Prognosis Of Concrete Corrosion Due To Acid Attack

Transcription

1 Prognosis Of Concrete Corrosion Due To Acid Attack HW Dorner and RE Beddoe Institute of Building Materials Science and Testing Technical University of Munich Germany Summary: A model is introduced for the prediction of the corrosion of concrete under acid attack at ph values between 4. and 6.5. The corrosion process is described by the diffusion of acid in the pore solution coupled to the reaction kinetics between the acid and the different hydration products. The effect of changing porosity is included. The input parameters are measured by exposing mortar containing quartzitic sand to acetic acid buffer solutions. An instationary method is used to determine the diffusion coefficients of the ions including, Fe 3 and Al 3 in the pore solution of corroded mortar. The rate constants for the dissolution of, Fe 3 and Al 3 contained in the solid phases are determined in solubility experiments. Experiments at ph 5. show that the dissolution of phases containing iron and aluminium is slower than C-S-H and calcium hydroxide. This implies that the iron and aluminium phases reduce the speed of corrosion, enhancing concrete durability. Keywords: Concrete, durability, corrosion, acid attack, model INTRODUCTION The degradation of concrete occurring when the unprotected concrete surfaces of sewer pipes, waste water treatment plants, cooling towers, animal houses etc. are subjected to aggressive acid solutions seriously limits service life. The resistance of concrete against acid attack depends on the concrete composition, the type of aggregate as well as the ph and type of the acid. The modelling of the chemical reactions and ion transport processes leading to corrosion is highly important for the prediction of durability. An overview of the various types of chemical reactions which can occur during corrosion is given by Samson et al. (). The present contribution introduces a model for the corrosion of unprotected concrete subjected to constant ph values between 4. and 6.5. At ph values below 4. concrete rapidly deteriorates. In this case the concrete requires a protective barrier. When a concrete surface is subjected to acid attack protons enter the concrete and neutralize the hydroxyl ions contained in the hydration products. This causes mainly calcium, iron, aluminium and sulphate ions to enter the pore solution. These ions then diffuse towards the concrete surface owing to concentration gradients between the pore solution and the attacking acid. The dissolution of the solid hydration products results in an increase in porosity. A corroded layer develops gradually. The speed of penetration of the corrosion front - which is decisive for concrete durability - is determined by (a) the rate of diffusion of the acid through the corroded layer to the reaction front and there (b) the reaction rate of the acid with the concrete. corroded layer, x acid H - X Fe 3 Al SO 3-4 uncorroded, i.e. alkaline concrete OH - Figure. Corroded layer due to acid attack. The corroded layer comprises zones of varying composition and structure which are determined by the distribution of protons within the corroded layer, the different acid resistances of the hydration products and the solubility limits for the dissolved 9DBMC- Paper Page

2 ions. As the ph value within the corroded layer decreases during the course of corrosion, calcium hydroxide (ph.5), ettringite (ph.7), C-S-H (ph 9) and finally the calcium aluminate and ferrite hydrates decompose successively until a silica gel layer remains at ph values below roughly. At ph values between 4. and 6.5 phases containing iron and aluminium are still present. The strength of attack depends on the ability of the acid to dissociate and the solubility of its calcium salt (Revertegat et al. 99). Although acetic acid dissociates far less readily than the mineral acids it is extremely aggressive owing to the high solubility of its calcium salt. Thus sulphuric acid is less aggressive than acetic acid at similar ph values because soluble gypsum fills surface pores and reduces the ingress of the acid (Pavlík 994). In the case of sulphuric acid, expansion and destruction of the surface concrete will finally take place. As opposed to strong mineral acids, weak acids lead to buffering effects which modify the zones described above. Changes in the corroded layer due to precipitation of salts are also possible. Thus Pavlík (994) attributed a brown zone, formed adjacent to the uncorroded material during the exposure of hardened Portland cement paste to nitric acid, to the diffusion of Fe 3 ions and the subsequent precipitation of ferric hydroxide at ph values above. Similarly, the diffusion of SO 4 - in the corroded layer can lead to the precipitation of gypsum. The corrosion resistance of concrete with