Structure and Physical Properties of Hardened Portland Cement Paste

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Research and Development Laboratories of the Portland Cement Association

RESEARCH DEPARTMENT

BULLETIN 94

Structure and Physical Properties of Hardened Portland Cement Paste

T. C. POWERS

MARCH, 1958

I CHICAGO

Authorized Reprint from JOURNAL OF THE AMERICAN CERAMIC SOCIETY

vol. 41, P. 1 (1958) VOL. 41, NO. 1 JOURNAL JANUARY I r 1958 of the American Ceramic Society

Structure and physical properties of Hardened portland Cement Paste

by T. C, POWERS

Research md Development Oivision, Portland Cement Association, Chicago, Illinois

Methods of studying the submicroscopic struc- The amount of water absorbed by dry lrasteindicated that ture of Portland cement paste are described, and the paste was highly porous, and at first the physical structure deductions about structure are presented. The of hardened paste was tborrghtof in terms of pores. ‘~heories main component, cement gel, is deposited in pertaining to capillaries were used. In about 1939 the con- water-611ed space within the visible boundaries cept changed, and pores were thought of as spaces among par- of a body of paste. Space tilled with gel con- ticles (interstitial spaces). This change marked the hegin- tains gel pores; space not tiled by gel or other nin,g of progress. The theory of Brunauer, Emmett, and solid material is capillary space. I-Iygroscopicity Tellerl was used to interpret data on adsorption of water of cement gel, and capillary pores, accounts for vapor by predried paste, and this application of the tbeory, various aapects of the properties and behavior still in constant use, turned out to be a most valuable tool for of concrete. Data on gel and paste strrrc- studying physical structure, tnre are used in discussing strength, permeabil- By the Brunauer-Emmett-Teller method, internal surface ity, volume stability, and action of frost. area was measured and then the order of size of the solid par- titles composing hardened paste was computed. This was 1. Introduction first accomplished in about 1940, The thermodynamics of adsorption and tbe freezing of water in hardened paste also N the parlance of the cement industry, a mixture of Port- land cement and water is called cement paste; the chem- were studied. Such studies were coordinated with experi- I ical reactions of the components of Portland cement with mental and theoretical studies of such physical properties as water are spoken of collectively as cement hydration; hydra- strength, permeabilityy, and volume change. tion of cement causes the paste to harden and thus there is After a wartime hiatus, work was resumed and new tech- the term 6‘hardened Portland cement paste.” niques were gradual]y added to the old ones. An experi Stndies of the strncture and properties of hardened paste mental study of permeability has been under way on a part- began in the Portland Cement Association laboratories in time basis for about 11 years, and studies of volume changes, about 1936. The purpose was to bridge a gap between especially those caused by freezing of water in hardened paste, cement chemistry and concrete technology, It seemed that have been especially intensive. X-ray techniques are now establishing the relation between properties of the paste and applied to almost all aspects of stndies of structure, Diffrac. chemical constitution of cement on the one hand and between tion has been effective in establishing tbe stoichiometry, and properties of paste and properties of concrete on tbe other structure of the solid phases of the paste, and small-angle hand might accomplish this purpose. Results are gradually scattering has been used recently for measuring specific sur- fulfilling that hope. face. Electron-optic and electron-diffraction techniques are now being applied.

Presentedat the Fifty-.VinthAnnual Meeting, The Americau Ceramic Society, Dallas, Texas, May 8, 1957 (Basic Science Division, N-o.44). Received May 7, 1957; revisedcopy received October2,1957. LStepheu Brunauer,Adsorptioll of Gases and Vapors, Vol. 1. The anther is manager,Basic ResearchSection, Researchand Princeton University Press, Princeton, 1943. 511 PP.; Cenwn. DevelopmentDivision, Portland Cement Association. .4b$tr,, 23 [11] 204(1944). 1 Ceramic Sociely-Powers Vol. 41, No. I

Ftg. 1. Smplafied model of porte rlruciure. Gel paiticles are represented or needles or platel; C designoler capillary cavltier. Co(OHIj cryrtols, unhydrated cement, and minor hydrates ore no1 represented.

