry , C h ic ag o 10, 111.
J N a v y D ept.., W a sh in g to n , D . C ., fo rm e rly R e se a rc h C h e m is t, P o r tla n d C e m e n t A ssn. R esea rc h L a b o ra to r y , C h ic ag o 10, 111.
(101)
Part 1.
Part 2.
Part 3.
Part 4.
Part 5.
102 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 8. M echanism of shrinking and swelling
9. Capillary-fiow and moisture diffusion
10. E stim ation of absolute volume of solid phase in hardened paste 11. Specific volumes of evaporable and non-evaporable w ater 12. Com putation of volume of solid phase in hardened paste 13. Lim it of hydration of portland cement
14. Relation of physical characteristics of paste to compressive strength
15. Perm eability and absorptivity
16. Freezing of w ater in hardened portland cem ent paste F O R E W O R D
This paper deals w ith the properties of hardened p o rtlan d cem ent paste. The purpose of th e experim ental work on which it is based was to bring to light as much inform ation as is possible by th e m ethods of colloid chemistry and physics. Owing to the war, th e original program , which included only a p a rt of the field to be explored, was n o t com pleted. M ore
over, the in terpretation of the d a ta is incom plete, p a rtly because of th e inability of the authors to comprehend th eir m eaning, and p a rtly be
cause of the need of d a ta from experim ents y et to be m ade.
Although the work is incom plete, it represents a considerable am o u n t of time and effort. Experim ental work began in a sm all w ay in 1934 and continued until Jan u a ry 1943. Some add itional w ork was done in 1945 during the preparation of this paper. T he first th ree years was a period of in term itte n t work in which little of p erm a n en t value was accomplished beyond the developm ent of a p p a ra tu s an d procedures.
This phase of th e work presented m an y problem s, some of w hich h av e never been solved to our com plete satisfaction.
The in terpretation of th e results of experim ents also presented m an y difficulties. D uring the course of our experim ents, im p o rta n t new developments in colloid science were coming to light th ro u g h a series of papers from other laboratories. I t was necessary to stu d y these papers as they appeared and to seek th eir applicationa to our problem s. T he result is th a t the theory on which m uch of our present in terp re tatio n is based is one th a t did no t exist when our w ork began and is one th a t is still in the process of developm ent. T he reader m ay note th a t m any of th e papers referred to in P a rt 3 were no t published u n til 1940 or later.
The theory referred to is th a t of m ultim olecular adsorption b y B ru- nauer, E m m ett, and Teller as first given in Í938 and as am plified in th e paper by B runauer, Deming, Deming, and Teller in 1940. In justifi
cation for the use of such a recent and unfinished developm ent, we m ay note in the first place th a t a rem arkable num ber of papers b y various a u th o rs have appeared since 1940 strongly supporting th e kind of use th a t we have m ade of th e theory, p articularly th e estim ation of surface area. In the second place, the basic conclusions reached thro u g h th e
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 103 use of th e theory m ight have been reached from a strictly em pirical analysis of th e data. However, it is difficult to imagine how a picture of th e hardened paste as detailed as th e one presented in th is pap er could have been draw n w ithout adopting theoretically justified assum ptions.
T he paper is composed of nine parts. P a rt 1 contains a review of previous work done in this field and discusses various experim ental methods. P a rt 2 elaborates on th e principal m ethod used in the present study, nam ely, th e m easurem ent of water-fixation. I t also presents the empirical aspects of th e d a ta so th a t th e reader m ay become fam iliar w ith facts to be dealt with.
P a rt 3 presents th e theories upon which an in terp retatio n of the d ata in P a rt 2 can be based. I t gives also a p artial analysis of m ost of th e experim ental d a ta given in P a rt 2 in th e light of th e adopted basis of in terpretation. P a rt 4 is a discussion of th e therm odynam ics of m oisture- conten t changes in hardened paste and th e phenom ena accom panying those changes. I t is th u s an extension of th e earlier discussion of theory.
In P a rt 5 d a ta are presented pertaining to th e volumes of different phases in th e paste. T he in terp re tatio n of these d a ta involves th e use of factors developed in th e preceding parts of the paper. T he final result is a group of diagram s illustrating five different phases, th e relative proportions of each, and how those relative proportions change as hy d ration progresses.
T he relationship between th e physical characteristics of th e h ard ened cem ent paste and compressive strength of m ortars is discussed in P a rt 6. A sim ilar discussion of perm eability and ab sorptivity is given in P a rt 7.
A stu d y of th e freezing of w ater in hardened paste is presented in P a rt 8. T he conditions under which ice can exist in th e paste are de
scribed and em pirical equations are given for th e am ounts of w ater th a t are freezable under designated conditions.
T he properties of portland cem ent paste as th ey appear in th e light of these studies are described in P a rt 9. This p a rt am ounts to a sum m ary of the outcom e of th e study, a t its present incom plete stage, w ithout details of experim ental procedures, or theoretical background.