quartzitic acid-resistant aggregate depends on the chemical composition of the cement. According to De Belie et al. (996) the vulnerability of concrete to attack by lactic and acetic acid decreases over the groups: Portland cement without C 3 A, ordinary Portland cement, Portland cement with fly ash, blast furnace slag cement. In view of the higher corrosion resistance of the calcium aluminate and ferrite hydrates, which decompose at ph values below approximately 4.5 (Biczók 968), cements with a high C 3 A or C 4 AF content should improve corrosion resistance. On the contrary, Shi and Stegemann () observed that hardened pastes made with high alumina cement corroded faster in nitric and acetic acids than pastes made with Portland cement. The corrosion resistance of concrete is affected by the use of mineral additions such as blast furnace slag or fly ash since these materials modify the chemical composition and pore system of the hardened binder as well the composition and permeability of the corroded layer. According to Shi and Stegemann () it is the nature of the hydration products rather than the porosity of the hardened binder that specifies the corrosion resistance. Fig. shows the effect of binder type and w/b ratio on the deterioration of concrete plates stored for 4 d in a solutions with a ph of 4.5 which contained. mol/l Na SO 4 (Dorner, ). Compared with samples made from German Portland cement () or German blast furnace cement (II), the samples made with high alumina cement (HAC) clearly exhibits the highest resistance against acid attack. [% O loss [wt.% FeO3 loss [wt.% w/b=.5; % SF: w/b=,4; % SF; w/b=.4; % SF; HAC w/b=.3; % SF; HAC w/b=.4; 8% SF; w/b=.4; 8% SF II B w/b=.4; 8% SF; HAC w/b=.3; 8% SF; w/b=.7; 5% SF; AlO3 loss [wt.% Figure. Loss of, Fe and Al from concrete plates ( 5 mm 3 ) stored for 4 d at 5 C in a solution with ph 4.5 containing. mol/l Na SO 4 (Dorner ) 9DBMC- Paper Page

3 CORROSION OF MORTAR CYLINDERS Fig. 3 shows the distribution of, Fe and Al within the corroded layer of a mortar cylinder exposed to a buffer solution with a ph of 5. for 4 d at 5 C (Dorner ). The mortar cylinder was prepared with acid-resistant quartz sand and German Portland cement ( 4,5 R) at a w/c ratio of.58. Following storage in the acid, a lathe was used to remove the corroded layer in steps of.5 mm down to the undamaged mortar at depth of 6.5 mm. By dissolving the material obtained from each step in hydrochloric acid and chemically analysing the solution composition, it was possible to determine the distributions of, Fe and Al remaining in the corrosion layer. The vertical axis of Fig. 3 gives the remaining amount of, Fe and Al as a percentage of the initial contents of these elements. The initial contents were calculated from the chemical analysis of the cement and the composition of the mortar. It is apparent that a large proportion of calcium has been removed throughout the thickness of the corroded layer. This is primarily due to the decalcification of C-S-H and calcium hydroxide. Thus the calcium in these phases rapidly dissolved and was able to diffuse through the corroded layer into the surrounding acid. In contrast to this, iron and aluminium were only partially removed from the corroded layer. It therefore appears that the rate of degradation of the hydrate phases containing iron and aluminium is slower. These phases can slow down the corrosion process. Content, % Al Fe Element,75,5,75,5,75 3,5 3,75 4,5 Depth, mm 4,75 5,5 5,75 6,5 Figure 3. Distributions of Al, Fe and in the corroded layer of mortar stored for 4 d at ph 5. and 5 C (Dorner ) 3 SIMULATION OF CORROSION The exact simulation of the corrosion process requires detailed knowledge, not only of the chemical reactions between the acid and the numerous solid phases, but also of the transport of ions in the pore system. Such a simulation would, ideally, require exact data describing the phases and their amounts as well as a three-dimensional description of the pore system. Schmidt-Döhl and Rostásy (999), for example, developed a model based on the minimization of the Gibbs free energy of the different solid phases and solution taking part in the chemical reactions leading to corrosion. The model requires knowledge of the chemical or phase composition of the concrete as well as the formation enthalpies of the different phases and the rate constants of the corrosive reactions. Since the present corrosion model should ultimately be applicable to the wide range of concrete compositions used in practice and the different types of acid environments encountered, such a fundamental approach would be extremely difficult. To avoid this problem, integral materials properties (effective diffusion and absorption coefficients, effective rate constants) can be used instead of basic thermodynamic data. This means that