Renewable and Sustainable Energy Reviews 135 (2021) 110147

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Renewable and Sustainable Energy Reviews

journal homepage: http://www.elsevier.com/locate/rser

Proposal of demolished concrete recycling system based on performance

evaluation of inorganic building materials manufactured from waste
concrete powder
Dayoung Oh a, Takafumi Noguchi a, Ryoma Kitagaki b, Hyeonggil Choi c, *
a
Department of Architecture, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8685, Japan
b
Graduate School of Engineering, Hokkaido University, Nishi-8-chome, Kita-13-jyo, Kita-ku, Sapporo-shi, Hokkaido, 060-8628, Japan
c
School of Architecture, Civil, Environment, and Energy Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu, 41566, Republic of Korea

A R T I C L E I N F O A B S T R A C T

Keywords: The rapid industrial development and global population growth of the past century have resulted in an expo­
Waste concrete powder nential increase of resource consumption and thus caused elevated CO2 emissions that, in turn, are held
Inorganic building material responsible for global warming and associated environmental problems that require urgent solutions. Specif­
Recycled cement
ically, increase of cement production causes CO2 pollution and generates a significant amount of concrete waste.
Hydrothermal synthesis
Tobermorite
Waste concrete, the major component of construction waste, can be efficiently recycled and is mainly used as a
Demolished concrete roadbed or backfill material. However, as no further resource recycling is expected for waste concrete, more
Recycling system efficient and productive recycling systems are sought after. Herein, waste concrete powder is used to produce
added-value inorganic building materials, namely recycled cement and solidification. The characteristics of
recycled cement (manufactured through calcination) are evaluated in terms of free lime content, mineral
composition, density, color, flow test and strength, and the performance of recycled cement is found to be
identical to that of ordinary Portland cement. X-ray diffraction and compressive strength analyses of the solid­
ification manufactured through hydrothermal synthesis show that blocks of the desired strengths can be pro­
duced by adjusting the degree of consolidation and curing conditions. Based on these results, this study proposes
a concrete waste recycling system to reduce the amount of construction waste and prevent resource depletion.

1. Introduction aggregate production accounts for 30–70% of the employed concrete,

depending on the production method. The production of this powder is
In 2017, the annual amount of waste generated by the Japanese expected to increase further because of the development of high-quality
construction industry amounted to 83.9 million tons, representing recycled aggregate production technologies. Studies on the recycling of
~22% of the total amount of Japanese industrial waste generated in this waste concrete powder (WCP) can be largely divided into those on its
year [1]. Moreover, as the final disposal sites of industrial waste are utilization as a concrete material [8–12] and as a cement raw material
expected to be saturated in the foreseeable future, there is a growing [13–15].
need for recycling waste concrete, the major component of construction Xiao et al. [9] evaluated the mechanical properties of concrete, using
waste. Recent technological advances have allowed one to use waste recycled powder as partial replacement of Portland cement, and up to
concrete for the production of high-quality recycled aggregates. Also, 30% replacement of recycled powder was found to have positive or
many studies have been conducted on the quality of concrete that used minor negative effect due to the micro powder (that has higher hardness
recycled aggregate [2–6]. These aggregates are suitable for use in actual and strength than C–S–H) obtained from the aggregates and bricks. Shi
structures [7] and are expected to become increasingly sought after in et al. [11] manufactured the artificial aggregates using concrete waste
the future. The powder generated as a byproduct during recycled powder and evaluated the properties with different curing methods. In

Abbreviations: WCP, waste concrete powder; f.CaO, free lime; XRF, X-ray fluorescence; XRD, X-ray diffraction; OPC, ordinary Portland cement; BFS, blast furnace
slag; FA, fly-ash.
* Corresponding author.
E-mail address: hgchoi@knu.ac.kr (H. Choi).

