ENCYCLOPEDIA OF CHEMICAL - US Environmental … · ENCYCLOPEDIA OF CHEMICAL ... + CaO — 3 CaO.Stt h '•_ ... the lime to form (.'sA, and with lime and I'erric oxide to form. 686

iLi2.cn,

Kirk-Othmer

ENCYCLOPEDIAOF CHEMICALTECHNOLOGYSecond completely revised edition

VOLUME 1 Calcium Compounds*: to

r Chloramphenicol

?*•-'••. ,;a.{'

684 CEMENT VoL

' ] CEMENT

Portland cement is a powdered mat trial \vhich, with water, form* a paste that .. " ,hardens slowly, bonding intermixed crushed rock or gravel and sand into rock-hard 'l!l " '.v,,,.., _. -~;.- ...... .... ._-„,,...>- • tM-j mt- yj K-oaa.- —— =aa —— wy™,,, —— _ „ i.«»a*«W-=V— . .;aSK4Kr~ • —— .. —— iposed mainly of calcium carbonate (as limestone) and the other ofalummum silicates(as clay or shale) (see Clays; Lime and limestone). During the sintering process. ° .chemical reactions take place producing nodules, called clinkers, which are composed 'U ." ' /"principally of calcium silicates and aluminates. When the clinker is pulverized with a m .esmall amount of gypsum (calcium sulfate, see p. 14), the resulting powder U the M' >it>.Portland cement of commerce. Portland cement is distinguished from other kinds of <l. ' T, "commercial cements by the different phases of which it is composed, and the meeting , 101.1' , ,of particular standard specifications laid down by established authorities. Under UUC °T, .these requirements, white cement and oil-well cement may be portlands but high- ,alumina and natural cement cannot be so classified. Some cements are mixtures of \°". . .Portland with other materials, as masonry, slag, and pozzolan cements. The common Ustypes of commercial hydraulic cements are described in this article.

Modem understanding of the requirements for producing a strong and durablehydraulic cement (one that will not disintegrate in water) was attained by two im-portant discoveries in the eighteenth and nineteenth centuries. The first was that ahydraulic cement can be made only by burning an argillaceous limestone, or a lime-stone with which clay or shale has been interground in certain proportions. If theclay is not present in. the raw mixture, the product will disintegrate in water. The • 1S a an, .second critical discovery was that a cement capable of producing concrete of superior barrel o -strength can be made only by burning the above mixtures at sintering temperature,producing hard clinkers whicjj. thereafter must be finely pulverized.

The name "portland cement" was. given by Joseph Aspdin in a patent, dated1824, because the cement, when mixed with water and sand, hardened into a block producedthat resembled a natural limestone quarried on the Isle of Portland in England. geneous yIt is possible, however, that the high-temperature portland cement as we know it • 'm tno rtoday was not made until about 184.) when introduced by I. C. Johnson (1). Institute

The growth of the cement industry hi the United States was stimulated by the the solutdemand early in the nineteenth century for cement to be used in the expanding canal Nationalsystems. Natural argillaceous limestones, discovered near the l-'.ric Canal in New EnglandYork and the Lehigh Canal in Pennsylvania, were found to be suitable for making iiivcstigahydraulic cement. These deposits contained about the right amount of clay in- propertytimately associated, jyith thg calcium carbonate so that the rock could be burned in '"'° st-'vthe vertical kilns without pulverizing or admixing with any other material. For that when mireason, thjsjiypyof lunestohc came, to be known as cement rock. And. since the agents e>cement was inatj rectiy rkim the naturally occurring rock, the product came to be 'known as liftttT ^ yiit; Portland cement (as distinguished, by sintering in the I'lilcinmkiln) was fifs ^ jrtlio/UnitpU States by David Savior in 1S71 in vertical kilns Liinc linin Copley, Penra ltania. Jlie first rotary kiln was introduced in 18!)'.) and within a residualdecade the vertical kiln Fof portland cement in the United States became obsolete.The efficiency of the vertical kiln has been greatly improved, however, and in many .. ft^countries this type of kiln is used extensively. Up to the turn of the century, low- ' t| • itemperature- natural, cements were produced in larger amounts than high-temperature .Portland but after that time thev liccaiue relatively unimportant and their production ,. . . , " , i i aliliniiiaverv limited.

Vol. 4 ; CEMENT 685

The demands of the engineer tend to keep ahead of the output of the producer.The engineer presses continuously for improvement in quality and in uniformity ofquality. His needs become1 varied and his demands require :dift'ercnt cements fordifferent uses. lie writes specifications that become increasingly restrictive. Thisnecessitates ever closer control throughout the manufacturing process. Some sourcesof raw materials once adequate are rendered inoperative and others require bencficiationor importation of deficient components. Research and development laboratorieshave mushroomed in government, college, and industrial organizations. It no longersuffices to know how to make a good cement; it is necessary to know why certain.alterations produce certain changes so that a product can be produced to any specifica-tion. Thus, the portland cement of today differs from that of previous years, andundoubtedly future requirements will necessitate a different cement.

Throughout this article, abbreviations for the cumbersome formulas of cementcompounds will be used in accordance with the general practice in the cement in-dustry, as follows: ;

CjS = 3 CaO.StOj = tricalrium silicate -C S = '2 CaO.SiCK •* dicalcium silicateCjA =» 3 CaO.AI»Oj = tricaleium aluminate

C«AF = 4 CaO.AloOj.FesOs = tetracalcium aluminoferrite

The commercial unit for measurement of portland cement tin the United Statesis a barrel of 376 Ib, equal to 4 bags of 94 Ib net. The unit formasonry cement is abarrel of 280 Ib.

PropertiesThe classical method for studying the nature and behavior of the phases that are

produced by any given thermal treatment of any system is by the discipline of hetero-geneous phase equilibrium. Pioneer studies on the system CaO.Al«Oj.SiOj wore madein the first quarter of this century at the Geophysical Laboratory of the CarnegieInstitution in Washington, as shown hi Figure 1 (2). Those studies were followed bythe solution of many other systems of two, three, four, and five components at theNational Bureau of Standards in Washington, the Building Research Station inEngland, and similar laboratories in many other countries. As a result of theseinvestigations, a clear picture is now available; of the phases, their compositions,properties, and fields of stability for the principal systems involved in portland cement.The several phases have been prepared and studied, thoircharacteristics ascertainedwhen mixed with water, and the physics and chemistry of their function as bondingagents established (3). '•

The principal reactions in the formation of portland cement are those by whichcalcium oxide combines with the acidic components to form (,'3S, C2S. C3A, and CiAF.Lime and silica read readily to form CaS, but this C-.S combines only slowly withresidual lime to form C'nS. The reaction,

2 CaO.SiO, + CaO — 3 CaO.Stt h '•_

is the essential reaction in the production of portland cement. That is because CjSis the principal active hydraulic compound of the cement. The ('...S is" slowly hydraulic\\hi-n in the beta form, but early strength is dependent almost entirely on C'uS. Thealumina combii.es with, the lime to form (.'sA, and with lime and I'erric oxide to form

686 CEMENT

Fig. 1. The system Cat >-Al;< )j-Si< >;, modified, fruiu liuiikin and U'riglil CZ).

CiAF. The C':;A is rapidly hydrated in water with the evolution of considerable heat,but the product has little cementing value. The CiAF has no cementing value.

it Magnesia remains in portland cement as MgO or periclase, which is limited in the' specifications because it may give rise to excessive expansions. A small amount of

lime also remains uneombined in the clinker as free C'aO. During burning in the kiln.about '!()-:',()' ( of liquid is formed, from which the constituent compounds will crys-tallixe if cooled slowly. Hut if the liquid is cooled rapidly, a considerable amount ofit will he supercooled and freexe to-Ii glassy phase which imparts a different set ofproperties ) the ciinker and the cement. The C.;A in this phase is less reactive, andMgO.djssolyi'd i" it is nonexpansive. Hence rapid cooling ofclinki-r is often demandedfor the control oi" (':t.V and MgO activity. IJeci-nt data from the I'ortland ('einentAssoc tk>n Laboratory indicate that there i-- no appreciable amount of true gla.-sin comniercially cooled clinkers. These results show that the interstitial phases areessentially completely crystalline to.\ ray.-, although in a very line state of subdivision.

