SULFATE ATTACK ON CONCRETE - SOLUTION CONCENTRATION AND PHASE STABILITY

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1 3- June 29, Toulouse, France SULFAT ATTACK ON CONCRT - SOLUTION CONCNTRATION AND PHAS STABILITY Wolfram Müllauer, Robin. Beddoe and Detlef Heinz Centre for Building Materials, Technische Universität München, Germany Abstract The mechanisms of sulfate attack on concrete are investigated with regard to the effect of sulfate concentration, duration and temperature on the solubility reactions between the hydration products and the pore solution. Finely ground hardened cement paste was added to 2- Na 2 SO 4 solutions with concentrations up to 4 g/l SO 4 and stored between 3h and 14d under rotation. The composition of the solid phases was investigated by XRD using the Rietveld method. The results indicate that the formation of ettringite is governed by the dissolution kinetics of monocarbonate as a primary source of aluminium and the ph of the solution. Gypsum forms only at higher sulfate concentrations and ph values below about 13.; it dissolves with monocarbonate to form ettringite as predicted by the solubility equilibria of these phases. Aluminium is also supplied for ettringite formation by other phases such as C-A-S-H. Hardened cement paste cylinders (w/c ratio.6) were also stored in different sulfate solutions. To obtain distributions of the solid phases, sample material was removed in thin layers on a lathe and analysed by XRD. As opposed to ettringite, gypsum formation is limited to the surface region. 1. INTRODUCTION Sulfate attack is the term used to describe the chemical reactions between sulfate ions and the components of hardened concrete, principally the binder paste matrix, occurring when concrete is exposed to sulfates and moisture. This can result in visible damage like spalling, delamination, macrocracking and, possibly, loss of cohesion [1]. The chemistry of sulfate attack is complex and involves numerous interdependent reactions [2]. As generally accepted, the main reasons for the deterioration of concrete attacked by sulfates are the formation of expansive phases such as ettringite and possibly gypsum and loss of strength caused by the deterioration of the hydration products [2]. In addition, the phase thaumasite can form [2, 3] in concrete components containing a source of carbonate and exposed to low temperatures. If the ingress of sulfate ions from an external source is constant in strength, the aluminium content, i.e. C 3 A, of the cement can be seen as the limiting factor for ettringite formation. Thus in many countries a maximum C 3 A content for high sulfate resistant cements is specified [4]. Currently, there is no standard German method for testing resistance to sulfate 18

2 3- June 29, Toulouse, France attack. Contemporary tests such as the ASTM [] or Wittekindt methods [6] attempt to accelerate sulfate attack by using concentrations higher than encountered in practice. This leads to reactions other than in the field resulting in less precise prediction of sulfate resistance. The present investigations focus on the changes which occur in the system binder matrix phases / pore solution during sulfate attack. Firstly, the effect of transport of the different species in the pore system was minimized by using ground hardened cement paste stored for different periods of time in sodium sulfate solutions with different concentrations. It was necessary to develop a sample preparation and storage procedure to reduce exposure of the powder specimens to air and thus carbonation effects as much as possible. Secondly, the combined effect of the transport of external sulfates into concrete along with phase dissolution and formation was investigated by storing hardened cement paste cylinders in sulfate solutions. 2. MATRIALS AND MTHODS Ordinary Portland cement with 12.3 wt.% C 3 A and a high sulfate resistant Portland cement with 1.9 wt.