Characterization of Pozzolanic Cement for Radioactive Waste Disposal

UNLV Retrospective Theses & Dissertations

1-1-1995

Characterization of pozzolanic cement for radioactive waste disposal

James Alan Kappes University of Nevada, Las Vegas

Follow this and additional works at: https://digitalscholarship.unlv.edu/rtds

Repository Citation Kappes, James Alan, "Characterization of pozzolanic cement for radioactive waste disposal" (1995). UNLV Retrospective Theses & Dissertations. 539. http://dx.doi.org/10.25669/4jz5-dnvw

This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself.

This Thesis has been accepted for inclusion in UNLV Retrospective Theses & Dissertations by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. INFORMATION TO USERS

This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer.

The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed through, substandard margins, and improper alignment can adversely afreet reproduction.

In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion.

Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book.

Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order.

A Bell & Howell Information Company 300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA 313/761-4700 800/521-0600

CHARACTERIZATION OF POZZOLANIC CEMENT

FOR RADIOACTIVE WASTE DISPOSAL

James A. Kappes

A thesis submitted in partial fulfillment of the requirements for the degree of

Master of science

Mechanical Engineering

Department of Mechanical Engineering university of Nevada, Las Vegas December 1995 UMI Number: 1377640

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI 300 North Zeeb Road Ann Arbor, MI 48103 The Thesis of James A. Kappes for the degree of Master of science in Mechanical Engineering is approved.

Chairman, Robert F. Boehm, Ph.D.

Examining com m ittee Member, Robert L. Skaggs, Ph.D

Examining com m ittee Member, David Stahl, Ph.D.

1/ CU d Examining com m ittee Member, William c. culbreth, Ph.D.

f , ( c ^ C ______Graduate Faculty Representative, clay crow, Ph.D.

interim Dean of the Graduate college, Cheryl L. Boyles, ed .d .

Material characterization studies were conducted to analyze the chemical and physical properties of pozzolanic cement. The principle objective of this research was to determine whether pozzolanic cement is a suitable material for permanently encasing high-level radioactive waste (HLW) and spent-nuclear fuel (SNF) in disposal canisters. This particular cement was produced by mixing lime or

Portland cement with volcanic tuff, a natural pozzolanic material extracted from Yucca Mountain, Nevada.

portland-pozzolan and lime-pozzolan mixtures were created, analyzed and tested for compressive strength, in addition, the pH of several mixtures was monitored and the values recorded periodically over the first eight days of curing. Particle size (coarse vs fine), crystal structure (welded vs nonwelded tuff) and composition were all varied to study the interactive effects on the pozzolanic reaction between the pozzolan and lime. The pozzolanic material varied from approximately

90% crystalline silica with the welded tuff to over 95% crystalline silica with the unwelded tuff from Yucca Mountain. pozzolanic cement is commonly used in Europe for structural

applications and has been used, to a limited extent, in the united states

both as a structural material and as a solidifying agent for low-level

radioactive waste. The application of pozzolanic cement encapsulation

for disposal of snf in the proposed geologic repository at Yucca

Mountain is a novel idea that could effectively utilize the natural

resources of the proposed geologic repository. A large amount of

volcanic tuff would no doubt be displaced during excavation of the

site. The use of pozzolanic cement would serve to utilize the displaced

tuff and produce a biocompatible wasteform that could potentially

provide an additional margin of safety for the engineered barrier

system by limiting the migration of radionuclides.

The results indicated that the particular volcanic tuff from Yucca

Mountain, Nevada, is not a good pozzolan because it has poor reactivity with lime. The compressive strength of portland-pozzolan and lime-

pozzolan mixtures were, on average, significantly lower than that of ordinary Portland cement. The data showed a strong correlation between the particle size of the pozzolan and the reactivity of the cement paste, in most of the coarse grained mixtures, for example, the cement never solidified, others partially solidified, but crumbled

iv with less than 100 psi of compressive stress. With the fine-grain mixtures, however, both the welded and nonwelded lime-pozzolan mixtures produced a solid cement matrix.

in theory, pozzolanic cement is an excellent engineering material with a low heat of hydration, better durability, and a lower permeability than ordinary Portland cement, in reality, however, the pozzolanic volcanic tuff from Yucca Mountain is mostly non-reactive, and would not (without extensive grinding to an extremely small particle size) make a good pozzolanic cement for encapsulating radioactive waste.

v TABLE OF CONTENTS

ABSTRACT...... Hi

LIST OF FIGURES...... Vlii

ACKNOWLEDGMENTS...... X

CHAPTER 1 INTRODUCTION...... 1

CHAPTER 2 POZZOLANIC MATERIALS...... 10 History of Pozzolanic cem ent ...... 13

CHAPTER 3 VOLCANIC TUFF SAMPLES FROM YUCCA MOUNTAIN...... 18 Thin section O bservations...... 20

CHAPTER 4 CEMENT MIXTURES & MATERIALS...... 25 Portland cement ...... 26 Pozzolan Type ...... 27 Particle Size...... 27 composition ...... 29 Analysis of cem ent Thin sections ...... 31

CHAPTER 5 PHYSICAL AND CHEMICAL PROPERTIES OF CEMENTITIOUS MIXTURES...... 41 Cement chemistry...... 41 portland-pozzolan cem ents ...... 42 Durability...... 47 Effect of crystal structure and Reactivity ...... 48 solubility of Silica in w a te r ...... 49 pH E ffect...... 51 particle size Effect...... 54 Heat of Hydration ...... 56

CHAPTER 6 COMPRESSIVE STRENGTH...... 60

CHAPTER 7 DISCUSSION ...... 70

V i Permeability...... 70 Radionuclide Leaching ...... 70 other Factors ...... 71

CHAPTER 8 CONCLUSIONS ...... 73

APPENDIX I PULVERIZING PROCEDURE...... 77

APPENDIX II SIEVE ANALYSIS...... 78

APPENDIX III COMPRESSIVE STRENGTH TEST PROCEDURE...... 80

BIBLIOGRAPHY...... 81

vii LIST OF FIGURES

Figure 1 Heat output Per snf Assembly...... 7 Figure 2 Heat output Per canister ...... 7 Figure 3 Colosseum in Rome ...... 15 Figure 4 Photomicrograph of welded Tuff @ 40x ...... 22 Figure 5 Photomicrograph of welded Tuff @ 200x ...... 22 Figure 6 Photomicrograph of unwelded Tuff @ 40x ...... 23 Figure 7 Photom icrograph of unw elded Tuff @ 200...... 23 Figure 8 ordinary Portland cem ent (OPC), 40x ...... 32 Figure 9 ordinary Portland cem ent (OPC), 100 x ...... 33 Figure 10 2FU OPC-Pozzolan cem ent, 40x...... 34 Figure 11 2FU OPC-Pozzolan cem ent, 100X...... 34 Figure 12 2FU OPC-Pozzolan cem ent, 200x...... 35 Figure 13 3FU OPC-Pozzolan cem ent, 40x...... 36 Figure 14 3FU OPC-Pozzolan cem ent, 100X...... 36 Figure 15 3FU OPC-Pozzolan cem ent, 200x...... 37 Figure 16 3FW OPC-Pozzolan cem ent, 40x ...... 37 Figure 17 3FW OPC-Pozzolan cem ent, 100X...... 38 Figure 18 3FW OPC-Pozzolan cem ent, 200x...... 38 Figure 19 4FU Lime-Pozzolan cement, 40x ...... 39 Figure 20 4FU Ume-Pozzolan cement, ioox ...... 39 Figure 21 4FU Lime-Pozzolan cem ent, 200x ...... 35 Figure 22 compressive strength of Pure OPC Paste as a Function of curing Time ...... 42 Figure 23 Relation Between Permeability and capillary Porosity of cement Paste ...... 43 Figure 24 Relation Between Permeability and water/cement Ratio for cement Paste ...... 45 Figure 25 Effect of Curing Age and Pozzolan % on calcium Hydroxide c o n te n t...... 48 Figure 26 Activities of Aqueous Quartz and Amorphous Silica in Equilibrium a t 25°c ...... 50 Figure 27 Measured pH values vs Time for Pozzolanic cement 53 Figure 28 Effect of substituting Natural Pozzolan on the Heat of Hydration of Portland cem ent ...... 58

viii Figure 29 compressive strength of #4 and §6 Lime-Pozzolan M ixtures ...... 64 Figure 30 compressive strength of coarse Tuff Samples w ith 0.6w/c Ratio ...... 65 Figure 31 compressive strength of Fine Tuff Samples w ith 0.6w/c Ratio ...... 66 Figure 32 compressive strength of coarse vs. Fine unwelded Tuff sam ples (0.6 w/C)...... 67 Figure 33 compressive strength comparison for 31-Day vs. 108-Day Aging (0.6 W/C) ...... 67

ix ACKNOWLEDGMENTS

I would like to express my deepest thanks to my wonderful wife,

Lisa, who endured the long hours and late nights necessary to finish graduate school. Without your encouragement and support, I would have never achieved this.

Thanks to Dr. Skaggs, my advisor and mentor, who pointed me in this direction and was always there to talk or lend a helping hand.

Dr. culbreth, your enthusiasm, support, and guidance over the last couple years was sincerely appreciated and will not soon be forgotten.

I’d like to extend a special thanks to Dr. Clay Crow and his colleagues in the Geosciences Department, who provided much of the facilities and expertise needed to carry out the research and analysis, and bent over backwards to accommodate a "lowly engineer".

x CHAPTER 1

INTRODUCTION

The purpose of this work was to determine the suitability of

pozzolanic cement as a solidifying agent for encapsulating and

stabilizing high-level radioactive waste (HLW) and spent-nuclear fuel

(SNF). in particular, this work was undertaken to analyze the potential

for utilizing natural volcanic tuff from Yucca Mountain, Nevada, to

create a pozzolanic cement with chemical and physical properties

superior to those of ordinary Portland cement (OPC). Each of the following properties was investigated and analyzed:

• compressive strength

• Microstructure

• Permeability

• Pozzolanic Activity

Pozzolanic cement is produced by mixing a natural (e.g. volcanic tuff) or artificial (e.g. fly ash) pozzolan with either OPC or calcined lime, various mixtures of both OPC-pozzolan and lime-pozzolan cements were studied. Theoretically, the active silica and alumina from the pozzolan will react with calcium hydroxide, Ca(OH)2, from hydrated lime or hydrated OPC in what is called the "pozzolanic reaction." The result is

1 2

a slow-hardening, refined-grain cement which is typically more durable

and impermeable than OPC1. For radioactive waste disposal

applications, a cement with low permeability and increased durability is

very desirable for stabilizing long-lived radionuclides and providing a

long-term barrier to radionuclide release.