As m entioned in the first paragraph, a particular point of view as to th e m eaning of th e d a ta has been adopted. Specifically, we have assumed, on th e basis of evidence given in th e paper, th a t th e various phenom ena discussed are predom inantly of physical ra th e r th a n chemical nature. T he result of the stu d y therefore constitutes a hypothesis, or series of hypotheses, ra th e r th a n a rigorous presentation of established facts. Considering th e present state of our knowledge, we believe th is policy to be more fruitful th a n one of trying to m aintain a strictly u n
104 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 biased view as to th e m eaning of the d ata. As w ritten, th e p ap er repre
sents th e thinking of one group of w orkers (influenced, of course, by m any others), and it im plicitly invites independent investigations of the sam e field b y any who m ay have good reason to ad o p t a different point of view. To this end, we have appended tab u latio n s of th e original data.
T hough we th u s concede th e possibility of o ther in terpretatio ns, we nevertheless feel confident th a t a large p a rt of th e present in terp re
tatio n will w ith stan d logical criticism. However, it seems very likely th a t corrections and changes of emphasis will develop as experim ental work continues, and as fu rth er advances are m ade in fun dam en tal colloid science.
T he paper is directed prim arily tow ard all who are engaged in research on p ortland cem ent and concrete. However, it m ay be of considerable in tere st to m an y who seek only to understand concrete as th e y w ork w ith it in th e field. S tudied in connection w ith earlier papers on th e characteristics of paste in the plastic state,* this paper affords a com prehensive, though incomplete, picture of th e physical natu re of p o rtla n d cem ent paste. It, therefore, pertains to any phase of concrete technology th a t involves th e physical properties of the cem ent paste. T his m eans th a t th e paper should find application to m ost phases of concrete technology.
F o r th e m ost p art, th e reader will find few item s of d a ta t h a t b ear directly on specific questions or problem s. Successful application of th is stu d y to research or practical technology requires some degree of com prehension of th e work in its entirety. Consequently, i t is n o t likely th a t a single, casual reading will reveal m uch th a t is of value to one n o t already fam iliar w ith the m ethods and background of this ty p e of investigation.
A C K N O W L E D G M E N T S
We are deeply indebted to M ark L. D annis and Harold Tarkow , no t only for th eir long and painstaking labor w ith the various experi
m ents, b u t also for th eir contributions to an understanding of th e results.
M ark D annis m ade m ost of the adsorption and specific-volume m easure
m ents and H arold T arkow perform ed th e freezing experim ents repo rted in P a rt 8.
We are grateful to Gerald P ick ett, whose constructive criticism was of great value th ro u g h o u t m ost of the period of study.
* B u ll. 2, " T h e B le e d in g of P o r tla n d C e m e n t P a s te , M o r ta r a n d C o n c r e te ,” b y T . C P o w e rs, P .C .A . R e s e a rc h L a b o r a to r y (1 9 3 9 ); B u ll. 3, “ R a t e of S e d im e n ta tio n ,” b y H a r o ld H . S te in o u r, P .C .A . R e s e a rc h L a b o r a to r y (1944), re p r in te d fro m I n d . E n g . Chem . 36. 618; 840; 901 (1 9 4 4 ); B u ll. 4, " F u r th e r S tu d ie s o f t h e B le ed in g of P o r tla n d C e m e n t P a s te ,” b y H a r o ld H . S te in o u r, P .C .A . R e se a rc h L a b o r a to r y (1945).
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 105 We are especially indebted to H arold H. Steinour, who took tim e from his own work to carry on the final volum enom eter work described in P a rt 5. Also, he helped prepare the discussion of therm odynam ics in P a rt 4 and gave m uch valuable criticism of various other p arts of the paper.
To Virginia A therton, who m ade m any of th e hundreds of com puta
tions, typed th e m anuscript, and corrected p rin ter’s proof we express our kindest thanks.
Part 1. A Review of M ethods That H a v e Been Used for Stu d yin g the Ph ysical Properties of H a rd en ed Portland Cement
C O N T E N T S
M ethods for studying the physical properties of the hardened paste 106 *
Microscopic exam inations... 106
The light-microscope... 106
The electron microscope... 108
X -ray exam inations... 109
W ater fixation... 110
Isotherm s and isobars... 110
Binding of w ater in hydroxides... 110
W ater bound by covalent b o n d s... 110
W ater bound by hydrogen b o n d s... 110
Zeolitic w a te r... Ill Lattice w a te r... Ill Adsorbed w a te r... 112
In terp retatio n of iso b ars... 112
Influence of surface adso rp tio n ... 112
In terp retatio n of isotherm s... 116
Isotherm s of h y d ra te s... 116
Isotherm s of gels... 116
W ater content vs. tem perature curves (isobars) from hardened portland cement p a s te ... 118
Review of published d a ta ... 118
Discussion of isobars... 122
W ater content vs. vapor pressure curves a t constant tem pera tu re (isotherm s)... 124
Relationship between isotherm s and isobars... 127
Significance of isotherm s... 127
Studies of w ater fixation by means of freezing te s ts ... 128
Sum m ary of P a rt 1 ... 128
R eferences... 131 S tartin g as a suspension of cem ent particles in w ater, portland cem ent paste becomes a solid as th e result of chemical and physical reactions betw een th e constituents of th e cement and w ater. A solidified p aste of typical characteristics is capable of giving up or absorbing a volum e of w ater equal to as m uch as 50 per cent of th e app aren t volum e of the
106 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 paste. These facts engender th e idea th a t w hatever th e chem ical con
stitu tio n of th e new m aterial produced by chemical reaction, th e new m aterial is laid down in such a w ay as to enclose water-filled, in te r
connected spaces. T h at-is, th e h y d ratio n prod uct appears to be n o t a continuous, hom ogeneous solid, b u t ra th e r it appears to be com posed of a large num ber of prim ary units bound to geth er to form a porous stru ctu re. I t seems self-evident th a t th e m anner in which th e p rim ary units are united, t h a t is, th e physical stru ctu re of th e paste, is closely related to th e q uality of th e paste and is therefore som ething ab o u t which we should be well informed.
