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Recycling of hardened cementitious material by pressure and
control of volumetric change
Yuya
Sakai
,
Biruktawit Taye Tarekegne
Toshiharu
Kishi
,
Journal of Advanced Concrete Technology,
volume ( ), pp.
14
2016
47-54
Sustainable Concrete made of Construction and Demolition Wastes using recycled Wastewater in the
UAE
Mohamed
Elchalakani
,
Elgaali
Elgaali
Journal of Advanced Concrete Technology,
volume ( ), pp.
10
2012
110-125
Improving the Quality of Recycled Fine Aggregates by Selective Removal of Brittleness Defects
Hideo
Ogawa
,
Toyoharu
Nawa
Journal of Advanced Concrete Technology,
volume ( ), pp.
10
2012
395-410
Durability of recycled aggregate concrete: An overview
Jianzhuang
Xiao
,
Deng
Lu
Jingwei
Ying
,
Journal of Advanced Concrete Technology,
volume ( ), pp.
11
2013
347-359
Effective Recycling of Surface Modification Aggregate using Microwave Heating
Heesup
Choi
,
Ryoma
Kitagaki
Takafumi
Noguchi
,
Journal of Advanced Concrete Technology,
volume ( ), pp.
12
2014
34-35
Method for Estimating Quantity of Non-Hydrated Cement in a Cement Recycling System
Daiki
Atarashi
,
Tetsuji
Kamio
Yutaka
Aikawa
,
,
Masahiro
Miyauchi, Etsuo Sakai
Journal of Advanced Concrete Technology,
volume ( ), pp.
13
2015
44-49
Journal of Advanced Concrete Technolog y Vol. 14, 47-54 February 2016 / Copyright © 2016 Japan Concr ete Institute47
Scientific paper
Recycling of Hardened Cementitious Material by Pressure and Control of
Volumetric Change
Yuya Sakai1*, Biruktawit Taye Tarekegne2 and Toshiharu Kishi3
Received 24 June 2015, accepted 13 February 2016 doi:10.3151/jact.14. 47
Abstract
This study proposes a novel method for recycling concrete waste by compaction. The advantages of the proposed method
include its high production speed and complete recycling; all of the concrete waste was used without segregation of gravel
or other components. First, hardened cement paste (HCP) was examined, crushed, milled, and compacted. These formed
compacts showed high strength. Compacts of hardened concrete (HC) were prepared after crushing and milling; however,
their strength was low. The processes of adding HCP or sludge cakes and performing carbonation treatment were inves-
tigated to increase the HC compact strength, and all treatments were found to be effective. Another advantage of this
compaction method is that the control of volumetric changes due to moisture loss or absorption can be achieved by
adequately drying the powder before compaction.
1. Introduction
Concrete is currently one of the primary materials used in
construction; however, it has many drawbacks and ex-
tensive effort is required to overcome or compensate for
these shortcomings. For example, recycling concrete
waste is not easily accomplished compared with other
materials. Because of this difficulty, in addition to the
CO2 emissions generated during cement production,
concrete is generally regarded as environmentally un-
friendly (Guggemos and Horvath 2005). Another chal-
lenge lies in obtaining quality aggregates (Buck 1997).
Under such circumstances, an epoch-making concept,
“perfect recycling concrete” (PRC), was proposed in the
1990s (Tomosawa and Noguchi 1996; Tomosawa et al.
1999; Noguchi and Tamura 2001). However, PRC has
not yet been practically achieved. In general, recycling of
concrete waste involves reusing the aggregate or using
the waste as a base course material (Li 2008; Hansen
2004). To reuse the aggregate, the cement paste on the
surface must be removed or it will result in a reduction of
strength and an increase in drying shrinkage of the ap-
plied concrete (Hansen and Boegh 1985). However,
complete removal of the attached cement paste is ex-
pensive since it consumes large amounts of energy and
generates large CO2 emissions (Karthik 2009). The im-
provement of aggregates has been investigated as a
method to simplify their reuse, but has not yet been fully
developed (Kameyama et al. 2014).