the values for the model input parameters should be determined in tests which are as closely linked to the model as possible. The model, the tests and the method of calculating the input parameters from the test results represent a system. Correction factors will, at a later date, be introduced to account for differences between the model and actual experimental results. These factors will be based on physically and chemically definable differences such as effects due to specific surface, additional chemical reactions and interactions between ions in the pore solution. Currently, the model is being developed by using a number of tests to quantify the corrosion process when cement mortar made with fine acid-resistant quartz sand is stored in an acetic acid / sodium acetate buffer solution. The results of these tests 9DBMC- Paper Page 3

4 are necessary to (a) confirm the mechanisms and parameters necessary for the simulation and (b) supply the basic input parameters. At a later date, tests will be carried out using strong mineral acids, such as HCl or HNO 3. Finally, the application of the model to concrete should only require the results of simple tests, e.g. corrosion depth after a defined storage time at the ph value of the attacking acid. As already mentioned, the decomposition of the different hydration products depends on the ph of the attacking acid. Thus the rate of a particular corrosion reaction is determined by the concentration of the acid and the type and quantity of the hydration product taking part in the reaction. For example at any given ph, is released from C-S-H and (OH) more rapidly than from the ferrite or aluminate hydrates. At first, it will be assumed that the corrosion reactions are of second order type as described by Eqn. (). The various reactions take place, in principle, simultaneously throughout the depth of the corroded layer. The dissolution processes result in a corroded layer which is more porous than the original concrete. The porosity is expected to decrease when moving from the surface of the concrete towards the corrosion front where the porosity of the original concrete is reached. This is because the duration of exposure is longest for the concrete surface and zero for the undamaged concrete. Furthermore, the ph of the pore solution increases from the value of the attacking acid at the concrete surface to approximately.5 at the corrosion front. This is because proton consumption increasingly dominates proton supply by diffusion. Corrosion experiments indicate that the main increase in ph takes place within a fairly narrow region next to the uncorroded material (Pavlík 994). The porosity at an arbitrary distance x from the concrete surface within the corroded layer is determined by the combined effect of the different reactions with the local variation of ph over the duration of exposure at x. In general, the ph value of the pore solution at x decreases with time from about.5 to the ph of the attacking acid. The rate of ph decrease is itself a function of the porosity changes within the corroded layer since the protons must permeate the corroded layer to arrive at point x and react. In addition, the consumption of protons at x by the corrosion reactions affects the local concentration gradient and thus the diffusion of the acid. 3. Transport and reactions concerning The following figure shows schematically the various processes occurring within a volume element of thickness x at a distance x from the concrete surface which lead to changes in the amount of in the solid hydration products and the pore solution. - solid: OH S H X - X HO pore solution x - V Figure 4. Processes affecting content of hydration products and pore solution in a volume element of thickness Dx at a distance x from the concrete surface The hydroxyl ions in the solid hydration products are neutralized by the protons causing ions to enter the pore solution. The rate of neutralization depends on the proton concentration of the pore solution [H (mol/l) and the content of potentially soluble calcium in the solid, [ S (mol/kg). The increase in [ also depends on the solution volume V and the initial mass of the solid phase m participating in the reaction. The latter specifies the total available amount of soluble calcium at the beginning of corrosion. During corrosion, the remaining amount of soluble calcium at x, i.e. [ S m, diminishes. The solution volume V is given by the local volume fraction of water-saturated pores into which acid ions are supplied by diffusion and convection. This is essentially the capillary porosity, P, if it is assumed that the pores are completely saturated of that diffusion and convection depend on pore size in the same manner. The reaction kinetics are to a first approximation 9DBMC- Paper Page 4

