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Revista ingeniería de construcción

versión On-line ISSN 0718-5073

Rev. ing. constr. v.24 n.3 Santiago dic. 2009

http://dx.doi.org/10.4067/S0718-50732009000300003 

Revista Ingeniería de Construcción Vol. 24 N°3, Diciembre de 2009 PAG. 245-258

 

Modelling of the carbonation of concrete (UR-CORE) from fractionafconversion data obtained through in situ monitored neutrón diffraction experiments

 

Marta Castellote* , Carmen Andrade

* Instituto de Ciencias de la Construcción Eduardo Torroja, lETcc, (CSIC). Madrid, ESPAÑA

Corresponding author:


ABSTRACT

One of the most important depassivation mechanisms of steel reinforcement in concrete is that caused by the neutralisation of the cement matrix. In this work, a summary of the development of a model for the carbonation of cementitious matrixes (UR-CORE) is presented. On one hand, in-situ monitoring of the changes that take place in the phase composition of cement pastes during accelerated carbonation (100% C02) for different binders, is reported, by taking Neutrón Diffraction patterns in parallel with the carbonation experiments. The variation of the intensity of chosen reflections for each phase along the experiment supplied data, in real time, for fractional conversión of different phases. On the second hand, fitting of these results allowed to make a quantitative approach to the kinetics of the carbonation of the different phases, and develop the UR-CORE model that is based on the principies of the "unreacted-core" systems, typical of chemical engineering processes.

Keywords: Accelerated carbonation, in situ monitoring, neutrón diffraction, modelling, UR-CORE


 

1. Introduction

Carbonation of concrete, as consequence of its interaction with the atmospheric C02, has been considered for years a subject of interest in researches of cement chemistry (Tokyay, 1997; Alonso et al., 1988). The process known as carbonation, is a complex physicochemical process that slowly modifies the structure of the concrete in the course of time and induces changes into its physical (Pihlajavaara, 1998; Houst and Wittmann, 2002) and chemical (Susuki et al., 1989; Richardson et al., 1993; Nishikawa y Suzuki, 1991; Kobayashi et al., 1994; Kim et al., 1995; Groves et al., 1991; Sergi, 1986; Berger, 1979; Sauman and Lach, 1972; Colé y Kroone, 1959; Colé and Kroone, 1960) properties.

Despite much research carried out on the subject, related to the prediction of the advance of the carbonation front, or on service life calculation of reinforced concrete, there is still fundamental doubts, as two main problems arise: On one hand, carbonation is assumed to be, mainly, a diffusional phenomenon, where the carbonation front moves inwards the concrete at a rate proportional to the square root of time (Rahman and Glasser, 1989; Venuat and Alexandre, 1968; Weber, 1983; Smolczyck, 1968). In addition to the simple square root of time expression, several service life models can be found in literature, being some of the most used those given in (Tuuti, 1982; Bakker, 1964; Parrot, 1994). Some of them use the diffusion coefficient as the basic transport parameter (Tuuti, 1982; Bakker, 1964), while others use the air permeability coefficient (Parrot 1994). More recently, a model based in the use of the resistivity has been also proposed (Andrade et al., 2000). However, no experimental results have been found in literature on the reliability of the short term results for predicting long term results, finding a general overestimation of the predicted results at long term (Sanjuan et al., 2003).

On the other hand, there is not a generally accepted test method, mainly due to the fact that the natural carbonation is a very time consuming process and accelerated carbonation, using higher C02 concentrations, is a common practice in most of laboratories, existing many doubts on the validity of the accelerated tests to extrapólate results in natural conditions. This is due mainly to the fact that the resulting microstructure of pastes carbonated using puré C02 is different that those of air-carbonated pastes (Suzuki et al., 1989; Groves et al., 1991; Bier et al., 1989; Sergi, 1986; Verbeck, 1958; Sanjuan et al., 2003; Catellote et al.,2008), and even the resulting amount of carbonated material increases when increasing the C02 concentration (Castellote (a) et al.).

