CONCRETE LIBRARY OF JSCE N0. 30, DECEMBER 2000 ASR GEL COMPOSITION, SECONDARY ETTRINGITE FORNIATION, AND EXPANSION OF MORTARS IMMERSED IN NaCl SOLUTION t (Translation from Proceedings of JSCE, No.641/V-46, February 2000) ¢_-. 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".3‘=‘Efii$30iEfiE5E=E1]€F?Ei%=E=5?ii - ° ' 1.‘€:'IE>§%=53."/';I¢1"l=§EiE1§31:§2=E=§E’ "1J:i.<EI';=*-5ii>.'%é=€' V §‘:)">‘.¢3ii‘§¢s-3-.r¢t= ' -=¢li1&&E=3%R€taré1§?I$€?<;i:<1i<l;:»:%21:i£1511‘R5195 Kenji KATAFUTA In this study, we pursue the changes in microstructural features and characteristics of ASR gel composition in mortars containing reactive aggregate after immersion in NaCl solution. These characteristics are compared with their expansion behavior. Mortars immersed in NaCl solution undergo greater expansion than those in NaOH solution, and this may be a result of the formation of ASR gel with a relatively low alkali content. However the coexistence of ettringite with ASR gel is indicated by EDS analyses, so it is not possible at this stage to infer whether the formation of secondary ettringite increases mortar expansion or not. Keywords: ASR, NaCl,- de-icing salt, ASR gels, ettringite, SEM, EDS-BSE analysis Mitsunori Kawamura is a Professor in the Department of Civil Engineering at Kanazawa University, Ishikawa, Japan. He has been active in cement and concrete research for 35 years. His reseach interests include the alkali-silica reaction, concrete durability, and the microstructure of cement-based matetials. He is a fellow of JSCE. . Noriyuki Arano is a researcher in the Division of Cement at Denki Kagaku Kogyo Co. Ltd., Niigata, Japan. He obtained his D.Eng. from Kanazawa University in 2000. His reserch interests relate to the reaction products and compatibility of aggregate in concrete. He is a member of JSCE. Kenjii Katafuta is with the Goyou Kensetsu Co.Ltd.. He obtained his M.S. Degree from Kanazawa University in 1999. —273--- 1. INTRODUCTION The effect of NaCl contributed by deicing salt or seawater on ASR damage to concrete is an important issue in the field of concrete structure maintenance. A recent survey of the effects of deicing salt on ASR-induced expansion reported that mortar containing a reactive aggregate may exhibit greater or lesser expansion when immersed in NaCl solution than in water, depending on the reactivity of the aggregate [1]. Another field investigation, this time on ASR-affected concrete piles in a partially submerged marine environment, has revealed that ASR causes significant damage to the submerged portions of even relatively newmarine structures, but not to the above-water portions [2]. Thus, if reliable repair of ASR-affected structures in saline environments is to be achieved, a better understanding of the mechanisms by which NaCl influences ASR is crucial [3]. The mechanism by which NaCl influences the expansion of hardened concrete containing reactive aggregate has been studied by a number of groups. At least part of the expansion in mortar containing reactive aggregate is certainly due to a rise in the OHion concentration in the pore solution, which is brought about by intrusion of NaCl, and Cl'ions themselves accelerate ASR in concrete [4,5], However, recent reports [6,7] have suggested that CaA and gypsum as well as alkalis in the cement play an important role in the expansion of mortar containing a reactive aggregate. In fact, SEM examinations of opal-containing mortars immersed in 1M NaCl solution for longer than a year showed that large amounts of chloride-bearing ettringite had formed in the mortars [8]. However, ambiguity remains as regards the mechanism by which the formation of secondary ettringite is related to mortar expansion in an NaCl solution. The aim of this study is to pursue the changes in microstructural features indicated by BSE-EDS analysis and the characteristics of ASR gel composition in reactive aggregate-containing mortars when immersed in NaCl solution, and to relate them to the expansion behavior of the mortars. 2. EXPERIMENTAL OUTLINE 2.1 Materials Calcined flint with a size fraction of 2.5 mmto 0.6 mmwas used as the reactive aggregate. Its dissolved silica (Sc) and alkalinity reduction (Re) in the ASTM chemical test (C 289) were 1063 and 70 mmol/1, respectively. Japanese Toyoura standard sand was used as a non-reactive aggregate. The CsA content of the ordinary and sulfate-resisting Portland cements used was 9.2% and 3. 