1. INTRODUCTION
⌅The alkali-silica reaction (ASR) is a major cause of concrete deterioration. During ASR, alkali-reactive siliceous aggregates react with the alkalis in the pore solution of the concrete to form an expansive alkali-silica reaction gel (ASR gel). Due to its hygroscopic behaviour, the ASR gel absorbs water and swells resulting in pressures often leading to a typical crack-network-pattern with a severe impact on concrete durability.
The chemical composition of ASR gels in
concretes mentioned in literature varies between molar (K+Na)/Si ratios
of 0.1 to 0.7 and Ca/Si ratios of 0.2 to 0.6 (1-31. Leemann, A. (2017) Raman microscopy of alkali-silica reaction (ASR) products formed in concrete. Cem. Concr. Res. 102, 41-47. https://doi.org/10.1016/j.cemconres.2017.08.014.
2.
Leemann, A.; Shi, Z.; Lindgård, J. (2020) Characterization of amorphous
and crystalline ASR products formed in concrete aggregates. Cem. Concr. Res. 137, 106190. https://doi.org/10.1016/j.cemconres.2020.106190.
3.
Dähn, R.; Arakcheeva, A.; Schaub, P.; Pattison, P.; Chapuis, G.;
Grolimund, D.; Wieland, E.; Leemann, A. (2016) Application of micro
X-ray diffraction to investigate the reaction products formed by the
alkali-silica reaction in concrete structures. Cem. Concr. Res. 79, 49-56. https://doi.org/10.1016/j.cemconres.2015.07.012.
).
The variation in the chemical composition of the ASR gel makes it
difficult to assign a uniform structural description to the gels. In the
literature, ASR products are distinguished by two main types, i.e.
amorphous and crystalline ASR products (11. Leemann, A. (2017) Raman microscopy of alkali-silica reaction (ASR) products formed in concrete. Cem. Concr. Res. 102, 41-47. https://doi.org/10.1016/j.cemconres.2017.08.014.
, 44.
Geng, G.; Shi, Z.; Leemann, A.; Borca, C.; Huthwelker, T.; Glazyrin,
K.; Pekov, I.V.; Churakov, S.; Lothenbach, B.; Dähn, R.; Wieland, E.
(2020) Atomistic structure of alkali-silica reaction products refined
from X-ray diffraction and micro X-ray absorption data. Cem. Concr. Res. 129, 105958. https://doi.org/10.1016/j.cemconres.2019.105958.
, 55.
Shi, Z.; Geng, G.; Leemann, A.; Lothenbach, B. (2019) Synthesis,
characterization, and water uptake property of alkali-silica reaction
products. Cem. Concr. Res. 121, 58-71. https://doi.org/10.1016/j.cemconres.2019.04.009.
).
The crystalline ASR products show similarities to the mineral shlykovite [KCa[Si4O9(OH)]·3H2O]. The structure of the crystalline ASR products consists of silicate sheets and is dominated by Q³ species (55.
Shi, Z.; Geng, G.; Leemann, A.; Lothenbach, B. (2019) Synthesis,
characterization, and water uptake property of alkali-silica reaction
products. Cem. Concr. Res. 121, 58-71. https://doi.org/10.1016/j.cemconres.2019.04.009.
, 66.
Shi, Z.; Leemann, A.; Rentsch, D.; Lothenbach, B. (2020) Synthesis of
alkali-silica reaction product structurally identical to that formed in
field concrete. Mater. Des. 190, 108562. https://doi.org/10.1016/j.matdes.2020.108562.
).
However, it is thought that crystalline ASR products, as opposed to the
amorphous products, do not lead to damage in concrete (77.
Leemann, A.; Shi, Z.; Wyrzykowski, M.; Winnefeld, F. (2020) Moisture
stability of crystalline alkali-silica reaction products formed in
concrete exposed to natural environment. Mater. Des. [195], 109066. https://doi.org/10.1016/j.matdes.2020.109066.
). Thus this study focuses on the amorphous ASR products.
The nanostructure of amorphous ASR gels can be considered as irregular cross-linked SiO44- tetrahedra. Hou (88. Hou, X.; Kirkpatrick, R.J.; Struble, L.J.; Monteiro, P.J.M. (2005) Structural investigations of alkali silicate gels. J. Am. Ceram. Soc. 88 [4], 943-949. https://doi.org/10.1111/j.1551-2916.2005.00145.x.
) described the structure of synthetic ASR gels as low-order Q3-dominated layers. The surface of the ASR gel is in thermodynamic equilibrium with dissolved cations (K+, Na+ and Ca2+) which are chemically bonded as surface groups (e.g. ≡Si-ONa). According to (99.
