ROLE OF ETTRINGITE, THAUMASITE AND MONOCARBONATE IN HARDENING AND DESTRUCTION OF PORTLAND CEMENT GYPSUM SYSTEM

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1 ROL OF TTRINIT, THAUMASIT AND MONOCARBONAT IN HARDNIN AND DSTRUCTION OF PORTLAND CMNT YPSUM SYSTM Konstantin Kovler National Building Research Institute Faculty of Civil and nvironmental ngineering, Technion Israel Institute of Technology, Haifa, Israel Abstract Hardening and destruction of cementitious material made of 5% Portland cement 5% gypsum blend was studied. The material composition was studied by XRD and DTA methods. The results show that such composition can be strong and durable in water, when the thaumasite formation (caused by carbonation) is avoided. In this case ettringite is assumed to freely expand in the pores of the material successfully contributing to its strength, together with C-S-H engulfing gypsum crystals. It is known that carbonation of Portland cement gypsum materials in humid and cold climate results in complete disintegration of the system where ettringite serves as a precursor of thaumasite. To stimulate thaumasite formation by promoting carbonation in wet environment, part of the specimens cured under still water at 2 C until 28 days were then exposed to wet/dry cycles. All the last specimens disintegrated after ~4 months and transformed into a white, incohesive mush. The disintegrated material contained thaumasite and in the later stages of carbonation monocarbonate and calcite. The formation of monocarbonate seems to be related to the initiation of ettringite disintegration, when portlandite is consumed completely. 1. DSTRUCTION OF YPSUM CMNT SYSTM: WHAT DO W KNOW? It is known that in general, that mixing of gypsum and Portland cement should be avoided. For example, a blend consisting of 75% gypsum plaster and 25% Portland cement after 4 5 months of water immersion expands significantly and cracks, despite the fact that the compressive strengths in wet and dry states and the wet/dry strength ratio are increased somewhat during the first month of water immersion [1]. The deterioration of gypsum - cement materials in water is usually attributed to the formation of ettringite, which induces high internal tensile stresses and results in disruptive expansion. As was shown in [2, 3], the deterioration of the gypsum - Portland cement system in a humid environment seems to be more related to the formation of thaumasite in the presence of ettringite, which is a "precursor" of thaumasite. Differential thermal analysis (DTA) and X-

2 ray diffraction (XRD) diagrams showed the peak characteristic of ettringite for both systems: gypsum - cement and gypsum - cement - silica fume pastes; however, the gypsum - cement paste showed additional peaks of thaumasite. The formation of thaumasite in the presence of calcium hydroxide is a result of the carbonation of the gypsum - cement system in a humid environment accompanied by the disintegration of the C-S-H gel. During this destructive reaction the C-S-H gel is consumed together with gypsum and calcium hydroxide [4]: Ca 3 Si 2 O 7 3H 2 O + 2{CaSO 4 2H 2 O} + Ca(OH) 2 + 2CO H 2 O Ca 6 [Si(OH) 6 ] 2 (CO 3 ) 2 (SO 4 ) 2 24H 2 O (1) Another route of thaumasite formation, via woodfordite (the name of the solid solution whose end members are ettringite and thaumasite), is suggested in [8]. There is an observation that ettringite is decomposed during the thaumasite formation into semi-stable poorly crystallised compound, and that Ca(OH) 2 retards ettringite decomposition [5]. Different kinds of calcium aluminate carbonate hydrates (monocarbonate and ettringite carbonate, mainly) have been found in gypsum - Portland cement systems containing thaumasite [rreur! Signet non défini.]. At the same time, the range of quantities of soluble sulfates required for TSA, the transformations occurring in the aluminate phase, the environmental conditions favourable for formation of ettringite and thaumasite and their role in destruction of Portland cement gypsum stone are not precisely known yet. The present paper tries to shed more light on both structure formation and destruction mechanisms involved in the interaction of Portland cement minerals and gypsum in different environmental conditions. Understanding the physical and chemical mechanisms resulting in the destruction of cement gypsum system can be useful in finding a method of successful creation of the new blend of gypsum cement, which possesses advantages of both gypsum and Portland cement: (a) early hardening and fine finish (gypsum) and (b) strength and enhanced durability in water (Portland cement), similarly to that suggested in [1]. 2. THAUMASIT FORM OF SULFAT ATTACK Thaumasite is an ettringite analogue; i.e. an AFt compound, containing silicate ions occupying the sites normally associated with aluminate, and carbonate ions in some of the sites normally associated with sulfate ions. Thaumasite, calcium carbosulfosilicate hydrate, Ca 6 [Si(OH) 6 ] 2 (CO 3 ) 2 (SO 4 ) 2 24H 2 O, or more simply CaCO 3 CaSO 4 CaSiO 3 15H 2 O, and the related phase ettringite (Ca 6 Al 2 (SO 4 ) 3 (OH) 12 26H 2 O or 3CaO Al 2 O 3 3CaSO 4 32H 2 O) are formed in cement and concrete by the action of aqueous sulfates and/or carbonates [6]. The formation of ettringite is limited by the alumina content of the cement and leads to expansion and cracking. The effects of thaumasite formation can be much more severe since it involves the reaction of the main binding phase of the cement, calcium silicate hydrate (C S H). In extreme cases the portions of a concrete member affected can lose all of its strength, and even fluidize. Similar deterioration can be observed in Portland cement gypsum systems, in which gypsum content is high. Thaumasite formation requires the transportation of ions like Ca 2+, CO , SO 4 and sufficient moisture through the hardened cementitious material. It is known, for example, that