https://doi.org/10.1016/j.rser.2020.110147
Received 9 January 2020; Received in revised form 15 July 2020; Accepted 24 July 2020
Available online 6 August 2020
1364-0321/© 2020 Elsevier Ltd. All rights reserved.
D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

their study, the artificial aggregates, after CO2 curing, had higher crush disposed waste. Specifically, WCP is used as a raw material, as its
strength, density, and lower water absorption than normal curing due to chemical composition is similar to that of cement. Recycled cement is
the reaction between CH and CO2 formed calcium carbonate crystals manufactured in a laboratory by clinker synthesis using the proposed
that reduced the overall pores. Abdel-Gawwad et al. [15] produced and combination, and the chemical, physical, and mechanical properties of
ready-mix alkali activated cement by the recycling of waste concrete this cement are evaluated.
powder at a lower temperature (1100–1200 ◦ C) than Portland cement
production (~1450 ◦ C). 2.2. Raw material combination of recycled cement
Increased CO2 emissions are one of the main causes of global
warming and an emerging social issue. In particular, the cement in­ The main chemical components of cement are CaO, SiO2, Al2O3, and
dustry accounts for >50% of the total material industry production and Fe2O3, the proportions of which determine those of minerals in calcined
~8% of the total CO2 emissions. In turn, the decarboxylation of lime­ cement. As the properties of cement depend not only on its mineral
stone, a cement raw material, accounts for >60% of the cement composition but also type and quantity of impurities, it is very important
industry-related CO2 emission [16]. As shown in Fig. 1, the global to quantitatively evaluate the chemical components of raw materials
cement production in 2018 amounted to ~4.1 billion tons [17,18] and is before calcination and to match them to the chemical components of the
expected to increase further because of the economic development and existing cement. For example, when the content of SO3 exceeds 2.6 wt%,
population growth in the countries of East Asia, including China. the amount of produced C3S decreases, and those of C2S and free lime (f.
Moreover, increased cement production generates a greater amount of CaO) increase, adversely affecting the stability and mechanical perfor­
concrete waste. Therefore, systems capable of producing eco-friendly mance of concrete [19]. Therefore, it is important to evaluate the
cement to replace the use of natural resources through more efficient amount of SO3 in cases where cement is produced from waste. For this
concrete waste recycling technologies are highly sought after. reason, the chemical components of WCP and limestone, to be used as
Oh et al. [13] estimated the annual amount of waste inorganic the raw materials of recycled cement, were analyzed by X-ray fluores­
building materials produced in Japan, using this waste to produce cence (XRF) spectroscopy. The SiO2 content of WCP was found to exceed
recycled cement, and evaluated the mechanical and environmental that of cement, whereas the reverse was true for CaO content, that was
performances of this cement, revealing that the employed technique ascribed to the influence of the aggregate included in concrete. Table 1
allows one to reduce both the generation of industrial waste and CO2 summarizes the results of chemical component analysis.
emissions. Based on these results, the present study focuses on Within the chemical composition range of ordinary Portland cement
explaining the mechanism of recycled cement production from WCP (the (OPC) [20], the content of WCP was increased as much as possible, and
discharge of which is expected to dramatically increase in the future) as the optimal combination of raw materials was determined by using
a raw material and analyzing the recycled cement from a chemical Bogue’s formula to evaluate the proportions of cement-constituting
perspective. Moreover, this study uses WCP to produce solidification, minerals. As a result, it was concluded that a 39 wt% WCP–61 wt%
that has lower energy consumption and is less sensitive to the pro­ limestone composition was most appropriate. The results of the corre­
portions of chemical components than cement, and evaluate its me­ sponding chemical component analysis are shown in Table 2.
chanical performance. Finally, the feasibility of a recycling system that
balances the amounts of discharged and recycled WCP is examined. 2.3. Clinker synthesis

