29Si MAS NMR spectroscopic investigation of alkali silica reaction product gels

CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 811-823, 1993. Printed in the USA. 0008-8846/93. $6.00+00. Copyright © 1993 Pergamon Press Ltd.

29Si MAS NMR SPECTROSCOPIC INVESTIGATION OF ALKALI SILICA REACTION PRODUCT GELS

Xian-Dong Cong and R. James Kirkpatrick Department of Geology, University of Illinois, Urbana IL 61801, U.S.A. Sidney Diamond School of Civil Engineering, Purdue University, West Lafayette, IN 47907, U.S.A. (Communicated by P.L. Pratt) (Received May 29. 1992; in trmalform March 30, 1993)

Abstract

29Si magic angle spinning NMR spectroscopy and 1H-29Sicross-polarization MAS results are reported for gels prepared by reactions of a Nevada opal, Beltane opal, and cristobalite with KOH in the presence and absence of Ca(OH)2. The extents of the reaction can be followed as functions of time. The opals react much more rapidly than cristobalite. In the presence of calcium, the predominantly Q4 sites of the starting materials change to predominantly Q2 and Q3 sites. Q~ sites are less abundant and in part transient. Cross-polarization experiments confirm that all the reaction products are hydrous. Poor signal to noise ratios for the crosspolarization experiments at room temperture and increased signal/noise at -80°C suggest that most of the protons are loosely bound at room temperature and may occur predominantly in water molecules. The chemical shifts of the Q~ and Q2 sites observed for these products suggest local environments surprisingly similar to those of the calcium silicate hydrate gel produced in portland cement hydration. Gel products produced in the absence of calcium yield broad peaks corresponding to Q3, Q2, and Q~ sites, but the spectra vary in a non-monotonic way with reaction time. Introduction The alkali silica reaction (ASR) in concrete involves reaction between dissolved alkali hydroxide in concrete pore fluid and reactive silica phases (opal, chert, cristobalite, strained quartz, etc.) in fine or coarse aggregate. The reaction produces a gel product of varying compositon, usually incorporating alkali, hydroxyl, and Ca ions, and often other ions as well. Some, but not necessarily all, gels produced by such reactions can cause local expansions leading to cracking of the concrete, with subsequent deterioration and sometimes failure. Because of its engineering significance, the effects of the alkali silica reaction have been extensively studied, but the structure of the gel and the relationship between structure and 811

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swelling tendency is not well understood. The gel is amorphous, although secondary crystallization to fine crystals of unknown structure has been reported in a few cases. Infra-red absorption spectra are not very helpful in characterizing the material, other than in indicating the presence of H-bonded OH ions and shifts of the main Si-O lattice vibrations to wave numbers lower than for the starting silica mineral. X-ray diffraction patterns show the expected broad band in the appropriate region for siliceous gels (centered at about 23 ° 20 for Cu Ks radiation). In this study we report results obtained for laboratory-produced gels using 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, with supplementary results obtained by IH-29Si cross polarization MAS NMR techniques (CPMAS) to provide information about the Si-environments and the state of the water in these hydrous gels.

29Si MAS NMR spectroscopy is a powerful tool for investigating aspects of the structure of both crystalline and amorphous silicates, especially nearest neighbor (NN) and next-nearest neighbor (NNN) atomic environments. In particular, detailed information can be obtained concerning the polymerization of the silica tetrahedra. A recent review was provided by Kirkpatrick (1). The CPMAS technique is especially useful for hydrous materials, because transfer of nuclear spin from protons to 29Si enhances the signal from those 29Si nuclei in close association with protons. Thus, this technique can be used to determine whether individual Sisites are in hydrated or unhydrated local environments. It can also provide information about the mobility of the associated protons.