5 m = K [ H [ S V where the variables [, V, [H, and [ S are functions of position x and time t. The mass m is constant. Using the rate constant K it is possible to calculate the increase in dissolved calcium in the pore solution. The neutralization of the various hydration product phases may be considered to be a reaction between the acid and (OH) contained in the phases (see Fig. 4). Consequently, stoichiometry yields the reduction in ph of the pore solution corresponding to Eqn. (), i.e. () H The reduction in the amount of soluble calcium in the solid phase is given by S = = V m () (3) The supply of acid into x and the composition of the pore solution depends on the diffusion flux of the various species. In the simplest form, the diffusion flux J is determined by the concentration gradient of the pore solution and the local watersaturated volume fraction P available for diffusion. Thus J H = D H H P and J x = D P (4) x The coefficient D is an effective value for the diffusion of ions in the pore solution. It differs from the bulk value for infinitely dilute electrolytes owing to the interactions between ions which decrease the activity coefficients. In addition, the diffusion potential due the different mobilities of the ionic species will tend to slow down the faster ions and speed up the slower ions, see Samson et al. (999), Tang (999). The quantity P is also an effective property since only the solution volume effectively contributing to diffusion is considered. It encompasses the effect of pore size distribution, tortuosity and constrictivity of the pore system. When dry concrete is exposed to an acid solution the ions generated will rapidly enter the concrete by capillary suction. The penetration rate for the front of capillary suction x C into the concrete and thus the convection of the acid ions is determined by the absorption coefficient A (kg/(m s / ) and the capillary porosity P i.e. xc ( t) = A P ρ W x ( t) C (5) Here ρ W is the density of water. 3. Transport and reactions concerning Fe 3 and Al 3 The transport processes and reactions involving Fe 3 and Al 3 can be treated analogously to. Owing to the neutralization reaction, Fe 3 and Al 3 ions are accompanied by when they are released from the hydrated ferrite and aluminate phases. Thus, as above, a ph reduction is calculated from the increase in the calcium concentration of the pore solution. However, it is likely that different ferric and perhaps aluminium oxide hydrate compounds precipitate or redissolve, thus affecting porosity, diffusion flux and capillary suction. It is also necessary to include this effect during the calculation of the ph of the pore solution. 3.3 The acid Acetic acid has been chosen for the experiments necessary to determine the model input parameters and verify the model. This acid represents the organic acids produced by the decay of organic matter in, for example, waste disposal sites or animal houses (De Belie et al. 996) and, owing to the high solubility of its calcium salt, attacks concrete with similar aggression to mineral acids. Acetic acid has the advantage that buffer solutions with sodium acetate can be prepared with ph values ranging from 4. to 6.5 which cover the very strong, strong and weak attacks as defined by DIN 43. Furthermore, acid attack is commonly in the form of large quantities of standing or slowly flowing solution in contact with a concrete surface so that the strength of the attack is not diminished by the reaction with the concrete. This requirement is easily fulfilled by the buffer solution. Another advantage is the high ionic strength of the buffer solution which means that the activity coefficients of ions in the pore solution will not change greatly as ions from the solid material enter the pore solution. 9DBMC- Paper Page 5

6 During the simulation of the corrosion process, the properties of the acid must also be taken into account. Dissociation is especially important for weak organic acids and their buffer solutions. The ph of the acid is determined by the equilibrium between the dissociated protons and acid anions with the non-dissociated acid molecules as defined by the dissociation constant of the acid K a, i.e. K a [ H [ X = (6) [ HX Differences in the diffusion coefficients of these species and the removal of protons due the reactions with the solid continually disturb the equilibrium according Eqn. (6) thus necessitating the calculation of new solution compositions. The conservation of the number of the components X and H in molecular and ionic form in the solution requires [ HX [ X = [ HX [ X Here denotes the initial unstable composition and the final equilibrium composition. [ HX and [ H = [ HX [ H The solution of Eqns 6 and 7 yield quadratic expressions for the equilibrium composition of the pore solution. It is necessary to calculate the new equilibrium composition of the buffer solution components of the pore solution after each time step t of the corrosion model. It is assumed that the equilibrium is not affected by other ions in the solution and that it is achieved instantaneously. 