2. Manufacturing of recycled cement through calcination and The mineral profile of calcination-produced cement depends on the
performance evaluation composition of raw materials and calcination temperature. Therefore,
chemical compositions within the ranges that can constitute cement and
2.1. General calcination temperatures high enough to decompose the minerals of raw
materials are required. Moreover, even when these conditions are met,
In this section, a raw material combination of recycled cement is the slow cooling of clinker after calcination affords γ-C2S with a sym­
proposed to (i) minimize CO2 emissions caused by limestone decar­ metric structure, i.e., without hydration performance, that adversely
boxylation during cement production and (ii) reduce the quantity of affects hydrate strength [21–23]. Therefore, it is important to generate
metastable and hydratable β-C2S by quenching clinker from 1200 ◦ C to
60–80 ◦ C. Fig. 2 shows the area of Portland cement clinker generation in
the CaO–Al2O3–SiO2 ternary diagram of cementitious materials.
Clinker synthesis was performed in a laboratory using the combi­
nation proposed in Section 2.2. An electric furnace was used instead of a
kiln employed in factories. A Pt vessel was used to contain the sample, as
Pt does not melt at 1600 ◦ C and is not subject to deformation upon rapid
cooling.
The raw materials of recycled cement, pulverized and sifted through
a 150 μm sieve, were mixed for 30 min, placed in the Pt container, and
heated in the electric furnace for 1 h at 900 ◦ C for dehydration and
decarboxylation. The sample was removed from the electric furnace,
cooled, and subjected to calcination, with the employed temperature
program provided in Table 3. Although calcination in cement factories is
performed at ~1450 ◦ C, a higher temperature of 1600 ◦ C was used in
this study to promote the occurrence of the desired reactions inside the
sample, because it was not rotated in the electric furnace as it would
have been in a kiln. After calcination, the sample was removed when the
electric furnace temperature reached 1300 ◦ C (Fig. 3), quenched to
induce the generation of β-C2S. Recycled cement was manufactured by
pulverizing the obtained clinker with a ball mill and sifting through a 75
μm sieve and supplemented with 37 g of gypsum per 1 kg of cement.
Fig. 1. Global cement production (1998–2018) [17]. Fig. 4 shows images of the synthesized clinker and the recycled cement.

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

Table 1
Chemical component contents (wt%) of raw materials.
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O TiO2 ig. loss

WCP 35.68 9.78 3.94 29.21 2.23 0.78 1.66 0.72 0.40 15.95
Limestone 0.26 0.04 0.02 45.03 0.42 0 0.01 0.04 0.04 53.18

Table 2
Chemical component contents (wt%) of recycled cement manufactured from WCP and limestone.
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O TiO2 ig. loss

WCP (×0.39) 13.92 3.81 1.54 11.39 0.87 0.30 0.65 0.28 0.16 6.22
Limestone (×0.61) 0.16 0.02 0.01 27.47 0.26 0.00 0.01 0.02 0.02 32.44

Recycled cement 23.03 6.28 2.53 63.59 1.84 0.50 1.07 0.50 0.30 –

zone and decreasing the extent of C3S generation. Therefore, the quan­
titation of f.CaO in cement is a task of high practical importance. Mtarfi
et al. [26] showed that the f.CaO content of cement should be decreased
to ≤ 2.0 wt% to reduce the influence of hydration-induced expansion.
In the present study, f.CaO content was measured as 0.70 wt% in
accordance with Japan Cement Association Standard (JCAS) I-01 [27]
(Eq. (1)) and was concluded to be sufficiently low not to affect concrete
expansion or curing rate.
f.CaO content (Wt%) = 100% × vE/m, (1)

where v = 1.32 mL is the volume of ammonium acetate solution, E =

5.338 × 10− 3 g/mL is a coefficient relating the volume of the ammonium
acetate standard solution to the amount of CaO in the sample, and m =
1.00011 g is sample weight.