F rey ssin et(1)* discerned th e need for knowledge of paste stru c tu re and devised a hypothesis ab o u t th e setting and hardening process and ab o u t th e stru ctu re of th e hardened paste. G iertz-H edstrom ,® an activ e contrib u to r to this subject, has given an excellent review of publications on th is subject. This review, together w ith B ogue’s{3) earlier one of a slightly different aspect of the subject, obviates th e necessity of an ex
tensive historical review a t this time.
A program of studies of th e properties of th e h ardened p aste was begun in this labo rato ry in 1934. I t has consisted m ainly of studies of th e fixation of w ater, b u t has also included m easurem ents of th e h ea t- effects accom panying th e regain of w ater by th e previously dried paste, m easurem ents of th e freezing of the w ater in th e sa tu ra te d paste, and various o ther related m atters.
T his w ork has yielded a considerable am o u n t of inform ation on th e physical aspects of hardened paste. I t contribu tes to th e knowledge of the chemical constitution of th e h y d ratio n products only in a negative w ay; th a t is, it shows th a t some of th e curren t inform ation on th e con
stitu tio n of th e hyd ratio n products m ust be incorrect. L ate r p a rts of this paper will give an account of these studies.
M E T H O D S F O R S T U D Y IN G T H E P H Y S I C A L P R O P E R T IE S O F T H E H A R D E N E D P A S T E f
T he question of s tru c tu re can be broken down into th ree p a rts : first, th e question as to th e chemical constitution of th e h y d ra tio n prod ucts, which includes th e question of stru c tu re of th e u ltim a te p a rts ; second, th e question of th e stru ctu re of th e sm allest prim ary aggregations of th e u ltim ate p a rts; th ird , th e question of how th e p rim ary aggregations are assem bled and how th ey are held together. A review of some of th e work done by earlier investigators follows.
M icroscopic examinations
The light-microscope. T he effectiveness of th e m icroscope as a m eans of studying th e stru ctu re of the hardened p aste is lim ited because th e
♦See referen ces en d of P a r t 1.
f S e e also th e re v ie w b y G ie rtz -H e d strd m (R ef. 3).
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 107 un its of th e essential p a rt of the stru ctu re are too small to be seen.
T he results obtained by Brown and C arlson(4) are typical. They, like others, observed th a t the hardened paste is predom inantly “ am or
phous,” so far as th e microscope can reveal. Em bedded in this am or
phous mass are th e rem nants of u n h y d rated clinker grains, crystals of calcium hydroxide, and sometimes crystals of other compounds.
K ü h l(5) reported th a t th in sections of hardened cem ent p aste showed . . a residue of undecomposed cem ent particles, separated by a gray and only slightly differentiated mass which gives a feebly diffused luminescence in polarized light. E ven under th e highest m agnification individual particles cannot be distinguished in this m aterial, which is obviously alm ost entirely ultram icroscopio in stru ctu re .” However, on examining th e sam e specimens 20 years later he found th a t . . the passage of years had resulted in fundam ental changes. No longer (was th e m aterial) uniform and only slightly differentiated under polarized light as was th e case a few m onths afte r th eir preparation. T hey now showed a definitely increased (polarized-light) transm ission and, m ost notew orthy of all, th eir properties were m arkedly different according to th e percentages of w ater w ith which they had been gaged. T he speci
mens gaged w ith th e greatest am oun t of w ater showed th e greatest changes while those mixed w ith th e least w ater had undergone consid
erably less m odification.” T he changes m entioned were such as to suggest th a t th e originally colloidal m aterial had gradually changed tow ard th e m icrocrystalline state, th e change being greater th e higher th e original w ater content.
Useful inform ation has been obtained by microscopic observations of the h y dration of cem ent in th e presence of relatively large quantities of w ater. B u t it is unlikely th a t th e stru ctu re developed under these conditions is th e same as th a t developed in pastes; hence, conclusions ab o u t th e norm al stru ctu re draw n from observations of this kind are open to question. (6> F or example, Le C hatelier(7) observed th a t when a large q u a n tity of w ater was used, needlelike crystals of microscopic dimensions soon developed. He concluded from th is th a t similar, though submicroscopic, crystals developed u nder all conditions.
