EFFLORESCENCE - MECHANISM OF OCCURRENCE AND POSSIBILITIES OF PREVENTION

Dr. Peter Kresse

Bayer AG, Krefeld-Uerdingen, West-Germany

&mnary

Taking concrete paving blocks into special account, the author discusses the mechanism of th formation of primary and secondary efflorescence and presents test methods of reproducing th phenanena on laboratory scale. All possibilities of reducing efflorescence practically boil dOol! to the production of a compacted as possible, waterrepellent concrete, the cement/sand ratio, th water/cement ratio and the curing conditions playing sane role. The use of additives as a reliabl means of obtaining a concrete stable to efflorescence is regarded with great scepticism. Only th impregnation of the concrete surface with a plastic resin emulsion has proved a certain success The paper concludes by explaining hOol! efflorescence disappears in the course of exposure t, weathering.

1 Chemical principles

Efflorescence is an unpleasant phenanenon familiar to any concrete expert, but to a lot. of. laymen .as well: hazy whitish layers to thick white crusts, spoiling concrete surfaces. Fran the purely optical point of view, they becane increasingly visible against darker backgrounds. Which is why they are a particular problem on coloured concrete; on the other hand, they are un­noticeable in white concrete.

The chemism behind the occurrence of efflo­rescence can be described in only a few words: as the cement 'sets, free calciun hy­droxide is fonned, this being soluble in wa­ter, even if only to a slight'extent. 'Conse­quently, it can migrate to the concrete sur­face either after already being dissolved in the mixing water of the fresh concrete, or through the hardened concrete when exposed to the effects of rain or dew. Having reached the surface of the concrete, the cal­cium hydroxide reacts with carbon dioxide in ,the air. to fonn water-insoluble calcium car­bonate. The following equation represents this mechanism:

It is not the water in the concrete which mi­grates to the surface and entrains' the calci­um hydroxide; rather, the calcium hydroxide diffuses up through the water-filled capilla­ry system of the concrete to the surface, where it reacts in accordance with the above equation. The precipitation of the calcium carbonate reduces the calcium hydroxide con­centration, thus creating a concentration difference in relation to the interior of the capillary system. New Ca(OH)2 is sup­plied to the surface. Efflorescence occurring during the hardening of the concre­te will be referred to as "primary efflores­cence", that resulting fran the weathering of the hardened concrete being referred to as "secondary efflorescence".

324

2 Test methOOs

Investigations of efflorescence on concret are made more difficult by the fact tha there were for a long time no reproducibl test methods. Today, effllorescence unde natural weathering conditions can easily b detected by brightness measurements on th concrete surface, using an electric reflec tance photometer. Flat, extruded concret slabs are best suited to this purpose. The are best coloured with iron oxide black i order to increase the contrast. Carbon blac pigments are not resistant to weathering an would falsify the results.

OUtdoor weathering, which primarily cause secondary effloresence, is time-consuming Consequently, there has been no lack of at· tempts to simulate these natural occurrence: in the laboratory. Primary efflorescenc. during the initial phase of hardening can b reproduced well, for example by compressin~ a slightly moist mixture of cement and sili· ca flour into specimens and allowing theSE to harden under different atmospheric condi· tions. In this case, ,the resultant efflores' cence can again be detennined quantitativel~ by way of brightness measurements.

The puddle test can be carried out on bod fresh and hardened concrete, as will bE shown later on. In principle, it involves ap­plying a puddle of distilled water to the surface of the concrete. The surface is sub­sequently assessed in relation to the forma­tion of efflorescerice after the water has evaporated. we have recently started tc surround the area to be tested with a perma­nently elastic sealing compound.