Experimental Svnthesis of ASR Gels Most of the gels studied in this work are laboratory products produced by reaction of dry mixtures of ground reactive silica and calcium hydroxide with potassium hydroxide solutions. Parallel products were produced starting from (a) an opal-CT, obtained from Ward's Natural Science Establishment from a deposit in Nevada, (b) Beltane opal, a commonly used standard alkali-reactive rock material from Sonoma County, California, containing opal-CT with accessory quartz and kaolinite, and (c) a synthetic cristobalite produced by calcining a relatively pure flint, and provided through the courtesy of Blue Circle Cement, Ltd. Characteristics of the Beltane opal have been described by Barneyback (2) and by Gutteridge and Hobbs (3); the preparation and properties of the cristobalite have been described by Lumley (4). The batch compositions were designed to simulate the proportions of these components that might be found in a standard ASTM C-227 mortar designed to contain the pessimum proportions of reactive aggregate. The batch compositions for the Nevada opal and Beltane opal mixes were opal: 112.5 g, KOH solution: 500 ml of 1 N solution of dissolved reagent KOH, Ca(OH)s: 57.0 g. The reactive aggregate:water ratio was 0.225. The amount of Ca(OH)2 used would produce a 2 molar solution, about 100 times the maximum solubility of this compound. The excess Ca(OH)2 assures a constant reservoir of Ca, as in actual hydrated cement or concrete. For cristobalite the pessimum effect is not pronounced, and maximum expansions occur at much higher proportions of reactive material to total aggregate. In this case the mix

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proportions used were cristobalite: 92.5 g, KOH solution: 100 ml of a 1 N solution, and Ca(OH)s: 82.5 g. The corresponding reactive aggregate:water ratio was 0.825. In all of the experiments the reactive silicas were ground to a mean grain size of - 3 0 #m and thoroughly dry blended with Ca(OH)2. The KOH solution was added slowly to the mixed powder in a large beaker over a magnetic stirrer to produce a homogeneous slurry. The slurry was then subdivided and packed into individual 10 ml plastic containers and individually sealed for storage at room temperature. Separate specimens were analyzed after storage periods ranging from a few days to nearly 1 year. After the desired storage period, the individual containers were opened and the contents washed at least three times with acetone to remove the uncombined KOH solution and to permit rapid air drying to minimize carbonation. The quickly dried specimens were sealed prior to actual analysis by X-ray diffraction and NMR. Some specimens were also submitted for chemical analysis by x-ray fluorescence to X-Ray Assay Laboratories, Don Mills, Ontario, Canada. It was found that the Ca(OH)2 used was initially slightly carbonated, despite being taken from a newly-opened bottle of reagent grade material, and that additional carbonation likely occurred during processing. All of the XRD patterns show calcite peaks. It is thought that the carbonation derived primarily from the uncombined Ca(OH)2 rather than from the reaction product gel. In addition to the preparations described above, additional preparations were made by reacting Nevada opal and Beltane opal starting materials with 1N KOH solution in the absence of any source of calcium. These Ca-free samples were mixed at solid:solution weight ratios of 0.5, and stored for 10, 71, and 170 day periods prior to washing and analysis.

NMR Analvses The MAS NMR spectra were recorded at room temperture using a home built pulseFourier-transform spectrometer equipped with an 8.45T superconducting magnet ('295i Larmor frequency = 71.48MHz; Oxford Instruments, U.K.) and a Nicolet 1280 data system (Madison, WI, USA). The MAS probe was made by Doty Scientific (Columbia, SC, USA) and can hold up to about 0.5 g of sample. The magic angle was set by maximizing the time-domain rotational echoes of the +(3/2,1/2) satellite transitions of 127I in crystalline KI. The spinning frequencies were about 3 kHz. ~-/3pulses with 50 s delays were used to eliminate signal saturation. 400800 scans were accumulated for each sample. All 29Sichemical shifts are reported relative to external TMS, with increasing shielding to more negative values. The NMR spectra were fit using the program NMRFIT (5). 1H-29Si CPMAS spectra were collected with the same spectrometer (1H frequency = 360 MHz) and sample probe at both room temperature and -80°C. Contact times ranged from 700 /~s to 90 ms, and 1,000-1,200 scans were collected per sample. The contact time is the time during which both nuclides are simultaneously excited. Generally, longer contact times allow spin transfer to Si nuclei further from the protons.

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X-Ray Diffraction, Chemical Analysis, and NMR Results: Ca Bearing Systems

XRD and Chemical AnaIvses No new crystalline phases were detected by X-ray diffraction in any of our experiments except for the calcite carbonation product previously mentioned. Thus any silica-bearing reaction product formed in significant amount must be amorphous, as expected. Table 1 presents chemical analyses for the starting silica materials and for the acetonewashed Ca-bearing preparations at ages of 7, 50 and 155 days. It should be noted that these analyses reflect the combination of residual starting material, reaction product gel, and carbonation product present in the reaction mixture after acetone washing. Table 1. Chemical compositions of silica reactants and of reaction mixtures at various ages.

