Research and Development Laboratories

of the

Portland Cement Association

RESEARCH DEPARTMENT

Bulletin 203

Surface Discoloration of

Concrete Flatwork

By

N. R. Greening and R. Landgren

Reprinted from the

Journal of the PCA Research and Development Laboratories

Vol. 8, No. % 34-50 (September 1966)

@ Portland Cement Association, 1966

N. R. Greening R. Landgren

Surface Discoloration

of Concrete Flatwork

ByN. R. Greening, Senior Research Chemisf

andR. Landgren, Research EngineerApplied Research SectionResearch and Development LaboratoriesPortland Cement Association

SYNOPSIS

Laboratorystudies of mottling discoloration of hori-

zontal concrete slabs ara dascrlbed, showing thatdiscoloration is increased by hard troweling, by usa

of celcium chloride admixtures, or by poor curing of

fhe dabs. Local discoloration can also be caused by

non-uniform curing conditions.

Avoiding the use of calcium chloride in hard-

frowelad flafwork would eliminafe much flatvmrk dis-

coloration. If calcium chloride is nacessary, curing

procedures recommandad fo minimiza local discolora-

tion ara water pending or use of sprayed membrana

curing compounds. The effect of calcium chloride is

depandent on the alkali content of the cement.

Immediate and thorough washing of the concrata

surface with water seams the aasiest way of “erasing”

discoloration. Special chemical treatments to arase

discoloration ara of additional benefit.

INTRODUCTION

Surface discoloration of concrete flatworkis frequently a problem of concern. Thesurface discoloration discussed here is thenon-uniformity of color or hue in a single

concrete flatwork job. This discolorationmay take the form of: (1) gross color changesin large areas of concrete caused as in Fig.l(a) by changes in the concrete mix; (2)spotted or mottled discoloration wherelight or dark blotches ap ear on the flat-

!work surface, as in Fig. 1( ); and (3) earlydiscoloration by light patches of “efflores-cence.” These discolorations appear soonafter the flatwork has been placed and aredue in the latter two cases to the proce-dures used to cast, finish, and cure the slab.

Some of the more obvious types of discol-oration, such as dirt being blown ortracked onto fresh concrete surfaces, willnot be discussed here. Stains caused byspilling oil, paint, or other liquids onconcrete are also beyond the scope of thispaper.

The investigation was undertaken todetermine the effects of various concretingprocedures and concrete materials on flat-work discoloration, to study the primarycauses of discoloration, to develop an un-derstanding of the mechanisms causingdiscoloration, and to explore the methodsof preventing or remedying discoloration.Field experience suggested that steel trow-eling, calcium chloride admixtures, andcuring conditions are of importance, andthese were chosen as primary variables.

34 Journal of The PCA Research and

(a) Ovarall View, Concrete in foreground containedcalcium chloride, that in back did not.

[b) Closeup of surface.

Fig, I - A Driveway with Gross Color Contrast and Mottling Discoloration.

Development Laborofories, Sepfember 1966 35

SCOPE

To attain better control of environment,and be able to explore more variables, thiswork was confined to the laboratory. Con-crete slabs, mostly 1 foot square and 3 inchdeep, were used to study the effects offinishing techniques, admixtures, curing,cement properties, etc.

BASIC FACTORS AFFECTING THE COLOROF A CONCRETE

Three concrete variables found to beimportant in establishing the color of con-crete are the original color of the cement,the water-cement ratio, and the extentand rate of hydration of the ferrite phasein cement.

Wiitar4emenf Ratio

Color of the Cement

Individual cements ‘may differ in color.Thus, substituting one cement for anothermay change the color of concrete.

Not all the ramifications of such colorchanges are as yet clearly understood. Thepresence and concentration of lime andchemical admixtures may affect the finalcolor of the ferrite hydrates and hydratedcement, as should such factors as carbona-tion or the temperature at which hydrationoccurs.

Calcium chloride is an established “ac-celerator” that speeds up the hydration ofthe silicates in cement. However, calciumchloride retards the hydration of the alum-inate and ferrite phases in cement. Retard-ed ferrite phases that remain unhydrated incement will remain dark.

A low water-cement ratio paste is al-most always darker than a high water-cement ratio paste made with the samepordand cement. This is evident in Fig. 2,which compares the color of mature pastes,both wet and dry, made with water-cementratios of 0.3, 0.4, 0.5, and 0.6 by weight.All pastes were made with the same ce-ment. Construction practices producinglocalized areas of variable water-cementratio within a slab are potential causes ofdiscoloration.

Hydration of Cement Ferrites

Unhydrated ferrite phases (iron com-pounds) in cements are blackish-brown.They are primarily responsible for the darkcolor of unhydrated cement. Hydrationlightens their CO1OVfully hydrated ferrites,prepared as slurries of the pure phases,range in color from white to dark red-brown. Thus, lightening of the ferritephase by hydration is apparently the majorcause of cements and concretes becominglighter in hue as they hydrate,

WET

Fig. 2 — Effect of Watei-Oement Ratio on Color of

Seven-Year-Old Pastes,

Cement alkalies moderate the actions ofcalcium chloride in concrete by reactingwith calcium chloride, thus precipitatingcalcium hydroxide and leaving sodium orpotassium chloride in solution. These re-action products do not significantly retardthe hydration of the ferrite phase in ce-ment, and thus should not greatly delay thelightening of the ferrites and cements byhydration.

