Engineering, 2013, 5, 251-267 Published Online March 2013 (
A Talc-Based Cement-Poor Concrete for
Sealing Boreholes in Rock
Roland Pusch1*, Laurence Warr2, Georg Grathoff2, Alireza Pourbakhtiar1,
Sven Knutsson1, Mohammed Hatem Mohammed1
1Luleå University of Technology, Luleå, Sweden
2Geography & Geological Department, Greifswald University, Greifswald, Germany
Email: *
Received December 20, 2012; revised January 29, 2013; accepted February 6, 2013
Deep investigation boreholes in crystalline rock for site selection of repositories for high-level radioactive waste are
proposed to be sealed by installing a series of dense concrete and clay plugs. These should prevent radionuclides from
leaking canisters at depth to migrate to the biosphere through the holes. The concrete seals will be installed where the
holes intersect water-bearing fracture zones to serve as stable and low-permeable supports for adjacent clay plugs. Low
porosity and microstructural stability must be guaranteed for many thousands of years and ordinary Portland cement
with organic superplastizer will not fulfill the requirements since the high pH will cause degradation of contacting clay
and the organic additive can produce colloids with a capacity to carry radionuclides up to the biosphere. Very ce-
ment-poor concrete (<8%) based on low-pH cement and with talc as plasticizer is an option but it matures more slowly,
which requires that the construction of seals is made so that sufficient bearing capacity for carrying overlying clay seals
is reached.
Keywords: Clay; Concrete; Ductility; Strength; Superplasticizers; Talc
1. Introduction
Deep boreholes made in conjunction with site investiga-
tions for locating repositories for high-level radioactive
waste (HLW) should be effectively sealed for up to a
hundred thousand years [1]. For holes passing through
low-permeable rock with fine fractures only, sealing with
clay is suitable, whereas concrete seals are proposed for
those parts of the holes that intersect permeable fracture
zones [2], cf. Figures 1 and 2. The average number of
clay and concrete seals in 500 to 1000 m long holes is on
the order of 10 - 20. The concrete, which is cast down in
the holes, does not have to be water-tight but serve as a
filter for preventing particles from adjacent clay seals to
migrate through it and further into the fracture zones
where they can be dispersed and lost. Since the concrete
seals require a certain time to mature and obtain suffi-
cient bearing capacity for carrying the clay segments
without yielding, the whole sealing campaign must be
planned with respect both to the time-dependent increase
in strength of the concrete and of the clay seals, which
establish bonds to the rock that ultimately makes them
carry their own weight.
2. Objective
The seals consist of a series of clay segments containing
highly compacted smectite-rich clay, and segments of
concrete cast on top of them where the hole is intersected
by a fracture zone. Casting of concrete with high density
requires addition of superplasticizers, which are com-
monly of organic type. Since they can release organic
colloids with a potential to transport radionuclides up to
the biosphere [3], attempts are made to find chemically
compatible, inorganic additives and a candidate material,
fine-grained talc, has been used in pilot testing of con-
crete for use in boreholes. Ordinary Portland cement
raises the pH of the concrete so much that it can degrade
contacting clay seals by cation exchange and substitution
reactions [4]. Low-pH cement can be a suitable binder
and the present study included investigation of the prop-
erties of talc-concretes with similar aggregate composi-
tions, based on Portland and low-pH cement of type
Merit 5000, respectively.
3. A New Concrete
3.1. Criteria
In addition to placeability, acceptable performance of the
*Corresponding author.
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Figure 1. Schematic borehole profile. Grouting is made for
reducing the inflow of water into the hole from fracture
zones and for stabilizing them.
Figure 2. Schematic picture of borehole seals consisting of
very tight clay where the rock is of low permeability and
concrete where permeable fracture zones are intersected.
concrete to be cast in boreholes requires that its bearing
capacity is sufficient to carry the load of clay plugs and
additional concrete seals. Therefore, a sufficiently high
compressive strength is needed both in the construction
phase and for at least one hundred thousand years [3].
Additional criteria are:
High erosion resistance: groundwater flow can cause
erosion of freshly cast concrete and reduce its sealing
and stabilizing functions. The best way of minimizing
the erodability is to prepare the concrete so that the
porosity and permeability are at minimum, which im-
plies high density. Experiments and theoretical con-
siderations show that the grain size distribution
should be parabolic (Fuller-type) in order to make the
aggregate grains form a filter that minimizes the risk
of erosive release and transport of particles [5].
Longevity: successive time-dependent changes in the
physical properties of the seals will be caused by
chemical and mineralogical reactions but this aspect
has not been fully recognized for clay plugs and al-
most neglected for concrete. The ultimate state of
contacting clay and concrete, implying complete de-
gradation and loss of coherence of the plugs, must be
defined and their function assessed [6,7]. It means, in
practice, that the remainder after complete loss of the
binder must still work acceptably with respect to the
filtering and strength criteria. All these items were
considered in the present study.