Efforts to simulate natural weathering pheno­mena in the laboratory probably failed for the most part through doing too much of a good thing: if concrete samples are constant­ly sprayed with water, the calcium hydroxide appearing at their surface is immediately washed off and cannot settle in the form of

1: CbOliog plate "ith ooncrete nx>firg am ther.oBtat

pdnclple of pttducirt} efflOlCesoence the flli.ni.Jn.a po6S1Dle """",lit. u( .... Le~ , ..

abo taken into IIOCCO.Int in the other meUlod ~19P'ld t<j 1.18 . In thi8 case . a s t ad< o f concrete pawrs is spra.yed ... 1th distillecl ""'­ter for 2(1 Olinll t es every day . a"oOI! in the omnirt} an::! lOgIIin in the !!'Venirg. The Vdter pe,.tn<,tes into the stadr. and is act I"" bet_n the OC>:'lCrete !aye... vi r tually all day Long . the top l ayer. on the other ha nd . layer. on the other hand . dries oot very rapidly, """""'l.Jently al'4YS displaying far less efflorescence than the layers nel"", (cf. FiO. 2). The test du ration ahould be bo;>t_n 9 and :4 days. n.e metlx:d s!rou!ates ~ract1cal conclitions very wall , since most p>:Obl_ witll of UOZ'<O~ en """" .... t .. pave .... probably QCCUr wdrg IltorS<)lt of the pallets i n the vard Ole on the Wildll"(! .Ite.

3 PrlJ\\ary effl.oreacence

h\;) shall """ turn to primary et florsoo"" ... I t '" .olw.ayc tI:UQ that v i"lhb ",fflo~ eM only occur i f the capillaries in the oon­crete are filled with _ter to the ""t}' tq>. Cnly then can calc:1un hydro. Ide read> the lIurface .

11>ill applies <q.Jally fOle l:oth fresh an::! hard­e nee! ooncrete. HC:IWC""r, If the cuw::rete dries rot near the ... rflOCe ... i th the t'&SI.Il t that the capil laries are only filled with ..... -t et to a level bel"", the surface of the c0n­c rete. a reaction will t ake pl ace i nsioo tile """"'tete between the caldun hydroxide and the cal"b::rl dioxide In the air. A li<jlt 2:01"18 bel"", the surf...,. can ~sioN.l1y be lleltn on fractuNd COr'ICrete surf"".!S. Figure 3 de­scrloo~ U ... d[Ol.-..nonUcned "ituotlOO"l in oc ...... matic tom.

b. R"'""", .,..1., 1 ..... 1 In Ih. capillary (drylng-out ot cone .. lo) : t .. nlf>O<1 ot ca .. ion. 10 lIMo .... 11"'. If Inhibit ...

Fi.g . 3, _tion of pria:l<Y e ffloo:eso.noe on cr:ncr.te surfaoaa

Therefore. since prlmory off loresceoce can only OCCUr i f tile capillar:es a re full to t he l:ru. with _ter <1Jrirg the initial ~" of hardening. gra<1lal drying will pn:xnote e ffl<XOSOl!nce . In o:>ntrast. a cl imate 1""," irg to rllQld drying o f t he CQ)Cre te i8 un­likely to cause efUorescenc'30 n.... decisive [""t=.., In thu. oonto~t a ... r>Ot; OO"I ly tNo ,.".. lath.., hunidity and t""",,,rature , wt alao air ~t. the test spe<:1mens of oem!nt arrl silica flour descriDed earlier on we ... rna1a here every day for 1 1/2 yea n< and allowed to harden In the C9"n. The differen­ce in brlohtr.ess In ~rhon wi th the re­ference ~les without efflorescer.oe ..... wbsequently .......... red . PIg\lt'& 4 stx:ws the results: particularly gnnot briQhtness differences , i. e . prlnl/l ry efflore~ , i n the Autumn months.

• • •

• .- -. <- . . y •••• , .. -.- ,., • : .. , . •.• '-,"" '!,'" " r

-.I-:.,..c;,,~,:'-':'--- ;-"~'::;';-~:"'c'--:'-.:,----;.;';-;-:,----;.-,.;-'-:.-"" ~,.-T-"':;,-;.-'c~ -- -

- - -Fig . t, Prinary efflo>:'esoor>ee am """ather oomi t i ons (the h ighe( Ry. the higl'er the degE_ o f etf~l

_ lorg does priOl.llry effloresoenoe occur on cor.orete? To fir>::! til ! .. rut . the pOOdle tllst 'oWl perforned every hco.Jc on t>oO series o f SpeeUoens <.Older dtffenll'lt Nroening con(li­tiOO$. Fill" re 5 aho:-.! the oosults, Ir""""",,­tivo of ..t ", tl'le r MC'd&ning took p!_ lit 28 ·c aod 95 , .:.1.4tivo hunidlty OC 2(1 · c and SO , rd .. tl~ 1" ... 1dity - virt ually no =re efUonSO(lJlO& occurnld in the puddle test Ilftu 8 hco.Irs . W!I ct>t.lo ined simi Llor .­sulta with C<:InC ..... te r:oofirt; tiles. III t his

-"",~'" .,

~.

