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Physical and chemical modeling of Portland cement pastes under seawater attack Emmanuel GUILLON, Micheline MORANVILLE Laboratoire de Mécanique et de Technologie, ENS de Cachan, Cachan, France. 1. Introduction Seawater is an aggressive natural aqueous environment for cement-based materials and one of the most difficult to study. Building materials like concrete and mortars are constituted of a rigid skeleton (composed of sand and aggregates) in a matrix, the hydrated cement. The cement paste is a porous medium, partially or completely saturated with a pore solution. Due to this porosity, material-environment mass exchanges can occur. In particular, diffusion of external ions through the porosity can alter the material integrity and its long-term behavior. For the past decades, experimental studies with microscopic observations and simplified models have shown the influence of some particular ions on the evolution of a cement paste. For example, external sulfate ions cause the precipitation of ettringite which can be expansive if it occurs within a rigid and confined porous matrix. Carbonate dioxide in solution dissolves the portlandite. Chlorine ions can react with monosulfoaluminate to form calcium chloroaluminate and also can be adsorbed on C-S-H. Finally, magnesium ions from chloride or sulfate are responsible for the precipitation of brucite, magnesium hydroxide. Seawater is a natural solution which contains the previous ions in different concentrations. So, the study of seawater attack on cement pastes is difficult because of the coupling effect of various ions which has to be taken into account. Moreover two opposite phenomena occur when a cement paste is exposed to seawater i.e. either a degradation by the precipitation of expansive ettringite within the material, or a superficial protection by superimposed layers of brucite, aragonite and calcite, closing the porosity. In function of the cement composition, experiments show different evolution of the microstructure [1, 2]. One approach for modeling the influence of seawater on a cement paste considers the principal physical and chemical phenomena involved during a chemical aggression by an external aggressive environment. When the material is fully saturated with water, the transport of

Abstract: Due to the presence of multiple aggressive ions, seawater is a complex natural environment to understand for the durability of cement based materials. Experimental studies and observations have given numerous advances in the conception of materials resistant to seawater, but the modeling of degradation phenomena is globally weak. Thus, a new modeling approach, CITRAR, consisting in the coupling of reactive and transport laws at the microstructure scale, is applied to understand the chemical degradations occurring in cement based materials due to seawater



species is mainly diffusive. Within the porosity of the cement paste, the transport of ions causes locally thermodynamical unbalances which immediately involve successive dissolutions or precipitations of solid phases in order to reach a new thermodynamical equilibrium. Theoretically, a modeling based on the equations which govern these phenomena can reproduce the effects of a given ion on the cement paste. This paper details our approach of the influence of seawater aggressive ions on the cement paste microstructure. We precisely focus on the effect of each ion taken independently and we compare modeled and experimental results. Finally, a study of the coupling effects of all these ions on the microstructure is presented. 2. Presentation of the reactive-transport approach : CITRAR 2.1. Main hypotheses relative to the modeling The main objective of the modeling is to reproduce with a general approach, based on thermodynamics, the phenomena involved during chemical external aggressions of cement-based materials. These phenomena mainly occur within the cement paste because the capillary pores, partially or fully saturated of solution, are distributed within the cement paste. Following studies only focus on the chemical behavior of cement pastes in saturated conditions. In such saturated conditions and in regards to intrinsic cement paste fluid permeability, the liquid flow is negligible compared to ionic diffusion. During the life-time of the material, the bi-phase system composed of the pore solution and the solid skeleton follows a succession of thermodynamical equilibriums. As a consequence, locally, undersaturations or supersaturations of the pore solution in regards to some components of the solid skeleton can occur. They tend to respectively dissolve or precipitate these components in the porous network. As a conclusion, the main hypotheses formulated in this study concern the transport of species, which is ionic diffusion, in saturated conditions. Reactions of dissolution and precipitation are ruled by a succession of infinitesimal local equilibriums. 2.2. Which representation of the cement paste? In order to represent the evolution of the microstructure of a Portland cement paste due to an external aggressive solution, we chose to use as initial data digital images of cement pastes generated by the 3D hydration model developed at NIST by D.P. Bentz and E.J. Garboczi, CEMHYD3D [3, 4]. The model is known to be really relevant in the numerical building of a digital tridimensional image of a hydrated cement paste, knowing the chemical composition and the granularity of the anhydrous cement, and also chemical rules of hydration. Each digital image corresponds to a ‘Representative Elementary Volume’ (REV) of a 100µm3 volume cement paste. The building process begins by positioning clinker spheres, in the REV. Each clinker sphere is composed of the classical four principal phases: C3S, C2S, C3A and C4AF. The distribution of these phases within the spheres respects the voluminal and surface proportions of real cements analyzed by X-ray diffraction. Then cellular automatons ‘hydrate’ the microstructure, by changing the role of each pixel in the microstructure, and in respect with the voluminal stoechiometry of the hydration reactions. Figure 1 illustrates the building process and shows sections of a Portland cement paste microstructure, before hydration and at a given stage of the hydration process. Each color represents a specific solid phase (i.e. C3S = red, C2S = light blue, C-S-H = orange, dark blue = portlandite…). The porosity full of solution is in black.