1%, respectively; the Na20 equivalent of the two was 0.67% and 0.59%, respectively. The chemical and mineral compositions of these cements, as calculated by the Bogue method, are given in Table 1. Table 1 Chemical S iO s Composition A ha of Ordinary O rd in a ry 2 2 .0 5 .2 F e 2O 3 2 .7 C aO 6 3 .6 S u lfa te -re si stin e 2 2 .2 4 .2 4 .6 6 4 .1 M eO and Sulfate-Resisting Portland Cement (%) IG O 0 .4 1 4 7 c zs 2 7 C sA 1 .9 N ifc O 0 .4 0 C 3S 1 .5 9 .2 C 4A F .2 0 .4 2 .2 0 .3 4 0 .3 8 5 1 2 5 3 .4 1 4 .0 S O 3 2.2 Mortar Specimens for Expansion Testing Mortar was prepared with a total aggregate to cement ratio of 2.0 and a water-cement ratio of 0.5. Reactive aggregate replaced 30% by mass of the non-reactive aggregate. Mortar bars measuring 25.3 by 25.3 by 285.5 mm,were cured in a sealed state for 28 days, then immersed in 1M NaCl solution at 38 °C. For expansion tests in 0.6M NaOH solution, a further series of mortars was prepared with an aggregate-cement ratio of 2.25 and a water-cement ratio of 0.6, with 10% by mass of aggregate replaced by reactive aggregate. The same cements were used [9]. 2.3 BSE-EDS Analysis Procedure At 14, 56, 119, and 406 days after immersion in NaCl solution, mortar slices were cut from the center of the mortar bars for BSE-EDS analysis. These were dried by ethanol replacement, followed by overnight -274- vacuum drying at room temperature. The dried slices were impregnated under vacuumwith ultra-low viscosity epoxy resin. The polished surfaces were sputter coated with a 30 nm layer of gold-palladium alloy. The samples were then examined using a Hitachi S-2250N SEM equipped with a back scatter detector and a Horiba EMAX-5770W energy-dispersive X-ray analyzer. 3. RESULTS 3. 1 Expansion Characteristics of Mortars in Nad and NaOH Solutions The expansion curves for mortars made with both the ordinary and sulfate-resisting Portland cements and immersed in 1M NaCl and 0.6M NaOH solutions are presented in Fig.l. The mortars are found to expand rapidly immediately upon immersion in both solutions. The rapid progress of expansion in NaCI solution was maintained up to about 120 days, reaching a maximumvalue of about 1.5% in the case of ordinary cement. In the later stages, the mortars continued to expand at a lower rate. Even at an early age, however, expansion rates in NaOH solution were lower than those in NaCl solution. The expansion rate for mortars in NaOH solution rapidly decreased after about 100 days of immersion, reaching an ultimate expansion of about 1.0%. As Fig.l demonstrates, the expansion of sulfate-resisting cement mortars was less than two thirds that of ordinary Portland cement mortars. 3.2 ASR Gel Composition in Immersed Mortars 100 200 300 400 500 600 700 800 TIME (DAYS) Fig.l Expansion curves for mortars in NaCl and NaOHsolution Careful SEM examinations over the whole of the polished face of mortar samples cured for 28 days in a sealed state at 38 °C showed that the cement paste developed the normal microstructural features and that large amounts of ASR gel were present within relatively wide cracks in reactive aggregate particles. Lowand high-magnification BSE micrographs of a reactive aggregate particle found in the polished section of a mortar sample immersed in NaCl solution for 56 days are presented in Photograph 1. Here, there are many cracks within the reactive aggregate particles and some cracks are filled with large amounts of ASR gel. The average mole percentages of SiOz, CaO, and (NaaO+BGO) obtained by BSE-EDS analysis at about 10 spots in ASR gel areas in five different reactive aggregate particles are plotted in the ternary phase diagram in Fig. 2. These particles were selected from mortars pre-cured for 28 days in a sealed state. Figure 2 also shows that little change occurred in the mole percentages of (Na2O+IGO) in the gels during immersion in NaCl solution. However,it is found from the plots of KzO/NasO content ratios at various immersion times (Fig.3) that potassium in the ASR gels produced in mortars during the pre-curing period was replaced by sodium during the relatively early stages of immersion in NaCl solution. Additional alkalis were not incorporated in the gels; only Na* and K+ions were exchanged. The replacement of potassium by sodium mayhave no effect on the expansion ofASR gels [10]. The results of SEM observations of mortars immersed in both NaCl and NaOH solutions show that more ASR gel is produced in mortars immersed in NaOH solution than in mortars immersed NaCl solution. However,although the initial content of reactive aggregate in mortars immersed in NaCl solution was three times that of mortars in NaOH solution (according to the mix proportions), the amount of reactive silica even in NaOH-immersed mortars may be high enough to maintain the progress of ASR. Unlimited amounts of OJTions would be supplied to mortars in NaOH solution and the OH ion concentration of the pore solution of these mortars is higher than that of mortars in NaCl solution. In