3 the presence of sulfate-bearing ground waters, which also contain high amounts of dissolved CO 2 /HCO 3, can supply both sulfate and carbonate ions needed to "fuel" thaumasite formation. The sulfates can be present as an internal ingredient of a concrete, for example if the cement or aggregate is contaminated with gypsum or similar sulfate-bearing material. In the majority of thaumasite occurrences in the field, the sulfates originated from an external source; e.g. ground water, sulfate-bearing bricks, gypsum plaster and gypsum-bearing historical mortars. It is known that the formation of thaumasite is favored at low temperatures and a temperature of around 5 C is most favorable. Nevertheless, a formation at temperatures up to about 25 C is also possible. In contrast to relatively limited damage potential for conventional sulfate attack, damage due to thaumasite form of sulfate attack (TSA) is in principle unlimited. Unlike ettringite, the amount of thaumasite that can be formed is not associated with, and thus not limited by, the alumina content of the cement. Thaumasite can continue to be formed as long as external sulfate and carbonate sources continue to be available. The required silica is supplied by the C S H itself, and if the reaction proceeds indefinitely all the C S H in a given concrete member can be totally destroyed [7]. Although powder X-ray diffraction is the most commonly used method for detection of thaumasite, this phase is very difficult to distinguish and quantify from ettringite because of the close similarity in unit cells and low-angle, high-intensity reflections for ettringite and thaumasite [8, 9, 1], with both having very similar crystal structures: in thaumasite Al(OH) 6 3- ions are replaced with Si(OH) 6 2- and (3SO H 2 O) with (2CO SO 4 2- ). Therefore the XRD patterns are very similar. Rigorous analysis of these, however, does reveal the differences in the unit cells, but it has been shown in [11] that electron diffraction is unable to distinguish the solid solutions. An unambiguous detection of thaumasite is also difficult to obtain by thermal analysis and infrared spectroscopy [1]. 3. XPRIMNTAL The experimental design was based on previous study [1], where disintegration of the blend consisting of 75% gypsum plaster and 25% of Portland cement was studied. In that study clear evidence was found for ettringite/thaumasite after 4 5 months of water immersion at 2 C. In the current study a blend of 5% calcium sulphate hemihydrate and 5% of Portland cement was investigated, in order to enhance the effects of possible ettringite/thaumasite formation and transformation. For this purpose it was decided (a) to favour the ettringite formation in the part of the specimens by preventing their carbonation in water and increasing curing temperature to 5 C, and (b) to favour the thaumasite formation in the part of the specimens by promoting their carbonation in water of 2 C. The pastes studied were of.5 water/binder ratio with 2% (by mass of total binder content) of Rheobild-1 superplasticizer of naphthalene formaldehyde sulfonate type. The cement was ASTM type I, having a standard compressive strength of 3 MPa and a specific mass of 3,1 kg/m 3 (a product of Nesher Industries, Israel). The calcium sulfate hemihydrate was of the β-type, produced by esher ypsum Works, Israel (93% pure). Cubes of 25 mm size were cast, demolded after 1 day and cured before testing for compressive strength at 7 days, 28 days and 8 months. Different curing conditions were applied: (a) curing under water in sealed bags (without access of CO 2,) at both 5 C and