Figure 1: sections of w/c=0.4 Portland cement ‘digital’ microstructures before (left) and after

(right) hydration processes, with CEMHYD3D. In our approach, a 100µm3 volume REV is not sufficient to be able to reproduce most chemical degradation phenomenology. For example, main durability studies on reinforced concrete materials (leaching, chloride penetration) focus on the coating zone of the reinforcement bars, which is a mostly 1 to 5 centimeters depth. Our idea is to consider the digital image as a ‘cell’, and our study domain as a parallelepiped composed of these cells. Each cell is characterized by its position in the study domain, its phase composition, its porous network and also its chemical composition of the pore solution. Figure 2 shows the representation of the study domain. In the x direction, the two first planes of cells represent the aggressive external environment, chemically known.

2.3. Setting the equations governing the phenomena Reactive-transport phenomena can be governed by a system of equations, each one being specified for a given species. For example, the evolution of the concentration of a given ion, i, at a given place, is expressed as:

( )( ) ∑=

Γ+=∂

∂ J

jjij

fii

fi ccgradDdivt

c

1

* , (1)

Figure 2: study domain used in the modelling. Each cell represents either the aggressive environment, of a cement paste cell, composed of a microstructure generated by CEMHYD3D

model and the pore solution

C3A C4AF C-S-H CH

C3S pore SC

C2S



where Di is the apparent diffusion coefficient, *ijc the stoechiometric coefficient of the ith ion in

the jth mass-law expression, and finally jΓ the jth reaction speed. As previously written, our approach of the phenomenology considers that dissolution and precipitation reactions occur when a local thermodynamical unbalance appears, consequently to the transport process. This remark joins the classical ‘Local Equilibrium Approximation’ formulated in most geochemical approaches. LEA can be formulated when reaction speeds are infinitely higher than diffusion processes, which is the case in our study. LEA also authorizes the decoupling of reaction and transport calculations in the study, because reaction calculations are the consequences of the transport ones. In this case, we consider a multi-scale calculation:

- (i) at the ‘study domain’ scale, diffusive transport of ions is calculated - (ii) at the ‘cell’ scale, locally, reactive calculations tend to re-equilibrate the

cement paste system, which is now the designation of the bi-phase system composed by the solid skeleton and the pore solution of the cement paste.