4 DETERMINATION OF THE MODEL PARAMETERS 4. Diffusion coefficients and porosity The ph value of the pore solution and the time of exposure to the acid vary over the corroded layer. Thus the corrosion model requires values for the diffusion coefficients of the various ions and porosity as a function of the time of exposure to ph values between 4. and, theoretically,.5. Thin mortar disks (thickness l = 3 mm, diameter d = 3 mm) are stored for different time periods t up to d in buffer solutions at ph values between 4. and nearly 8.5. Following this, the corroded disks are transferred to distilled water so that the ions contained in the pore solution can diffuse out of the disks into the surrounding water. One-dimensional diffusion conditions are obtained by sealing the disk edges with epoxy resin. The concentration of the various ions in the storage water is monitored for up to four days, see Fig. 5. Afterwards the porosity and matrix density of the disks are determined gravimetrically. In order to determine diffusion coefficients from the experimental data, Fick s second law of diffusion has been solved for the boundary conditions imposed by thin semi-infinite plates to yield an expression for the increase in concentration of an ion in the storage water as a function of the diffusion coefficient. The increase in, for example, the calcium concentration of the water is given by [ P [ = ( t) 4v n= P π d l,3,5.. 4 π n π ( cos( πn) ) exp D t l Here [ is the initial concentration of calcium in the pore solution and v the storage water volume. Fig. 5 shows examples for the increase in the concentration of ions in the storage water according to Eqn. (8). The curves were calculated for a disk 3 mm in diameter and 3 mm thick immersed for h in 5 ml water. The initial concentration of ions dissolved in the pore solution of the disk was. mol/l and the porosity of the disk was 3%. The maximum concentration given in the figure is reached when the pore solution and the surrounding storage water have the same concentration. n (7) (8) 9DBMC- Paper Page 6

7 ,4, max. concentration Concentration, mmol/l,8,6,4 D = - m /s D = - m /s, Time, h Figure 5. Diffusion of calcium ions from thin disks into surrounding water Values for the diffusion coefficients of the relevant ions, including, Fe 3 and Al 3, are obtained by fitting Eqn. (8) to the appropriate experimental data. 4. Rate constants for dissolution The rate constants are determined by solubility measurements with finely ground mortar in order to minimize the effect of diffusion. Samples of initial mass m are stored for various lengths of time in buffer solutions of volume V with ph values ranging between 4. and 8.5. At the end of the storage period the storage solution is chemically analysed and the dry residue weighed. The solution of Eqn. for a buffer solution ([H is constant) yields the calcium concentration of the solution as a function of time. ( exp( K [ H t ) m [ = [ S, (9) V Here [ S, is the initial content of potentially soluble calcium in the solid hydration products at the beginning of the acid attack (mol/kg). Values for the rate constants governing the release of, Fe 3 and Al 3 and the corresponding initial contents of these ions in the mortar are determined by fitting Eqn. (9) to the experimental data. Fig. 6 shows results obtained from preliminary solubility measurements. A specimen of finely ground mortar weighing 5 g was placed in ml buffer solution with a ph of 4.5 at C. The concentrations of calcium, iron and aluminium in the buffer solution were recorded a number of times over a h storage period. In the figure, the concentration of calcium, iron and aluminium (symbols) in the solution is plotted against the square root of time. Based on the initial contents, it was estimated that after h more than 95% of the calcium in the sample had entered the solution. Around 95% of the iron and 65% of the aluminium were in the solution. [, mmol/l K = 3. l/(mmol s) [ S, = 3 mmol/kg Time, min / [Fe 3, [Al 3 mmol/l K Al3 =.7 l/(mmol s) [Al 3 S, = 5 mmol/kg K Fe3 =.3 l/(mmol s) [Fe 3 S, = 65 mmol/kg Time, min / Figure 6. Increase in concentration of, Fe 3 and Al 3 in a buffer solution (ph 4.5) containing finely ground mortar 9DBMC- Paper Page 7

