THE EFFECT OF SULFATE CONCENTRATION ON THE TIME OF THAUMASITE FORMATION

THE EFFECT OF SULFATE CONCENTRATION ON THE TIME OF THAUMASITE FORMATION

M. K. Mohamed

Faculty of Science, Helwan University, Cairo, Egypt

Kamalm925@gmail.com, mohaed.kamal@science.edu.eg

Key Words:Thaumasite; clinker; limestone; magnesium sulfate;PH

ABSTRACT

The formation of thaumasite from limestone cement in different sulfate concentration was investigated. The limestone cement consists of 70% clinker with very low sulfate content and 30% pure calcium carbonate. The pastes were cured for 4months in different concentrations of magnesium sulfate solution 1, 2, 4, and 6%.

After specific times the reactions were terminated and the surfaces of samples scratched and analyzed by infra-red spectroscopy, x-ray diffraction and scanning electron microscopic analysis. Also the PH and sulfate concentration are measured with time. Analysis indicates that thaumasite can be obtained with low sulfate content at PH 10-10.5 and the rate of its formation increased with the increasing of sulfate concentration in solution where is formed after 60 days from curing in 4%.

1. INTRODUCTION

Formation of thaumasite in concrete destroys the main cementitious binder phases in concrete (e.g. calcium silicate hydrates(C–S–H) [1]. Collepardi summarized the three main types of chemical sulfate attack on cement, the first sulfate attack occurs in presence of magnesium sulfate MgSO4 on calcium hydroxide(C-H) and C-S-H to form gypsum, brucite and silica gel, while the second attack on calcium aluminate hydrate(C-A-H) and monosulfate occurs to form ettringite, thethird sulfate attack on calcium silicate hydrate and calcium hydroxide in the presence of carbonate ions to form thaumasite [2]. The formation of thaumasite requires presence of calcium, silicate, sulfate, and carbonate ions under low temperature and high humidity [3]. Also ettringite salt can be used as a template for initial nucleation of thaumasite because they both have similar structure [4]. Thaumasite salt was detected at different PH values where it was observed in presence and absence of calcium hydroxide. However, its formation is enhanced in alkaline conditions (pH ≥ 12.5) while at lower pH values (pH ≤ 8.0) the amount of thaumasite decreased and amount of gypsum increased [5-7]. On the other hand thaumasite was observed and remained stable at pH 6–8 [8]. Sulfate ions are an essential chemical component of Thaumasite formation that can originate from internal constituents or external environments. Also, they might be generated from the oxidation of sulfides, for instance pyrite (FeS₂), and the atmosphere (gaseous SO2) [9]. Common sulfate salts are sodium, potassium, magnesium, and calcium sulfates. Among them magnesium sulfate has more deleterious effects on the hydrated cement paste. In an experimental study, higher degree of deterioration was detected in Portland limestone cement pastes immersed in magnesium sulfate solution compared to corresponding specimens immersed in sodium sulfate [10]. The time of thaumasite formation is not defined depending on the conditions of its  formation where is formed quickly in mortar than in paste, possibly because the transition zone at the paste aggregate interface introduced more porosity, or the silica sand contributed some extra silicate to the system. Thaumasite was formed as a material deposited in a cement mortar containing 35% calcium carbonate filler after about 7 months s [11],and formed also after 14 months from the carbonation of sodium silicate- ettringite mixture [7] .The amount of sulfate required to form thaumasite is not clear where Oberholster reported that thaumasite formation was enabled at low sulfate content (0.05 molar)[12]. On the other handJuel found that thaumasite only forms in cements systems where enough sulfate (2.1-3 molar)has been added to transform all available aluminum into ettringite [13].In this paper, the required sulfate concentration to form thaumasite and the effect of sulfate ions concentration on the rate of its formation, also pHof thaumasite are investigated.

2. EXPERIMENTAL

Cubes from clinker(FLSMDITH CEMENT COMPANY) and calcium carbonate (PHARMACEUTICAL-Chemical-CO.) are casted with dimension 1*1*1 inch. The composition of each cube is 70% clincker with very low sulfate content and 30%calcium carbonate. The surface area of clinker was 2850 cm2/g Blaine and water/cement ratio of 0.38. After 24 hours all cubes are demoulded and cured at 7˚c in 1, 2, 4, and 6% Magnesium sulfate solution (MgSO4.7 H2O) for four months. Periodically the cubes are removed from the solution and the surface of each cube was scratched and washed by mixture from acetone and isopropyl alcohol to stop the curing reaction. After washing the powders were filtered off and dried at 50˚c for 4 hours. The powders were investigated by infra-red spectroscopy, x-ray diffraction, and scanning electron microscopic analysis. Also, the change in PH and sulfate content is monitored during the curing time. The chemical and phase composition of the clinker is shown in table 1 [14].