Most of the clinker compounds take up small amounts of other component.- tnform solid solutions. IV-r known of thesi- phases is the' C.,S solid solution called

Vol. I CEMENT

Fig. 2. EhiitiMimTcujniiili »( ;i polished and etched si'>-tioii of typical purtland cement clinker.showing clc:ir rrystnllim- grains of C..S. roundotl :unl twinned crystals of 3-C-S. dark interstitialmaterial which is C^A and glass, and light interstitial mati-rial which is C^F (X500) (.5).

I-'ig..3. Electron micrograph <>t h\dratfd porthttul ccmi-nt ! X HHl.tUH)

688 CEMENT

"alite." which has been assigned the formula ".4CaO.10iSiO.j.Al,03.A[gO. This mayhe understood as eighteen molecules of CiS in which one SiO* has been replaced bvAIjOj, and another by MgO (4). The clinker phases are best observed by examining^

s Wffrfr faWBg mffi hr'ff-c r5 itwinned grains, the C3A and glass as dark interstitial material, and the C4AF as lightinterstitial material. An electron micrograph of a hydra ting cement is shown inFigure M. Hexagonal crystals of C;SA hydrate and long needles of calcium sulfoalu-minate. :;C'aO.Al->03.:>CaSO4.:UH2O, are shown.

Some of the optical and thermal properties of the clinker compounds are givenbelow. Tricalcium silicate occurs as small, equidimensional colorless grains having

, refractive indexes a 1.718 and 7 1.724, and a very weak birefringence. The grainsappear uniaxial or biaxial with a small axial angle; the optical character is negative.The C'aS is not stable with liquid of its own composition. It was formerly believedthat CsS dissociates at 1900°C into C£> and C'aO, but recent work at the BuildingResearch Station in England indicates that this compound does not decompose at

J 1900°C but, rather, melts incongruently at 2070°C, forming CaO and liquid. On. cooling below 1000-1300°C, C3S decomposes slowly but, if cooling is not prolonged,, it comes down to ordinary temperature in a practically unchanged state and remains{ relatively stable.I Dicalcium silicate exists in several forms, the interrelationship of which is not

yet completely resolved (4). The a form melts at 2130°C and inverts to the a'form at 1425°C, which in turn inverts to the 0 form at 670°C. The 0 form mayinvert to the 7 form at any temperature below 525 °C, but may be stabilized to resistthat inversion.* The. P-C£> forms rounded grains usually showing polysynthetictwinning and having refractive indexes a 1.717 and 7 1.735, with medium birefringence.

, | The optic axial angle isJarge, and the optical character is positive. The 7-CsS is' * prismatic in habit, refractive indexes a 1.042, 0 1.645, 7 1.654, birefringence medium,

optic angle about 5,2°, and optical character negative. .__.Tricalcium aluminate forms isometric crystals, having a refractive index of 1.710.

It dissociates below ite.meltirrg point forming Ca0 and liquid at 1535°C. The ironphase approaches C.tAF in_composition but takes up other components in solid solu-tion. It melj jpjigvujntly at 1415°CT- The compound is biaxial negative with amedium optic axial angle,, and refractive indexes_of a 1.90, 0 2.10, and 7 2.04. Otherphases that may be "present In clinker include isometric periclaso (MgO), refractiveindex 1.737; isoinetrj& CaO, refractive index C$8; and lass which varies with thecomposition." . "fjr. ..; " "" . •

The bonchiig properties of a cement-water paste arc all assoi-iated with thereactionsjisf"the..cement phasos during hydration. Our understanding'of cementhydrstmiOjas Iw'ii immeasurably enhanced by studies in many laboratories in recent,year£ j|jias.been found (0) that the principal constituent of cement paste is a tober-moritegei whidLis prodticeiby the hydration of the calcium silicates according tothe following reactions:" ..- '.f.-T ' .\~_ ^ ~-~ .-

2:& CIic>.»*!(vt -f 0 fljt) -*":> C'aO.'JSK>».3 IIzO 4*3(

A unique property of this gel is its specific surface which ha.- been reported to hoon the order of :>()() m- g. Electron micrograph.- show predominantly mng fibers,

Vol. 4 ; CEMENT 6«<Jf

may often rolled up in sheets. The dimension in the c direction is colloidal and -.vrge.- on''I by the molecular but that in the /> direction is on theporder of 1 M or more. X-ray difl'rac-inins; tion patterns indicate that the gel is very poorly crystallized, having *fthHisity of• re 2. 2.70 g cm3, and an interlaver distance of 10.3 A.. The mean diameter of the srel poresi and has been reported as 20-40 A (7). :light The adhesion of the tobermoritc particles to'each other and to the embeddedn in aggregates is responsible for the strength of concrete. More definitively, the surface>alu- area and porosity arc responsible for the important engineering properties of strength.

shrinkage, permeability, and resistance to the stresses of freezing and thaui;;^. Theiiveu tobermoritc; sheets are two"or throe molecular layers thick and admit the entry ofvihc; water between them or adsorbed on their surface. The entry of water moleculesa in.- causes an expansion of the layers resulting in swelling of the structure. Tin.- removaltive. of the water causes contraction of the layers and shrinkage1 of the structure, with••ved possible cracking. ,ding The hydration of the C3A proceeds in different ways depending on the temper-e at ature. Above 21°C, isotropic crystals arc forrhed having the composition 3C'aO.-On AlsOj.GHaO and a refractive index of 1.G04. Below 21 °C, hexagonal crystals areged. formed of di- and tetracalcium aluminate hydrates, 2CaO.Al20s.5-9HiO and.aius 4CaO.AU03.12-14H20. The latter compound has been reported to be biaxial negative

with refractive, indexes a 1.522, /3 1.538, and 7:1.542. The tetracalcium alumino-uot ferrite phase hydrates slowly to form the calcium aluminate hydrates as above, and' a' an amorphous calcium ferrite phase. ;

The rate of development of strength in a cement paste is determined chiefly bythe CjS content and the fineness of the cement. : Tests indicate (8) about a fivefold

•>tic increase in compressive strength of concrete specimens after one day by increasingice. the €38 in the cement from 30 to 70%. The /3-C2S gives low strength at early ages•; is • but within a year it attains a strength comparable to that of C3S. The strength of;m. CsA by itself is low, but the introduction of 15^ of C3A to CaS has been found to

raise the strength of concrete at ages up to three days. At later ages, the strength is10. sharply reduced by large amounts of C3A. The iron compound develops no appreciable)ti strength at any age. The introduction of gypsum has little effect.on the strength of'u- the silicates but markedly improves that of the aluminate when added in the optimuma amount. ;er During the process of setting and hardening, considerable heat is liberated by,ve the reactions of hydration. This heat is usually qf no special concern for it is rapidly;1(> dissipated, but in some cases it is of great importance. In massive concrete struc-

tures, such as large- dams, the heat so liberated could raise the temperature/ to suchi[e vahugj tha,t, on cooling,, thermal contraction would produce serious damage. In;,t cold\veathcr, the heat liberated during hydration Jnay be of value in preventing freez-•it. ing"fif the grout and in accelerating the setting and hardening of the concrete. Thei-- rat<vf>t heat liberation is determined principally .by the composition of the cement.;O Thus, at three to seven days, the heat liberated from one cement may be mure than

.double that liberated from another cement. The (VV evolves heat more rapidlythan the other cement compounds; the amount isjoO^t greater at two days than thatof CiS. The heat evolved from the C'-jS and CiAFFis relatively small. From the aboveinformation, type IV and type II cements have been designed (see below).