3.2. Test Program
3.2.1. Major Issues
The scope of this study was to develop an optimal con-
crete recipe with respect to fluidity for casting in deep
boreholes, and with appropriate compressive strength and
low potential to cause degradation of contacting clay.
Pilot tests had shown that talc can serve as a superplasti-
cizer but it remained to be shown that this concrete
component does not raise pH beyond that of concrete
with low-pH cement and no talc. The present study in-
cluded such testing. For comparing the performance of
concretes with organic and inorganic superplastizers (SP)
two brands were prepared and tested, one with Portland
cement and organic SP (Glenium 51) and one with Merit
5000 low-pH cement and inorganic SP (talc). The ag-
gregate components were crushed quartzite, and fine-
grained quartz.
3.2.2. Concrete Components
The concrete seals to be cast in boreholes should have a
minimum amount of cement for reducing the mutual
chemical impact on contacting clay seals. The use of talc
was firstly for decreasing the viscosity of the mixture at
preparation and secondly for finding out if it can contrib-
ute to the mechanical strength of the concrete by chemi-
cal interaction with the cement component.
The very low content of cement (<4.5%) will make the
concrete sensitive to chemical and physical disturbances
and the expected ultimate loss of binder will reduce the
strength of the concretes. Only the aggregate grains will
remain by then and for maintaining as much strength as
possible the aggregate should have a high internal fric-
tion, which was achieved by using freshly crushed
quartzite as a main aggregate component. For obtaining a
suitable overall grain size distribution of Fuller type,
adequate finer material was added in the form of fine
quartz-rich sand.
3.2.3. Superplasticizers
The following materials were used as superplasticizers.
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3.2.5. Preparation of Test Samples Glenium: Glenium 51, a commonly used organic sub-
stance manufactured by BASF Chemical Co, UK. Three concretes mixtures, one with Portland cement and
two with low-pH Merit 5000 cement, were prepared by
using a laboratory mixer.
Talc: very fine white mineral powder with the che-
mical formula 3MgO·4SiO2·H2O. It is chemically
stable in ordinary groundwater and has no impact on
the environment. It is hydrophobic and low-viscous
and does not form gels. The manufacturer of the ma-
terial was VWR International Company, UK. The
grain size distribution is shown in Figure 3.
Intense agitation was used for reaching a high degree
of homogeneity. The semi-fluid concrete mixtures were
poured in plastic tubes of 10 cm length and 5 cm diame-
ter and slightly vibrated. The tubes, which had been pa-
per-lined for providing the maturing samples with tap
water, were stored under water in an aquarium.
3.2.4. Aggregate The recipes for the Glenium and talc concretes using
Portland cement are given in Tables 1 and 2 and for the
concrete using low-pH Merit 5000 cement as described
by Table 3. The amount of water was selected so as to
make them equally fluid for casting.
The aggregate, delivered by Forshammar AB, Sweden,
consisted of crushed and ground quartzite to which very
fine milled rock flour was added. This fraction, which
contained 75.6% quartz and 14% Na/Ca feldspars, has a
specific density of 2600 kg/m3. The complete granu-
lometry of the aggregate material is shown in Figure 4.
One notices that for Portland cement the highest den-
sity was obtained when the amount of cement was very
Figure 3. Grain size distribution of talc powder used in the study. Weight percent smaller than the respective size is given on
the vertical axis.
Figure 4. The grain size distribution of the aggregate material consisting of 70% “coarse” and 30% “fine”, quartz-dominated
Table 1. Glenium (G) concrete recipe for Portland cement in weight percentages of total mass.
Cement/aggregate w/c Cement% G% solid Aggregate% Water% Density (kg/m3)
0.055 2.0 4.5 1 85.0 10.5 2292
Table 2. Talc (T) concrete recipe for Portland cement, weight percentages of total mass.
Cement/aggregate w/c Cement% T% Aggregate% Water% Density kg/m3
0.062 3.8 1.7 9.5 69.0 19.8 2125
Table 3. Talc (T) concrete recipe for Merit 5000 cement.
Cement/aggregate w/c Cement% T% Aggregate% Water% Density kg/m3
0.078 3.6 0.8 7.6 68 23.6 2028
small. An increase in cement content gave a significant
drop in density and increase in water content for reaching
the same fluidity. For Merit 5000 cement the same
weight fraction of the aggregate required somewhat more
water to get it fluid, which led to a lower density. The
potential of talc to cause fluidity is significantly lower
than for the organic superplasticizer.
4. Test Results
4.1. Fluidity
The fluidity of concrete is traditionally determined by
conducting slump tests that give the height of the pile
formed by letting the freshly prepared concrete flow out
from the standard metal cone in which it is prepared. The
results for the concretes with Glenium and talc composed
as in Tables 1 and 2 are given in Table 4. The figures
represent the height of the slumps after the outflow.