Weight Percent (%) Nevada Opal

Beltane Opal

Cristobalite

unreacted

7d

50 d

155 d

unreax:ted

7 d

50 d

155 d

unreax:ted

7d

50 d

155 d

SiO2

90.60

63.70

46.00

41.70

89.90

44.30

39.70

29.90

99.20

89.40

86.80

70.80

Ti02

0.03

0.01

0.01

0.01

0.35

0.19

0.18

0.17

0.02

0.02

0.02

0.02

A12 03

0.80

0.59

0.45

0.42

3.58

2.22

2.26

1.70

0.05

0.06

0.06

0.04

Fe203

0.85

0.47

0.30

0.34

0.18

0.26

0,40

0.21

0.05

0.09

0.09

0.09

MnO

0.02

0.02

0.01

0.02

0.01

0.01

0.01

0.01

0.01

0.02

0.01

0.01

CaO

0.73

14.20

21.50

22.30

0.03

25.00

25.40

14.70

0.28

6.26

7.96

7.52

MgO

0.16

1.32

0.38

0.40

0.02

1.30

1.35

0.22

0.01

0.10

0.11

0.09

Na20

0.02

0.01

0.25

0.32

0.08

0.01

0.01

0.17

0.01

0.01

0.01

0.02

K20

0.08

1.41

3.05

5.24

0.22

1.41

1.77

2.46

0.01

0.01

0.03

0.10

P20s

0.01

0.01

0.02

0.02

0.03

0.01

0.01

0.02

0.03

0.03

0.03

0.02

H20 +

4.50

7.30

9.30

9.20

3.60

7.30

9.10

6.30

0.20

0.60

1.80

1.80

H20-

2.30

3.30

12.00

9.60

0.50

2.50

10.30

36.60

0.10

0.10

0.50

14.10

C02

0.32

6.38

5.98

8.43

0.01

13.30

7.52

4.20

0.01

3.68

2.44

3.08

Sum

100.42

98,72

99.25

98.00

98.51

97.81

97.65

96.66

99.98

100.38

99.86

97.69

SiO2

98.81

86.71

72.30

71.57

97.45

77.62

65.89

70.46

99.70

97,75

94.35

94.83

A1203

0.51

0.48

0.40

0.42

2.28

2.32

2.18

2.37

< 0.01

< 0,01

< 0.01

< 0.01

CaO

0.36

8.89

23.39

21.24

0.06

15.13

26.86

23.73

0.30

2.25

5.65

5.15

MgO

0.26

2.70

0.86

1.03

0.06

3.35

3.19

0.79

< 0.01

< 0.01

< 0.01

< 0.01

1(20

0.05

1.22

3.06

5.73

0.15

1.37

1.86

2.65

< 0.01

< 0.01

< 0.01

< 0.01

Sum

99.99

100.00

100.01

99.99

100.00

99,79

99.98

99.99

100.01

100.02

100.01

100.04

Mole Percent Normalized to 0% H20 and CaCO3 (%)

Bound CO2 is present to various extents in the different preparations. On the assumption that this CO2 is present only in calcite, it and the appropriate percentage of Ca were separately assigned. The residual components were then re-expressed on a 0% H20 basis, and then reexpressed as mole %. This ultimate tally is taken to represent the molar composition of the non-water part of the reaction product gel plus residual unreacted starting solid material (Table

1).

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The progress of the reaction with time can be followed in terms of increasing proportions of bound K and Ca. The calculated mole % K20 increases with time to about 5.7% in the Nevada opal mixture and to about half this value in the Beltane opal mixture. The cristobalite mixtures show little evidence of reaction by this criterion, but considerable Ca appears to be incorporated into the gels, as confirmed by the NMR data described below. However, certain inconsistencies are present in the chemical data reported in Table 1, and they may not fully reflect the ongoing reaction processes.

NMR Spectra 2 9 S i MAS NMR spectra are provided in Figure 1 for the starting silica materials and for reaction mixes after 7, 50, 155, and 300 days of storage.

cristobalite

Nevada opal

Beltane opal

-85

-85

-85

300 days

155 days j

50days

L__

7 days J

original 11 -60

I I I I I I I I I I IO -60 -100 -120

I

I I I , I = r J I I I I i

-60

-60

-100

420

-60

-60

-100

-120

i i pprn

Figure 1. 29Si MAS NMR spectra of starting silica and selected reaction mixes with Ca(OH)2 and KOH solution. Changes in the NMR spectra are readily understood as showing the effects of reaction of the starting silica with KOH and Ca(OH)2 to produce reaction product gel. In all cases the