Hou, X.; Struble, L.J.; Kirkpatrick, R.J. (2004) Kanemite as a model
for asr gel. 12. International Conference on Alkali-Aggregate Reaction
in Concrete. Beijing, China, 115-123.
, 1010.
Kirkpatrick, R.J.; Kalinichev, A.G.; Hou, X.; Struble, L. (2005)
Experimental and molecular dynamics modeling studies of interlayer
swelling: water incorporation in kanemite and ASR gel. Mater. Struct. 38, 449-458. https://doi.org/10.1007/BF02482141.
), the structure of the ASR gel is comparable to the structure of kanemite [NaSi2O5·3H2O].
However, the structure of the ASR gel is more complex due to its higher
water absorption capacity which lead to greater swelling pressures than
kanemite (1010.
Kirkpatrick, R.J.; Kalinichev, A.G.; Hou, X.; Struble, L. (2005)
Experimental and molecular dynamics modeling studies of interlayer
swelling: water incorporation in kanemite and ASR gel. Mater. Struct. 38, 449-458. https://doi.org/10.1007/BF02482141.
).
The swelling behaviour of the ASR gels is controlled by their calcium content. Mansfeld (1111.
Mansfeld, T. (2008) The swelling behaviour of alkali silicate gels
considering their composition. Bauhaus-Universität Weimar, Dissertation.
)
classified the swelling capacity of natural and synthetic ASR gels
according to calcium content. Only gels with a calcium oxide content
between 5 and 30 wt.% (e.g. Ca/Si molar ratio = 0.07 - 1.27) are able to
develop damaging swelling pressures. 29Si NMR spectroscopic studies on synthetic ASR products have shown that in the presence of calcium, resonances corresponding to Q1 and Q2 sites can occur, similar to those observed in C-S-H phases (22.
Leemann, A.; Shi, Z.; Lindgård, J. (2020) Characterization of amorphous
and crystalline ASR products formed in concrete aggregates. Cem. Concr. Res. 137, 106190. https://doi.org/10.1016/j.cemconres.2020.106190.
). According to Leemann et al. (22.
Leemann, A.; Shi, Z.; Lindgård, J. (2020) Characterization of amorphous
and crystalline ASR products formed in concrete aggregates. Cem. Concr. Res. 137, 106190. https://doi.org/10.1016/j.cemconres.2020.106190.
) the amount of Q² sites in the structure of synthetic ASR products increases with increasing Ca/Si molar ratio.
In
the case of concrete, a sufficient amount of ASR gel cannot be obtained
experimentally to characterize the gel with the usual laboratory
methods. However, Raman spectroscopy enables the fast investigation of
small sample quantities, without preparation and at a better cost
effectiveness. A number of authors have confirmed the suitability of
Raman spectroscopy for assigning Q-species to the structure of ASR gels (11. Leemann, A. (2017) Raman microscopy of alkali-silica reaction (ASR) products formed in concrete. Cem. Concr. Res. 102, 41-47. https://doi.org/10.1016/j.cemconres.2017.08.014.
, 1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 1313.
Ling, T.C.; Balachandran, C.; Muñoz, J.F.; Youtcheff, J. (2018)
Chemical evolution of alkali-silicate reaction (ASR) products: a Raman
spectroscopic investigation. Mater. Struct. 51, 23. https://link.springer.com/article/10.1617%2Fs11527-018-1151-x.
).
The
present study investigates the effect of different compositions on the
structure of synthetic ASR gels. Raman spectroscopic measurements were
performed on synthetic ASR gels with different compositions, i.e. Na/Si
and Ca/Si molar ratios. The Raman spectra were evaluated with the
PeakFit software and compared with data from the literature and in-house
databases specID and MSC-RD (1414.
Drozdovskiy, I.; Ligeza, G.; Jahoda, P; Franke, M.; Lennert, P.;
Vodnik, P.; Payler, S.J.; Kaliwoda, M.; Pozzobon, R.; Massironi, M.;
Turchi, L.; Bessone, L.; Sauro, F. (2020) The PANGAEA Mineralogical
Database. Data Br. 31, 105985. https://doi.org/10.1016/j.dib.2020.105985.
). The band assignments were validated by comparing the results with 29Si NMR spectroscopic measurements.