Brownmiller® described th e results of microscopic exam inations of hardened pasté by m eans of reflected light. T he m ethod is a modifica
tion of th a t described by T avasci(9) and Insley (10) for studying th e con
stitu tio n of clinker. In addition to the use of etchants to bring different phases into contrast, Brownmiller tre a te d th e surface w ith a dye which was tak en up by the so-called am orphous m aterial and th e m icrocrystal
line phases. A lthough Brownm iller’s prim ary object was to develop th e experim ental technique, some of his conclusions concerning th e n atu re of hardened paste and the h ydration process are of considerable
108 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 interest. He found th a t there was . . no microscopic evidence of channeling of w ater into th e interior of cem ent particles to selectively h y d ra te an y single m ajor constituent. H y d ratio n seems to proceed by th e g radual reduction in the size of the particles as a function of th e surface exposed.” T his conclusion seemed to be based on th e ap p ear
ance of th e coarser particles th a t rem ained u n h y d ra te d a fte r th e first day.
B rownm iller found th a t a T ype II I* cem ent having a specific surface of 2600 appeared to be alm ost com pletely h y d ra te d afte r one d a y in a sealed vial and six days in w ater. A T ype I cem ent having a specific surface of 1800 showed an u n h y d ra te d residue of ab o u t 15 per cent afte r one d ay in a sealed vial and 28 days in w ater. T he w ater-cem ent ra tio of th e original paste was 0.4 by w eight in b o th cases.
T he only m icrocrystalline h y d ra te m entioned by Brow nm iller was calcium hydroxide. T his was found as clusters of fine crystals em bedded in w h at Brownm iller called th e hydrogel.
Brownm iller found th a t th e etched surface of th e 7-day-old p aste m ade of T ype I cem ent showed “ an extrem ely com plicated b u t in te re st
ing structure. A close exam ination . . . shows th a t th e cem ent hydrogel is no t a formless mass b u t has an in tricate stru c tu re .”
The electron microscope. EiteK11) used th e electron m icroscope for photographing th e hyd ratio n products of some of th e co n stitu en ts of P ortland cement. T he article consulted gives alm ost no details concern
ing th e m ethod of preparing th e sam ples th a t were photographed. I t appears th a t th e samples were tak en from d ilu te suspensions of th e hyd rated m aterial. Presum ably, samples of Ca(OH)2 were ta k e n from satu ra te d or su p ersatu rated “ m ilk of lim e” and hy d ratio n p ro du cts of CsS and C3A from satu ra te d solutions of w ater in isobutyl alcohol. T he isobutyl alcohol was used to dilute th e w ater and th u s m ake possible a relatively high concentration of th e solid w ith respect to w a te r and a t th e sam e tim e a dilute suspension. Since th e isobutyl alcohol was sa tu ra te d w ith w ater, th e chemical ac tiv ity of th e w ater was unaffected by the presence of the isobutyl alcohol.
Several photographs of these preparations (m agnifications ranging from 7200 to 36000) were published. T he Ca(OH)2 tak en from m ilk of lime as well as th a t formed from th e hydrolysis of C3S appeared as hem ispheres ranging in size from ab o u t 0.1 to 0.5 m icron. T he calcium silicate h y d ra te appeared as th in crystalline needles ab o u t one-half
*See A .S .T .M . D e s ig n a tio n C 1 5 0 -4 4 ,w h ere five ty p e s of P o r tla n d c e m e n t a r e d e fin e d as fo llo w s:
T y p e I — F o r u se in g e n e ra l c o n c re te c o n s tru c tio n w h en th e sp ecial p ro p e rtie s sp ecified fo r T y p e s I I I I I , IV , a n d V a r e n o t req u ired .
T y p e I I — F o r u se in g e n e ra l co n c re te c o n s tru c tio n ex p o sed to m o d e ra te s u lf a te a c tio n , o r w h ere m o d e r a te h e a t of h y d r a tio n is req u ire d .
T y p e I I I — F o r u se w h en h ig h e a rly s tr e n g th is req u ired . T y p e IV — F o r u se w h en a low h e a t of h y d r a tio n is req u ired . T y p e V — F o r u se w h en h ig h s u lf a te re s is ta n c e is re q u ire d .
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 109 m icron long. T he C3A appeared m ainly as rounded particles (“ roses” ) ab o u t 0.1 m icron in diam eter. R eferring to these and app aren tly to o ther observations, E itel concluded th a t although the hydration p rod
u cts of p o rtlan d cem ent are predom inantly colloidal, th ey ap pear crys
talline— n o t am orphous— to th e electron microscope.
Sliepcevich, G ildart, and Ivatz,(12) reported th e results of a tte m p ts to photograph th e h yd ratio n products of portlan d cem ent and th e m ajor constituents h y d rated separately. M ost of th e photographs published were of samples prepared as follows: 0.5 to 0.75 g of po rtlan d cem ent or a cem ent co nstituent was mixed w ith about 10 cc of purified w ater and allowed to stand. A t th e desired age th e specimen for photographing was obtained by taking a drop of th e su p ern atan t liquid and allowing it to evaporate on a collodion film previously prepared. T he m aterial on this film was th u s w hatever dissolved or suspended m aterial th e drop contained.