4.2. pH
The pH of the freshly prepared concretes was determined
by use of litmus strip papers since pH glass electrodes
could not be inserted because of the dominance of coarse
aggregate grains. The freshly prepared concrete with
Portland cement and Glenium had a pH value of 12 while
the concrete with talc had a pH value of 13. In contrast,
the talc concrete with Merit 5000 had pH = 10.
4.3. Strength
4.3.1. Compressive Strength of Cement Pastes
The cement component of dense concrete generates co-
hesive bonds that are mobilized at small strains and de-
termine the peak strength. Bulk failure is initiated by
successive breakage of these bonds by shearing. The
shear and compressive strength of the cement paste con-
trols, in principle, the strength of the respective concretes
and unconfined compression tests of pastes of the two
cements were made for comparison.
They were prepared with different water/cement ratios
and allowed to mature under water for 10 days for
reaching complete hydration. The values are given in
Table 5, from which one finds that the Portland cement
pastes were 5 to 30 times stronger than the Merit 5000
The strength of the Portland cement paste with a wa-
ter/cement ratio of 3 was almost the same as for the ratio
1, indicating that the lower water content did not give
complete hydration. The same was found for Merit 5000
cement. The study shows that the Portland paste was
considerably stronger than the Merit, which suggests that
the strength of the respective concretes would also be
very different. If Merit concrete would turn out to be
stronger, chemical reactions involving the talc compo-
nent would be the reason.
4.3.2. Compressive Strength of Concrete
The unconfined compressive strength was determined
after 2, 7 and 28 days. The compression rate was 1.5% of
the sample height per minute until failure appeared in the
form of cracks. For ductile concrete with no obvious
breakage, failure was taken to be the time to reach 10%
strain. Two to three series of tests were made for each
material. The data are collected in Table 6. The differ-
ence in strength is most obvious for the 2-day and 28-day
samples of Portland concrete with Glenium while the
7-day samples were much stronger than those tested after
2 days.
This suggests that the chemical reactions that gave the
ultimately relatively high strength were rather slow.
Portland concrete with talc reached a peak strength after
7 days and did not undergo further strengthening. Table
7 gives the compressive strength in MPa of concretes
with Portland and Merit 5000 using talc as superplasti-
cizer. It shows that while the Portland concrete was
stronger in the first weeks the Merit concrete was
Copyright © 2013 SciRes. ENG
Table 4. Slump test for glenium and talc concrete pastes
using portland cement. The height of the pastes before the
outflow was 14.5 cm.
Glenium concrete Talc concrete
4.5 cm 3.5 cm
Table 5. Compressive strength after 10 days of cement
pastes without superplasticizer. Average values from two
test series.
Test Cement
Compressive strength
1 Portland 1 9.0
2 Portland 3 8.9
3 Merit 5000 1 1.4
4 Merit 5000 3 1.1
Table 6. Compressive strength of Portland concrete com-
posed according to Tables 1 and 2.
Glenium Talc
time Density
2 days 2200 1.25 - 1.28 2125 0.21 - 0.34
7 days 2200 1.60 - 2.90 2100 0.89 - 1.10
28 days 2100 5.10 - 5.68 1990 0.82 - 0.91
Table 7. Comparison of the compressive strengths of Port-
land and Merit 5000 concretes with talc as the super-
Portland concrete
Merit 5000 concrete
2 days 0.52 - 0.57 0.01
7 days 0.62 - 0.70 0.10 - 0.12
28 days 0.76 - 0.88 2.62 - 2.65
significantly stronger after four weeks. The two cement
types obviously interacted differently with the talc com-
ponent, the Portland concrete earlier and only to a minor
degree, the Merit slower but much more extensively.
A separate test series was conducted with Portland and
Merit 5000 concretes with Glenium as superplasticizer
for investigating if the Merit concrete would undergo
significant strengthening with time. In this study fine-
grained quartz powder of silt size was used as the only
aggregate component. As shown by the data in Table 8
the compressive strength of the Merit concrete did not
significantly increase during 28 days of maturation.
4.3.3. Tensile Strength
The stress/strain behaviour of the talc concretes is dif-
ferent from those containing Glenium. Testing was made
by using the so-called “Brazilian” test technique (ISRM)
according to which a disc sample with radius R and
thickness L is diametrically loaded by the force P in
Figure 5. At failure, which takes place when the tensile
stress in the loaded plane becomes critically high, the
tensile strength is given by Equation (1).
Testing was made after 30 days of maturation and
gave the results in Table 9. The Merit concrete was sig-
nificantly stronger than the Portland concrete and the
ratio of compressive and tensile strength was higher.
4.3.4. Stress/Strain Behaviour of Concretes
Figures 6 and 7 show the stress/strain behaviour at com-
Table 8. Comparison of the compressive strengths of
Portland and Merit 5000 concretes with Glenium as super-
plasticizer. The concentration was the same as given in
Table 3.
Portland concrete
Merit 5000 concrete
2 days 1.57 - 1.58 Not measurable
7 days 2.85 - 2.92 Not measurable
28 days 5.05 - 5.17 0.053-0-061
Figure 5. Test arrangement with typical tensile fracturing
of brittle material (Portland concrete).