2. METHODS
⌅2.1. Synthesis of ASR gels
⌅The ASR gels were synthesized from the following starting materials: Ca(OH)2, colloidal SiO2 (40 % suspension in H2O), deionized water and NaOH or KOH pellets. The NaOH or KOH was dissolved in the deionized water and the colloidal SiO2 and the Ca(OH)2 were then added. Six blends were prepared at room temperature in high-density polyethylene bottles and stored for three weeks in an overhead rotating mixer at a mixing speed of twelve revolutions per minute. After mixing, a portion of the sediment was removed and dried in a vacuum desiccator for seven days. It was not possible to prevent a small degree of carbonation. The sample designations and compositions are listed in Table 1. The exact molar ratios of the synthetic and dried ASR gels were determined by ICP-OES.
| Sample name | Na/Si | K/Si | Ca/Si |
|---|---|---|---|
| Molar ratio | |||
| KS0.3 | - | 0.30 | |
| NS0.3 | 0.30 | - | - |
| NS0.6 | 0.60 | - | - |
| NS1.2 | 1.20 | - | - |
| NSC0.3_0.22 | 0.30 | - | 0.22 |
| NSC0.7_0.46 | 0.70 | 0.46 | |
2.2. Raman spectroscopy
⌅The
measurements were carried out with a Raman spectrometer Senterra I
(Bruker) at the Fraunhofer IBP, Valley, Germany. The Raman spectrometer
is equipped with an Olympus BX 51 confocal polarizing microscope. The
samples were measured using frequency doubled Nd: YAG laser with a
wavelength of 532 nm, a maximum laser power of 10 - 20 mW and 20x
objective lens (laser diameter 5 μm). The measuring process was carried
out with the software OPUS 7.5. The measurements were performed in a
spectral range of 50 - 4000 cm-1 with a spectral resolution of 2 cm-1.
At least five measurements were carried out at different positions on
each sample. During each measurement, a background correction and spike
suppression were applied. The measured spectra were processed with using
a baseline correction and a smoothing function. The phase assignments
of the Raman spectra were performed by comparison with recorded spectra
of the databases specID and MSC-RD (1414.
Drozdovskiy, I.; Ligeza, G.; Jahoda, P; Franke, M.; Lennert, P.;
Vodnik, P.; Payler, S.J.; Kaliwoda, M.; Pozzobon, R.; Massironi, M.;
Turchi, L.; Bessone, L.; Sauro, F. (2020) The PANGAEA Mineralogical
Database. Data Br. 31, 105985. https://doi.org/10.1016/j.dib.2020.105985.
). The Raman spectra were evaluated using the program PeakFit (version 4.06, Systat Software, Inc.). The R2 values of the fitted Raman data ranged between 0.99 - 0.999. The settings for the evaluation are listed in Table 2.
| Derivation | I. (first) | Peak type | Gauss+Lor Amp |
|---|---|---|---|
| Baseline correction | Hyperbolic, D2 | Add residues | ✔ |
| Tol.% | max. 3,0 % | Vary width | ✔ |
| Smoothing | Savitsky-Golay | Vary shape | ✔ |
2.3. 29Si NMR spectroscopy
⌅The 29Si NMR experiments were performed with a Bruker Avance 300 spectrometer (magnetic field strength 7.0455 T, resonance frequency for 29Si: 59.63 MHz) in MAS mode (magic angle spinning) using the single pulse technique (90° pulse). The samples were placed in 4 mm zirconia rotors and spun at 8 kHz. About 5000 scans were recorded for each spectrum with a repetition time of 45 s. The chemical shifts were set relative to the external standard tetramethylsilane. The signal patterns of the spectra were deconvoluted with Bruker WINNMR software.
3. RESULTS AND DISCUSSION
⌅The vibrational band assignments of Raman spectra are listed in Table 3. In general, the results of Raman and 29Si NMR spectroscopy show that the synthetic ASR gels have a disordered silicate framework with different degrees of connectivity of the silicate units (Qn).