Pozzolanic cement could potentially be used as a solidification

agent to encapsulate and immobilize high-level radioactive waste and

spent-nuclear fuel in the proposed geologic repository currently being

studied at Yucca Mountain, Nevada. As part of the Nuclear waste Policy

Amendments Act (NWPAA) of 1987, congress directed the Department

of Energy (DOE) to study only one site for the proposed high-level waste

geologic repository. The chosen site was Yucca Mountain, located approximately 100 miles Northwest of Las Vegas.

cement encapsulation of radioactive waste is a viable option which is widely used in the UK and Europe for solidifying low-level (LLW) and intermediate-level radioactive waste (ILW), and has been explored to a limited extent for LLW disposal applications in the united states.2 cement encapsulation of hlw could potentially provide the following benefits:

1) isolate mobile radionuclides and limit migration to the accessible environment by binding them in the cement matrix. 2) Minimize potential leaching of radionuclides by creating an impermeable barrier.

3) Provide long-term physical and chemical stabilization of the w aste.

4) Chemical and physical properties of cement are weii known.

5) cement imparts good shielding properties and radiation resistance.

Pozzolanic cement has several unique properties that

distinguish it from OPC or commercial concrete, and lend credibility to

its proposed use as an encapsulant material for nuclear waste disposal.

as previously mentioned, pozzolanic cement has a slow curing rate and,

as a result, a lower heat of hydration than OPC. some forms of

pozzolanic cement are reported to have a heat of hydration 60% lower than OPC3. A low heat of hydration is an important feature since heat will also be generated inside the disposal canisters due to radioactive decay of the waste.

Pozzolanic cements also have excellent long-term durability, a key feature since the radioactive waste must be isolated for up to

10,000 years. Ten thousand years is the approximate length of time for long-lived radionuclides in SNF to decay to background levels. The improved durability of pozzolanic cement is generally attributed to the 4 increased volume of hydrous calcium silicate (C-S-H) which is a product of the reaction between a pozzolan and calcium hydroxide:

Pozzolan(SiOl) + Ca(OH)2 + H lO slow >C-S-H

The chemical composition of C-S-H is somewhat variable, but it contains lime (CaO) and silicate (Si02) in a ratio on the order of 3 to 2.a The C-S-H particles are so minute that they can only be seen under an electron microscope. As a result, it is often referred to as an amorphous “gel." in hardened cement paste, these particles form a dense, bonded matrix between the other crystalline phases and the un-hydrated cement grains. The C-S-H content is extremely important in cement because it’s responsible for setting, hardening, and strength development.5

By comparison with OPC, pozzolanic cements typically have smaller pores, less free water and less free lime in the cement matrix.

All of these factors improve the durability of the cement. Free lime is the major cause of cement degradation because it’s soluble in water and can leach out of the matrix, leaving weak porous voids.6

Many of the ancient creek and Roman buildings and structures provide a natural analog for pozzolanic cement and lend proof to its long-term durability. The Roman Aqueduct and the coliseum in Rome are good examples of pozzolanic structures that have proven to be extremely durable and are still structurally intact after 2,000 years of weathering7. The ancient Chinese also used lime-pozzolan mortars to build the Great wall, and the ancient Egyptians reportedly used calcined gypsum and pozzolans to build the Great Pyramid of Cheops.

Another benefit is that the permeability of hydrated cement can be significantly reduced with the addition of pozzolanic material.

Permeability and leach resistance are important characteristics for a filler material which must encapsulate radioactive waste and retard the migration of radionuclides for thousands of years.

At the present time, cement is not being investigated as a filler material for hlw disposal applications. This is due primarily to the

Multi-Purpose canister (MPC) designs that are being pursued by the doe 's Office of Civilian Radioactive waste Management (OCRWM). With the current MPC designs, cem ent is not a feasible filler material, despite the obvious containment advantages, because of its low thermal conductivity. The high thermal load MPC's, for example, are designed to hold up to 24 pressurized water reactor (PWR) spent fuel assemblies or 40 boiling water reactor (BWR) spent fuel assemblies8. A typical pw r spent fuel assembly weighs about 650 kg and is comprised of unused fuel, fission products, activation products, actinides and daughter 6 products. A single canister with 24 pwr assemblies is designed to contain in excess of 17 tons of waste! The net result is that a significant amount of heat is generated inside the canisters and heat transfer becomes a major issue9. Heat generation for snf is particularly important during the first 30 years out of the reactor (see Figure 1), when the relatively short-lived radioisotopes Strontium-90 and cesium-

137 significantly increase the decay heat. Encapsulating the disposal canisters with cement, which has a low thermal conductivity of approxim ately 2 w/M-°c, does not allow sufficient heat transfer to keep the maximum temperature within the required thermal limits. Figures

1 and 2 below illustrate the amount of heat generated by a single 10 year old pwr fuel assembly, and for canisters housing multiple assemblies.10 Watts 1000 2000 3000 4000 6000 5000 7000-S' I 100 -/' 0 4 1 120 60-'' -' 0 8 1 -' - - * Figure 1 - Heat Output Per SIMFPer Output 1- Heat Assembly Figure Figure 2 - Heat Output Per Canister Per Output 2 - Heat Figure 10 5 Years Out of Reactor of Out Years Years Out of Reactor of Out Years 0 30 10 050 30 16-PWR■ 1-PWR■ 21-PWR□ 04-PWR 100 7 8

in spite of the DOE concentrating its efforts on the MPC canisters, pozzolanic cement encapsulation may eventually find a use in the future for disposal of high level nuclear waste, or perhaps with low level waste, mixed waste, or solid toxic waste that cannot be chemically reduced, cement encapsulation provides a low cost, effective barrier to retard the migration of radionuclides and also serves as an excellent shield for penetrating neutron and gamma radiation. Pozzolanic cement in particular offers a unique opportunity to utilize the natural resources of Yucca Mountain and coincidentally serve a useful function in the permanent disposal of high-level radioactive waste at the proposed geologic repository. 9

CHAPTER 1 NOTES

1 Mehta, Kumar, Concrete Structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, inc., 1986.

2 Boynton, R.S., Chemistry and Technology of Lime and Limestone, 2nd Edition, John Wiley & Sons, inc., 1980.

3 Kosmatka, Steven H. & Panarese, William c., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, 1988.

4 Kosmatka, Steven H. & Panarese, William c., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, pg. 3,1988.

5 Kosmatka, Steven H. & Panarese, William C., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, pg. 3,1988.

6 Davis, Raymond E„ "A Review of Pozzolanic Materials and Their use in Concretes," American Society for Testing Materials, San Francisco, Calif., October 10-14,1949, pg. 4.

7 Neville, A.M., Brooks, J.J., Concrete Technology, Longman scientific & Technical, Essex, England, 1987.

8 Multi Purpose Canister, Conceptual Design Report, OCRWM, 1994.

9 B&w Fuel Company, Design Drawings, 1993.

10 R.Scott Moore and Karl J. Notz, LWR Radiological Database, CDB_R/V1.1, QA-M04- 2002.021 .C077, U.S. Departm ent of Energy OCRWM, July, 1992. CHAPTER 2

POZZOLANIC MATERIALS

A pozzolan is defined as any siliceous or alumino-siliceous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of water, chemically react with calcium hydroxide to form compounds possessing cementitious properties.11 Pozzolans are separated by astm c 618 into several categories including Class N, class F, and Class c.

Class F artificial pozzolan, which includes fly ash with pozzolanic properties, is by far the most commonly used pozzolan in cement. Fly ash is a fine powder which results from the combustion of pulverized coal in fossil-fueled power generation plants. During combustion, the coal's mineral impurities (i.e. clay, feldspar, quartz and shale) fuse in suspension and are carried away in the exhaust gas. The fused material cools and solidifies into spherical particles called fly ash. Fly ash is highly reactive with hydrated lime (Ca(OH)2) for two reasons: 1) its structure is almost entirely amorphous (versus crystalline); and, 2) the finely divided, spherical particles provide a large surface area to react

10 11

with. Ground granulated blast-furnace slag and silica fume are also

commonly used in the united states as pozzolanic admixtures to

partially replace cement. Most of the research and available literature on pozzolanic materials is focused on the artificial pozzolans. use of artificial pozzolanic cem ent has increased rapidly in recent years due to concerns with energy conservation and cost, utilization of fly ash as a cement admixture provides an excellent use for material which is currently disposed of as waste, at a significant cost to the producers, in addition, pozzolans can reduce cement costs since inexpensive fly ash or blast-furnace slag is used to partially replace Portland cement.

astm c 618, Class N consists of raw or calcined natural pozzolans including diatomaceous earths, opaline cherts and shales, volcanic tuffs, volcanic ashes or pumicites, and some calcined clays and shales.12

The volcanic tuff used in this study is a natural pozzolan and would be classified as Class N. Natural pozzolans are widely used in

Europe but have not yet caught on in the U.S., in spite of their clear advantages in cost and durability, in Europe, portland-pozzolan mixtures are frequently used for marine and hydraulic structures because of their lower permeability in water and their increased resistance to aggressive waters, in this country, investigations of pozzolans by Bates, Phillips, and wig were begun as early as 1908, and it 12

was shown as early as 1912 that Portland cements containing

pozzolanic materials exhibited certain desirable properties.13

An early example of portland-pozzolan cement use in the united

states is the Los Angeles aqueduct, where 600,000 bbl. was employed

between 1910-1912. in 1935, a portland-pozzolan cement was used to

construct the Bonneville Dam, and in 1942, a pozzolanic cement was

employed in the construction of Friant Dam, both in California.14

More recently in 1977-1978, the use of portland-pozzolan

cement was studied on an experimental, laboratory-scale program for

solidification of high-level radioactive waste sludge at doe 's savannah

River Plant.15 Results of the study indicated that the waste was

chemically compatible with the cement and that the wasteform exhibited excellent properties including: 2,000-3,000 psi compressive strength, low cesium and strontium leachability, and good radiation and thermal stability.