The cement paste system is constituted of a large amount of different solid components. Cement hydrates, and also eventual remaining anhydrous constituents, have to be considered. Each constituent, taken separately, has its specific chemical equilibrium and its specific ions involved in the chemical equilibrium. In the pore solution, main species in solution are calcium, sulfur, aluminum, silica, and also alkaline ions like sodium and potassium. The pH of the pore solution in a sound cement paste nears the 13.5 value. In this basic environment, dissolved species like calcium are represented by a principal ion, Ca2+, but also by its complex forms, like CaOH+. The complex ion concentrations may not be negligible relatively to the principal ones, which imply to consider all forms of ions in the composition of the pore solution and especially in the transport calculation. In order to limit calculations and to simplify the study, the diffusion process is considered for the total concentration of each species. The number of diffusive species by this way is equal to six (calcium, silica, aluminum, sulfur, sodium, potassium) plus the species potentially present in aqueous environments but not initially in the cement paste like carbonate, magnesium or chloride. The precise calculation of the chemical composition of the pore solution can be deduced from a speciation calculation. In a tri-dimensional study domain, the diffusion equation does not have an analytic solution. A finite-volume scheme is chosen to represent the evolution of concentration of total species within the study domain. In order to improve convergence and stability of the numerical resolution, the Crank-Nicholson semi-implicit scheme is used [5]. Time steps range from 900 to 3600 seconds, near the stability domain of an explicit scheme with such a cell size. In every time step, apparent diffusion coefficients are re-evaluated in relation of content of the capillarity porosity, due to dissolutions and precipitations in the solid skeleton. The relationship taken to evaluate the diffusion coefficient is [6]:

( )( )22

0

17.017.07.103.004.0

−−++= φφφββ

HDD

, (2)

where D and D0 are respectively the apparent diffusion coefficient and the water diffusion coefficient, φ the capillarity porosity, H the Heaviside function and β a parameter which is function of the silica fume content.



Boundaries conditions for this problem are: diffusive fluxes equal to zero on the two surfaces perpendicular to the x direction, and periodicity on the four other surfaces. Diffusion equation in a tri-dimensional mesh with a semi-implicit scheme needs to write a matrix system which gives a linear relationship between the concentration vector at a given time step and one at the next time step. The resolution of this linear system for all time steps is done by evaluating iteratively a solution vector using the conjugate-gradient algorithm [5]. As apparent diffusion coefficients appear to evolve in each time-step due to dissolutions and precipitations, the matrix system has to be written for all computations. Dissolution and precipitation phenomena occur when a chemical unbalance appears after ionic transport. Reaction calculations lead to determine the number of moles of each solid component to evolve in order to add ions to or take off ions from the pore solution until a new chemical equilibrium is reached. Relative corresponding equations are:

- Total mass conservation, for each species (like calcium). In one cell and for the reaction calculations, the total number of moles of this species remains constant and is subdivided in a solid part and a solution part:

∑∑∈∈

+=solidsj

sj

solutioni

fispecies nnn , (3)

where n is the number of moles and the exponents s and f refer respectively to the solid and the solution. nspecies can evolve from one time-step to one other, due to diffusive exchanges. With a speciation calculation, the nf value permits to determinate all ions concentrations.

- Mass-action laws. For each component, whatever it is a solid or a complex ion, at an equilibrium stage, the product of the activities of all species involved in the equilibrium equals a thermodynamic equilibrium constant, Kj. If the product value is lower than Kj, the solid component is undersaturated in regards to the pore solution and tends to dissolve, and is supersaturated if the product is greater than Kj. As reactive calculations tend to determinate local equilibriums, mass action laws are expressed here:

∏=

=I

i

cij

ijaK1

*

, (4)

where ai is the activity of the species i and *

ijc its stoechiometric coefficient in the equilibrium relation with the solid j.

- Electroneutrality of the pore solution. The sum of the electrical charges of all ions in

the pore solution must be equal to zero. This relationship is used to determinate the pH value of the solution.