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observed 29Si chemical shifts indicate an overall reduction in the average number of Si-O-Si bonds per silica tetrahedron, i.e., an overall partial depolymerization of the starting silica. Except for the cristobalite, the peaks for both starting material and altered material are quite broad, consistent with the amorphous nature of both the reactive silica and the products produced. We interpret the NMR spectra using the standard Q~ nomenclature (see, e.g., Kirkpatrick, 1). In this nomenclature the Q stands for a given tetrahedron, and the exponent n for the number of associated bridging oxygens. Thus a Q4 tetrahedron has four bridging oxygens, and describes the local state of fully-cross-linked silica, such as the state of most silicon atoms in the starting materials. In contrast, a 03 tetrahedron has three bridging oxygens and one non-bridging oxygen, a Q2 tetrahedron has two bridging oxygens and two non-bridging ones, etc. The non-bridging oxygens may be bonded to a low-charge cation different from Si, or to a proton as an OH group. Because the 295i NMR chemical shifts which are used to determine the local silicon environment are sensitive only to the NN and to the NNN structural features, our data cannot be used to determine the extended arrangement of the tetrahedra. Peak assignments are made by comparison of the observed chemical shifts with those of known amorphous and crystalline model compounds. 29Si MAS NMR chemical shift data for hundreds of silicate materials are now known and provide a solid basis for these assignments. For Al-free silicates, Q4 sites resonate in the range -103 to -115 ppm, 03 sites in the range -91 to -98 ppm, Q2 sites in the range -79 to -85 ppm, and Q~ sites in the range -62 to -83 ppm. A proton rather than an alkali or alkaline earth cation as an NNN to a Si (e.g. an OH group bonded to the silicon) usually results in increased shielding, i.e., a more negative chemical shift, than would otherwise be the case. For example, 03 sites with a single OH in amorphous silicas generally have chemical shifts in the -98 to -103 ppm range rather than the -91 to -98 ppm range for anhydrous alkali or alkaline earth silicates.

NMR Results for Cristobalite and Ca-Bearing Cristobalite Reaction Products The spectra of the cristobalite and cristobalite reaction products are the simplest among our samples. The spectrum of the starting cristobalite (Figure l) consists of a single sharp peak at -109.1 ppm with full width at half height (FWHH) of 1.5 ppm. This peak is clearly assignable to the Q4 sites in cristobalite (Smith and Blackwell, 6). The slight tailing toward more negative shifts at the base of the peak may indicate the presence of some tridymite-like sites associated with stacking disorder in the cristobalite. It is evident that the cristobalite reacts only slowly in this system. The first evidence of reaction occur at 21 days (spectrum not shown) with the appearance of two new very small peaks at -79.6 and -84.9 ppm. These peaks are slightly larger at 50 days, but are first really resolved on the scale of presentation of Figure 1 in the spectrum for 155 days. The -84.9 ppm peak is very much larger than the -79.6 ppm peak, and in this spectra a small unresolved peak at about -83 ppm also seems to be present. There is some indication of a broad peak near -95 ppm in the 300-day spectrum, but it is difficult to see at the scale of Figure 1.

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One major advantage of 29Si NMR spectra collected with sufficiently long recycle times (as ours were) is that the percentage of nuclei with a given environment is accurately reflected in the percentage of the total peak area corresponding to that environment. In the cristobalite mixture at 300 days the combined percentage of the reaction product peaks is 27% of the total, indicating slow, but eventually significant reaction. The two dominant new peaks (at -84.9 and -79.6 ppm) are at the same positions as the peaks for calcium silicate hydrates found in hydrated cements by, e.g., Young (7) and Grutzeck et al. (8). These authors have unambiguously assigned these peaks to Q2 and Q~ sites, respectively. In the process of hydration of portland cement or tricalcium or dicalcium silicate, the reaction converts previously isolated silica tetrahedra (Q0 species) to QI and Q2 species. In the present reactions, the conversion runs in the opposite direction; fully cross-linked Q4 species are partly depolymerized. For the reaction of cristobalite, this process appears to have produced mostly Q2 and a few QI species, with only a very low concentration of Q3 species as indicated by the small amount of intensity in the -95 ppm range.

