Effect of composition, environmental factors andcement-lime mortar coating on concrete carbonation

The paper describes the physicochemical processes of concrete carbonation and presents asimple mathematical model for the evolution of carbonation in time, applicable under constantrelative humidity higher than 50%. The model is based on fundamental principles of chemicalreaction engineering, and uses as parameters the ambient concentration of CO2, the molarconcentrations of the carbonatable constituents, Ca(OH)2 and CSH, in the concrete volume,and the effective diffusivity of CO2 in carbonated concrete. The latter is given by an empiricalfunction of the porosity of hardened cement paste and of relative humidity, derived fromlaboratory diffusion tests. The validity of the model for 0PC or p0220lanic cement concretesand mortars is demonstrated by comparison of its predictions with accelerated carbonationtest results obtained in an environment of controlled CO2 concentration, humidity andtemperature. The mathematical model is extended to cover the case of carbonation of thecoating-concrete system, for concrete coated with a cement-lime mortar finish, applied eitheralmost immediately after the end of concrete curing or with a delay of a certain time.Parametric studies are performed to show how the evolution of carbonation depth with time isa;ffected by cement and concrete composition (water/cement or aggregate/cement ratio,percentage 0PC or aggregate replacement by a p022olan), environmental factors (relativehumidity, ambient concentration of CO2), the presence and the time of application of alime-cement mortar coating and its composition (water/cement, aggregate/cement andlime/cement ratios of the mortar, percentage OPC or aggregate replacement by a p0220Ian).

Corrosion of steel reinforcement is the most seriousdurability problem of reinforced concrete structures.Normally reinforcement is protected from corrosion bya thin layer of ferrous oxide that forms on its surface dueto the high alkalinity of the surrounding concrete mass.This alkalinity corresponds to a pH value somewhatgreater than 12.5 and is provided by the dissolution inthe pore water of even a very small amount of the solidCa(OHh in the hardened cement gel. A reduction in thepH value of pore water below 9 causes depassivation ofthe steel bars, due to dissolution of their protective surfaceoxide layer. The pH of the pore water drops at a certainpoint in the concrete mass to an equilibrium value ofabout 8.3, i.e. below the depassivation threshold, whenall the quantity of dissolved Ca(OH)2 in the pore waterand of solid Ca(OH)2 in the cement gel is converted toCaC03, due to reaction with the atmospheric CO2 thatdiffuses from the environment into the concrete poresystem. The process of conversion of calcium hydroxideto calcium carbonate is termed carbonation. For relativehumidities exceeding 50% but below 100%, soon aftercarbonation (and the associated reduction of pH)penetrates the concrete cover to the reinforcing bars,corrosion of the latter may start [1,2]. For such relative

humidities, experimental measurements [1,2] for a widerange of parameters have demonstrated that the rate ofsteel corrosion in carbonated concrete is much higherthan that of carbonation. Therefore, the time requiredfor carbonation to reach the reinforcement may beconsidered as a lower bound to the corrosion initiationperiod.

Concrete carbonation has been studied enough in thepast but the models so far developed have only anempirical or semi-empirical basis [1-8]. Development ofa quantitative model for the prediction of the initiationtime of carbonation-induced corrosion, in terms of designparameters which are known to, or controlled by, theengineer, would be of great practical value. The lessphenomenological such a model is, the greater itsapplicability and value would be. In this paper afundamental carbonation model is briefly described, andits practical application is demonstrated through para-metric studies. The first part of the paper presents thephysicochemical processes involved in carbonation ofconcrete or mortar, the physicochemical characteristicsaffecting its durability, a simplified mathematical modelwhich describes the carbonation phenomenon for relativehumidity values between 50 and 100%, and experimentalverification of the model. Furthermore, a sensitivityanalysis is performed on the effect of various compositionand environmental parameters on carbonation depth. Inthe second part of the paper, the effect of a lime-cement

mortar coating of concrete on carbonation is examined.A mathematical model is developed for the coating-concrete system, and applied to perform sensitivityanalyses on the effect of various composition parametersof the mortar coating on concrete carbonation.

2. CARBONAnON OF CONCRETEOR MORTAR

2.1 Physicochemical processes

The main reactions of concrete or mortar carbonationare those describing the conversion of Ca(OH)z (denotedalso by CH) and of CaO in the CSH compound ofhardened cement, into calcium carbonate:

Ca(OH)2+CO2 -7 CaC03 + H20 (la)

(3CaO·2Si02·3H20) + 3C02 -7 (3CaC03'2Si02'3HzO) (lb)

The calcium hydroxide reacting in Equation 1a isproduced by hydration of the calcium silicate compoundsof the clinker. In pozzolanic cements, or in concrete inwhich pozzolan is mixed as an additive, part of thiscalcium hydroxide is consumed by the pozzolanicactivity, and only the left-over quantity is available forcarbonation. In the presence ofpozzolans, the CSH whichcarbonates according to Equation Ibis not only thatproduced by the hydration of clinker, but also the oneresulting from pozzolanic activity of those CaO and SiOzoxides in the pozzolan which are in amorphous form [9].The chemical reactions of cement hydration and ofpozzolanic activity, along with their rates, are presentedin detail elsewhere [9,10].