| KS0.3 | NS0.3 | NS0.6 | NS1.2 | NSC0.3 _0.22 | NSC0.7_0.46 | Assignments | Ref. |
|---|---|---|---|---|---|---|---|
| 462 | 452 | 450 | 447 | 450 | 446 | ONBO-Si-ONBO bending | (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h. , 1616. Garbev, K.; Stemmermann, P.; Black, L.; Brenn, C.; Yarwood, J.; Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation in air-A Raman spectroscopic study. Part I: fresh phases. J. Am. Ceram. Soc. 90, 900-907. https://doi.org/10.1111/j.1551-2916.2006.01428.x. ) |
| 533 | 530 | 548 | 551 | 584 | 601* | SB Si-O-Si of Q3 units | (1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644. ) |
| 597 | 596 | 600 | 600 | 603* | 630 | SB Si-O-Si of Q2 units | (1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644. ) |
| - | - | - | - | 657 | 669 | SB Si-O-Si of Q2 units for C-S-H | (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h. , 1818. Higl, J.; Köhler, M.; Lindén, M. (2016) Confocal Raman microscopy as a non-destructive tool to study microstructure of hydrating cementitious materials. Cem. Concr. Res. 88, 136-143. https://doi.org/10.1016/j.cemconres.2016.07.005. ) |
| 779 | 780 | 780 | 776 | 784 | 772 | Si against tetrahedral O motion | (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018. ) |
| 897 | 900 | 921 | 928 | 892 | 930 | SS Si-O (Q1) | (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018. , 1919. Black, L.; Breen, C.; Yarwood, J.; Garbev, K.; Stemmermann, P.; Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation in air-A Raman spectroscopic study. Part II: carbonated phases. J. Am. Ceram. Soc. 90, 908-917. https://doi.org/10.1111/j.1551-2916.2006.01429.x. ) |
| 990 | 985 | 984 | 994 | 984 | 996 | SS Si-O (Q2) | (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018. , 1717. McMillan, P. (1984) Structural studies of silicate glasses and melts - applications and limitations of Raman spectroscopy. Am. Min. 69, 622-644. ) |
| 1040 | 1041 | 1042 | - | 1027* | 1035 | SS Si-O (Q³) | (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018. ) |
| 1065 | 1065 | 1068 | 1067 | 1065 | 1065 | ѵ1 [CO32-] | (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018. , 2020. Hurai, V.; Huraiová, M.; Slobodník, M.; Thomas, R. (2015) Geofluids, 1st Edition, Elsevier, https://doi.org/10.1016/C2014-0-03099-7. ) |
| 3415 | 3420 | 3397 | 3407 | 3415 | OH stretching | (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h. , 1818. Higl, J.; Köhler, M.; Lindén, M. (2016) Confocal Raman microscopy as a non-destructive tool to study microstructure of hydrating cementitious materials. Cem. Concr. Res. 88, 136-143. https://doi.org/10.1016/j.cemconres.2016.07.005. ) |
|
| *Q² or Q³ units | |||||||
3.1. Synthetic silica gels without calcium
⌅Two characteristic Raman peak pattern of the sample KS0.3 are illustrated in Figure 1. The vibrational bands in the ranges 400 - 650 cm-1 and 850 - 1200 cm-1 correspond to Si-O-Si and Si-O bonds, respectively. The evaluation of Si-O-Si group reveals one main peak at 533 cm-1 for Q3 sites (1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644.
). The small peak at 431 cm-1 is characteristic for SiO4 tetrahedra vibration of Q4 sites and the peak at 462 cm1 corresponds to symmetric bending (SB) vibration of ONBO-Si-ONBO (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h.
, 1616.
Garbev, K.; Stemmermann, P.; Black, L.; Brenn, C.; Yarwood, J.;
Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation
in air-A Raman spectroscopic study. Part I: fresh phases. J. Am. Ceram. Soc. 90, 900-907. https://doi.org/10.1111/j.1551-2916.2006.01428.x.
). At 597 cm-1, there is one SB vibration band, which is attributed to Q² sites (1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644.
). The dominant peak for Q3 silicate sites at 1040 cm-1 corresponds to symmetric stretching (SS) vibration. The SS vibrational peak at 990 cm-1 is linked to Q² sites. The flat and broad band at 897 cm-1 is related to the symmetric stretching vibration of Q1 sites (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644.
). The peak position at 1065 cm-1 can be linked to CO32- vibration (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 2020. Hurai, V.; Huraiová, M.; Slobodník, M.; Thomas, R. (2015) Geofluids, 1st Edition, Elsevier, https://doi.org/10.1016/C2014-0-03099-7.
). The peak position at 1108 cm-1 can be assigned to Q4 sites (2121. Ortaboy, S.; Li, J.; Geng, G; Myers, R.J.; Monteiro, P.J.M.; Maboudian, R.; Carraro, C. (2017) Effects of CO2 and temperature on the structure and chemistry of C-(A-)S-H investigated by Raman spectroscopy. RSC Adv. 7 [77], 48925-48933. https://doi.org/10.1039/C7RA07266J.
).
The 29Si NMR spectrum of the potassium silica gel (KS0.3) is shown in Figure 2. Four main peaks are apparent and attributed to different tetrahedral Qn environments for Si in silicates. The main fitted peaks are centred at -80 ppm, -88 ppm, -97 ppm and -107 ppm and assigned to Q1, Q², Q³ and Q4 sites, respectively (88. Hou, X.; Kirkpatrick, R.J.; Struble, L.J.; Monteiro, P.J.M. (2005) Structural investigations of alkali silicate gels. J. Am. Ceram. Soc. 88 [4], 943-949. https://doi.org/10.1111/j.1551-2916.2005.00145.x.
, 2222.
Tambelli, C.E.; Schneider, J.F.; Hasparyk, N.P.; Monteiro, P.J.M.
(2006) Study of the structure of alkali-silica reaction gel by
high-resolution NMR spectroscopy. J. Non. Cryst. Solids. 353 [32-35], 3429-3436. https://doi.org/10.1016/j.jnoncrysol.2006.03.112.