NaOH-immersed mortars, the formation of ASR gel with low viscosity and higher fluidity, as a consequence of the higher alkali content associated with increased OH ion concentration, may be lead to greater expansive stress relaxation. -275- (b) Photograph 1 BSE micrographs for reactive aggregate particle (a) and ASR gel (b) in ordinary cement mortar immersed in NaCl solution for 56 days portland C o Si02 o-A-i oo Mole Percent 0.4 cc LU 100 Na20+K20 Fig.2 ^i ob Ternary phase diagram for ASR gels in mortars 3 IMMERSION Fig.3 Mole Percent 106 NaaO+KaO Fig.4 Ternary phase diagram for ASR gels in mortar s -276- 00 400 TIME (DAYS) KzO/NaaO ratios of ASR gel in mortars immersed in NaCI solution As shown in Photograph 2, gel remained inside reactive aggregate grains even in mortar immersed in NaCl solution til the age of 120 days. On the other hand, it is confirmed that gel in NaOH solution was carried outside the reactive aggregate grains as a result of its low viscosity. The CaO content of the gel was also investigated, and shown to be the same level in both NaCl and NaOH immersion. This demonstrates that CaO content has little effect on the expansion behavior of mortars. In a separate study [9] the authors confirm this limited effect of CaO content in the ASR gel on the expansion of mortar. (a) NaCl solution Photograph 2 3.3 Microstructural SEM micrographs (b) NaOH solution for ordinary cement mortars immersed in NaCl solution solution for 120 days and NaOH features of Cement Paste in Mortars in NaCl Solution SEM observations turned up no particularly characteristic features in the microstructure of cement paste in mortars immersed in NaCl solution for 14 days. However, after 56 days in NaCl solution, many ASR-induced microcracks were found to be partly filled with ettringite. In the BSE images in Photograph 3, belts of ettringite are seen in cracks approximately 10 um wide by 50-200 um long. No rim cracks filled with ettringite were found around aggregate particles, and particles of ettringite appear to partly fill ASR-induced cracks propagating through the cement paste. However, not all cracks were filled with ettringite, although cracks caused by ASR cannot be differentiated from those caused by sample preparation. Welooked at 10 different areas of ettringite deposits and carried out EDS analysis for about 5 spots in each individual area for a total of about 50 spot analyses of ettringite deposits in the polished face of a mortar sample. A typical EDS result was that relatively large amounts of (NazO+BGO) were detected when analyzing string-like ettringite areas. The mole percentages of (NazO + l&O) and SiOa obtained by EDS analysis are plotted in Fig.5. Part of the SiCh and (NazO + IGO) content may result from some contamination that occurred during analysis. It should be noted that the alkali contents obtained in these analyses were considerably higher than the 2% found as an impurity in ettringite [1 1]. The CaO, AhOa, and SOs contents, obtained by excluding other elements being normalized to 100 percent , are plotted in a ternary phase diagram shown in Fig. 6. The plots lay on and around a line connecting the stoichiometric composition of ettringite with points obtained for ASR gel in air voids within the mortars. Thus, these results show that string-like ettringite deposits found in cracks are mixtures of ettringite and ASR gel. The movementof plotted points towards the CaO apex over time means that the proportion of ASR gel in the string-like deposits and/or the CaO content of the ASR gel itself increases with time. It should be noted that we found no such ettringite deposits in ASR-induced cracks in the sulfate-resisting cement mortars. -277- 55fc£<? (a) reactive aggregate particle (b) cracks filled Photograph 3 with ettringite deposits BSE micrographs for ordinary cement mortar immersed in NaCl solution for 56 days (c) ettringite 12 \- Immersion O^ 6 Time 56 days V^" . %à" ^? o 5 SiO, Fig.5 Alkali and silica 10 (MOLE%) contents in cracks of ettringite deposits Fig.6 Ternary phase diagram of ettringite in cracks 4. DISCUSSION As observed above, some cracks in the cement paste phase were filled with ettringite. The formation of ettringite like this requires a supply of Ca2+,SO42",and Al(OH)4" ions. It has been shown that the solubility of ettringite in the system consisting of ettringite-water-NaCl is about three times that in the ettringite-water system, and that monosulfate-water-NaCl systems are less stable than ettringite-water-NaCl systems [12]. It 275- Table 2 Expansion, Ettringite String-like Formation in Cracks, ASR Gel Composition, Width Ettringite Areas in Ordinary Portland Cement Mortars T im e(D ay s) E x pan sion String in -likCerack