One reason why pozzolanic cements have not achieved commercial success in the U.S. is the fact that short-term strengths are somewhat lower than Portland cement, because of the slower cure rate. The long-term strength (i.e. after one year) of pozzolanic cement, however, will often exceed that of ordinary Portland cement (see

Figure 3). Another postulated reason for the minimal use of pozzolanic 13

cement in this country is the difficulty in determining the rate at which a pozzolan will react with calcium hydroxide. The reaction is not clearly

understood and there is no simple method to determine whether a

pozzolan will be reactive or not, based solely on its composition, it appears that any siliceous material, regardless of its mineral structure or composition, if of sufficiently high fineness will combine with calcium hydroxide at normal atmospheric temperatures; however, for some materials the rate of combination may be very slow16

History of Pozzolanic cem ent

The Romans are generally credited with the discovery of pozzolanic cement. They were the first to discover that when calcined lime was mixed with a siliceous material such as volcanic ash, the mixture would harden underwater. From this discovery came the development of Roman cement, a.k.a. hydraulic lime, made from calcined lime and natural pozzolans.17 The word pozzolan gets its name from the village of Pozzuoli, near Mount Vesuvius in Italy, where extensive deposits of volcanic ash existed.18 The term pozzolanic cement is used to this day to describe cements obtained simply by grinding natural materials at room temperature.19 14

There is some evidence that pozzolan mortars were discovered much earlier than the Romans, however. Tests on an uncovered concrete slab in southern Galilee suggest that the invention of lime- pozzolan cement dates back to the Neolithic period, or 7,000 bc.20

Early writers such as Vitruvius described the preparation of hydraulic mortars from slaked lime, sand and pozzuolana. When a hydraulic mortar was needed for river or harbor works, the recommended proportions were two parts pozzolan to one part lime.

The Romans were masters at the art of properly grinding, proportioning, and mixing the cement ingredients into a homogeneous mixture. The fact that lime-pozzolan cement would harden under water was a tremendous technological advancement for the Romans because it allowed them to build massive canals, bridges and aqueducts that were previously unthinkable. Prior to the discovery of hydraulic lime-pozzolan cement, structures were constructed with stones and mortar, as the Romans discovered, it was much harder to chip rock and stones into the desired shapes than it was to cast cement. The ability to pour cement into a pre-formed mold made it much easier to build large, solid structures and paved the way for the revolution in Roman architecture. Many of the ancient Roman structures were made from

pozzolanic cement, including the coliseum in Rome, the Pont du Gard

near Nimes, the Roman aqueduct, and the ruins at Pompeii. Numerous

Roman monuments such as the bridges of Fabricus, Aemilius, Elius and

Milvius, th e arches of Claudius and Trajan a t ostia and Nero a t Antium, and many maritime works built during the time of the Roman emperors, were constructed with pozzolanic cement mortars, in fact, some are still in use today. The fact that many of these structures are still stable and structurally intact after withstanding the ravages of time for more than 2,000 years is a testament to the performance and durability of pozzolanic cement.

Figure 3 - coliseum in Rome

unfortunately, with the fall of the Roman Empire, much of the pozzolanic cement technology was lost, it wasn't until the late 18th century, when Britain's John smeaton was commissioned to rebuild the

Eddystone Lighthouse off the Cornish coast, that hydraulic cement technology resurfaced.21 smeaton discovered that the best mortar was produced when pozzuolana was mixed with limestone containing a high proportion of clay.

Today, pozzolanic cement is widely used in many European countries for industrial applications as well as radioactive waste solidification, in addition to the performance benefits of pozzolanic cement, there are cost benefits as well. Addition of pozzolanic material as an admixture to Portland cement is typically cheaper than using

100% OPC, since natural and artificial pozzolans are both inexpensive and abundant. 17

CHAPTER 2 NOTES

11 Kosmatka, Steven H. & Panarese, William C., Design and Control of Concrete Mixtures, 13th Edition, Portland cement Association, pg.69,1988.

12 Kosmatka, Steven H. & Panarese, William C., Design and Control of Concrete Mixtures, 13th Edition, Portland cement Association, PG.68,1988.

13 Davis, Raymond E., "A Review of Pozzolanic Materials and Their use In Concretes," American society for Testing Materials, San Francisco, Calif., October 10-14,1949, pg. 3.

14 Davis, Raymond E„ "A Review of Pozzolanic Materials and Their use in concretes," American Society for Testing Materials, San Francisco, Calif., October 10-14,1949, pg. 3.

15 Holcomb, William F„ "A Survey of the Available Methods of Solidification for Radioactive wastes,” Office of Radiation Programs, U.S. Environmental Protection Agency, pg. 12, November 1978.

16 Davis, Raymond E., "A Review of Pozzolanic Materials and Their use in Concretes," American Society for Testing Materials, San Francisco, Calif., October 10-14,1949, pg. 4.

17 Boynton, R.S., Chemistry and Technology of Lime and Limestone, 2nd Edition, John Wiley & Sons, Inc., pg. 442,1980.

18 Boynton, R.S., Chemistry and Technology of Lime and Limestone, 2nd Edition, John Wiley & Sons, inc., pg. 441,1980.

19 Neville, A.M., Brooks, J.J., Concrete Technology, Longman Scientific & Technical, Essex, England, 1987.

20 Shi, caijun and Day, Robert L, chemical Activation of Blended cements Made with Lime and Natural Pozzolans, Cement and Concrete Research, vol.23, pp.1389, Pergamon Press Ltd., 1993.

21 Boynton, R.S., Chemistry and Technology of Lime and Limestone, 2nd Edition, John Wiley & Sons, inc., 1980. CHAPTER 3

VOLCANIC TUFF SAMPLES FROM YUCCA MOUNTAIN

Samples of two types of volcanic tuff were received from the sample Management Facility at the Nevada Test site, near Beatty,

Nevada. The samples consisted of approximately 90 kg (200 lbs.) each of welded tuff and unwelded tuff, in general, the welded tuff was much denser and solid in appearance, compared with the unwelded tuff which was very porous and pumicitic. Both the welded and unwelded tuff samples were extracted from the Topopah Springs member of

Yucca Mountain, although the exact location and depth was unknown.

The exact mineral composition of the two tuffs is also unknown, but is inconsequential for the purposes of this study.

The volcanic tuff from Yucca Mountain is primarily composed of crystalline silica (quartz) and the silicate minerals, sanidine and feldspar.

Over 90% of the tuff is comprised of silica and alumina. The average composition of Yucca Mountain tuff from the Topopah springs

Member, as determined from a chemical and mineralogic study by

Broxton et al.22, is shown below in Table 1:

18 19

Table 1 - Average Composition of Topopah Springs Tuff

SIOa Tl02 AI20j Feox* Mno Mgo cao Na^o k2o PA

77.3% 0.10% 12.7% 0.85% 0.06% 0.16% 0.53% 3.64% 4.97% 0.01%

* X = 1.3-1.5

The tuff samples were ground to two levels of fineness to study

how particle size would affect the pozzolanic cement mixtures studied.

The two variations of particle size will hereinafter be referred to as

coarse and fine. A wet sieve analysis of the coarse and fine tuff

indicated that approximately 85% of the fine tuff samples passed a

#325 sieve (45pm diameter opening), while only 25% of the coarse tuff

samples passed (see Table 2 below). For more detailed information on

the pulverizing procedure and sieve analysis, refer to Appendix I and

Appendix II, respectively.

Table 2 - Sieve Analysis

Sample Description initial weight After Moisture weight After wet Percent * weight, w, Dehumidifylng content,% Sieving & Drying, passing #325 igramsi & Drying Wr igramsi {grams)

1 Coarse welded 28.3185 28.2733 0.16 20.8310 26.32 2 Coarse unwelded 26.2304 26.0981 0.50 17.7620 31.94 3 Fine welded 16.1435 16.1162 0.17 2.6424 83.60 a Fine unwelded 19.1763 19.0917 0.44 1.7572 90.80 20

Thin section observations

Thin sections of the welded and unwelded tuff were developed at the Geosciences Department of the university of Nevada, Las Vegas.

The thin sections were viewed under a petrographic microscope and provided valuable insight into the crystal structure of the welded and unwelded tuff samples.

The welded and unwelded tuff thin sections indicated a predominately crystalline structure with very little amorphous phase.

With the help of Dr. Clay crow and Dr. Eugene smith of the unlv

G eosciences Dept., it was estimated that approximately 95% of the unwelded tuff and 99% welded tuff was crystalline. The crystalline phases of silica included quartz and the silicate minerals, sanidine and feldspar.

The unwelded tuff appeared to have about 5% microcrystalline glass shards in a crystalline matrix. The welded tuff showed evidence of devitrification, which is a transformation from the amorphous glassy phase to crystalline silica over geologic tim e scales.

The pozzolanic reaction is much more active and complete with amorphous silica versus crystalline silica because of the higher bond energy required to dissociate silicon from oxygen when silica is in the crystalline state, on an atomic level, crystalline silica (i.e. quartz) is in its 21 lowest energy state and prefers to remain there; whereas amorphous silica is in the equivalent of an unstable, excited energy state.

The photomicrographs shown below were taken from thin sections of the welded and unwelded tuff samples. The photomicrograph in Figure 4 illustrates the welded tuff at a magnification of 40x, and depicts four large sanidine crystals in a matrix of devitrified glass shards. Figure 5 shows a close-up view of the welded tuff at 200x magnification, in Figure 6, the unwelded tuff at 40x magnification illustrates the notable difference between the welded and unwelded tuff microstructure. The unwelded tuff is much more porous, as indicated by the numerous air pockets, and shows less evidence of the dense, devitrified glass shards. Figure 7 is identical to

Figure 6, but at 200x magnification. 22

sanidine crystals

Figure 4 - w elded Tuff Thin section, 40x

Figure 5 - Welded Tuff Thin Section, 200x 23

Porous matrix of crystalline Silica

Figure 6 - unwelded Tuff Thin Section, 40x

Air Pockets

sanidine crystal

s'?