Table 9. Tensile strength data from Brazilian tests.
Concrete type with Glenium Tensile strength (MPa), 30 days
Portland concrete 0.04 - 0.05
Merit 5000 concrete 0.10 - 0.11
Copyright © 2013 SciRes. ENG
pression of the Portland and Merit concretes matured
under water for 7 days, the peak values representing the
compressive strength. The Portland concrete showed 3%
compressive strain giving a peak value representing brit-
tle failure. In contrast, the Merit concrete showed ductile
behaviour and progressive strain (Figure 7). After 28
days the Merit concrete was as stiff as the Portland con-
crete and showed axial fracturing that is typical of brittle
material (Figure 8).
4.4. Chemical and Mineralogical Reactions
4.4.1. XRD
The function of the concrete is to provide axial support to
the clay seals for at least one hundred thousand years,
according to Swedish specifications [3]. During this time
the concrete must offer sufficient bearing capacity and
tightness, which requires high density and a minimum
content of cement. A further criterion is that it must not
significantly alter the contacting clay seals. Such impact
is caused by the alkali nature of the concrete solution that
Figure 6. Uniaxial compressive test of Portland concrete
with talc after 7 days.
Figure 7. Uniaxial compressive test of Merit concrete with
talc after 7 days.
Figure 8. Failed Merit concrete with talc matured for 28
originates from the alteration of Portland cement as con-
cluded from a three year field experiment [4]. This study
showed that extensive alteration of the smectite clay oc-
curs at the interface by dissolution and cation substitution,
with transformation to less expansive clay minerals and
amorphous siliceous compounds. Such reactions were
found to lead to significant and early loss in the strength
of the concrete within a distance of a few centimetres
from the clay/concrete contact.
The present study was confined to investigate chemi-
cal changes of the two concrete versions without interac-
tion with clay. 25 g of Portland or Merit 5000 cement
were mixed with 15 g of water, and prepared either with
or without 2.2 g of talc. XRD analysis of the starting
powders, as well as the products that matured, was made
after 7 days (Figures 9-11).
The talc peaks in the XRD pattern in Figure 10 are
labeled. The other peaks all belong to cement minerals,
namely portlandite (Port), ettringite (E), brownmillerite
(B) and larnite (L) as well as quartz (Q) and calcite (C).
The most intense peaks are labeled. The XRD analysis
was made using the instrument parameters: Cu-Kα radia-
tion, 40 kV 30 mA, Ni filter and a Lynx Eye detector us-
ing a Bruker D8 diffractometer.
4.4.2. Scanning Electron Imaging
The formation of talc coatings of aggregate grains was
evident when observed at the micron-scale but no direct
textural evidence of talc dissolution could be seen for
Portland concrete (Figures 12 and 13). For the Merit
some changes were seen (Figures 14 and 15).
The scanning electron microscopy verified two notable
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Figure 9. Powder XRD patterns (Cu Kα radiation) of the starting dry Merit, talc, and cement after 7 days of setting with and
without added talc. The two cements contain a large amount of amorphous components as well as calcite and minor amounts
of crystalline phases that could not be identified with certainty. The added talc does not change the mineralogy of the cement.
Figure 10. Powder XRD patterns (Cu Kα radiation) of the starting Portland cement with and without talc after 7 days of
maturation. The cements contains ettringite, portlandite, larnite, brownmillerite and calcite. The cement with added talc
contains the same minerals and that with additional talc, which therefore does not seem to have effected the mineralogy of the
Figure 11. Powder X-ray diffraction patterns of the two cements (without added aggregate) matured for 60 days. The top
pattern is solely cement, the middle is cement with 8.5% talc. The bottom plot is the difference between the top and middle
pattern, reflecting the added talc.
Figure 12. Accumulation of talc particles oriented around a rounded quartz grain (lost) in concrete.
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Figure 13. Scanning electron micrograph of talc concrete with Merit 5000 cement showing fibrous cement phases between
aggregate particles.
Figure 14. Talc particles in Merit concrete. The smooth, slightly rounded edges of the talc may be taken as an indication of
some dissolution.
features: 1) the formation of fibrous cement phases be-
tween aggregate particles, and 2) the alignment of talc
particles between larger aggregate particles.
Table 10 summarizes the element concentrations in
Merit concrete with and without talc after 7 days matura-
tion in initially electrolyte-free water.
The major difference between the cement with and
without talc was a loss in F, Al, S, and K and an increase
in Na and Mg in the presence of talc. Compared with
virgin cement the significant increase in Ca, Mg and Si
and the decrease in Al is the most obvious finding.
It indicates that the talc component was partly dis-
solved and the elements incorporated into new cementing
compounds that formed in the first week of reaction.
Further reactions caused the strengthening that was re-
corded after 28 days. The EDX results also imply that the
Figure 15. SEM photo of Merit cement with talc. The spectrum was taken from the spherical object, which is a cement grain
that collapsed during dissolution of the cement balls.