Other shortcomings include a long curing period in
order to achieve sufficient strength and drying shrinkage.
To shorten the curing period or reduce shrinkage, re-
searchers have developed a high-early strengthening
agent (Matsunaga et al. 2001) and dry-
ing-shrinkage-reducing additives (Collepardi et al. 2005;
Weiss et al. 2008); however, these countermeasures still
have shortcomings regarding their effectiveness and cost.
Problems associated with curing and shrinkage arise
from the fact that hardened cement paste (HCP) is the
product of the hydration reaction of cement. Here,
however, according to a pioneering study conducted by
Soroka and Sereda (1968), a hydration reaction is not
necessarily required to produce HCP of sufficient
strength. They reported that when HCP is crushed and
pressed, its stiffness and hardness are the same as those
of HCP before crushing if the total pore volume is the
same. Yoshimoto et al. (1978) crushed and pressed
hardened mortar (HM) and reported, “the strength [was]
much less than intact” HM, but that it “bonded com-
pletely.” These experiments were conducted to explore
the bonding and interactions of HCP and the plastic de-
formation of concrete under high pressures. Here, if we
interpret their results from a different viewpoint, they
suggest that the strength of concrete waste can be re-
gained through compaction.
In this study, we recycle concrete without separation of
the aggregate or production of waste. Although our ap-
proach is unsuitable to form a complex-shaped products
(ordinary concrete is well-suited for such purposes), the
results of this study enable us to make use of an under-
utilized property of the cement paste matrix—its plastic
deformability. As will be discussed in detail later, our
results showed that the volumetric change of the com-
pacts can be controlled by adjusting the moisture content
of the powder before compaction. Because compaction
requires only 10 min, it can be used to avoid time-related
shortcomings inherent to concrete, such as a long curing
1Assistant Professor, Institute of Industrial Science, The
University of Tokyo, Tokyo, Japan.
*Corresponding author, E-mail: ysakai@iis.u-tokyo.ac.jp
2Guraduate student, Graduate school of Engineering, The
University of Tokyo, Tokyo, Japan.
3Professor, Institute of Industrial Science, The University
of Tokyo, Tokyo, Japan.
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 48
period and extensive drying shrinkage.
In this study, we crushed and milled HCP and hard-
ened concrete (HC) and compacted the prepared powder
to form compacts. The compressive strengths of the
samples were subsequently measured. First, the effect of
compaction pressure and the volume ratio between the
cement paste and aggregate were examined. To study the
properties of the compacts from a different viewpoint,
gravel and chemicals (calcium hydroxide and gypsum)
were also compacted and examined. The compacts were
observed by scanning electron microscopy (SEM) to
determine what happened to the particles during com-
paction. Improvements of compact strength achieved via
the following two approaches were investigated. In the
first approach, sludge cake, i.e., waste material from a
concrete plant, was used. The cake was dried, crushed,
milled, and added to the crushed concrete waste. In the
second approach, carbonation treatment was applied to
the prepared compacts. Arinaga et al. (2004, 2005) and
Norimasa et al. (2011) prepared compacts of calcium
hydroxide and demonstrated that subjecting the com-
pacts to carbonation treatment significantly improved
their strength. Finally, the possibility of controlling the
volume change of the compacts caused by moisture loss
or absorption was investigated by changing their mois-
ture content. This was made possible by the fact that the
preparation of the compacts did not require water; i.e.,
their moisture content was changed by drying the pow-
ders before compaction.