If a portland cement clinker is ground without the addition of a retarder. itsinteraction with water is usually rapid, the temperature ri.-es sharply, and a flash set

r690 CKMKNT

occurs. This is due to the rapid hydration of C:!A. accompanied by cry.-tallix.ationof the calcium aluminate hydrates that conueal the paste, (iypsinn, added as aretardcr of set. reacts rapidly with the dissolved aluminate in the presence of cal-cium hydroxide to form calcium sulfoahuninate, 3(1aU.Al3Oa.3CaSO:.:>lHj<). The

aiVd- CC• is*-'--*""" ——- T» «T.—«v~TO«jr——e;

" --•"•—-'—-••••—-paste to congeal and so th<- set i.- retarded. An excess of ttypsum is detrimental tothe concrete and hence the amount permitted in cement is limited by all specifications.The optimum gyp.-ii'm requirement can be determined (!)), and is found to be influenced _not only by the CY\ content but also by the fineness of the cement and its alkalicontent. Tests on strength, expansion in water, contraction in air, and other proper- ~ties all indicate that the best results are obtained when the optimum amount of algypsum is present. fe

The durability of concretes in sulfate waters has been found to be determined n;j largely by the content of crystalline C3A, which reacts rapidly with soluble sulfates sl

to form calcium sulfoaluminate hydrate. This is the same compound that is formedby the gypsum in the retardation of set. The significant difference is that, in retarding 1,set, the reaction takes place before set occurs and so does not affect the volume of the irpaste a/fter set. But, when a hardened concrete is attacked by sulfate ions, the crys- ("tals form in1 the pores of the structure with a volume increase of about 227* , which £.may bring about a disintegration of the structure. -

Types of Cement vPortland Cement. In the United States, five general types of portland cement ^^ r

are recognized in the usual specifications. These are designated by ASTM Specifica-tion C 150-63 as follows:

Type I. For use in general concrete construction when the special propertiesspecified for types II, III, IV, and V are not required.

Type II. For use in general concrete construction exposed to moderate sulfateaction, or where moderate heat of hydration is required

Type III. For use when high early strength is required.Type IV. For use when a low heat of hydration is required.Type V. For use when high sulfate resistance is required.Types I, II, and III may be specified also to contain materials interground with

the clinker to provide air entrainment in the concrete. Such air eiitrainmcnt impartsto the concrete a greatly increased resistance 1o disruption under the stresses offreezing and thawing:. Other organizations such as the Federal Specifications Boardand the American Association of State Highway Officials, often write their own speci-ficationsjjnjt these normally conform closely with those of the American Society forTestiriljTthd Materials." The cements produced in other countries often includevarion&rtypes of "quite different character, as noted in the sections which follow.The specifications and methods of test also vary widely, which demands multipletesting for international trade. Effort is now being made by international testingsocieties to normalize the testing procedures..

The principal chemical requirement.- for portland cement- of the live typos arcshown in Table 1. and tho phy.-iral requirement.- in Table 'J. The typical oxidecompositions of several types of cement are given in Table 3, and the potential com-

Vol. 4

pound compositions in Table 4. The equation

fFI

s emplnyodl for

CEMENT

the calculation of

691

thepotential compound composition arc as follows (10) : j

C,s =c.s =C'jA =

C.AF =

Table 1. Principal

silicon dioxide. c,'(, minaluminum oxide, f£, maxferric oxide, fj, maxmagnesium oxide, 9c. maxsulfur trioxide, rc, maxwhen CjA is S(. i or lesswhen C»A is more than .Sf

loss on ignition, ft, maxinsoluble residue, r<, maxCiS, c't, maxC;S, c ', , minCsA, r<- max

4.07CaO-7.(JO>it):,-0.72.,S7Si()5-0.7.54C;1S ,2. Go AM'i-l.Oli Fe2(>,:V04Fe»(\

Chemical Reciuirements for

Type I Type II

21.00.00.0

5.0 5.0

i:,5 2.5~i 3.0

3.0 3.00.75 0.75

. .8

•2AM),-.I.4S1-V,

:lfEf

Portland Ceinciit

Type lit;^

[5.0 I

•3.0 ;4.0 ;3.0 ;0.75;

!, 15' '

< )»-•-'.; soSUs

, ASTM C 150-03

Type l\' . Type

a

6.5• 5.0 4.0

2.3 2.3

2.5 3.0

V

0.75 0.7535407 5

" The CiA shall not exceed 5?c, and the C4AF plus twice the amount of CjA shall not exceed 20? .b When moderate sulfate resistance is required for type III cement, CjA may be limited to 89c.

Wlien high sulfate resistance is required, the CjA may be limited to o%.

Table 2. Principal Physical Requirements for Portland Cement (Abbreviated), ASTM C 150-63

fineness, fin- ft. minturhidimeter:iir permeability

soundness, autoclave expan-sion, Ci . max

minimum time of set, Vicat, minair content of mortar, vol fc,max

c-ompressive strength, psi, min"1 day!! days7 days28 days

tensile strength, psi, min°1 dav --.-Bgeisfc_ . " -sasese:— *3 days ^ w .•" 1 '"1 aa\s . v,.?/28 days "xf

heat of hydration, eal/g, max7 days2S days

Tj-pe I

10002.SOO

O.fSO45

12.0

12002100350O

150275350

Type II •

1000•JSOO

0.8045

12.0

1000isod3500

125250325

70SO

Type III Type IVf '; 1000[ ' 2SOO;

It. SO ', 0.8045 I 45 .

12.0 j 12.0

1700 '3000 ;

1 soo; 2000

275 ;375 •

F , — -i !<;>1 • 300\

'

1 -

Type V

1(5002SOO

O.SO45

12.0

loOO3000

• >sn*.Ov'

325

" The purchaser slKMil<l specify t.lie type of strenjith test ref|iiir>>d: otherwi.ip, the rompressivestrength lest shall govern. The strength at any age shall be higher than the strength at the nextpreceding nee. I'nless otherwise- specified, the strength tests fur typejs I ami II cements will lie madeonly :il '! and 7 days. f

SR30525Q

(i'J2 CKMIiNT

Table.'!. Ts p.- ii ('.. i.p..^n ions of V-in.. H (Vmmi

\l i

portland. type I, ' , 21 ".\ ii II J 7portland, type 11, ', U2 :( 17 1 '.'<

Portland, typo V, ', 25 0 :! 1 J.xPortland white, ', 2." 5 5 'i (I iinatural, ' ,' 2:> 7 II Ushigh alumina, ' ', •">.•! •!'.! 1 l . l i

" Nut, determined.

Table 4. Typ cal Potential Compount

C,,S (V* C..A .

portland, type I, '.', 45 27 11Portland, type II, '"( 44 31 5portland, type HI, < , 53 HI 11portland, type IV, ' t 2S 4!) 4Portland, type V, Tc 3S 43 4

r.:: -j j u is i :; n _•tin i j :, -|7 us n i

ill i i M i ii n ii ii L!I'M (I II 1) 1til ."i I n 2 2 :; n:;:; ."i I :; o I n i s

Composition.- of Various Cements

C.AI-- CaSO, Mgi) |.'reeCu«)

S :i i 2 !> 0.513 2.. 2,.-. 0. I '!t 4.0 2.0 0.712 3.2 l.s O.-J9 2.7 !.!> 0.5

Other types of cements are recognized on the basis of composition, character-istics, or uses.

White portland cement is one in which the iron oxide is so reduced in concentrationand the manufacture so controlled that the product is practically white. This requiresthe selection of limestone and clay of exceptional purity. Unusual precautions againstiron contamination are necessary throughout the operations in the plant. Slightlyreducing conditions in the oil-fired kiln and rapid quenching of the clinker tend tomaintain the iron in the ferrous state; thus, the intense coloration produced by ferriciron is avoided. In France a white cement is made from an iron free clay-limestonewhich is burned at a relatively low temperature. The product is a hydraulic limeinstead of a portland cement, and the free lime in the product is partially hydratcdbefore shipping. White cement is of special importance in architectural concretewhere a pure white is required or where brilliant colors are sought by the addition ofpigments or colored aggregates.

Colored cements are usually made by intergrinding o-lO^c of pigment with whitecement. The pigments must be unaffected by the constituents of the cement anddurable in the exposures to which they will be subjected. Iron oxides arc used to givefed, yellow, brown, and black colors, manganese dioxide for brown and black, cobaltblue for blue, chromium oxide for green, and carbon black for black. The addedpigments?, sometimes with inert fillers, lower the strength, and a fading often occursowing to the formation of a film of calcium carbonate on the surface. Kor thesereasons, many advantages are realized in attaining color effects in concrete by the useof colored aggregates; the product is known as exposed aggrpyaic concrete.

