assessment of the durability of cementitious materials in repository environment 1/46 assessment of...

Assessment of the durability of cementitious materials in repository environment

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Roberto Vicente

with contributions fromJulio Takehiro MarumoHissae MiyamotoVera Lucia Keiko IsikiEduardo Gurzoni FerreiraLuciano Gobbo(1)

Institute of Energy and Nuclear Research, IPEN-CNEN/SP

(1) Engineering School – University of São Paulo

Sao Paulo - Brazil

IAEA CRP meeting on cement, Bucharest, November 24 – 28, 2008

IAEA Research Contract No. 14206/R0


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Background

The concept of a deep borehole for disposal of Disused Sealed Radiation Sources (DSRS) is under development at IPEN.

DSRS come from decommissioning of radioactive lightning rods, oil well logging probes, nucleonic industrial gauges, irradiators, teletherapy and brachytherapy equipment, etc.

Brazilian inventory of SRS is about 270,000 sources

Main radionuclides are 60Co, 90Sr, 137Cs, 226Ra, 241Am

Total activity of sources is estimated at 26 PBq

Most sealed sources in the Brazilian inventory would be unacceptable for disposal in near surface repositories.


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General view of the concept of the borehole repository

Cement paste is used to backfill the space between the steel pipe and the geological formation


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In the long term, hydrated Portland cement paste (Pcp) is unstable

recrystallization and chemical reactions with other materials change paste chemistry, microstructure, and mineralogy

Required service life in repository → thousands of years,

much longer than conventional, civil engineering experience.

More data needed to predict long term performance of Pcp products as an engineered barrier in deep repositories

Objectives of the present research project: to evaluate the durability of Pcp under the conditions that are expected to prevail in a deep borehole for DSRS.

Introduction


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Research Methodology

Method

Evaluate service life of Pcp, by extrapolating empirical results from accelerated laboratory tests

1.Identify degrading factors (temperature, corrosive media, radiation, etc.) present in repository environment

2.Estimate extreme levels of the factors, deemed to prevail in actual repository conditions

3.Design multifactorial exposure experiments to assess the effects of single factors and their interactions

4.Run accelerated tests; measure changes in properties; model the long term performance; estimate durability.


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Assumptions

1. Degrading processes can be accelerated by test conditions

2. Measured properties can be associated with material performance under repository conditions

3. Performance can be modeled based on test results

4. Short term results can be extrapolated to long term


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Product properties changes

Observed effects:a) loss of mechanical strength; b) swelling/shrinkage; c) variation in hydraulic permeability/porosity; d) changes in mineralogy.

1. Pcp composition:a) cement types; b) water/cement ratios; c) cement admixtures.

2. Repository exposure environment:a) ionising radiation; b) high temperature; c) groundwater ions

Factors under consideration in the multifactorial experiments


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Brazilian ABNT standard Portland cement type II (OPC: ordinary Portland cement) and type V (HES: high early strength cement.

Water to cement ratio: 0.35; no additives.

Geometry and size: cylindrical samples, [ =2.5cm h=5cm] selected to allow irradiation with even dose distribution, while maintaining size as large as possible.

Standard size [=5cm h=10cm] cylindrical specimens, with same composition, used as reference for checking suitability of procedures and testing equipment with small samples.

Sample description


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Sample description

2.5 cm

5 cm

Left: acrylic, reusable, small moulds (2.5 x 5) and two Pcp samples in the foreground.Right: polystyrene, disposable, standard size moulds (5x10) with Pcp samples being cast.


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Radiation: Irradiation produces radiolysis and radiolytic gases that affect Pcp chemistry.

Planned: 8 MGy ≡ dose delivered by the most active and long lived sources of the inventory(corresponding to complete decay of 56 TBq - 137Cs or

2 GBq - 226Ra sources

Realized: 400 kGy (dose rate 4 kGy/h) in a multipurpose compact irradiator with 3.4 TBq of 60Co.(because of availability of irradiation facility and dosimetry)

Higher doses and different dose rates will be tested later.