This paper presents the summary of a model for the carbonation of cementitious matrixes based on the principies of the "unreacted-core" systems, in which the reacted product remains in the solid as a layer of inert ash, adapted to the specific case of carbonation, UR-CORE (Castellote y Andrade, 2008). The development of the model has been possible through the in situ monitoring, by neutrón diffraction, of accelerated carbonation tests, at 100% C02 concentration, of cement pastes with different binders (Castellote et al., 2008). In Castellote et al., 2008 and Castellote and Andrade, 2008, detailed explanation of the in-situ neutrón diffraction experiments and on the development of the model are given. In this paper, a summary of the essentials of the model in conjunction with the f ractional conversión, for different phases of the cement pastes, obtained through the in-situ experiments is presented.

2. Experimental procedure

2.1 Materials and preparation of specimens

For the in-situ carbonation experiments [54], cement pastes (with three different types of binder) were prepared by hand mixing with deuterated water from 99.95% purity to a w/c ratio of 0.5. The first binder was plain OPC cement type I/45A/SR-MR, having a 5% of addition of lime. The second one, mix OPC-FA, was prepared using the same cement substituted in a 35% by fly ash. The third type, mix OPC-MS, was used again the same cement with a substitution of a 10% by micro-silica. The chemical analyses of the Cement, Fly Ash and Micro-Silica, are given in Table 1.

These paste specimens were cast in cylindrical plástic moulds, which were sealed, in order to avoid carbonation, and allowed to cure for 28 days at room temperature (22°) in a humid chamber (HR> 95%). The resulting specimens were cylindrical, 9 mmΦ; and 32 mm height.

Table 1. Chemical analysis of the cement, fly ash and micro-silica used

2.2 Techniques and procedures

After curing and demoulding, the deuterated specimens of every mix were kept sealed, in order to avoid carbonation, until the in-situ experimental triáis, that took place 130 after casting them.

A glassed mini-carbonation-chamber was designed for the carbonation experiments, consisting in a cylindrical body with a tube and a valve in its bottom. It has a perforated glass separating the body of the device and the valve that allows filling in the bottom part with the 65% HR regulation solution (saturated solution of NaN03) keeping it separated from the specimen. The device has a hermetic cap, where the sample was suspended, with three inlets (inlet of C02, outlet of C02 and aeration). The C02 gas was delivered at a concentration of 100%, through a tube, covered by a sheet of cadmium (for avoiding interferences of the plástic tube with the neutrons), at the bottom of the cylindrical device, in order to assure the right circulation of the gas through the sample.

The in-situ carbonation was followed on line by simultaneous acquisition of diffraction patterns at the D20 instrument of the Institute Max von Laue - Paúl Langevin (ILL), in Grenoble, France, and the powder diffraction patterns were analysed by a standard procedure. Crystalline phases were identified by a search-match manual procedure and selected peaks for Portlandite, Calcite, Ettringite and a Calcium silicate hydrate were fitted to Gaussian curves for the whole series. The variation of intensity of a chosen reflection for a particular phase along the experiment (related with the concentration) has been used to monitor concentration changes.

2.3 Carbonation Model

In a simplified way, carbonation of a cementitious matrix can be considered as a heterogeneous reaction system in which several solid phases of the sample react in parallel with the CO2 (gas). However, as most real processes, this is a quite complex system, that can differ considerably from the stoichiometric equations. In fact, in this type of heterogeneous systems, the overall rate expression becomes complicated because of interaction between physical and chemical processes. This is introduced, in the models, by the requirement that reactants in one phase have to be transported to the other phase containing other reactants where the reactions take place.

There are various mathematical models, in the field of the chemical engineering, for the description of the gas-solid reactions.

Among them, simplified models of the type "unreacted-core" have proved to represent the actual behaviour closely, in most cases in which the reacted product remains in the solid as a layer of inert ash, as it is the case of carbonation. However, itt is necessary to be aware that it has been proved, by TG profiles, that there is a transitional zone rather than a sharp interface (Rahman and Glasser, 1989; Parrott, 1992; Houst and Wittmann, 2002) in the carbonation front. However, testing the carbonation front using chemical indicators (phenolphthalein) is also satisfactory for standard depth of carbonation tests and probably gives the best indication as far as the overall depassivation of the steel is concerned (Rahman y Glasser 1989).