2. Experimental
2.1 Materials and preparation of powders for
compaction
This study primarily used powders prepared by crushing
and milling HCP and HC. The specifications of the ma-
terials used in this study are listed in Tabl e 1 . The wa-
ter-to-cement ratio (W/C) of the HCP was 40%, and the
HCP was cured under sealing for 1 year. The W/C ratio,
cement content, and sand ratio of the HC were 0.4, 435
kg/m3, and 0.4, respectively. After a sealed curing of 28
days, the HC was cured for 2 years in a room at 20 °C,
where the uncontrolled humidity averaged 60%. The
crushing was executed with a jaw crusher and a disk mill,
and the powder that passed through a 200-μm sieve was
collected and used in the tests. The collected powder was
stored in sealed containers until the compaction process.
The materials were stored and all procedures were con-
ducted in a room at 20 °C. To measure the moisture
content, a 5 g powder sample was heated in an oven at
105 °C and dried until the weight decrease of the drying
became less than 0.01 g over 24 h. The obtained moisture
content of the HCP and HC were 20.8% and 15.6%,
respectively. Here, the moisture content was calculated
by dividing the weight change from drying by the final
weight after drying.
Sludge cakes were filter-pressed at a concrete plant
and dried outside for 3 days (Max. and min. temperatures
were 20 °C and 10 °C, respectively) and dried at 40 °C
for 24 h. The dried cakes were crushed and milled, and
the powder that passed through a 200-μm sieve was
collected. The moisture content was 50.8% and 9.7%
before and after the drying process, respectively. The
calcium hydroxide and gypsum used in this study were
commercially available extra-pure reagents; they were
not subjected to crushing or milling. For some of the
cubic compacts, carbonation treatment was performed by
placing them in a chamber filled with CO2 gas at 20 °C
for 48 h after compaction.
2.2 Preparation of compacts and performance
of compression tests
The compressive strengths of the compacts of crushed
and milled HCP, HC, and gravel were measured. The
effect of the volume fraction of cement paste on strength
was studied by adding HCP powder to HC powder. The
compacts were prepared by applying a unidirectional
load or hydrostatic pressure.
For the unidirectional pressing, a set of steel molds
(SS400) with a cylindrical cavity 52.5 mm in diameter
and 120 mm in height was used. The molds were com-
posed of four separate pieces (Fig. 1) that were con-
nected with bolts. Powder was poured into the cylindrical
cavity, and a steel cylinder with diameter of 52 mm was
used to compress the powder within the cavity. The cyl-
inder was controlled by a universal testing machine. The
applied pressures were 50–200 MPa for HCP and 200
MPa for HC, and each pressure was maintained for 10
min. Here, pressure is defined as the applied load divided
by the cross-sectional area of the compact. After each
compaction, the molds were cleaned with a nylon
scrubber. An example of a prepared compact is shown in
Fig. 2. Because of the mold size, the prepared compacts
were cylinders with a diameter of 52 mm and a height of
60 mm. Because the height of the cylinders could be
varied depending on the applied stress, the amount of
powder was adjusted such that the compacts had the
Table 1 Materials specifications.
Cement Ordinary Portland cement (density: 3.15 g/cm3)
Sand Fujigawa river sand (surface dry density: 2.62
g/cm3; water absorption: 2.1%; FM: 2.55)
Gravel
Chichibu-ryogami crushed stone (surface dry den-
sity: 2.72 g/cm3; water absorption: 0.5%; maxi-
mum diameter: 20 mm; FM: 6.67)
Fig. 1 Separable steel mold.
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 49
same height. Two compacts were prepared for each case.
In the process of pressing by hydrostatic pressure, a
silicon mold with a cubic cavity having 10 mm edges was
used (Fig. 3). Powder was poured into the space and
covered with a silicon plate, and the mold was vac-
uum-packed. Hydrostatic pressure was then applied for
10 min. As shown in Fig. 4, the prepared compacts were
cubes with edges 10 mm in length. The dimensions of the
compacts were measured using a digital caliper. The
sizes of the compacts prepared under 130 MPa and 400
MPa were the same, even though the same amount of
powder was used. Three compacts were prepared for
each case. After each compaction, the molds were wiped
with a cloth.