The expression of the ionic activities is not trivial and depends on the concentration of a given ion and the ionic strength, I, of the pore solution, i.e. the concentration of all the other ions. Different interactions models have been developed, like Debye-Hückel, extended Debye-Hückel, Davies, and Pitzer. Their differences come from the evaluation method but give similar results for a same range of ionic forces. For electrolytes like pore solution where the



ionic strength is high, the Pitzer interaction model and the Debye-Hückel model give the best approximations. In our modeling, the Debye-Hückel model is chosen. The calculation of the ionic activities is ruled by the following expressions:

fiii ca γ= , (5)

( ) IzA ii2.ln −=γ , (6)

∑=

=n

i

fii czI

1

25.0 , (7)

where A is a temperature-dependent parameter, γi the activity coefficient of the ith ion, I the ionic strength and zi the number of electric charges of the ith ion. Knowing the total concentration of some principal species, speciation calculations to evaluate exact concentrations of all ions, follow the same strategy. Indeed, mass action laws between a principal ion like Ca2+ and its complex forms, like CaOH+ and electroneutrality relations permit to write the remaining non-linear equations relative to the ionic concentrations and finally to get a non-linear system which has exactly the same number of equations and unknowns. This system is evaluated by iterations using the Newton-Raphton algorithm [5]. During iteration, activity coefficients are corrected in regards to the evaluated ionic concentrations. This calculation strategy for reactive calculations is applied to the geochemical speciation code, PHREEQC [7]. To resume, the approach is based on the coupling of diffusive transport and diffusion and precipitation reactions. A typical time-step in the numerical simulation is composed of the succession of transport and reactive calculations. Within the cement paste, represented here as a parallelepiped subdivided in cubic cells, a difference in the chemical composition between the ‘external environment cells’ and the pore solution is the driving force of ionic exchanges, ruled by diffusion laws. The diffusion of total species is done independently and for each species a finite volume algorithm developed with Matlab code permits to calculate concentrations profiles at the study domain scale. Then, in each cell, new total species concentrations induce chemical unbalances in regards to solid compounds inside the cell. Reactive calculations, done in each cell with the PHREEQC speciation code, permit to evaluate for each solid phase its quantity which dissolves or precipitates. After reactive calculations new parameters like apparent diffusion coefficient are evaluated in order to prepare the next time-step. This approach has been successfully applied to the simulation of pure water leaching of CEM I cement pastes [8, 9]. Simulations have shown similar degradation fronts as observed experimentally and previously already modeled [10, 11, 12], but have also shown the same influence of the water-to-cement ratio influence on leaching kinetics as experiments [13]. The objectives, presented in this paper, are to investigate the influence of seawater on cement pastes. To do this, we first present the influence of some seawater ions, and then we study the seawater itself.