The COz that reacts with the carbonatable constituentsof hardened cement paste according to Equations 1diffuses from the environment into the air-filled fractionof the pores of the concrete or the mortar according toFick's law, from regions of high COz concentration (theenvironment) to those of low. There, it is dissolved intothe film of water that covers the wall pores and reactsaccording to Equations 1. To react with COz, solidcalcium hydroxide is dissolved from the cement gel intothe pore water, and diffuses in it from regions of highalkalinity to those of low, i.e. where it reacts accordingto Equation 1a. 'Quantification of the rates of theprocesses of diffusion of COz in the gas phase and ofCa(OHh in the aqueous phase of the pores, as well asof the rates of the dissolution process of COz and of solidCa(OHh into the pore water, is given elsewhere [11,12].In the same references, the rates of the carbonationreactions (Equations 1) are also given.

The pH of the concrete mass, which controls depassiva-tion of the steel bars, depends on the concentration ofhydroxyl ions (OH -) in the pore water. When at a certainpoint within the concrete mass all the amount of dissolvedCa(OHh has been converted into CaC03, the pH valueof the pore water drops to an equilibrium value of 8.3,i.e. below the threshold of steel depassivation.

The evolution of the concrete carbonation processeswith time depends on the composition parameters of the

concrete and on the environmental conditions. The maincomposition parameters of concrete are the chemical andmineralogical composition of cement, and the water/cement (w/c) and aggregate/cement (a/c) ratios ofconcrete. The main environmental factors are the ambientCOz concentration ([COzJo, mol m - 3), the ambientrelative humidity (RH, %) and for outdoors unshelteredconcrete, the cycles of rain, snow, etc.

The degree of pore saturation is a controlling factorfor carbonation, as water blocks the pores and reducesthe diffusion of COz. On the other hand, water providesthe medium in which dissolved COz carbonates theCa(OH)z and the CSH according to Equations 1. So,experimental results, field observations and mathematicalmodels [13J show that carbonation is maximum atrelative humidities around 50%. Pore water is equallyimportant for the eventual corrosion of steel in carbonatedconcrete, as on the one hand it blocks oxygen diffusionand on the other it provides the electrolyte for thecorrosion process. So, corrosion in carbonated concreteis maximum for relative humidities around 90%, whereasit is extremely slow if the relative humidity drops below50%. In completely saturated concrete the blocking ofgas diffusion arrests both carbonation and corrosion.

The degree of pore saturation varies with time andlocation within the concrete, as the ambient relativehumidity changes and the external concrete surface maybe exposed to rain and snow [14]. Then water may movein by capillary suction and vapour diffusion-condensationon the pore walls, and evaporate later on, with a certainhysteresis after the relative humidity drops again [10].Water is also produced by the carbonation reactions(Equations 1) and consumed by hydration and pozzolanicactivity. So, a complete mathematical model of thecarbonation process should also account for the variationof pore saturation with location within the concrete andwith time. To avoid this additional and significantcomplication, it is assumed herein that the degree of poresaturation within the concrete is constant in space andtime, and is determined from hygrothermal equilibriumwith the average ambient humidity. This is a goodapproximation for indoor conditions and for outdoorconcrete sheltered from rain.

A general and comprehensive mathematical model of thephysicochemical processes involved in the carbonationof concrete or mortar has been developed [10-12]. Thismodel, which can be solved only numerically, is basedon the mass conservation of COz, Ca(OHh and CSH inany control volume of the concrete mass. In the limit ofa COz-diffusion controlled carbonation process, which isthe case for the usual range of relative humidities(RH > 50%), certain simplifying assumptions can bemade. These assumptions lead to the formation of acarbonation front separating the concrete (or mortar)volume in two regions: a completely carbonated regionand a non-carbonated one. The evolution with time t of

the distance of this front from the surface, calledcarbonation depth and denoted by Xc, is given by

X = (2[C02]ODe,co2 t)1/2c [CH] + 3[CSH]

where De, cO2 is the effective diffusivity of CO2 mcarbonated concrete or mortar (m2 S-l) and [CH],[CSH] are the molar concentrations of Ca(OH)2 andCSH at the end of the hydration period (mol m ~3),

Equation 2 is valid for constant effective diffusivity ofCO2 in both time and space, i,e, if there is hygrothermalequilibrium between a constant relative humidity environ-ment and the pore system of the concrete. For indoorapplications and for exterior concrete sheltered fromprecipitation, the variation of ambient relative humidityis limited and slow. Then one can still apply Equation 2with an average value of De, C02 in space and time. Anothercondition for the validity of Equation 2 is that cementhydration and any pozzolanic activity are (almost)complete. For normal curing, this condition is fulfilledabout 3-6 months after concrete casting.

The time tc required for the carbonation front to reachthe reinforcement can be obtained from Equation 2 bysetting Xc equal to the concrete cover of the reinforcement,c, and solving for t:

c2([CH] + 3[CSH])t = --------c 2[C02]ODe,co2

The information required for the application of Equation2 or 3 for predictive purposes comprises (a) the effectivediffusivity De. CO2' which depends on the compositionparameters of concrete, that determine its pore volumeand structure, and on the ambient relative humidity,which determines the degree of pore saturation with waterand is assumed constant herein; and (b) the concentrationsof the carbonatable constituents [CH] and [CSH], whichdepend on the composition parameters of cement, andon those of concrete or mortar. These dependencies aregiven below.

2.2.1 Molar concentration of carbonatable constituentsof concrete

The molar concentration of constituent k of cement perunit volume of mortar or concrete, [k], is given by theexpreSSIOn

[k] = Pc(l - Gair) Pk (4)M Wk[1 + (w/c)(pclPw) + (a/c)(pclPa)]

where Pc, Pw' Pa are the mass density of cement, waterand aggregates, respectively (kg m - 3), M ~ is the molarweight of constituent k (kg mol- 1), Gairis the volumefraction of entrapped or entrained air in concrete ormortar, and Pk is the weight of constituent k per unitweight of cement.