NMR Results/'or Nevada Opal and Ca-beating Nevada Opal Reaction Products The 29Si MAS NMR spectrum of the unreacted Nevada opal consists of much broader peaks than found for cristobalite, indicating a wide range of local Si-environments. As indicated in Figure 1, the spectrum consists of a broad peak at -112.2 ppm (FWHH 7.2 ppm); a broad shoulder centered at about -103 ppm (FWHH about 8 ppm), and an extremely weak peak at about -94 ppm. This spectrum is very similar to recently published 29Si MAS NMR spectra of opals by Adams et al (9), and also to spectra of silica gels (e.g., Sindorf and Maciel (10) and Brinker et al. (11)).

"i/

A CPMAS spectrum of this opal at a contact time of 10 ms is shown in Figure 2 and confirms that the -103 and -94 ppm peaks are significantly enhanced by cross polarization, whereas the -112 ppm peak is relatively less intense. Accordingly, the appropriate assignment for the -112 ppm peak is Q4; for the -103 ppm peak, Q3 with one OH group bonded to each silicon atom; and for the -94 ppm peak, Q2 with 2 OH groups bonded to each silicon atom. These assignments are in agreement with previous workers.

-112

I

-60

I

I

I

I

-80

I

,

I

I

- 1 O0

I

I

ill " 20

,

i ppm

The Nevada opal is much more reactive than the cristobalite. There is substantial Figure 2.29Si CPMAS spectrum of Nevada opal reduction in the peak intensity for the -112 with 10 ms contact time. ppm (Q4) peak even at 7 days. Quantitative data for the percentage of Q4 silicon atoms in the reacting mix vs. time are shown in Figure 3. By one month the fraction of such silicon atoms is reduced from about 86% of the total to about 30% of the total, and by 2 months to only about 20% of the total. After this time there is no further decrease in the Q4 signal.

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100

i

i

Vol. 23, No. 4

i

i

B0I D..

E O.

•

Nevada. Opal

~'

Beltane Opal

Q.

60 "6 40

• 2o m twO

....

0

I .... 50

1 ....

100 Time

l

,

150

,

.

.

I

=

200

,

.

,

I

,

250

,

,

,

300

CDays)

Figure 3. Relative intensities of peaks due to Q4 sites as functions of time for Nevada opal and for Beltane opal reaction mixtures with Ca(OH)2 and KOI-I solution. Reduction in the -112 ppm Q4 peak is accompanied by the appearance of broad new peaks at less shielded chemical shifts indicating less polymerized Si-environments. Two of these peaks are at about -79 and -85 ppm, the same positions as the peaks developed in the cristobalite reaction mixture at later ages and attributed to Q2 and Qt Si sites in the reaction products. In addition, the present spectra contain a peak or broad band of intensity centered at -92 to -95 ppm and some contain a resolved shoulder at -84 ppm. The -79 ppm peak for Q1 species is weak at first (7 days), increases in magnitude by 50 days, and then decreases at later times, suggesting that some of the Qz sites are transient in these reaction products.

=

i

i

I -80

i

I

i

I -100

i

i

t

i

l

l

-120

ppm

Figure 4. 29SiCPMAS spectra for Nevada opal reaction mix at 13 days: (A) - 80°C, CPMAS spectra acquired for the reaction 900 #s contact time; (]3) room temperature, mix at 13 days are shown in Figure 4. At room 900/xs contact time; (C) room temperature, temperature the signal to noise ratios are poor, for 30 ms contact time. both short and long contact times. However, the spectrum collected at -80°C has a relatively good signal to noise ratio. This spectrum contains the -85 ppm peak, a shoulder at -79 ppm, and an unresolved band of decreasing intensity from about -88 ppm to about -112 ppm. There is little intensity in the -112 ppm peak (Q4), which is still large in the MAS NMR spectrum of the same sample. These results confirm that the silicon sites in the developing reaction products are

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closely associated with protons, i.e. that the phases formed are hydrated, but that the species involved do not produce a good cross-polarization effect at room temperature.