3. Study of the influence of some ions on a Portland cement paste 3.1. Data relative to the cement paste system A hydrated cement paste is studied. Using CEMHYD3D model, a Portland cement paste with a 0.4 water-to-cement mass ratio is numerically generated. The same microstructures have been formerly used for water leaching simulations [12, 8, 13]. As anhydrous constituents have a very high solubility in water [14] and the modeling, for the moment, only uses mass proportions and not surface proportions of reactants, in order to prevent in the first reactive calculation the ‘brutal’ hydration of the remaining anhydrous constituents. So, we consider these anhydrous constituents as inert and non-reactive inclusions. Here, the hydration degree of the ‘digital’ cement paste is 73%. Voluminal proportions of principal phases are respectively: 18% capillarity porosity, 17% portlandite, 3% ettringite, 2.5% calcium monosulfoaluminate and 48% C-S-H. The remaining 11% volume are anhydrous constituents, distributed in 5% C3S, 4% C2S and 2% CaAF. As external ions are diffusing through the porosity, the four main hydrates are reactive and may precipitate, dissolve and eventually form new phases. These new phases in the approach are: calcium chloroaluminate (Friedel’s salt), calcium chloride, brucite, calcite or aragonite, calcium carbonaluminate and gypsum. Gypsum is part of the anhydrous cement and so is not really a new phase but is completely consumed during the hydration process if the cement formulation is correct. Except for C-S-H, all these solid phases have a homogeneous dissolution and are not ruled by surface reactions. In this particular case, the C-S-H stoechiometry can evolve and the calcium/silica ratio of the C-S-H tends to be modified in function of the chemical aggression. In order to represent the decalcification of the C-S-H, a simple approach chosen for this study is to consider two types of C-S-H with specific equilibrium constants and Ca/Si ratios: 1.65 and 1.1. Nevertheless, this approach is not sufficient to represent the adsorption of other ions at the surface of the C-S-H like sulfate, chloride, aluminum or magnesium, or at the surface of other phases [15]. All equilibrium constants are taken from the literature [16, 17, 18, 19]. 3.2. Description of the seawater composition Seawater composition may vary with the geographic location. For example, the total mass of dissolved salts can be multiplied by four between Baltic Sea and Red Sea (10 grams par kilogram of water to 40 grams). Here, we consider as seawater the mean composition of the Atlantic Ocean. Its composition is quite similar to the one proposed by Wilson [20] and also to the ‘standard seawater’ which is taken from the surface of the North Atlantic Ocean. Its composition is corrected for having a salinity value of exactly 35.000 grams per kilogram. ‘Standard seawater’ chloride content is 19.374 [21]. In the following study, the total concentration for concerned species, expressed in millimoles of species by liter, are given table 1. Calculated pH with these concentrations is 8.8.



Table 1: total concentrations taken for the composition of the seawater _______________________________________ _______________________________________ Species Concentration (mmol.l-1) _______________________________________ Chloride 565.70 Sodium 485.40 Magnesium 55.07 Sulfate 29.26 Calcium 10.66 Potassium 10.58 Carbonate 2.41 Silicate 0.074 _______________________________________ _______________________________________ As written in introduction, external ions like chloride, carbonate, magnesium or sulfate are responsible for an evolution of the cement paste microstructure [23, 24] More precisely, they are responsible for the formation of new precipitated minerals which may damage the material if their precipitation occurs in a rigid porous medium (Fig.3). In order to identify the role of each ion, preliminary studies concern the influence of some dissolved salts constituting the seawater and taken independently. Concerned salts are: magnesium sulphate, sodium chloride, and magnesium chloride. Sodium sulphate influence is not studied here but has already been modelled in [22], and is responsible for the formation of ettringite, due to the dissolution of the monosulfoaluminate, portlandite and the diffusion of external sulphate ions. Then, gypsum can precipitate [23]. Finally, a study of the influence of carbonate ions is made before studying the coupling effect of these species.

Figure 3: micrographies of crystallized ettringite in cement pastes: left, fiber crystals in a pore with sufficient volume for the precipitation (23) and, right, massive crystals in the matrix when

precipitation is confined (24). 3.3. Study of the behavior of a Portland cement paste subjected to NaCl solution Sodium chloride is the major salt dissolved in seawater. Its interaction with a typical Portland microstructure has been previously studied with our model [26]. The phenomenology of reactions is reminded here. Figure 4 shows for a 0.1 mole per liter concentration of sodium chloride the time evolutions of solid phases in the first cell.



Figure 4: voluminal evolutions of solid phases in the first cell of the cement paste versus the