Fig. 3 illustrates the effect of calciumchloride and alkalies on ferrite hydration.Shown are curves of conduction calorimeterrate of heat release for mixtures of synthet-

36 Journal of The PCA Research and

1-aIdr

5.

~

Compound C4AF:Ca(OH)2 :CaS04.2H20

Mole Ratio I : 4 : 0.2Water/total salids = 0.40

4 -

<H20 IIlcal /G at 72hrs

3 -

92 Cal/G ot 72hrs,

2 -

I -

J <29 Cal/G at72hrs.

OJo

18 16 24 32 40 48 56 64 72

TIME, HOURS

!%g.3-Effect of CeC12and NaOH on Hydration of CiAF.

ic C4AF* (of the fineness of cement),gypsum, and excess calcium hydroxide re-acting in the presence of the indicatedsolutlons. These data show that the C4AFin this system hydrates rapidly after about8 hours in water, When the same mixturereacts in 4 percent calcium chloride solu-tion instead of water, the hydration rate isgreatly reduced. After 3 days, the hydrationin calcium chloride solution is only one-fourth that of the same mixture in water.However, if 2 percent NaOH is added tothe 4 percent calcium cfdoride solution, thealkali almost overcomes the retardationcaused by the calcium chloride alone,

If the ferrite phases are initially undulyretarded, there may be difficulty in subse-quently hydrating them enough to lightenthem. Rapidly hydrating silicates, accel-erated by calcium chloride admixtures,preempt much of the water otherwiseavailable for hydrating the ferrite phases.Physical compaction, such as troweling, candecrease the water–cement ratio of thetop surface of the slab to the point wherewater and space necessary for the hydration

‘The formula C4AF is cement chemist’s shorthandfor 4 Ca0.A1208.Fe203 (tetracalcium aluminafer.rite), considered to represent fairly well the com-position of the ferrite phase in portland cemenL

of ferrite phases are not available. Thissituation may produce indefinite delays inhydrating and lightening this surface por-tion of the concrete.

MATERIALS AND TEST PROCEDURES

Concrete Materials

Six Type I cements were ‘used in thisstudy. Their alkali contents are given inTable 1.

Well-graded Elgin sand and Elgin dolo-mitic gravel (?)/4-inch maximum size) werethe concrete aggregates.

For this work, most concrete mixes had acement factor of 5 bags er cubic yard of

Econcrete and a slump of a out 3 inches.Tests indicated that the air content of

the concrete was not an important factor indiscoloration. Most concretes for thesestudies contained 5 percent of air entrainedthrough the use of neutraltied Vinsol resinadded at the mixer.

Unless otherwise stated, concretes with“calcium chloride admixture” contained 2percent flake calcium chloride (CaC12”2H20) by weight of cement. This calciumchloride was dissolved in the batch water.

All concrete was mixed in a small pan.type concrete mixer in a laboratory having

Development Laboratories, Sepfember 1966 37

TABLE 1—ALKALI CONTENTS OF TYPE I CEMENTSUSED

(ASTM Standtwd C 114-65)

Water Soluble Alkalies, YOCement Cement

Alkalies, ~0Alkall

Designoilon No. Designation Total TotalNa*O K,O as NIJPO Na*O K*O as No*O

A LTS 13 LOW 0.003 0.02 0.01 0.04 0.19 0.17B LTS 14 High 0.01 0.66 0.44 0.06 1.30 0.92

c 20132 Medium 0.05 0.20 0.18 0.26 0.50 0.59

D 20133 Low 0.02 0.04 0.04 0.10 0.14 0,19

E 20134 High 0.05 0.96 0.68 0.11 1.38 1.02

F 20135 Medium 0,09 0.29 0.28 0.26 0.55 0.62

a temperature of 73 F and relative humid-ity of 50 percent,

CastingAlmost all specimens were concrete slabs

1 foot square. These small slabs could behandled easily, yet were large enough tofinish with ordinary hand tools.

Most of the slabs were cast in 3-inch-deep,waterproofed plastic-coated plywood molds.However, to explore the effects of slabthicktless and water absorption from a slabby a dry subbase, l-foot-square molds ofsuitable depth were used to accommodateslabs 3 inches and 6 inches deep withoutany subbase, and slabs of these depths castabove a 3-inch-deep compacted dry sandsubbase.

Several of the slab surfaces were “jitter-bugged” to establish whether this pre-fin-~shing procedure (which pushes down thelarger aggregate particles, thus bringingmortar to the surface) has a significanteffect upon concrete discoloration. A wirepotato masher with openings of Y2 inchserved as the miniature jitterbug. By thismeans, a mortar layer averaging s/s inchesin de th was brought to the surface of the

1!jitter ugged slabs.

Finishing

Most slabs were given a hard trowelfinish, using the following procedure. Im-mediately after consolidation by rodding,the concrete in the mold was struck off andgiven a quick, rough finish with a corkfloat. At this time the slabs were lightly

Yed ed to embed the larger aggregate ~ar-tlc es at the slab borders. At about the timethe bleed water receded into the slabsurface, the slabs were given a Ii ht prelim-

kinary steel troweling and anot er edging

pass. Unless the time of troweling wasunder study, the final troweling was doneat the earliest time at which a suitablefinish could be obtained. Only hand toolswere used for finishing.

Curing

Experience has indicated that curing ofconcrete influences the tendency to dis-color. The curing procedures used in theselaboratory tests were:

(1) A iT curing. After finishing, concretewas left uncovered in the molds for about16 hours, the molds were stripped, and theconcrete slabs were allowed to dry and curein the 73° I?, 50~0 R.H. environment, Con-crete that received this treatment is here-after called air-cured.