2.4.2. Physical performance evaluation

Cement becomes yellow with increasing Al2O3 and Na2O contents
and with decreasing Fe2O3 content [28]. The Al2O3 (6.28 wt%) and
Na2O (1.07 wt%) contents of recycled cement exceeded those of OPC
Fig. 2. CaO–Al2O3–SiO2 ternary diagram of cementitious materials [24].
(5.28 and 0.28 wt%, respectively), while the Fe2O3 content (2.53 wt%)
of recycled cement was lower than that of OPC (2.91 wt%). Therefore,
the color of the recycled cement was yellowish (Fig. 4).
Table 3
Calcination temperature program used for clinker production.
The density of recycled cement was measured by a pycnometer as
3.05 g/cm3 that was lower than that of OPC (3.14 g/cm3) [20]. This
Temperature (◦ C) Time (min) Heating rate (◦ C/min)
result was ascribed to the low contents of high-density Fe2O3 and CaO
100–1000 9 100 and the high contents of low-density Al2O3 and SiO2 in recycled cement.
1000–1600 12 50
1600 180 –
1600–1300 6 − 50
2.4.3. Mechanical performance evaluation
The mechanical performance of recycled cement was assessed by a
cement strength test conducted in accordance with JIS R5201 [29] for
2.4. Evaluation of recycled cement performance mortar specimens with dimensions of 4 × 4 × 16 cm. A 1:2:6 (w/w/w)
water:cement:standard sand ratio was employed, and the produced
2.4.1. Chemical performance evaluation specimens were subjected to water curing at 20 ◦ C.
The mineral composition of recycled cement was probed by X-ray In the flow test, values of 20.1 cm and 14.2 cm were obtained for the
diffraction (XRD) analysis (Fig. 5). The corresponding patterns featured OPC control group and recycled cement, respectively, i.e., the latter
peaks of major cement minerals such as C2S, C3A, C4AF, and C3S, that sample exhibited relatively low fluidity. This behavior was ascribed to
are the largest contributors to cement hydraulic properties. Thus, the the fact that recycled cement had a narrow particle size range, while
chemical components of raw materials and calcination conditions were factory-produced cement exhibited an excellent particle size distribu­
concluded to be appropriate. tion. Moreover, the sifting of recycled cement through the 75 μm sieve
Next, f.CaO content was examined. As shown in Fig. 6, C2S is could result in the retention of particles larger than those of OPC.
generated by the reaction of CaO with SiO2 during clinker synthesis. If Fig. 7 shows the images of specimens used for strength testing. The
the CaO content is sufficient, all SiO2 is converted into C2S starting from strength of recycled cement was lower than that of OPC at early ages but
~1200 ◦ C, and the reaction between C2S and CaO produces C3S at became almost equal to that of OPC over time (Fig. 8). This finding was
temperatures of 1300–1400 ◦ C, causing most f.CaO to disappear. There explained by the presence of large particles in the laboratory-produced
are cases, however, where f.CaO is retained because of the influence of cement and the resulting low early-age reactivity. The strengths, how­
impurities, low calcination temperature, and/or short calcination time ever, became similar at a time of 28 days, as the influence of particle size
[19]. f.CaO accelerates the hardening of cement and expands to form Ca decreased with time.
(OH)2 by reacting with water, potentially inducing the cracking of hy­
drated concrete. Moreover, this expansion may also adversely affect
concrete strength by facilitating microcracking around the transition

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

Fig. 3. Quenching at ~1300 ◦ C.

Fig. 4. Images of (a) as-synthesized clinker and (b) recycled cement.

Fig. 5. Clinker XRD pattern.

3. Manufacturing of solidification through hydrothermal Fig. 6. Effect of calcination temperature on clinker mineral composition [25].
synthesis and performance evaluation

3.1. General

Next, this study considered the increase in the amount of waste

concrete due to demolition and attempted to balance demand with
supply by manufacturing solidification that is less sensitive to chemical
composition than cement. An autoclave characterized by low heat and
low energy consumption was employed to reduce the associated envi­
ronmental load. According to Ishida [30], hydrothermal synthesis at
150 ◦ C requires a sixth of the energy consumed during calcination.
Herein, solidification was manufactured from WCP, blast furnace Fig. 7. Images of cement mortar specimens (a) OPC, (b) recycled cement.

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

combination, consolidation, and hydrothermal synthesis.