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Corrosion: reactions with dissolved ions in granitoid rock pore water degrade Pcp. Reported low-, high-, and average-concentrations (g.L1):

low high average Ca2+ 0.0011 1.89 0.27 Na+ 0.001 2.1 0.37K+ 0.000156 0.0251 0.01 Mg2+ 1.92x10–5 0.0734 0.01 Cl– 0.002 6.34 0.99 F– 0.0001 0.00627 0.0013 HCO3

– 0.01 0.309 0.10 SO4

2– 0.0009 0.56 0.11 Si 0.00297 0.039 0.01 Fe 0.000056 0.0016 0.0005 NO3

– 0.0008 0.0015 0.0011

concentrations used in the test


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Temperature: long term exposure to the temperature prevailing at the depth of the repository can negatively affect Pcp

•induces loss of pore water (?)•accelerates groundwater chemical reactions•influences (positively or negatively) reactions with radiolytic gases T = 60 oC, initial test condition, considering

•geothermal gradient 0.075 oC.m–1, in top 400 m of the earth crust, •mean local earth surface temperature of 30 oC.

These figures are quite arbitrary but reasonable as a first approach, taking into account that the geothermal gradient varies between 10 and 200 oC.km–1, with an average, or normal, value 25 oC.km–1.


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Results

1. Mechanical strength - preliminary

ABNT Standards accept a coefficient of variation (CV) of less than 6% (for n = 4 specimens) for cement mortars. Expected values for Pcp was about 10%.

Preliminary trials, instead, showed a CV as high as 50%.

This could hamper the effects of stress factors being observed on Pcp samples, because the large pre-exposure variation could mask any post-exposure changes in material properties.


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Effects of each one of the experimental conditions on pressure of rupture (MPa) (confounding interactions)

No. of samples

Mean SD CV

Cement type

OPC 29 23 9 40

HES 30 24 11 44

Sample size

standard 20 30 10 34small 39 20 8 40

Mould type

disposable 39 25 10 41reusable 20 21 9 42

Sample cap

Top only 30 20 6 31 both sides 29 27 12 43

Pressure of rupture (MPa) of each sample set

Mean SD CV

A 23 5 20

B 35 9 25

C 15 3 19

D 21 9 41

E 25 10 39

F 19 7 38

G 22 2 8

H 42 6 14

I 21 7 34

J 22 9 41

K 17 6 34

L 23 11 49

Results of preliminary tests


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cement type sample size


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The density of the paste, on the other hand, is uniform in all samples, the observed variation being the result more of error in the geometric volume measurements than in the actual quantity.

All results fall inside an envelope around the mean of %4,2%8,2X


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Investigation of possible causes of the observed variability:

1. mistakes in sample preparation (for instance, too many or too large entrained air bubbles)

2. poor cement quality or aged packages (partially hydrated)

3. inadequate plastic moulds (low deviation toward a conical shape)

4. bad storage conditions during setting and hardening of samples (hardening in sealed moulds vs hardening in saturated moist air)

5. defective test machine or test setup (off center sample holder)


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Actions taken

1. Recalibration of test machine and check of test conditions

2. Agreement with the Institute of Technology Research (ipt), a recognized Brazilian laboratory of cement: a series of samples was cast and tested, aiming at finding the causes of the observed large variation in the strength of the Pcp samples.

set number cement origen mould type casting lab testing lab

A Supplied by retailer plastic ipen ipenB Supplied by retailer plastic ipen iptC Supplied by retailer plastic ipt ipenD Supplied by retailer plastic ipt iptE Supplied by retailer steel ipen ipenF Supplied by retailer steel ipen iptG Supplied by retailer steel ipt ipenH Supplied by retailer steel ipt iptI Supplied by ipt plastic ipt ipenJ Supplied by ipt plastic ipt iptK Supplied by ipt steel ipt ipenL Supplied by ipt steel ipt ipt


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Comparision of results from ipen and ipt labs


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LOW, HIGH AND MEDIAN PRESSURE OF RUPTURE (MPa)EFFECT OF EACH FACTOR - OTHER FACTORS CONFOUNDED

0

10

20

30

40

50

60

ALL IPEN IPT PLASTIC STANDARD IPEN IPT IPEN IPT

CAST LAB MOULD TYPE CEMENT ORIGEN TEST LAB

Comparision of results from ipen and ipt labs


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Discussion of preliminary results

Results of mechanical strength of Pcp, as measured by axial compression of cylindrical samples, showed a much larger variation than the expected CV of around 10%, even those cast and tested by the reference laboratory personnel.