Cement paint is a dry powder consisting of portland cement interground withpigments, fillers, accelerators, and water-repellent agents. A white color is mostfrequently used, containing additions of titanium oxide or y.'mc snlfide. The fillersare usually hydrated lime or siliceous material. Calcium chloride is used as an ac-

Vol. J , ( J'MF.NT

celeratnr, :uul calimmi^or jjluiijimmi^-tearah's ;is water-repellent asy-nN. ('t-nieiiti;i "•. ~" paints are u.-ed principally lor painl ing concijete ami concrete ()i'oilucr^.

•••—:— (til-ii'i'H criiiriil i.- a cement i|c-iji'iiei| for cementing casings in deep'wells and lorsealing the wells after drilling. A particular coinposition is required to keep the groutfluid for a period of several hours al Icmperatures up to 17-VC and pressures up toIS,000 psi. The slow set under these uuu-iuil conditions is attained by reducing the('.iA nearly to zero or by the use of a rotardoiiadapted for that purpose. Such ivtardersmay include starches or cellulose products, sugars, or acids or acid salts that containthe HO'C-Hgroup. I

___ Hydraulic Limes. U'hen a limestone is1 heated to SO PC at at mo-phcric pressure.flpciirbonation occurs with the formation of lime.

CaC'O:, — CaO -j- CO.;-___ _ . . I -l,-m. (.a() If the limestone is relatively pure calcium jcarbonate, the product, known a? fat or————— high-calcium lime, slakes rapidly with the evolution of considerable heat. When the

"" ' resulting calcium hydroxide is mixed with waiter to form a putty, it hardens in the air() - to form the carbonate, but will not harden .under water. However, it' the limestoneo;;> contains considerable clay, the heating below sintering temperature causes some coin-u.5 • bination to take place between the lime and the oxides of the clay to form compounds

that hydrate slowly but are hydraulic and harden under water. Such products,therefore, are called hydraulic limes. In France, the burning process in vertical kilns

character- sometimes produces lumps that are separated out and sold, after grinding, as grappiercement. .

centration Natural Cement. Xatural cement is a product made by burning below sinteringfcfrecmires temperature an argillaceous limestone or a mixture of limestone with flay or shale.sis IBnst This cement represents, therefore, a type of hydraulic lime. Xatural cement is madeSlightly in both vertical and rotary kilns at temperatures of 8">0-1000°C, with the use of

r tend to variously sized rock. The cement is not restricted in magnesia content, which allowsi by ferric the use of dolomitic rock. The calcined product is an unsintered, chalky, porouslimestone material, which is pulverized and sometimes partially hydrated before shipment.mlic lime This type of cement finds use in masonry projects that do not require the specialhydrated finalities of the portlands. Up to the turn of the century, natural cements wereconcrete . produced in much larger quantity in the United States than portlands but, since thatdition of time.they have been relatively unimportant and their production has been very limited.

Masonry Cement. Masonry cement refers to an end use in mortar for bonding•h white brick and masonry, and not to any particular composition. Many kinds of cementing"itt and materials have been used in masonry cements including, as reported in 1934, hydraulic! to give limes, hydrated limes, natural limes, blast-furnace slag cements, and cements whose... cobalt compositions could not be positively determined, as well as portland cements with»• added . and lthout admixtures (11). At the present time in the United States, masonry'.i occurs ~o.eirf6fi.ts consist mostly of a finely interground mixture of portland cement and lime->r these stone, given high plasticity and water retention with an air-entraining agent, andthe use regulated in setting time by gypsum. Serious failures have sometimes resulted from

the blending of portland cement with incompletely hydrated dolomitic lime, expansionnl with lioiiii"' caused by the delayed hydration of the free magnesia. The use of carefully's most hydrated lime or a low-magnesia lime has been recommended to avoid this problem.• fillers Pozzolan Cement. Extensive deposits^ occur in many countries of variousan ac- geological formations which, from I'oman times, have been used successfully wirh

A R 30-5252

69 1 CEMENT Vo| ,

limes or cements to produce structures having superior qualities. Thosi; defxi-ti-are referred to as po/zolaas and are defined (iL'.i us "material.- which, though notcementitious in themselves, contain constituents which will •.•ombiiic with [hue aiordinary temperatures in the presence of water to form stable insoluble compoundspossessing cementing properties." When intorground with portland cement clinker.

"" _. ,, ^ .--g- .(sciBi*!»CTa«uj-T«aMg'»«feaa» ^ • ^

AR305Z53

ns' IncTude thTTvoTeanTc tli$s*~of Italy, Sonfonn earth of Greece, tra.-sof Germany. Humania. and the U.S.S.R.: rhyolitic tuffs and diatomaceous earth- ofthe United States, the siliceous sedimentary rock of France called "gaizc," and many -phc <.other rocks. Artificial pozzolans have been made of clay or shale burned at temper- -'('aO \1atnres of 600-!JOO°C'. fly ash (finely divided ash from coal burned in boilers), a. mixture . •>\fn'0 S:of crushed brick and burnt clay used in India called "surkhi." and from other mate- Qrilrials. Granulated blast-furnace slag has sometimes been described as a pozzolan but 0|- (vl-t;i'the slag is better classified as a "latent cement." .Many minerals have been identified pa.-ti-. itin these diverse materials, and various explanations advanced to account for their ,IS conticementing activity. In many cases, the active mineral seems to be a hydrated silica, helpful iopal, rhyolitic glass, or an amorphous phase haying a gel-like structure and high ju tno \internal surface area. • fide),

Pozzolans are used in making concrete both by Intel-grinding with the cement and modulusby direct additions to the concrete mix. It is only when the two materials are inter-ground that the mixture can bo referred to as pozzolan cement. The concrete pro-duced could be identical in the two cases and would bo referred to as a pozzolan con-crete. In the United States, considerable concrete has been produced by the additionof fly ash or other pozzolan at the mixer, but relatively small amounts of pozzolan Raneeshave been interground with clinker to produce pozzolan cement. The proportion ofpozzolan so admixed with clinker has varied from 15 to more thai? 30%. In Germany,trass is interground with clinker to the extent of 30-50%, and with slag cement to theextent of 20%. In Italy, cements w.ith a 30-40% pozzolan content are widely usedfor all types of construction. When employed for sea water or sulfate water construc-tion, a clinker of low Al:0:1 to Fe.jOa ratio is used for greater resistance to salt actionand, when employed for general construction where high early strength is required, •a clinker of high C3S content is used.

The value of pozzolans lies in their ability to react with portland cement andwater to form ccmentitions products. The chemistry of that action has been studiedextensively but has not been fully resolved. It is accepted, however, that the criticalelement in the pozzolan is an active silica which reacts, with the calcium hydroxideliberated during the hydration of the cement. This is the recognized criterion of In the 'pozzolanic action. The products arc complex gel-like hydrates of the calcium silicates. \ ariety'aluminates, ferrites, and sulfates that are present in the paste. Slat

The addition_of a pozzolan to portland cement reduces the early strength of the latticoncrete but thai loss in strength is largely regained by the end of a year. The heat some pievolved during hardening is reduced, but resistance to expansion arising from the i-orro-io"alkali-aggregfl|0 reaction" is often greatly improved, as is the resistance to the •"•>'"/corrosive actionof sea and sulfate waters. or anhy>

Slag Cements. It is common practice in some European countries, but rarely low dryin the United States, to intergrind portland cement clinker with granulated blast- e-pecia!furnace slag to produce a port land blast-furnace slag cement. For this use. the slag Anmust be chilled very rapidly as it comes from the iron furnace by quenching iti a large 70'',' el

Vol.4 ; CEMENT 695

excess of water or by subjecting the slag to high-pressure jets of water and air. Theobjective is to cool tho slag so rapidly that cry.-ta!l./:ltion i- prevented and the productis a supercooled liquid or glass. Tho lightweight i'iioth or sandliki- product that re-sults is called granulated slag. The composition "vane* considerably but. for usewith cements, usually falls within the following percentage raniii-.

4(i-.-iO MS;I > [

The compounds present will vary with the !oxide composition, but include2CaO.AlAs.SiO-., 2Ca0 rgO.2SiO.-, 2CaO.SiO,; CaO.Sin. ('ai >..\|.t >..-jSi< >...