(2) Hot room curing, After finishin1!uncovered slabs were stored in the mol s

for about 16 hours in a hot room main-tained at 100° F and 20~0 R. H., thenstripped and stored completely uncoveredin the hot room.

(3) Polyethylene cure. After finishing,the slab and mold were wrapped with

rolyethylene film, to minimize moisture

OSS, with no contact between the poly-ethylene film and the slab surface. At 16hours, the molds were removed and thespecimens were stored uncovered in thesame room in which they were cast. In someinstances, the specimens were put backunder the polyethylene covers for an addi-tional two days.

(4) Burlap-polyethylene cure. After fin.ishing, the slabs were put under a frame-work on which pieces of wet burlap weredraped. Moisture was sealed in by an outercovering of polyethylene film. After either1 day or 3 days of burlappolyethylene cure

38 Journal of The PCA Research and

the’ specimens were exposed to the roomenvironment of 73° F and 50~0 R.H.

(5) Moist room cure. After finishing, theslabs were put in a moist room at 73° F and100~o R,H. Until demolding on the dayafter casting, the specimens were storedunder tenting material that kept the sur-faces of the slabs from being disfigured bywater droplets.

(6) Membrane curing. Within 1 hourafter finishing, proprietary membranecuring compound was flowed onto thesurface with a fine paintbrush. No otherprotection was

$rovided for the concrete,

which was store m the laboratory at 73° Fand 50~0 R.H. The membrane was even-tually removed by use of a solvent in orderto study the surface.

Subsequent Sxposure or Treetment of Slebs

The uniformity or discoloration of dryspecimens was considered to be the crite-rion of greatest importance; therefore thespecimens were dried before examination.

After the initial extent of discolorationhad been determined, the specimens weretreated by various procedures to determineif the discoloration could be relieved. Suchtreatments usually consisted of washingwith water, treatment with strong alkalisolutions, acid washing, or the applicationof other chemicals.

A number of the dried specimens werestored in the Skokie Outdoor Ex osure

[Plot to determine the effect of weat eringon discoloration.

PRIMARY FACTORS CONTRIBUTING

TO DISCOLORATION OF FLATWORK

In these studies, no single factor seemedto cause discoloration. However, combina-tions of factors caused very severe discolora-tion. Factors found to influence discolora-tion were calcium chloride admixtures,cement alkalies, hard-troweled surfaces, in-adequate or inappropriate curing, con-creting practices and finishing proceduresthat cause surface variation of water-cementratio, and changes in the concrete mix. Thefollowing discussion describes the influenceof these factors on concrete discoloration,

Calcium Chloride and Alkalies

These studies disclose two major types ofmottling discoloration that can result fromthe interaction between cement alkaliesand calcium chloride, or from the separate

effects of these two components. The firsttype consists of light spots on a darkbackground and is characteristic of mix-tures in which the ratio of cement alkaliesto calcium chloride is relatively low. Thesecond consists of dark spots on a lightbackground and is characteristic of mix-tures in which the ratio of cement alkaliesto chlorides is relatively high. These typeswill be called “light spot” and “dark spot”discoloration, respectively.

The detailed discussion to follow willshow that in addition to the initial ratio ofthese two factors, certain aspects of placin$,finishing, and curing appear to affect thisratio and thus influence the type, degree,and location of mottling discoloration thatmay develop. Whether an area will be lightor dark depends upon the amount anddegree of formation and deposition ofalkali chlorides and alkali carbonates atthe surface, and u on the hydration of theferrite base in t e particular area. Both

i?!types o discoloration are shown in Fig. 4.All slabs in Fig. 4 were hard troweled andair-cured (conditions that promote discol-oration).

Fig. 4(a) shows the surface of a concreteslab made with low-alkali cement A with-out calcium chloride. The slab is free ofdiscoloration. Washing did not alter itsappearance (see Fig. 4(b) ).

Fig. 4(c) shows a similar concrete slabmade with high-alkali cement B, also with-out calcium chloride. This slab is definitelydiscolored with dark spots, but the discol-oration is temporary and disappears whenthe slab is treated five times by ahern,atelyhosing it thoroughly with water and dryingit overnight (see Fig. 4(d)).

The slab of Fig. 4(e) is exactly like thatof 4(a) except that it contains calciumchloride. The extreme discoloration ischaracterized by light spots directly overcoarse aggregate particles near the concretesurface. The dark surface matrix is overrelatively deep mortar. Severe discoloration(Fig. 4(f) ) was still evident on this slabsurface after the 5-day washing and drying,treatment just described.

Fig. 4(g) shows the surface of a concreteslab made with high-alkali cement B andwith calcium chlorlde. Except for the cal-cium chloride, the slab is identical with the(c) slab. The washing and drying treat-ment alleviated the original discoloration(see Fig. 4(h) ), but not as easily or success-fully as for slab (c).

Development Laborofodes, Sepfember 1966 39

Slabs (e), (f), (g), and (h) of Fig. 4contained 2 percent flake calcmm chlorideby weight of cement. Higher percentages ofthis accelerator are rarely intentionallyused in concrete. Available evidence indi-cates that the magnitude and permanenceof discoloration increases as the calciumchloride concentration increases from O to 2percent, provided other factors remain con-stant.