The strength of each prepared compact was measured
using a universal testing machine. The loading rates were
30 and 1.5 kN/min for the cylinders and cubes, respec-
tively.
2.3 SEM observations of compacts
As previously discussed, crushed cement paste is known
to regain its strength when compacted (Soroka and
Sereda, 1968); however, the mechanisms responsible for
changes to the powder under pressure are unclear. We
used SEM to observe changes in the powder under the
applied pressure. The observed samples were prepared as
follows. First, 3 g of crushed and milled HCP were
placed on a glass plate measuring 26 × 20 × 1.5 mm3. The
sample was vacuum packed, and a hydrostatic pressure
of 5–400 MPa was applied for 10 min. After the sample
was pressed, the glass plate was removed and the side
that was in contact with the plate was observed by SEM.
Powders of calcium hydroxide and gypsum were ob-
served using the same procedure.
2.4 Strain measurements of compacts
The compacts formed in this study do not require water
for preparation; thus, the water content of the compacts
was varied by changing the water content of the powder
before compaction. Such manipulation of the water
content may enable control of the volumetric changes of
the compacts due to moisture loss or absorption. To study
the strain of the HCP compacts with different moisture
contents, powder dried for 24 h at 40 °C or 105 °C and
non-dried powder were compacted under unidirectional
pressure of 200 MPa for 10 min. The weight reductions
due to drying at 40 °C and 105 °C were 10% and 17%,
respectively. The prepared compacts were circular plates
5 cm in diameter and 1 cm thick, as shown in Fig. 5.
After compaction, chips used to measure the strain with a
contact strain gage (4 cm measurement length) were
attached to the compacts, which were then placed in a
chamber at 20 °C and 60% relative humidity. Two
compacts were prepared for each case, and chips were
attached to both surfaces. The reported values are the
averaged strains of the two compacts. The weight
changes of the compacts dried at 105 °C were measured
to monitor moisture absorption while in the chamber of
20 °C and 60% relative humidity.
Fig. 4 Compacts prepared under hydrostatic pressure.
Fig. 5 Compact for strain measurement.
Fig. 2 Compact prepared by unidirectional pressing.
Fig. 3 Silicon mold.
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 50
3. Results
3.1 Compressive strength of HCP compacts
First, the compressive strengths of the HCP compacts are
shown in Fig. 6. In the figure, the two results for each test
case are connected with solid vertical lines, and the av-
erages for each pair are connected with broken lines. The
figure shows that the compressive strength trend of the
compacts under unidirectional loading is proportional to
the applied stress. In Fig. 6, the compressive strengths of
the compacts prepared with hydrostatic pressures of 130
MPa and 400 MPa are shown. For comparison, the figure
also shows the measurements of cubic HCP samples with
1-cm edges that were cut from a lump of cement paste
weighing 10 kg. The W/C ratio of the cement paste was
0.5, and it was cured under sealed condition for 1 year.
The compacts prepared using hydrostatic pressure ex-
hibited a compressive strength of 60 MPa, which is three
times greater than that of the compacts prepared by uni-
directional pressing. Carbonated compacts prepared with
a hydrostatic pressure of 130 MPa exhibited an average
compressive strength of 150 MPa, three times greater
than that of the samples without carbonation treatment.
3.2 SEM observation of compacts
The SEM images of HCP powder after compaction are
shown in Fig. 7. When a pressure of 5 MPa was applied,
the change in the particles was not significant; they
contacted each other at points but did not exhibit a no-
tably altered appearance. When the pressure exceeded 10
MPa, the particles deformed significantly, filling the gaps.
The boundaries between the particles became vague. In
Fig. 7(d), most of the gaps are filled, whereas some
remain discontinuous. Fig. 8 shows a magnified version
of the image in Fig. 7(b). At a relatively low applied
pressure of 10 MPa, granular and needle-like hydrates
appear flattened and the boundaries with surrounding
hydrates appear to be vague. An SEM image of calcium
hydroxide is shown in Fig. 9. Compared with HCP, the
calcium hydroxide exhibited less particle deformation
Fig. 6 Compressive strength of crushed and compacted
HCP.