- 2.\fgO.SiO.,,, CaS, and others. I . '•Granulated slag alone has no appreciable cementing action.but in the pre-ence

of certain activators, such as Ca(OH).. and CaSO., always pre-eiit in portland cementpaste, it shows marked cementitious properties. This is the characteristic slau activityas contrasted with pozzolun action. Certain composition ratio.- ha-ve been foundhelpful in evaluating the quality of a slag, and limitations are placed on certain oxiile.-Ifi the United States, limits are specified as follows: .MgO, "/"' : SO., •_'..">', : S iassulfide), 2%; and Mn-A:i, 1-5%. Also, the slust should conform to the followingmodulus: . ;

C:il > ~ M«< > ±_"jLV-'_!:- ;>

The amount of slag permitted to be interground with the portland cement clinkeris 2">-G.")% in the United States, and not more than 0")% in the United Kingdom.Ranges permitted in France and Germany are as follows r

France I Slay. %'' Grade B ciment artificial 10' ciment de fer | . 20-30

ciment metallurgique . r . ~>0ciment de haut forneau- , (".")-7">ciment de laitier an clinker SO

GermanyPortlandzement 30

; _ . wEisejiportlandzement 30-70• s f'JHoehofenzement • \ . " 70-85'

In thefiJI.iiS.R,, 1;"-20% of slag is normally interground with clinker, and a largevariety of staiidards allow for all types of product. ; -

Slag cements ha\-e lower early strengths than port-lands but at a year may equalthe latter. The heat evolved on setting is relatively [ow, which has given slag cementssome preference in mass concrete construction. They have superior resistance tocorrosion in sea and sulfate waters and have found :'a\;or in such exposures.

Xuix-rxulfnlril cement ({'•'<} contains about S0r' .-lag interground with !.">% gypsumor anhydrite and ">% portland cement clinker, ft has a very low heat of hydration andlow drying shrinkage. It has been used in Europe for mass concrete construction andespecially for structures exposed to sea and sultat'- waters.

An i-.r/iiin-lini/ n mi nt has been developed in France (I Ii. made by ititergrinding70'',' clinker with •_'(!', <lasi and III'(' so-ca'!ei| "Calcium -itKoah'imiitate cement."

AR30525I4

fiflO CEMENT \ol. 1

The latter component s prepared by burning a mixture of .">()' c nyp-uin, 2.""% bauxite. ixychloiand2.}jlrqj| '''|^

! ^ i j i ! i ^ ! t ^ ^ — - • » * — • = " — r " - • - • - • • - * = « * . - = - ^ ? r ? i r T h i 'concrete. It has been used on bridges and tanks for the .-'to rage of water and gasoline. turpeiir

Iron Ore and Bauxite Cements. In (lei-many, iron ore is sometimes substitutedfor clay or shale in portland cement and the product, high in ferric oxide and low inalumina, is known a- et-z cement. It is slow in setting and hardening but highly re- ,,sistant to salt action. A somewhat similar product produced in France and Italy is ' ' • • ! .known, for it- inventor, as Ferrari cement. Another product made witli bauxite sub--tituted for clay or .-hale, producing a cement high in alumina an I ferric oxide but . ,low in silica, i.-known as baiixif land cement. . ,.Tcr

High-Alumina Cement. This product, known also as aluminous cement and. •• lit cin Knrope, as ciment fondu, is not a portland cement. It is made by fusing a mixture of ', • .- <()limestone and bauxite with small amounts of silica and titania. In Europe, the process . .is usually carried out in an open-hearth furnace having a long vertical stack into which . ,ci,,.itthe mixture of raw materials is charged. The hot gases produced-by a blast of pul- 5.verized coal and air pass through the charge and carry off the water and carbon dioxide. , ^Fusion occurs when the charge drops from the vertical stack onto the hearth at a -pi mutemperature of about 142.~)-1~>000C. The molten liquid runs out continuously into • .steel pans on an endless belt in which the melt solidifies. Electric-arc furnaces also vjrecnt-have been used where electric power is cheap. In the United States, the mixture isburned in a rotary kiln similar to that used for portland cement but provided witha tap hole from which the molten liquid is drawn intermittently. The black solidifiedsinter is dumped onto a storage pile from which it is transferred to crushing and grind-ing mills where it is reduced without additions to a fine powder.

The cement is composed of about 36-42% AI-Oj, about the same amount ofCaO, 7-18% oxides of iron, 5-10% SKX and small amounts of TiO->, MgO, and alkalies. , uu- ferricA number of compounds are formed, most important of which (15) are CaO.AUOs,6CaO.4Al-A3.FeO.SiO... 2CaO.Al-A3.SiO:., and ferrites. The setting and hardening areprobably brought about by the' formation of calcium aluminate gels, such as CaO.-AIa0.i. 10HA. 2CaO.AlA3.8HaO. and 3CaO.Al20,.6H20. "'" Th

One of the notable properties of high-alumina cement is its development of very . ,high strengths at early ages. It attains nearly its maximum strength in one day, " . .which is much higher than tho strength developed by portland cement at that age. . .At higher temperatures, however, the strength drops off rapidly. Heat is also evolved °rapidly on hydration and results in high temperatures; long exposures under moistwarm conditions may lead to failure. The resistance of the cement to corrosion in ^"i",.!'sea or sulfata watpr i, as well as its resistance to weak solutions of mineral acids, isoutstanding. The-'cement is attacked rapidly, however, by alkali carbonates. An "l '"„"important, nsrv of high-alumina cement is in refractory concrete for withstanding . ,temperatures up" to 1 *>00°('. For this use, special procedures and refractory aggre-gates are necessary. Refractory concretes are used for foundations of furnaces, kilns,coke ovens, etc. A white calcium aluminate cement has been produced in France .which, with a I'jHed au'ttreuate of pure alumina, will withstand temperatures up to

Magnesium Oxychloride Cement. This product, also known as sorel cement, is "",obtained when mau'iie-ia i- mixed with a solution containing about 20% magnesiumchloride. A reaction occur- with the evolution'of heat and the formation of magnesium

8R305255

Vol.1 CEMENT 697

o.xychloride, 3.MgO..\lgt.'l3.1 j HA- The product is hard and strong but is attackedby water. Its principal use is in magncsitc flooring, -applied in admixture with aninert filler and a pigment, and protected against water by polishing with wax andturpentine.

Manufacture ;

Raw Materials. The raw materials for producing portland cement containlime and silica as principal components, and alumina ami ferric oxide as fluxingcomponents. Other oxides will always lie present in small amounts as impuritiesin the rock but are not essential, and .-till other components are proscribed beyondspecified limits because of harmful effects induced by them. The chief sources of limearc limestone, cement rock, chalk, marl, shell residues, and blast-luruace slag. Thechief .sources of argillaceous material are clay, shale, slate, cement" rock, and blast-furnace slag. The chemical limitations on the limestones are highly exacting andpreclude the use of a wide variety of rocks which may In-suitable for other purposes.

Specifications for the five types of portland cement carry limitations which arebased on the chemical composition of the raw-materials and method of production.The multiplicity of limitations narrows to a restrictive range the permissible variationin chemical composition of the raw materials'. The chemical requirements (maximumpercentages) of the limestone for the five types of portland cement arc listed below,on the assumption that the rock is the whole raw material, and that it has an ignitionloss of 36%, which is an average figure. . The. limitation on alkalies may apply whenthe cement is to be used with reactive aggregates. •

Type I Type II Type HI Type IV Type Valumina 4.8 3.8 4.8 3.8 2.6ferric oxide 3.8 3.8 3.8 4.2 2,6magnesia 3.2 3.2 3.2 3.2 2.6alkalies, as Xa-A 0.4 .0.4 0.4 0.4 0.4

The remainder of the composition must consist, essentially of lime and silicain such proportions as are needed to meet the requirements for C3S and C2S. Xotconsidered above is a practical and necessary limitation in phosphoius pentoxideof about 0.2%, which rules out phosphatic limestone. The limitation of S0:i in the

• cement to 2.~i-3.0% excludes limestones containing 'appreciable, concentrations ofgypsum. • The 1'initation on magnesia is especially restrictive because of tho commonand extensive occurrence of limestones containing dolomite. In the United Kingdom,flints form a valuable by-product and arc .sold mainly for ceramics.