The dark s ot discoloration shown in!Figs, 4(c) and (g) appears to be caused by

alkali salts that migrate to the dryingconcrete surface and concentrate in themore porous or checked areas of the sur-face. These deposits of salt are relativelytransparent and continuous. Their opticalbehavior is apparently similar to that ofwater or clear oil, which will darken pastewhen absorbed. Microscopic examinationindicated that the materials causing darkspots on slabs without calcium chloride(Fig. 4(c) ) were alkali carbonates – reactionproducts of cement alkalies and carbondioxide from the air. Dark spots on slabscontaining calcium chloride (Fig, 4(g) )are primarily crystalline potassium andsodium chloride —reaction products of ce-ment alkalies and calcium chloride, Thesimplest remedy for either type, of dis-coloration is to wash away the discolor@gsalt with water. One washing was oftensufficient for the dark spot type of dis-coloration which occurred in the absenceof calcium chloride. If the concrete con-tained calcium chloride, repeated washingswere necessary to remove the dark spotscompletely. Somewhat inconclusive datasuggest that this washing treatment be-comes less effective as the concrete ages.

The light spot type of discoloration (Fig.4(e) ) is particularly difficult to remedy.However, washing or weathering causes thedark background areas of these slabs tolighten very slowly, approaching the colorof the lighter spots. It appears that discol-oration by salts, such as that described forthe dark spot type of discoloration, isrelatively minor. The first washdown withwater perceptibly lightens the slab, proba-bly by removingthe relatively small amountof salt at the slab surface.

The large areas of extremely tenaciousdark background remaining after the firstwashdown do not appear to be the result ofsalt deposits. Apparently, these are areas ofpaste containing large amounts of retatded

dark ferrite phases, because trowelin hasfdensified the surface making it difficu t for

hydration to continue.

The light spots on the surface of Fig.4(e) coincide exactly with particles ofcoarse aggregate close to the slab surface.These relatively impermeable particles in-terfere with the normal migration of chlo-ride salts toward the drying surface of aconcrete slab, resulting in differences in theextent of hydration of both the ferrite andsilicate phases. This produces light spots(low calcium chloride content) over theagg~egate, which contrast with dark (highcalcmm chloride content) surface areasover deep mortar. The migration of chlo-ride salts toward the drying surface of theslab significantly increases the surface con-centration of these salts. Limited chlorideanalysis indicates that surface mortar of air-dried slabs has three to four times theoriginal concentration of chloride.

Immediate and thorough flushing withwater tends to lessen the light spot type ofdiscoloration. Complete eradication mayrequire special chemical treatments dis.cussed later,

In summary, the type of discolorationproduced is influenced by both the alkalicontent of the cement and the calciumchloride content. For example, air-curedslabs made with low-alkali cement A and1/2prcent calcium chloride have dark spotdiscoloration, and ones made with 2 per-cent calcium chloride have light spot dis-coloration. At the other extreme, air-curedslabs made with high-alkali cement E andvery high percentages of calcium chloride(about 5 percent or greater) show lightspot discoloration in contrast to the darkspot discoloration with lower amounts ofcalcium chloride. The factor mainly de-termining the type of discoloration in theslab is the ratio of the alkalies to thecalcium chloride present in the concrete.High and low ratios produce dark spot andlight spot discolorations, respectively.

Additional confirmation of the effects ofthe ratio of alkali to calcium chloride wasobtained by adding sodium hydroxide (0.8percent as Na20) to air-cured slabs madewith low-alkali cements A and D. Darkspot discoloration resulted. Discolorationsproduced by adding sodium hydroxide tothe mix were similar to those encounteredwith cements having a high content of

40 Journal of The PCA Research and

Development Laboratories, Sep+ember 1966 41

potassium oxide. This suggests that sodiumand potassium are equivalent in their ef-fects on discoloration.

The conversion of calcium chloride topotassium or sodium chloride appears to bethe probable cause of basic changes fromlight spot to dark spot discoloration, Thiswas investigated by using 1 and 2 percentsodium chloride in slabs made with low-alkali cement A. Dark spot discolorationresulted, which was remedied by four wash-ing and drying treatments.

Further complications are introducedbecause the ratio of alkali to calciumchloride at the concrete surface is theimportant factor. Curing effects, tempera-ture of the slab, and other factors shouldaffect the rate of migration of salts to thesurface and the rate of hydration of thecement, both of which may govern the typeof discoloration. One good example of suchrate effects is the variable discolorationnoted for concretes made with calciumchloride and medium-alkali cements C andF. Air-cured slabs had dark spot discolora-tion, Hot-room-cured slabs of the sameconcrete had light spot discoloration.

Finishing

Hard Troweling. Discoloration is pri-marily a problem with hard-troweled slabsthat have been either left smooth orbroomed lightly. It is rarely of importancein concrete flatwork with rough-texturedsurfaces produced by floating, burlap drags,or rough brooming. Dark and light spot-ting that typifies discoloration may appearon rough-textured flatwork, but is easily lostamong the closely grouped highlights andshadows on rough-finished concrete. More-over, the rather porous, uncomp”acted sur-faces of rough-textured concrete flatworktend to have less spotting than well-com-pacted, hard+troweled surfaces of similarconcrete.

Fig. 5, showing slabs that were cast at onetime, with identical concrete, illustrates theeffects of variations in finishing time uponthe eventual discoloration of the concrete.Low-alkali cement A and calcium chlorideadmixture were used in these air-curedslabs.