The weight fractions PCH and PCSH of Ca(OH)2 andCSH, respectively, are determined from the stoichiometry

of the reactions of cement hydration and of pozzolanicactivity, given in detail elsewhere [9,10]. For the weightfraction of calcium hydroxide, if

PCH = 1.32(pc,c + YcPc,p) - 1.85(ps,c + YsPs,p)

- 2.91(PA,c + YAPA,p) - 1.86(PF,c + YFPF,p)

+ 0.43pgy (5a)

PCH = 1.32(pc,c + YcPc,p) - 1.85(ps,c + YsPs,p)

- 2.18(PA,c + YAPA,p) - 2.32(PF,c + YFPF,p) (5b)

In Equations 5 and 6 Pi,c and Pi,p are the weights of theoxide i originating from clinker or pozzolan, respectively,per unit weight of blended cement (i = C, S, A or F, i.e.CaO, SiOz, Al203 or Fe203). Yi is the weight fraction ofoxide i in amorphous form in the pozzolan, and Pgy isthe weight fraction of gypsum in the cement. The weightsPi,c and Pi,p are determined from the weight of oxide iper unit weight of clinker or pozzolan, by multiplying itby the weight fraction of clinker or pozzolan in thecement, Pel and Ppo'

The information required for the application ofEquations 5 and 6 is the chemical composition of cement,i.e. the oxide analysis of the clinker and of the pozzolan(for the determination of Pi, c and Pi, p), the mineralogicalanalysis of the pozzolan (for yJ, the pozzolan fractionand the gypsum content. Application of Equation 4requires in addition knowledge of the mix proportionsof concrete or mortar.

2.2,2 Effective gas diffusivity and porosity of concreteor mortar

The effective diffusivity of CO2 in concrete or mortar,De,co2' depends on the volume, the structure and thedegree of saturation of the pores. It has been measuredas a function of the (constant) ambient RH, and of thecomposition parameters of cement and concrete, using aWicke-Kallenbach type of apparatus [10]. Measuredvalues were fitted by the empirical expression

De,co,Cm2 S-l) = (1.64 x 1O-6)G;8[1 - (RH/100)]22

(7)

in which Gp is the porosity of hardened cement pastecomputed from the total porosity of the concrete or themortar, G,through the expression

G = G 1 + (w/c)(pclPw) + (a/c)(pclPa) (8)p 1 + (w/c)(pclPw)

The porosity G of concrete or mortar, carbonated or not,defined as the volume fraction of the pores, can becomputed from the following expression, which has beenexperimentally verified elsewhere [9,10]:

G = Go - LlGH - LlGp - LlGc (9)

in which Go is the porosity of fresh concrete:

and LlGH, LlGp, LlGc are reduction terms due to hydrationof Portland cement, pozzolanic activity and carbonation,respectively. These terms are due to the fact that themolar volume of the solid products of hydration,pozzolanic and carbonation reactions exceed that of thesolid reactants, and are given below as a function of thechemical and mineralogical composition of cement.Using the molar volumes of the solid reactants and thecarbonation reaction products, term LlGc is computed as

LlG = 1O-3pc(l - GaiT)

c 1+ (W/c)(Pc/Pw) + (a/c)(pclPa)

The similarly computed values of the other two termsdepend on the amount of gypsum, in comparison to thatof C3A of the clinker and of the pozzolanically activeAl203 in the pozzolan. Ifpgy < 1.69PA,c - 1.08PF,c, then

10 - 3pc(l - Gai.)LlGH = ------------

1 + (w/c)(Pc/Pw) + (a/c)(pclPa)

x (0.252pc,c + 0.183ps,c + 1.054PA,c + OA11PF,c

+ 0.035pgy) (l2a)

LlG = 1O-3pc(l - GaiT)

p 1 + (W/C)(Pc/Pw) + (a/c)(pclPa)

x (O.289yc Pc, p - O.032ys PS, p + 1.121YAP A,p

+OA07YFPF,p) (13a)

Otherwise

A 1O-3pc(l - GaiT)LlGH = ------------

1 + (w/c)(pclPw) + (a/c)(pclPa)

X (0.252pc,c + 0.183ps,c + 1.112PA,c + O.374PF,J

(12b)

and for LlGp we have two different cases, if

YAPA,c- O.64YFPF,p> O.59pgy - PA,c + O.64PF,c

then

LlG = 1O-3pc(1 - GaiT)

p 1 + (w/c)(pclPw) + (a/c)(pc/P.)

X (O.289yc Pc, p - 0.032ys Ps, p + 1.121YAPA, p

+ OA07YFPF,p + 0.035pgy - 0.059PA,C + O.038PF,J

(13b)

LlG = 1O-3pc(l - GaiT)

p 1 + (W/c)(pjpw) + (a/c)(pclPa)

X (0.289yc Pc, p - 0.032ys PS,p + 0.370YFPF, p

+ 1.180YAPA,p) (13c)

For Portland cement, LlGp is zero, and for non-carbonatedconcrete, LlGc is zero.