Compaction stress (MPa)
Compressive strength（MPa）
Unidirectional, cylinder
Cut HCP
Hydrostatic pressure, cube
Hydrostatic pressure,
carbonated cube
Fig. 7 HCP powder after compaction at applied pressures of (a) 5, (b) 10, (c) 100, and (d) 400 MPa.
(a) 5 MPa
20 μm
(b) 10 MPa
20 μm
(c) 100 MPa
20 μm
(d) 400 MPa
20 μm
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 51
under an applied pressure of 10 MPa. When a pressure of
100 MPa was applied, the particles deformed and the
boundaries between them became vague. However, in
the case of gypsum, deformation was not observed even
though the applied pressure was high (100 MPa, Fig. 10).
The compact prepared at 10 MPa was too weak for ob-
servation by SEM.
3.3 Compressive strength of compacts of
crushed and milled concrete
The compressive strengths of the compacts composed of
crushed HC prepared with unidirectional pressing were
measured. The applied pressure was 200 MPa. The ob-
tained average compressive strength was 8.5 MPa, three
times less than that of the HCP discussed in section 3.1.
The compressive strengths of the compacts with different
cement paste volume ratios are shown in Fig. 11. The
compact composed of crushed and milled gravel corre-
sponds to the point where the cement paste volume is
zero. The cement paste volume of HC is approximately
30%. As shown in the figure, the compressive strength of
the compacts linearly increases as the cement paste
volume increases to 80%.
Figure 12 shows the compressive strengths of the HC
compacts, carbonated HC compacts, and HC compacts in
which half the volume was replaced with HCP or sludge.
The compacts were prepared under a hydrostatic pressure
of 200 MPa. The HPC compacts exhibited substantially
different strengths depending on the compaction method
(Fig. 6); however, the HC compacts exhibited similar
strengths of slightly less than 10 MPa irrespective of the
compaction method. When HCP was replaced with
sludge, the compacts exhibited a compressive strength of
18 MPa, approximately double that of the HC-only case
and 30% greater than that of the case where half the HC
volume was replaced with HCP. The carbonated com-
pacts exhibited the largest compressive average strength
of approximately 32 MPa.
3.4 Volumetric change in compacts after dif-
ferent drying treatments
The measured strains of the compacts dried under dif-
ferent conditions prior to compaction are shown in Fig.
13. In the figure, the solid and broken lines indicate the
strains of the upper and lower sides, respectively. The
results indicate that the strains of both sides are almost
equivalent. The compacts composed of the non-dried
powder exhibited extensive shrinkage. In contrast, the
compacts prepared using powder dried in chambers of
40 °C or 105 °C expanded. The weight changes of two
compacts dried at 105 °C are shown in Fig. 14. The
increase in strains and weights of the compacts dried at
105 °C show similar trends.
Fi
g
. 8 Ma
g
nified version of the ima
g
e in Fi
g
. 7
(
b
)
.
(a) 10 MPa (b) 100 MPa
Fig. 9 Calcium hydroxide powder after compaction. (Numbers below images indicate applied pressure.)
10 μm
10 μm
50 μm
Fig 10 Gypsum powder after compaction. (Applied
pressure of 100 MPa.)
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 52
4. Discussion
4.1 Effect of compaction method on compact’s
strength
We first discuss the difference in the strengths of the
compacts with respect to the compaction method. As
shown in Fig. 6, the compacts of HCP prepared under
unidirectional pressure exhibit a three-fold lower
strength compared with that of HCP prepared under
hydrostatic pressure. This reduction in strength can be
attributed to their size difference; however, other possi-
ble causes include frictional resistance between the
powder and the mold wall and non-uniformity in the
compacts due to non-uniform stress during pressing.
