On the durability of Portland limestone cement: Effect of pH on the thaumasite formation

HBRC Journal (2017) xxx, xxx–xxx

Housing and Building National Research Center

HBRC Journal http://ees.elsevier.com/hbrcj

FULL LENGTH ARTICLE

On the durability of Portland limestone cement: Effect of pH on the thaumasite formation Hanaa Y. Ghorab *, Fouad S. Zahran, Mohamed Kamal, Amr Said Meawad Chemistry Department, Faculty of Science, Helwan University, Cairo, Egypt Received 17 December 2016; accepted 13 April 2017

KEYWORDS Thaumasite; Mechanism; Ettringite; Silicate; Carbonation

Abstract The mechanism of thaumasite formation is studied in a solution of sodium silicate and ettringite stored for 12 months at 7 °C. After 7 months, the mix was carbonated by bubbling CO2 gas and the pH decreased from 11 to 9.5; at the 9th month the pH was raised again to 12.5 by adding lime water. The phases formed at the different pH ranges were identified by means of X-ray diffraction and infrared spectroscopy. The results indicate that a certain reaction occurred in the mix stored for 7 months at pH 11: A shoulder appeared at frequency of 1030 cm 1 in the infrared spectrogram of the mix and the band in the frequency region 500–400 cm 1 broadened; the X-ray patterns show, however, unchanged ettringite patterns with weak calcite phase. The ettringite phase disappears by lowering the pH to 9.5, and an amorphous phase forms instead. Diffraction lines of aragonite and calcite are identified in this sample, and its IR spectra indicate the transformation of the shoulder at 1030 cm 1 to a strong broad band at 1027 cm 1 and the appearance of infrared frequencies characteristic of the carbonate phases. The amorphous phase formed is a carbonated complex of hydrated silicate and of decomposed ettringite. It converts to thaumasite with the supply of lime and the rise of pH to 12.5. Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

* Corresponding author. E-mail addresses: [email protected] (H.Y. Ghorab), [email protected] (F.S. Zahran), [email protected] (M. Kamal), [email protected] (A.S. Meawad). Peer review under responsibility of Housing and Building National Research Center.

Production and hosting by Elsevier

The production of Portland limestone cements is of technical and economic advantages. The European standards EN 1971 and the Egyptian specifications ESS 4756-1 allow up to 35% limestone (by mass) in CEM II/A and CEM II/B. The Egyptian Concrete Code of Practice [1] approved first in 2014 the use of a maximum of 10% limestone in cement for structural concrete. Higher limestone content was limited to avoid failures due to uncontrolled construction procedures in small projects which may threaten the service life of concrete. A series of investigations was therefore carried out to gain

http://dx.doi.org/10.1016/j.hbrcj.2017.04.002 1687-4048 Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: H.Y. Ghorab et al., On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC Journal (2017), http://dx.doi.org/10.1016/j.hbrcj.2017.04.002

2

H.Y. Ghorab et al.

experience on the use of Portland limestone cement and to understand the mechanism of thaumasite formation [2–7]. The thaumasite phase forms in the presence of calcium ions, silicate, sulfate, carbonate, and sufficient water to permit the transport of potentially active species. Low temperatures favor its formation but it was detected at room temperature as well [8,9]. The mechanism of the thaumasite formation is still under debate. Few works discussed the carbonation process and role of pH [10,11]; these two parameters are the subject of the present investigation. Experimental Liquid sodium silicate (SL), Na2Si2O5, of pH 13 was supplied from Fisher Scientific Company. The ettringite phase was prepared in the laboratory from the reaction of chemically pure calcium hydroxide with hydrated aluminum sulfate at room temperature. It was characterized by means of X-ray diffractometer X’PERT MPD Philips and by Fourier Transmission Infrared spectrometer FTIR -6300 Type A. The mechanism of thaumasite formation was investigated in a solution composed of liquid silicate and ettringite mixed with a mole ratio 1:1 in 50 ml water and stored for 12 months in plastic bottles at 7 °C. The pH of the solution was 11. After 7 months, the pH was lowered to 9.5 by bubbling with CO2 gas, and at the 9th month the pH was raised again to 12.5 by adding 20 ml of lime water. The phases formed during the first 7 months, as well as after 9 and 12 months were identified by means of X-ray diffraction and infrared spectroscopy. At the prescribed times, the solids were filtered off, washed with isopropyl alcohol to stop the reaction, dried in the desiccators and analyzed.

Fig. 1b

The infrared spectra of ettringite.