Fig. 5(a shows a slab that was steeltroweled k efore the concrete was stiflenough to take a slick trowel finish. Slab5(b) was finished 15 minutes after the (a)slab. Both slabs had a rou h “orange peel”

%finish. The (c) and (d) sla s in Fig. 5 werefinished 45 mmutes and 60 minutes, respec-

tively, after the (a) slab. While both theseslabs took an acceptable trowel finish, anintermediate time would have been best.The sequence of photographs 5(a) through5(d) shows that overall concrete discolora-tion of the light spot type gradually be-comes more pronounced as a greateramount of compactive effort is expendedin troweling. The progressively darker back-ground discoloration with harder finishingmight be caused by the progressive com-pacting of the paste, which leaves less waterfor hydration of the ferrite (which the cal-cium chloride retards),

Limited data indicate that concretesmade with high-alkali cements are lesssusceptible to accentuation of dark spotdiscoloration by hard troweling.

Trowel Burning. Extreme discoloration iscaused by trowel burning, a blackening ofthe surface resulting from attempts to hard-trowel concrete after it has become muchtoo stiff to trowel pro~erly, Trowel metalrubbed off onto the stiff concrete is a con-ven tional explanation for trowel burns.Some of the trowel burn discoloration un-doubtedly is due to abraded metal. How-ever, since a troweled surface can be“burned” by rubbing it vigorously withplate glass, densifying a paste by trowelingto a point where the water-cement ratio isdrastically decreased appears to be the mostimportant cause of trowel burning. Eventhe best curing does little to lessen trowelburn discoloration. Such burns are alsonearly impossible to remedy by subsequenttreatments,

Since it is difficult to remove trowelburns, ways to avoid burning are very im-

fortant. Concrete that will stay finishable

ong enough for proper troweling shouldhelp in avoiding the conditions leading totrowel burns. A “finishable” concrete canbe described as one with a buttery texturethat is sufficiently stiff so that a slick trow-eled surface does not degrade to a rough“orange peel” surface before the concretesets, and et is still plastic enough to fill

7small sur ace irregularities without exces-sive work. Cement composition is one fac-tor that may influence concrete finishabil.ity. For example, high calcium aluminateportland cenients produce surfaces that arebuttery for relatively long periods. At theother extreme, the acceleration of hydra-tion of cement silicates by calcium chloridecauses concrete to be buttery and finishablefor only short time periods.

42 Journal of The PCA Research aIId

(a) Troweled 2 hours after casting. [b) Troweled at 2 hours and 15 minutes.

(c) Troweled at 2 hours and 45 minutes. (d) Troweled at 3 hours.

Fig. 5 — Delayed Hard Troweling Increasas Troweling Effort and Concrete Discoloration.

To minimize trowel burns: (1) have anadequate finishing crew, (2) reduce evapo-ration losses with sunshades and wind-breaks, and (3) avoid the use of calciumchloride where possible,

Curing Procedures

Curing procedures have a significanteffect on discoloration. This studv indicatesthat the curing procedures that’ areefficient in preventing evaporation

Development Laboratories, September

mostfrom

1966

the entire concrete surface also produce themost uniform slabs.

Effectiveness of Various Curing Proce-dures. Fig. 6 shows the effects of differentcuring procedures on the mottling discol-oration of cement B and A concretes con-taining calcium chloride admixture. Bothair-cured specimens, (a) and (e), were bad-ly discolored. High-alkali concrete slabsthat were given 1 day of polyethylene cur-ing, (b), or burlappolyethylene curing,

43

Air Cured

(a)

High-Alkali

Cement

Concrete

(e)

Polyethylene Film Cure1 day

(b)

(f)

Fig. 6—Effect of Curing on Oiseolorafiorr of Hard Troweled Slabs

Containing 2 Percan+ Calcium Chloride Admixture.

Moist Room Cure

(d)

(c), or were moist room cured, ~d), dis-played little discoloration, indicating theeffectiveness of most curing procedures inpreventing such discoloration.

One day of burlappolyethylene curing,(g), and mol?t cm’htg, (h), appeared rea-sonably effective in preventing discolora-tion in low-alkali concrete slabs, One dayof curing under a polyethylene film, (f),was not sufficient. The inadequacy of the 1-day polyethylene cure in this instance sug-gests that thoroughness of curing is essen-tial to prevent discoloration of low-alkalicalcium chloride dabs.

Fi~ 7(a) and 7(b) show the results ofmem rane curing concretes made withhigh-alkali B and low-alkali A cement anda calcium chloride admixture. About 1hour after trqweling, half of each slab waspainted with a membrane curing com-pound. After 11 day, the curing compoundwas removed ~with a solvent.

Originally, the membrane-coated portionof each slab ~as extremely uniform in col-or. When the 1membrane was removed, thelow-alkali slafj (b) remained free of discol-

!!oration, exce t for three membrane filledpinholes. Sla (a), however, shows smalldark. alkali-sa!lt smudges that kept reap-pearing after Ithey had been washed awaywith water. F@r washings were re uired to

\permanently eliminate these smu ges.

Early remo~al of membrane coatings isnot normal for concrete flatwork. Pro-

1longed mem rane protection appears tomake the coat d portions even less suscepti-ble to later silt discoloration if the mem-brane is subs~quently removed.

Uneven Cu#ing. The overall color of theair-cured por~ion of the slab in Fig. 7(a)stayed noticeably darker than the mem-brane-coated Iportion, This condition is

?

probably per anent. It is a good exampleof discolorati n froti uneven curing. Toprevent disc loration from this cause,

:“membrane cu mg compound should be applied evenly @d adequately, moist cover-ings should be kept uniformly wet, andplastic film covers should be thoroughlyanchored.