_~"""'" H --. --"' .. , . wit • ..,.

- " .. . ~ .. .. .. .. .. Fil;j. 5: Rlddle t.e6t.s as funct ion of. a:>O ",u,te .,. ce.uatioo of efflorescence I n t hot p.dcIle te:rt after mote than e ho..H:s due ele ly to t>tog .... ""i .... l\o<d •• 'drq, 0<" doo~ ,,,.tt.>c<1 01-""ide react . 'ith t he c .. l c-I .... hydt'QX ide on the surf_ ruting th is t..... that the e.!O­pill.orles of the conc:rete ato Soe<Iled off at the surface? To a rt5'oOGr t his questioo , 'o(!

alla...ed the p" .... ('8 to .... rden in nit..roQen, I.e. without CXl<: . The p.dcI l e t est then proved ~itive even Afte" several days . '!he aouont of <:artQn dio>elde required to _1 o ff the caplllaries c:an be -determined. 1b do so , 'o(! AIlco.oed concrete pavers t o harden In sealed crAtes ::>C<'ltaining a "'I'Ilg,oo qua.nt l t ly of solid c-arixn diOlCide . It ""'" fOUr<! that as little as 0 ,5 .g <X>:2 pe r pave r , correspord­Irq to 270 "'1, suffice largely to t>re ..... nt ef­flo~tIC:Je (eI. r io . 61.

----, -.r.:.

" 10 . 6 , Puddle t eo!;; .... t-Jn.::U" " of Cl.'2 po IOenOI! during curing

The resul ts cbtained to (late are parada: <::&1 : wh ile CO:l is r>eQ!uary tor eUlon <:ene:<!, i t .. leo does its best to prevent j If the ..... is II fila of ..... ter en the freah '" <;re t e , ll>eaning that cal<;iU'n hydr:odde c ttSCbpe frail the mClRh$ :>f the capillad "rw:I re4Ct with cartx>n di:xdde oYer the • ti re ... a"face of the conc:rete, prift\/lry effl resoenc:e occurs; I'Iawew... if the ~i takes p~ In the ""'-' th!! of the e.!OpUl.o les , they a"" r apid ly c-lcwed I:y the ""l c­... ""rbonate formed ar<:I the f~tion of po: .... ry efUoresoena. oanes to a !\alt. Th is (: be vl .. udheo _n o::o::rete I" ""red i n t presence of ~, 1. e . In air , and In the ~ sence of 0'l2. L e. in a pure nit~n a .....,phere. Figure 7 shows the electron mkll graph of the 9Jrfaoe of M ex trude<l oon<:re' roofing tile which underwent curl rg In e cl: matic dwlt>er at 60 ·C arC 98 , .. . h. , ..nell lIS Figure 8 eh0w9 .. ~ile fran the samo bit t< which se t at roa.o tmlpera:.ure In a nl t rogt a t:no::lephe .... . AplIn f ran a f ..... l a"'JQr f.O<"e . the surface of Figure 7 i s !>'l>C"h snootht than that o f Figure 8, .... _e re the fIO.tths ( the capil l a ries are obYioosly ro::>to c-l OJgt! due to the .. bsence of 00,. Therefore , whe ther prl ... ry e f flOl'escence b C-/IO.IeOO C ['I....".",t.-d doop<;-n<b on the QU<U,tLty aM tilt" of occurrence of the t...:. &)ents r equ ired fa e fflorescence. aJ:2 and 'O'/Iter .