, 2323. Walkley, B.; Provis, J.L. (2019) Solid-state nuclear magnetic resonance spectroscopy of cements. Mater. Today Adv. 1, 100007. https://doi.org/10.1016/j.mtadv.2019.100007.
). The two smaller resonances at -89 ppm and -99 ppm can be assigned to Q2 and Q3 sites (2424.
Dedecek, J.; Balgová, V.; Pashkova, V.; Klein, P.; Wichterlová, B.
(2012) Synthesis of ZSM-5 Zeolites with defined distribution of al atoms
in the framework and multinuclear MAS NMR analysis of the control of Al
distribution. Chem. Mater. 24 [16], 3231-3239. https://doi.org/10.1021/cm301629a.
). The fitted relative integral intensities are about 49 % for Q³, 19 % for Q², 16 % for Q4 and 4 % for Q1.
The evaluation of the 29Si NMR spectrum of KS0.3 agrees with the band assignments of the Raman spectrum. The four Q units can be determined by both measurement techniques.
Figure 3 shows the Raman spectra for sodium silica gels with different Na/Si
molar ratios. All recorded Raman spectra exhibit a strong vibrational
band in the range of 2500 - 3750 cm-1. The band position around 3420 cm-1 corresponds to OH stretching vibration of H2O (1818.
Higl, J.; Köhler, M.; Lindén, M. (2016) Confocal Raman microscopy as a
non-destructive tool to study microstructure of hydrating cementitious
materials. Cem. Concr. Res. 88, 136-143. https://doi.org/10.1016/j.cemconres.2016.07.005.
).
The results for KS0.3 and NS0.3 are almost identical. Therefore, the
results for NS0.3 will not be discussed further. The main Raman peak of
the sample NS0.6 in the Si-O-Si range (400 - 750 cm-1) appears at 600 cm-1 and is linked to SB vibration of Q2 sites. There is a peak shoulder at 548 cm-1 which is related to SB vibration of Q3 sites. In the higher frequency region, there is a pronounced peak at 1042 cm-1 which is attributed to SS vibration (Q³) (2525. Huang, Y.; Jiang, Z.; Schwieger, W. (1998) A vibrational study of kanemite. Micropo. Mesopo. Mat. 26, 215-219. https://doi.org/10.1016/S1387-1811(98)00270-4.
, 2626. Kalampounias, A.G. (2011) IR and Raman spectroscopic studies of sol-gel derived alkaline-earth silicate glasses. Bull. Mater. Sci. 34, 299-303. https://doi.org/10.1007/s12034-011-0064-x.
). The peak position at 1065 cm-1, dominant for samples NS0.6 and NS1.2 which is linked to CO32- vibration, indicates the presence of trona [Na3H(CO3)2·2H2O] (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 2020. Hurai, V.; Huraiová, M.; Slobodník, M.; Thomas, R. (2015) Geofluids, 1st Edition, Elsevier, https://doi.org/10.1016/C2014-0-03099-7.
). The Raman spectrum of the sample NS1.2 contains two main peaks in the lower frequency region: one peak at 600 cm-1 related to SB vibration of Q² units and one peak at 450 cm-1 linked to SB of ONBO-Si-ONBO (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h.
). In the higher frequency region, a pronounced peak occurs at 776 cm-1 that is attributed to an internal deformation vibration of Si-O. Another peak is found at 928 cm-1 (SS of Q1) (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 1717.
McMillan, P. (1984) Structural studies of silicate glasses and melts -
applications and limitations of Raman spectroscopy. Am. Min. 69,
622-644.
).
On comparing the Raman spectra of the sodium silica gels the following trends were determined. The dominant peak in the low frequency region changes from 530 cm-1 (Q3) to 600 cm-1 (Q2) with increasing Na/Si molar ratio. The dominant vibrational band at 530 cm-1 for the sample NS0.3 disappears in the case of the sample NS0.6 and a shoulder at 548 cm-1 (Q³) remains. This shoulder linked to Q³ sites then disappears in the spectrum of NS1.2.
It is also apparent that the Raman peak around 450 cm-1 becomes more prominent with increasing Na/Si molar ratio. This indicates an increasing amount of non-bridging oxygen (NBO) and hence a decreasing degree of cross-linking in dependence of the sodium content. The internal deformation vibration band of Si-O at approximately 780 cm-1 increases in intensity. Another indication for the decrease in silicate linkage with increasing Na/Si ratio is the intensity increase of the Raman band around 930 cm-1 (Q1). The vibrational band at 430 cm-1 (Q4), similar to that of KS0.3 in Figure 1, is not present in the Raman spectra of the samples NS0.6 and NS1.2.