“Greenhous& Efiect.” Careless placing ofplastic curing~ film on the surface of theflatwork concfete containing calcium chlo-ride may cau~e unsightly efflorescence de-posits in addftion to mottling discolora-

/

tion. A high old in a waterproof curingsheet can serv as a little “greenhouse.” On

High- AlkollCement

(b) Low-AlkallCement

Fig, 7 — High- and Low-Alkali Cements Usad in

Concrate Slebs with 2 Parcant Calcium Chloride.

A Portion of Each Slab Wes Painted with Membrane

Curing Compound, Which Was Removed with SoI-

vant Aftar One Day.

a hot day, under direct sunlight, the foldbecomes the location of a water evapora-tion-condensation cycle. The heat of thesun, aided perhaps by the heat of hydrationof the concrete, evaporates water from theconcrete under the fold. The water vaporthen condenses on the cool high part of thefold and eventually runs down the sides ofthe film to collect at the points of intersec-tion of the concrete and the film, or in lowplaces in the concrete surface. Such local-ized dry and wet areas on fresh concretesurfaces may cause concrete discoloration.

Laboratory tests were made to evaluatethis “greenhouse effect.” Fig. 8(a) shows around hard-troweled slab, 41/2 in. thick, be-ing cured beneath a folded sheet of clearpolyethylene. The concrete in this slab wasmade with high-alkali cement B and calci-um chloride, Application of the polyethyl-ene sheet to the concrete was delayed untilthe concrete surface was firm enough not toadhere to the polyethylene. A heat lampshining on the concrete surface from a dis.tance of about 4 feet provided the radiant

Ilevefopment Lobqroforles, Sepfember 1966 45

energy needed for the evaporation-conden-sation cycle; a fan cooled the high part ofthe fold,

Fig. 8(a) is a photograph taken about 5hours after the start of curing. The darkspots in the figure are due to Ii uid that

1filled thenarrow spaces betweent ecuringsheet and the concrete surface. Slight ebbsand flows, which were apparent during ob-servation, indicate that the water wasdefinitely cycling beneath the polyethylenecover; however, at this stage of the curerelatively stable boundaries were estab-lished between water-filled dark spaces incontact areas and the light, droplet-speckled tented areas where evaporationwas taking place.

Continuous curing under the heat lampwas maintained for about 17 hours. Fig.8(b) shows the top surface of the slab short-ly after the curing sheet was removed.White areas in this photograph match ex-actly the areas where a thin liquid layercontacted both the curing film and the con-crete. Dark areas occur wherever the poly-ethylene remained sufficiently out of con-tact with the concrete surface to permitevaporation to occur. Subsequent scrub-bing with water, using a stiff bristle brush,removed some of the concentrated whiteefflorescence deposits, as shown in Fig. 8(c).Application of very dilute hydrochloricacid removed the remaining efflorescence,as shown in Fig. 8(d). The dark mottlingunderneath the efllorescence was not re-moved by the acid wash. Soaking in 10 per-cent caustic soda solution or allowing waterto stand overnight on the slab surface wereonly moderately successful treatments forreducing effkorescence and mottling discol-oration.

.Limited tests indicate that it is difficultto greatly discolor, calcium chloride-freeconcrete by the “greenhouse effect.”

Miscellaneous Factors that Affect

Discoloration of Flatwork

Certain construction procedures maytend to lessen or increase the discolorationof concrete. Such procedures include jitter-bugging, subgrade preparation, protectionof fresh concrete from evaporation, andchanges in concrete materials during cast-ing or finishing of the slab.

.Jitterbugging. The use of a jitterbug tobring mortar to the surface of a slab slight-ly decreased mottling discoloration.

46

Subgrade preparation. Theoretically, themoisture condition of the subgrade can beconsidered a factor in discoloration. A dryspot on the subgrade will absorb more wa-ter from the overlying concrete slab than amoist section of subgrade, Thus, a slabplaced on a subgrade of highly variable ab-sorptive capacity will have areas of relative-ly high and low water-cement ratio, withconsequent light and dark hues.

It was anticipated that .3-inch-deep slabscast on a 3-inch-deep dry sand subbasewould be relatively dark, the 6-inch-deepslabs cast on the same type base a littlelighter, and the comparable slabs cast inwatertight molds lighter yet, Certain slabgroups of this type cast from the same mixbehaved in just this way, but there weresufficient exceptions to raise doubts aboutsubgrade absorption as a cause of strongcolor contrasts. Nevertheless, moisteningthe subgrade should improve the appear-anceof the finish, since color variations dueto unequal absorption will be moderatedor elimmated. Another advantage is thatthe finish will be more uniform due togreater uniformity of setting, for variablesetting can occur if there are surface patch-es of wet and dry concrete.

Protecting concrete surfaces from windand sun. Heat accelerates hydration of ce-ment. Heat and wind will dry a concretesurface, decreasing the water-cement ratioand increasing the concentration of salts atthe slab surface. Discoloration is possible ifone portion of a slab has a different expo-sure to the wind and sun than another por-tion. Thus, protecting fresh concrete fromthe wind and sun are measures that help toreduce discoloration.

Gross mix changes. Sometimes the con-cretemix fora jobmu~t readjusted. If theadjusted mixhasa color different from thatof the original mix, a two-toned job is aninevitable result. The contrast can be mod-erated by providing a construction joint atthe mix change line.