In some respects the results were like those found by Eitel. P hoto
graphs of calcium hydroxide appeared like those of E itel b u t whereas E itel concluded th a t the particles were hemispheres, Sliepcevich, G ildart, and K a tz concluded th a t th ey were spheres. F rom each m aterial th e la tte r investigators usually found m aterial of several geometric forms.
Some of th e m aterial appeared am orphous and some crystalline. As did Eitel, those investigators found th e m ajority of crystals to be in the colloidal size range.
T he significance of these results is open to question until it is known definitely ju st w hat relation th e samples obtained in th e m anner de
scribed have to the h y dration products m aking up th e mass of a h ard ened cem ent paste.
X -ra y examinations
T he results of X -ray exam inations were sum m arized by Giertz- IIed stro m (13) as follows: “ X -ray exam inations of hardened cem ent have so far given little beyond a confirmation of w hat has been shown b y th e microscope. T he presence of clinker rem ains and crystallized calcium hydroxide is th u s confirmed by B randenburger.(u) T he struc
tu re of th e m ain mass, th e “ cem ent gel,” is, however, such as to give, a t least for th e present, no clear guidance in th e X -ray diagram s. T his m ay be due to its lacking a crystalline stru ctu re or other regular fine stru ctu re or to the crystals being so small or deform ed (for example b en t needles) th a t no definite interferences are obtained.”
Bogue and L erch(15) m ention th e use of X -ray analysis in connection w ith microscopic exam ination. T he X -ray confirms th e microscopic indication th a t unaltered b eta or gam m a dicalcium silicate rem ained in pastes afte r 2 years of curing. X -ray diffraction p attern s from b o th
110 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 h y d ra te d tricalcium silicate and h ydrated dicalcium silicate showed a t th e end of 2 years no evidence of th e developm ent of a new crystalline stru ctu re such as would be expected if th e hydrates were to change from th e theoretically unstable gel sta te to th e stable m icrocrystalline state.
As m entioned above, evidence of the beginning of such a change w ithin a period of 20 years was reported by Kiihl.
W ater fixation
Because direct observation fails to answ er m any questions concerning th e stru ctu re, properties, and behavior of hardened p o rtlan d cem ent paste, indirect m ethods of stu d y have been used. T he principal one is th a t of studying th e m anner in which w ater is held in th e h ardened paste.
Isotherms and isobars. T he fixation of w ater b y w ater-containing solids is usually m easured in term s of th e am ounts of w ater held a t various vapor pressures w ith tem p eratu re constant, or in term s of th e am ounts held a t various tem peratures w ith pressure con stan t. T he curves obtained b y th e first m ethod are called isotherms. Those ob
tained b y th e second m ethod are called isobars. B oth of these m ethods have been used in th e stu d y of hardened p o rtlan d cem ent paste. T h e n atu re of th e hyd ratio n or dehydration curve depends on th e m an n er in w hich th e w ater is com bined and on other factors to be discussed.
B inding of water in hydroxides. In m etallic hydroxides, which repre
sent one class of com pounds th a t m ay be included am ong h y d rates, th e elem ents of w ater are present as OH-groups th a t are strongly bound by th e m etallic ions. This is usually recognized in w riting th e form ulas of m etallic hydroxides; thus, calcium hydroxide is usually given th e form ula Ca (OH )2 ra th e r th a n C a0.H 20.
Water bound by covalent bonds. In m an y hydrates, th e w ater m olecule retains its id e n tity to a large degree, i.e., H 20 is a u n it in th e stru ctu re.
An example is MgCl2.6H20. In this h y d ra te th e six molecules of w ater are bound to th e m agnesium ion b y covalent bonds and are arranged around th e m agnesium ion in an octahedral grouping. To in d icate this, th e form ula should be w ritten M g(H 20 ) 6Cl2, for this m ore nearly rep re
sents th e structure.
Water bound by hydrogen bonds. There is an o th er ty p e of com pound in which th e w ater molecule rem ains in ta c t and is bound to th e com pound by one or both of its hydrogen atom s. T he molecules so held are said to be bound by hydrogen bonds. C u S 0 4.5H20 and N iS 0 i.7 H 20 are hy d ra te s in which one of th e w ater molecules is bound in th is way.
* * * * *
W ater held in a com pound by an y of th e ty p es of bond described above is properly regarded as being chemically bound. T he rem oval of
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 111 w a te r from such hydrates necessarily gives rise to a new solid phase and hence th e isotherm s and isobars of these hyd rates should show well m arked steps in accordance w ith the phase rule. Fig. l a gives, for example, th e relationship betw een w ater content and vapor pressure for the hydrates of copper sulfate.
Fig. 1 - Univariant and bivariant dehydration curves (a) Isothermal p-x curve for CuS0 4 .5 H i0 (b) Isobaric T-x curve for Cr2(SO03.1 8H 2O
The first 1 5 h h O come off along a uni
variant curve, the remaining 3 H 2O being zeolitic
(c) Ze o litic dehydration of green C r2 < S 0 4 )3 .1 5 H 2 0
(Curves and caption from Emeleus & A n d erso n )
Zeolitic water. One type of m icrocrystalline hyd rate, which com
prises- th e zeolites and several basic salts and hydroxides of bivalent metals, gives sm ooth isotherm s or isobars. Fig. lc is an example of an isobar from Cr^SO^zAUHzO. W ater held in th is ty pe of com pound is called zeolitic w ater.