'Io- 8, £lectn'l'l Jlicr'cgrll(i> of a OXWlCt"ete -..rl1lOll, ~ in ~ o f 'll2 Sut this Ie not the only appar ent contradlc­ttcn d:lser....cJ in relatlcn to priJMry effloo:--8SCence: FI!]Urtr 3 $ha.oOO that calciun carbo­roate Is foomed iMide the CQ'ICrete If the ..,rfaoe of the latter <lries oot nopi<lly. If> this case , ~".tr . the capilluies are not _led ott at the surfaoe, bo.It at l ower layers of the ooncrete, meanil'O'j th!Ot e xtra­IIeooJg .... t"'" occurlng later on can cause pri­IIIIIry .. fflores:enoo. The calci,," hydroxido require:! th<,m c:ones fran the upper l ayer o r i~etely ha:deM<l OJ<>Crete . This 'hal'(X!M It the concrete I\l10...,.,,.,,, in .. ~u .... ." wi r<! oc while the cela t ive I-o.ni<lity is very low. COI'l­ccete PII""CS .. Uowed to harden in tho cliJn.:l­tic dlarrber at 95 , relative /'uOIi<llty 00 not react to the flU.Id le lest ~fte ... harden;ng .

Betonpflastersteine lConcrete pavers Zement /Sand 1:6, 5%8ayferroK 318 Pfutzentest I Puddle test

Sand : 60"10 Q-4mm /IJ.,. 4- Srnrn WZ 0.38

Sand ; S5't. 0-4mm 45 "10 4-Smm

WZ 0.37

mil ~t2schkht l Mth toeing

"" gs·/.d , "" 3O"Ior.l, Hiif t'-"!lICuring

Pig. 9 , Pl.r:ldle test as f...etlan of curing ...,..j i U .,..

~ve", the S¥ne paV9"" 00 ceact If a llc:ooe<l to harden If> the .Laboo:'atory atnoosphere with 35 t relative Iuni<lity lct. ?III. 9). A nillh­e r o:ellltive huoidity, rut In the presceflt:e of a s t rong draUQht, ~rated by a fan tex eK~le, can Ie&:! t o the SOJllIl resul t. 'the ...,vers p.\\JIII!nte<l with iron ""Ide black then 10:::1< ...,rtlc:ularly blad< a t the start , since there 4 00 calc i ...., carlxlnat" In the Il'nIths of the capillllries. th4 otIle ...... ise having a certain l1ght "ul, .. .. ffe<:t . ,.,...,.,..,r, t:.tw:.o to.­.;one aU the li<jlter when ex~ to the pu:ldle or dew test . If> other >oms, prevent­i ng pdmary e fflorescence can cause an In­crease<! level of seoordary e ::'floresoence.

4 Secxn:!ary effloreeoenoe

One rea..,., for the occurrence of M<n'>dary e fflorescence has ~lre~y been mentione<l: the drying oot o f the cor'IttI'Ote .. t too su .... face . A secor>:! cause can also be described in wry feo< >ord,,: If ~ ooncrete Is pXIl'l y ~ted to 9tllrt with, .... Ming that extra­neous water is rspi<lly ab$Orbed afte r harden­lrg, this .... tet can 11180 dissolve calc! ... ny­droxide aoo cllUSe e tUorescence . w..l"~ ir. C<.>r>:::rote with very <lOa rse p:>re~ can <lis!!Ol"'1 large quantities of calcl .... hydroxldll, thb resulting In sint ering. On the other haM , if tho cap i llaries are n.w""""'r. m<!an i ng

l:hIJt the .... tOlr can peneuatOl into the c0n­crete but oot. fb., thn:ugh it {noely, efflo­re~ will oxur """"r the entire surfaoe. In the CMe of (:OOrly OJIIV1>Ct.ed, p'loI'CU< cr>n­

crete, the"" Is li ttle point In ...... klrg a dlsUroctloo bot-. p";"",r)' aOO !I&OOndary e f­flo.esceooe, sl""" the _I'll"" of fOl"T'Mt loo Is vlttually identic~l.