Table 10. Element concentration (weight%) of Merit 5000 cement with and without talc measured by energy dispersive
X-rays (EDX).
Material F Na Mg Al Si S K Ca Ti Fe
Raw - - 2.63 1.57 4.46 0.11 - 1.37 - -
Matured without talc 1.12 0.07 1.14 1.11 6.28 0.32 0.13 8.84 3.00 -
Matured with talc 0.08 0.60 3.06 0.60 7.66 0.08 0.03 8.15 2.47 0.23
dissolution of talc provided additional Mg for the ce-
menting minerals.
4.5. Hydraulic Conductivity
In this study the hydraulic conductivity was determined
on 2 cm thick samples, matured for different periods of
time days, by mounting them in oedometers and expos-
ing them to a hydraulic gradient of 25 m/m (meter water
height per meter flow length).
For the Portland concrete the conductivity was found
to be 4E10 m/s. This is in fair agreement with values
reported by Powers [8], who found the conductivity of
fresh Portland concrete paste to be E10 m/s after 6 - 8
days and to drop to E13 m/s after a very long time. For
the concrete with Merit cement and talc the conductivity
of a 7 days old sample was 5E9 m/s and 8E10 m/s for
a three week old one. The two concretes hence behaved
similarly with respect to permeability.
The conductivity is controlled by the degree of filling
Copyright © 2013 SciRes. ENG
of the voids in the cement paste through the progressive
replacement of the original cement minerals by precipi-
tating hydration products, initially in the form of cement
gel. The volume of the gel including gel pores produced
by hydration is approximately 2.3 times the volume of
the cement elements, which leads to a considerable re-
duction in free pore space according to Powers [8]. The
present authors believe that the hydraulic gradient has a
considerable impact on the gel structure, which develops
over a long period of time. Thus, high gradients are ex-
pected to cause piping and creation of channels that may
not self-seal.
5. Microstructural Modelling
5.1. Principles
Statistical treatment can be applied to define microstruc-
tural parameters [9]. For the talc-concrete the solid ma-
trix consists of four major components, 1) aggregate par-
ticles, 2) cement grains, 3) talc particles, and 4) water.
The aim was to define the granulometric size distribution
of these components, for which the sieve curves were
utilized. For the microstructural modelling the largest
grains were all assumed to have an equivalent “diameter”
of 1 mm and all aggregate grains to be tetrahedrons mo-
tivated by the angular shape caused by the crushing.
Each grain had an edge length representing the “equiva-
lent diameter” and a volume that being the base area
multiplied by the height and divided by 3. Its weight was
the volume multiplied by 2700 kg/m3, which made it
possible to calculate the number of grains belonging to
the respective size fractions, for which the equivalent
diameter was taken to be 0 - 0.8 mm (Fraction 1), 0.4 -
0.8 mm (Fraction 2), and 0 - 0.4 mm (Fraction 3). All
particles in the respective fraction were assumed to have
the same size and the number of them was calculated
from the weights given by the sieve curve. The size of
the quadratic REV1 in 2D was defined as 2000 × 2000
μm2. The thickness of it was taken to be 400 μm for giv-
ing fair representation of larger particles. The distribution
of tetrahedrons was made randomly using four examples,
which all lead to nearly the same particle and void dis-
tributions indicated in Figure 16. The thickness of the
“2D” section was assumed to be 400 μm and hence rep-
resent a pseudo-3D box, which made it possible to in-
clude complete aggregate grains of the 0 - 0.4 mm frac-
tion, i.e. the most important population.
5.2. Derivation of Calibrated Model
5.2.1. Objective
The principle of converting 3D to 2D versions of par-
ticulate systems was made using a method employed by
[5] for applying the Hagen-Poiseuille model used for
calculating conductivity. A first issue was to define a
model with basic components, the aggregate grains, for
comparing it with empirical data in order to find whether
the size and variation of the primary pores is reasonable.
Omitting grains larger than 1 mm and also talc, and tak-
ing cement grains to be spheres, a first step was to calcu-
late the number of grains belonging to selected size frac-
tions in the 400 μm thick elements with 2000 μm edge
Four cases representing different distributions of ag-
gregate grains were analyzed [9], using the densities ρtalc
= 2750 kg/m3, ρquartzite = 2650 kg/m3, ρcement = 2900 kg
The basis of the modeling was the weight percentages
of the three grain fractions 1 mm, 0.8 mm and 0.4 mm,
omitting grains larger than 1 mm. The number of 0 - 0.4
mm grains in Fraction 1 of the REV of the concretes was
0.58, while the number of Fraction 2 grains was 1.86,
and 28.9 grains of those belonging to Fraction 3. The
cement grains were assumed to form 30 clusters, each
with 1000 tightly grouped particles.