After the HPC compacts were prepared using unidirec-
tional pressing, some HPC was adhered to the mold wall
(Fig. 15). The attached HCP was difficult to remove with
a cloth. We used a steel scrubber to remove it. In contrast,
the attached HC could be removed with cloth. This dif-
ference indicates strong sticking of HCP to the mold wall,
which may cause large frictional resistance during uni-
directional pressing.
4.2 Compaction pressure and compact’s den-
sity and strength
According to Fig. 6, the compressive strength of the
prepared compacts increased with increasing compaction
pressure. When the hydrostatic pressure used to compact
HCP was increased from 130 MPa to 400 MPa, the
compressive strength increased by 20%; however, as
mentioned in section 2.2, the resulting compacts had the
same size, i.e., they exhibited the same density. Chino et
al. (1982) compacted inorganic salts and reported that
when KCl was compacted at 150–200 MPa, the density
of the resulting compacts did not substantially change,
although the strength increased by 40%. When Chino et
al. increased the pressure to 300 MPa, it was observed
that the strength of the compacts reached a plateau. In our
study, the strength of the compacts prepared under 400
MPa may have reached a plateau. We could not increase
the pressure further because of limitations of the testing
machine. In Fig. 6, the plots for unidirectional pressing
extrapolate to the origin, whereas those for hydrostatic
pressure do not. Chino et al. (1982) reported the same
tendencies.
4.3 Deformation of particles due to pressure
and strength development
As shown in Fig. 7, the particles deformed, filling gaps
Strain (μ)
Elapsed time after compaction (h)
Fig 13 Measured strain after compaction.
No drying
40 °C drying
105 °C drying
Weight increase (%)
Elapsed time after compaction (h)
Fig 14 Weight increase of the compacts of powder dried
at 105 °C
Fig. 15 Attached HCP on the mold wall.
Volume fraction of cement paste (%)
Gravel
Concrete
Cement paste
Compressive strength (MPa)
Fig 11 Compressive strength of crushed and compacted
HC. (Unidirectional pressing.)
(a) (
b
)(c) (d)
Fig. 12 Compressive strengths of compacted HC under
different conditions. (Half of the volume of the concrete is
replaced with cement paste or sludge.)
Concrete
only
With
cement paste
With
sludge cake
Carbonation treatment
(No replacement)
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 53
under increasing compaction pressure. The boundaries
between particles were vague. In the process of cold
welding, metals are bonded by plastic flow resulting
from pressure (Bay 1979). Bonded metals exhibit
strengths similar to those of intact metals. In the study on
compaction of inorganic salts, Chino et al. (1982) sug-
gested that gap filling due to plastic deformation of the
particles is the mechanism underlying the strength in-
crease. Ultimately, the increase in contact area results in
strengthening. This mechanism is supported theoretically.
At the contact face between two particles, bonding forces
such as van der Waals forces and electrostatic forces are
present. When sufficient compaction pressure is applied,
the compacts will exhibit strengths similar to those of
their single crystals (Chino et al. 1982). Fig. 7 shows that
crushed and milled HCP deformed and the contact area
amongst the particles increased. When HCP was com-
pacted under sufficient pressure, the compacts exhibited
mechanical properties similar to those of the intact com-
pacts (Soroka and Sereda 1968); this phenomenon cor-
responds to the aforementioned examples of metals and
inorganic salts. On the basis of these considerations,
during compaction of crushed and milled cementitious
material, similar to metals and inorganic salts, the parti-
cles not only become mechanically interlocked, but
presumably still experience attraction forces such as van
der Waals and electrostatic forces. In Fig. 7, considerable
filling is evident; however, some gaps still exist. Ac-
cording to the results discussed in section 3.2, these gaps
are attributable to the difference in the stress required for
deformation of hydrates. Some hydrates are deformed at
low pressures and fill gaps, whereas others require addi-
tional pressure to be deformed.