A """'to difficult f~ct to ~"phin ie that any highly ~cted COJX: .ete, ~"" as l:hIJt in ccnc:.ete moB'"'! t iles , ..ti l"" tflactA I'Iegati­vely to the puddle test .. fter hardeni",) , ,.,.,.. sequently displays secor-dary efflorescence ~f ter _r~l In:JI"Iths 'o f _thorlllQ. tt <Xl­cu .... Irrespective of loc.otlon, even In tho desert cillllllte of MI..:ona, n.e r.wcl....., 15 .. 1-.... 1"1 reached .. ithin a yeor, and I t takes aoother year before It Is "",atheto<.\ ~"'''y again. AH~~ tM t tim<> , i~ I. vlr~v&lly

never seen "9"in, as re .. suromenta o ver a per k<:l of sb y<!ars ha"e <le<r\:xIstrated . "There ~re n ...... <"COJS Indlcatior>s that this 8OOC<'WdIIry efflorescence Is caused by the oontlnulrg hardenil'YJ of the ~t, which is knc:w atiH to t:e In prt>;JrefS ......., after _ral nr;Jnth. CUrillQ this pro:ess, calch .. hydracide is f~ aU the time , then rwctlOQ with car­ben dioxide In the .. ir . This theory at least corrIIl~tM with the fact l:hIJt ~ry ef­flore.,.;enoe is m lor>;J<!r obee""'ld ~fter t"" year:!!, that Is tc ""I' , after the !IIlttl llQ pro­(:(ISS of the cer:>ent has probably OCJT'E! to M end. the ' fac t l:hIJt """ere secondary "fflores­cence even OCC\IO in Ari:rooa ' lrdlcstee l:hIJt dew ""fflees for Its L'onMtlon, altho.qh not for subtIequem:.ly _thering It _yo f'i<;JUre 10 IOQIlln ...-rhos the <xcot'"C9I"OCe o f efflo­resc:ence as 10 function of the .... t.,.. .,.., car-00n diadde 9Jpply .

... '

1'i9. 10, Effloo:tS<:et.... .. fun:t.IOl'l of HlO and 00:z supply during euril'>',! of the ...... .... ete

S Ooo.o'rtea.-..~

"!he mechanism o f oecun:"eB::<! of prltna.ry ~M secondary "fflorescence has bcon oos<:ti bed In such c:lGtall bec~use it CM in itsolf pro­vide i nfO<lMtion on tKw e~ ten.si ..... .,.-o""ntiOil can be IIChloved. If ca<JSe<l by t he ~t hardenin;j of the concrete, there Is sa !ar

virtually no ""'Y at all of ~entlng 5eOJn­dIory efflorescence. n.e ?r<>lOtltr part of OOJr attention RUst ooocent rlOtOl on ~tirg. pr l_ ""''Y .fflO<"~.

S.I Concrete fOOlUlatian

J;:vcryth ir-o;l 'otlich i"{)!lil'S the perrre .. blllty of the concre t e , that is to say , ~avou rs the po­r.etration of ""'ter Into the hl.rdencd <X>I'>:;t;O­

t it , ft'OJst be avoided . Concrele consists of the IIOggregate lind the hartlene:l <:e:ent paste which birxls i t. In the fresh concrete, the llOtter, In turn, consists ct. corrent, the wry tine porticles of the a);Irlt'Jate, po8I!Il_ ble plonent.s, ... ter .....:I 10 cettain ...."..,t of ~I r OQ I<'Is. If the vol...,., of this paste is in­suffiCien t to fill <:O::'(>letel\' the ca"ities left by the "'.)9re<)ate after c;t;mpI>C tioo, the oonc...,~e b cert<oin tv I ... "" " lJi'.lh ""ter aD­~rption c~i>"City l ater on . If the =nent/ag­Qregate ratio o f the oonC('9:e is changed frem very rich to very l'X'r in cement, eHlo­reaoence Increases "ith dlldin! .... cer:>ent c0n­tent. f'igure 11 sh<Jws 10 $(Irles o f ccnc:re­te paver>! ..;tII oemenVIIOggre<pte ratios of I , 5 to 1 : 10, the Wit rat:.o also """il'9 to be d'IIInged for P"'X'essllY,l cusoos. n.e ef-