square root of time, due to a 0.1 mol.l-1 concentration sodium sulfate aggressive solution. Sodium ions diffuse through the porosity but do not react with existing solid phases in the cement paste. Their sole influence is the modification of chemical equilibriums, by modifying the pH of the pore solution. A diffusion of alkaline ions increases the total concentration of positive ions, which results in the increase of hydroxyl ions to get the electroneutrality. Inversely, the release of these alkaline ions tends to decrease the pH value, as already modeled in the case of pure water leaching [8, 13]. When the chloride ion diffuses through the porosity, it reacts with monosulfoaluminate to form calcium chloroaluminate. As the calcium concentration in the external solution is equal to zero, portlandite dissolves. Simultaneously, sulfate ions released from the ‘substitution’ monosulfoaluminate-chloroaluminate and calcium ions coming from the portlandite dissolution tend to precipitate ettringite. Once all monosulfoaluminate is totally consumed, the contents of ettringite and chloroaluminate do not evolve any more, only portlandite dissolves. When all portlandite has totally dissolved, ettringite and chloroaluminate dissolve but with slower kinetics. Previous calculations [26] have shown for different NaCl concentration same phenomena but with different kinetics. In particular, if the NaCl concentration gets lower than a ‘limit’ value (estimated between 0.001 and 0.005 moles per liter) total dissolution of portlandite can occur before the substitution monosulfoaluminate-chloroaluminate. As a recall, these calculations are carried out without considering the chemical behavior of anhydrous constituents. In the case when anhydrous constituents remain, more chloroaluminate can precipitate, by the dissolution of anhydrous C3A. 3.4. Study of the behavior of a Portland cement paste subjected to MgSO4 solution. Magnesium sulfate content in seawater is quite weak (2.2 grams per kilogram) but the effects of both magnesium and sulfate ions may be sensible for cement based materials. In this study, we consider a 0.05 mol.l-1 concentration solution of magnesium sulfate and we follow, first, the evolutions of the voluminal proportions versus time in the cell which is directly in contact with



the aggressive solution, and, secondly, the voluminal profiles in the depth of the material after 30 days of chemical ‘numerical aggression’. The phenomenology of the microstructural evolutions in the other cement paste cells remains the same but with a time-dephasing and different amplitudes. Figures 5 and 6 are deduced from calculations made by the model. Due to the diffusion of magnesium ions (principally Mg2+) and sulfate ions through the porosity, chemical equilibriums between solid phases and pore solution are shifted. Figure 5 shows that, first, precipitation of ettringite occurs, by simultaneous dissolutions of monosulfoaluminate and portlandite. Secondly, when all monosulfoaluminate is consumed, the diffusion of sulfate ions and the dissolution of the portlandite cause the precipitation of gypsum. Simultaneously, magnesium ions diffuse within the porosity and precipitate into brucite, which is the magnesium hydroxyde. If we focus on the porosity evolution in the profile (figure 6), there are three distinct zones in terms of evolution of the solid species. From the sound zone, on the right of the profile, the evolution of the porosity is due to the difference in volume between precipitated ettringite and the dissolved species, i.e. monosulfoaluminate and portlandite. In the second zone, when all monosulfoaluminate is consumed and gypsum has not precipitated yet, the total volume of porosity does not evolve. Finally, when gypsum begins to precipitate, the volume balance between portlandite, brucite and gypsum becomes negative, which results in the lower value of the porosity and the risk for the material to expand and crack.

Figure 5: voluminal evolutions of solid phases in the first cell of the cement paste versus the

square root of time, due to a 0.05 mol.l-1 concentration magnesium sulfate aggressive solution.



Figure 6: voluminal solid profiles in the material after 30 days of chemical aggression with a

0.05 mol.l-1 concentration magnesium sulfate aggressive solution. 3.5. Study of the behavior of a Portland cement paste subjected to MgCl2 solution Magnesium chloride is considered as one of the most aggressive salts contained in the seawater. The two previous studies have shown the influence of magnesium ions and chloride ions in the cement paste system. Results from calculations with CITRAR of the influence of a 0.05 moles per liter concentration of magnesium chloride are not really surprising. Figures 7 and 8 reveal two simultaneous processes. Brucite precipitates with the diffusion of magnesium ions while the substitution monosulfoaluminate-chloroaluminate occurs when chloride ions come through the porosity. In the same time, ettringite precipitates, due to the sulfate ions released in the substitution and the portlandite dissolution process.