According to Em eleus and A nderson(16> zeolitic w ater is regarded as being packed between th e layers of th e crystal or in th e interstices of th e structure. A distinguishing characteristic of zeolitic w ater is th a t it m ay be rem oved w ithout giving rise to a new solid phase. I ts removal m ay, however, change th e spacing betw een successive layers of the crystal.
W ater held in such a way as to exhibit th e behavior described above is som etimes referred to as being in a state of zeolitic solution or sohd solution. <17>
Lattice water. Emeleus and A nderson(16) distinguish a type of h y d rate in which there is w ater of crystallization “ th a t cannot be supposed to be associated chemically w ith the principal constituents of th e crystal lattice.” As an example, th ey cite potassium alum , K A l(S0i)2.12H 20.
There is little question th a t six of the twelve molecules of w ater are linked to the alum inum ion by covalent bonds. The rem aining six mole
cules are known to be arranged octahedrally around the potassium ion b u t a t such a large distance from it as to suggest to Em eleus and A n
derson th a t th e in teraction is very weak and hence th a t the w ater is not chemically bound to th e potassium ion. This perhaps represents a borderline case between chemically bound w ater and zeolitic w ater, which is supposed no t to be chemically bound. The fact th a t exactly six molecules of w ater are associated w ith th e potassium ion is accounted for by th e geom etry of th e crystal lattice. T he rem oval of these six molecules of w ater presum ably gives rise to a new crystalline phase,
(a) (b ) (<=)
0 25 SOMM
W a ter V apor P r e s s u r e
) so ioo iso°c T e m p e r a t u r e
3 75 ISO 2 2 5 ° C
T e m p e r a t u r e
112 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 however, and hence th e isobars and isotherm s should be stepped. W hen m ore inform ation on this ty p e of h y d ra te becomes available, perhaps m any of th e examples cited will have to be placed in one of th e classifica
tions listed above.
Adsorbed water. In addition to anjr w ater held by th e chem ical forces m entioned above, a sm all am ount per u n it of surface is held b y surface forces. These forces, physical ra th e r th a n chemical, are know n collec
tively as van der W aal’s forces.(18) If th e specific surface of th e solid phase is small, th e am ount so held is usually undetectable. B u t if th e specific surface is very large, as it is for colloidal m aterial, th en physically adsorbed w ater can be a large fraction of th e to ta l held u n d er given con
ditions; indeed, anhydrous solids such as quartz pow der can hold rela
tiv ely large am ounts of w ater b y surface adsorption if th e pow der is extrem ely fine. Zeolitic w ater can be regarded as adsorbed w ater, th e
“ surfaces” in this case being certain planes in th e crystal, as described above. T he subject of adsorption will be tre a te d m uch m ore fully in la te r sections of this paper.
Interpretation of isobars
Influence of surface adsorption. F rom w hat was said above i t m igh t ap pear th a t by m eans of isobaric or isotherm al deh y d ratio n d a ta w ater held in m icrocrystalline h y d ra te s could readily be distinguished from th a t held as zeolite w ater, in solid solution, or b y adsorption. I t will be developed below th a t such a distinction can be draw n u n d e r some circum stances b u t no t under others. T he com plication can b est be illustrated by describing th e work of H agiw ara.(I9) Theoretically, th e v apor pressure of th e w ater in a very sm all crystal of a h y d ra te should be g reater th a n th a t of a larger crystal of th e sam e substance a t th e sam e tem perature. C onsequently, a m ixture containing various sized crystals should exhibit a range in v apor pressures according to th e range in particle size. Practically, th e effect is n o t noticeable unless th e particle size range extends into th e range of colloidal dim ensions. T heo
retically, very sm all n a tu ra l crystals or very sm all fragm ents of large crystals should behave sim ilarly, though n o t necessarily identically.
C onsequently, th e results of experim ents w ith preparatio n s m ade by pulverizing m acrocrystals should be indicative of th e general effects of changing particle size and particle-size range.
H agiw ara pulverized crystalline h y d rates and o btain ed th e isobar for each preparation. In one series of experim ents, he used A l20 3.3H 20 prepared by th e m ethod of Bonsdorff.(20> T he original crystals were of microscopic size. T he preparation was dried to co n stan t w eight in a desiccator over concentrated H 2SOi to establish th e initial w ater co ntent, which was determ ined by igniting a portion of th e m aterial. Sam ples th u s dried were th en heated in an electric oven a t 100C for 30 m inutes
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 113 afte r which th ey were cooled in a desiccator and weighed. Finally, th e am oun t of residual w ater was found b y ignition. This was repeated on o ther sam ples a t tem peratures ranging from 90 to 220C as indicated in Fig. 2. H eating a t 170C and a t higher tem peratures was continued u n til fu rth e r heating caused no m ore change in th e dry weight. The to tal heating period a t these higher tem peratures was n o t less th a n 5 hours and a t 210C was 20 hours.