Therefore, the main premise of an unreacted-core model is considered to be accomplished enough. Thus, it can be applied and the overall rate expression may be formulated by considering the successive steps involved in the process, that are the following ones:

a)  Diffusion through the gas film: Diffusion of gaseous reactant, C02, from the bulk of the first phase (phase gas) to the interface between the two phases (gas-solid).

b)  Penetration and diffusion of the gas through the ash shell to the surface of the core not yet reacted (Diffusion of C02 through the external carbonated part of the sample).

c)  Chemical reaction between the reactants.

d)  Diffusion of the products formed that do not precipítate to form the ash shell (water vapour/liquid released in the reaction) from the surface of the unreacted core towards the external surface.

e)  Diffusion of these non solid products through the gas film.

These steps have been schematically represented in figure 1, in which the transitional zone in the surface of the unreacted core has also been depicted. In the left part of the figure, the change in the concentration of the reactant gas (C02) have been depicted for the steps a), b) and c) while the right part of the figure shows the steps c), d) and e) for the water formed.

Considering the resistance of all this steps would make the problem very difficult to solve, and that the resistance of the different steps normally is quite different, the slowest step is the controlling one and will determine the rate of the global process.

Figure 1. Schematic representation of the successive steps involved in the unreacted-core model. In the left part, change in the concentration of the reactant gas (C02) for the different steps (a, b and c ). In the right part, steps c), d) and e) for a non- solid product formed

In the case of carbonation of cementitious matrixes, the steps of the diffusion through the gas film (steps a) and e)) can be directly neglected. Concerning step d), most of the producís of the carbonation process are solid phases. The only fluid product is the water formed, whose reléase depends on the relative humidity of the environment where carbonation is taking place, which is known to strongly influence the global speed. Provided that the experiments reported here have been carried out at constant HR, this step will not be taken into account at this time. However, this is an important point to be incorporated in the model.

Therefore, the steps to be taken into account are: step b) diffusion of C02 through the carbonated part and c) Chemical reaction. These steps have to be expressed as differential material balances, in which the diffusion through the ashes is represented by the first Fick's law and considering the chemical reactions of first order.

3. Resutts and discussion

3.1 Neutrón diffraction patterns

The analysis of the neutrón diffraction patterns has allowed the identification of 4 main crystal phases: Portlandite, Calcite, Ettringite, and a form of crystalline Hydrated Calcium silicate (CSH gel). No anhydrous cement was clearly resolved (Castellote et al., 2008).

These peaks have been fitted to Gaussian curves for the whole series. The variation of intensity of a chosen reflection for a particular phase along the experiment (related with the concentration) was used to monitor concentration changes. As an example, Figure 2 shows the evolution of the normalised intensity as a function of the time of carbonation for the OPC mix and for the four crystalline phases analysed. In Figure 2, it can be seen that carbonation implies the simultaneous reduction of the Portlandite, Ettringite and CSH gel while Calcite increases progressively (note that the initial normalised intensity for calcite is not zero due to an initial amount of lime in the cement).

Figure 2. Evolution of the normalised intensity for the OPC mix as a function of the time of carbonation for the four crystalline phases analysed

3.2 Development of the model

Even though several models have been proposed in literature to predict the advance of the carbonation front in cementitious matrixes, the validity of the accelerated tests to extrapólate results in natural conditions continúes to be very doubtful, not having found in literature any validated model able to use data of tests, in any condition, for predicting the rate of carbonation in any other situation. This is the objective of the UR-CORE model (Castellote y Andrade, 2008) of the carbonation rate, that has been undertaken in three steps:

a)  Establishment of the controMing step in the global carbonation rate.

b)  Adaptation of the model to the specific problem of cementitious matrixes

c)  Validation of the model

3.2 a) Establishment of the controMing step in the global carbonation rate

According to the unreacted core model proposed, the overall rate expression is formulated by considering a series of successive steps. However, in order to have a solvable problem, it is necessary to simplify the physical phenomena and identify the slowest step that is the controMing one, and will determine the rate of the global process.

As explained in the experimental section, in the case of carbonation of cementitious matrixes, the steps to be analysed, that could control the process, are the steps labelled as b) diffusion of C02 through the carbonated part, and c) chemical reaction, having each case different integrated equations (Castellote y Andrade, 2008).

The in situ neutrón diffraction results provide the kinetic data of fractional conversión for the different phases, making it possible, for the first time, to discrimínate the controMing step by comparing the experimental results with the corresponding theoretical equations. For the sake of clarity, the comparison has been done in a graphical way: The theoretical equations for a cylindrical specimen have been depicted in Figure 4, where the best fit of the experimental data in comparison with the theoretical curves has been depicted for mix OPC-FA, as an example of the behaviour for the different mixes.