E ttring s ite 0 N o ne N o ne 14 56 119 V ery A ctiv e V ery A ctiv e V ery A ctiv e N o ne P resent P resen t A v erag e W id th o f B a n d A re a s o f E ttrin eite A v erag e L en gth of B a nd A rea s of E ttrin eite S tate o f A S R G el N o ne N o ne 9.4 u m N o ne N o ne S o lid S olid and Length 26 6 S lo w ly P resent 40 6 V ery S low ly P re sent 12 .4 um 17 .8 um 14 .O u m 37 0u m 2 34 um 2 72 um 2 13 u m S olid S o lid P a r t ly E v acu ated M o s t ly E vacu ated of is known that SO/" ions are released during the conversion of the monosulfate to Friedel's salt under NaCl attack. So, taking into consideration the fact that secondary ettringite deposits were hardly found in cracks in the cement mortar samples made with a sulfate-resisting cement, despite immersion in NaCl solution up to 406 days, the monosulfate can be concluded to be a major source of SO/" ions. As shown in Fig.l, the expansion of the sulfate-resisting cement mortar was much less than that of mortar containing ordinary cement. The equivalent percentages of NasO in the two types of cement are little different, but the amount of unreacted CsA in the sulfate-resisting mortar when immersion began would have been much less than in the latter. As a result, it is impossible to infer whether the much lower expansion of sulfate-resisting cement mortar in NaCl solution was due to the absence of secondary ettringite formation or not. In order to elucidate the role of ettringite formation in the expansion of mortars, we tabulate the rate of expansion, ettringite formation, average width and length of ettringite-filled cracks, and the state of ASR gel within aggregate particles for various immersion times in Table 2. The width and length values given in this table are averages of 10 measurements for different cracks filled with ettringite. The width of these ettringite-filled cracks appears to increase with increasing expansion. However,even after immersion for 406 days, an expansive ASR gel with relatively low alkali content was present within reactive aggregate particles, although a great portion of this gel had been evacuated from some of the particles. Thus, it is unclear if secondary ettringite formation led to increased mortar expansion. Grattan-Bellow [13] and Davis & Oberholster [14] have reported that the increased expansion in concrete supplied with NaCl is induced not only by ASR as promoted by the increasing concentration of Na+and OH"ions formed in the reaction between NaCl and CsA or Ca(OH)j duced, but also by the production of expansive monochloro-aluminate and chloro-surfo-aluminate. As far as these results of immersion tests in NaCl solution up to 406 days are concerned, it is not possible to infer whether the formation of secondary ettringite in many pre-existing ASR-induced cracks increased mortar expansion or not. The greater expansions of mortars in NaCl solution than in NaOH solution found in this study could result from the production of low-alkali ASR gel with high viscosity, which would give rise to lasting expansive pressure within the mortar over a long period. 5. CONCLUSIONS The conclusions reached in this investigation can be summarized as follows: (1) Mortar immersed in 1M NaCl solution expanded more than that in 0.6M NaOH solution. The greater expansion of mortar in NaCl solution might result from the formation ofASR gel with relatively low alkali content and high viscosity. (2) Many ASR-induced micro-cracks were found to be partly filled with ettringite in mortars containing reactive aggregates and immersed in NaCl solution for periods longer than 14 days. (3) String-like ettringite deposits found in cracks were mainly a mixture of ettringite and ASR gel. (4) From this work, it is impossible to infer whether the formation of ettringite in ASR-induced cracks increases mortarexpansion or not. 279 - References [1] Miura, T., et al., "Report on Damage to Concrete by Deicer Salt," JCI , 1999 (in Japanese) [2] Spencer, T.E. and Blaylock, A.J., "Alkali-Silica Reaction in Marine Piles," Concrete Vol.19, No.l, pp.59-62, International, 1997. 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[12]Dorner, H., Rippstain, D., and Gottmann, H., "Einwirkung wafiringer Natriumchloridlosung auf Monosulfat, Teil 2," TIZ-Fachberichte, Vol. 1 10, No. 7, pp.477-481, 1986. [13]Grattan-Bellew, P.E., "Preventive Measures to Counteract Expansion of Concrete Containing Alkali-Reactive Aggregates," Durability of Building Materials, pp. 363-376, 1983. [14]Davis, G., and Oberholster, R.E., "The Effect of Different Outdoor Exposure Conditions on the Expansion due to Alkali-Silica Reaction," Proc. 8th Intl. Conf. on Alkali-Aggregate Reaction, Kyoto, pp. 623-628, 1989. -250-