Preparation for Burning. Two general procedures are in use for the preparationof the kiln feed in cement plant operation, known as the "dry process" and the "wetprocess." In the former, the raw materials an- ground in the dry state, which maynecessitate a special drying operation before entering the mills. In the wet process,the raw materials are ground with water to produce a slurry of creamy consistency.When chalk is a raw material, it is often di.sintet;rated;hy violent agitation in a washmill from which the finely dispersed overflow is conducted either to a mill for furtherreduction, to a h\ ilroclone for further, separation of oversi/i- particle-, or to tankswlieie it is m'ixed with a clav .-lurrv. The residue- in the wash mill consist maimv of

•AR305256

fi!Ks <'EMENT

f Scale Shale, randstone. ore

Proportioning scalesRakeoverflow

TTT. ^Feed-weight scales ' TT TTrf > To packing

and loadingFig. 4. Flow shc'c-t for wet-process portland cement pl.-int (courtesy of Reinhold Publishing ^

Corporation). V• tl

flinty material varying in size from fine sand to large logs. These arc cleaned out ^eperiodically and are usually discarded. '°

In both processes, the limestone is extracted from the quarry or mine by the biusual processes of drilling and blasting or, if sufficiently friable, by shovels or scrapers. ccLarge blocks are crushed in one or more operations to 1- or 2-in. pieces which arc """hauled to a storage pile. Other raw materials are treated similarly except that, clay semay be extracted with power shovels and require drying, but is frequently made into ''"a thick slurry with water by violently agitating in a wash mill, and pumped to storage a""tanks. Material from the several piles or bins is then withdrawn in carefully pro- P.'-portioned ratios through a weighing machine or table feeder to form the mill feed, and 'sdeposited in the bin supplying the mill. In the wet process, water is introduced as the c'°mixture enters the mill or, if a clay slurry has been prepared, this slurry is introducedto the mill at that time. :l'

Cottoil joF'.the composition of the clinker starts in the quarries with systematic u'-core drillings and selective quarrying in order to utilize the deposits economically. ^eRock i ygycr of identical composition throughout the quarry and it is common pruc- *''tice toliEleet the supply of rock going to the primary crusher so as to give a first ap- • ^proximatloh of uniformity. Hut the first major control point is at .the proportioning 4-oi the several raw materials as they enter the mills for grinding. As the mixture passesthrough -these mills, two objectives arc .-ought: line grinding and uniform intimate I"iMixinsi. If either of these conditions is not fulfilled, I he clinker will be of nonuniform 'composition. The present practice of grinding in closed circuit effectively prevent* ' '''l In-out (low of grains larsi'er than a pre-crihed sixe. '"

Vol.4 CK.MKNT 6i><)

Fig. 5. Typical portland cement kilns in a modern plant.

When dry-ground material leaves the mills, it goes to a series of correction bins,the function of which is the final precise adjustment of the composition of the kilnfeed. The CaO concentration is established on each bin and some method is employedfor the homogenization of its content. Material from two or more bins is then com-bined in the kiln feed bin in such proportions as are required to give the designedcomposition. Wet-ground material may contain an excessive amount of water whichmay be reduced by filtration through rotating drums covered with filter cloth, or by aseries of hydroseparators, classifiers, and thickeners. Sometimes separated size.fractions are treated by flotation in a series of 'cells with a fatty acid and a frothingagent for the purpose of removing undesirable constituents, such as mica, quartz,pyrite, or feldspar, and increasing the CaO concentration. The bencficiated materialis then recombined with untreated fractions and the mixture is pumped to a series ofcorrection tanks.

The water requirement of the slurry for effective handling is sometimes reducedand the fluidity increased by the addition of 0.0.~>-0.109c of certain agents, such aswaste sulfite liquor, sodium carbonate, sodium silicate, sodium tripolyphosplmte, ortetrasodium pyrophosphate. From the correction tanks of established C'aO concen-tration, slurry is pumped into the kiln feed tank in such proportion as to give thedesigned composition. A typical (low sheet of a wet-procbss plant is shown in Figure4. •

Burning. The heart of a cement plant is the kiln where the hnely ground andprecisely composed dry feed or wet slurry is ''burned" to portland i-t-ment clinker.The rotary kiln is a steel cylinder usually l~>0-f>flO ft or more in length and S - I i ! ft indiameter, lined in the different sections with different types of refractory brick. \modern kiln i.- shown in Figure ."i. The kiln i- set. at an inclination I mm the hoi-i/ontal

r 700 CEMENT ~ Vol.

of a few tenths of an inch per foot and rotated at a speed of about I rpm. Fuel in the mnform of powdered coal, oil, or gas is blown in with hot air, heated by passing over I'"'clinker in the coolers, at the lower end of the shell: the raw feed is introduced at a ..__ _ . "}"•!"'

'i' i rn {&if' •' ">•'.'*i'»»'.-n"*"''r V'1*' la-e'''-~J:'iJsC'«tf'''*'iM;la.jm"F't--"py--- '•«••?" •"*"—— -i-j-'fur— **»* - ., - ..w-- -»•— •*••• -* — s- •~<*rviKV~xf. t'- ^ _ js~-. iw *-•*-" ' ••*•* »*&.»* iis*.

STe .'ij.r ud .Thii.bur.n'iig.zoiifi.is xuaxJJie discharge end-whew—-. - ----- —-——temperatures from 14.~>0to IIiOO°C are reached.

The rotation and inclination of the shell cause the feed to travel slowly down-ward. Water is first evaporated, with the aid in wet-process operations of varioustypes of heat exchangers. A bank of steel chains in the upper section of wet-processkilns is the commonest kind of heat exchanger. As the charge is heated to highertemperatures, organic matter is burned out, sulfates are decomposed, and chlorides lf °and alkali salts are partially volatilized. Most important, near the middle of the l'|1(-'1kiln, the calcium and magnesium carbonates are decomposed with the liberation of "vh''carbon dioxide and formation of CaO and MgO. In the burning process, about a tad'.third of the original dry weight of the feed is lost. In the hot zone, about 20-30% of carrthe charge is converted to liquid, and it is through this medium that the chemical tnereactions principally proceed. In the U.S.S.R., it is common practice to use water *necooling on the outside of the kiln at the burning zone, which favors the formation of a . lmnthicker protective coating of sinter over the lining. tne

The lining serves two important functions. First, it insulates the shell from the su"'burning gases that otherwise, in the burning zone, would distort or melt the steel. cau~Second, it serves as a medium of heat transfer between the burning gases and the "Mraw materials of the charge. Different kinds of brick are required in the different t(':rsections of the kiln. Ordinary refractive types may be used in the cooler sections but, ^^in the burning, zone, alumina or magnesia brick are usually employed. The former rcontain fluxible components that tend to fuse the individual bricks'into a monolithic S1"esheet and permit a coating of sinter, essential in kiln operation, to be built up. The an"magnesia brick are more difficult to bond together and a coating does not so easily m"°form on them. To offset these disadvantages, metal shims are commonly inserted n'£nalong one face of each brick, usually extending to the kiln shell. During operation, 8lvethe shims become partially melted and partially oxidized, which aids in the fusing an^together of the bricks, in the formation of a coating, and in the production of a morerigid lining. - Pl'°P

The cement-burning process consists of a series of reactions that take place ''"'between the finely divided grains of the different constituents, and between these a''01solid grains and a liquid. Hence, successful burning depends not only on composition rna<but also on the physical and chemical condition of the components. The time of 'iU''<passage through a kiln will vary with the length, pitch, and speed of rotation, but *"*' 'may be from 2 to 4 hr._ ('n:u'

The calcium carbonate dissociates under atmospheric pressure at 8i>4°(". The tn'vclay minerals begin to decompose at about 98()°C with the formation of -/-alumina ''"'"and mullite, 3AM>s.-Si()-j. The initial interactions begin before any liquid has formed, '""''but liquid formation takes place at the surface and extends into the grains only by r'u%diffusion, which proceeds very slowly. Interaction between the C'aO and SiO^ be- -^ '"conies appreciable at 1 HHrC and is rapid at II'TOT with the formation of ('.jS. in thepresence of liquid which first appears at about. l2SOr('. Interaction of the CjS withmore CaO to form C..S is slow, even at higher temperatures, but the presence ofalumina and ferric oxide increa.-es markedly its rate of formation. If the temperaturefails to exceed 1370"C, only small amounts of CV> will be formed and the C-S will