The main vibrational band in the high frequency region in the spectrum of sample NS0.3 at 1040 cm-1 (Q3) disappears with increasing Na/Si molar ratio; only a sharp Raman peak remains at 1065 cm-1, indicating the presence of trona, in the case of the sample NS1.2. In the Raman spectrum of the gel sample NS0.6, a shoulder, corresponding to SS vibration of Q³ sites, is still apparent. This shoulder disappears in the spectrum of NS1.2 and only a shallow peak shoulder at 994 cm-1 (Q2) remains. The peak at 928 cm-1 (Q1) emerges as new prominent Raman band. Accordingly, the high-frequency prominent Raman band shifts with increasing Na/Si molar ratio to lower wavenumbers. The carbonate band around 1065 cm-1 is sharper at higher Na/Si molar ratios.
In Figure 4,
the position of the prominent peak of the Raman spectra with varying
(Na,K)/Si molar ratio is shown including results of synthetic ASR gels
of Balachandran et al. (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
). A linear trend between the Q³ peak position in Raman spectra and the (Na,K)/Si molar ratio is apparent (Figure 4, left). With increasing (Na,K)/Si molar ratio from 0.2 - 0.8 the peak position of Q³ shifts from 520 cm-1 to 565 cm-1. In Figure 4,
the relationship between the prominent peak positions and the (Na,K)/Si
molar ratios is shown. The prominent peak position of the sodium and
potassium silica gels changes from Q³ to Q² with increasing (Na,K)/Si
molar ratio.
).
3.2. Synthetic sodium-calcium silica gels
⌅The Raman spectra of the sodium-calcium silica gels are compared with a Raman spectrum of a C-S-H phase in Figure 5. The C-S-H phase was synthesized in a previous study with a Ca/Si molar ratio of 0.8 (C-S-H_0.8) (2727. Irbe, L.; Beddoe, R.E.; Heinz, D. (2019) The role of aluminium in C-A-S-H during sulfate attack on concrete. Cem. Concr. Res. 116, 71-80. https://doi.org/10.1016/j.cemconres.2018.11.012.
). Both synthetic ASR gels exhibit a vibrational band at 780 cm-1 which corresponds to an internal deformation vibration of Si-O (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
). The sharp vibrational band at 1065 cm-1 indicates the presence of trona (2020. Hurai, V.; Huraiová, M.; Slobodník, M.; Thomas, R. (2015) Geofluids, 1st Edition, Elsevier, https://doi.org/10.1016/C2014-0-03099-7.
).
The
characteristic vibrational bands of the sodium-calcium silica gel
NSC0.3_0.22 in the low frequency region are at around 600 cm-1 and 660 cm-1 (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 1919.
Black, L.; Breen, C.; Yarwood, J.; Garbev, K.; Stemmermann, P.;
Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation
in air-A Raman spectroscopic study. Part II: carbonated phases. J. Am. Ceram. Soc. 90, 908-917. https://doi.org/10.1111/j.1551-2916.2006.01429.x.
). The exact assignment of the peak at 600 cm-1 is not possible. This Raman peak can be linked to SB of Si-O-Si in Q³ or Q² sites. The present 29Si
NMR spectroscopic studies indicate that Q³ sites dominate the structure
of the sample NSC0.3_0.22. Based on the relative integral intensities,
the NMR Q³:Q² ratio is roughly 5:3. According to studies (11. Leemann, A. (2017) Raman microscopy of alkali-silica reaction (ASR) products formed in concrete. Cem. Concr. Res. 102, 41-47. https://doi.org/10.1016/j.cemconres.2017.08.014.
, 1616.
Garbev, K.; Stemmermann, P.; Black, L.; Brenn, C.; Yarwood, J.;
Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation
in air-A Raman spectroscopic study. Part I: fresh phases. J. Am. Ceram. Soc. 90, 900-907. https://doi.org/10.1111/j.1551-2916.2006.01428.x.
) on C-S-H phases, the Q3 peak lies within a range of 594 - 628 cm-1. The peak around 660 cm-1 is at a characteristic position for chain silicates as in the case of C-S-H phases (1818.
Higl, J.; Köhler, M.; Lindén, M. (2016) Confocal Raman microscopy as a
non-destructive tool to study microstructure of hydrating cementitious
materials. Cem. Concr. Res. 88, 136-143. https://doi.org/10.1016/j.cemconres.2016.07.005.