Figure 7 - unwelded Tuff Thin Section, 200x (Magnified view of Figure 6) 24

CHAPTER 3 NOTES

22 Broxton, David E., Warren, Richard G., and Byers, Frank M., Chemical and Mineralogic Trends Within the Timber Mountain-Oasis valley Caldera Complex, Nevada: Evidence for Multiple Cycles of Chemical Evolution in a Long-Lived Silicic Magma System, Los Alamos National Lab, American Geophysical Union, 1989. CHAPTER 4

CEMENT MIXTURES & MATERIALS

Twenty one unique mixtures were developed using three independent variables: pozzolan type, particle size, and cement composition. The various cement mixtures and their compositions are shown in the following table:

Table 3 - Pozzolanic Cement Mixtures

Sample# Mixture time Portland Pozzolan Sand Water/Cement Cement Ratio flWC+P)

1 Control Mixture - — 125 g — 344 g 75 ml Portland Cement (26.7 %) (73.3 %) (0.60)

2 4:1 — 100 g 25 g 344 g 75 ml OPC:Pozzolan (21.3 %) (5.3 %) (73.3 %) (0.60)

3 1:1 — 62.5 g 62.5 g 344 g 75 ml OPC:Pozzolan (13.3 %) (13.3 %) (73.3 %) (0.60)

4 2:1 41.75 g — 83.25 g 344 g 75 ml Pozzolan: Lime (8.9 %) (17.8 %) (73.3 %) (0.60)

5 2:1 83.25 g — 41.75 g 344 g 75 ml Lime:Pozzolan (17.8 %) (8.9 %) (73.3 %) (0.60)

6 1:1 62.5 g — 62.5 g 344 g 75 ml Lime:Pozzolan (13.3%) (13.3 %) (73.3 %) (0.60) NOTES: 1) Four types of pozzolan: coarse welded (CW), coarse unwelded (CU), fine welded (FW) and fine unwelded (FU). 2) Total number of samples: 5x4+1 = 21 3) Cement mixtures were cured at room temperature. 4) Water-Cement plus Pozzolan [W/(C+P)J ratio constant at 0.60 5) All mixing done at approx. 80°F; water density = 1.0 g/cm3 6) Lime used was Type N - high calcium hydrated lime. 7) Type l-ll Low Alkali (moderate sulfate resistance) portland cement used

25 26

Portland cem ent

The Portland cement used for these mixtures was the commercially available Type h i, p er astm C150, stan d ard specification for Portland cement. Type I is designated as normal, Type M has moderate sulfate resistance. Type i-ii meets the specification requirements for both types, and represents about 90% of the

Portland cement produced in the united States.23

Over 90% of Portland cement is comprised of the following four com pounds:

Tricalcium silicate (C3S) 3Cao«sio2 Dicalcium silicate (C2S) 2Cao*Si02 Tricalcium alum inate (C3A) 3Ca0*AI203 Tetracalcium aluminoferrite (C4 af ) 4Cao*AI2o3«Fe2o3

The chemical and compound compositions for Type I and Type II

Portland cem ent are shown below in Table 4:

Table 4 - Chemical and Compound Composition of Portland Cements24

Type Of Chem caicorr IpQSiti on,.% Poteni:ial compound, % Portland Si02 AIA FeA Cao m o CjS CjA cem ent ■ I I Type I 20.9 5.2 2.3 64.4 2.8 2.9 55 19 10 7

Type ll 21.7 4.7 3.6 63.6 2.9 2.4 51 24 6 11 Pozzolan Type

The two types of pozzolans utilized were the welded and

unwelded tuffs from Yucca Mountain. Both materials were essentially the same in composition, but differed greatly in appearance. Because there was such a distinctive difference in appearance and since both types of tuff are found at varying depths throughout Yucca Mountain,

it was important to study them independently, initially, it was thought that the welded tuff consisted mostly of amorphous silica phase and that it would, therefore, react thoroughly with lime. The unwelded tuff appeared to be more crystalline, and certainly more porous.

There was little difference between the two tuffs in how they reacted and formed cementitious compounds with lime. The noticeable difference in compressive strength between the coarse and fine tuff mixtures was attributed to a particle size effect, since the unwelded tuff was quite a bit finer than the welded tuff in both the fine and coarse samples (see Table 2 - Sieve Analysis), as the fineness of the pozzolan was increased, so did its reactivity with lime.

Particle Size

Two levels of fineness were used to study the effects of particle size on the pozzolanic reaction with Yucca Mountain tuff and lime. 28

Since the pozzolanic reactivity is an important measure of the potential strength of a pozzolanic cement, varying the particle size was very important to understanding this effect.

The coarse tuff samples ranged in fineness from 26% of the welded tuff passing a #325 sieve to 32% for the unwelded. The fine tuff samples ranged from 84% of the welded tuff passing a #325 sieve to

91% of the unwelded. For comparison, the standard fineness for OPC particles is 85-95% passing #325. Table 5 lists several examples of the

OPC-pozzolan and pozzolanic cem ent mixtures developed.

Table 5 - Examples of Pozzolanic Cement Mixtures

Mixture Particle Size Pozzolan Type Cement Proportions

2CW Coarse welded 80% OPC - 20% Pozzolan

2CU Coarse unwelded 80% OPC - 20% Pozzolan

2FW Fine welded 80% OPC - 20% Pozzolan

2FU Fine Unwelded 80% OPC - 20% Pozzolan

4CW Coarse w elded 67% Pozzolan - 33% Lime

4CU Coarse unwelded 67% Pozzolan - 33% Lime

4FW Fine w elded 67% Pozzolan - 33% Lime

4FU Fine Unwelded 67% Pozzolan - 33% Lime 29

Composition

Six different cement compositions were used to study their

effect on strength. The control mixture was astm standard Type n

Portland cement, purchased at a local Home Depot store. Type 11

Portland cement is normally defined as a moderate sulfate resisting

cement with a low heat of hydration.25 Much is known about the

cement reactions of Portland cement and there is an abundance of

literature and data on its properties.

pozzolans are often used as a partial replacement for OPC,

typically 5%-30% pozzolan, since natural and artificial pozzolans are

abundant and cheaper than Portland cement. Many countries,

particularly those in Europe, are using pozzolans to reduce cement

costs and utilize their natural resources, in Jordan, for example, studies

on the pozzolanic activity of Jordanian oil shale ash indicated that up to

20% of cement could be replaced by ash.26 in this study, sample

mixtures of OPC with 20% and 50% pozzolan were analyzed.

The other common method for utilizing pozzolans is as an admixture with calcined lime. Typically, proportions ranging from 2:1 to 3:1 pozzolan to lime are used, in some cases, good results were achieved with pozzolan to lime ratios as high as 4:1 and as low as 1/2:1.

Most of the references cited herein used calcined lime with a high 30

calcium hydroxide c o n te n t (i.e. ASTM C207 Type N). The following th ree

lime-pozzolan mixtures were chosen to better understand the effects

of varying the lime-pozzolan ratio with Yucca Mountain tuff:

(1) 67% pozzolan, 33% lime (#4)

(2) 50% pozzolan, 50% lime (#5)

(3) 33% pozzolan, 67% lime (#6)

in most mixtures, a high calcium (Type N), commercial hydrated

lime was utilized, in several special cases, lime-pozzolan mixtures were

m ade w ith ASTM C207, Type s lime to see what the effect would be.

Both types of lime were obtained from the Chemical Lime plant in

Henderson, nv , courtesy of Dr. Robin Graves. Type s lime is specially

developed to have high plasticity without soaking, i.e. it’s high in

magnesium hydroxide.27 As shown in Table 6 below, Type i\i lime is almost entirely (94%) calcium hydroxide, whereas Type s lime has about

41% magnesium hydroxide. 31

Table 6 - Chemical Analysis of Type l\l and Type s Lime28

Compound Type N Lime Types Lime ca(OH)2 94.2 % 52 32 % Mg(OH)2 0.0 40.57 CaC03 0.4 3.96 MgCOj 0.6 0.0 Free Mg 0.2 0.90 Free Ca 2.5 0.0

h20 0.3 0.15

A water/cement (w/c) of 0.6 was chosen because it was found to

be the lowest ratio capable of attaining a sufficient workability, ideally, the w/c ratio should be as low as possible to maximize the strength gain, in commercial Portland cement, the typical w/c ratio for high strength and good workability is in the range of 0.4-0.5. However, good workability is a necessary tradeoff and, in this case, required a higher water content.

Analysis of cem ent Thin sections

Thin sections were prepared for several of the pozzolanic cement mixtures, as well as the Portland cement control sample. Thin sections were not prepared for all of the cement mixtures because some were simply not hard enough to be cut into a 30pm section, and time didn't 32 allow for the preparation of all 21 thin sections. Thin sections were successfully prepared for the following mixtures: OPC, 2FU, 3FU, 3FW, and 4FU.