5.2.2. 2D Model for Calculating the Hydraulic
Using the calculated number of grains of each fraction
and the numbers of talc and cement clusters, a great vari-
ety of microstructural constitutions can be derived as
exemplified by Figure 17 in which one recognizes the
quartzite and quartz tetrahedrons represented by triangles
with 3 different sizes corresponding to the three size
fractions mentioned. The talc particles appear as thick
stacks marked red in the 2D sections, sticking to the ag-
gregate grains. The figure to the right shows the distribu-
tion of voids, represented by circles of two sizes, 150 and
75 μm.
5.2.3. Hydraulic Conductivity
Calculation of the hydraulic conductivity of the model
was made by using the Hagen/Poiseuille law, adopting
here the way of conceptualizing an individual pore as an
equivalent capillary tube in which flow occurs according
to the law for capillary flow, cf. [10,11].
The volumetric flow rate, Qh can be written as the sum
of the flow rates of individual saturated capillaries within
the ith fraction of the porous medium. One hence gets
where qt is the volumetric flow rate for a
single tube channel (cm3/s) and Npi being the number of
pores of the ith pore size fraction [12].
For a system consisting of N capillaries with radius r,
and a total cross section area of Atot, the porosity n can be
derived from Equation (2).
nNrA (2)
1Representative elementary volume.
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Figure 16. Schematic view of the REV for talc concrete in 2D. The section is taken to be 400 μm thick and host aggregate
grains representing the size fractions 0.8 - 1 mm, 0.4 - 0.8 mm, 0 - 0.4 mm. The non-hydrated cement grains are assumed to
be 20 μm in diameter. The talc particles are assumed to cover the whole surface of the indicated aggregate grains and to have
an average thickness of 100 micrometer.
Figure 17. Left: 2D model with tetrahedral aggregate grains, circular cement grains and talc patches (thick lines) in the left
picture. The cement clusters are circular, each of them representing 1000 cement particles. Right: Location of the voids
before hydration of the cement: 12 with 150 µm diameter and 39 with 75 µm diameter.
By using the Hagen-Poiseuille law (Equation (3)) one
gets for the flow rate v:
π8vr PL
 (3)
where, η is the fluid viscosity, ΔP is the pressure differ-
ence between in- and outflow ends, L is the total length
of the capillary.
Applying this to the four versions of the model the
flow rate q of water was found to be 8.7E4 to 1.2E3
cm3/s. Using Darcy’s law tot
, the average
conductivity K was in the interval 3.3E5 to 5.9E5 m/s
The K-values refer to the case of non-hydrated cement,
neglecting tortuosity, which is a measure of the influence
of twisted or dead-ending pores [13-15]. Applying the
tortuosity factor π/2, meaning that the diameter of the
flow paths are reduced by about 60%, the net hydraulic
conductivity of no more than E6 m/s was obtained,
which is typical of sandy/silty soils of high density. The
models are therefore concluded to be relevant from the
point of hydraulic conductivity.
The microstructure will change significantly from the
considered “dry” state to the “wet” stage when cement
hydration has taken place, implying absorption of water
and swelling of the cement particles [8]. Following Pow-
ers one can assume that the volume of the hydrated ce-
ment grains increases by 100%. This reduces the size and
interconnectivity of the voids, which will be occupied by
cement gels. Thus, the geometry of the ultimately formed
system of pores and capillaries depends strongly on the
changes of volume of the cement hydration products
Estimating the size of the individual water-bearing
channels of cross sections, like those in Figure 17, to be
reduced by 50% the net average hydraulic conductivity
of the matured modeled concretes will drop from E7 to
less than E10 m/s, which is hence in agreement with
Powers’ and or own results. The ultimate conductivity
after complete formation of cementing reaction products
goes down further but dissolution and loss of sealing
components will make it rise in a very long time perspec-
5.2.4. Maturation and Strength Increase
Cement hydration and hardening: two processes lead to
the hardening of cement by chemical reaction when
mixed with water: primarily the reaction between trical-
cium aluminate, and secondly the hydration of the cal-
cium silicates [17]. Since the concretes in the current
study have very small amounts of cement, their strength
is significantly lower and their hydraulic conductivity
higher than for typical construction concretes, which
have a water/cement ratio of 0.23 to 0.4. The relative
volumes of the aggregate grains, talc, cement hydration
products and excess water determine the distribution of
bonds and hence the strength of the microstructural net-
work. They are determined by processes influenced by
phase transitions [17], and by the successive dispersion
and loss of the binders.