Careful choice of experimental conditions could not avoid that variation and its reason is as yet unexplained.

It seems as if the variability in mechanical strength of Pcp were intrinsically large, a result that was not found in the examined literature.

However, we continue to pursue an explanation to this finding as well a means of avoiding this large uncertainty in further experiments.


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It follows the initially established experimental plan of exposing samples to the deteriorating factors. 144 samples, randomly distributed among 24 sets

Sample set ID

factor value A B C D E F G H I J K L M N O P Q R S T U V W X

immersion

distilled water x x x x x x x x

salt solution x x x x x x x x

dry storage x x x x x x x x

temperature20 oC x x x x x x x x x x x x

60 oC x x x x x x x x x x x x

immersion time

30 d x x x x x x x x x x

60 d x x x x x x x x x x x x x x

irradiation0 kGy x x x x x x x x x x x x

400 kGy x x x x x x x x x x x x


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Results of the multifactorial experiment - strength


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Median Mean ± sd

19 22 ± 10

20 23 ± 10

21 23 ± 9



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Median Mean ± sd

21 24 ± 10

19 21 ± 9


20


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Median Mean ± sd

19 20 ± 7

24 25 ± 11



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Results of the multifactorial experiment – strength

distilled water

Median Mean ± sd

22 25 ± 10

20 22 ± 9


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Results of the multifactorial experiment – Shrinkage/swelling

0 = after setting 1 = after immersion3 = before irradiation 4 = after irradiation

Distilled water


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0

4

8

12

16

0.62-1.32 1.32-2.02 2.02-2.72 2.72-3.42 3.42-4.12 4.12-4.93

weight increase (%)

fre

qu

en

cy

30 days

60 days

0

2

4

6

8

10

12

14

0.62-1.32 1.32-2.02 2.02-2.72 2.72-3.42 3.42-4.12 4.12-4.93

weight increase (%)

fre

qu

en

cy

distilledwatersaltsolution

0

4

8

12

16

20

0.62-1.32 1.32-2.02 2.02-2.72 2.72-3.42 3.42-4.12 4.12-4.93

weight increase (%)

fre

qu

en

cy

20 oC

60 oC

0

5

10

15

20

25

0.05 - 0.49 0.49 - 0.93 0.93 - 1.37 1.37 - 1.81 1.81 - 2.25 2.25 - 2.69

weight loss (%)

fre

qu

en

cy

notirradiated

irradiated

Results of the multifactorial experiment – water absorption on immersion (1, 2, 3) loss after immersion and irradiation (4)

1 2

3 4


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0

4

8

12

16

20

0 - 2 2 - 4 4 - 6 6 - 8 8 - 10 10 - 12

weight loss (%)fr

eq

ue

nc

y

20 oC

60 oC

0

2

4

6

8

10

12

0-2 2-4 4-6 6-8 8-10 10-12

weight loss (%)

fre

qu

en

cy

notirradiatedirradiated

Results of the multifactorial experiment – water loss after dry storage and irradiation


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Sample No. Immersion Temperature (°C) Immersion time (d) Irradiation

3 Salt solution 60 60 NO

4 Salt solution 60 60 NO

5 Dry storage 20 - NO

6 Dry storage 20 - NO

41 Salt solution 60 60 YES

42 Salt solution 60 60 YES

45 Dry storage 20 - YES

46 Dry storage 20 - YES

49 Dry storage 60 60 YES

50 Dry storage 60 60 YES

Results of the multifactorial experiment – X-Ray Diffraction50 samples analyzed – results of 10 samples presented here


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Ettringite peak – not observed in samples 49 and 50