Fig. 111 Effloresoonoe &'I func:tian of mixing "lOt io ( .. M. v.l

flore.,.;enoe locreased "ith decreas i ng cemen t oontent, altl>a..ql thb no longer applies for the 1 : 10 mixing ratio. '1Ma XJnCrete is SO ]lOrOUII that, ..non sprlOye(l "ith ... ter, the __ tee drlOined thrOlJ9h, "'sl\,1'9 the dissolved. calc! ... 1>yd>"ClIC i de dvwo, ... oJ <.OJt. Th is plct".., and the ne>:t clearly 1l1"atr1OU that the top rQof of pavel'll in tre spray i ..... :est, ""Idl Is also at the tc:p in thf! plctuC1Oa , sh<::ws less effloreSC(!noe than the pa""l'S l""",r ~ in thll stack . Thi s t e s t llO.sted 9 d,ys .

f'ig. 12 .shows coocrete pavera wi t h a c0n­

stant <*!lent/aggregate ratio of I : 6, I1I t three different gra<linga ot the aggregates and W/C ratios ranging bttween 0 . 24 lOrd 0.38. At the very l ow W/C rattoo;, the vol...e of the hardened cement p.>ate _ OOIIIously not. ero::o.q, to fill wt the agg~te structu­re completely .

12: Q:ncrete p.!MIU wi th i ncreMing W!C lU"II:! thAe diffen>nr;. gradings of /IIO',IOt'8""

",..-_ "ftfJr _ina test .

In addition. the a::q:.act4bility or these .ilm •• IeS i s ""ry poor. In case o f ~ 0.24. the oc:nc:rate is 00 pot"t0.J3 that W<ltee drains thro.lQh very e~"i1y. thus leach i,," 8Ol<Jble calciU'n h;ojro>< ide ~r(ls 00 that thes(! pavers teem to hlave little efUoresceT'lC<! . It is the s.wre I*>e!'lCre1'lCOl as "ith "'i xin;! ratio I , 10 in the pmvi(;lle pictu~e . Aj»rt iron t.hege far too dry lIli ~tu"..". the pavers ab6crl>. with incr<Jasil'll) \o(fC. _l1er <pJantities of exlrar.ec;:ue _ter aro display cor(8S(lCndlngl~ less eHlor~nce In the :oprayiN;! test . The l!d<:litiC>'l of oc:nc:rate plas­ticizera <My be ber.ef1cial. if they produoa a ""'no .... oooe""'U. In oe,..ul. r.:-....... "uch agetI~ CllMOt be "".,..::tod to hIa .... ""'I poaiti­"" effe<:t.

5.2 ~tion. l'wIroenino /II'>d stor_

G:xld I!IIIC:hlJni<:al o::oopact.iQ'l Is essential. par­ticularly at the surface. It can frequently b!! ~a:ovod wi th the al6 of a fj"" facl....,. 1a~r.

I4entlon ha$ .lI. ready been .-Ia 0{ the dsk of the surface ex: the oc:nc:r<lte dryin;! (;lit dur­In;! hardening. Althct.l<)h this prevents the oc­~ 01. pri ... ry efflorosoenre aro yields particularly brilliant colours , it also In­<:r<l_ the lik<llihQod of seoon<lary dflo­«lllCer'l<;(l . Fi\O"lre 13 shc>oos the influenoc of various types of Mmcnlrg In mixtures with ratios of I : 5 ~r>d I , 6 . Very high IItr1\OS­phede ru...ldi t y dur1no hardeflirq produces the beSt results In the spraying test. fUr-

,

.. .. .. .. .. .. - ...

Pig . Il : Effl(ll'e:!lOOnQe .. function of curllY,; conditkn<

the r \""""dg"ticno _ ...... u...:l at .. t..nl1..t>­Ir>;! whether Irw:boc storage c#. the pOvers for as l.c:aQ 11$ possible hils tJnY "Uf>Ct C>'l s..t>ee­qvent efflo""scence. IlarO<lning ""~ carrlee OIlt at 24 · C a"" 50 , reh :iV<! luni~lty , if one case. II"" a t 28 ·c M<I 95 , "!ati", ttu.idity . in the other. '!'he ~ ... .... n subje<:t.ed to the sprayloo test a f tu L J. 7, 14 am 28 days. the res.>lts :!lI'aowing thaI the different ~torage periods had no e f­fect (F!," . 14). This contradic t s the ol~