Table 1.Thechemical and phase composition of clinker (XRF Analysis).

3. RESULT

3.1 PH and sulfate determination.

Figure 1a shows the change of PH of different samples with time where the samples soaked in 0, 1, 2, 4, and 6% magnesium sulfate (MS) solution. In 0% MS, the PH was11.5 after one day and become stable at 12.5 for 4 months due to the elaboration of calcium silicate hydrate and calcium hydroxide. But in 1, 2,4, and 6% MS after one day of curing, the pH was in range (10-10.5) because the PH  of magnesium sulfate solution is 8  and with time after one month, the PH decreased to 9.2 for 2, 4, and 6% and to 10.2 for 1%  due to the formation of magnesium hydroxide (brucite) Mg (OH) 2 . After two months, the PH of the solution increased from 9.2 to 10 with the detection of thaumasite (T) salt in 4 and 6% only. In the fourth month, the PH of 1, 2, 4, and 6% increased to 10.5, and thaumasite is also detected in 1and 2%.

Figure 1b illustrates the change in the concentration of sulfate of paste surface with time in the different percent of magnesium sulfate. Its show that the concentration of sulfate increase with time, where the thaumasite salt is detected after two months of curing in  4 and 6% MS solution with 0.315 and 0.47 mole per liter sulfate concentration and after four months of curing in 1 and 2% MS with 0.092 and 0.182 mole per liter sulfate concentrations.

3.2 FTIR analysis

The powders were investigated by means of FTIR-6300 Type A by mixing and pressing with KBr, then exposed to radiation and recording spectrum the analysis was performed within wave number range of 300 to 2000 cm^-1.

Figure 2a illustrates the infrared spectra of paste surfacesoaked in 1% magnesium sulfate for 4 months at 7⸰C . Analysis indicates that the bending vibrations of water appear at 1625 cm^-1within the four months, After four months the stretching vibrations of C-O is shifted from 1420 cm^-1to 1400cm^-1due to the formation of thaumasite while the bending vibrations of C-O bond not changed where is observed at all months at 875cm^-1 which overlap with AL-OH of ettringite salt. In the fourth month the stretching vibration of S-O is observed at 1110cm^-1rather than 1120cm^-1but the bending vibrations at 680 and 600 not changed. In the second month, a weak Si-O band of calcium silicate hydrates is observed at 980cm^-1and a weak band of C-O band of carbonate ions observed at 715 cm^-1. After four months from curing a new band is detected at 500 cm^-1which belongs to octahedron silicon of thaumasite salt. The bands at 429cm^-1and 450cm^-1are attributed to the tetrahedron silicon. Figure 2b shows infrared spectra of pastesoaked in 2% magnesium sulfate for 4 months which there is no critical difference between it and that in 1%, where the stretching vibration of C-O is shifted from 1430 cm^-1to 1400cm^-1 and the octahedron silicon of thaumasite salt also observed at 500cm^-1 after four months.

Figure 3a demonstrates the spectrogram of paste surface cured in 4 % magnesium sulfate at different times at 7⸰C .It demonstrate that the bending vibrations of water molecules is observed at 1625 cm-1 in all times, but the stretching vibration mode of C-O band is perceived at 1400 cm-1 after the third month instead of 1430 cm-1. The stretching and bending vibrations of S-O band of sulfate ions are observed at 1120 cm-1 and (670,700) cm-1 respectively. The bending vibration of C-O which usually appears at 810 cm-1 overlaps with the aluminum spectra of ettringite and appears at 875 cm-1. After the first month a weak Si-O band of calcium silicate hydrates is observed at 990 cm-1 and in the third month disappeared due to the transforming of calcium silicate hydrates into thaumasite salt. After two months of curing new bands are detected at 750 and 500 cm-1 which attributed to octahedron silicon of thaumasite. Figure 3b shows the spectrogram of surface of paste cured in 6 % magnesium sulfate at different times at 7⸰C which is not has vital difference from that of 4%, where it proves that the octahedron silicon is observed after 2 month of curing the paste at 750 and 500 cm^-1.

