Thermodynamic Stability of Sulfate Ions on Calcium Aluminosilicate Hydrate Microstructure

Abstract

The thermodynamic stability of sulfate ions on synthesized calcium aluminosilicate hydrate (C-A-S-H) microstructure with different Ca/Si ratios and Al/Si ratios was investigated by XRD, SEM-EDS, ²⁹Si and ²⁷Al nuclear magnetic resonance (NMR) and thermodynamic modeling. The results indicate that sulfate attack leads to both decalcification and dealumination for C-A-S-H gels, and the amount of corrosion products (gypsum and ettringite) decreased gradually with decreasing Ca/Si ratios of C-A-S-H. Sulfate ions can also promote the polymerization degree of C-A-S-H gels, improving its resistance to sulfate attack. Moreover, the 4-coordination aluminum (Al[4]) in C-A-S-H, 5-coordination aluminum (Al[5]), 6-ccordination aluminum (Al[6]) in TAH (third aluminum hydrate) and Al[6] in monosulfate or C-A-H (calcium aluminate hydrate) can be transformed into Al[6] in ettringite by sulfate attack. Furthermore, through thermodynamic calculation, the decrease of Ca/Si ratios and increase of Al/Si ratios can improve the thermodynamic stability of C-A-S-H gels under sulfate attack, which agrees well with the experiment results.

Keywords

sulfate attack / calcium aluminosilicate hydrate / thermodynamic stability / microstructure

References

[1] Taylor H F W. Cement Chemistry, 1997 2nd ed London: Thomas Telford.

[2] Irassar E F, Bonavetti V L, Gonzalez M. Microstructural Study of Sulfate Attack on Ordinary and Limestone Portland Cements at Ambient Temperature[J]. Cem. Concr. Res., 2003, 33(1): 31-41.

[3] Kalousek G L. Crystal Chemistry of Hydrous Calcium Silicates: I. Substitution of Aluminum in Lattice of Tobermorite[J]. J. Am. Ceram. Soc., 1957, 40(3): 74-80.

[4] Richardson I G, Brough A R, Brydson R, et al. Location of Aluminum in Substituted Calcium Silicate Hydrate (C-S-H) Gels as Determined by ²⁹Si and ²⁷Al NMR and EELS[J]. J. Am. Ceram. Soc., 1993, 76(9): 2 285-2 288.

[5] Richardson I G. The Nature of CSH in Hardened Cements[J]. Cem. Concr. Res., 1999, 29(8): 1 131-1 147.

[6] Richardson I G. Tobermorite/Jennite and Tobermorite/Calcium Hydroxide Based Models for the Structure of CSH: Applicability to Hardened Pastes of Tricalcium Silicate, ß-dicalcium Silicate, Portland Cement, and Blends of Portland Cement with Blast-furnace Slag, Metakaolin, or Silica Fume[J]. Cem. Concr. Res., 2004, 34(9): 1 733-1 777.

[7] El-Hachem R, RozièRe E, Grondin F, et al. New Procedure to Investigate External Sulphate Attack on Cementitious Materials[J]. Cem. Concr Compo., 2012, 34(3): 357-364.

[8] Lothenbach B, Bary B, Bescop P L, et al. Sulfate Ingress in Portland Cement[J]. Cem. Concr Res., 2010, 40(8): 1 211-1 225.

[9] Kunther W, Lothenbach B, Skibsted J. Influence of the Ca/Si Ratio of the C-S-H Phase on the Interaction with Sulfate Ions and Its Impact on the Ettringite Crystallization Pressure[J]. Cem. Concr Res., 2015, 69(1): 37-49.

[10] Lippmaa E, Maegi M, Samoson A, et al. Structural Studies of Silicates by Solid-state High-resolution ²⁹Si NMR[J]. J. Am. Chem. Soc., 1980, 102: 4 889-4 893.

[11] Justnes H, Meland I, Bjoergum J O, et al. NMR-a Powerful Tool in Cement and Concrete Research[J]. Adv. Cem. Res., 1990, 3(11): 105-110.

[12] Sun G K, Young J F, Kirkpatrick R J. The Role of Al in C-S-H: NMR, XRD, and Compositional Results for Precipitated Samples[J]. Cem. Concr Res., 2006, 36(1): 18-29.

[13] Andersen M D, Jakobsen H J, Skibsted J. Incorporation of Aluminum in the Calcium Silicate Hydrate (C-S-H) of Hydrated Portland Cements: a High-field ²⁷Al and ²⁹Si MAS NMR Investigation[J]. Inorg. Chem., 2003, 42(7): 2 280-2 287.

[14] Grimmer A R. Colombet P, Grimmer A-R. Structural Investigation of Calcium Silicates from ²⁹Si Chemical Shift Measurements[M]. Application of NMR Spectroscopy to Cement Science, 1994 London: Gordon and Breach.

[15] Brough A R, Dobson C M, Richardson I G I S, et al. Solid-state NMR Studies of Ca₃SiO₅: Hydration at Room Temperature Using ²⁹Si Enrichment[J]. J. Mater Sci., 1994, 29(15): 3 926-3 940.

[16] Hu C G, Hu S G, Ding Q J, et al. Effect of Curing Regime on Degree of Al³⁺ Substituting for Si⁴⁺ in C-S-H Gels of Hardened Portland Cement Pastes[J]. Journal of Wuhan University of Technology-Mater. Sci. Ed., 2014, 29(3): 546-552.

[17] Faucon P, Delagrave A, Petit J C, et al. Aluminum Incorporation in Calcium Silicate Hydrates (C-S-H) Depending on Their Ca/Si Ratio[J]. J. Phys. Chem., 1999, B103: 7 796-7 802.

[18] Renaudin G, Russias J, Leroux F, et al. Structural Characterization of C-S-H and C-A-S-H Samples-Part II: Local Environment Investigated by Spectroscopic Analyses[J]. J. Solid State Chem., 2009, 182(12): 3 320-3 329.

[19] Andersen M D, Jakobsen H J, Skibsted. A New Aluminium-hydrate Species in Hydrated Portland Cements Characterized by ²⁷Al and ²⁹Si MAS NMR Spectroscopy[J]. Cem. Concr Res., 2006, 36(1): 3-17.

[20] Thoenen T, Kulik D. Nagra/PSI Chemical Thermodynamic Database 01/01 for the GEMS-Selektor (V.2-PSI) Geochemical Modeling Code, PSI, Villigen[EB/OL]. http://les.web.psi.ch/software/GEMS-PSI/doc/pdf/TM-44-03-04-web.pdf, 2003