' ." , , • , , I

., , , , • • ,

Ft,.. 14: Efflor<!SceTlOe as function 0 ' _U.., bel ief that exposure to rain. I"'ttleulad~ <).ld ..... the first f(IW ~ of hardening. Ls, p r ill9 CIIUse of efn~. Fathu. tho P!'''9re $hcW.d be prote<:te:l "9"-inst extr ... reou.s ""'te.- for aa l ong lIS they a" s tad<ed since . once water had .,enetrated Into t n. I""""~ RUcks, it harclly (l'/awr~te9 at all It I. <b.lbtful ..nether !ItIrink-.... /lPPiOO 0 pallets Is the best 8Olution , sl,..,. CQn

<lensed .... ter can .--In ...-.der the ..... awin for a considerable time. " sheet of pla.t! or strorY) paper place:! belcw the fim laye ot 1"' .... .-. co..a1d ~ better. since It no only affords the palLetOO st~ck rreehllnlca stability. t..Jt al so prevent~ tile penetntio of ...,ter into the ~tack , .tIile at the 8M time pra:r;>ting drying (Ft'" ' 15) .

5.3 Cerrent type

OJr experience to date hM S"","", that the type of cement is relatively uni!tp)rtant in relatic:n to the C<X:Urrence of efflorescence. '!he ce.oont in 1 m' of concrete should c0n­tain up to 100 kg lime, this <>etin;! as a ~e.servoir to surply ca(Ctl)2' :tn vi"", of this figure, it is surprising that efflorescence <Ioes rIOt =r fa~ =re frequently) afte~ all, a mere 0.1 mg cacoycm' Is sufficient to cause visible effloresce1'lC<l. That is equi­valent to cnly 6 g, referre<l to the surface of I m' of cOI'ICrete! ~ .... r, this also sOOws tNot minor fluctuatiC<ls in the oernent c~ition should te irrelevant in relatiC<l to the OCCUn-enoe of efflorescence.

5 . 4 unemlcal aoaiti .... s

The <lreMl of every concrete manufacturer is an additi .... "n!ch reliably prevents efflores­cence -.hen 00<Ie<I to the concrete . llhat could the effective pdociple of such a suootance be? In vi"", of the afurer.:mtiC<Je<l vast quan­tity of calci"" h.,.:lro:<lde poltentially avail­able for efflorescence, an additive capable of binding this calcl"" h.,.:lroxide ...;.ul(\ be lnoonceivable.

~regnation of the concrete with silicc:nes or stearates also holds no p<t:rnise of suc­cess, since they onfy prevent penetrat ion of water in it.! li<)lid st~te . These agent.! have no oUcct on tile conde~ti-:;fl of .... toer ""'­pour in t he c/lPillaries and pores of the cc<>­crete. On too cthe~ hand, the condensation of water vafXlUr 00 and in the surface of the concrete is of consi<lerable irrport~r>Ce fOI" secondary efflorescence, i n particular.

5.5 Spray-ooating with plastic dispe.-sions

Sea1ir>g of the concrete surface with transpa­I"ent, unplgrenteo:l plastic disrersions , base<! on acrylics fo. exaq,le, was rrore success­fol. The sealant film thickness should be so thin that the film COIltil'>g' is hardlyeffie­rent. Iletwcen 10 and 20 m' of o:JnCrete paver can be sealed ...ith I kg of a catmO<"cially av"naMe a1spe<ston . '1he thin tilm th i Ck­ness ensures that the concrete surface oc.es not re<nain stidy . NoI" does it gather dust and dirt, and co:;t.! are reduced as "",11. The dispersioo sh::<JJ.d te mixe<l with water at "

ra tio of I , 1 for spraying. On tile other hand, a sealing coat sl.>Ch "" that sno.", in Fig. 15 can only be applie<l in rrono-layer ' I'~sses.

Spraying of flat o:JnC .... te an aqueous acrylic <'fm..Il.sie<>

n:e results are illostrated in Fig. 17. The right-hand side of the jYIoto;waph sro.-s un­sealed blocks a f ter a short spI""ying test, the sealed blocks teing 00 tt.e left. The ef­florescence-preventing effec: is obvictls. :tTllldequacies of the concrete manufacturing pl.""OCess, hc<;ever, Cannot be !1"6de 900d l:<i ooaling . The best it c"n do is prevent secon<laty efClorescence, not that caosed l:<i peer a;;q:>/Ictloo of the 00nCrete .