Fig. l(a) shows an attempt made in thefield to shorten overall finishing time, byswitching to concrete containing calciumchloride, after first placing concrete with-out an accelerator. The photograph wasmade five years after casting. The dark areais a classic example of chloride discolora-tion. Even if the mottling discolorationhad not occurred, the overall dark color ofthe concrete containing calcium chloride

Journal of The PCA Research and

- Containing 2 Pkrca;t Calcium Chlorida.

Development Laboratories, Sepfember 1966

would have clashed with the lighter colorof the portion of the concrete that did notcontain this accelerator.

Retarded concrete has occasionally beenused in the same slab with unretarded con-crete. This procedure can also result in atwo-tone effect.

The practices of dusting surfaces withdry cement to speed finishing, or of smear-ing mortar or cement paste on the surfaceof concrete already too hard to trowelproperly, will generally result in discolors.tion.

OutdoorExposure of Laboratory Specimens

Both discolored and clear slabs werestored outside during the winter and springof 196.5-66. Clear slabs showed no appreci-able discoloration after this exposure. Dis-coloration that could not be remedied byfive laboratory washing and drying treat-ments was not appreciably affected by theoutdoor exposure,

CHEMICAL REMEDIES FOR DISCOLORATION

Some success has been achieved in im-proving the appearance of discolored slabsthrough the use of a strong lye wash. Drydiscolored slabs were covered with a 10 per-cent solution of sodium hydroxide (causticsoda) for a day or two and then thoroughlywashed to remove the caustic solution.During the subsequent drying period, thediscolored portion of the concrete oftenblended with the rest of the slab surface.The older the discolored concrete is, theless effective is this or any other knownremedy.

Strong acid washes were found to be apoor treatment for mottling discoloration.Strong acid washes were hard to control,and usually either caused no perceptibleimprovement in the appearance of the sur-face or etched so much of the paste awayfrom the slab surface that the finish waslost and considerable aggregate was exposed.

One chemical, with an action much likethat of a very mild acid, appears to havereal potential in “erasing” discoloration,Di-ammonium citrate, (NH.&HCoH~07,slowly attacks calcium carbonate, calciumhydoxide, and other cement constituents.The chemical is a diuretic, but otherwiserelatively harmless to man. When appliedto a dry discolored surface, the ammoniumcitrate penetrates the surface, digests the in-dicated paste constituents, and makes thesurface more porous. After treatment, wa-

47

ter can penetrate more readily into the sur-face region to promote hydration andlightening of cement constituents. An im-mediate lightening of the surface by thistreatment results from the formation anddeposition of silica gel and calcium citrate,formed by the reaction of the ammoniumcitrate and cement paste. After drying, thegel becomes a very tenacious light-coloredcoating.

Fig. 9 showsa badly discolored slab, thelight portion of which resulted from twoammonium citrate treatments (describedbelow). Some of the lightening is caused bythe gel “whitewash,” but there has alsobeen a pronounced change in the nature ofthe discoloration. Further treatments withthe chemical, alternated with wetting anddrying, produced even more lightening ofthe concrete surface and made the treatedsurface even more uniform in color.

The slab in Fig. 9 was given what ap-pears to be the most efficient ammoniumcitrate treatment. The procedure is: (1)Apply a 20 to 30 percent water solution ofdi-ammonium citrate on dry concrete. (2)Continuously and lightly brush the treatedarea to maintain a uniform film of clearliquid on the surface. About 5 minutes aft-er application, the liquid on the concretesurface will start to turn into a white gel.More water must be applied so the gel doesnot stiffen or dry. (3) Continue to stir andbrush this coating around on the concretesurface for about 15 more minutes aftergelation. (4) Thoroughly clean all the gelfrom the surface with a stream of water andvigorous brushing.

Because the citrate solution must pene-trate the concrete surface for efficient re-moval of discoloration, the second treat-ment should be deferred until after theconcrete has had an opportunity to dry.After ammonium citrate treatment, theuniformity of the surface can be improvedstill further by alternate wetting with wa-ter and drying. Use of the above treatmenton portions of a slab is not recommended,since contrasting light patches of surfacewill result. The whole slab should be treat-ed to obtain maximum uniformity.

Ammonium citrate treatment has alsobeen used for cleaning concrete surfacesthat were discolored by form-oil spots. Itmay also be useful in treating other blem-ishes of formed concrete surfaces.

Fig. 9 — Half of This Slab Was Traatad with Di-

Ammonium Citrate to Ramove Light Spot Discoloration.

CONCLUSIONS

1. The discoloration of a slab may beminimized or prevented by moistening ab-sorptive subgrades, proper scheduling ofplacing and finishing, good finishing prac-tice, protection of concrete from drying bythe wind and sun, and proper curing.

2. Calcium chloride in concrete is a pri-mary cause of concrete discoloration. Thechances for discoloration are much less ifcalcium chloride is not used.

3. High-alkali contents moderate discol-oration in concretes containing calciumchloride by counteracting the retardingeffect of the chloride on the hydration ofthe ferrite phases.

4, Discoloration becomes more pro-nounced and more permanent with harderfinishing.

5. Trowel burns are the most severe andpermanent type of discoloration of concreteflatwork,

6. If discoloration occurs because of in-adequate curing, intermittent washing withwater and drying of the slab surface shouldbe the first remedy tried, Such treatmentshould be initiated as soon as possible,since the treatment is more effective at anearly age.