% C 3 A were used in four test series covering different storage conditions. Polyethylene bottles (Ø 1 mm 3 ) were filled with Portland cement paste prepared at a w/c ratio of.6, sealed and stored at 2 C. During the first three days the bottles were rotated to avoid sedimentation. Table 1 shows the mineralogical composition (analysed by XRD) of the cements used in the test series. Table 1: Composition of the cements in wt.% (XRD) cement anhydrite arcanite C 2 S C 3 S C 4 AF C 3 A calcite gypsum periclase 1 C C CM I 32. R, 2 CM I 42. N HS (high sulfate resistant cement) At an age of 28 days, the hardened cement paste cylinders were demoulded and ground to < 63 µm in isopropanol and dried for about 2 hours at 4 C before transferring to an artificial pore solution (prepared with 36 mmol/ L KOH, 42 mmol/l NaOH and.68 mmol/l Ca(OH) 2 in distilled water, see [7]) for 14 days further hydration at 2 C. This additional storage step was necessary to enable the hydration of the clinker fracture surfaces produced during grinding and reduce the amount of clinker phases in the ground material. Afterwards the powder was separated from the solution by filtration and dried at 4 C for about 3 min. Batches weighing 1 g were added to either 8 or 4 g Na 2 SO 4 solution with concentrations up to 4 g/l SO 4 2- and then stored between 3h and 14d under rotation at either 8 or 2 C. The solid phases were filtered off after different storage periods, the chemical composition of the filtrate investigated by ICP OS and the ph analysed by titration. The solid phases were ground (<4 µm) and analysed quantitatively by XRD (Cu-Kα) using the Rietveld method. To obtain some information about the content of amorphous phases, the samples were spiked after drying at 4 C with 1 wt.% ZnO (with respect to total weight) as an internal standard. The addition of a known amount of ZnO (a standard material widely used in cement XRD analysis) permits the estimation of the amount of non-crystalline phases by the Rietveld fitting procedure, see for example [8]. In test series, hardened cement paste cylinders made with cement C1 were subjected directly to the above storage procedure at 19

3 3- June 29, Toulouse, France 2 C. Afterwards the cylinders were transferred to L sulfate solutions with different concentrations. The solution volume to specimen surface ratio was 2 L/m 2. After storage periods of, 14, 28, 6, 9 and 18 days, the specimens were removed from the solutions and sample material removed in layers approximately 1. mm in thickness by turning on a lathe. By analysing the phase composition of the powder specimens, it was possible to obtain distributions of the solid phases over the depth of the cylinders. The chemical composition of the storage solution was also analysed by ICP OS and the ph determined by titration. The present contribution presents the major results obtained for the development of phases observed by XRD. The combined analysis for the evolution of phase and solution composition for all test series will be presented elsewhere, see also [9]. Table 2: Storage conditions in the test series series 1 series 2 series 3 series 4 series cement C1 C2 C1 w/c ratio.6 pre-storage 28d sealed in P bottle, 2 C specimen form powder (< 63 µm) cylinder storage 14d in artificial pore solution, 2 C - storage temperature 2 C 8 C 2 C 2 C storage solution, 1., 1, 3 and 4 g/l SO 2-4 (Na 2 SO 4 ) duration up to 14d up to 18d amounts 1:8 (s:s*) 1:4 (s:s*) 2 L/m 2 *s:s: solid/solution ratio by weight 3. RSULTS AND DISCUSSION 3.1 Ground Hardened Cement Pastes Table 3 shows the phase composition for hardened paste made with cement C1 after 28 days hydration and 14 days further hydration in the artificial pore solution. Whereas the amounts of C 2 S, C 3 S, and C 4 AF decreased due to further hydration, the amounts of ettringite, portlandite, monocarbonate and hydrogarnet increased somewhat. C 3 A, which is important for sulfate attack, is no longer present. The amount of calcite increased from 4.1 wt.% (Table 1) to 7.9 wt.% (Table 3) during sample preparation and drying. However, during the subsequent storage in the sulfate solutions (including a control solution without sulfate), a small increase in the amount of calcite was observed likely resulting from the dissolution of monocarbonate. Hence carbonation should have little effect on the observed changes in the phase composition. Table 3: Quantitative XRD phase analysis after 28 days hydration and 14 days further hydration. (cement C1, phase contents in wt.%) amor. C 2 S C 3 A C 3 S C 4 AF cc. ett. hyg. monoc. port. 28 d d amor.=amorphous, cc.=calcite, ett.=ettringite, hyg.=hydrogarnet, monoc.=monocarbonate and port.=portlandite 2