A lthough th e corners are som ew hat rounded, there is a well defined step in th e isobar* a t approxim ately 205C, where th e w ater content decreases from 3 molecules to one molecule. This is in good agreem ent w ith th e result obtained by Weiser and M illigan.(21) The rounded corner is th e usual result in such experim ents; very sharply defined corners are the exception.
M
I I
t'crocrysfoHtne (A/, 0 j -3 H z 0
?oO o o
o
F i g . 2
o
Fig. 2 & 3 - Isobars for
AI2O3.3H2O
Data from T. H a g iw a ra A le xa n d e r: C o llo id Chemistry
V o l. I, pp. 6 4 7-6 5 8 The Chem ical C a ta lo g C o .
Inc. ( N .Y .) 1 9 26
♦ A lth o u g h th e cu rv es in F ig . 2 a n d 3 a r e ca lled iso b ars a n d th e te x t in d ic a te s t h a t iso b aric c o n d itio n s w ere in te n d e d , i t seem s p ro b a b le t h a t iso b aric co n d itio n s w ere n o t a c tu a lly m a in ta in e d . I f th e h e a tin g of th e s a m p le w as d o n e in a n o v en in th e p re se n c e of ro o m a ir, th e a c tu a l v a p o r p re ssu re in th e o v en w o u ld v a r y w ith th e h u m id ity of th e ro o m a ir a n d w o u ld b e d iffe re n t a t d iffe re n t te m p e ra tu re s . H o w e v e r, o v e r th e te m p e r a tu r e ra n g e u sed in th e e x p e rim e n t th e v a ria tio n s in p re s s u re w ere p ro b a b ly sm a ll. A t a n y r a te i t is n o t lik ely t h a t h a d s tr ic tly iso b aric co n d itio n s b ee n m a in ta in e d th e o u tc o m e w o u ld h a v e b een sig n ifi
c a n tly d iffe re n t.
114 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 H agiw ara th en ground a portion of th e dried p re p ara tio n in an agate m o rtar for V/2 hours, using fine qu artz pow der as a grinding aid, th u s g reatly reducing th e particle size. T he experim ents were rep eated w ith this finely ground m aterial w ith the results shown in Fig. 3, where th e results shown in Fig. 2 are reproduced for com parison. A com parison of th e curves in Fig. 3 shows three significant effects of reducing th e size of the crystals:
(1) All sem blance of a step is absent in the curve for th e finely ground sam ple.
(2) T he initial w ater content of th e finely ground m aterial is higher by 0.25 mole th a n th a t of the unground m aterial. T his ad dition al w a te r m u st have come from th e atm osphere during th e grinding. (The in itial w ater content is th a t of th e sam ple a fte r it is dried to co n stan t w eight in a desiccator over conc. H^SOi.)
(3) T he w ater is lost from th e finely ground sam ple a t a m uch lower tem p eratu re th a n from th e unground m aterial.
H agiw ara m ade a sim ilar series of experim ents w ith FezOs.HzO w ith th e results shown in Fig. 4. T he curve for th e unground m acrocrystalline m aterial shows a well defined step. T he curve for “ G rind A ,” th e shorter period of grinding, still shows a step th o ug h th e corners are som ew hat m ore rounded th a n for th e unground h y d ra te an d th e step occurs a t a lower tem perature. W hen th e grinding was m ore p ro longed, “ G rind B ,” th e step disappeared and th e in itial w a te r co n ten t increased to 1.54 molecules. G rinding h ad little effect on th e tem p era
tu re a t which th e final w ater co n ten t was reached.
I t appears from H agiw ara’s results th a t a h y d ra te in w hich th e w a te r is bound by chemical bonds m ay still yield a sm ooth isobar if th e sam ple is m ade up of a m ixture of particles of various sizes, th e sm allest particles being very small. T his has a significant bearing on th e in te rp re ta tio n of isobars in general. W hen a solid th a t has w ater for one of its con-
Fig. 4 - Effect of pro
longed grinding on the isobars of F e 2 0 3 .H -0 H a g iw a ra , C o llo id Chemistry
A le x a n d e r E d .; V o l. I.
pp. 6 4 7 -5 8 (1 9 2 6 )
G r i n d B
I20
T e m p e r a t u r e °C
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 115
stitu e n ts yields a stepped isobar, it can usually* be concluded th a t the solid is a well crystallized hydrate. When, however, the isobar is a sm ooth curve w ithout steps, it m ay represent a sam ple of th e hy drate comprising particles of various sizes or it m ay represent a m aterial in which th e w ater is no t bound by chemical bonds in the usual sense of this term . I t appears also th a t a lack of steps in an isobar is no t sufficient evidence th a t the h y d rate has a zeolitic structure, for neither of the hydrates w ith which H agiw ara experim ented was of th a t nature.
T he o ther effect of fine grinding, nam ely, th e increase in the initial w ater content brought ab o u t by grinding, is fully as significant as the one ju s t m entioned. I t m ust be assum ed th a t th e w ater in excess of th a t required by th e form ula is held by forces of a kind different from those th a t hold the h y d rate w ater. T he effect is clearly a surface effect, for the initial w ater content is m uch higher after th e longer period of grinding th a n afte r th e shorter period. (Com pare “ G rind B ” w ith
“ G rind A ” in Fig. 4.) I t seems reasonable to suppose th a t this excess w ater is held b y adsorption forces, i.e., forces which reside in the sur
face of th e crystal and which came into prominence after th e specific surface of th e h y d rate had been greatly increased by grinding.