7. Strong caustic soda solution pondedon discolored concrete sometimes moder-ates the discoloration. To date, the most

48 Journal of The PCA Research and

effective chemical treatment found for dis.coloration has been application of di-am-monium citrate solution.

RECOMMENDATION TO PREVENT

DISCOLORATION OF FLATWORK

1. Avoid use of calcium chloride in theconcrete, if possible. Calcium chloride ac-celeration aggravates discoloration of hard-troweled flatwork. (Calcium chlorides maybe present either because added intention-ally as an accelerator, or unintentionallythrough use of certain proprietary chemicaladmixtures that contain it.)

2. Cure the concrete properly and atonce, for bad discoloration may occur asearly as 16 hours after placing. Curing isessential to prevent discoloration. For con-crete flatwork without calcium chloride ad-mixture, ordinary curing procedures, suchas coverings of wet canvas, burlap, or sand;pending with water; or use of a white-pig-mented membrane compound appear quiteadequate. The “greenhouse effect” is rob-

1ably minor in the absence of chlorl e inthe concrete; thus curing with plastic sheetsshould be sufficient so long as overlappedareas are properly taped, the sheet is as flatas ossible, and the edges are adequately

~anc ored.

For flatwork concrete that must containcalcium chloride admixture, positive pre-vention of water eva oration during curing

cl’is necessary. Sunsha es and windbreaks aredesirable to keep the flatwork from dryingin localized areas during finishing. Wetcoverings of burlap, sand, and canvasshould not be allowed to dry in patches.Water pending and membrane curingcompounds are apparently the best curingprocedures to avoid discoloration. Eventu-ally, traffic ma wear curing compounds

[from portions o the surface, causing a kindof discoloration. Removing the remainingcuring compound with a suitable solventwill restore uniform color to the flatworkslab,

8. To finish properly and prevent trowelburns, the pace of operations must be lim-ited to the finishing capacity of the crewand equipment at hand. Since the time forfinishing any concrete that contains calci-um chloride is limited, the schedule shouldbe cut back accordingly, or the size of crewincreased. The harmful practice of trowel-ing additional cement paste onto the sur.face should not be permitted.

4, Differently colored concretes shouldbe separated by construction joints. Suchcolor differences should be anticipatedwhen there is a gross change in the mix, achange of cement brands, or an intermit-tent use of accelerating or retarding chemi-cal admixtures.

RECOMMENDATIONS TO ERADICATE

DISCOLORATION

1, For the types of discoloration de-scribed in this report, the first (and usuallyeffective) remedy that should be tried is animmediate thorough flushing with water.Permit the slab to dry overnight, then re-peat the flushing and drying process untilthe discoloration is eradicated. The soonerthis is done, the better are the chances forsuccess. Use plenty of water —turn thehose on the flatwork and flush it gently forat least a half hour. If possible, use hotwater.

2. The first flushing with water ma notiremove all white effforescence (pro ably

calcium carbonate) from concrete surfaces.Scrubbing the wet surface with a very stiffnonmetal brush may remove this efflores.cence coating. If, and only if, this fails,acid washing may be used. Very dilute so-lutions of hydrochloric acid (about 1 to 2percent) will remove carbonate efflores-cence from hard-troweled flatwork. Higherconcentrations ag~egates,Higher concentra~o~s ~fx~a~er acids suchas 3 percent acetic acid or 3 percent phos-phoric acid can be used to remove efflores-cence. While not as effective as di-ammo-nium citrate, the weaker acids just men-tioned do lessen mottling discoloration.Weak acids me cheap and treatment withthem is easier than treatment with di-ammonium citrat,e, Trials with weak acidon small test patches in unobtrusive loca-tions on a discolored slab are advisablebefore adopting the more effective di-ammonium citrate treatment.

3. The type of discoloration that consistsof light spots on a dark background mayrespond less to the water flushing treatmentthan do other types of discoloration, Lim-ited success has been obtained by treatingthe dry slab with a 10 percent solution ofsodium hydroxide (caustic soda), allowingthe solution to remain on the surface a dayor two, then thoroughly washing to removethe caustic solution. During the subsequentdrying period, the discolored portion oftenblends with the rest of the slab surface.

Development Laboratories, Sepfember 1966 49

4, The best remedy to date involvestreating the dried surface with a 20 to 30percent water solution of di-ammoniumcitrate. The solution behaves like a slow-working acid. One treatment consists of ap-plying the solution to the surface of a dryslab for about 15 minutes. White gelformed by the solution should be dilutedwith water and continuously agitated bybrushing during the treatment. The gelshould be scrubbed off with water after thetreatment. Water curing between or aftertreatments increases the treatment’s effec.tiveness. Two or three treatments should

be enough; more may tend to expose un-due amounts of aggregate, though theymay be beneficial with respect to removalof discoloration.

ACKNOWLEDGMENT

The authors wish to acknowledge thehelpful work of William Perenchio, for-merly Research Engineer of the Ap liedResearch Section, in the early bases o this

frwork on discoloration, and o Bernard Er-lin of the Chemical and Petrographic Re-search Section for his microscopical exami-nations,

PCA.R&D.Ser.1235.2

50 Journal of The PCA Research and Development Laboratories, Sepfember 1966

mlPORTLAND CEMENT I I ASSOCIATIONAn organization of cement manufacturers to improve and extend the uses of portland cement

and concrete through scientific research, engineering field work, and market development.

Old Orchard Road, Skokie, Illinois 60076