Results Fig. 1a proves that all the d-value-lines characteristic of the ettringite phase are detected in the X-ray diffractogram of the salt prepared. The infrared spectra of ettringite are illustrated in Fig. 1b and indicate the presence of the stretching and bending frequencies of water at 3633–3433 and 1675 cm 1. The main stretching vibration of sulfate (S-O) appears at 1115 cm 1, and its bending vibration is 614 cm 1.

Fig. 1a

The X-ray diffraction patterns of ettringite.

Fig. 2a The X-ray diffraction patterns of the ettringite-silicate mix at pH 11.

The Al-O group is identified at 850 cm 1. The CO band in calcite is weakly recognized at 1427 cm 1 and proves that the sample is slightly carbonated. No change was observed in the X-ray diffractogram of the phases present in the silicate-ettringite mix (pH = 11) stored for 7 months at 7 °C. Representative patterns of these phases are shown in Fig. 2a and indicate a clear presence of ettringite beside weak d-value line of calcite at 3.03 A. A small hump is detected in the 2h range of 20–40° attributed to the amorphous hydrated silicate. In this system, the functional groups of the reactants and products are better identified in the frequency range 1500– 500 cm 1 of the infrared spectrogram: Fig. 2b shows the appearance of the calcite bands at 1431–1384 and 869 cm 1; the sulfate frequencies are at 1113 and 615 cm 1. An interesting shoulder is observed at 1030 cm 1 together with the broadening of the band in the frequency region 500–400 cm 1 This indicates that a certain reaction is taking place in the mix which could not be recognized in the X-ray diffractogram. After 2 month storing the mix at pH 9,5, the ettringite phase disappeared in the X-ray diffractogram. An amorphous phase formed instead with a clear hump in the range of 2 theta

Please cite this article in press as: H.Y. Ghorab et al., On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC Journal (2017), http://dx.doi.org/10.1016/j.hbrcj.2017.04.002

On the durability of Portland limestone cement

Fig. 2b 11.

The infrared spectra of the ettringite-silicate mix at pH

20–40° (Fig. 3a). The diffraction lines are observed at 3.4, 3.27 and 1.98 A attributed to the aragonite phase. The calcite phase is identified by its diffraction lines observed at 3.03, 2.49 and 1.87 A. The IR spectra of the carbonated mix are illustrated in Fig. 3b. It shows a broad CO frequency at 1446 cm 1, beside other CO bands at 873 and 709 cm 1. The observed broadening in the v3/CO at 1446 cm 1 band means that a mixture of calcite and aragonite exists: the v3 vibrations of pure calcite and pure aragonite being at 1430 and 1470 cm 1 respectively. Furthermore, the band observed at 709 lays between that of the pure calcite (at 713 cm 1) and the duplicate of aragonite which appear at 713/700 cm 1 [12]. In this sample, the shoulder detected at 1030 cm 1 in the previous spectrogram of Fig. 2b is transformed to a strong broad band at 1027 cm 1 lying beside the sulfate frequencies of sulfate 1129 cm 1 and indicating that a significant reaction occurred in the sample. The thaumasite salt formed upon increasing the pH value of the carbonated silicate-ettringite mix to pH 12.5 and storing the alkaline mix for another 2 months. All the characteristic d-

3

Fig. 3b The infrared spectra of the carbonated ettringite-silicate mix at pH 9.5.

Fig. 4a The X-ray diffraction patterns of the thaumasite formed from the addition of lime to the carbonated ettringite-silicate mix at pH 12.5.

value lines of thaumasite are identified in the X-ray diffractogram of the salt obtained as shown in Fig. 4a. Significant amount of calcite (Cc) is found in the sample beside thaumasite. The infrared spectra of the thaumasite formed are in accordance with those reported in the literature [2,13] (Fig. 4b). The functional groups detected are the CO bands of the calcite phase at 1419, 879, and 710 cm 1, the SO-sulfate bands at 1104 and 638 cm 1, and the two bands characteristic of the octahedral coordinated silicon at 750 and 500 cm 1. Discussion

Fig. 3a The X-ray diffraction patterns of the carbonated ettringite-silicate mix at pH 9.5.