4 3- June 29, Toulouse, France After mixing the initial material with the sulfate solutions, the amount of ettringite was, in general, observed to increase with storage time whereas the amount of the AFm phase monocarbonate and portlandite decreased, Figure 1. For concentrations of 1. to 3 g/l SO 4 2- and a solid/solution ratio of 1:8, the amount of ettringite rapidly increased up to 72 hours and then decreased somewhat. At a concentration of 4 g/l SO 4 2-, a reduction in the amount of ettringite was not observed after 72 hours. At a solid/solution ratio of 1:8 the ph reached values above 13.2 after 72 hours for concentrations of 1 g/l SO 4 2- and higher, Figure 2. After this time no further ettringite formation was observed whereas at a solid/solution ratio of 1:4 the ph is lower and ettringite continues to form, Figures 1 and 2. ttringite is stable between a lower limit ph of 1. to 1.7 [1, 11] and an upper limit of 12.6 to 13.2 [1]. Thus an increase in ph may explain the dissolution of ettringite after 72 hours. Matschei et al. [12] noted that calcium and aluminium are not readily soluble at high Na 2 SO 4 concentrations. At a solid/solution ratio of 1:4 the final amount of ettringite was between 21 and 22 wt.% after 14 days, independent of the initial concentration of the sulfate solution. Figure 1 demonstrates the differences between an OPC (C1) and a high sulfate resistant cement (C2) during sulfate attack. Comparing test series 2 and 4, it is apparent that the maximum ettringite content after 14 days is much higher for the OPC with a high C 3 A content. The formation of ettringite during storage coincides with the dissolution of monocarbonate. Compared with cement C1, the initial hydration products for cement C2 contain less monocarbonate which is consumed within the first 3 hours; the subsequent increase in ettringite is relatively small. For cement C1, the consumption of monocarbonate and thus ettringite formation continues over a longer period of time. Since gypsum formation obviously does not require aluminium, it is not strongly dependent on the type of cement test series 1: C1, s:s = 1:8, 2 C ettringite calcite portlandite monocarbonate gypsum portlandite test series 3: C1, s:s = 1:4, 8 C ettringite calcite monocarbonate portlandite test series 2: C1, s:s = 1:4, 2 C monocarbonate gypsum calcite ettringite calcite monocarbonate test series 4: C2, s:s = 1:4, 2 C portlandite gypsum ettringite Figure 1: ffect of storage time on phase contents for test series 1 to 4 with a sulfate concentration of 3 g/l in wt.%. 21

5 3- June 29, Toulouse, France 13, 13,4 13,3 13,2 13,1 ph 13, 12,9 12,8 12,7 12,6 12, test series 1: C1, s:s = 1:8, 2 C H 2O 1. g/l 1 g/l 3 g/l 4 g/l , 13,4 13,3 13,2 13,1 13, ph 12,9 12,8 12,7 12,6 12, test series 2: C1, s:s = 1:4, 2 C H 2O 1. g/l 1 g/l 3 g/l 4 g/l Figure 2: volution of ph in test series 1 and 2 during storage in different sulfate solutions. The trend of the ph observed in test series 3 and 4 is roughly similar to that of test series 2. Figure 3 shows the changes in sulfate concentration observed during storage for the solutions 2- with initial concentrations of 1. and 3 g/l. At a concentration of 1. g/l SO 4 and a solid/solution ratio of 1:4, the consumption of sulfates was much higher for the OPC (test series 2) than for the high sulfate resistant cement (test series 4). Since in both cases gypsum did not precipitate, this is obviously due to the formation of ettringite. At a solid/solution ratio of 1:8 (test series 1), the sulfate consumption is more rapid because the total amount of sulfate in the