NMR Results 1"orBeltane Opal and Ca-Bearing Beltane Opal Reaction Products Beltane opal is known to be a multiphase mixture of opal-CT, 10 or 15% quartz, a few % kaolinite, and traces of other phases. The bulk material is not homogeneous, and different samples have slightly different compositions. The MAS NMR spectrum for the Beltane opal starting material (Figure 1) reflects this multiphase composition. The largest peak is the broad one at about -112 ppm due to the Q4 sites in opal-CT. In addition, there is a shoulder at about -109 ppm, assignable to cristobalite-like Q4 sites, a narrow peak at -107.8 ppm due to the Q4 sites in quartz, a broad shoulder at about 103 ppm attributable to Q3 sites (with 1 OH) in the opal-CT and a small peak at -92 ppm attributable to the Q3 sites of kaolinite. Thus, this spectrum is consistent with the expected mineralogical composition of this sample. The Beltane opal also reacts rapidly, and there is significant signal for reaction product even by 7 days. As indicated in Figure 3, the reaction consumes the opal CT component responsible for the -112 ppm peak at a rate even faster in the early stages than for the Nevada opal. Again, however, the reaction appears to cease by about 2 months, leaving a similar percentage of unreacted Q4 sites. The nature of the reaction products is similar to those produced from the Nevada opal. The Q2 peak at -85 ppm quickly develops and is retained indefinitely. The QI peak at -79 ppm again increases slowly for several months but subsequently decreases. A small but sharp peak at -82 ppm is formed quickly and persists. A broad peak centered at -92 to -95 ppm develops, and at later stages a narrow peak at -97 ppm develops. The original Q4 peaks and the original Q3 peak for kaolinite remains throughout but at reducing intensities. CPMAS spectra were collected for Beltane opal reaction mixes at room temperature, but also have poor signal to noise ratios.

X-Ray Diffraction and NMR Results: Ca-Free Systems The reaction mixes prepared using Nevada and Beltane opals without Ca(OH)~ did not produce crystalline reaction products that could be detected by XRD.

29Si MAS NMR spectra of the Ca-free reaction mixes of the Nevada opal are shown in Figure 5 for ages of 10, 71, and 170 days. The rate of reaction is much less than in the experiments with Ca(OH)2. The major -112 ppm Q4 peak for the opal starting material persists throughout, and there is also some unresolved signal at the ca -103 ppm position suggesting partial retention of the Q3 sites in the opal. The peaks for the reaction products vary with time. At 10 days there are broad peaks at -95 ppm, 88 ppm, and -79 ppm assignable to Q3,Q~, and Q1 sites respectively. At 71 days these have largely disappeared, and only a -103 ppm peak is present besides the residual -112 ppm peak.

820

X.-D. Cong et al,

-112

/(~ ~0~ days'

MAS

71 days,MAS

-9S.5

-88_/

-

~

10

days,MAS

-99

10 days,CPMAS

Vol. 23, No. 4

At 170 days there are peaks for the reaction products at about -98.5 and -88 ppm. The 98.5 ppm peak is probably due to overlap of signal for Q3 (1OH) sites in the opal and Q3 sites in the gel, and the -88 ppm peak to Q2 sites. An important difference between these gels and those produced when Ca(OH)2 is present is that the relatively narrow peaks at 85 and -79 ppm are not present here. The CPMAS results show that the new peaks are due to hydrous reaction products. Figure 5 shows a room temperature CPMAS spectrum for a 10-day old reaction mixture taken at a short contact time of 900/~s. The opal peak at -112 ppm is relatively suppressed, and there is a broad peak centered at -98 ppm and a shoulder near -90 ppm. The large, broad peak probably results from the overlap of signal from ( f sites in the reaction product at -95 ppm and the 103 ppm peak of the Q~(IOH) sites of the starting opal. The effectiveness of the crosspolarization process at room temperature is apparently greater than for the Ca-bearing reaction mixtures, possibly implying more SiOH sites in the Ca-free products.

The Ca-free reaction mixes with Beltane opal (not shown) are similar to those with the Nevada opal. The kaolinite and Figure 5. 29Si MAS NMR spectra of reaction quartz peaks persisted indefinitely, and the mixes of Nevada opal with KOH solution at 112 ppm peak for opal-CT is not reduced The same non-monotonic several ages, and a 29Si CPMAS spectrum at 10 significantly. behavior with time is found as with the days. Nevada opal. Broad but definite reaction product peaks appear in the 10 day specimen (here at -96, -87, and -79 ppm), do not occur at 71 days, and only as a broad peak centered at around -95 ppm at 170 days. ,

I

I

-80

'

i

l

I

-100

I

I

I

|

-120

1

I

ppm

Discussion

It is clear that 29Si MAS NMR spectroscopy can follow the progress of the reaction of silica, Ca(OH)2 and dissolved alkalis by detecting both the reduction in intensity of the peaks for the original silica and the increase in intensity of the peaks for the reaction products. The silicon sites in the starting materials are mostly fully cross-linked (Q~) species. The products inevitably contain sites with a smaller average number of bridging oxygens per silicon site. For the peaks due to reaction products, there seems to be no question about the assignment of the ca -85 ppm peaks to Q2 sites and the ca -79 ppm peak to Q1 sites. The

