EFFECT OF MECHANICAL DISTURBANCE ON THE HYDRATION OF CEMENT SUSPENSIONS

EFFECT OF MECHANICAL DISTURBANCE ON THE HYDRATION OF CEMENT SUSPENSIONS

By

F. GROF

Department of Hydraulic Engineering.

Technical C niver"ity, Budapest (Rc('('iwd ?lIareh 31, 1969)

III tlw La!.uratur, of th,· of

• 1

Budal)cst Technical l:-ni\-ei'~ity a test ~f~rlC~ \\-a~

with shaft "inking. The problem ,,-as as foUo"w",:

Carl'lt"O

1)/' the out 111 connectioll In the course

or

^shaft ~inking often watpl' lwaring :"t1'ata 11<n-(' to he (Tossed. PUlllPing m ay not he economic a! or pos;.;ihle any more. In ~uch cases formation of a ,:u],Jl](']'gecl cem('nt plug at the giyen In-PI of the shaft uncleI' construction may stop \\-ater flow from lwlow: or at least reduce it to a rate that can }w handled hy continuous pumping. Aquiferuus cracks of the rock al'(' ':.'aj"d hy grouting through this hardened ('f;ment plug at the :::haft hottom.

Tts (Iualily strength and impermeability is primordial for the dt;sired result_ especially when an important water pressnre i" acting from helow

OJ] the cpment plug.

In gcrwral. the cement plug is prepared of a cement suspension eonn;,ypd through a yertical pip!' "ith.~r under pressure or hy grayity to the pit hottom Ulldpl' water.

In "haft construction practice. yariatio!1:3 in tll{, tluality of the (,f'I11,'11t

plug lurn, }wen ohservecL making determination of the neC!'5SaIT cement plug thickness incertailL leading to overdimt'l1sioning. In other eaSCE_ miEsed setting of the eement plug has also hecn ob;:;Pl'yerL though no harmful water- ehernical effects were prei'pnt.

Therefore investigation into the circumstallcP~ of cement plug p1'q)<11'a- tion and sptting hecame n('ceSEary.

Jlodel tests 5howed that quality variations of the cement plug prepan'd undn water were not independent of the motion during grouting. Cement hydration begins already during the motion period: but no setting can occur.

because the cement particles are in constant moyemcnt. There are two kind"

of motion: primarily the technological procedure of grouting inY()h-e~ motion.

then after standstill. cement particles in the grout settle. as an internal motion. Both types of motion may he con,:idpred as dii'turbances to setting.

228 F.GRUF

while the chemical process of hydration is left unhindered by motion and even intensified.

Beyond the actual tcchnical problem justifying the investigation, similar or analogous phenomena may occur in other domains of practice, disturbing the setting motion.

The most important among them are:

a) In grouting works, grout or mortar conveyed by pipeline. and thereafter during the ~routing process, is in movement for quite a long time, causing both hydration and development of texture to rather differ from those at rest.

b) In the "prepacked concrete" process, especially in mass concreting, the mortar is in motion until saturation of the individual sections. To determine both the volume to he grouted at once and the optimum effect of grouting under pressure, the relation between motion time and strength has to be known.

c) It is important as well to kno,': the efft'ct on ;.:trength of the mixing car time of ready-mixed concrete.

2. Disturbed setting phenomena

HydratioIl of cement is known to start immediately in the presence of water and to continue for a certain time. Hydrates of clinker minerals hegin to form on the cement particle surfaces just upon contacting water, then the hydration proceeds inward" at a gradually slowing rate. So the hydrates forming on the particle surface are gel-lilet' and are characterized hy a large surface. The, hond of the particles i.e. the strength relies primarily on large surfaces; namely the hinding forces are essentially surface forces. A further determinant is the increase of density in a paste at rest, namely the hydration products occupy ahout twicf' as much place as th\' compact cem\'nt ]wfon' hydration.