3.2XRD analysis

Figure 4a shows the XRD pattern of paste surfacecured in 1 % magnesium sulfate at different times. Pattern indicate that after one month and for three months lines of ettringite(9.66, 5.6, 3.87, 2.78, 2.50, 1.80, 1.60 A), calcite (3.04, 2.29, 2.09, 1.91 A), gypsum (7.70, 4.30, 3.08, 2.89, 2.69 A) and brucite (4.82, 2.36, 1.80, 1.57 A) are observed. In the fourth month the lines of thaumasite are observed at 9.52, 5.48, 4.82, 3.63, 3.65, 2.72, 2.48, 1.83 and 1.60 A   beside to lines of gypsum, calcite and brucite. Figure 4b represents the paste surface that cured in 2% magnesium sulfate, where also the ettringite, calcite, gypsum and brucite were formed for three months and thaumasite is formed after the fourth month with strong lines of gypsum.

(E=Ettringite, Cc=calcite,B=brucite, G=gypsum,T=thaumasite)

The ettringite salt are observed at d values 9.2, 5.65, 4.4, 3.32, 1.86, 1.69 A with brucite at 4.8, 2.75, 2.34, 1.79, 1.50 A, gypsum at d values 7.4, 4.28, 3.64, 2.46, 2.39A and calcite at 3.90, 3.00, 2.27, 2.08, 1.99, 1.94 A from curing the paste in 4, 6% magnesium sulfate for one month only. In the second month the thaumasite salt is pronounced at d- spacing lines 9.51, 5.56, 4.49, 4.54, 3.81, 2.48, 2.36, and 1.65 A in 4% solution and also appeared at 9.48, 5.46, 4.54, 3.36, 2.46, 2.39, 1.65 A in 6% figure (5a, 5b).

3.3SEM analysis

 Picture (1) illustrates the scanning electron micro graph of paste surface cured in 4% MS solution for 2 month at 7⸰C with EDX analysis.The SEM micrograph illustrates the morphology of thaumasite salt in needle shape and EDX analysis emphasis that the needlethat formed on paste surface is thaumasite salt where itconsist of calcium, silicon, oxygen, sulfur, and carbon.

Picture (1): The scanning electron micrograph and EDX of paste surfacecured in% MS solution for 2 month at 7⸰C.

4. DISCUSSION

This resultdiscusses PH of thaumasite formation and the effect of sulfate concentration on the rate of thaumasite formation. Thaumasite salt was detected at different PH values where it was observed in presence alkaline conditions (pH ≥ 12.5). However, its formation is enhanced in while at lower pH values (pH ≤ 8.0) and remained stable at pH 6–8 [5-8]. In this work the PH firstly decreased after one day from 12 to (10.2-10.5) due to the immersion in magnesium sulfate solution , then after one month decreased to 9.5 resulting from precipitation of brucite on the surface of mortar. Secondly after two months PH increased from 9.5 to 10 with formation of thaumasite in 4 and 6% only. Thirdly the PH increased from 10 to 10.5 with detection of thaumasite in 1, 2, 4 and 6% figure 1a.Juel et al in 2003 found that thaumasite only formed in cements systems that have enough sulfate must be added to transform all available aluminum into ettringite, where Thaumasite to be observed to must the sulfate contents (2.1- 3 mole per liter)[13].. In figure 1b the thaumasite was obtained with very low sulfate content, where was formed with concentration from 0.09 -0.47 molar.Hartshorn [11] reported that thaumasite formed more quickly in mortar as a material deposited in a cement mortar containing 35% calcium carbonate filler after about 224 days. Analysis indicates that thaumasite can be formed in mortar containing 30% calcium carbonate after 60 days. X-ray diffraction analysis illustrates that phases that formed in the first stage are ettringite, brucite and gypsum and with time the ettringite phase is depleted and the thaumasite phase is pronounced and this confirm thatettringite salt can be used as a template for initial nucleation of thaumasite because they both have similar structure[4].

5. CONCLUSION

• Thaumasite can be formed at different PH where it formed at PH 10-10.5.
• Thaumasite can be formed with very low sulfate concentration (0.09-0.47 molar).
• Thaumasite can be detected faster with increasing the concentration of sulfate.
• Thaumasite is formed in limestone cement aftercuring for 2 months in 4 and 6% magnesium sulfate solution
• Thaumasite is formed in limestone cement after curing for 4 months in 1 and 2% magnesium sulfate solution.
• Ettringite salt can be used as a template for initial nucleation of thaumasite.