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essentially identical chemical shifts of these peaks to those found for hydrating cement systems suggests that many of the silicon sites in the gels produced here in the presence of Ca(OH)2 have quite similar local environments to the silicon sites in calcium silicate hydrate gels. That is, their next-nearest neighbor cation is Ca. The broad peak centered at -92 to -95 ppm in both the Ca-free and Ca-containing samples is readily assignable to Q3 sites. Such sites are not present in Ca-silicate hydrate (7,8), but are expected in K-containing systems, large, low-charge cations such as K bond less strongly to the oxygens of Si04 tetrahedra than do smaller, higher-charge cations such as Ca. Thus, large cations are normally found in more polymerized structures (Dent Glasser, 12). The products formed in the absence of Ca(OHh are clearly different from those formed in its presence. The most important difference is the lack of the Q~ and Q2 peaks at -79 and -80 ppm, which are clearly associated with Ca. For the Ca-free samples, the peaks at ca -95 ppm are assignable to Q3 sites, the peak at ca -88 ppm to Q2 sites, and the peaks at ca -80 ppm to QI sites. It is also possible that some reaction product Q4 sites might have been present but unresolved from the other Q4 sites. The spectra for the Ca-free products do not necessarily reflect all of the reaction products formed. Struble (13) and others have shown that in the absence of calcium, alkali hydroxides react with reactive silicas to produce mostly soluble species, which may have been lost during the acetone washing of these specimens prior to drying and examination. The chemical shifts observed for our K-bearing but Ca free products for the different Q" species are more negative than corresponding shifts observed for anhydrous K-silicate glasses by Maekawa et al (14). These authors reported ranges of -102 to -105 ppm for Q4 sites, -89 to 94 ppm for Q3 sites, -76 to -83 ppm for Q2 sites, and about 71 ppm for Q~ sites. This difference is probably due to the hydrous nature of the present gel products. Yang and Kirkpatrick (15) have shown that hydration of glasses causes more negative 295i chemical shifts (without depolymerization), probably due to weaker interaction of the cations with the Si tetrahedra thus increasing the mean Si-O-Si bond angles. The role of water or other proton-bearing species in the gels is critical to understanding their chemical and physical behavior. Various proton-bearing species might exist in these gels: OH groups attached to silica tetrahedra, OH groups coordinating Ca or K counterions, and water molecules in a variety of environments (bound, zeolitic, and "free"). Infrared spectra do show the presence of bonded OH groups in alkali silica gels, and indeed some of the Q3, Q2, or Q1 sites could have one or more attached OH groups. However, the behavior of our Ca-bearing samples under cross polarization conditions (poor signal/noise ratio at room temperature) suggests that many of the protons are in water molecules that are in rapid motion at room temperature. Protons in such water molecules would not be very effective in cross-polarization. Hydrous amorphous silicas that have extensive Si-O-H linkages are known to cross-polarize effectively at room temperature (10, 1i). If protons and silicon atoms are in relative motion at frequencies of the order of kHz, the motion causes decoupling of the spin systems (16). The much improved cross-polarization for the spectrum taken at -80°C (Figure 3) is consistent with a much reduced rate of motion at that temperature. It should be noted that similarity between the reaction products produced in these experiments and reaction product gels produced by alkali silica reaction in concrete is not clearly established. The usual concrete environment surrounding reactive silica includes ready access

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to calcium hydroxide, but not necessarily to calcium hydroxide as finely divided as the precipitated reagent chemical used here in intimate mixture with the silica. In addition, to facilitate reaction, the reactive silica here was ground to a particle size considerably less than that of the sand or coarse aggregate sized grains of concrete.

Conclusions 1. 29Si MAS NMR can be used to follow the rates of formation of alkali silica and calcium-alkali silica reaction gels from mixtures of reactive silica "aggregates", alkali hydroxides, and calcium hydroxide. 2. The observed reaction rates of Nevada opal (a relatively pure opal-CT) and Beltane opal (a relatively inhomogeneous material containing quartz and some kaolinite in addition to opal-CT) are similar to each other and much more rapid than that of cristobalite. 3. In the experiments with calcium hydroxide in excess, the predominantly Q4 Si polymerizations of the starting materials rapidly give way to Q2 and Q' polymerizations similar to those in Ca-silicate hydrate and Q3 polymerizations not present in CSH. The Q~ sites are in part transient. 4. The similarity of the positions and widths of the Q2 and Qt peaks for our Ca-bearing gel reaction products to those of calcium silicate hydrates derived from the hydration of cement minerals (ca -85 and -79 ppm) suggests that similar local environments with Ca next-nearest neighbor exist in both materials. 5. The spectra of gels obtained in the absence of calcium hydroxide contain peaks at somewhat different positions attributable to Q3, Q2, and Q1 sites. However, the peaks observed vary in a non-monotonic manner with age, and further investigation is needed. 6. Evidence from IH-29Si cross-polarization spectra indicates that many of the protons present in air-dried calcium-alkali silica reaction products are likely to be in rapid thermal motion at room temperature. This observation suggests that the protons occur primarily in relatively loosely bound water molecules and not primarily as OH ions bonded to Si nuclei.