2.2.3 Simplified carbonation depth formula forope concrete

Equations 4-13 can be used in Equation 2 to express theevolution of carbonation depth with time, in terms ofRH and of the composition parameters (a) of concrete:w/c, a/c, GaiT; (b) of cement PcI' Ppoand Pgy; and (c) of theclinker, Pi,c and of the pozzolan, Pi,p and Yi (i = C, S, Aand F). For non-air-entrained OPC concrete, and forcommon values of gypsum content in the OPC and ofthe composition of the clinker, Equations 4-6 yield thefollowing simple expression for the total molar concentra-tion of carbonatable constituents of concrete (mol m - 3):

[CHJ + 3[CSHJ :;::: 33000 (4a)1 + (pclPw)(w/c) + (pclPa)(a/c)

whereas Equations 8-13 yield the following for theporosity offully hydrated and carbonated cement paste:

(Pc) (w/c) - 0.3Gp:;::: Pw 1 + (Pc/Pw)(w/c)

Furthermore, a simpler, yet less accurate, alternative toEquation 7 has been proposed [10]. In this alternativeexpression the values 1.8 and 2.2 of the exponents arereplaced by the value 2.0, and coefficient 1.64 byapproximately 104. If this alternative expression is usedin Equation 2, along with Equations 4a and 8a and withthe value 42Yc02 (mol m - 3) for [C02J (Yc02 is theambient CO2 content by volume), then the followingsimplified expression is obtained for the carbonationdepth Xc (in mm) in OPC concrete:

X :;::: 350(PC) _(_w_/c_) _-_0_.3_ (1 __RH_)c Pw 1 + (pclPw)(w/c) 100

In Equation 2a, time t is in years.It is worth comparing Equation 2a with an expression

empirically fitted by Morinaga [1,2J to limited carbona-tion test data. For w/c ::; 0.6, and with the same units asin Equation 2a this phenomenological expression is

For w/c > 0.6, the term in parentheses containing thewater/cement ratio is replaced [1J by

(w/c) - 0.25[1 + 2.6(w/C)J1/2

For values of w/c between 0.5 and 0.65 these empiricallyderived expressions are in good agreement with Equation2a, developed herein on a fundamental basis. For highervalues of w/c, Equation 2a predicts lower carbonationdepths than the expressions of Morinaga [1,2J, thedeviation being greater for high water/cement ratios.

2.3 Experimental results and comparison with theory

As in normal atmospheric environments with 0.03-0.05%of CO2 the evolution of carbonation depth with time isvery slow, an accelerated carbonation chamber has beendeveloped and used in this study. In this chamber,concrete specimens have been exposed to 50% CO2-50%air mixtures under controlled temperature and relativehumidity conditions. Carbonation depths were measuredperiodically using a phenolphthalein pH indicator.Details of the apparatus, the test procedure, the specimensand the materials used are given elsewhere [9,12].

Fig. 1 shows the measured evolution of carbonationdepth with time, in concrete made with OPC orpozzolanic cement. This latter blended cement, denotedby AHPC, had a pozzolan (fly ash of low CaO contentand of high pozzolanic activity) content of 20%. Fig. 1shows also the effect of water/cement ratio and aggregate/cement ratio. The solid lines correspond to Equation 2,and are in excellent agreement with the test results. Ithas been found also that Equation 2 is valid for normalexposure tests as well [12].

Equation 2 is in good agreement with experimentaldata only for RH values above 50%. For lower relativehumidities it overestimates carbonation depth, becausethen the rate-controlling processes are the carbonationreactions rather than the diffusion of COb as presupposedfor the derivation of Equation 2. Under low relativehumidity conditions, one has to solve numerically thegeneral mathematical model of concrete carbonationpresented elsewhere [11-13].

2.4 Parametric studies

Sensitivity analyses have been performed of the effect ofvarious parameters on concrete carbonation. Thesesensitivity analyses (Table 1) refer to fully hydratedcement, without entrained or entrapped air, and to aconstant temperature of 20°e. The baseline case for thisanalysis is an OPC concrete (65.3% CaO and 23.6% Si02

in the clinker and 5% of gypsum) with w/c = 0.65 anda/c = 5, at a constant relative humidity of 65% and in a0.05% CO2 environment (average urban conditions).

The very significant effect of water/cement ratio isshown in Fig. 2a. When w/c increases, Xc increases dueto the increase in concrete porosity and, consequently,

4,.....

507.C02 657.RH :30'CE

N type w/e olea:3 .OPC 0.50 :3

)(

0.65 :3::I: .OPCl- .OPC 0.80 :3a..~ 2z01-1I-

<Z0

(a)IIIII:::<U

0 2 :3 4 5HIME (~dOY5)

4,.....

507.C02 657.RH :30'CE

N type w/e olea:3 .OPC 0.65 :3

)(

::I: .OPC 0.65 5l- .OPC 0.65 7a..~ 2 • AHPC 0.65 :3

z01-1I-

<Z0 (b)IIIII:::<U

0 2 :3 4 5HlME (~dOY5)

Fig. 1 Effect of exposure time and concrete compositionparameters on carbonation depth. Symbols: experimentalresults, lines: Equation 1.

Table 1 Scheme of parametric studies of effect of concretecomposition and of environmental factors

wlc ale Ppo Pro RH Yea,(% OPC) (% aggr.) (%) (%)

Baseline 0.65 5 0 0 65 0.05Fig.2a 0.45-0.85 5 0 0 65 0.05Fig.2b 0.65 3-7 0 0 65 0.05Fig.3a 0.65 5 0-40 0 65 0.05Fig.3b 0.65 5 0 0-8 65 0.05Fig.4a 0.65 5 0 0 50-90 0.05Fig.4b 0.65 5 0 0 65 0.02-0.10

in effective diffusivity as well. The effect of varying a/c isminor, as shown in Fig. 2b: when a/c increases, Xc

increases slightly because the molar concentration ofCa(OHh and CSH in the concrete decreases.

