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Synthesis of Tobermorite: A Cement Phase Expected Under Repository Conditions

Sue I. Martin

This paper was prepared for submittal to the American Nuclear Society's

International High Level Radioactive Waste Management Conference Las Vega, NV May 1 - 5,1995

November 1994

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Synthesis of tobermorite: A cement phase expected under repository conditions

Sue I. Martin

Introduction

In this study I have synthesized tobermorite, Ca&016(OH)2.4H20, a principal

crystalline phase expected to form in cementitious materials subjected to elevated

temperatures in a potential nuclear waste repository. Fluids interacting with these materials

may have a profound effect on the integrity of the waste package and on transport of

radionuclides. At ambient temperature, Portland cement reacts with water to form an

amorphous calcium-silicate-hydrate (C-S-H) gel.1 At elevated temperatures, crystalline

phases of various hydration states form. The C-S-H system has not been well

characterized at elevated temperatures up to 250 OC, which has been considered a

bounding temperature for the potential Yucca Mountain repository. Physical, chemical,

and thermodynamic data for these cement minerals that are predicted to be stable at these

temperatures must be obtained from synthetic or natural samples to help predict fluid

chemistry. For some of these minerals natural samples are difficult to obtain in sufficient

quantity and purity. Therefore, monomineralic phases must be synthesized in order to

unambiguously define their behavior. The synthetic or natural phases will be characterized

as part of a comprehensive study to define the behavior of cementitious materials in a

repository environment?

Materials and Methods

The starting materials for synthesis were CaO and an aqueous suspension of fumed Si@

(Cab-o-sperse@) in a molar ratio of 5 6 . The CaO was prepared from CaC@ heated to

1050 OC for >4 hours. The highly reactive Si02 had a surface area of 100 m2/g. In one

case hydrochloric acid (HC1) was added before mixing with CaO to neutralize the Si02

suspension which had a pH of 9.8. The constituents were mixed in an argon filled glove

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box to inhibit C02 uptake. C02 free water was added in excess to obtain a homogeneous

sougel. The gels were autoclaved at 90 and 120 O C in pressure bombs. Individual bombs

were sampled multiple times over the total length of the run in the argon glove box. Post

run treatment of the aliquots included washing the product with milli-Q water and ethanol

before drying in desiccators. To hinder calcite formation and remove excess CaO in the

product some of the reacted samples were titrated with HC1 to a pH of 7-8.

Results

Powder x-ray diffraction 0) was used to identify the mineralogical phases. Two types

of run products were obtained, a mixture of 1.1 and 1.4 nm tobemonte at 90 OC, and a

1.1 nm tobermorite at 120 OC. For the mixed phase runs the intensity of the 1.4 nm peak

decreased and the 1.1 nm peak increased over time as the tobermorite became less hydrated

(Fig 1). Mixed phase samples heated and dried at 100 OC produced a 1.1 nm tobermorite.

X-ray diffraction of the higher temperature run indicated a single phase, 1.1 nm tobermorite

(Fig 1). In general, tobermorite became more crystalline over time and at higher

temperature.

1.

Scanning electron microscopy was used to determine particle morphology.

Photomicrographs of the reacted material showed an aggregation of platy particles similar

to those shown by Suzuki and Sinn? No distinct morphological differences were noted

between the 1.1 nm tobermorite and the mixed 1.1 and 1.4 nm tobermorite.

Measures were taken to preclude calcite formation but calcite was detected in the product

and was probably due to unreacted CaO reacting with atmospheric C02 @H measurements

of the run products were high, 11.3). The formation of calcite was not instantaneous. X-

ray diffraction patterns indicated an increase in calcite concentration in the longer runs and

was most pronounced in the longest run of 161 days. Although aliquots were taken in the

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argon glove box, they were cooled outside of the box in the laboratory. Carbon dioxide

may have diffused through the Teflon lined bombs into the run product.

Carbon dioxide concentrations were measured with a C@ analyzer and indicated equivalent

CaC03 concentrations of up to 6%. Calcite concentrations were the highest in samples

exposed to atmospheric C02 for longer periods of time. Samples titrated with HC1

immediately after synthesis did not contain calcite, and XRD analysis of shelved samples

containing calcite indicated that the calcite was removed from the sample with acid titration

without affecting the tobermorite. The volume of HCl added to reduce the pH to

approximately 8 was equivalent to an excess CaO concentration of approximately 8%,

which correlates to a CaCO3 concentration of approximately 14% if completely reacted.

,1.1 nm

-- L calcite- I

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\ 120 'C, 57 days

I J

I 90 'C, 161 days J A L

2 12 22 32 42 52 Degrees, 2-9

Figure 1. XRD patterns of tobemorite run products; the 90 OC XRD patterns show a peak shift from 1.4 to 1.1 nm tobermorite with time; the 120 OC XRD pattern shows only a single phase tobermorite; crystallinity increased with longer run times and higher temperame.

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Conclusions

Fluids interacting with the large volume of cementitious material to be used in the

construction of a potential nuclear waste repository at the Yucca Mountain Site may impact

waste package performance. To predict fluid composition, thermodynamic, kinetic, and

sorption properties of pure cement mineral phases must be obtained first.

I have developed techniques for consistent synthesis of pure, well crystallized tobermorite,

a cement phase expected to form under repository conditions. The run products were

titrated with HC1 to remove excess CaO to assure and preserve their purity. X-ray

diffraction analysis indicated that the titration procedure did not affect the crystal structure

of the tobermorite. The pure tobermorite phase is now available for further

thermodynamic, kinetic, and sorption studies needed to determine the effects of cements on

radionuclide solubility and transport in a repository environment.

Acknowledgement

Prepared by Yucca Mountain Site Characterization Project (YMP) participants as part of the

Civilian Radioactive Waste Management Program. The Y M P is managed by the Yucca

Mountain Site Characterization Project Office of the U.S. Department of Energy, Las

Vegas, Nevada. Work performed under the auspices of the U.S. Department of Energy by

Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.

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References

1. Taylor, H.F.W. (1990) Cement Chemistry, Academic Press, London and San

Diego, 123.

2. Bruton, C.J., A. Meike, B.L. Phillips, S. Martin, and B.E. Viani. (1993)

Thermodynamics and structural characteristics of cement minerals at elevated

temperatures, Proceedings of Focus '93 Conference, Las Vegas, NV, 150.

3. Suzuki, S. and E. Sinn. (1993) 1.4 nm tobermorite-like calcium silicate hydrate

prepared at room temperature from Si(OH)4 and CaC12 solutions, J. of Mat. Sci.

Let., 12, 542.

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