solution is lower. At an initial concentration of 3 g/l, the differences between the two cements are less pronounced because sulfate is consumed in both cases by gypsum formation. concentration [mmol/l] test series 1 1. g/l test series 4 test series 2 test series Figure 3: volution of the sulfate concentration for 1. and 3 g/l SO concentration [mmol/l] 3 g/l test series 3 test series 2 test series 4 test series 1 During storage at 8 C the same reactions were observed as at 2 C (cf. test series 2 and 3 in Figure 1), but the changes in the amounts of ettringite and monocarbonate were less rapid. A well-defined correlation between the increase of ettringite and the decrease of monocarbonate was observed in all test series (Figure 4). In sulfate solutions, monocarbonate is metastable with respect to ettringite. Monocarbonate like monosulfate can be regarded as an important aluminium source for ettringite formation, see [1, 13]. The ettringite formation was also accompanied by a reduction in the amount of portlandite. However, no significant change in the amount of calcite was observed. Thus, in this case, the formation of ettringite is governed primarily by the dissolution of monocarbonate and portlandite. The ettringite formation was also accompanied by a reduction in the amount of portlandite. Thus, in this case, the formation of ettringite is governed primarily by the dissolution of monocarbonate and portlandite. 22

6 3- June 29, Toulouse, France 12 decrease of monocarbonate [%] g/l 1 g/l 3 g/l 4 g/l increase of ettringite [%] Figure 4: Correlation between ettringite formation and monocarbonate dissolution during storage for cement C1. The values are with respect to the initial phase contents. In all test series, the amount of portlandite decreased rapidly during the first 3 hours. Gypsum was only observed for a solid/solution ratio of 1:4 and initial sulfate contents of 1 g/l and higher. As can be seen in Figure 2 for the solid/solution ratio of 1:8, the ph reaches values above 13. within the first 3 hours provided sufficient sulfate was present. Since the ph values were lower for the solid/solutions 1:4 compared with solid/solutions 1:8, the observed gypsum formation reflects the solubility of gypsum which increases with ph [14, 1]. After 24 hours the amount of gypsum reached a maximum after which it started to decrease. At the same time the ph reached a constant value near 13. and the portlandite was completely consumed. Since the decrease in the amount of gypsum was accompanied by a reduction in the amount of monocarbonate and the ph of the solution did not change appreciably, it appears that monocarbonate is transformed into ettringite by the solubility reaction with gypsum. A small increase in the amount of calcite results from the reaction. However, if portlandite is no longer available for ettringite formation, calcium can also be supplied by the decalcification of C-S-H [1]. This results in a reduction of the C/S ratio of the C-S-H phases and loss of strength of the binder paste matrix. The experiments performed with cement C2 (Figure 1, test series 4), showed an increase in the amount of ettringite during the first 3 hours after which time the monocarbonate had disappeared. Although the monocarbonate was exhausted, the amount of gypsum continued to decrease and, at the same time, the amount of ettringite increased slightly. This behaviour may be explained by ettringite formation due to the dissolution of gypsum and C-S-H phases which contain aluminium. Since 7.3 wt.% monocarbonate (initial content of monocarbonate measured for hardened paste C1 by XRD) can, theoretically, be transformed into about 16 wt.% ettringite, the initial amount of ettringite (8.2 wt.%) may increase to at most about 23 wt.%, cf. Figure 1. Based on the initial C 3 A content of the cement (Table 1), the amount of aluminium not bound in ettringite, monocarbonate and hydrogarnet was estimated to be at most approximately 3%. According to [16], aluminium can replace 6 to 7% Si in C-S-H phases or may occur in layers of AFm intimately mixed with those of C-S-H [17]. This aluminium is, in principle, available for ettringite formation. The formation of the so-called U-phase was observed for sulfate concentrations above 3 g/l SO 4 2- and a solid/solution ratio of 1:8, Figure. At a concentration of 3 g/l it first 23