REFERENCES

[1]Taylor, H.F.W. (1997):Cement Chemistry, Thomas Telford, London.

[2] Collepardi, M. (2003):A state-of-the-art review on delayed ettringite attack on concrete,Cement and Concrete Composites., 25: 401–407.

[3] Collett, G. ;N.J. Crammond ;R.N. Swamy and J.H. Sharp(2004): The role of carbon dioxide in theformation of thaumasite,Cement Concrete Research.,34:1599–612.

[4] Kohler,S. ;D. Heinz andL. Urbonas (2006): Effect of ettringite on thaumasite formation, Cement Concrete Research.,36:697–706.

[5] Hobbs, D.W. and M.G. Taylor(2000): Nature of the thaumasite sulfate attackmechanism in field concrete.Cement Concrete Research.,30 (4):529- 533

[6] Zhou,Q. ; J. Hill ; E.A. Byars ; J.C. Cripps ; C.J. Lynsdale and J.H. Sharp(2006): The roleof pH in thaumasite sulfate attack, Cement Concrete Research., 36(1): 160– 170.

[7] Ghorab, H.Y. ; Fouad S. Zahran ; A. Mohamed Kamal andAmr Said Meawad(2018): On the durability of Portland limestone cement: Effect of pH on the thaumasite formation, HBRC journal 14, no. 3, 2018, 340-344.

[8]Gaze,M.E. and N.J. Crammond(2000): The formation of thaumasite in a cement:lime:sand mortar exposed to cold magnesium and potassium sulfatesolutions, Cement and Concrete Composites. 22,3, 2000, 209– 222.

[9] Blanco-Varela,M.T. ;J. Aguilera ;S. Martı ´ nez-Ramı´rez ;F. Puertas ;A. Palomo andC. Sabbioni(2003):Thaumasite formation due to atmospheric SO2–hydraulic mortar interaction,Cement and Concrete Composites., 25(8):983–90.

[10] Hartshorn,S.A. ;J.H. Sharp andR.N. Swamy (1999):Thaumasite formation in Portland limestone cement pastes. CemConcr Res.,29(8):1331–40.

[11]Hartshorn,S.A.(1998): Sulfate attack of Portland limestone cements, in PhD in Mechanical Engineering, The University of Sheffield: Sheffield, 1998, p. 214.

[12] Oberholster,R.E. (2002): Deterioration of mortar, plaster and concrete: South Africa laboratory and field studies. First International Conference on Thaumasite in Cementitious Materials, Garston UK,

[13]  Juel, I. ; D. Herfort ;R.Gollop ;J. Konnerup-Madsen;H.J.Jakobsen and J. Skibsted(2003):A thermodynamic model for predicting the stability of thaumasite. Cement Concrete Composites, 25(8): 867-872.

[14] Mohamed,M.K. Studies on some important salts formed in Portland cement: Thaumasite and ettringite-similar phases, Ph.D. Thesis. Helwan University, Cairo Egypt.Ongoing.

تأثیر ترکیز الکبریتات على وقت تکوین التاومازیت

محمد کمال محمد محمد

مدرس مساعد بقسم الکیمیاء -  کلیه العلوم -  جامعه حلوان

Kamalm925@gmail.com, mohaed.kamal@science.edu.egh

تم دراسه تکوین ملح التاومازیت من معالجه اسمنت الحجر الجیرى فى ترکیزات مختلفه من الکبریتات  حیث یتکون أسمنت الحجر الجیری من 70% کلنکر بمحتوى کبریتات منخفض جدًا و 30٪ کربونات کالسیوم نقی. تمت معالجة مکعبات الاسمنت  لمدة 4 أشهر بترکیزات مختلفة من محلول کبریتات المغنیسیوم 1 و 2 و 4 و 6٪.

بعد أوقات محددة تم وقف التفاعلات وخدش أسطح العینات وتحلیلها بواسطة التحلیل الطیفی بالأشعة تحت الحمراء ، حیود الأشعة السینیة والتحلیل المجهری الإلکترونی. کما تم قیاس الاس الهیدروجینى و ترکیز الکبریتات لکل العینات مع الوقت. أشارت النتائج على انه یمکن تکوین ملح التاومازیت بترکیز قلیل من الکبریتات عند اس هیدروجینى 10.5-10  ویزداد معدل تکوینه مع زیادة ترکیز الکبریتات فی المحلول حیث تم تکوینه بعد 60 یومًا من المعالجة فی 4 ٪.