A theoretical paper about freezing of water in hardened paste based a results of studies of physical structure was published in 1945,' but a comprehensive statement about the structure and physical properti'& did not appear until 194i.' Since then the program has produced other papers. The following is a brief statement about the principal con- cepts developed during the course of this work. The Brunauer-Emmett-Teller method gives the specific surface of the solid part of the gel as about iM) m.' per cm.Jof II. Structure of Paste solid. This is equal to the specific surface of a sphere having Fresh cement paste is a network of particles of cement in a diameter of 86 a.u. The figure for specific surface was con- water. The paste is plastic, and it normally remains thus firmed recently by s&-angle scattering of X rays. for an hour or more, during which period it "bleeds"; i.e., As seen with the electron microscope, cement gel consists there is a small amount of sedimentation.' After this rela- mostly of fibrous particles with straight edges. Bundles of tively dormant period, the plastic mass sets and thereafter such fibers seem to form a cross-linked network, containing the apparent volume of the paste remains constant, except some more or less amorphous interstitial material. for microscopic but technically important variations caused The structure of paste is not identical with the structure of by changes of temperature or moisture content, or by reac- gel. Space within the visible boundaries of a specimen of tions with atmospheric C01. paste contains gel, crystals of calcium hydroxide, some minor Chemical reactions between components of cement and components, residues of the original cement, and residues of water produce new solid phases.6 One of them is crystalline the original water-filled spaces in the fresh paste. These calcium hydroxide and another, the predominant one, micro- residues of water-filled space exist in the hardened paste as scopically amorphous, is "cement gel." interconnected channels or, if the structure is dense enough. Cement gel is composed of gel particles and interstices as cavities interconnected only by gel pores. These residual among those particles, called gel pores. The solid part of the submicroscopic spaces are called capillary pores, or capillary gel contains approximately 3Ca0.2Si0,.3H20. Its crystal cavities. structure, although highly disorganized, approximates that of Thus two classes of pores within the boundaries of a body tobermorite. Cement contains Al and Fe atoms as well as of paste are recognized: (1) gel pores, which are a charac- calcium and silicon atoms. They seem to play a relatively teristic feature of the structure of gel, and (2) capillaty pores minor role as structural units but a more important role in or cavities, representing space not filled by gel or other solid determining rates of reaction. components of the system. Figure 1 shows a model of this concept of structure. All the spaces, gel pores arid capillary cavities, are submicroscopic. This fact, together with the hpdrophile charac- 2 See refereure (4) of Bibliography on p. 6, this issue. a See reference (7) of Bibliography. ter of the solid phase, amounts for the hygroscopicity of paste; See reference (1) of Bibliography. water content is a function of ambient humidity. $(a) J. D. Bernal, "Structures of Cement Hydration Com- Capillary porositv is greatest in a given paste when the pounds." Proc. Intern. Syntposium Chemistry of Cement. 3rd Sym- posium. London, 1952, pp. 2lFt-1% (19.54); Ccron~.Abrlr., 1956, paste is fresh. It is least when all the cement bas become repternher, p. 184r. hydrated that can become hydrated under existing conditions. (h) H. 11. Steinour. "Reactions and Thermochemistry 01 At any given stage of hydration, capillary porosity depends on Cement Hydration at Ordinary Temperature." Proc. Inlem. the original proportion of water in the paste. which is usually Synrposium Chemistry of C~mmt,3rd Symposium, London, 195?, pp. 261-333 (1954): Ccram. 4hslr.. 1956, September, p. expressed as the ratio of water to cement in the original mix- 1s-lh. ture. January 1958 Profierties of Hardened Portland Cement Paste 3 1 0 - Mix

x- II

A- ,’

Capillary Porasity fig. 4. Permeability vs. c.pillory porosity for cement paste. Different symbol, desion.ate different cements,