Results of the multifactorial experimentDiffractogram of the 10 samples


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Results of the multifactorial experimentDiffractogram of sample ‘5’ with main peaks described


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Diffractograms of 10 different samples

Results of the multifactorial experimentCluster Analysis - the 10 samples were grouped in 4 clusters by similarity


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Results of the multifactorial experimentDendrogram of the XRD results


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Results of the multifactorial experiment:scanning electron micrographs - sample 1 surface, after immersion in salt solution;needlelike crystals are probably ettringite because of form and sulfur presence.

needles

grains

1


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Results of the multifactorial experiment:scanning electron micrography- sample 2 surface, after immersion in salt solution;Ettringite evidence is consistent with SO4

2- ions presence in immersion solution

2

needles

grains


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Results of the multifactorial experiment:scanning electron micrography- sample 3 surface, after immersion in salt solution;

grain 1

grain 2


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Results of the multifactorial experimentImmersion bath composition analyses

(g.L1) Ca2+

(x102)Na+

(x102)K+

(x102)Mg2+

(x104)Si

(x104)Fe2+

(x104)Al

(x104)Cl

(x103)F

(x105)SO4

2

(x103)NO3

(x104)

DW / 20oC 5.90 5.52 20.4 0.94 22.8 1.80 1,760 1.45 13.5 3.13 0.50

DW / 60oC 13 13.9 45.2 605 6.00 1.80 1,540 0.20 5.0 22.4 41.9

SS (initial) 173 121 6.38 445 40.5 1.50 - 4,920 500 564 90.0

SS / 20oC 148 127 34.4 3.35 3.11 4.50 - 4,610 600 380 185

SS / 60oC 146 137 59.9 4.84 2.58 9.25 7.75 7,370 214 575 40.4

DW = distilled water; SS = salt solution


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Results of the multifactorial experiment: Immersion bath composition analyses

CaNa

K

Mg

Si

Fe

Al

Cl

F

NO3

SO42

KFe

ClNaSO4

2

CaNO3

F

Si

Mg

Concentration of species in g.L1

Ratio final/initial concentrations

[Ca, Na, K, Mg, Si, Fe, Al] by ICP-OES[Cl, F, SO4, NO3] by ion chromatography


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CONCLUSIONS

The Pcp samples were exposed to environmental conditions expected to induce changes in composition and structure which degrade the macroscopic properties of tested specimens.

The level of exposure were set as to reproduce the worst conditions expected to prevail in the environment of a repository for disused sealed radiation sources disposed of in a deep bore-hole.

In most cases the analytical techniques employed were not able to detect changes in sample properties because the variance of pre-exposure results was large enough to overshadow the effects of those changes in measured sample properties. Up to now, we could neither understand the large variability in fresh sample properties nor be able to avoid it.

However, about the question on whether to proceed with the research work, results thus far seem to give support to the chosen methodology.


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Annex: Cluster Analysis of XR Diffractograms

Cluster analysis simplifies the analysis of data, automatically sorting closely related scans into separate clusters and marking the most representative scan of each cluster as well as outlying patterns.It is a three step process:

1. Compare all scans with each other and build a correlation matrix representing the similarity of any given pair of scans.

2. Put the scans in different classes defined by similarity, the branches of a dendrogram, by agglomerative hierarchical cluster analysis.

3. Estimate the number of clusters by the KGS test or by the largest relative step on the dissimilarity scale, and determine the most representative scan within each cluster.


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Cluster Analysis of XR DiffractogramsThe KGS test describes a penalty function which can be plotted against the number of clusters.The minimum of this penalty function indicates the optimum cut-off value and thus the “right” number of clusters according to this method.The minimum represents a state where the clusters are as highly populated as possible, whilst simultaneously maintaining the smallest spread.The advantage of both methods with respect to other methods is that they always show a “best” cut-off value as a result.

KGS test stands for: Kelley, L.A., Gardner, S.P.,Sutcliffe, M.J. (1996) An automated approach for clustering an ensemble of NMR-derived protein structures into conformationally-related subfamilies, Protein Engineering, 9, 1063-1065.

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