Figure 7: solid profiles in the material after 14 days, due to the influence of a 0.05mol.l-1

concentration magnesium chloride.



Figure 8: voluminal evolutions of solid phases in the first cell of the cement paste versus the square root of time, due to a 0.05 mol.l-1 concentration sodium sulfate aggressive solution.

An interesting point is the evolution of the capillarity porosity content. At the beginning, its value goes down because of the positive volume balance between formed products (i.e. ettringite and chloroaluminate) and the dissolved products (monosulfoaluminate). Then, as the total volume of portlandite to dissolve is higher than precipitated brucite, the porosity content increases and so the global degradation kinetics gets faster. 3.6. Study of the behavior of a Portland cement paste subjected to carbonate ions Carbonate ions are known to be prejudicial and also beneficial to cement based materials. Beneficial because these ions react with the phases containing calcium and form calcium carbonates, calcite and aragonite, which precipitate at the surface of the material. This precipitation comes with a closing of the surface porosity and so a limitation of the ionic exchanges with the external environment, source of chemical degradations in the material. Carbonate ions are also prejudicial because of the acid pH of a solution containing such ions. An acid pH can be the origin of the initiation of the corrosion of reinforcement steels in concrete. A solution with a 0.1mol.l-1 concentration of carbonate ions is the external solution applied for the material. Figure 9 and 10 show voluminal solid profiles after respectively 14 days and 29 days of chemical aggression. As expected, the diffusion of carbonate ions causes the formation of calcite and simultaneously the dissolution of the portlandite, but also a monosulfoaluminate-carboaluminate substitution which causes the precipitation of ettringite. During the degradation, the capillarity porosity content and so the global degradation kinetics increase, whereas the precipitation of calcite on the surface should slow down this kinetics due to the impermeabilization of the surface.



Figure 9: voluminal solid profiles of the cement paste obtained after 14 days, due to the

interaction with a 0.1mol.l-1 carbonate solution.


interaction with a 0.1mol.l-1 carbonate solution.

3.7. Study of the behavior of a Portland cement paste subjected to the seawater Figures 11 and 12 give the solid profiles after 4 and 9 days of chemical aggression with seawater, whose composition is given table 1.




interaction with the standard seawater.


interaction with the standard seawater. Different degradation stages can be found with figures 11 and 12. At the beginning, monosulfoaluminate dissolves and forms simultaneously chloroaluminate and ettringite, due to the presence of chloride ions, similarly as observed for NaCl or MgCl2 solutions. Then, when all monosulfoaluminate has dissolved, the chloroaluminate is no more stable and dissolves itself, probably because of the diffusion of sulfate ions from the seawater and the partial dissolution of portlandite. The consequence is a precipitation of ettringite. Its content inside the