In concret<'", wh!T(~ particles are densely adjacent. 'with an optimum hydration water there is a strong hond hetween cement particles, the increas- ing hydration process is accompanied by increasing concrete strength.

In high water/cement grouts. however, the eement particles are "s,,-im- ming" in the carrier liquid: the indiyidual particle;; art' quite apart. Also here, the quantity of hydration products is increasing during hydration. but as the cement particle~ are far from each other. the cement sets as a low- density material and not eyen the ,,-hisker structure of the hydrate., deyelops.

The hydration hOlld between the cement particles deyelops here after a long time and in a rather loose form. This explains also for the wen-known phenom- ena of protracted :-ptting and strength decrea~e in ca;;p of high 'water!c(~mf'nt

ratios.

EFFECT OF ,HEr.H.'/.'dCAL DIST("JUl.,lSCE 229 A cement suspension at rest howeyer high its ",-ater/cement ratio sets sooner or later, because the cement particles settle and form a loose, but confined bulk: in this wav hydration bond_ /'Yl'Il if loose_ exists het'ween the particles_

Cement particles kept in motion show_ howeyer_ a phenomenon of a different tendency_ Gel formation on the particle surface is started by hydra- tion upon contact with water in this case too, hut agitation of the suspension counteracts the bond between the particles throughout the motion_ It depends on the duration and the eharactn of the motion, whether at the time of still- stand, gel formation continues on the particle surface, or it is accomplished_

If the hydration can p(lIltinut', then further bonds between particles may form. hut the hydration that ha::: oecurred during motion exhibits a minor cffect and tlwre is no suh,-tantial hardening hut as result of the final stage of the proccss as a rule. For a motion of a giyen extreme duration. the entire particle bulk may become hydrated without bond between the cement parti- cles. In such ca"es tllf' end-product i" a cement gcl-slurry without strength.

During motion of the eemen t particles. !'id(' effect" affecting the strength of the end-product may occur.

Motion. e.g. mixing inten~ifies "'wetting" hence hydration in fact, favourable for ~trength. Protraction of the process is, howeyer. of negative effect with ach-anced hvdration. lVI('chanical effect of the motion causes the particles to collide and inYoh'ps abrading. This abrasion "uses up" primarily the new hydration products. Remoyal of thf' gel. formed ill the initial stage of the hydration. i" known to be ach-antageous for tIlt' strength. A further result of tIlt' ahradin!! effect is, 11oweyer. to decrease the quantity of "hydra- tion products able to bond". Th(' abrasion effect depends on the type and intensity of motion. A motion of sueh an intensity could he conceived as to continuously grind the cement particles. i.e. to illcr(,<1se the specific surface.

resulting in increased strength. proyided it i5 not maintained beyond a stage of rather advanced hydration.

, Hence tIlt' f'trength of a cement suspension kept in constant motion depends on tht> gel t('xturf' and position. rather than on the hydration rate.

These circum"tances do not pro\-ide for the gels to penetrate capillaries, there- fore progress of the hydration is not accompanied 1Iy increasing density. This may be responsible for a low ;;trength.

Among mechanical disturhances of the cenH~n t suspcnsion setting, a type of disturbance. due to other than outside influences, namely to sedimenta- tion. a natural motion of the suspension. has to be considered.

The cement grout of high w/c ratio shows sedimentation at rest, i.e.

water bleeding. After standstill, tll(' partick,;: of the suspension - homo- geneous during mixing hegin to settle. The sedimentation rate of the parti- cle;; i~ proportional to the square of the particle size (Stokps theorem).

230 ^{F. GR()F}

During sedimentation the particle" are ill motion. though t hc suspension itself is already at rest. Duration of the particles' motion is a function of the rate of sedimentation and the path length. During sedimentation of the particles there is no setting hut the hydration process is undisturhcd.

During sedimentation along a long path. the particles segregate accord- ing to fractions, i.e. the cement particles settle in upwarcl~ decreasing "ize order in the space holding the cement slurry.