The different mechanisms for the thaumasite formation reported in the literature can be summarized as follows: The thaumasite salt may form from a direct reaction of sulfate with carbonate, silicate, and excess water in the presence of calcium ions at low temperature (0–5 °C) [8]. Its formation through the solid solution series between ettringite and thaumasite, the

Please cite this article in press as: H.Y. Ghorab et al., On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC Journal (2017), http://dx.doi.org/10.1016/j.hbrcj.2017.04.002

4

Fig. 4b The infrared spectra of the thaumasite formed from the addition of lime to the carbonated ettringite-silicate mix at pH 12.5.

woodfordite route, was not proved because of the gap found between the two minerals which makes the series discontinuous [14,15]. In other words, the thaumasite does not form through a topochemical replacement of ettringite. Other researchers found, that the thaumasite formation does not necessarily depend on ettringite but takes place in areas of pure calcium silicate pastes not previously occupied by ettringite [16]. However, the ettringite salt catalyzes the thaumasite formation [17] and was suggested to be a precursor for thaumasite. The beneficial action of alumina was the use of ettringite as a template for the nucleation of thaumasite, which once nucleated, additional thaumasite continues to form from solution. Evidence has shown that any alumina present will be incorporated into the thaumasite structure [18,19] but the absence of alumina does not prevent its formation [20]. The thaumasite formation is, however, strongly dependent on lime and extra lime is always needed to form either ettringite or thaumasite. The carbonation process reduces the pH value and favors the formation of thaumasite at a suitable pH range of 13–10.5 [10]. A transition intermediate state must exist to permit an octahedral arrangement of OH ions around the highly polarizing Si [8]. This can arise at low temperatures where atomic and molecular vibrations are relatively slow. The [Si(OH)6]2 groups formed are distorted because the carbonate ions delocalize the high charge of the strongly polarizing Si ion. None of these mechanisms approached the details of the reactions occurring during the carbonation process of the calcium silicate hydrate system which happens easier at low temperatures because of the higher dissolution of the CO2 gas. At pH value 11, the silicate phase in the studied mix is not present in the form of calcium silicate because of the absence of the respective IR spectra at 980 cm 1. It is a slightly carbonated silicate mix existing beside the ettringite and showing an IR shoulder at 1030 cm 1. This band magnifies through bubbling with CO2 and then converts to thaumasite with the supply of lime. The dissolution of CO2 in water of pH 7 leads to the formation of carbonic acid, in the presence of lime (pH 11–12), and bicarbonate ions form which is the case of the present study.

H.Y. Ghorab et al. The bicarbonate ions formed from bubbling CO2 gas to the mix decomposed the ettringite phase and consequently a carbonated silicate hydrate phase incorporating the relicts of the carbonated ettringite is formed. This reaction occurred at pH 9.5 and the ettringite phase being unstable at pH less than 10, decomposes to calcium carbonate, aluminum hydroxide and gypsum. One concludes that the intermediate phase needed to form the thaumasite as suggested in the literature [8] is the amorphous carbonated silicate phase incorporating the relicts of the decomposed ettringite. The formation of aragonite beside calcite in the carbonated phase indicates that the bubbling process has created a localized pressure suitable to form the high-pressure calcium carbonate polymorph. Under these conditions of highly localized pressure, the octahedral silicon might have formed with the residual OH ions in the mix. The supply of lime offers a strong nucleophilic agent to silicon already polarized by the carbonate ions and is further polarized by the OH ions. This results in bonding the lime with the octahedrally polarized silicon and the formation of thaumasite. The identification of an octahedral silicon in the amorphous phase by NMR shall support this theory. In this work, the thaumasite formed despite the low concentration of sulfate which is the only source in ettringite. This would rise the question of whether a destructive and a nondestructive thaumasite exist depending on the concentration of sulfate in a way analogous to the previous finding of sulfate deficient ettringite which becomes harmful with the excess of the sulfate supply [21,22]. Conclusions  The mechanism of thaumasite formation is studied in a mix composed of sodium silicate and ettringite with pH 11 at 7 °C. The mix was subjected to CO2 bubbling; the pH decreased to 9.5 and then was raised again to 12.5 by adding lime water.  At pH 9.5, the ettringite decomposed and an amorphous phase of a carbonated hydrated silicate phase incorporating the ettringite relicts is formed. The thaumasite phase is appeared by adding limewater to the carbonated mix.  The presence of the high-pressure calcium carbonate polymorph, aragonite, in the amorphous phase, indicates a localized pressure created in the mix. This is most probably due to the CO2 bubbling at 7 °C. Under these conditions the octahedral silicon might have been formed and was bound to the calcium ions with the rise of pH to 12.5.