7 3- June 29, Toulouse, France appeared after 7 days, at a concentration of 4 g/l SO 4 2- already after 3 hours. The U-phase was not observed in the entire test series performed at a solid/solution ratio of 1:4. The U- phase is a Na-substituted AFm phase which forms at ph values above 13.1 and high sodium sulfate concentrations [18]. It can clearly be identified by XRD with its main two peaks at scattering angles 2θ of 8.8 and g/l SO4 Duration: 7 days Intensity / cps Test series 4 C 2 S:S=1:4 T=2 C Test series 3 C 1 S:S=1:4 T=8 C Test series 2 C 1 S:S=1:4 T=2 C Test series 1 C 1 S:S=1:8 T=2 C U MS 1 M G CF 1 U P 2 M G G 2 Theta 2 Cc G P 3 G Z CS CF Z P G 3 Z CS Cc 4 C 1 = cement 1 C 2 = cement 2 s:s = solid/solution ratio T = temperature = ettringite M = monocarbonate MS = monosulfate G = gypsum P = portlandite U = U-phase cc = calcite CS = C 3 S, C 2 S CF = C 4 AF Z = ZnO Figure : Powder XRD diagram for test series 1 to 4 after 7 days in a 4 g/l SO 4 2- solution. 3.2 Hardened Cement Paste Cylinders Figure 6 shows the distributions of ettringite, monocarbonate, portlandite and gypsum in hardened cement paste cylinders (C1) after 9 days in sulfate solutions with initial concentrations up to 4 g/l. 2 ettringite 9d monocarbonate 9d H 2O 1. g/l 1 g/l 3 g/l 4 g/l depth [mm] H 2O 1. g/l 1 g/l 3 g/l 4 g/l depth [mm] 24

8 3- June 29, Toulouse, France 2 18 portlandite 9d 1 9 gypsum 9d H 2O 1. g/l 1 g/l 3 g/l 4 g/l depth [mm] H 2O 1. g/l 1 g/l 3 g/l 4 g/l depth [mm] Figure 6: Distributions of ettringite, monocarbonate, portlandite and gypsum in hardened C1 cement paste cylinders after 9 days in different sulfate solutions at 2 C. Integral values for 1 mm steps In general, the same basic reactions occurred as observed in the experiments with ground hardened cement paste where the effect of diffusion can be neglected. At higher initial sulfate concentrations, more sulfate diffused deeper into the cylinders precipitating ettringite at greater depths. The initial amounts of monocarbonate and portlandite decreased in proportion. Again a well-defined correlation between the formation of ettringite and the dissolution of monocarbonate was observed. At initial sulfate concentrations of 1, 3 and 4 g/l gypsum was found to precipitate in the surface region of the cylinders (see Figure 6). After 9 days storage, gypsum was not found deeper than 2 mm. This indicates that the ph of the pore solutions inside the cylinders was too high for a gypsum precipitation and/or the concentration of sulfates was too low. Hence gypsum was only able to form at the surface of the cylinders. The ph of the storage solution increased to at most 12.8 for an initial sulfate concentration of 4 g/l SO After 9 days ettringite formed at depths up to about mm for initial concentrations of 1, 3 and 4 g/l SO After 6 days, small cracks in the cylinders were first observed in all sulfate solutions - with and without gypsum precipitation. 4. CONCLUSIONS When sulfate ions penetrate hardened Portland cement paste, rapid portlandite dissolution results in gypsum precipitation if the ph of the pore solution is below 13. and the sulfate concentration is sufficiently high. ttringite forms at a slower rate governed by the dissolution kinetics of monocarbonate as a primary source of aluminium and the ph of the pore solution. If gypsum is present it converts with monocarbonate into ettringite at constant ph as predicted by the solubility equilibria of these phases. Gypsum dissolution and ettringite formation continue after exhaustion of monocarbonate so aluminium is supplied by other phases such as C-A-S-H. For a high sulfate resistant cement, the aluminium content of the hydration products is consumed more rapidly than with regular Portland cement so less ettringite is able to form; the maximum amount was reduced by % in these investigations. Gypsum formation is limited to the surface region (2 mm for w/c =.6) owing to the effect of sulfate concentration and ph on its solubility equilibrium. It is possible that the U-phase forms in concrete at the sulfate concentrations commonly used testing for sulfate resistance. 2