According to Jing et al. [31,32], the C/S ratio allowing for maxi­
mized tobermorite generation ranged from 0.83 to 1.17. Based on this
result, raw material combinations were chosen to achieve a solidifica­
tion C/S ratio of unity, with the corresponding chemical compositions
listed in Table 4.
As the CaO content of FA is almost zero, the raw material formulation
was designed considering only the SiO2 component. To harden the
powder through consolidation, moisture was added at a loading of 5 wt
%. Combination 1 corresponded to WCP:BFS = 1:1 (w/w) + 5 wt%
water, and combination 2 corresponded to FA:BFS = 1:9 (w/w) + 5 wt%
water.
The raw materials were filled into a φ4 × 5 cm steel mold and
consolidated using a φ4 cm circular cylinder (Fig. 10(a)). The pressure
was raised to 30 MPa, maintained for 1 min, and then slowly removed.
Consolidated specimens were subjected to 12 h autoclave curing at
200 ◦ C to generate tobermorite (Fig. 10(b) and (c)). As autoclave curing
is generally performed at the low temperatures of 150–200 ◦ C, the heat/
Fig. 8. Results of mortar compressive strength testing. energy consumption of this method is lower than that of calcination at
temperatures above 1000 ◦ C.
slag (BFS), and fly-ash (FA) as raw materials by consolidation and hy­
drothermal synthesis (autoclave curing). The chemical and mechanical 3.4. Evaluation of solidification performance
performances of the manufactured solidification were evaluated. Also,
the applicability of WCP as a building material was examined by esti­ 3.4.1. Chemical performance evaluation
mating the annual WCP generation amount and the amount of solidifi­ Minerals present in the two solidification types were identified by
cation produced using WCP as a raw material. XRD analysis (Fig. 11), revealing that the solidification phase compo­
sition was independent of raw material type or crystal structure.
3.2. Tobermorite generation requirement
3.4.2. Mechanical performance evaluation
Tobermorite (Ca5Si6O16(OH)2∙4H2O) is a calcium silicate hydrate Autoclave-cured specimens were dried at 80 ◦ C for 40 h and sub­
generated from CaO and SiO2 during hydrothermal synthesis. Usually, jected to compressive strength measurements, revealing that the
tobermorite formation requires the use of high-purity crystalline silica compressive strengths of BFS50% + WCP50% (60.90 MPa) and BFS90%
and lime as raw materials. + FA10% (58.15 MPa) were almost identical. As described in Section
Recently, studies on the hydrothermal production of high-strength 3.2, the generation and crystal structure of tobermorite are significantly
building materials from CaO-rich materials, such as BFS, have been affected by raw material crystallinity. The fact that similar results were
conducted for byproduct utilization [30–34]. Jing et al. [31] hydro­ obtained, despite the different crystallinities of raw materials, implied
thermally synthesized tobermorite after adding FA or quartz with a high that the tobermorite generation rate increased because of the long
SiO2 content to BFS, revealing that highly soluble FA was more appro­ autoclave curing time (12 h). Thus, this study concluded that tober­
priate for tobermorite generation than quartz. morite can be effectively generated if the chemical composition of so­
As silica crystallinity affects the crystal structure of tobermorite, this lidification is adjusted so that the C/S ratio equals unity when conditions
study analyzed the crystal structure of raw materials by XRD (Fig. 9), such as consolidation pressure, curing time and temperature, and drying
demonstrating that, whereas the pattern of WCP was indicative of a time are the same. It was also found that under long-term curing con­
crystalline structure and featured peaks of several minerals, the pattern ditions, tobermorite formation is more influenced by the chemical
of FA was indicative of an amorphous state. The difference in the crys­
tallinity of tobermorite generated from the two raw materials with Table 4
different crystal structures was then probed by XRD and compressive Chemical compositions of raw materials used for solidification production.
strength measurements. SiO2 Al2O3 Fe2O3 CaO MgO

WCP 35.68 9.78 3.94 29.21 2.23

3.3. Solidification production BFSa 33.8 13.4 – 41.7 7.4
a
Source: Nippon Slag Association data.
Solidification was manufactured by going through raw material

Fig. 9. XRD patterns of WCP (a) and FA (b).

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

Fig. 10. Manufacturing of solidification: (a) consolidation, (b) autoclave curing, and (c) solidification.

Fig. 11. XRD patterns of solidifications obtained using (a) WCP + BFS and (b) FA + BFS as raw materials.

composition of raw materials than by their type or crystal structure. aggregate and the amount of WCP generated during recycled aggregate
production. In Japan, ~30 million tons of waste concrete are generated
annually. Fig. 12 shows the dependence of the WCP generation amount
3.5. Estimation of solidification production amount according to the above two factors, revealing that at a concrete recycling
rate for recycled aggregate of 30% and a WCP generation efficiency of
To calculate the amount of solidification produced using WCP as a 50%, 4.5 million tons of WCP are produced annually.
raw material, this study estimated the annual WCP generation amount, The method of obtaining raw material combinations for
that depends on the recycling rate of concrete waste for recycled

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

changes in the WCP generation amount and the WCP input fraction
according to the C/S ratio, with the results shown in Fig. 13.
At a 30% rate of concrete waste recycling for recycled aggregate, a
WCP generation rate of 50%, and a WCP C/S ratio of 0.7, the producible
amount of solidification was obtained as 14.8 million tons. Fig. 13 shows
the results calculated, assuming that the amounts of BFS and FA were
sufficient. Considering that the annual amount of BFS generated in
Japan is ~23 million tons [35], the practical solidification production
amount is expected to be smaller than the predicted value. Herein,
additional studies on the use of CaO- and SiO2-containing inorganic
building materials, other than WCP, BFS, and FA (that were considered
in this study), as raw materials are required to reduce the final amount of
disposed waste.