material becomes constant when all chloroaluminate is consumed. During this monosulfoaluminate-chloroaluminate-ettringite substitution, brucite and calcite precipitate, mostly at the surface of the cement paste. This results in a closing of the porosity, like a protective layer which slows down the reaction kinetics. Finally, gypsum precipitates, due to the diffusion of sulfate ions, when the ettringite content has reached its maximum value. Qualitatively, these calculations are in agreement with the phenomenology mostly observed and described [1, 25, 27]. We can also remark two kinds of precipitated solid phases. Ettringite can be designed as a phase which precipitates voluminally, whereas brucite and calcite have a surface precipitation. The first type of precipitated phases can be prejudicial to materials, causing expansions and cracking whereas the second one is mostly protective, closes the porosity and so limits ionic exchanges. This illustrates the influence of aluminates in the durability of cement-based materials in the case of sulfated aqueous environments like seawater, and so is the strategic choice of the cement composition in function of the environment, as it was already observed in [2]. Figure 13 presents sections of Portland cement pastes with different aluminate content, exposed to seawater. In the left sections, where aluminate content is the highest, cracking is visible and may be the consequence of the precipitation of ettringite, although layers of magnesium hydroxide and calcium carbonate have precipitated. Nevertheless, as already remarked in the studies of the influences of chloride ions, the role of anhydrous constituents have to be taken into account because the remaining anhydrous aluminates can react with gypsum newly formed or external sulfate ions to precipitate more ettringite. 4. Conclusions Our approach, based on the coupling effect on diffusive transport and reactions of dissolution and precipitation, agrees with site observations. The influences of principal aggressive ions, i.e. sulfate, chloride, magnesium and carbonate, on the microstructure of a w/c=0.4 Portland cement paste are qualitatively well evaluated and remind former studies [1, 25, 27]. For example, the modeling clearly illustrates the competition between the ettringite precipitation and the formation of the protective layer of brucite and calcite in the immerged zone of the material exposed to seawater. Nevertheless, some limitations have to be formulated, in particular in terms first of porosity profiles. Due to the representation using 100µm3 cement paste cells, proportions of solid phases and porosity are averaged in each cell. Precipitation and dissolution of some solid phases like calcite, brucite, are considered to be homogeneous within each cell, which is the case of low-permeability materials. This is a consequence of the choice of the expression between the apparent diffusion coefficient and capillarity porosity [6], resulting in statistical computations on hydrated microstructures. In particular, this relationship does not consider whether the porosity is spatially localized or not. So, such a relationship is not able to represent the influence of a brucite or aragonite and calcite precipitated layers. Thus, in order to improve the predictions and to enhance the coupling between reactions and apparent diffusion coefficient within a cell, a possible approach could be to take into consideration the effects of surface precipitations and voluminal precipitations separately.



Figure 13: Electron probe microanalysis of Portland cement pastes exposed 6 months to seawater [2]. Light zones correspond to a high concentration of the considered element. - left: CEM I high in C3A., cracks due to the formation of expansive ettringite in the matrix- right: CEM I low in C3A, protective layer of calcium carbonate and magnesium hydroxide above a sound matrix.



Other limitations are relative to the non-consideration of cement anhydrous phases and the simplification of the C-S-H chemical behavior. Indeed, taking into account remaining anhydrous aluminates would increase the ettringite content in the case of sulfate attacks or let the possibility to form new hydrates. A more realistic chemical behavior of the C-S-H in presence of magnesium should also consider the transformation of this hydrate into M-S-H, with no binding properties, as observed in extremely deteriorated materials in marine environment [1]. As a concluding remark, this approach offers encouraging results and permits at this stage of development to give pertinent information on the long-term chemical behavior of a cement paste subjected to different external environments. The future developments will be focused on the improvement of the apparent diffusion coefficient evaluation (in relationship with voluminal capillarity porosity and surface porosity) and the extension of the reactive species to the anhydrous constituents. These perspectives would offer a useful tool for the conception of cement-based materials in specific aqueous environments. 5. Acknowledgments Authors would like to thank the ATILH (Association Technique de l’Industrie des Liants Hydrauliques) for their technical and financial support in this study. 6. References 1. Regourd, M., 1975, L’action de l’eau de mer sur les ciments, Annales de l’Institut

Technique du Bâtiment et des Travaux Publics, 25:86-102 (in french). 2. Regourd, M., 1977, Résistance chimique du ciment à l’eau de mer, Revue Internationale

des Hautes Températures et Réfractaires, 3 :139 (in french). 3. Bentz, D.P. 1997. Three-dimensional simulation of Portland hydration and microstructure

development, Journal of the American Ceramic Society 80(1):3-21. 4. Bentz, D.P. 2000. CEMHYD3D: a three dimensional hydration and microstructure

development modelling package, National Institute of Standards and Technology NISTIR 6485.

5. Press, W.H., Flannery, B.P., Teulosky, S.A., & Vetterling, W.T., 1992, Numerical Recipes, C version. The art of the scientific computing, 2d edition, Cambridge University Press.

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