Sedimentation as a disturhed setting is a highly differentiated process.

when' also effects othe r than the rl1lOted phellomena haye to he taken into

eUll sideration.

From the Stokl'~ theorem it follows that in a concrete hardplled from a sdtlrd ceml'llt .-lurry. the particles settle according to fractions and so they harden. The enarser tll{' partidp. thp longer is the path al1(l the higlwr I;; ti]l' rate of sedillwntation. hut thp shorter its clur~,tion. This i~ al:,o true inYcrseh-.

n'snlting in a prolonged disturhance of :-etting. Thi;;: effect absolutpJy bear~

on t ht, final ~trength.

., Cl. of test~

The test program eonsi;;:tetl of thre(' test senes (all invoh-ing cel1l('nt C 500 ofTata).

The first test series clf~an-d np phenomena occurring ill the technological process of cementation of the shaft foot. Thi;: te:::t series of no genpral interest aimed at analysing an actual technological process. It was important hy having directf'd thi' attention to the phenomellon of disturbed setting and hy being starting point for detaikd tests of a more general character. Hf}wCYcr, sonw basic phenomena were already cleared up at this ~tagp, therefore major items uf thp test Eeries are pn~i'ented in the folIo-wing as an introduetion.

An original scale model representing a ;;:haft waO' tested in the arrange- men t seen in Fig. 1. Different density cement suspensions were fed by graYity into a \,-ater tank through an axially connected pipe. Another yal'iahle para- meter was the distance het,,-een the pipe end and the hottom of the tank.

rp5nlting in different grades of turbulence of the introduced 5uspension.

After the introduced cemcnt suspension scttled, specimens were taken to determine the 7 days strengths. Compressive Etrength data were proce;;:sed in function of thc spatial position of the Epecimen. Fig. 2 "hO\\-5 the spatial position of sampling, Fig. 3 the compressive strength in function of both the spatial position and the density of the sU8pension. Fig. J shows relation of the density of the su;;:pension to the 7 days comprC'ssive strength. omitting disturbances.

In thc first test series the relation between strength and spatial position of the specimen prohabilizcc1 the basic relation to be that between the strength

EFFECT OF lIECHASICtL DTSTGIW.·t:,;n:

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F. GROF

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120 Average strengtrs

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EFFECT OF ,11ECIIASICAL DISTl.CRKLVCE 233 and the time of setting disturbance by motion. a VIew supported by observa- tion of both the turbulence for different feeding rates and thl' time necessary for reaching the standstill.

To enhancc this cffect. a further test was madl~ by continuously blowing in air during 8 hour", through the bottom of the model, to keep the cement

;;:uspen"ion in motion, Fig. ;:; sho"'8 that disturbance by air bIo'wing decrease;;

the strength of specimens taken of th(' same place in proportion to the dis- turbance tinH'. Theorl'tically th('r(' ('xist~ a disturbance time where th(' strength if' zeroed!

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The second part of the test series ~tr()\"f' after a more exact iln'estigation of the disturhance effect.

It was desired to dear up the trend of phenomcna occurring in the cour,;e of disturbed hydration in fUl1ction of the kind, the intensity and the duration of motion, as well as to det('l'lnine tIlf> critical time of di,-turlH1l1CP where strength is completely zl'roed. 7 days strength was testt'd.

Types of motion used a" distnrhanc(' were as follows:

a) Constant air blou'ing: the grout was kept in constant motion by intTO(lucing eompr('~::,ed air. This motion. though of low intensity. caused agitation throughout the> suspension. Of the suspension in motion, samples were taken hourly to make ~ 3 specimen::, for the s(-dimcntation te:-'t::<.