Vol. 4 CEMENT 701

invert, from the active beta to the inactive gamma form with a large increase in vohmve.This inversion and volume increase are referred to as ''dusting." The; action is dis-astrous to high-quality cement because the dusted C«S Is without value in concrete.Some of the reactions in the kiln are exothermic and soine are endothennic. It hasbeen reported (1U) that the heat of formation of CsS is exothermic between 1300 and1900°C. On heating below or above that range, the C3S dissociates into CjS and C'aO.The heat of formation of C.iS from CaO and C«S has been reported to be 143."i cal '»'(17). ;

The clinker drops from the lower end of the kiln onto a drag chain which dumpsit onto a clinker pile, or into some form of cooler where its temperature is quicklyreduced. The early coolers consist of cylindrical steel shells provided with lifterswhich raise and cascade the hot clinker through a draft of air. Integral coolers at-tached to the kiln below the burning zone are frequently used. More recent designscarry the clinker on a perforated grate through which air is forced. In either case,the hot air is drawn from the cooler to the front of the kiln, where it is reintroduced withthe fuel. From the cooler, the clinker may be taken to a storage pile or transferredimmediately to the finish mills where it is ground with a small amount of gypsum forthe control of set. The finish mills are sometimes sprayed with water to keep themsufficiently cool to minimize dehydration of the gypsum, for such material mightcause the cement to have a "false" set. This is a rapid stiffening of the cement pastedue to the hydration of the dehydrated calcium sulfate. .The cement is finally trans-ferred to storage silos for shipping either in bags or in bulk.

The vertical or shaft kiln was used for producing limes and cements before theadvent of the rotary kiln, and continues to be used extensively in many countries out-side of the United States. The vertical kiln is favorably known for economy of fueland space but the product, during the earlier operations, was generally found to beinhomogeneous and -undependable, and the kiln was found to have low capacity andhigh labor costs. In recent years, improved designs and operating technology havegiven it a new status of reliability. It is frequently used for limes, natural cements,and portland cement. . ,

The usual practice for making portland cement includes the same care in theproportioning and grinding of raw materials that is eoinmon with the rotary kiln.The conventional procedure for burning requires the introduction to the raw mix ofabout 16f anthracite or coke in buckwheat size. The irregular concentrations ofcoal ash at the spots where each lump of coal has been burned was one of the reasons forlack of uniformity of the clinker, and has led to developments in the intergrinding ofthe coal with the raw materials. Further problems, resulting from contraction of thecharge during burning and the pulverization of the clinker on the grate, have led tothe development of funnel-shaped sintering zones and improved types of grate.Those radical changes, either singly or in combination, together with the use of a dishnodulizer for the preparation of the feed, have produced marked improvements inthe uniformity and quality of. the product and the economy of the operation (18).A burning temperature of about 14.">0°C, a fuel consumption of about ~>'2(>,000-(i.'O.OOO Htu bbl. and a production capacity of about 1300 hbl 'day have been re-ported. A procedure for burning in a vertical kiln with oil or gas is being developed.

In (iermany a xiutfr-ijrnlr prticexi* is in use which consists of a moving perforatedsteel grate about li'-j ft wide that travels a distance of about 40 ft. The pelletizedfeed is spread on the grate over a l-tyer of1 fine clinker/ and passes under an ignition

flR305260

702 CEMENT Vol.

hood where a burning oil flame impinges on the surface. The hot air is drawn through fabrthe charge by a vacuum under the grate, created bv an exhaust fan, which raises .

&rtfET?r"ii"Tni"?~T'n*T!i>e air". ToifoweTTlTj'r crushing and grinding into cement with the Typusual addition of gypsum. The heat consumption is reported to be about 82/5,700 unitMtu bbl at a capacity of 2300 hbl 'day. grea

The rotary kiln is notoriously wasteful in fuel consumption, usually requiring onlymore than 1,000.000 Mtu bbl of clinker. Many devices for conserving or reducing or terv.recovering heat have been introduced, especially in Europe. Outstanding among these rectidevices i.- the Lepol kiln, known in the United States as the ACL kiln. Here the exit convgas is used to lu-at a layer of pelletized raw feed that is spread on a traveling grate. out iBy a double pass of the gases, first through a heated portion of the mix and then contthrough the moist pellets, the fuel consumption has been reduced to about 700,000' Bttt 'bbl. The rotary kiln into which the preheated pellets are discharged can be costreduced in length by a half to a third, and the dust entering the stack is reduced to specabout l^c °f the weight of the clinker. In Argentina, the process has been adapted rawto wet-process operation (19). com)

The Humboldt prcheater was first introduced in Germany but this equipment wou;is now found in many plants around the world. The process provides a number ofcyclones through which the kiln exit gases pass before reaching a battery of dust . but •collectors. The dry feed enters the top chamber and falls consecutively through each sitiv*cyclone, swept by upward-moving gases; the dry feed is heated to about 7oO°C by thetime it reaches the kiln. Claims are made for fuel economy, increased production,low manpower, and conservation of space. .

The Mieg process allows the exit gases and dust from the kiln, at about 7oO°C, com!to pass through a slowly rotating drum containing heat-exchange members such as is bu4 X 4-in. cast-iron rings. The slurry is poured slowly into the drum; by this process to prthe water may be reduced from about 30 to 7%. ' • thec

The Vickers desiccator consists of an enlarged section at the feed end of the kilnin which a double screw is attached to the shell over which the slurry passes on enter- p|lfiing the kiln. The screw is followed by a section of similar length containing chainswhere the diameter tapers to normal. In-this desiccator the water of the slurry may beilr-creased from about 40 to S J.

The Holderbank heat exchanger is an installation of lifters which raise the charge a v_and cascade it back through the hot gases. By means of a row of guide vanes, a vortex desigis produced that .increases the rate of flow of the gases. A saving of 21% in the fuel nrol<consumptiorYTiasKeetf claimed. ir.lliu

In sofflTrltifopoan plants, a. practice known as insufflation is in use whereby kiln ' fi , t.dust is nTf§!|Jjw]i|.h JHi£ powdered coal and blown back into the kiln through the fuel r(ispipe. f' jjPJftt-—way, blast-furnace slag and coal slack are introduced with the burnfuel. A fuel conservation of SfJ" has been claimed. In other plants, dust is returnedto the kiln at a'point jifst below the prcheater.

Dust Collection. One of the startling developments in cement plant operation is .1the removal of dust from the plant by collection in glass bags. The coarse fraction is n-ii-iusually collected in a cyclone and the fine fraction is blown into glass bags through anexhaust duct. A typical installation has been described (20) which uses 1280 silicone-impregnated. idas,- fabric bans, each ll'_' in. in diametel' by 2"> ft high, arranged in iten sections of 12S bags each. Mag cleaning is done by reversing the pressure on the

AR30526I

Vol. 4 CEMENT 7031

fabric, each section being cleaned automatically .in. a timed sequence. As each bagcollapses, its dust drops to the bottom and is removed by a scresV conveyor.

Automation. Automation is making rapid headway hi the cement indu.-try.Typically, a sensing device operates a relay which transmitslpower to the neee.-saryunit by which the desired change is effected'to precisely the extent required. Thegreat difference between manual control and automation is that a man senses the needonly for gross changes and makes relatively large corrections at relatively long in-tervals, whereas-an instrument can sense minute changes anil make very small cor-rections at very short intervals. There has not yet evolved. Jiowever, any definitiveconventional highroad for cement plant automation. Kach 'interested plant carvesout its own direction of effort and applies new principles of phtnt operation or productcontrol or economy to meet best its particular needs. i

Linear programming may be accomplished by means of ja computer; the lowestcost combination of available raw materials that can be combined to give a mix of anyspecified composition is given. Assuming eight different raw materials and a requiredraw mix with specified percentages of SiO-j, Ala03, l'V.j();i, Cat), MgO, and alkalies, acomputer may solve the problem- in 10-30 min, whereas a mau with a desk calculatorwould require several days (21). '•.