Figure 8 illustrates the thin section for the opc control mixture at40x. Figure 9 shows the same OPC section at 1 0 0 x. Notice the matrix of blackish material surrounding the crystals of quartz. This is the cementitious microcrystalline c-s-H, which provides strength and durability to the cement paste.

c-s-H cel

Large Quartz crystal

Figure 8 - Ordinary Portland Cement (OPC), 40x 33

Crystalline Silica A ggregate

cementitious Matrix

Figure 9 - Ordinary Portland cem ent, 100x

The OPC-pozzolan and lime-pozzolan mixtures looked surprisingly similar under the microscope, as indicated by the photomicrographs in

Figures 10-21. Figures 10-12 show the 2FU mixture at increasing magnification of 40x, 100xand 200x. Notice the fairly homogeneous distribution of silica crystals and the relatively large amount of dark cementitious material interspersed between them. The 2FU photomicrographs look very similar to the OPC photomicrographs, as they should, since 2FU is 80% OPC. 34

some large quartz crystals, som e small

Figure 10 - Photomicrograph of 2FU OPC-Pozzolan Mixture, 40x 80% OPC/20% Fine Unwelded Pozzolan

High density of c-s-H cel

Figure 11 - Photomicrograph of 2FU OPC-Pozzolan Mixture, 100x 80% OPC/50% Fine Unwelded Pozzolan 35

C-S-H Matrix

Figure 12 - Photomicrograph of 2FU OPC-Pozzolan Mixture, 200x 80% OPC/20% Fine unwelded Pozzolan

increasingly finer distribution of particles

Figure 13 - Photomicrograph of 3FU OPC-Pozzolan Mixture, 40x 50% OPC/50% Fine unwelded Pozzolan Figure 14 - Photomicrograph of 3FU OPC-Pozzolan Mixture, 1(30x 50% OPC/50% Fine unwelded Pozzolan

Figure 15 - Photomicrograph of 3FU OPC-Pozzolan Mixture, 200x 50% OPC/50% Fine Unwelded Pozzolan Figure 16 - Photomicrograph of 3FW OPC-Pozzolan Mixture, 40x 50% OPC/50% Fine welded Pozzolan

Figure 17 - Photomicrograph of 3FW OPC-Pozzolan Mixture,100x 50% OPC/50% Fine Welded Pozzolan 38

Figure 18 - Photomicrograph of 3FW OPC-Pozzolan Mixture, 200x 50% OPC/50% Fine welded Pozzolan

Figure 19- Photomicrograph oMFU Lime-Pozzolan Mixture, 40x 67% Fine Unwelded Pozzolan/33% Lime < 3j

Figure 20 - Photomicrograph of 4FU Lime-Pozzolan Mixture, 100x 67% Fine unwelded Pozzolan/33% Lime

Figure 21 - Photomicrograph of 4FU Lime-Pozzolan Mixture, 200x 67% Fine Unwelded Pozzolan/33% Lime 40

CHAPTER 4 NOTES

23 Kosmatka, Steven H. & Panarese, William c., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, pg.19,1988.

24 Kosmatka, Steven H. & Panarese, William c., Design and control of Concrete Mixtures, 13th Edition, Portland cement Association, pg. 21,1988.

25 Kosmatka, Steven H. & Panarese, william c., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, 1988.

26 Taisir Khedavwi, Asim Yeginobali and Mohammad Smadi, "Pozzolanic Activity of Jordanian Oil Shale Ash,” Department of Civil Engineering, Jordan University of Science and Technology, Cement and Concrete Research, vol.20, pg. 843,1990.

27 Boynton, R.S., Chemistry and Technology of Lime and Limestone, 2nd Edition, John Wiley & Sons, Inc., 1980.

28 Chemistry data courtesy of Chemical Lime, Henderson, IW, 1995. CHAPTER 5

PHYSICAL AND CHEMICAL PROPERTIES OF CEMENTITIOUS MIXTURES

Cement Chemistry

The strength of hardened Portland cement is derived from a

cementation reaction between water and the cement minerals. The

primary strengthening compounds in Portland cement are the calcium

silicates and aluminates: tricalcium silicate (3Cao-Si02), dicalcium silicate

(2Cao-si02), tricalcium aluminate (3Cao-AI203), and tetracalcium

aluminoferrite (4Cao-AI2o3-Fe2o3)29. in the cement industry, these

compounds are referred to as C3S, C2S, C3A and C4AF, respectively, see

Table 4 for the compound compositions of Type I and Type n Portland

cement, when water is added to a dry Portland cement mixture, the

following reactions occur spontaneously:

2(3CaO • 5*02) + 6H lO => 30*0 • 25*02 • 3# 20 + 3Ca(OH)2 (1) crricalcium Silicate) (Tobermorite (C-S-H) gel) (Calcium Hydroxide)

2(20*0 • 5*02) + 4#20 => 30*0 • 25/02 • 3H20 + 3Ca(OH)2 (2)

Figure 22 illustrates the strength development for each of these

reactions as a function of time.30 From the graph, we see that early strength is attributed to the tricalcium silicates (reaction 1), but the

long term strength is due to the dicalcium silicates (reaction 2).

41 42

Tricalcium aluminate has a constant, but much lesser effect on

strength, if pozzolanic material is present in the Portland cement

paste, it will react with the calcium hydroxide [Ca(OH)2] produced from

reactions (1) and (2), and increase the amount of total c-s-H in th e

cement matrix.

Figure 22 - Compressive Strength of Pure OPC Paste as a Function of Curing Time

10000

'in 8000 ■ * a. ■Tricalcium Silicate ■Dicalcium 5 6000 ■ • Silicate ■Tricalcium % 4000 ■ ■ Aluminate

U 2 0 0 0 • •

0 20 40 60 80 100 Time, days

Portland-Pozzolan cem ent

fast Portland Cement Reaction: c 3 S + h2 O > C-S-H+CH

slow Portland-Pozzolan Reaction: PozzolaniSiOl) + C a{O H )i+H2O +C-S-H

The pozzolanic reaction is distinguished from the typical

Portland cement reaction by three features.31 First, the Portland- 43 pozzolan reaction is slow. Therefore, the rate of heat liberation and strength development are correspondingly slow. second, the pozzolanic reaction is lime (CH) consuming instead of lime producing, which has the benefit of improving durability and resistance to leaching (to be discussed later). Third, the pore size distribution is significantly finer with portland-pozzolan mixtures.32 a s a result, the replacement of CH (a byproduct of the Portland cement reaction) with additional C-S-H gel from the pozzolanic reaction is very effective in filling up large capillary pores, which improves the strength and impermeability of the cem ent (see Figure 23 below).

Figure 23 - Relation Between Permeability and Capillary Porosity of Cement Paste33

- - LU 8 T-* X

> 1 £ 2 6 •» O)ro §Q> a. o ac o E as

0 10 20 30 35 40 Capillary Porosity, %

Pore size distribution studies were conducted by p .k . Mehta e t al. on portland-pozzolan cements containing 10, 20 and 30 weight 44

percent creek santorin earth. Results indicated that with 20 or 30

weight percent pozzolan, there were no pores larger than 0.1 mm

found in the pastes cured for 1 year.3*

water permeability tests by Mehta showed that portland-

pozzolan cements were much more impermeable than ordinary

Portland cement.35 The permeability of a material may be described as

the ease with which liquids or gases can travel through it. in the case

of cement, low permeability is very important for resistance to

leaching and chemical attack. Simply stated, the more dense and

impermeable the cement is, the less likely it is to leach, erode or

otherwise be subjected to chemical attack.

For pozzolanic cement encapsulation or solidification of nuclear

waste in a geologic repository, water intrusion is a primary concern, if,

for example, waste packages were encapsulated with pozzolanic

cement and placed underground in a repository, cement permeability

would be a key factor in determining the containment of such a system, in the unlikely event that saturated water did find its way into the repository and flooded the canisters, a cement with a low

permeability would help to ensure that soluble radionuclides were not

leached out of the canisters and transported to the accessible environment. The permeability of cement is very much dependent upon the

water/cement (W/C) ratio of the cement paste during mixing. For

maximum strength, the ideal W/C ratio is typically 0.4 to 0.6. As the

water content is increased and the w /c increases, workability increases

but strength decreases, with a low w /c ratio, strength may be very

high but poor workability may limit its practical use. The w /c ratio

should be balanced to provide an optimum mix of strength and

workability. Figure 24 shows the relationship between permeability

and th e w /c ratio in cement paste.

Figure 24 - Relation Between Permeability and Water/Cement Ratio for cement Paste36 140 ■ Lii T- 120 X 100 jan

o 0)c 40 u Ea o o

0.2S 0.3 0.4 0.4 0.6 0.7

Water/Cement Ratio

in the pozzolanic cement mixtures developed at unlv , it was found that a minimum w /c ratio of 0.6 was required to obtain sufficient workability, with w /c ratios of less than 0.6, th e cem en t 46 paste was too thick to manipulate and furthermore, it was nearly impossible to obtain a homogeneous mixture of cement and sand. The high lime-pozzolan mixtures (i.e. 2:1 lime-pozzolan) and the coarse tuff mixtures generally required a greater amount of water to achieve the same workability as OPC-pozzolan and fine tuff mixtures. This behavior is due to the fact that 1) lime-pozzolan mixtures had a higher specific surface area than OPC-pozzolan mixtures and, 2) coarse particle mixtures absorbed more water than the fine grain mixtures.

common sense would lead one to believe that mixtures with finer grains, and therefore a greater surface area, would require more water than coarse-grain mixtures to attain the same workability.

However, the opposite was found to be true. The fine-grain pozzolan- lime mixtures such as 6FU and 6FW had good workability with a water/cement ratio of 0.6, whereas the same coarse-grain mixtures (6CU and 6CW) required ratios of 0.8-1.0 to attain the same workability. My hypothesis is that the packing density of the fine grain mixtures was so high that uncombined water molecules were not able to occupy the void spaces between cement particles, in theory, the amount of water absorbed into the hydrated cement matrix should be similar, if not greater (due to the increased reactivity), for the fine-grain cement than for the coarse-grain cement. However, it appears that the void spaces 47 between the coarse-grain mixtures were large enough to allow water

molecules to be held in suspension.

Because this paper proposes that pozzolanic cement be used to encapsulate spent fuel assemblies in a waste package, it will be necessary to have a fairly high degree of workability, in a typical pw r nuclear fuel assembly, for example, the gap between fuel pins is about

1 mm. Therefore, it would be necessary to use a fine grained pozzolan to create the pozzolanic cement for encapsulation of SNF.