8 In Fig. 6 the experimental results are compared with curves calculated according to Eqn. (9). The values for the rate constant and the content of soluble ions in the mortar are next to the curves. Although the figure illustrates the method by which rate constants can be obtained from the results of solution measurements, a better description of the corrosion reactions is clearly necessary to provide more accurate rate constants. In principle, the rate constant in Eqn. (9) is valid for the complete range of ph values (4. to.5) and is therefore an integral value incorporating the effect of many different chemical reactions, precipitation and dissolution processes. A more accurate description of the kinetics requires knowledge of the effect of ph on the various rate constants. In view of the brown zone adjacent to the uncorroded material observed by Pavlík (994), lower solubility of phases containing iron is expected at higher ph values. This also explains why at ph 4.5 the amount of dissolved aluminium in the ground mortar sample appears to reach a maximum (Fig. 6) well below the total aluminium content, which would completely dissolve at, for example, ph. Eqn. (9) assumes that the rate constant is independent of the ratio of the initial mass of solid to the volume of the liquid in contact with it, m /V. However, the true ratio within the corroded layer will be much larger than the ratio used in the preliminary solubility measurements. This will affect ionic strength and the solubility limits of the solution. At present experiments are being conducted to ascertain the effect of the ratio m /V on the rate constants. Eqn. (9) also assumes that all the soluble ions contained in the mortar particles are in permanent contact with the acid, i.e. the time needed for the protons to reach any point within the particles is negligible. However, the corrosion reactions will also have some topological character, i.e. K will also depend on specific surface. Since the mortar particles ( 6 µm) contain capillary pores, their specific surface is determined by the external and internal areas at which the corrosion reactions occur. This will vary as the dissolution reactions proceed. The change in external specific surface during corrosion is currently being investigated using laser granulometry. 4.3 Verification In order to test the model, mortar cylinders are stored for up to three months in buffer solutions with ph values ranging from 4. to 6.5. The calcium, iron and aluminium content of the external buffer solution and the corroded layer, resolved in.5 mm steps, are determined by chemical analysis. The thickness of the corroded layer is measured. Finally, the data are compared with the results of the simulation. 5 CONCLUDING REMARKS A model is presented for the prediction of the corrosion of concrete under acid attack at ph values between 4. and 6.5 as a function of time. The model considers the diffusion of the acid within the corroded layer and the rate of reaction with the solid phases. Changes in porosity due to corrosion as well as precipitation and dissolution are also considered. An important part of the model is the simulation of the release of, Fe 3 and Al 3 ions into the pore solution and the diffusion of these ions through the corroded layer into the attacking acid. This approach enables the quantification of the corrosion mechanisms based on experimental observations. The main input parameters of the model are diffusion coefficients for the acid as well as, Fe 3 and Al 3, rate constants for the release of, Fe 3 and Al 3 into the pore solution and the associated change in porosity. The parameters are integral, i.e. effective, material properties rather than fundamental thermodynamic quantities, which are chosen in order to simplify the application of the model to practice concretes. The parameters are determined in special experiments tailored to the model. The values for the diffusion coefficients are determined under instationary conditions by corroding thin fine mortar disks in acetic acid / sodium acetate buffer solutions or other acids, transferring them to distilled water and monitoring the concentration of ions in the water. The porosity of the disks is determined gravimetrically. The rate constants are determined by storing finely ground mortar specimens in buffer solutions and observing the concentration of ions in the solutions. It is necessary to take account of the effect of ph on the corrosion reactions. The latest experimental results, in particular for high alumina cement, will be presented at the conference. 6 ACKNOWLEDGMENT The authors thank the Deutsche Forschungsgemeinschaft for financially supporting this work. 7 REFERENCES. Biczók, I. 968, Betonkorrosion, Betonschutz, Bauverlag, Wiesbaden-Berlin, in German. De Belie, N., Verselder, H. J., De Blaere, B., Van Nieuwenburg, D. and Verschoore, R. 996, Influence of the cement type on the resistance of concrete to feed acids, Cem. Concr. Res., 6, Dorner, H. W., Säurewiderstand von Hochleistungsbetonen, Deutscher Ausschuss für Stahlbeton. 38th Research Colloquium, Tech. Univ. Munich, Germany, -3 March, in German 9DBMC- Paper Page 8

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