Table 1. Comparison of Permeabilities of Rocks and Cement Pastes

Perz;o:p

“o 0,2 0.4 0.6 0.8 1.0 Kind of rock (dawm) W.ter.cenlentrauo* —— Gel-Space Ratio (X) Dense trap 2,57 X 10+ 0.38 Quartz diorite 8.56 X 10-9 .42 Fig. 3. Compresske strength w. gel-space raiio for cement-sand mortars. Marble 2.49 X 10-8 ,48 f. = compressive strength (lb, per s.+ in,); x = gel-space ratio. Marble 6.00 X 10-7 .66 Granite 5.57 X1O+ 70 Sandstone I,2SX IO+ :71 Granite 1.62 X 10-6 .71 * Water-cement ratio of mature paste having same permeability asrock.

The products from 1 cm,~ of cement require a little more than 2 cm.s of sp~ce. Therefore, the volume of water-filled space in fresh paste must exceed twice the aheolute volume of cement, or some of the otiginal cement must remain unhydrated. Cement gel can be produced only in water-filled capil[ary cavities, and when all those cavities became full, no sented there contain aggregates, and whatever effect the further hydration of cement can occur, Figure 2 illustrates aggregate+$ on strengthis also reflected in tbe characteristics bow hydration products gradually reduce the amount of capil- of the empmcal curve. It is evident that the gel-space ratio lary space, and in some cases eliminate it. is the dominant variable, and that strength increases in direct proportion to the cube of the increased gel-space ratio, Ill. Strength The numerical coefficient probably depends principally on the As just indicated, cement gel is regarded as a solid sub- intrinsic strength of the gel produced by this particular stance having a characteristic relatively high porosity. From cement, and it would be different for a different cement, the assumption that this substance has intrinsic strength de- As to the source of strength of tbe gel itself, there is no ade- pending on its composition and structure, and that the quate theory. It is perhaps a fair speculation to assume that strength of tbe gel is the sole source of the strength of hard- strength arisesfrom two general kinds of cohesive bonds: (1) ened paste, it follows that the strength of a specimen of paste physical attraction between solid surfaces and (2) chemical should he related to the amount of gel witbin its boundaries. bonds. Since gel pores are only about 15 au. wide on the Furthermore, an assumption that the relative strength of the average, it seems that London-van der Waals forces ought to paste depends on the degree to which gel fills the space avail- tend to draw the surfaces together or at least to hold the par- able to it leads to the establishment of an empirical relation- ticles in positions of least potential energy. In either case, ship between the porosity and the strength of a paste. those forces give rise to cohesion, Since water cannot dis- The degree to which gel fills available space can be ex- perse gel particles, i.e., since cement gel belongs in the limited- pressed as a ratio of volume of gel to volume of available swelling categosy, it seems that the particles are chemically space. A typical relationship between compressive strength bonded to each other (cross-linked). Such bonds, much and gel-space ratio is shown in Fig. 3. The specimens repre- etronger than tbe van der Waals bonds, add significantly to 4 Journal of The American Ceramic Society--l’owers Vol. 41, No. 1 over-all strength; there is good reason to believe, however, that only a small fraction of the boundary of a gel particle is Specimen OV chemically bonded to neighboring particles and that physical bonds are prrhaps the more important. Pertinent evidence w/c = 0.58 is that converting gel to well-organized crystals by curing in 2.4 ~ 72% hydrated steam at about 400” F. destroys cohesion. I Vs = 0.49