From Figure 3, it can be deduced that the controMing step is that of diffusion of the C02 through the carbonated part of the sample, fitting very well the theoretical equation for the three types of cement. This type of behaviour is the same for the other two phases: Ettringite and CSH (only available for OPC).

It was expected (in (Parrott, 1992), it was stated that under normal environmental conditions of C02 and relative humidity, carbonation was controlled by gas diffusion through the empty pores in the surface layer) that the rate of the carbonation was a diffusion controlled process. However, here it has been demonstrated that the controlling step in the carbonation rate is the diffusion of the C02 through the external carbonated part of the sample, that revalidates the simplified use of the simple law of the square root of time (Rahman y Glasser, 1989; Venuat y Alexandre, 1968; Weber, 1983; Smolczyck, 1968), to describe the advance of the front in given conditions, and therefore, the set of equations to apply for a cylindrical specimens are:

Figure 3. Craphical presentation of theoretical equations for a cylindrical specimen and best fitting of the experimental data obtained by neutrón diffraction, for portlandite and OCF-FA mix, to these equations

 

 

 

Where:

Xs: Fractional conversión of the solid reactant, s, at time t.

ρs : Molar fraction of the reactant, s, in the solid (mol/cm3)

t: Time of experiment (seg)

τ : Time for complete conversión of the reactant s (seg) D: Effective diffusion coefficient of C02 through the

carbonated layer (cm2/seg)

b : stoichiometric coefficient for the reaction: b S(s) + C02 -->.....

CCG2: Concentration of C02 in the gas phase (mol/cm3)

r: Radius of unreacted core (cm)

R: Radius of the cylinder (cm)

3.2 b) Adaptation and application of the model to cementitious materials using different concentrations of C02

From the experimental data and the set of equations 1, 2 and 3, it is possible to determine the time for complete conversión of each phase, from which it is possible to deduce that even though for the same mix, the rate for the different phases is different, there is the same proportionality between each phase in the different samples (Castellote y Andrade, 2008). Providing that the diffusion of C02 through the external carbonated part of the sample is the controlling step, the different τ are not due to different chemical reaction rates, but they are attributed to the different molar fraction of each phase reactant, being the D a common parameter for the different individual isolated phases. However, from a practical point of view, the application of the model to the cementitious materials encounters three main points to so I ve:

1) Usually, the amount of the different phases able to carbonate in the original sample is not known. In addition, the amount of calcite formed when the sample is fully carbonated is neither known. However, a linear relationship between the percentage of CaO in the binder and the mol/cm3 of calcite formed when fully carbonating at 100% C02 (series b) has been found (Castellote et al., 2008). In (Castellote (a) et al., 2008), it was reported that when carbonating using different C02 concentrations, the total amount of calcite formed was different. Therefore, from the data given in (Castellote (a) et al., 2008) by the same authors (obtained with OPC samples), the concept of "reductions factors", RF, was introduced (Castellote y Andrade, 2008). It is necessary to apply these factors to the model when carbonating at other C02 concentrations different from 100% C02.

It has to be remarked that if the sample were mortar or concrete, it would be necessary to introduce an additional factor correcting the amount of paste in the sample, CFP, (this factor is explained in more detail in the next section) (Castellote y Andrade, 2008).

2)  On the other hand, in order to have a global parameter to use it is necessary to know the averaged fractional conversión of each phase, as well as the averaged "b". Provided that most of the carbonated fraction of the paste is made of Portlandite and CSH, it seems a reasonable approximation consider b as 1 for the global process. In addition, considering that from a practical point of view the most convenient and used procedure for measuring the carbonation is the phenolphthalein indicator, it could be considered that, even though there is some degree of bias, it should be quite cióse to the fractional conversión for portlandite (Rahman y Glasser, 1989).

3) Calculation of D. There is not a direct relationship between the D and the total porosity neither for the carbonated samples nor for the uncarbonated one (Castellote 2008). The only relationship found between the diffusion coefficient and the porosity (% vol) has been, for the carbonated samples, in the range of pores between the 0.1 and 0.05 Hin. This confirms the controlling step through the carbonated layer, and makes a bit difficult to be able to predict a diffusion coefficient just from porosity data. For the uncarbonated sample, the most approximated relationship found has been for the range of pores smaller than 0.05 μm.