4 2 C, (b) immersing in water at 2 C, (c) promote carbonation of the material under water, when a part of the specimens cured under still water at 2 C until 28 days were then exposed to cyclic drying (1 day) and water immersing (6 days) this treatment provided access to CO 2 to promote carbonation. First and last types of curing conditions were favourable for ettringite and thaumasite formation, respectively. Some cubes were tested in their wet (water-saturated) state; the others were oven-dried until achieving constant mass. The specimens were dried "gently" (at 5 C) to prevent gypsum from partially dehydrating, which could start before 1 C. ach series of testing consisted of three companion specimens, and the results were averaged. The material composition was studied by XRD (Philips PW 172, CuKa radiation) and DTA methods. DTA tests were carried out on powder samples of 1 mg at a heating rate of 1 K/min, to a temperature of 25 C; the reference sample was anhydrous calcium sulphate. To identify more reliably the DTA peaks of ettringite and thaumasite expected at the temperatures below gypsum dehydrating, some samples were heated, and their XRD diagrams were compared with those of unheated materials. 4. RSULTS AND DISCUSSION The DTA curves at early ages (minutes and hours after addition of mixing water) always showed the two peaks characteristic of decomposition of gypsum () at ~15 C and calcium sulphate hemihydrate (H) at ~2 C. No peak characteristic of ettringite or other hydrates was observed. The DTA curves (Figure 1) and XRD patterns (Figure 2) obtained after 7 days of water curing at 5 C (sealed specimens) showed clearly the formation of ettringite (). 1 Temperature difference ( C) H Temperature ( C) 7d/5 C/sealed 7d/5 C/sealed/125 C 7d/5 C/sealed/15 C Figure 1: DTA curves after 7 days of water curing at 5 C (sealed specimens), and heating at 125 C and 15 C ( gypsum, H hemihydrate, ettringite) The amount of ettringite increased from 7 to 28 days, but did not increase on prolonged curing of 8 months (Figure 3, Figure 4). Perhaps, this maximum at 28 days depends on the availability of aluminate phase in the cement. Similar results were obtained for the cubes

5 cured at 2 C water in both sealed bags and immersed in water, although ettringite formed more slowly than at 5 C. For example, at 2 C it is not seen at 7 days (see Figure 5), or has a very small peak (see Figure 6). ttringite formation was accompanied by consumption of portlandite (P) and gypsum: during the curing period the peaks of P and decreased by 3 and 1.5 times, respectively (Figure 4). An additional C-S-H formation is readily seen, as suggested by the increasing height of the broad halo in the left part of the XRD diagrams, which is typical of amorphous hydration products (Figure 4). P H ϑ 7d/5 C/sealed 7d/5 C/sealed/125 C 7d/5 C/sealed/15 C Figure 2: XRD patterns after 7 days of water curing at 5 C (sealed specimens), and heating at 125 C and 15 C ( gypsum, H hemihydrate, ettringite, P portlandite) As a result of C-S-H and ettringite formation the strength increased significantly in both wet and dry test conditions (Figure 7); but the most surprising result was the relatively high wet to dry strength ratio for 8 months of the cubes cured in sealed bags filled with water at 5 C. This ratio exceeded.7, the value typical for water-resistant materials (Figure 8). This result clearly shows that ettringite formation alone does not result in the destruction of Portland cement gypsum composites, and even can enhance their strength. The porosity of the studied Portland cement gypsum pastes is relatively high, about 3 vol. % (at age of 7 days). These trends suggest that ettringite may expand freely in the pores and not result in the destruction, as it usually happens in relatively dense Portland cement pastes containing a few percent of sulfates only. In order to stimulate the process of thaumasite formation by accelerating the carbonation of the material under water, a part of the specimens cured under water at 2 C until 28 days were then exposed to wet/dry cycles The cubes disintegrated after ~4 months and transformed into a white, incohesive mush. The XRD patterns presented in Figure 9 indicate thaumasite formation in the deteriorating system. In addition to the growing peaks of thaumasite, the following peculiarities are observed:

6 (1) thaumasite formation was accompanied by formation of calcium aluminate monocarbonate hydrate or simply monocarbonate, 3CaO Al 2 O 3 CaCO 3 11H 2 O; (2) ettringite remained in the system, however its amount decreased; (3) the absence of the peak characteristic of portlandite suggests that it has been consumed completely in the deteriorating material; (4) C-S-H gel was partly disintegrated (the height of the XRD halo decreased); (5) calcite was formed at later ages in the deteriorating system; (6) gypsum was consumed; Figure 11 shows a large gypsum crystal serving as a source and "fuel" for small thaumasite needles growing nearby; (7) carbonate ettringite (3CaO Al 2 O 3 3CaCO 3 32H 2 O) was not observed. 1 Temperature difference ( C) H Temperature ( C) 7d/5 C/sealed 28d/5 C/sealed 8m/5 C/sealed Figure 3: DTA curves after 7 days, 28 days and 8 months of water curing at 5 C (sealed specimens) ( gypsum, H hemihydrate, ettringite) When portlandite is consumed completely, carbonation and thaumasite formation continue. In this stage, apparently, ettringite starts disintegrating in addition to C-S-H, triggering the formation of monocarbonate. It has to be emphasized that ettringite was found in the system exposed to all the environmental conditions applied. When ettringite is formed alone in the porous Portland cement gypsum system (not accompanied by thaumasite), enhances its strength, together with C-S-H. However, it is still difficult to resolve the contribution of each of them, ettringite and C-S-H, to the strength increase. At the same time, ettringite can serves as a trigger to thaumasite formation, when the system starts carbonating; this process can be fatal. In order to identify accurately the DTA peak of thaumasite disintegration, the specimen of the deteriorated paste at age of 6 months was heated at 12 C, and its XRD diagram was compared with that of the specimen before heating (Figure 1). Heating significantly reduced the peaks of both thaumasite (T) and monocarbonate (MC). Therefore, the wide peak on the

7 DTA curve of the deteriorated paste, which consists probably of two peaks lying close to each other, can be characteristic of thaumasite and monocarbonate decomposition (Figure 6). P ϑ 2h 7d/5 C/sealed 28d/5 C/sealed 8m/5 C/sealed Figure 4: XRD patterns of fresh (2 hours age) and cured gypsum cement pastes for 7 days, 28 days and 8 months under water of 5 C (sealed specimens) ( gypsum, H hemihydrate, ettringite, P portlandite) 1 Temperature difference ( C) H Temperature ( C) 7d/2 C/sealed 28d/2 C/sealed 8m/2 C/sealed Figure 5: DTA curves after 7 days, 28 days and 8 months of water curing at 2 C (sealed specimens) ( gypsum, H hemihydrate, ettringite)

8 1 8 Temperature difference ( C) T+MC H Temperature ( C) 7d/2 C 28d/2 C 8m/2 C 6m/2 C/water changed weekly Figure 6: DTA curves of Portland cement gypsum pastes after 7 days, 28 days and 8 months of immersion in ditch water and after 6 months of wet/dry cycles ( gypsum, H hemihydrate, ettringite, T thaumasite, MC monocarbonate) 3 25 Compressive strength (MPa) C/sealed/dry 5 C/sealed/wet 2 C/sealed/dry 2 C/sealed/wet 2 C/immersed/dry 2 C/immersed/wet 2 C/water+CO2/dry 2 C/water+CO2/wet Time (days) Figure 7: Compressive strength of Portland cement gypsum pastes after 7 days, 28 days and 8 months of water curing at different temperatures, 5 C (in sealed bags) and 2 C (sealed and immersed curing) tested in both wet and dry conditions; the specimens exposed to wet/dry cycles deteriorated quickly (at ~4 months)