IV. Porosity and Permeability 1 kry solid composed of particles randomly aggregated is 20 - I both porous and permeable. Since cement paste has such SIrucbure, it is intrinsically porous and permeable. The densest possible completely hydrated cement paste has a porosity of about 2(YY0. The porosity of paste as a whole is 16 - usually greater, and it depencls on the original water content 0 and cm Lhe extent to which space bas become filled with hydration produCtS, It depends, therefore, on the original wate.r-cementratio and cm the conditions of curing. 1.2 The permeability of a granular solid depends on porosity and on the size and shape of the pores. In such solids, size of pore can be expressed in terms of hydraulic radius, which is the quotient of water-filled space by the boundary area of 08 that space. Knowing the porosity of a paste and the specific 8 surfiaceof the gel it contains, one can calculate the hydraulic radius, The hydraulic radius of the pores in the gel itself is found to he about 5 au. Resistance to flow through pores so 04 - small is exceedbIgly high. Measurements show that the coetilcient of permeability of the gel itself is about 7 X 10–’1 dareys.“ The permeability of paste as a whole depends mostly on its 0 0 .2 .4 .6 ,8 1.0 capillary porosity, for the resistance tu flow through the h capillary cavities is nmch smaller than that through the gel. Kg,5. Drying shrinkage of cement pcxte. AVIV = fractional volume The relationship between permeability and capillary porosity change; V, = solids per unit volume of paste; h = relative humidity. is shown in Fig. 4. Paste such as is produced normally in concrete of good quality hx.s a capillary porosity of 30 to 40~o and, as seen in Pig, 4, is from 20 to 100 times as permeable as cement gel itself. It is, however, less permeable than many n~lural racks, as maybe seenby the data shown in Table I. 40 ~ c V. instability of Volume As with othci colloidal hydrophilic materials, cement gel shrinks and swells with changes in moisture content, and its response to chaugc in temperature is complex, Noncolloidal components of paste, a,ld the mineral aggregate of concrete, ~ -40 - ,0 restrain most of tbe shrinking and swelling of gel, but the z remainder, which accounts for some characteristic volume =.- /“ changes of concrete, is commercially significant. Typical ~ -8o - ,/ shrinkage of putt at constant temperature, caused by drying from the saturated state, is shown in Fig. 5, Shrinkage is & : manifestly a complex function of the change in relative :-120 humidity in the pores of the paste. Clmn#e in volume caused by change in temperature also is 5@ complex, In Fig. O the dashed line indicates the change in .5’160 - A vohnne produced by a slow change in temperature with the specimen kept fully saturated at all stages of the chmge, Specimen under Mercury This line represents the ordinary thermal contraction shown, -200 - i [or example, by meial]ic solids. during test The solid line 4 B is ihe locus for a specimen of paste not c~uite saturated with water, When such a specimen is -240 - mmlcd, it undergoes ordinary thermal contraction a“d irl : addition a ~!,rinka~e that is called hygrotkerwudshrinkage, The rmujI.o:t of such shrinkage is indicated by the vertical Fig. & Hydrothermal effect. !. cement pmt., distance from x point on line AB tothe corresponding point directly above it on the dashed line, The locus BC’, showinj~lack of reversibility, and residual expansion, is imlic:ltive of still more complexities of behavior tbat are not dismissedhere. The stiatcof shrinking or swelling depends on the amount of water adsorhcd by the gel, This may range from none to a * A flow rate of 1 cm,, per second through m srca of 1 sq. maximum which represerlts a state of saturation. Tbe cm. under a pressure gradient of 1 atm. per cm. with a flaid amount of water that gel’ is able to adsorb jucreases as tem- having a viscosity equal to 1 centipoiseequals 1 darcy, January 1958 ~+opertiesOfHardened Portland Cement ~asle 5 f+”’o’ 1600

., Fig. 8, Effect of entrained air in cement paste. Upper curve shows dMlion prod. cad in paste containing no bubbles. lower curve shows same paste with entrained air, Al/l = fractional length change.