). Evaluation of the vibrational band reveals a peak around 584 cm-1, which is presumably related to Q³ sites. The high frequency region contains a peak shoulder at 983 cm-1, linked to SS of Q² sites and a peak at 1027 cm-1,
which probably linked to Q³ sites. Due to overlap of the prominent
trona vibrational band, band assignment is difficult here. The prominent
peak of the sodium-calcium silica gel NSC0.7_0.46 is at 670 cm-1. This peak position is linked to SB of Q2 sites. A shoulder around 600 cm-1 is linked to SB vibration of Si-O-Si and presumably due to Q3 sites. A broad Raman peak around 772 cm-1 is explained by an internal deformation vibration of Si-O. In the higher frequency region, two shallow peaks occur at 932 cm-1 and 996 cm-1, which are attributed to SS vibration of Q1 and Q² sites, respectively (1212. Balachandran, C.; Muñoz, J.F.; Arnold, T. (2017) Characterization of alkali silica reaction gels using Raman spectroscopy. Cem. Concr. Res. 92, 66-74. https://doi.org/10.1016/j.cemconres.2016.11.018.
, 1616.
Garbev, K.; Stemmermann, P.; Black, L.; Brenn, C.; Yarwood, J.;
Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation
in air-A Raman spectroscopic study. Part I: fresh phases. J. Am. Ceram. Soc. 90, 900-907. https://doi.org/10.1111/j.1551-2916.2006.01428.x.
).
As
mentioned above, a synthetic C-S-H phase with a Ca/Si molar ratio of
0.8 was measured for comparison purposes. The dominant vibration bands
are at 669 cm-1 and 1006 cm-1 corresponding to SB of Si-O-Si and SS of Si-O, respectively, in Q² sites (1818.
Higl, J.; Köhler, M.; Lindén, M. (2016) Confocal Raman microscopy as a
non-destructive tool to study microstructure of hydrating cementitious
materials. Cem. Concr. Res. 88, 136-143. https://doi.org/10.1016/j.cemconres.2016.07.005.
). The origin of the vibrational Raman band at 798 cm-1 is not certain. According to Black et al. (1919.
Black, L.; Breen, C.; Yarwood, J.; Garbev, K.; Stemmermann, P.;
Gasharova, B. (2007) Structural features of C-S-H(I) and its carbonation
in air-A Raman spectroscopic study. Part II: carbonated phases. J. Am. Ceram. Soc. 90, 908-917. https://doi.org/10.1111/j.1551-2916.2006.01429.x.
), this peak is due to the translation motion of Si compared with O in Q4 units. The Raman peak at 890cm-1 is related to SS of Si-O in Q1 units. The vibrational band at 1081 cm-1 is linked to SS of CO32- and indicates the presence of calcium carbonate (1515.
Black, L. (2009) Raman spectroscopy of cementitious materials. In
Spectrosc. Prop. Inorg. Organomet. Compd., J. Yarwood; Douthwaite, R.;
Duckett, S. B. https://doi.org/10.1039/b715000h.
).
The
comparison of the two different sodium-calcium silica gels shows a
shift of the dominant Raman peak in the low frequency region from 600 cm-1 to 670 cm-1. In the Raman spectrum of NSC0.7_0.46 the vibration band at 600 cm-1 almost disappears and a dominate peak at 670 cm-1 emerges. In the high frequency region, the dominant peak around 1030 cm-1 shifts to lower wavenumbers around 1000 cm-1 with increasing Ca/Si molar ratio (from 0.22 to 0.46). It is concluded
that the main vibrational band position of the sodium-calcium silica
gels in the Raman spectra shifts from a Q3 position with increasing calcium content towards a Q2 position. The shifts of dominant Raman bands with changing Ca/Si molar
ratio point to a structural change in the ASR gel. In particular, the
structure of the ASR gel becomes more like that of C-S-H phases, a
change which has also been suggested by other authors (28-3028. Cong, X.D.; Kirkpatrick, R.J.; Diamond, S. (1993) 29Si MAS NMR spectroscopic investigation of alkali silica reaction product gels. Cem. Concr. Res. 23 [4], 811-823. https://doi.org/10.1016/0008-8846(93)90035-8.
29.
Leemann, A.; Le Saout, G.; Winnefeld, F.; Rentsch, D.; Lothenbach, B.
(2011) Alkali-silica reaction: the influence of calcium on silica
dissolution and the formation of reaction products. J. Am. Ceram. Soc. 94 [4], 1243-1249. https://doi.org/10.1111/j.1551-2916.2010.04202.x.
30. Hou, X.; Struble, L.J.; Kirkpatrick, R.J. (2004) Formation of ASR gel and the roles of C-S-H and portlandite. Cem. Concr. Res. 34 [9], 1683-1696. https://doi.org/10.1016/j.cemconres.2004.03.026.
).
29Si NMR spectroscopic investigations on synthetic ASR products by Hou et al. (3030. Hou, X.; Struble, L.J.; Kirkpatrick, R.J. (2004) Formation of ASR gel and the roles of C-S-H and portlandite. Cem. Concr. Res. 34 [9], 1683-1696. https://doi.org/10.1016/j.cemconres.2004.03.026.