D u rab ility

compared with OPC, portland-pozzolan cements have superior durability in sulfate and acidic environments due to the combined effects of reduced permeability and a lower calcium hydroxide content in the hydrated cement paste. The reduced permeability is a function of the fine pore size, while the reduced calcium hydroxide is a result of the pozzolanic reaction between the free lime and the pozzolan. The calcium hydroxide content as a function of curing age for OPC-pozzolan cem ent is shown in Figure 25 below:37 48

Figure 25 - Effect of Curing Age and Pozzolan % on Calcium Hydroxide Content 8

5 3-Days 4

1 0 0 10 20 40 60 80 100 Pozzolan %

The durability of cement is primarily due to the C-S-H content of the solidified, hydrated cement. Because the pozzolanic tuff from

Yucca Mountain is over 70% silica, more C-S-H is formed with OPC- pozzolan or lime-pozzolan cements than with typical OPC. Also, with the OPC reactions, hydrated lime (CH) is produced. The CH effectively

"ties up" the calcium in a weak, porous phase which has poor durability, when active silica and alumina are present, however, the pozzolanic reaction utilizes the extra CH to form more of the desirable amorphous

C-S-H.

Effect of crystal structure and Reactivity

The chemical effect of adding pozzolans to lime or opc is predominately based on the reaction of Sio2 and ai2o3 with calcium hydroxide, in general, this reaction results in a higher proportion of 49

calcium-silicate-hydrates (C-S-H), which increases strength. The degree

of strength increase, however, is very dependent upon the pozzolanic

reactivity. Many studies have been undertaken to analyze the reactivity

of different pozzolanic materials, unfortunately, there is no guideline

for predetermining which pozzolans are reactive, some pozzolans

simply react more than others and many don't react at all. Cement

specifications such as ASTM C 311 address this issue by providing an

activity index to determine the pozzolanic activity with lime. Although

pozzolans can have a wide range of reactivities, ideally they should

have a high percentage of amorphous silica and/or alumina. The fact

that amorphous pozzolans are much more reactive than crystalline

pozzolans is strongly supported in the cement literature. The increased

pozzolanic reaction with amorphous silica is influenced by the

following:

• Amorphous silica grains are generally finer and have a higher surface area.

• The non-crystalline structure is in a higher energy state than the stable crystalline phase and no chemical bonds need to be broken to dissociate the silicon from the oxygen.

Solubility of Silica in w ater

The driving force for dissociating the silica into its constituents, silicon and oxygen, is the pH of the lime-pozzolan or OPC-pozzolan system. The pH of pure lime is 12.45 @ 25°. it is the high pH of lime and

the presence of water that allows the silicon to dissociate from Si02,

then recombine with calcium to form c-s-H. Both amorphous and crystalline silica begin to rapidly dissolve in water at a pH of 9. At a pH

of 12, amorphous and crystalline silica are almost completely soluble in

water, as shown in Figure 26 below.

Figure 26 - Activities of Aqueous Quartz and Amorphous Silica in Equilibrium at 25°C38

-1 ■ ■

■D ■—♦ —•Amorphous Silica —H —Quartz

2 4 6 8 10 12 13 14

As shown in the diagram above, quartz (crystalline silica) has a lower solubility in water at all pH values. Above a pH of approximately

13, however, both amorphous and crystalline silica are completely soluble in water. 51

Below a pH of 9, the primary reaction of silica with water is:39

Si02 + 2H20 o H +1 + H4Si04

if the silica phase is quartz, the equilibrium constant Kq at 25°c is 10-4; if

it is amorphous silica, Kqa=2xl0‘3. Above a pH of 9, the following two

additional reactions contribute to dramatically increase the solubility of silica in water:

H4Si04o H +1 + H3Si04 and H3Si04» H+1 + H2Si04

pH E ffe c t

with pozzolanic cement, the pH value would be expected to decrease with time, as the pozzolanic reaction progresses and the free lime is consumed, t o validate this hypothesis, four pozzolanic cement samples were mixed and monitored for eight days. The mixtures included #3FU (50% pozzolan/50% OPC), #4FU (67% pozzolan/33% lime),

#6FU (50% pozzolan/50% lime) and 100% o pc . as expected, th e pH values for all the samples decreased over time as the free lime was consumed and incorporated into the cement matrix. Results of the pH analysis are shown in Table 7 below: 52

Table 7 - pH values Over Time for Pozzolanic Cement Mixtures

Timemrs) OPC #3FU #4FU #6FU

0 11.5 11.0 12.0 11.5

0.33 11.5 11.0 12.5 11.5

0.66 11.5 11.0 12.0 11.5

1.0 12.0 11.0 12.0 12.0

1.5 12.0 11.0 12.5 12.0

2.0 12.0 11.0 12.0 12.0

5.0 11.5 11.0 11.5 12.0

17.5 10.5 11.0 11.0 12.0

24.5 10.0 11.5 10.0 11.5

40.0 9.0 11.5 10.0 10.5

112.0 9.0 10.5 8.5 10.0

138.0 9.0 9.0 9.0 9.0

162.0 8.0 8.0 9.0 8.0

188.0 9.0 8.5 9.0 9.0

212.0 9.5 9.0 8.0 9.5

The pH results confirmed that in fact there was a reaction taking place between the lime and the pozzolan. in each case, the pH decreased over time. The pH drop over the eight-day test period was fairly similar for all of the mixtures, except for #4FU. interestingly, mixture #4FU had the highest initial pH and the lowest pH at the end of 53 eight days, its ending pH of 8.0 was significantly lower than all the others, which ranged from 9.0 to 9.5. This phenomena could indicate that the 2:1 pozzolan to lime ratio was producing the best pozzolanic reaction and most completely utilizing the free lime. The pH values as a function of log time are shown graphically in Figure 27 below:

Figure 27 - Measured pH values vs Log Time for Pozzolanic Cement Mixtures

13 n

OPC

111 v

pH io-

t r

0 3.07 3.38 3.56 3.73 3.86 4.26 4.8 4.95 5.16 5.61 5.7 5.77 5.83 5.88 Log Time (sec)

Most waste package designs for permanent geologic disposal of hlw and SNF, require the waste to be inserted into a metal canister made from carbon steel, stainless steel, or a combination of both.

There is a legitimate concern regarding the corrosion of these canisters over a long period of time. A cement solidified wasteform could potentially decrease the corrosion rate of steel by increasing the pH of the near-field environment. The question with pozzolanic cement is 54 whether the reduction in pH would negate the corrosion protection or even drop the pH to such a level that the corrosion rate would be accelerated. From the limited data collected in Table 7, it appears this would not be the case, iron forms a protective oxide film in a pH environment between 9-13, therefore, the reduction in pH due to the addition of a pozzolan is not expected to have a significant effect on the corrosion protection of ferrous materials.

Particle Size Effect

The addition of a finely ground pozzolan, whether it be natural or artificial, will increase the percentage of fine grain particles and tend to reduce the cement's water demand. The stability and durability of pozzolanic cements are improved as a direct result of this behavior, as previously discussed, the reduced water demand with fine-grain cements is attributed to the high packing density of fine particles, which prevents the free water molecules from occupying the void space in the cement matrix. The water then becomes available as additional "skidding agent" for mixing.40 The result is a cement paste with better workability and a denser matrix of c-s-H gel.

The grain size effect was a predominate factor with the Yucca

Mountain pozzolan. with both the welded and unwelded tuff samples, there was a noticeable difference in the workability between the coarse and fine tuff. The fine welded and fine unwelded samples were much more fluid and workable than the corresponding mixtures using coarse welded and coarse unwelded tuff. The difference was particularly noticeable with the lime-pozzolan mixtures because the lime seemed to "suck up" the water much more rapidly than the OPC- pozzolan mixtures, with the lime pozzolan mixtures using coarse tuff

(4CW, 4CU, 5CW, 5CU, 6CW, 6CU), it was very difficult to g e t a workable paste w ith th e standard 75 ml of w ater (0.6 w /c ratio). However, the lime-pozzolan mixtures with the fine grained tuffs (4FW, 4FU, 5FW, 5FU,

6FW, 6FU) were much easier to mix into a workable paste because the reduced absorption left more water for mixing.

A lesser particle size effect was noticed between the welded and unwelded tuff mixtures, in general, the unwelded tuff contained more fine grain particles than the welded tuff, therefore, it was slightly better in terms of workability and reduced water demand.

Fly ash is an exceptionally good artificial pozzolan because its made up almost entirely of microfine, amorphous spherical particles of silica. The spherical particles provide a maximum surface area for the pozzolanic reaction to occur, and the amorphous structure allows for 56 the silicon to be easily dissociated from the silica, thus causing it to be more reactive.

Heat of Hydration

Heat of hydration is the heat produced as a result of the cement reactions. Excessive heat generation during the curing stage has been known to induce cracking and degradation of the cement. Heat of hydration is particularly important when cement is considered as a solidification agent for radioactive waste, spent-nuclear fuel, because of the presence of long-lived radionuclides like plutonium and uranium, generates a significant amount of radioactivity and decay heat (see

Table 8). Although the majority of decay heat is liberated within the first 50 years after being removed from a reactor (refer to Figure 1), many of the long-lived radionuclides continue to decay for thousands of years (4.5 billion years in the case of u-238).

Table 8 - Long-Lived Radionuclide inventory of Spent Fuel41

Radionuclide Half-Life (years) Radioactivity (Ci/1000 MTU)*

Am-241 458 1.6 X106 PU-241 13.2 6.9 X107 PU-240 6.58 X103 4.5 X105 PU-239 2.44 X 104 2.9 X 10s PU-238 86 2.0 X106 U-238 4.51 X109 320 57

Radionuclide Half Ufe (years) Radioactivity (Ci/iooomtu )*

U-236 2.39 X107 220 L U-235 7.1 X108 16 CS-137 30 7.5 X 107 Sr-90 29 5.2 X 107 TC-99 2.15 X105 1.3 X104

* Ci = Curie, a measure of radioactivity; MTU = Metric tons uranium in spent fuel

pozzolanic cement is an attractive binding material because of its relatively low heat of hydration, which can be up to 40% less than ordinary Portland cement.42 A study done by p .k Mehta e t al. confirmed a significant decrease in the heat of hydration when natural *» pozzolan is partially substituted for Portland cement.43 As shown in

Figure 28 below, the heat of hydration decreases as the percentage of pozzolan is increased. Figure 28 illustrates the heat of hydration as a function of curing time, at 7,28 and 90 days.1501 Heat of Hydration, Cal/g 85 IK 85 Figure 28 - Effect of Substituting Natural Pozzolan on on Pozzolan Natural Substituting of 28 - Effect Figure 10 th e Heat of Hydration of Portland Cement Portland of Hydration of Heat e th 20 Pozzolan % Pozzolan 040 30 —B — — —B 28-Days 90-Days 7-Days 500 58 59

CHAPTER 5 NOTES

29 R.H. Bogue and w. Lerch, industrial and Engineering Chemistry, 26, 837,1934.

30 Young, J.F., Educ. Module Mater. Sci., Journal of Materials Education, 3:420,1981. 31 Mehta, Kumar, Concrete structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, inc., pg. 201,1986.