Strength distribution in the microstructure of concrete:
the microstructural models described in the preceding
section can be used for developing hypotheses concern-
ing the distribution of the strength-producing bonds pro-
vided by the cement component and of the rate of
strength growth. For this purpose one can consider a
model version with 30 cementing units in a volume of
1.6 mm3, corresponding to 1.6E9 bonding units per cubic
meter of concrete. A large number of them, approxima-
tely 50% in the model, are separated from aggregate
grains by stacks of talc lamellae, and since virgin talc has
negligible shear strength the bulk strength of talc con-
crete is predicted to be low compared with ordinary con-
crete with no superplasticizer. However, chemical reac-
tions involving dissolution the talc component involving
dissolution of talc and precipitation of cementing com-
plexes, are concluded to give a successive increase in
strength as we see from the SEM pictures of the Merit
concrete (Figure 14), but not in those of the Portland
cement. A possible explanation of this difference is that
the dissolution/precipitation processes takes place quickly
and locally in the high-pH Portland concrete and slower
and more wide-spread in low-pH Merit concrete with
6. The Bearing Capacity of the Concrete
with Respect to the Actual Loading in
Deep Boreholes
6.1. Stress Distribution in Boreholes with Clay
and Concrete Seals
The role of the concrete seals is to support the clay plugs
and to serve as a filter for preventing clay particles from
migrating out into the fracture zones. The concrete con-
tains very little cement for minimizing the clay-degrad-
ing impact of pH caused by cement, which does therefore
not generate significant wall friction. The concrete ma-
tures slowly and does not harden until after one week.
Before this, it acts as a viscoplastic material with sub-
stantial internal friction. The clay also behaves viscoplas-
tically and expands by absorbing water from the sur-
rounding rock. It stiffens as a function of time after in-
stallation and establishes sufficient bonding to the rock to
carry itself after a couple of days.
The bore sealing procedure is as follows: 1) concrete is
cast to fill the hole from the bottom up to 5 m above the
lowest fracture zone and left to mature for 2 days to one
week; 2) the first clay plug is placed with its upper end 5
m below the second lowest fracture zone and left to ma-
ture for 2 days: 3) concrete is cast over the clay plug and
left to mature for a further 2 days; 4) the second clay
plug is installed and left to mature for 2 days, and so on.
A 1000 m deep hole will be intersected by 5 - 20 fracture
zones, meaning that the entire effective time for sealing
the hole is about 20 - 100 days, assuming that each in-
stallation takes one day. In principle, all the seals must
have hardened sufficiently to carry the overlying material
throughout the sealing campaign. The clay plugs can
expand axially but not be compressed because the clay is
confined in perforated copper tubes that form stiff units,
meaning that only the clay “skin” formed between the
tubes and the rock needs to be considered. The concrete
seals can be compressed but not expanded.
For determining whether each concrete seal can carry
the clay plug placed above it without yielding, the pres-
sure distribution in the entire series of seals must be
known. If the bearing capacity of a concrete plug is not
sufficient it will be pressed up along the tube with clay,
by which the sealing capacity of the clay is strongly re-
Copyright © 2013 SciRes. ENG
Copyright © 2013 SciRes. ENG
plug materials depends on the length, density and friction
angle of the plug units under consideration. The pressure
distribution is not linearly increasing with depth but fol-
lows some exponential law because of the impact of wall
duced and the strength of closest part of the adjacent
concrete reduced. This phenomenon was observed in a
field experiment [4].
6.2. Pressure Conditions
The seals are assumed to be installed sequentially in
one day and left to mature for 2 days during which the
respective plug materials are assumed to retain their
original, soft condition. For this early state both the con-
crete and the clay behave as heavy liquids characterized
only by their density and a low angle of internal and wall
friction ϕ [18].
6.2.1. Borehole Seal Systems
Holes bored in rock for locating a radioactive waste re-
pository most suitably, typically extend to about 1000 m
below the ground surface. They all penetrate a number of
fracture zones that form a network of steep, oblique and
more or less horizontal, permeable discontinuities. For
illustrating the principle of constructing seals, called
plugs after maturation, a reference borehole of 1000 m
length with four concrete and three clay seals is used
here (Figure 18).
Mathematical expressions for lateral and vertical pres-
sures in silo fillings were proposed by [19] and are still
commonly used. The vertical pressure has the following
form (Equation (4)).
The calculation of the effective axial stress conditions
is based on the premise that the borehole is kept water-
filled throughout the sealing campaign. The “Early Case”
with 1 day for installation and 2 days for rest of each
plug is considered here.
where, ph: Horizontal pressure, ρ: Density of the mate-
rial, R: Hydraulic radius, equal to the area/perimeter ratio
of the straight section in meters, ϕ: (Effective) angle of
wall friction of the material.
The vertical pressure at the contacts between different
1000 m
Figure 18. Profile of the reference borehol with installed plugs of clay and concrete. e
The concrete plugs will undergo some slight shrinkage
and compression under their own weight. This generates
axial shortening and thereby generation of wall friction
that increases with the age of the cast concrete.
The clay plugs undergo evolution in the form of ex-
pansion of the clay through the perforation of the tubes
(cf. Figure 3) that initially contain highly compacted
expandable (smectite) clay blocks. A clay “skin” is formed
around the tubes, providing an increasingly strong bond
between the rock and the clay. The shear strength of the
clay skin increases with time caused by the increasing
swelling (effective) pressure, meaning that the “wall fric-
tion” increases.