9 3- June 29, Toulouse, France The experimental procedure developed enables investigation of the phase changes occurring inside concrete (pore solution / binder matrix) with and without the effect of transport (diffusion of species in the pore solution). As well as phase changes during exposure to sulfates, data on the corresponding changes in the chemical composition of the storage solutions are also available. To gain more insight into the mechanisms of sulfate attack on concrete, the distribution of phases in the cylinders after 14, 28, 6, 9 and 18 d will be compared with the observed changes in storage solution and phase composition of cement powder using an appropriate simulation model This methodology will then be applied to binders containing secondary cementitious materials such as fly ash. ACKNOWLDGMNTS The authors express their thanks to the German Research Foundation (DFG) for supporting this project financially. RFRNCS [1] Skalny, J., Marchand, J. and Odler I., 'Sulfate attack on concrete', 1 st dn Taylor and Francis, London, 22 [2] Monteiro, P.J.M., Kurtis, K.., 'xperimental asymptotic analysis of expansion of concrete xposed to Sulfate Attack', ACI Materials Journal, 28 [3] Schmidt, T., Lothenbach, B., Romer, M., Scrivener, K., Rentsch, D. and Figi, R., A thermodynamic and experimental study of the conditions of thaumasite formation, Cem. Concr. Res. 38, 28, [4] DIN N 197-1/A2, 'Composition, specification and conformity criteria for common cements, Amendment A2 (Sulfate resisting cement)', Normenausschuss Bauwesen, 26 [] American society for testing and materials, 'Standard test method for length change of hydrauliccement mortars exposed to a sulfate solution', ASTM C112-2, 22 [6] Wittekindt, W., 'Sulfatbeständige Zemente und ihre Prüfung', Zement-Kalk-Gips 13, H.12, 196, 6-71 [7] Odler, I., Stassinopoulos,.N., 'Über die Zusammensetzung der Porenflüssigkeit hydratisierter Zementpasten', TIZ Fachbericht, Vol. 16, No. 6, 1982, [8] Westphal, T., 'Quantitative Rietveld-Analyse von amorphen Materialien', PhD thesis, Martin- Luther-Universität Halle-Wittenberg, 27 [9] Müllauer, W., Beddoe, R.., Heinz, D., 'Mechanismen des Sulfatangriffes auf Beton. influss von Bindemittelzusammensetzung und Umgebungsbedingungen', DFG Forschungsbericht, 28 [1] Damidot, D., Glasser, F.P., 'Thermodynamic investigation of the CaO-Al 2 O 3 -CaSO 4 -H 2 O System at 2 C and the influence of Na 2 O', Cem. Concr. Res. 23, 1993, [11] Gabrisova, A., Havlica J., 'Stability of calcium sulphoaluminate hydrates in water solutions with various ph values', Cem. Concr. Res. 21, 1991, [12] Matschei, T., Glasser, F.P., 'Influence of sodium sulfate on the degradation of portland cement, Part I: Carbonate-free conditions', Materials and Structures, Manuscript draft, 27 [13] Brown, P.W., Taylor H.F.W, 'The role of ettringite in external sulfate attack, in J. Marchand and L. Skalny (eds) Materials Science of concrete; Sulfate attack mechanisms', The American Ceramic Society, Westerville OH 1999, [14] Bellmann, F., Möser, B., Stark J., 'Influence of sulfate solution concentration on the formation of gypsum in sulfate resistance test specimens', Cem. Concr. Res. 36, 26, [1] Bellmann, F., Stark J., 'New findings when testing the sulphate resistance of mortar', ZKG International, No. 6, 26, Vol. 9,

10 3- June 29, Toulouse, France [16] Hong, S.Y., Glasser, F.P., 'Alkali sorption by C-S-H and C-A-S-H gels, Part II. Role of alumina', Cem. Concr. Res. 32, 22, [17] Taylor, H.F.W., 'Cement chemistry', Academic Press Limited, London, 199, [18] Moranville, M., Li, G., 'The U-phase formation and stability', in J. Marchand and L. Skalny (eds) Materials Science of Concrete; Sulfate Attack Mechanisms, The American Ceramic Society, Westerville OH 1999,