In Fig. 8, granular and needle-like hydrates are ob-
served to be in contact with other hydrates. The bounda-
ries between the hydrates are vague even though the
applied pressure was low (10 MPa). The needle-like
hydrates are presumed to be ettringite because of their
shape. In contrast, calcium hydroxide and gypsum were
not substantially deformed under pressures of 10 MPa.
As shown in Fig. 11, under the same pressure, HC
compacts exhibited much lower strengths than HCP
compacts. A possible reason for this phenomenon is
insufficient compaction pressure: more stress may be
required to induce plastic deformation and result in ag-
gregation and bonding between the different materials
(cement paste and aggregate).
4.4 Strength improvement of HC compacts
The compacts of the crushed and milled HC were pre-
pared in a short time (10 min). However, the compressive
strength was slightly less than 10 MPa, as shown in Fig.
11, which is relatively lower than the 17 MPa compres-
sive strength value required for blocks in concrete block
pavement (JIS A 5371:2010). Here, as shown in Figs. 11
and 12, when 80% of HC was replaced with HPC or 50%
of HC was replaced with sludge, the strength of the pre-
pared compacts exceeded 17 MPa. Moreover, although it
requires more time, a 48-h carbonation treatment was
very effective in improving the strength. This improve-
ment is attributed to the filling of gaps resulting from the
conversion of calcium hydroxide into calcium carbonate
(Matsuya et al., 2007). Strength improvements from
carbonation have also been demonstrated for various
compacts containing calcium hydroxide (Arinaga et al.
2004, 2005; Norimasa et al. 2011). Thus, practicable
compacts can be prepared by compacting crushed and
milled HC containing added HPC or sludge, or by per-
forming carbonation treatment.
4.5 Volumetric change of compacts
The measured strains on the top and bottom surfaces of
the HCP compacts were approximately the same (Fig.
13). This indicates that the prepared compacts were
uniform. The compacts of non-dried HCP exhibited large
shrinkage soon after being prepared. This shrinkage is
attributed to a loss of moisture, which was squeezed from
the compacts. When a pressure of 200 MPa was applied
unidirectionally to the crushed and milled HCP, some of
the moisture was left on the mold wall. When the HCP
powder was dried before being compacted, the prepared
compacts expanded. This expansion was likely caused by
water absorption as seen in Fig. 14. When the powder
was dried, the mold wall was not wet after compaction.
These results indicate that the volumetric change of the
compacts can be controlled by varying the moisture
content of the powder before it is compacted.
5. Conclusion
In this study, we recycled concrete without separating the
aggregate or producing waste. A compaction method was
applied to crushed and milled HC. The compacts were
prepared in 10 min, and no waste was produced. How-
ever, the compressive strengths of the resulting HC
compacts was insufficient. Consequently, HC was par-
tially replaced with HPC or sludge, or carbonation
treatment was applied. The strengths of the obtained
compacts was sufficiently improved to satisfy the
minimum strength requirement for blocks in concrete
block pavement. As demonstrated, practicable compacts
can be prepared through appropriate treatment. In addi-
tion, we controlled the volume change of the compacts
due to moisture loss and absorption by drying the powder
before compaction. However, currently, the required
pressure to provide high enough compact strength is
large and the compaction duration of 10 min is long for
practical production. Further studies on the effect of
particle size distribution, moisture content, and sample
size could be employed to address these problems in
order to achieve perfect recycling of concrete by com-
paction.
Acknowledgments
This study was supported by JSPS KAKENHI grant
number 26630201. The authors thank Sumitomo Osaka
Y. Sakai, B. T. Tarekegne and T. Kishi / Journal of Advanced Co ncrete Technology Vol. 14, 47-54, 2016 54
Cement and Tokyo SOC for providing cement and sludge
cakes. The authors are also grateful to Mr. Manish Keshri
of the Indian Institutes of Technology for assisting with
the preparation of the crushed and milled HC and HCP.
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