9 .8.7 Wet/dry strength ratio days 28 days 8 months 5 C/sealed 2 C/sealed 2 C/immersed Figure 8: Wet/dry strength ratio of Portland cement gypsum pastes after 7 days, 28 days and 8 months of water curing in different conditions (sealed and immersed) MC T T +T+MC +T +T +C ϑ 4m/2 C/water+CO2 6m/2 C/water+CO2 Figure 9: XRD patterns of disintegrating Portland cement - gypsum system under wet/dry cycles, at age of 4 and 6 months ( gypsum, ettringite, T thaumasite, MC monocarbonate, C calcite)

MC T +T+MC T +T +T +C 8 1 12 14 16 18 2 22 24 2ϑ 6m/2 C/water+CO2 6m/2 C/water+CO2/12 C Figure 1: XRD patterns of disintegrating Portland cement - gypsum system under wet/dry cycles,

10 MC T +T+MC T +T +T +C ϑ 6m/2 C/water+CO2 6m/2 C/water+CO2/12 C Figure 1: XRD patterns of disintegrating Portland cement - gypsum system under wet/dry cycles, at age of 6 months, before and after heating at 12 C ( gypsum, ettringite, T thaumasite, MC monocarbonate, C calcite) Figure 11: Thaumasite needles growing from a nearby gypsum crystal

11 5. CONCLUSIONS Testing of specimens made of 5% Portland cement 5% gypsum blend shows that such composition can be strong and durable in water, when the thaumasite formation (caused by carbonation) is avoided. In this case ettringite is formed as a result of interaction between Portland cement and gypsum and successfully contributes to the strength, together with C-S-H. Carbonation of Portland cement gypsum materials in humid climate results in complete disintegration of the system caused by the thaumasite formation. ttringite serves as a precursor of thaumasite formation. Monocarbonate formation seems to be related to the initiation of ettringite disintegration, when portlandite is consumed completely. Calcite and monocarbonate accompany thaumasite formation at later stages of carbonation. ACKNOWLDMNTS The author gratefully acknowledges Prof. Arnon Bentur for help and valuable comments. RFRNCS [1]. Bentur, A., Kovler, K. and oldman, A., 'ypsum of improved performance using blends with Portland cement and silica fume', Advances in Cement Research, 6 (23) (1994) [2]. Alksnis, F.F., 'Hardening and destruction of gypsum-cement composite materials' (Leningrad, Stroyizdat Publ., 1988) (in Rissian). [3]. Kovler, K., and Bentur, A., 'Differential thermal analysis of hydration and hardening in gypsumportland cement systems', Proc. Israeli - Hungary Binational Conf. on Therm. Anal. and Calorimetry of Mat., d. S. Yariv (The Hebrew Univ., Jerusalem, 1996) [4]. Kovler, K., 'nhancing water resistance of cement and gypsum-cement materials', ASC J. Mater. in Civil ngineering, 13 (5) (21) [5]. Pajares, I., Martínez-Ramírez, S. and Blanco-Varela, M.T., 'volution of ettringite in presence of carbonate, and silicate ions', Cement and Concrete Composites, 25 (8) (22) [6]. Taylor, H.F.W., 'Cement chemistry', Thomas Telford Publishing, London (1997). [7]. Macphee, D.. and Diamond, S., 'Thaumasite in Cementitious Materials', Cement and Concrete Composites, 25 (8) (22) [8]. Moore, A.. and Taylor, H.F.W., 'Crystal structure of ettringite', Acta Cryst. B 26 (197) [9]. dge, R.A. and Taylor, H.F.W., 'Crystal structure of thaumasite', Acta Cryst. B 27 (1971) [1]. Bensted, J., 'Thaumasite direct, woodfordite and other possible formation routes', Cement and Concrete Composites, 25 (8) (22) [11]. Lachowski,.., Barnett, S.J. and Macphee, D.., 'Transmission electron optical study of ettringite and thaumasite', Cement and Concrete Composites, 25 (8) (22)

Journal of Engineering Sciences, Assiut University, Vol. 34, No. 4, pp , July 2006

Journal of Engineering Sciences, Assiut University, Vol. 34, No. 4, pp. 1061-1085, July 2006 COMPRESSIVE STRENGTH AND DURABILITY OF CEMENT CONCRETE CONTAINING ALKALI WASTES OF OIL AND CELLULOSE PAPER INDUSTRIES