\\ \II Vi. Freezing Water can be caused to freeze in capillary cavities, but it 01 i o .2 .4 .6 .8 1,0 cannot freae in gel pores. Gel pores apparently are too small Relattve Humidity to permit nucleation of ice crystals. The fraction of total evaporable water that can be frozen is a function of tempera- fig. 7. Hywo!herm.1 swelling of cement post.. Two fop c.r.es, data of ture and time, as would be expected from dimensional factors Meyers (see footnote 6); boflom curve, data of Virronncwd and van and the effects of solutes in the freezable water. Thanh [see f.ac.tnote 7). Freezing of water in a saturated paste causes the paste to dilate destructively unless special steps are taken to protect the paste from the pressure that causes dilation. The pressure that causes dilation comprises two kinds: (1) hydraulic pressurethat, during freezing, forces water away from freezing sites (the water-filled capillary cavities) and (2) osmotic pressure produced by water tending to enter partly frozen capillary cavities. Either kind of pressure can be controlled by filling the fresh paste with microscopic air bubbles which re- perature decreases, When temperature drops and no extra main in the hardened paste. The bubbles must be so numer- water is available, the gel becomes relatively less saturated, ous that they are separated by layers of paste only a few and it shrinks. The amount of shrinkage thus induced de- thousandths of an inch thick. Effects of such bubbles rm pends on the state of saturation of the gel and hence on the dilation during freezing are shown in Fig. 8, internal humidity of the specimen, as intilcated in Fig. 7, When the air hubbies are sufficiently close together, freez- In F,g, 7 the amount of hydrothermal volume change is ing produces shrinkage rather than dilation. Under these shown in relation to the internal humidity of the specimen. circumstances shrinkage is caused by tl-ansferof water from It is expressed as millionths per degree and is tberefmw the paste to the air bubbles by osmosis. numerically comparable with the ordinary thermal coefficient, Since a typical value for a thermal coefficient is 11 millionths WI. Other Properties ,; per ‘C., these figures indicate that the maximum hygrother- Cement gel surrounds and isolates each nmrcolloidal par- mal swelling effect may be two to three times as great as the ticle in concrete. Mechanical properties of concrete are normal thermal coeklicient. / therefore characterized by the mechanical properties of the Such e~ects appear to be understandable consec!uences of gel, to an important degree. Stress-strain time relationships the colloidal state of the hydration products of Portland are to be explained largely in terms of the characteristics of cement, A comprehensive hypothesis about the mechanism cement gel, Most of tbe research needed in this field is yet of volume changes produced by changes in temperature and to be done. in the humidity is now being developed. VIII. Summary Research started in the Portland Cement Association 6 S, L, Meyers, <‘Thermal ExpansionCharacteristicsof Hard- laboratories about 20 years ago on the properties of Portland ened Cement Pastes and of Concrete,’% Highway Research Board, cement paste produced important basic knowledge about the Proc., 30, 193-203 ( 1950). properties and behavior of concrete. The principal resewch 7 L. Virrormaud and N. van Thanh, ‘ ‘Dilatometer with an 0P- ticd Tripod: Tests and Results of Experiment s,” ,4XX. inst. technique was water-vapor adsorption interpreted by the tech.bdtiment et tmu. @bL, 7, 522-40 (1954) (ill French) Brunauer-Emmett-Teller theory. More recently, other tech- 6 Journal of 7%e American Ceramic Society—Powers Vol. 41, No. 1 niques including X-ray and electron microscopy have been of the major product of the reactions between Portland introduced. Knowledge of the physical and chemical consti. cement and water, and to the spatial concentration of this tution of cement paste provides useful conceptual models for product (cement gel). The mechanics and physical chemistry dealing with practical problems, Strength, permeability, and of frost action in concrete wae established in terms ~ tbe volume instability are basically related to the colloidal state pbyeical characteristics of cement paste,