) and Leemann et al. (22.
Leemann, A.; Shi, Z.; Lindgård, J. (2020) Characterization of amorphous
and crystalline ASR products formed in concrete aggregates. Cem. Concr. Res. 137, 106190. https://doi.org/10.1016/j.cemconres.2020.106190.
) have shown that in the presence of calcium new Raman bands occur corresponding to Q1 and Q2 sites as in C-S-H phases. The Raman spectra of sodium-calcium silica gels and the C-S-H phase both possess bands around 670 cm-1 and 1000 cm-1. Differences in the spectra are for the band position at 800 cm -1 (in C-S-H gel) and 780 cm-1 (in N-S-C gel). The Q1 position of the C-S-H phase is at around 890 cm-1, whereas this band occurs at 930 cm-1 in the case of N-S-C gels.
Figure 6 shows the 29Si
NMR spectrum of the sample NSC0.7_0.46. The characteristic resonances
at -79 ppm, -84 ppm, -88 ppm and -95 ppm correspond to Q1, Q2(I), Q2(II), and Q³(I) sites of silicate tetrahedra (2222.
Tambelli, C.E.; Schneider, J.F.; Hasparyk, N.P.; Monteiro, P.J.M.
(2006) Study of the structure of alkali-silica reaction gel by
high-resolution NMR spectroscopy. J. Non. Cryst. Solids. 353 [32-35], 3429-3436. https://doi.org/10.1016/j.jnoncrysol.2006.03.112.
, 3131. Lothenbach, B.; Nied, D.; L´Hôpital, E.; Achiedo, G.; Dauzéres, A. (2015) Magnesium and calcium silicate hydrates. Cem. Concr. Res. 77, 60-68. https://doi.org/10.1016/j.cemconres.2015.06.007.
). The resonances at -99 ppm and -107 ppm can be assigned to Q³ (II) and Q4 sites (2424.
Dedecek, J.; Balgová, V.; Pashkova, V.; Klein, P.; Wichterlová, B.
(2012) Synthesis of ZSM-5 Zeolites with defined distribution of al atoms
in the framework and multinuclear MAS NMR analysis of the control of Al
distribution. Chem. Mater. 24 [16], 3231-3239. https://doi.org/10.1021/cm301629a.
).
The assignments of the two different resonances for Q² and Q³ can be
explained by different chemical environments. Depending on the =Si-O
counter ion (e.g. Ca2+, Na+), a chemical shift occurs due to different shielding effects (3232.
Myers, R.J.; Bernal, S.A.; Gehman, J.D.; van Deventer, J. S.J.; Provis,
J.L. (2015) The role of Al in cross-linking of alkali-activated slag
cements. J. Am. Ceram. Soc. 98 [3], 996-1004. https://doi.org/10.1111/jace.13360.
). The resonance at -84 ppm (Q2(I)) can also indicate that a C-S-H phase is present (3333. Sevelsted, T.F; Skibsted, J. (2015) Carbonation of C-S-H and C-A-S-H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy. Cem. Concr. Res. 71, 56-65. https://doi.org/10.1016/j.cemconres.2015.01.019.
).
However, in this study there is only one peak for pairing or bridging
positions, the second peak for a C-S-H dreierkette is missing here. This
suggests that the ASR gel is not in coexistence with a C-S-H gel, but
becoming as a whole structurally more like that of a C-S-H phase.
4. CONCLUSIONS
⌅The structure of synthetic potassium, sodium and calcium-sodium silicate gels was investigated using Raman and 29Si NMR spectroscopy. The Raman spectroscopic investigations on the ASR gels reveal a change of prominent peak position depending on the (Na+K)/Si and Ca/Si molar ratios. The experiments with sodium silicate gels revealed a shift of the low frequency band from 530 cm-1 (Q3) to 600 cm-1 (Q2) with increasing Na/Si molar ratio. Moreover, the results show that increasing the Na/Si molar ratio leads to more non-bridging oxygen atoms in the structure and therefore a reduction in the degree of cross-linking. The Q3 peak position of the sodium silicate gels (Na/Si ratios 0.2 - 0.8) in the Raman spectra (520 - 565 cm-1) can be used to estimate the Na/Si molar ratio of the gel. The low frequency band of the sodium-calcium silica gels shifts towards higher wavenumbers and, with increasing Ca/Si molar ratios, becomes more like that of C-S-H phases. The 29Si NMR investigations confirm the Raman spectroscopic results and indicate local environments similar to those of C-S-H phases for the sodium-calcium gels. Thus, from the overall evaluation, it can be concluded that Raman spectroscopy is a useful analytical method to gain new insights into Qn species present in the ASR gel structure. The systematic analysis of synthetic ASR gels provides new information on the role of alkalis within the structure of ASR gels.