To undertake the practical inconveniences, the assumptions that have been introduced when explaining the three different points, have been analysed and tested as follows: First of all, the D for OPC with the actual valúes (averaged conversión, averaged b and actualps,) has been calculated, which gives a valué of 5.1 E-6 cm2/s. Then, it has been obtained the valué for D using the proposed approximation (fractional conversión for portlandite, b=1, and ρs estimated from the percentage of CaO in the binder, which gives D=6.4 E-6, which is quite close to the real one, and of the same order of magnitude as those given in (Houst y Wittmann 2002).

3.2 c) Final Validation

The final validation of the model was carried out with data from concretes specimens, in order to valídate also the extrapolation from paste to concrete (Castellote 2008). For doing that, as already said, in order to obtain the righρs (mol/cm3) it is necessary to introduce a factor correcting the amount of paste in the sample, CFP, provided the mix proportion of the concrete is known.

In addition, a generic valué for the density of the paste fraction of 1.7 gr/cm3 has been used for all the cases.

The case of validation presented here uses data from literature (Sanjuan et al., 2003). Mix proportions for two different compositions of concrete (type A and B, series I in Sanjuan et al., 2003) and two very different conditions for the carbonation tests are given: 5 days at 100% C02 and 2 years at natural exposure. From the data obtained from the accelerated condition, the model has been applied to determine the depth of carbonation in natural conditions after two years.

For these two concretes, the calculated CFP's have been of 0.25 and 0.31 cm3 paste/cm3 sample, and DC02 of 2.11E-4 cm2/s and 1.1OE-4 cm2/s for concretes A and B respectively. These coefficients are lower but of the same order of magnitude than that obtained by application of the models Tuuti, 1982 and Bakker 1964 (calculated in Sanjuan et al., 2003).

The results for the validation are given in Figure 4, where it can be seen that even though the conditions of the two different conditions are the extreme ones, the experimental data and the predicted ones for both concretes are very good.

Figure 4. Validation of the model for concrete samples: Data from accelerated experiment and prediction for natural conditions (data from Sanjuan et al., 2003)

4. Conclusions

This paper presents a summary of the essentials of a model for the carbonation of cementitious matrixes (UR-CORE), whose development has been possible through the in situ monitoring, by neutrón diffraction, of accelerated carbonation tests, at 100% C02 concentration, of cement pastes of different binders.

On one hand, carbonation experiments with simultaneous acquisition of neutrón diffraction patterns has allowed to monitor the major features of the experiments. On the other hand, the first step for developing the model has been the establishment of the controlling step in the global carbonation rate, by adjusting the experimental fractional conversión curves to the theoretical equations. As a result, it has been demonstrated that the controlling step is the penetration and diffusion of the gas through the ash shell to the surface of the core not yet reacted (diffusion of C02 through the external carbonated part of the sample). Then, the model has been applied to the cementitious materials using different concentrations of C02. For doing that, some assumptions and factors have been introducing: the Reduction Factors, RF (due to the fact that more amount of alkaline material is carbonated as higher is the concentrations of C02) and the CFP, factor correcting the amount of paste in the sample, if mortar or concrete is used. Finally, the model has been validated with laboratory data at different concentrations (taken from literature) and it seems to be reliable enough to be applied to cementitious materials being able to extrapólate the results from accelerated tests to predict natural conditions.

 

 

5. Acknowledgements

The experiments reported here were done thanks to the beamtime (Exp 5-25-50) granted by the Institute Max von Laue - Paúl Langevin (ILL). The authors are especially grateful to the staff of D20. The authors also acknowledge the funding provided by the Spanish government through the project CONSOLIDER-SEDUREC

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E-mail: martaca@ietcc.csices

Contribución. Mejor artículo Conpat 2009

X Congreso Latinoamericano de patología de la Construcción y XII Congreso de Control de Calidad en la Construcción. Congreso Internacional de Patología, Control de Calidad y Rehabilitación de Estructuras y Construcción. 29 de Septiembre al 2 de Octubre de 2009. Valparaíso-Chile.

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