Bibliography

The following bulletins of the Research and Development (11) L. E, Copela.nd and J. C. Hayes, ‘rDetermination of Laboratories of the Portland Cement Association describe more Nonevaporable Water in Hardened Portland Cement Paste,” fully the work reviewed in the text. Bulletin No. 47, 9 pp. (December 1953); reprinted from A.VM (1) T. C. Powers, “Bleedi”g of Portland Cement Paste, B?M., No. 1P4,76-74( December 1953). Mortar, and Concrete Treated as a Special Case of ,%dimentat ion,” (12) T. C. Powers, “Void Spacing as Basis for Producing Bulletin No. 2, 160 pp. (July 1939) (appendix by L. A. Dahl). Air-Entrained Concrete,” Bulletin No. 49, 20 pp. (July 1954); (2) H. H. Steinour, “Rate of Sedimentation: 1, Nontlcccu- reprinted from J. .4 m. Concrete Inst. (May 1954); Proceedings, 5q pp. 7AI-I?n lated Suspensions of Uniform Spheres”; “11, Suspensions of Uni- !=. ““. f orm-S1~e Angular Particles”; “III, Concentrated Flocculated (13) J, E. Backstrom, R, W. Burrows, V, E, Woikodoff! a“d Suspensions of Powders,” Bulletin No. 3,52 PP. (October 1944); T.’ C.” Powers. discussion of the mmer. “Void %acim as Basw for reprinted from Ind. Eng. Chem., 36 [7] 618-24; [9] MO-47; Producing Air-Entrained Con&e~ej; Bufie& ““No,” ~9A,-i6 pp~ .—–,rlol 901-907.– (1944). (December 1954); reprinted from J, Ant, Ccmcrele I?ML ( Decem- (3) H, H: Steiriour, “Further Studies of Bleeding of Portland ber 1954, Part 2); Proceedings, 50, pp. 760-1-760-15, Cement Paste,” Bulletin No. 4, 88 pp. (December 1946), (14) L. E. Copeland and R, H, Bragg, “Self-Desiccation in (4) T. C, Powers, “A Working Hypothesis for Further Portland Cement Pastes,” Bulletin No. 52, 15 pp. (February StuIdies of Frost Resistance of Concrete, ” Bulletin No, b, 27 pp. 1955); reprinted from ASTM BzdL, No, 204, 34-39 (February (Februz ary 1945); reprinted from J. Am. Concrete Inst. (Febrw 195.5). ar y 1945 ); Proceedifigs, 41, pp. 245-72. (l~j T. C, Powers, L. E. Copeland, J, C. Hayes, and H. M, (5) Ru\h D. Terzaghi, Douglas .McHenry, H, W. Brewer, Mann, “Permeability of Portland Cement Paste, ” Bulletin No, A. R. Colhns, and T. C. Powers, discussion of the paper, “A 53, 14 pp. (April 1955); reprinted from J. Am. Concrete Inst. Working Hypothesis for Further Studies of Frost Resistance of (November 1954); Proceedings, 51, pp. 285-98. Concrete,” Bulletin No. 5A, 20 pp. (March 1946); reprinted (16) T, C, Powers, “Basic Considerations Pertaining to from J. Am. Concrete Inst. Supplement (November 1945); Pro. Freezing-and-Thawing Tests, ” Bulletin No. 58, 24 pp. (September ceedkgs, 41,,.n ’—.,272-1— -. 1955); reprinted from Proc. Am, SW, Testing Materials, 55, ,., -.. ald. Plckett, “ShrinkageStressesin Concrete,” Bulle- 1132-55( 1955), tin No. 11, 78 pp. (Marc ch 1946): reprintedfrom 1. Am. ComMk (17) T. C. ‘Powers, “Hydraulic Pressure in Concrete,” Bulle. ~~A~January and February 1946) ~Proceedings, 42, pp. 165-204, tin No. 63, 12 pp. (April 1956); reprinted from Proc. Am. .SOc, Civil B2K7s., 81, Paper No. 742 ( Iulv 1955).

(ij ‘T.-”C. Powers, “Nonevapor@e Water Co”tent of Hardened Portkmd Cement Past* Its Significance for Concrete Researth and Its Method of Determination;,, Bulletin No, 29, 8S pp. (June 1949); reprinted from ASTM BW., No, 1S8, 68-76 (May 1949), (10) T, C, Powers and R, A, Helmuth, “Theory of Volume Changes in Hardened Portland Cement Paste During Freezing,>, Bulletin No. 46, 13 pp. (September 1953); reprinted from Highway Research Board, Proc,, 32, 285(1953).