References [1] Egyptian Code of Practice for Design of Concrete Structures ECP 203:2007, Housing & Building National Research Center, 2009. [2] Y.A. Osman, Studies on the Chemical and Thermal Stability of the Thaumasite Salt and its Expansion Behavior Ph.D. Thesis, Helwan University, Cairo Egypt, 2015. [3] K.I. Ismail, Investigations on the properties and performance of limestone Portland cement Master Thesis to be submitted in 2015; Helwan University, Cairo Egypt, 2015 [4] H.Y. Ghorab, M.R. Mabrouk, S.F. Abd Elnaby, K.M. Yousri, H.H. Ahmed, D. Herfort, K.I. Ismail, Y.A. Osman, The

Please cite this article in press as: H.Y. Ghorab et al., On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC Journal (2017), http://dx.doi.org/10.1016/j.hbrcj.2017.04.002

On the durability of Portland limestone cement

[5]

[6]

[7]

[8] [9]

[10]

[11]

[12]

suitability of Portland limestone cement for use in construction applications in Egypt: International Conference. NonTraditional Cement & Concrete (2014), Brno, Czech Republic and Cement International, vol. 6, 2014, pp. 70–77. H.Y. Ghorab, M.R. Mabrouk, D. Herfort, Y.A. Osman, Infrared investigation on systems related to the thaumasite formation at room temperature and 7oC, Cement. Wapno. Beton. 4 (2014) 252–261. H.Y. Ghorab, M. Rizk, D. Herfort, Y.A. Osman, On the chemistry of thaumasite formation, in: 19. Internationale Baustofftagung. IBAUSIL Weimar, Germany, vol. 1, 2015, Journal Cement and its Applications (2017) (In press). M.K. Mohamed, Investigations on the Thaumasite Formation in Cement Systems, M.Sc. Thesis, Helwan University Cairo Egypt, 2017. J. Bensted, Thaumasite––direct, woodfordite and other possible formation routes, Cement Concr. Compos. 25 (2003) 873–877. S. Martinez-Ramirez, M.T. Blanco-Varela, J. Rapazote, Thaumasite formation in sugary solutions: effect of temperature and sucrose concentration, Constr. Build. Mater. 25 (2011) 21–29. M.E. Gaze, N.J. Crammond, The formation of thaumasite in a cement lime sand mortar exposed to cold magnesium and potassium sulfate solutions, Cem. Concr. Compos. 22 (2000) 209–222. C. Yangi, B. Liui, X. Xiangi, J. Zhang, The influence of pH on the formation and stability of thaumasite, Appl. Mech. Mater. 174–177 (2012) 268–274. Flemning A. Andersen, Ljerka Brecevic, Infrared spectra of amorphous and crystalline calcium carbonate, Acta Chem. Scand. 45 (1991) 1018–1024.

5 [13] M. Rospondek, A. Lewandowska, Mineralogical Society of Poland-Special papers, vol. 20, 2002, pp. 187–190. [14] S.J. Barnett, C.D. Adam, A.R.W. Jackson, Solid solutions between ettringite, Ca6Al2(SO4)3(OH)12 26H2O, and thaumasite, Ca3SiSO4CO3(OH)6 12H2O, J. Mater. Sci. 35 (2000) 4109–4114. [15] S.J. Barnett, D.E. Macphee, N.J. Crammond, Solid solutions between thaumasite and ettringite and their role in sulfate attack, Concr. Sci. Eng. 3 (2001) 209–215. [16] N.J. Crammond, The thaumasite form of sulfate attack in the UK, Cem. Concr. Compos. 25 (2003) 809–818. [17] R. Lachaud, Thaumasite and ettringite in building materials, Ann ITBTP, vol. 3, 1979, pp. 370 [18] M.A. Halliwell, N.J. Crammond, Deterioration of brick work retaining walls as a result of thaumasite formation, in: Durability of Building Materials and Components 7, Sweden, vol. 1, 1996, pp. 235–244. [19] N.J. Crammond, M.A. Halliwell, The thaumasite form of sulfate attack in concretes containing a source of carbonate ions, in: 2nd Symposium Advances in Concrete Technology, AC, vol. 154-19, 1995, pp. 357–380. [20] F. Bellmann, J. Stark, The role of calcium hydroxide in the formation of thaumasite, Cem. Concr. Res. 389 (2008) 1154– 1161. [21] H.Y. Ghorab, A.M. Zayed, A.S. Mohamed, M.R. Mabrouk, The Formation of the Ettringite from Sulfate Solutions, in: 12th International Congress on the Chemistry of Cement (ICCC), Montreal, Canada, 2007, pp. W4-09.3. [22] A.S. Mohamed, Studies on the Expansion Behavior of the Ettringite in pure Systems and its Application in Cement Mortars and Concrete, M.Sc. Thesis, 2006.

Please cite this article in press as: H.Y. Ghorab et al., On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC Journal (2017), http://dx.doi.org/10.1016/j.hbrcj.2017.04.002