!'"ig. 11, Spt""aying test: Influence of the im­pr-egnatiQ'l wi th aq.>eO.IS <>Ctyl Ie emul.sie<>

6 we"tlJedng away of efflorescence

~ efflorescence has occurred, it can only be removed l:<i ~cid washing. This sho.lld be <.lex .. ,..itl, <lilute<l acia, so.>Cl> as J ~ hyaro­chiodc acid. /lque<JJs solutions of organic " cids are c<:moorcially a vailable, arrl these are less risky to hardle . H is recarrnerrl-

to _ k the concrete In water before-­. Inca this .. ill prevent the acid pene­

to:> '-p into the oonc~te. Acid shcWd be followed I1f I<IMhlnQ with

can bo cleaned of ef­well by" such p::>et-treat-

~o huny, natural weatheri ng tile ~ as IIdd wash ing. cal­

"'I>Cts with the cartwon dicll<i -

~~:~~~~:~'~'~;oduallY fonning ealeh.rn hy-a _ter--.oluble pro<b:t . that efflo~ disap­

a cer tai n tlr.oe le t. Equa-

lC """"" co"<a'oto p_wu wi> i ~~ hIod seven! effloresceTlC(t after the SprllY­

t eet . Tho8y reqained thd r odglnal , TTUCh ooloo r made "fter 12 """ths of

.",;0:

Pig . 18, _thering away of e{flot'e9Ol!ooe

sfUoreeoence needs only be e~pecteo:'l to survive Iong&r in llrellS with very little rainfall , III! in A,rizonll, for exOfT9le . At lin other EIlCpoeurl! siOO5 used for cur ",,"thering tests In the Federal. Rep.j;>lle o f Ve"""ny, the Netherl"rds, oelqhn , Italy, France, sweden, ErqIand , florid!> a nd S<:o.Ith "trica, tho o(Oon>OOC""" .:01_:.0- d l .. PP"'"~ within II pedcd of t'oO yellrs . ~<QUS t rials h<ove alllO !Shown that ~ry effloreeoeooe (l'l\.y occu .. once .

1 Concl\lllion!!

The priNry .. tflorescence CCCIJri"ll during .h/I-edenlrQ ~ be attrit..ltable to the concre­te £OI'fII.Ilatloo , ~tlon or the hIoroenill\l oonditiona . 'DIe first ~rflqJlsite tor .u­nlaidnQ etflocescencoe i 8 " dens<! concrete ,.,id> abIIiorbe as little ... ter a!l possible a fter hardeniB,l. It ...... t allIO be e~ tt....t the freS> e<:ro::ret e does n<:lt. dry rut on the IlUrfaoe, t..lt al!lO thIIt ro ..... ter <X>I>de<,... $e.! ,,4> 10.:11 C>Juld drip onto t ho curh"", a t tho> concrete duri ,," the first few t-oJn of haed­enl,.... lila achie\1ed th i!l l:>f IIelltllY,l the ceil­I,... in OJr ho-edenl .... clwrt>er to prevent o::on-6e...,tl<:n of '''"''ter "lIPOUr at that point. ru .... thenn:>r<l. fre access of .. lr and, thus. .

"

carbon di<»c ide should also be ensu~ed wrirq In i t ial kIordening. n- CO'>I1 ldons help ,.J...:> t " .............. ",,~"'nt ~ry .. fflo­resoenoe . IIc)o,Iever. s ince i t is probably also a t trlb.lt;able to the ""tting of the cement, e process wh ich t a kes _~al """the, It oennot bo avoided enti rely . '!he only limlte<l chanoe i s to IIpply a traMPSrent plS!ltic: OOIItlng as SC<>I1 as the blC:CkS rone rut of the pnlss . f'enetrllticn o f wat er Into the n.aock of p"vers wring !ltor &Qe of the blcd<s before hying should be av:;.ided if lit all poeslble .