Fig. 3 shows the effect of OPC (Fig. 3a) or aggregate(Fig. 3b) replacement by a pozzolan (AHP: fly ash of lowcontent in CaO). In the former case w/c is calculated on

20 40TIME (years)

OPC w/c-0.65

0.057.C02 6570RH

20 40TI~ (years)

Fig. 2 Effect of concrete composition parameters oncarbonation depth: (a) the effect of water/cement ratio,(b) the effect of aggregate/cement ratio.

the basis of the total quantity of blended cement, whereasin the latter case a/c is calculated on the basis of thequantity of clinker and gypsum alone. The significantincrease in Xc when pozzolan replaces part of the clinkeris due to the combined effect of increased porosity andthe reduced concentration of carbonatable constituents[9]. The latter case (the use of pozzolan as an additivethat replaces aggregates, Fig. 3b) works in the oppositedirection, but to a much lesser degree.

Fig. 4a shows the effect of relative humidity: asignificant reduction in Xc is obtained when relativehumidity increases, due to the reduction of effectivediffusivity. Finally, Fig. 4b shows the moderate effect ofchanging the ambient CO2 content within its usual rangeof variation.

3. CARBONA nON OF CONCRETE WITH ALIME-CEMENT MORTAR FINISH

The application of surface coatings to concrete has beenproposed by many as a means of reducing the rate of

20 40TIME (years)

20 40TIME (years)

Fig. 3 Effect of pozzolan addition on carbonation depth: (a)effect of ope replacement by fly ash (AHP), (b) effect ofaggregate replacement by fly ash (AHP).

carbonation. For example, Hankins [15J applied andexamined more than 10 alternative coatings (among thema 6 mm thick layer of waterproof OPC mortar, with anestimated alc of 6 and a wlc of 1.8) with respect to theireffectiveness as carbonation retarders. Among thesecoatings only one, consisting of three brush-applied coatsof soluble organic silicone resin or siloxane acrylic resins,and another consisting of three brush or roller appliedcoats of vinyl acetate copolymer or pure acrylic emulsion,were found effective in that respect. As another example[IJ eight different surface coatings were tested ascarbonation retarders. A vinyl wallpaper was found veryeffective, a cement mortar and a resin plaster were foundfairly effective, whereas acrylic or epoxy-based sprayfinishes were less successful. Cement-based spray finishesor a lithium silicate surface hardener had almost no effect[1].

In many countries the ceiling and wall surfaces of mostbuildings are finished by covering them with plaster orrender, a mixture of a cementing material, an inert fineaggregate, and water [16,17]. For interior surfaces thecementing material is sometimes gypsum or hydrated

OPC w/c-0.65

0.057.C02

20 40TIME (years)

20 40TIME (years)

Fig. 4 Effect of environmental conditions on carbonationdepth: (a) effect of ambient relative humidity, (b) effect ofambient CO2 content.

lime. Mixtures of cement and hydrated lime are used ascementing material for exterior or hard-usage surfaces,but also sometimes for interior ones. In what follows wewill concentrate on this latter case, i.e. on the applicationof a lime-cement mortar coating and on its effect on therate of carbonation. We will consider only lime producedby hydration of high-calcium quicklime, as that originatingfrom the burning of limestone. Finally, in the followingwe will call dry hydrated lime, i.e. Ca(OH)z withoutexcess water, simply 'lime', and we will denote by I itsweight in the mixture in a water-saturated, surface-dryform.

3.2 Mathematical model

The mathematical model of carbonation of concretewith a mortar coating is developed with reference to Fig.5: superscripts (1) and (2) are used for plaster and forconcrete, respectively. The thickness of the coating isdenoted by d and the distance from the outer surface ofthe coating by x. The model applies to one-dimensionalgeometry, i.e. to concrete walls, slabs, beams or columns

with planar external surfaces, with the exception of cornerregions near the intersection of external surfaces, and ofthe vicinity of macroscopic cracks. A major hypothesismade is that the simplifying assumptions made for thecarbonation of pozzolanic cement concrete or mortar[11-13J, which have led to the formation of a carbonationfront and to the simple Equation 2, can also be madefor the carbonation of lime-cement mortar. So the onlydifference is that the physicochemical characteristics, suchas De,co2' [CHJ and [CSHJ are different for thelime-cement mortar due to the presence of lime. Thecarbonation depth, measured from the outer surface ofthe coating, is denoted by Xc> whereas that in concrete,measured from the coating-concrete interface, is denotedby XC"

The carbonation of mortar-coated concrete consists oftwo phases. In the first, carbonation is limited to thecoating and concrete remains unaffected. The end of thisphase occurs at time td (at which carbonation depthequals d), given by

So, during the first phase, i.e. for 0 ~ t ~ td, thecarbonation depth Xc in the coating is given by Equation2, with values of the parameters De,co2' [CHJ and [CSHJfor the coating mortar, i.e. with superscript (l). Duringthe second phase, i.e. for t > td, CO2 is diffused accordingto the equation

within the coating (0 ~ x ~ d), which is fully carbonated,and within the carbonated region of concrete (d ~ x ~ XJ.The boundary conditions are

at x = 0: [C02J = [C02JO

at x = Xc: [C02J = 0

(l6a)

(l6b)

The value of the concentration of CO2 at the coating-concrete interface, denoted by [C02Jd, is an unknown to

Integration of Equation 15 yields a bilinear concentrationprofile for COz, between the pairs of values given byEquations 16a, 16c for 0:::;; x :::;;d and 16c, 16b ford:::;; x :::;;Xc:

d o (x) 0[COzJ = ([COzJ - [COzJ ) d + [COzJ

d X - XC[CO?J = [COzJ --

- d - Xc

The value of [COzJd is found from continuity of flux atthe coating-concrete interface:

_ D(l) (d[COzJ)(l) = _ D(Z) (d[COzJ)(Z)e, C02 dx e, C02 dx

[CO Jd - [CO JOD(l) d - Xc ( 9)Z - Z e,C02D(l) (d-X)-D(Zl d 1

e, co, c e, C02

The location of the carbonation front Xc or Xc as afunction of time is determined then from the differentialequation of the front, derived elsewhere [9,10]:

:x (D~:t02d[~~zJ) + :t ([CHJ(Z) + 3[CSHJ<Zl) = 0

at x = Xc (20a)

dXc D~~~o,[COzJOdt {Xc - d[l - (D~~~o,/D~~~o,)J}([CHJlZ) + 3[CSHJlZ})

(20b)

which can be integrated again, from t - td and Xc = dto the current values of t and Xc:

[2D(Z) [CO JO (D(2) )ZJIIZX - d + e.CO, Z (t _ t ) + dZ e,CO,

c - [CHJlZ) + 3[CSHJlZ) d D~~~o,

D(Z)_ d e,CO,

D~~~o,

from which Xc = Xc - d can easily be obtained.From Equation 21 one can obtain the time required for

the carbonation front to reach the reinforcement at

t = (eZ + 2deD~:to,/D~~toJ([CHJ(Zl + 3[CSHJ<Zl)c 2D~:toJCOzJo

dZ([CHJ(ll + 3[CSHJ(l»

+ 2D~~toJCOzJo

In the above it has been assumed for analyticalconvenience that the mortar coating is applied almostimmediately after the end of the concrete curing. Inpractice, however, a relatively long period of time, of theorder of months, elapses between the end of concretecuring and the application of the finishing coat. In othercases, the concrete surface maybe left initially uncoated,and after several years it may be decided to apply thefinish, for reasons of architectural appearance, or formaintenance and protection from further deterioration.During the period of time, denoted here by ta> in whichthe concrete surface remains exposed to the environment,the concrete is left to carbonate and the carbonation frontreaches a depth xc,a obtained from Equation 2 for t = taand for parameter values equal to those of the concrete(superscript (2». Right after its application at time ta' thecoating mortar starts to carbonate according to Equation2, with the values of the parameters superscripted by (1),until the carbonation depth in the coating reaches itsthickness d at the time td (equal to that resulting fromEquation 14 plus the time of delay of the coatingapplication ta)' In the meantime the amount of COzentrapped in the already carbonated depth of concrete,xc,a (which follows a linear profile from [COzJo at theinterface to 0 at xc,a) diffuses backwards into the mortarcoating, and inwards beyond xc,a' carbonating thecorresponding materials. However, because of its verysmall concentration per unit concrete volume relative tothat of the carbonatable constituents CH and CSH, thisdiffusion of COz causes very little propagation of thecarbonated region beyond the one limited by x = 0 andx = xc, a' On the contrary, as in this latter fullycarbonated region the concentration of dissolved CH inthe pore water is zero, dissolved calcium hydroxide willbe diffused in this region from neighbouring yet-un carbonated areas. Since diffusion of very little quantityof dissolved CH is required for the equilibrium concen-tration of CH in water to be attained, the pH value inthe already carbonated depth of concrete will go backto about 12.5, possibly passivating again the previouslydepassivated bars (provided that corrosion has not yetstarted). However, as the total quantity of Ca(OH)z inthis previously carbonated concrete depth is very small,shortly after arrival of the carbonation front at thecoating-concrete interface this total quantity of dissolvedCa(OHh will react with the new COz that diffuses in,and the carbonation front will jump to xc,a' The evolutionof carbonation depth beyond xc. a is easily obtained byintegration of Equation 20b from t = td and Xc = d + xc•a

to the current values of t and Xc' Then, the carbonationdepth in the concrete (measured from the interface) is

[2D(Z) [CO JO ( D(Z) )ZJ1/2

- e,CO, Z (t t) + d •. co, +Xc - [CHJ(Z) + 3[CSHJlZ) - d D~~~o, Xc .•

D(2)_ d e.CO,

D~~~o,

t = (e2 + 2d(e - Xc.a)D~2~o,/D~:~02)([CHJ(2) + 3[CSHJl2)

c 2m:~02[C02JO

d2([CHJ(1) + 3[CSHJl1l)

+ 2D~:~02[C02JO

3.3 Physicochemical characteristics of the coating

The molar concentrations of Ca(OH)2 and CSH, [CH](lland [CSH](l) respectively, to be used in Equation 24,can be obtained by a simple modification of Equation 4as follows:

1 + ~ (Pc) + ~ (Pc) + ~ (Pc)e Pw e Pa e PI

to include the lime content in the mortar coating, in whichPI is the mass density of lime (kg m - 3), and l/e is thelime/cement ratio by weight. The denominators ofEquations 10-13 and the numerator of Equation 8 aremodified in the same way.

2. The lime content of the mortar is added to theright-hand side of Equations Sa and 5b to yield the totalcontent of CH, pgJ, after completion of hydration, etc.,of the coating.

It is further assumed that the effective diffusivity of CO2in a carbonated mortar coating, D~:b2' can be computedfrom Equation 7, which has been empirically fitted to alarge data set derived from Portland or pozzolaniccement mortar or concrete. Point No.1 above should betaken, though, into account in Equations 8 and 10-13.