Crushing strength was tested on 7-day cuhes of 7.06 cm edgp~. III tIw follow- ing tests, samples were taken and tested in the same way.

b) Air blowing; cvcles: the L(TOnt was brouzht into motion at deter-_{' - . / <.... 0.,;,..'} mined cycles by compressed air. Iik,> under !l). Three degrees of disturbanc(~

were applied: air ','as blown in for 2 minutes eyery 10, ~O or 30 l11il1ute~.

c) Continllous mixing: tlw grout wa" kept in constant motion by a

234 ^{F. GRuF}

motor-drivcn laboratory mixcr. Three tcst seric,; weri" made, the mIXIng

\'elocitip5 heing 1800 rpm. 360 rpm, and 60 rpm.

d) -"'fixing cycles: the suspension ,,-as brought into motion 111 gIven intervals by the equipment described under c), at a rate of 60 rpm; 3 degree!"

of disturbance werc applied, i.e. mixing for 2 minutps cvery 10. 20 and 30 millutps.

e) Sedimentation: thc cement suspension was poured into tpst tubes, without disturbance of the hydration~ and expo:3ed to disturbances according to a). b), c) and cl) in tUl"!!. Sedimentation of the suspemion was oh:3erved for 8 hour,..

Fig. 6. Effect of continuous mixing on crushing ,trcngth

For all tests ce111ent C 500 was used and the described tests were carried out on suspensions of t111"pe different densities, 50 that the densitv effect could a180 he evaluated.

Test results arc shown in Figs. 6 through 10, leading to the following- conclusions:

Disturbance by mixing not only impedes the bond between the hvdrat- ing cem('nt particles by ke('ping them in motion, but also ahrade" the forming hydrate film by successive collisions.

Air blowing essentially acts similarly as above, but the particle;; ('oIlid ..

with much less energy :30 the effect of "grinding" is les8 pronounced.

Cyclical application of the above motions aimed at eliminating tb,

"grinding" effect, i.e. at establishing the effect of "pure" setting disturbance,

frp\~ from any side effect.

It m a)' be concluded from the test series that in all four kinds of cli,,- turhance the effect on the strength is of the same order and tendency. For a

EFFECT OF JIECH.·LYICAL DISTURBA.YCE

disturbance of about 30 to 40 hours there is no setting, only a hydrated cement slurry develop". Strength lo"s is proportional to the disturhance time.

As it was expected, the more inten"e disturbance of 1 or 2 hours -- con- ceivable as wet milling resulted in maximum strength. This effect did not appear for cyclic air blowing involving le8s mechanical work, and was moderate for cyclic mixing.

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Fig. 7. Effect of eyclie mixing on crushing strength

The trend of intermediate strength values clearly reflects the effect I)f volume and intensity of the applied mechanical work: the more intense is the disturbance, the more marked the decrease in strength (mixing rpm, frequency of disturhance cycles).

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2 6 8 ;L 12 ;~ <6 '!8 2C: 2': r':- r::;:, 28 3C 32 ~0~rs

Time oir-blowinc;

Fig. 8. Effect of constant air-blowing on crushing strength

236 F, CHar-

The scope of the third part of the test sene;; was to investigate the sedi- mentation effect.

During the sedimentation process the cement particles being ll1

motion - hydration without setting occurs. thus. also this phenomenon can be conceived as a disturbed setting.

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The cIuoted experiment,- proved that a disturbed setting resulted in strength decrease. Moreover, after a certain disturbance time. the cement might completely hydrate without ,:.;etting: in:;:tead of a hardened cement stone. a slurry consisting of hydrate'd eeme'ut particle'S would rp,mlt. The

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tests also demonstrated inte115ity and duration of the disturhance to be relaterl to the final strength.

Sedimentation as a disturbance to setting gave similar results. Tests combined the already reported types of motion, applied before sedimentation.

An accessory feature of the mentioned phenomenon trouhling some- how it:3 effect -- was the circumstance, 'well-kno'I'll from th(, literature, that

increa~e of tilt' f(rinding fiIH:l1es:, of cement resulted hy itself in a higher con- crete ftrength. This meant ct'Ttainly an oppo1'itt' effect. as the sediulentation lasted longer for the fint'r cement particles and so did disturbance of the

~etting.

RelC'vant tesb \\·,'re de~tir!f:d to c[Par up the H'su1t[,nt effect.