Progress has also been made in rapid chemical analysis by x-ray sperd'ography.but this innovation still lacks full development. The instruiiient is exceedingly sen-sitive to many variables, and absolute constancy of conditions is demanded for com-parable results. But difficulties are being attacked and gradual introduction intooperational control is being made. [

A number of processes have been used in a limited number of operations for thecombined production of portland cement with other products.; Anhydrite or gypsumis burned in a rotary kiln with the required amounts of clay, sand, iron ore, and coketo produce portland cement and sulfuric acid. The calcium sulfate is decomposed bythe coke by the following reaction: ;

2 CaSO< + C —• 2 CaO ~ 2 SOj + C( >• !The gases are drawn out of the kiln and the SO-> is .recovered for conversion into sul-furic acid. The CaO interacts with the silica, alumina, and ferric oxide in the residueto form a portland cement clinker. , [

In the lime-sinter process for producing alumina, a CaS-rich residue is obtained asa by-product. Limestone and kaolin or bauxite are burned together in a rotary kilndesigned to fire from the upper end so that the cooling clinker will be subjected to aprolonged annealing. This causes the CaS in the sinter to invert from the beta to thegamma form, which pulverizes the sinter. The calcium aluminates are extracted fromthe "dusted" clinker with a solution of sodium carbonate and sodium chloride, leavinga residue rich in C*S which is then available for making into portland cement by re-burning with additional limestone. ^ [

When limestone Is burned in a rotary kiln with iron ore and.coke, together with therequired additions of sand and clay, molten iron is produced; which can be run outthrough a t'ap hole. The residual clinker can be processed into portland cement in theusual way. This operation is known as the Bassett process. |

Uses ICement is an intermediate product in the fabrication of niany types of material.

These include principally concretes and mortars and products made from them,i

Vol. -t CEMENT 70")

Table 11 u-mi/i;/»<r/i

Korea. Nc.rtli I'{.!i3lKorea. Republic -1,032Lebanon . 5,04ScstdMalaya. Federation I, MMPakistan S.170Philippines 5, (533Saudi Arabia SOISyrian Arab Republic 3,.312Taiwan 10,5)00Thailand 5.«4<iTurkey. - "3,.V.)7Yiet-Nam. North 2,ToCu-sUl

Tutt.il • 371,25sAfnc'.t

Algeria 3,81lesldAugi 'la 950Congo. Republic ' UuOestdEthiopia ' 240Kenya ' 2,02'.)Morocco 4,003Mozambique 1,085Nigeria . ' 2,814Rhudesia and Nyusaland, I'uUcmtiuu of 2,345Senegal 1,073Sout h Africa, Republic of 15,591Sudan 498Tunisia 2,128Uganda 328United Arab Republic (Egypt) 12,606

Total 50,547Oceania

Australia 17,197New Zealand 3,700

Total 20,897World total estd 2,095,654

Bibliography"Cement, Structural" in ECT 1st ed.. Vol. ;>, pp. 411-438, by R. H. lioguo, Portland Cement Associ-ation Fellowship i Portland Cement): .1. L. Miner and F. \V. Aahton, Universal Atlas Cement Company(Cak-Uun-Aluminate Cement); and C't. J. Fink, Osychloride Cement Association, Inc. (,MagnesiaCement"}, _ ' ~ '

1. IT. VT. Lesley. History of the Portland Cement Industry in the United Stales, International TradePress, Chicago, New York, London, 1924, p. 35.

•>. CK A.Uankin and F. Ii. \Vright, Am. J. Sci. 39, I (1915).3. R. H. Bo«ue, Tlir Clumiatrii of Portland Cement, 2nd ed., Reinhold Publishing; Corp.. New York,

llt")5.4. I!. \V. Nurse. I'HIC. Inti-rn. Hi/nip. Client. Cemenl*, 4t>>, Washington, 1060, p. ',t.:.. H. tn*ley. ./. Hi*. .V.i//. /<»/-. Slrt. 17, 353 (i!)3t\).«. S, MrunaiiiT. Am. Sc((ii<i'.v/ 50. 210 t.l-11'-'1-7. T. C. I'.'Ui-rsamlT. I,. Hmu'iiyard../. .li». I'ttncrrle lust. 43, '.177 i,!"''"'-.S. I1'. Si. Le:i;iu<lC'. II 1 )esi-h, Tin ('himMnj n! ('vini'ill unit.Cmwnti. revised eil.. I'.ilward Arnolil

Ltd., Lniidon, I'.l.'ili, p. l(>5.

'AR305263

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710 CENTRIFUGAL SEPARATIONi

9. W. Lerch, Am. Soc. Testing f^faier. Proc. 46, 12.VJ (.'IfMfij.10. L. A. Dahl, Rock Prod. 50 '.Jan. 1947). j11. J. S. Rogers and R. L. Blaine, J. Res. A"«(". liur.XM, 13, SU (I!I34).

._ ._. i. i nil, jj i i ijiiniM«'«nir..,P,!p.iiJHiiiJLiii j i'Tii " 11 i " , j n 11. .iim.ni LI. ILL.

.a -s.A-y*a -:w-»i ^14. H. Lossier. //it;. Cii-ils France (1948). i15. T. W. Parker. /"/•or. Intern. Symp. ('hem. t'nnrntss, Jsd, London, 1952, p. 485.

I 16. W. Jaiider, 7.. Angt >.r. Chnn. 51, 696(1938). |17. (). K. Johurmson and T. Thorvaldson, J. A m. Client! Soc. 56, 2327 (1034).18. R. Ironmun, Rock Prod. 65, 81 (Aug. 1962). K. Spohn, Zemenl-Kalk-Gi^ 11, 1954. H. 11.

Hughes, Mining Engr. (Dec. 1956). !19. ,1. Bniso. Rock Prod. 57, 6S i,Dec. 1954). '20. E: Meschter, Rock Prod. 62, 103 (Sept. 1959). '.21. LeRoy Weeks, Kock'Prod. 63, 85 (April 19601 122. Minerals Yearbook: Cement, U.S. Bureau of Minus; 1962.23. Bo Xikander, Rock Prod. 63, 102 (June I960;. :

General References

Proceedings of the International Symposia on the Chemistry of Cement: Snd Stockholm, 1H38, RoyalSwedish Inst. for Engineering Research, Stockholm. 193t); 3rd. London, 1U52. Cement and ConcreteAssn., London, 1954; 4th, Washington, I960, Government Printing Office, Washington, 1962.

F. M. Lea and C. H. Deseh, The Chemistry of Cement and 'Concrete, revised ed., Edward Arnold Ltd.,London, 1956. I

R. H. Bdgue, The Chemistry of Portland Cement, 2nd ed., Reirihold Publishing Corp., New York, 1955.11. W. Lesley, History of the Portland Cement Industry in the United States, International Trade Press,Chicago, New York, London, 1924. i

Hans Ktihl, Cement Chemistry in Theory and Practice, tranSl. by J. W. Cliristelow, Concrete Publica-tions Ltd., London, 1931. [

ROBERT H. BoorsConsultant to the Cement Industry

CENTRIFUGAL SEPARATIONTheory of centrifugal separation in liquid merlin .Classification and applications of centrifiination equipment.Gas centrifugal separation.........................L .....Nomenclature.................r,..........Bibliography..............................

122!)555658

In centrifugal separation, the components of a mixture are separated mechanicallyhy accelerating the material in a centrifugal field which operates on the mixture inthe same manner a* a gravitational field. The, centrifugal field, however, can he

.; vartgS af required hy changes in rotational speed or dimensions of the equipmentwhereas;, gravity is essentially constant: accelerations of L'O.fXX) times gravity areobtainable in commercial centrifugal ion equipment and up to .''('0,000 0 (d is thoratkfof centrifugal acceleration to acceleration of gravity) in laboratory equipment.Most centrifugation equipment is intended to separate immiscible or insoluble com-ponent? from a liquid medium, although the ultracentrifuge and the gas centrifugerepresent special cases which establish separation gradients on a molecular scale.The usual gravity operations, .-uch as sedimentation or flotation of solids in liquid.-,drainage of liquids trom .-olid particles, and ,-tratification of liquids according todensity, are more quickly ami completely .icrompli.-heti in a centrifugal Held.