32 Mehta, Kumar, concrete structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, Inc., pg. 202,1986.

33 Neville, A.M., Brooks, J.J., concrete Technology, Longman scientific & Technical, Essex, England, 1987.

34 Mehta, Kumar, Concrete structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, Inc., 1986.

35 Mehta, Kumar, Concrete Structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, inc., pg. 202,1986.

36 Neville, a .m.,Brooks, J.J., Concrete Technology, Longman Scientific & Technical, Essex, England, pg.264,1987.

37 Mehta, Kumar, Concrete Structure, Properties and Materials, university of California, Berkeley, Prentice-Hall, Inc., pg. 205 Fig.6-17,1986.

38 Richardson, Steven M., Geochemistry: Pathways and Processes, Prentice-Hall, Inc., pg. 167-168,1989.

39 Richardson, Steven M„ Geochemistry: Pathways and Processes, Prentice-Hall, inc., pg. 167-168,1989.

40 Michael Schmidt, Klaus Harr, and Raymund Boeing, "Blended cement According to ENV 197 and Experiences in Germany," Cement, Concrete, and Aggregates, CCAGDP, Vol.15, No.2, Winter 1993, pp. 156-164.

41 R.scott Moore and Karl J. Notz, lw r Radiological Database, CDB_R/V1.1, QA-M04- 2002.021 .C077, u.S. Department of Energy OCRWM, July, 1992.

42 Kosmatka, Steven H. & Panarese, William C., Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, pg. 70,1988.

43 Mehta, Kumar, Concrete Structure, Properties and Materials, University of California, Berkeley, Prentice-Hall, inc., 1986. CHAPTER 6

COMPRESSIVE TESTS

Compressive strength is the most useful physical property of cement and an excellent measure of its performance. in waste disposal applications, however, compressive strength is not considered a critical characteristic, cement stabilization and/or encapsulation of waste typically involves pouring the cement into a pre-formed canister or over a shallow earthen trench, in either case, there is not a significant load being carried by the cement. Of greater importance is the structural integrity of the cement, i.e. the physical and chemical stability.

in the case of high level radioactive waste and spent fuel disposal, the critical characteristic of a cement wasteform is its ability to stabilize and contain radionuclides for thousands of years. The initial criterion for structural stability as defined in the

Nuclear Regulatory commission’s iNRO "Technical Position on waste Form" was that cement-solidified waste-forms must exhibit a mean compressive 28-day strength of 50 psi. This was later

60 61

raised to 60 psi to reflect an increase in burial depth to 55 ft at the Hanford, Washington site.44 Although the NRC requirement

refers to land disposal of low-level waste, it illustrates the point that compressive strength is not the primary concern. For comparison, a typical Portland cement will have a 28-day compressive strength of 5,000-4,000 psi.

in spite of the above discussion, compressive strength is still the most effective and reliable method of verifying the structural integrity of cement. Research indicated that cements with a compressive strength less than 100 psi were found to be very weak and porous, either from too much water or void space, or not enough cement reactions taking place. A fairly extensive test program was conducted to determine the compressive strength of the pozzolanic cement mixtures analyzed in this study. Results of the pozzolanic cement compression tests are shown in Table 9 below: 62

Table 9 - Compressive Strength Results for Pozzolanic Cement Mixtures

Cement Water/cement compressive Mixture Ratio strength, psi 100% OPC 0.60 1814 100% OPC 0.80 748 2CW 0.60 1549 2CW 0.80 630 2FW 0.60 1991 2CU 0.60 1286 2FU 0.60 1976 3CW 0.60 818 3FW 0.60 915 3FW 0.80 512 3CU 0.60 424 3FU 0.60 1497 3FU 0.80 3451 4CW 0.60 0 4FW 0.60 87 4CU 1.00 0 4CU - Type 1.00 0 4FU 0.60 215 4FU-Type 0.60 245 5CW 0.60 0 5FW 0.60 0 5CU 0.60 0 5FU 0.60 0 6CW 0.60 0 6FW 0.60 28 6CU 0.60 0 6FU 0.60 213 6FU 1.25 177 6FU Paste 0.60 102 63

The compressive strength results illustrated below in Figures 29-

33 are for curing ages greater than 28 days. The actual curing ages varied from 28 to 180 days, however, the majority of the strength gain is expected to occur within the first 28 days.

Figure 29 illustrates the compressive strength of the lime- pozzolan mixtures, #4 and #6. Mixture §5 is not shown on the graph because none of the samples were hard enough to be tested. The #4FU mixture was the strongest, with a strength of about 250 psi. Relative to the other mixtures, however, the strength of the lime-pozzolan mixtures was significantly lower. Figure 29 also shows the difference in strength between #4FU made with Type N lime and #4FU made with

Type s lime, contrary to expectations, the Type s lime was a bit stronger. The difference was negligible, however, and the result is probably due more to the preparation of the mixture or test sample, than to an increased pozzolanic reaction with the Type s lime. Type N lime is higher in calcium hydroxide than Type s, and should produce a cem ent with more C-S-H in the matrix.

Figure 30 compares the compressive strength of welded tuff mixtures with those of unwelded tuff, in this case, the particle size was held constant (coarse), and the type of tuff varied, with the coarse tuff samples, only #2 and #3 hardened enough to test for compressive strength. The compressive strength due to mixtures §2 and #3 is due primarily to the OPC content, however, the pozzolan would be expected to contribute some to strength, contrary to what I would have expected, the coarse welded mixtures exhibited higher strengths than the coarse unwelded, although the differences were small. I would have expected the unwelded tuff to have a higher strength since the average particle size, as determined by sieve analysis, was somewhat smaller than the welded tuff (32% passing #325 vs 26%).

Figure 29 - Compressive strength of § 4 and §6 Lime-Pozzolan Mixtures

200

150 HType S Lime psi

100

4CW4FW 4CU 4FU 6CW 8FW6CU 6FU Cement Mixtures 65

Figure 30 - Compressive strength of Coarse Tuff Samples with 0.6 W/C Ratio

B Coarse Welded

Coarse Unwelded PSI 800

Cement Mixture

Figure 31 shows the compressive strength of the fine welded and fine unwelded tuff mixtures, with a constant water/cement ratio of 0.6. For the #2 mixture, the strengths were almost identical with th e fine w elded and unw elded tuff. However, m ixtures #3, #4, and #6 all exhibited higher strengths with the fine unwelded tuff than with the fine welded. 66

Figure 31 - Compressive Strength of Fine Tuff Samples with 0.6w/c Ratio

H Fine Welded

HFine Unwelded

psi

#2 #3 m #5 #6 Cement Mixture

Figure 32 compares the strength of coarse unwelded tuff vs. fine unwelded tuff mixtures, in each case, the fine unwelded tuff mixtures exhibited higher strengths than the coarse unwelded tuff mixtures. This is a clear sign that the smaller pozzolanic particles are providing greater pozzolanic activity and resulting in increased stren gth. 67

Figure 32 - Compressive Strength of Coarse vs. Fine Unwelded Tuff Samples (0.6 W/C)

B Coarse Unwelded (CU)

psi 1000 Q Fine Unwelded (FU)

#3 m Cement Mixture

Figure 33 shows the difference in compressive strength between opc and OPC-pozzolan mixtures 3FW and 3CW, aged for 31 days versus

108 days.

Figure 33 - Compressive Strength Comparison for 31-Day vs. 108-Day Aging (0.6 W/C)

B31-Day Age

B 108-Day Age

psi 1000

OPC 3FW 3CW Cement Mixture 68

cements will typically gain in strength between 31 and 108 days,

particularly cements with a pozzolan which effectively slows down the cure rate. However, in the case of OPC and 3FW, the results were opposite. This is probably due to the inconsistencies of sampling and testing the cement, rather than any extraordinary phenomena, if a statistical sampling was done, within the confines of a well-equipped laboratory using a standard procedure, the results would most likely be different. CHAPTER 6 NOTES

44 Akers, D.w., Kraft, W.C., and Mandler, J.W., "Compression and immersion Tests and Leaching of Radionuclides, Stable Metals, and Chelating Agents From Cement-Solidified Decontamination waste collected From Nuclear Power Stations," NUREC/CR-6201 EGG- 2736, INEL, pg. 2,1994. CHAPTER 7

DISCUSSION

Permeability

Permeability and chemical stability are actually more desirable

with respect to long-term stabilization and containment of waste

material. Permeability studies were not conducted in this research due

to a lack of facilities and resources. However, determination of

permeability would be absolutely necessary to fully characterize

pozzolanic cem ent for use as a nuclear waste-solidification material.

Radionuclide Leachability

The radionuclides of primary concern with respect to waste-form leaching include 55Fe, “Co, “Ni, and 14C.45 cesium-137 is another important leachate due to its relatively high solubility. Leachability is a complex function dependent upon the radionuclide, its chemical form, the solidification agent, and the final solidified matrix, site hydrology and groundwater chemistry also influence leaching rates, as do cyclic wet and dry conditions.46 The complexity of these interactions

70 71

precluded leachability from being studied in the lab. it would,

however, be prudent to conduct a study of the leachability of

radionuclides in pozzolanic cement before using it to solidify nuclear

w aste.

other Factors

There is little research on the physical and chemical stability of pozzolanic cement exposed to the following environments:

• Low levels of gamma radiation over a prolonged time period.

• High temperature oxidizing environment.