Tlw test ~erief consisted in pourIng cement slurry of three different d .. ;; "it 1(-0'· (1.:::: 1.5: 1.7 pcm³⁾ into ^H cleaved plexi tulw of 50 mm tiirr. After a disturhing prdn'atment. the sedimentation proC(;E~ in the plf'xiglas,~ tube 2.50 m high has ^h(,Pll recorded.

After the Ce111t'nt slurry ha" set. the plcxiglas5 tube was opened, the cenwllt concrete column cnt into cylindrical specimens 75 mm high and tested for crushing strength at -; daYE. Compre!'siYc strengths Wt're (''';aluat(~d in function of the distance from the tube hottom.

Test results are sho'wn in Figs. 11. 12 and 13.

Protracted sedimentation can he stated to be analogous to mechanical di!'turhance and n,sults in a i'trength loss proportional to the duration of

t he process.

Summary

The le,t ",.nes cleart'd up settinp: behaviour of cement suspem;ions upon disturbance by motion.

The chemical-physical process of hydration of a cement suspension in miltion was found to continue undisturbed. and even accelerated by a more intelbive motion, setting was getting ever weaker. After the motion stopped, the setting begins, provided duration and intensity of the motioll are below a critical value. Even so, the strength is reduced in proportion to the duration and the intensitv of the motion.

In a suspension kept in motion beyond a critical time - 30 to 40 honrs there is no setting at all and a loose cement sh!rry structure, consisting of hydrated cement particles, develops.

The phenomenon of sedimentation inside the cement suspension in rest involves also distnrbcd setting. The cement particle motion dnring sedimentation has the same effect as an externally applied motion.

Test results explain for the backward phenomena occurring in shaft foot cementation (low-strength or unset cement plugs). At the same time, rnles concerning grouting, slurry conveyance by pipeline and prcpacked concrete emerge, of fundamental importance from quality aspects.

EFFECT OF .iIECHA.YICAL DISTnW.·!SCE

References

AGROSKI);, J. DL\IITRIEY, G.-PIKALOY, E.: Hydraulics.* Tankonyvkiado, Bp. 1952.

BOLDIZS.~IL T.: :'Ilining :'IIanual. IV." 2I-luszaki Konyvkiado, Budapest 1965 .

241

. H.von. A.: :'IIodern methods of pOllllllon shaft sinking. * :'IfT1. report, Tankonyvkiado, Buda- pest 196·L

KARSAI, P.: Deep drilling. :'1nl. report, Tankonyvkiado, Budapest 1968.

LUIPL, H.: Presmre grouting." }fTI. report, Tankonyvkiado, Budapest 194.7.

LITYAI. E.: Selected ('hapter" of fluid mephanics." :'IIT1. report, Tankonyvkiad6, Bndapest 1965.

:\"AGYP.~L, S.: Special concreting prohlem" in civil engineering." :'lIT!. report, Tankonyvkiad6, Budapest 1965.

:'\:E~IETH, E.: Hydromechanics." Tankiil1yvkiado Bndapest 1962.

PALOT.-\S, L: Building materials H. ~ Akademiai Kiad6. Budapest 1961.

PALO'Lts, L. BALAzs, Gy. -KILL~", J.: Practical guide'; on building materials. C niversity notehook. Tank6nyvkiado, Budapest 1965.

TRl'l'AK, :'\. A.: Cementation of cracked roeks." '\ehezipari Konyvkiado. Budapest 195';.

Z.bmo, J.: -'lininf'." -'!iiszaki Ki)nyvkiadr.. Budapest 19S7.

10 years of Hungarian shaft Sillkill~ and mine construction. Shaft Sinking: Co. Pllhlicatioll.

'Budapest 19'62. .

Spf'cifications for conereU, and mortar making:. '.' EDOK. Budapest 1962.

In HUJlf'arian.

Fen'nc GROF. Rest'arch Engineer. Budapest XI., "\Hlegyetem rkp. 3.

Hungar~'