6.2.2. Evolution of the Effective Vertical Stress in the
Seals in a Deep Hole
The hole is assumed to be located in rock with the
groundwater level coinciding with the ground surface.
The pressure generated by the own weight of the seals is
gravitational and hence directed downwards while the
wall friction acts upwards and hence reduces the gravita-
tional force. Figure 18 specifies the levels for which the
effective pressure (P1, P2, etc.) shall be determined. All
segments have the same height, i.e. 142.85 m (1000/7 m).
The calculation refers to the successive construction
stages, starting with casting concrete to 142.85 m height
and leaving it for two days to rest (Stage 1), after which a
clay plug consisting of jointed 12 m long perforated
copper tubes filled with highly compacted clay are in-
serted with the top at 2 × 142.85 m = 285.70 m (Stage 2).
After letting the clay plug rest for two days on top of the
first concrete plug, the second concrete plug is cast with
its top 428.55 m above the lower end of the hole (Stage
3). It is left to rest for two days after which the second
clay plug is inserted, its upper end being 571.40 m above
the base of the hole (Stage 4). Two days later concrete is
cast over it with the upper end being 714.25 m above the
base of the hole (Stage 5). This third concrete plug is left
to rest for two days after which the third clay plug is in-
serted reaching up to 857.15 m above the base of the hole
(Stage 6). After two days of rest the fourth concrete plug
is cast up to the ground surface (Stage 7).
Calculated pressure distribution: calculation of the
vertical pressure according to this principle gave the
pressure distribution in Figure 19, from which one con-
cludes that the pressure increases successively with the
depth, ranging from 0.30 MPa2 at the base of the up-
permost concrete plug to 7.76 MPa at the base of the
borehole. The pressure increase is not linear for Stage 4
and subsequent stages because of the assumed increase in
internal and wall friction angles. The compressive strength
for each stage is consistently higher than the effective
stress, meaning that stable conditions prevail throughout
the sealing campaign. The only possibly critical condi-
tion is represented by Stage 2 for which the strength of
the clay “skin” has to be relied on for avoiding failure of
the concrete where the clay plug is in contact. In practice
one can raise the safety factor by letting the clay plug
hang on the drill rig during the required 2 days of matu-
Figure 19. Distribution of the effective pressure in the respective construction stages. The bearing capacity is represented by
he upper curve for each stage and the effective pressure by the lower curves. t
2Minus indicates pressure, plus is for tension.
Copyright © 2013 SciRes. ENG
Impact of volume change: the concrete plugs will un-
dergo some slight shrinkage and compression under their
own weight. This generates axial shortening and thereby
generation of wall friction that increases with the age of
the concrete. With increased age of the plugs follows
increased shear and compressive strengths, implying that
the system of plugs becomes stronger and that axial
strain under their own weight will be small. The ductility,
expandability and self-sealing capacity of the clay plugs
will make the whole series cohere and effectively tighten
the holes.
7. Discussion and Conclusions
The study led to new types of concrete for sealing deep
boreholes using crushed and milled quartzite with very
fine quartz-rich material as aggregate material, and Merit
5000 cement with talc as the superplasticizer. A recipe of
such a concrete is proposed in Table 11.
The cement content (0.8%) is very low in order to
limit the growth in void size that results from dissolution
and loss of this component, and also for minimizing the
impact of high pH on the chemical stability of contacting
clay seals in the boreholes. The density of the concrete is
believed to be sufficiently high to make the concrete
perform acceptably for very long periods of time even
after complete loss of the cement since the aggregate
matrix is as stiff and moraine soil. The pH of the Merit
concrete is also around 10 and hence much lower than
that of Portland concrete. The major conclusions from
the study are:
When using talc as a superplastifier, the Merit con-
crete has a low compressive strength after 7 days, i.e.
about 1/5 of that of Portland concrete, but is more
than 3 times higher strength after 28 days, which in-
dicates that the chemical reactions involving dissolu-
tion of the talc and neoformation of silicious cement-
ing compounds provides the high ultimate strength;
The Merit concrete is tough and ductile in the early
maturation phase as indicated by tensile stressing, but
shows brittleness after a few weeks. This makes the
concrete perform acceptably both in the construction
phase and later;
Talc serves to reduce the viscosity in the preparation
stage and to contribute to the strength of the maturing
Merit concrete with properly composed quartz-rich
aggregate and with talc additive fulfils the criteria set,
except that its placeability has not yet been demon-
8. Acknowledgements
This study profited from using a Bruker D8 diffractome-
ter and a Zeiss Auriga FIB-SEM crossbeam microscope
Table 11. Talc (T) concrete recipe for Merit 5000 cement,
weight percentages.
aggregate w/c Cement%T
%Aggregate% Water%Density
0.078 3.60.8 7.668.0 23.6 2028
at the University of Greifswald both of which were fi-
nanced by the German research foundation (DFG). We
also thank Manfred Zander for his technical assistance in
operating the Auriga microscope.
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