3.4 Parametric studies

A series of sensitivity analyses has been performed toexamine the effect of mortar coating compositionparameters, etc., on the concrete carbonation. Thesesensitivity analyses (Table 2) refer to the same baselinefor the concrete and for the environmental conditions as

Table 2 Scheme of parametric studies of effect oflime-cement mortar coating composition

Ppo Ppo ta(% OPC) (% aggr.) (years)

Baseline 0.8 6 0.5 0 0 0Fig.6a 0.6-1.0 6 0.5 0 0 0Fig.6b 0.8 4-8 0.5 0 0 0Fig.6c 0.8 6 0-1.0 0 0 0Fig.7a 0.8 6 0.5 0-40 0 0Fig.7b 0.8 6 0.5 0 0-20 0Fig. 8 0.8 6 0.5 0 0 0-20

Concrete parameters: OPC, wle = 0.65, ale = 5; mortar coatingthickness 20 mm; environment: 65% RH, 0.05% Yeo" 20°e.

the previous analyses (see section 2.4), and to alime-cement mortar coating thickness of 20 mm. Thebaseline case for the coating is an OPC mortar withw/e = 0.8, a/e = 6 and l/e = 0.5, applied almost simulta-neously with concrete casting (ta = 0).

The results of this series of analyses are presented inFigs 6-8. Carbonation depth is taken to be the depth ofthe front from the coating-concrete interface, i.e. XC" Thedashed lines in the figures correspond to uncoatedconcrete (d = 0) and the solid lines to coated. As shownin the figures, addition of a 20 mm thick layer of mortarcoating postpones the onset of carbonation in theconcrete for more than 20 years (sometimes for 60 yearsor even longer), after which the advancement ofcarbonation within the concrete itself is much slower thanduring the initial period of carbonation of uncoatedconcrete (note that the horizontal scale in Figs 6 and 7is proportional to the square root of t), because CO2 hasto diffuse through an additional 20 mm of mortar coating.The effects of varying the values of w/e and a/e of themortar coating (Fig. 6a and b, respectively) are similarto those observed in the previous sensitivity analyses forconcrete. The very significant effect of varying the ratiol/e (Fig. 6c) is due to the increased concentration ofCa(OHh Similar results for the effect of pozzolanaddition to the coating are shown in Fig. 7a and b. Thedifference with the concrete case, as far as the effect ofaggregate replacement by a pozzolan is concerned, is thatin the case of mortar coating the higher Ca(OH)2 contentallows greater replacements than in concrete. Fig. 8 refersto the case of delayed application of the coating, forvalues of the delay time ta from 0 to infinity. An interestingresult is that even for very late application of the coating,of the order of 10 or 20 years, the carbonation depth inthe concrete increases beyond its initial value prior tothe application of the coating only when the structureapproaches its conventional life-time (typically of theorder of SO years). So, the late application of a mortarcoating to initially exposed and already carbonatedconcrete turns out to be a very effective technique forarresting carbonation.

No experiments have been conducted within theframework of this study to verify the proposed model forthe carbonation of the plaster-concrete system. Never-theless, the two tests by Morinaga [1] on the evolutionof carbonation in concrete specimens with a cementmortar or a resin plaster finishing exhibit the samepattern as that depicted in Figs 6 and 7: onset ofcarbonation in the concrete itself only after a significantdelay of time, and from then on, evolution of thecarbonation front almost parallel to that in uncoatedconcrete, on an Xc versus tl/2 plot. Unfortunately, th~ in-formation given [1] is not enough to support an attemptto reproduce those test results by the present model.

In this paper a fundamental, yet very simple andself-contained, mathematical model for the time evolution

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Fig. 6 Effect of mortar coating composition parameters onconcrete carbonation depth: (a) effect of water/cement ratio,(b) effect of aggregate/cement ratio, (c) effect of lime/cementratio.

4

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Fig. 7 Effect of pozzolan addition in the mortar coating onconcrete carbonation depth: (a) effect of ope replacementby fly ash (AHP), (b) effect of aggregate replacement by flyash (AHP).

4

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of carbonation of concrete or mortar is developed, as asynthesis of segmental quantitative information presentedelsewhere on all the physicochemical processes [11,12]and the physicochemical characteristics and parameters[9,10] involved in, or related to, carbonation. Theapplicability and accuracy of the simple form of the modelpresented herein is experimentally verified for both OPCand pozzolanic cement concrete and mortar, but only forconstant relative humidities above 50%. The simpleversion of the carbonation model is extended herein to(lime-cement) mortar coating-concrete systems, in whichthe mortar coating is applied to the concrete surface eitheralmost immediately after the end of concrete curing, orafter the lapse of a certain time. The information requiredfor the application of the simple form of the model isreadily available to the designer of a concrete structure.It consists of (i) the mix proportions of the concrete andof the mortar coating, if any, and the chemicalcomposition of the OPC or pozzolanic cement; (ii) the(constant) relative humidity and CO2 concentration; and(iii) the thickness and time of application of any mortarcoating. Parametric analyses performed with the model,to study the sensitivity of the time evolution ofcarbonation to the above composition parameters andto other design variables, show that the most criticalparameter is the water/cement ratio of concrete (and ofmortar coating, if any), which determines porosity,followed by the OPC-pozzolan replacement, whichdetermines the content of Ca in the concrete or in thecoating in a carbonatable form, and by the lime/cementratio of the mortar coating, if any.