Acknowledeements This paper is a contribution from the Center for Advanced Cement Based Materials, a Science and Technology Center supported by the National Science Foundation and consisting of research groups at Northwestern University, the University of Michigan, and the National Institute of Standards and Technology as well as Purdue University and the University of Illinois. The present work is an example of inter-institutional collaboration fostered by the Center, and we are indebted to the National Science Foundation for its support.

References 1. R. J. Kirkpatrick, "MAS NMR Spectroscopy of Minerals and Glasses," in Reviews in Mineralogy, 18, Spectroscopic Methods in Mineralogy and Geology, pp. 341-403 (1988).

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2. R.S. Barneyback, Jr., "Alkali Silica Reaction in Portland Cement Concrete," Ph.D. thesis, Purdue University, 352 pp (1983). 3. W . A . Gutteridge and D. W. Hobbs, "Some Physical and Chemical Properties of Beltane Opal Rock and its Gelatinous Alkali Silica Reaction Product," Cem. Conc. Res. 10, 183 (1980). 4. J. S. Lumley, "Synthetic cristobalite as a reference reactive aggregate," Proc. 8th Internation. Alkali Conf., Kyoto, Japan, 561-566 (1989). 5. B. L. Phillips, R. J. Kirkpatrick, and A. Putnis, "Si,AI Ordering in Leucite by HighResolution 27A1MAS NMR Spectroscopy," Phys. Chem. Minerals, 16, 591 (1989). 6. J. V. Smith and C. S. Blackwell, "Nuclear Magnetic Resonance of Silica Polymorphs," Nature, 303, 332 (1983). 7. J. F. Young, "Investigations of Calcium Silicate Hydrates Structure Using Silicon-29 Nuclear Magnetic Resonance spectroscopy," J. Amer. Ceram. Soc., 71, C-118 (1980). 8. M. Grutzeck, A. Benesi, and B. Fanning, "Silicon 29 Magic Angle Spinning Nuclear Magnetic Resonance Study of Calcium Silica Hydrates," J. Amer. Ceram. Soc. 72, 665 (1989). 9. S . J . Adams, G. E. Hawkes, and E. H. Curzon," A Solid State 29Si Nuclear Magnetic Resonance Study of Opals and Other Hydrous Silicas," Amer. Mineral., in press (1992). 10. D. W. Sindorf and G. E. Maciel, "29Si NMR Study of Dehydrate/Rehydrated Silica Gel Using Cross Polarization and Magic-Angle Spinning, J. Am. Chem. Soc., 105, 1487 (1983). i1. C. J. Brinker, R. J. Kirkpatrick, D. R. Tallant, B. C. Bunker and B. Montez, "NMR Confirmation of Strained "Defects" in Amorphous Silica, J. Non-Cryst. Solids, 99, 418 (1988). 12. L. S. Dent Glasser, "Non-existent Silicates," Z. Krist., 149, 291 (1979). 13. L. J. Struble, "The Influence of Cement Pore Solution on Alkali Silica Reaction," Ph.D. thesis, Purdue University, 260 pp. (1987). 14. H. Maekawa, T. Maekawa, K. Kawamura, and T. Yokokawa, "The Structural Groups of Alkali Silicate Glasses Derived from 29SiMAS-NMR," J. Non-Cryst. Solids, 127 53 (1991). 15. W.-H.A. Yang and R. J. Kirkpatrick, "Hydrothermal Reaction of Albite and a Sodium Alumniosilicate Glass in a Solid-State NMR Study," Geochim. Cosmochim Acta, 53, 805 (1989). 16. C. S. Yannoni, "High Resolution NMR in Solids: The CPMAS Experiment," Acc. Chem. Res. 15, 201 (1982).