• A saturated, chemically aggressive environment of 80-90°c.

The physical environment of a high-level waste geologic repository could involve any or all of the above conditions over its expected lifetime. Any materials counted on to perform a function in this type of aggressive environment would need to be studied, over time, under simulated conditions mimicking those of a potential repository. 72

CHAPTER 7 NOTES

45 Akers, D.W., Kraft, N.C., and Mandler, J.W., "Compression and immersion Tests and Leaching of Radionuclides, stable Metals, and Chelating Agents From Cement-Solidified Decontamination waste collected From Nuclear Power stations," NUREC/CR-6201 EGG- 2736, INEL, pg. xiv, 1994.

46 Akers, D.w., Kraft, N.C., and Mandler, J.W., "Compression and immersion Tests and Leaching of Radionuclides, Stable Metals, and Chelating Agents From Cement-Solidified Decontamination waste Collected From Nuclear Power Stations," NUREG/CR-6201 EGG- 2736, INEL, pg. 2,1994. CHAPTER 8

CONCLUSIONS

This research was undertaken to analyze the potential for

pozzolanic cement to be used as a solidification matrix for permanent

disposal of radioactive waste and spent nuclear fuel. A cement-

encapsulated wasteform is an attractive option because it offers the

benefits of long-term stability, low permeability, positive ion-exchange

properties, and a good physical barrier to contain the migration of

long-lived radionuclides. Pozzolanic cement mixtures using natural volcanic tuff from Yucca Mountain, Nevada, were developed and

characterized. The microstructure was examined and the compressive strength recorded for samples with varying compositions of Portland cement, lime, sand, water, and pozzolanic material.

A literature search on the uses of pozzolanic cement indicated that it can be effectively used to stabilize and solidify radioactive waste.

Pozzolanic cement has a number of advantages over ordinary Portland cement, including a lower heat of hydration, potentially greater long term strength, and a denser, more impermeable cement matrix.

73 74

Two general types of volcanic tuff were studied: welded and

nonwelded, in addition, pozzolan samples were ground to two

different particle sizes to determine the effect of tuff material and

particle size and on the pozzolanic reaction between pozzolan and

lime.

Based upon results of the thin section analysis and compressive

strength tests, it was determined that pozzolanic cement made from

Yucca Mountain pozzolan would not be an ideal encapsulant material

for solidifying radioactive waste or spent nuclear fuel. This

determination is primarily due to the fact that Yucca Mountain

pozzolan is highly crystalline, on the order of 95-99%, and mostly

unreactive with lime.

one important characteristic of a reactive pozzolan is an

amorphous silica structure. Another is a an extremely small (microfine)

particle size. The natural pozzolan from Yucca Mountain had neither.

Although it was shown that both the welded and nonwelded tuffs were somewhat reactive, and achieved compressive strengths up to

3,500 psi by simply grinding to a very small particle size, the costs of grinding on a large scale could possibly be prohibitive.

The portSand-pozzolan mixtures, which consisted of 50% and 80% ordinary Portland cement (OPC) by weight, did show some promise. The highest compressive strength attained by any of the mixtures

studied was with #3FU (0.8 w/c ratio), which consisted of 50% OPC and

50% fine, unwelded pozzolan. The average compressive strength of

this mixture, 3451 psi, was more than four times that for OPC with a 0.8

w/c ratio. This alone is a compelling indication that portland-pozzolan

cement mixtures have the potential for higher compressive strengths

than o pc . whether or not portland-pozzolan cements can significantly

reduce the permeability of water or inhibit the migration of

radionuclides remains to be proven.

if used as a partial admixture to OPC, ground volcanic tuff from

Yucca Mountain could provide value as a pozzolanic material, consistently strong and solid cements were produced with pozzolanic cement mixtures §2 and #3, which consisted of 20 w/o and 50 w/o pozzolan, respectively.

Before pozzolanic cement could be seriously considered as an encapsulant material for radioactive waste disposal in the U.S., further characterization studies would need to be conducted, including:

• Permeability and leach testing

• compressive strength properties

• ion-exchange properties with mobile, long-lived radionuclides 76

• Chemical stability and long-term durability

• Degradation and cracking under simulated repository conditions

it is the authors hope that the results of this work will serve to stimulate interest in the use of pozzolanic cement, in some way, shape or form, for the proposed geologic repository at Yucca Mountain.

Pozzolanic cement could easily be used as a repository backfill material, or even to construct the pads on which the waste canisters would ultimately sit. Pozzolanic cement would not only help to reduce material costs, but would also provide maximum utilization of the natural resources at Yucca Mountain. APPENDIX I

PULVERIZING PROCEDURE

Approximately twenty pounds each of the welded and unwelded tuff were separated and ground in a BICO chipmunk pulverizer to a particle size of 1/2 inch or smaller. The roughly-ground tuff was then proportioned into four ziplock bags for further reduction, two of the samples were then passed through a BICO rotary disc mill to reduce the tuff to a fine powder. These samples were referred to as the "coarse" samples. The remaining two tuff samples, one welded and the other unwelded, were passed through a bico vibratory pulverizer (a.k.a. shatterbox). The shatterbox produced samples of a very fine particle size which resembled common talcum powder, samples ground in the shatterbox were referred to as "fine."

The end result of the pulverizing procedures was the following four tuff samples:

Sample #1 coarse, welded Tuff

Sam ple #2 coarse, unwelded Tuff sample #3 Fine, welded Tuff sample #4 Fine, unwelded Tuff

77 APPENDIX II

SIEVE ANALYSIS

The four tuff samples were wet sieved through a #325 (45mm diameter openings) sieve. The general sieving procedure was as follows:

1) Place approximately 25 grams of each sample in tin containers.

2) Dry each sample at 105-115°C for 10 minutes.

3) Place samples in dehumidifier overnight to achieve constant hum idity.

4) weigh each sample (including container) on a balance to four decimal places.

5) wet sieve each sample until no more grains pass (approximately 3 minutes) or until the water is clear.

6) Dry the remaining material in the sieve at 105-115°C for 10 m inutes.

7) Pour the remaining material back into the tin containers and place them in the dehumidifier overnight.

8) weigh the remaining material (including container).

78 79

Results of the sieve analysis are included in Table ii-a below:

Table ii-a - Sieve Analysis

San pie Description initial weight After Moisture weight After wet percent * weight, w, Dehumidffvlng content % Sieving & Drying, passing ,*325 igramsi & Drying w , igramsi (grams)

1 Coarse welded 28.3185 28.2733 0.16 20.8310 26.32 2 coarse unwelded 26.2304 26.0981 0.50 17.7620 31.94 3 Fine welded 16.1435 16.1162 0.17 2.6424 83.60 4 Fine unwelded 19.1763 19.0917 0.44 1.7572 90.80

The percentage of material passing through a No. 325 sieve was calculated by subtracting the final weight (Wf) from the initial weight

(W,)( then dividing by the initial weight and multiplying by 100, per the following equation:

(W|-Wf/W|) x 100 APPENDIX III

COMPRESSIVE STRENGTH TEST PROCEDURE

Prior to 12-30-94, cement mixtures were poured into paper rolls lined with paraffin wax. The nominal diameter of the paper molds was

1.375 inches. Beginning on 12-30-94, cement mixtures were poured into plastic molds made from thin-walled (0.125" thick) Putyrate tubing with plastic end caps. The inside diameter of the tubing was 1.75 inches.

compressive strength testing was conducted on the Tinius Olson machine in UNLV’s Multifunction Laboratory, where possible, two or three samples were tested for each unique mixture and the results averaged. Preparation of the test coupons consisted of:

1) Mixing a fresh batch of the particular cement, per Table 3 (pg.23)

2) Tamping the paste thoroughly until all the air pockets were displaced

3) Placing a plastic cap over the top of the test cylinder

4) Drying in air for 1-24 hrs

5) immersing in a can of water to cure.

80 BIBLIOGRAPHY

Mehta, Kumar, Concrete Structure, Properties and Materials, University of California, Berkeley, Prentice-Hall, inc., 1986.

Boynton, R.S., Chemistry and Technology o f Lime and Limestone, 2nd Edition, John Wiley & Sons, Inc., 1980.

Kosmatka, Steven H. & Panarese, William c., Design and control of Concrete Mixtures, 13th Edition, Portland Cement Association, 1988.

Neville, A.M., Brooks, J.J., Concrete Technology, Longman scientific & Technical, Essex, England, 1987.

Broxton, David E., Warren, Richard G., and Byers, Frank M., Chemical and Mineralogic Trends Within the Timber Mountain-Oasis valley Caldera complex, Nevada: Evidence for Multiple Cycles of Chemical Evolution in a Long-Lived Silicic Magma System,Los Alamos National Lab, American Geophysical Union, 1989.

Richardson, Steven M., Geochemistry: Pathways and Processes, Prentice-Hall, inc., 1989.

Akers, D.W., Kraft, N.C., and Mandler, J.W., "Compression and immersion Tests and Leaching of Radionuclides, Stable Metals, and Chelating Agents From Cement- Solidified Decontamination waste Collected From Nuclear Power stations," NUREG/CR-6201 EGG-2736, INEL, 1994.

Davis, Raymond E., A Review of Pozzolanic Materials and Their use in concretes, American Society for Testing Materials, San Francisco, Calif., Oct. 10-14,1949, pg. 4.

Holcomb, William F., A Survey of the Available Methods of Solidification for Radioactive wastes, Office of Radiation Programs, U.S. Environmental Protection Agency, pg. 12, November 1978.

Stabilization/Solidificationcercla of and RCRA wastes - Physical Tests, Chemical Testing, Procedures, Technology Screening, and Field Activities, Center for Environmental Research information, U.S. Environmental Protection Agency, EPA/625/6-89/022, May 1989.

Flinn, Richard A., and Trojan, Paul K., Engineering Materials and Their Applications, Fourth Edition, Boston, Houghton, Mifflin Company, 1990.

Nuclear Materials and Radiation Effects, Nuclear Engineering 116/217 Manual, Department of Chemical and Nuclear Engineering, university of California Santa Barbara, Fall 1989.