Provided that its water/cement ratio is relatively lowand its lime content high, a lime-cement mortar coatingof the usual thickness (around 20 mm) is found to be anextremely effective means of postponing or even preventingcarbonation-induced corrosion initiation: it postponesthe onset of concrete carbonation by the time requiredfor carbonation to fully penetrate the coating, and itretards its further penetration into the concrete becauseatmospheric CO2 has to travel further to reach thecarbonation front. Its effectiveness is not reduced verymuch even if it is applied to the concrete surface severalmonths or even a few years after the end of the concretecuring period. Application of such a surface coating tocarbonated concrete several years old arrests advance-ment of the carbonation front in the latter, and reinstatesalkalinity of the already carbonated concrete region toits initial high value above the threshold of steeldepassivation. In fact, such a belated application of amortar coating may appreciably prolong the service-lifeof already carbonated concrete, and hence may beconsidered as an effective, yet easy and inexpensive, repairmethod for carbonation-afflicted concrete.

1. Morinaga, S., 'Prediction of service lives of reinforcedconcrete buildings based on rate of corrosion ofreinforcing steel', Special Report No. 23 (Institute ofTechnology, Shimizu Corporation, Tokyo, 1988).

2. Idem, 'Prediction of service lives of reinforced concretebuildings based on rate of corrosion of reinforcing steel "in Proceedings of 5th International Conference onDurability of Building Materials and Components,Brighton, UK, November 1990, pp. 5-16.

3. Hamada, M., 'Neutralization (carbonation) of concrete andcorrosion of reinforcing steel', in Proceedings of 5thInternational Symposium on Chemistry of Cement,Tokyo, 1969, Vol. 3, pp. 343-369.

4. Schiessl, P., 'Zur Frage der zuliissigen Rissbreite und dererforderlichen Betondeckung im Stahlbetonbau unterbesonderer Beriicksichtigung der Karbonatisierung desBetons', Deutsch. Auss. Stahlbeton 255 (1976).

5. Tuutti, K., 'Corrosion of steel in concrete', CBI ForskningResearch (Swedish Cement and Concrete ResearchInstitute, Stockholm, 1982).

6. Ying-yu, L. and Qui-dong, W., 'The mechanism ofcarbonation of mortar and the dependence of carbonationon pore structure', in Proceedings of Katharine andBryant Mather International Conference on ConcreteDurability, Atlanta, ACI SP-100 (American ConcreteInstitute, 1987) pp. 1915-1943.

7. Nagataki, S., Mansur, M. A. and Ogha, H., 'Carbonationof mortar in relation to ferrocement construction', ACIMater. J. 85(1) (1988) 17-25.

8. Richardson, M. G., 'Carbonation of Reinforced Concrete'(Citis, Dublin, 1988).

9. Papadakis, V. G., Fardis, M. N. and Vayenas, C. G.,'Hydration and carbonation of pozzolanic cements',ACI Mater. J. 89(2) (1992).

10. Papadakis, V. G., Vayenas, C. G. and Fardis, M. N.,'Physical and chemical characteristics affecting thedurability of concrete', ibid. 88(2) (1991) 186-196.

11. Idem, 'A reaction engineering approach to the problem ofconcrete carbonation', AIChE J. 35(10) (1989) 1639-1650.

12. Idem, 'Fundamental modeling and experimental investiga-tion of concrete carbonation', ACI Mater. J. 88(4) (1991)363-373.

13. Idem, 'Experimental investigation and mathematical model-ing of the concrete carbonation problem', Chem. EngngSci. 46(5/6) (1991) 1333-1338.

14. Bijen, J. M. J. M. (ed.), 'Maintenance and repair of concretestructures', Heron 34(2) (1989).

15. Hankins, P. l, 'The use of surface coatings to minimizecarbonation in the Middle East', in Proceedings of 1stInternational Conference on Deterioration and Repairof Reinforced Concrete in the Arabian Gulf, Bahrein,1985, Vol. 1, pp. 273-285.

16. Huntington, W. C. and Mickadeit, R. E., 'BuildingConstruction. Materials and Types of Construction', 5thEdn (Wiley, New York, 1981).

17. Hornbostel, c., 'Construction Materials. Types, Uses andApplications' (Wiley, New York, 1978).

Effet de la composition, des facteurs ambiants et durevetement de mortier chaux-eiment sur la carbonatationdu beton

On decrit ici les processus physico-chimiques de carbonata-tion du heton et on presente un modele mathematique simpled' evolution de la carbonatation en fonction du temps,applicable a une humidite relative constante de plus de50%. Ce modele s 'appuie sur les principes fondamentauxdu processus de la reaction chimique, et utilise commeparametres la concentration ambiante en CO2, les concen-trations molaires des constituants carbones, CaOHz etCSH, dans Ie volume de heton, de la diffitsivite effective deCO2 dans Ie heton carbonate, qui est obtenue par unefonction empirique de la porosite de la pate de ciment durciet de I' humidite relative, derivee des essais de diffusion enlaboratoire.

On demontre la validite du modele pour les mortiers et

hetons de ciment pouzzolanique (OPC) en comparant sesprevisions avec les resultats d' essai obtenus dans uneambiance de concentration de CO2, d' humidite et detemperature contra lees. On etend Ie modele mathematiqueau cas de la carbonatation du systeme revetement-beton,pour un heton recouvert d 'une couche de finissage demortier de chaux-ciment appliquee presque immediatementapres la fin de la prise du beton ou apres un certain temps.

Les etudes des parametres montrent comment I' evolutionde la profondeur de carbonatation enfonction du temps estaffectee par la composition du ciment et du heton (rapporteau/ciment ou granulats/ciment, pourcentage d'OPC ouremplacement du granulat par une pouzzolane), lesfacteursambiants (humidite relative, concentration en CO2), lapresence et la duree d' application d 'un revetement demortier de chaux-ciment et sa composition (rapportseau/ciment, granulat/ciment et chaux/ciment du mortier,pourcentage d'O PC oU de remplacement du granulat parune pouzzolane).