This aspect of th e effect of fine grinding is fu rth e r emphasized by the results of experim ents m ade by Kelley, Jenny, and B row n.(22) These authors studied th e effect of grinding on the isobars of clay minerals.
See Fig. 5. The isobar for th e 100-mesh sample shows a well m arked step n ear 500C and th u s gives unm istakable evidence th a t th e m ineral is a m icrocrystalline hydrate. Com paring this w ith the isobar for the finely ground pyrophyllite, we see th a t the isobars are very nearly
* B u t n o t a lw a y s. S ee re m a rk s b elo w o n silicia gel.
116 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE October 1946 identical above 400C. E ach exhibits a well m arked step ; th e steps occur a t very nearly the same tem p eratu re; however, th e heigh t of th e step is slightly less for th e finely ground sam ple. Below 250C, th e finely ground sam ple shows a larger w ater co n ten t th a n the 100-mesh sample. T h a t is, ju s t as w ith th e m aterials H agiw ara used, a fte r grind
ing there is initially a large am o u n t of w ater in th e finely ground sam ple in excess of th e h y d ra te w ater represented b y th e nearly v ertical portion of th e isobar. This additional w ater m ust have been acquired from th e atm osphere during grinding and m ust be held b y some m echanism other th a n th a t which holds the h y d ra te w ater. I t seems m ost unlikely th a t th e excess w ater is zeolitic w ater, since fine grinding could h ard ly increase th e to ta l in terp lan a r area of zeolite crystals.
Interpretation of isotherms
Isotherms of hydrates. The usual behavior of a h y d ra te w hen th e w ater-vapor pressure around it is varied a t co n stan t tem p eratu re is illustrated in Fig. 6. This isotherm shows well defined steps w ith th e corners slightly rounded, as indicated b y th e d o tted lines. W h a t th e effect of pulverizing such m aterial on th e shape of th e isotherm m igh t be is no t known directly from experim ent, for a p p a ren tly no experim ents of this kind have been published. P resum ably, a sufficient am o u n t of grinding would produce a sm ooth isotherm , b y v irtu e of th e change in particle size and particle-size range. I t m ig ht also be presum ed t h a t in a sa tu ra te d condition th e m inute crystals would re ta in m ore w a te r th a n corresponds to th e highest hydrate. These presum ptions follow from considerations given above in connection w ith th e isobars.
Isotherms of gels. T he appearance of a well defined step in th e isotherm of a solid is n o t always positive proof of th e existence of a h y d ra te of definite chemical composition. Fig. 7 illustrates th is point. (23> As th e arrow indicates, th e isotherm was obtained by progressively lowering th e w ater vapor pressure. J u s t below a pressure of 4 m m Hg, th e iso
therm becomes very steep, a fact which m ight be tak en to in dicate th e existence of hy drates having th e form ulas S i0 2. i y 2H20 and S i0 2.H20.
Fig. 8 shows th e same isotherm as well as th a t o btained by progressively increasing the v ap or pressure, th e “ rehydration iso th erm ” as i t is some
tim es called. N ote th a t the la tte r gives no evidence of a h y d rate. W eiser, Milligan, and Holmes investigated th is m a tte r fully b y preparing silica gel from the sam e m aterials a t various tem p eratu res ranging from 0 to 100C and from o ther m aterials a t various tem p eratu res an d u n d er various conditions. N o t all preparatio ns exhibited th e v ertical section in th e dehydration isotherm . Gels prepared a t low tem p eratu res ex
hibited a step a t a higher w ater content th a n gels p repared a t a high tem perature. The sam ples prepared a t 100C and aged a t th is tem p eratu re for a few hours, and hence u nder conditions favoring crystal
PHYSICAL PROPERTIES OF HARDENED PORTLAND CEMENT PASTE 117
5
- Ï 3
$U)
Cl
---
<S04.SH4t’ Y
CuSO+,3Ht °
Tennp era tur<? 25°C
CuS04.Hp0
Fig. 6 - W afer content vs.
vapor pressure curve of Cu SO s
C o llin s and M enzies, J . Phys.
Chem . v. 40, pp. 3 7 9-9 7 (1 9 3 6 )
0.2 0.4 0.6 0.8
R e la t iv e V apor P ressu re, p/ps
l.O
I3'C Iso th e rm I3'C Is o th e rm s
W ater V ap o r Pressu re, mm. Hg 4 6 8 io iz
W ater V ap o r Pressure, mm. Hg [ Fig. 7— (left) Dehydration isotherm for silica gel prepared from water glass at 25C
W eise r, M illig a n and Holmes, J . Phys. Chem . v. 46, p. 5 8 6 (1 9 4 2 )
Fig. 8 (right)— Dehydration and rehydration isotherms for silica gel prepared from water glass at 25C
W eise r, M illig a n and Holmes, J . Phys. Chem . v. 4 6 , p. 5 86 (1 9 4 2 )