4. System of demolished concrete recycling

Based on the results of Chapter 2 and 3, this study proposed an

Fig. 12. WCP generation amount as a function of concrete recycling rate for effective system for waste concrete recycling.
recycled aggregate and generation rate of WCP. The service lifetime of concrete is approximately 50 years [13,36].
As concrete structures with expired service lifetimes are constantly
solidification according to the C/S ratio of WCP is as follows. demolished while construction work is simultaneously performed, the
The chemical composition of WCP depends on the method of recy­ reuse of demolished concrete to produce new concrete structures is the
cled aggregate production and base cement type. Therefore, raw mate­ most effective method of reducing the amount of waste and preventing
rial combinations for solidification production were chosen under the resource depletion.
assumption that the C/S ratio that varies with aggregate content, ranges As mentioned in Chapter 1, there are many studies which have been
from 0.4 to 1.1 (the C/S ratio of the WCP used in this study was 0.82; see conducted on the quality of concrete that used recycled aggregate. These
Table 4). After setting the C/S ratio for tobermorite generation to unity, recycled aggregates are suitable for use in actual structures and are
BFS and FA byproducts were used to increase the insufficient CaO and expected to become increasingly sought after in the future.
SiO2 contents. Herein, this study proposed to recycle the powder which is generated
The WCP input fraction was calculated using Eqs. (2) and (3). as a byproduct during recycled aggregate production by manufacturing
recycled cement and solidification. The recycled cement through high-
{xCaOWCP + (100 − x)CaOBFS } / {xSiO2WCP + (100 − x)SiO2BFS } = 1.0, (2)
temperature calcination at ≥1450 ◦ C is in high demand but sensitive
(xCaOWCP ) / {xSIO2WCP + (100 − x)SIO2FA } = 1.0, (3) to chemical composition. Whereas the solidification through hydro­
thermal synthesis at a relatively low temperature of 200 ◦ C is less sen­
where x is the WCP input fraction, CaOWCP is the CaO content of WCP, sitive to chemical composition but is in lower demand than cement.
CaOBFS is the CaO content of BFS, SiO2WCP is the SiO2 content of WCP, Thus, two types of recycled materials which are manufactured
SiO2BFS is the SiO2 content of BFS, and SiO2FA is the SiO2 content of FA. depending on concrete waste generation locations and conditions of
The following assumptions were made to estimate the WCP input WCP could be used in suitable applications. In other words, recycled
fraction and the input amounts of raw materials for tobermorite cement can be applied to concrete structures after mixing with recycled
generation. aggregate, and the powder originating from demolished concrete can be
used to manufacture solidification as an effective building material.
• The CaO + SiO2 content of WCP is 65 wt% (Table 4). Based on the results of Section 3.4, solidification can be used to produce
• The CaO and SiO2 contents of BFS are 40 wt% and 35 wt%, respec­ blocks with high compressive strength. Moreover, low-strength solidi­
tively (Fig. 2). fication produced by inputting low energy, through a reduction in the
• The CaO and SiO2 contents of FA are 0 wt% and 60 wt%, respectively degree of consolidation and in the autoclave curing temperature and
(Fig. 2). time can be used as sidewalk blocks. In the proposed sustainable system
for demolished concrete recycling (Fig. 14), component analysis is
Based on the above assumptions, CaO and SiO2 contents for different conducted when the service lifetime of solidification used as building
WCP C/S ratios and WCP input fractions were obtained using Eqs. (2) material is over, and the solidification is reused as a raw material for
and (3) (Table 5). cement production to construct new concrete structures.
The producible amount of solidification was calculated based on
5. Conclusions
Table 5
CaO and SiO2 contents determined for different WCP C/S ratios and raw mate­ The lack of final disposal sites and the need for natural resource
rials combinations for solidification. conservation increase the demand for resource recycling systems.
Herein, this study proposed methods of recycling waste concrete powder
C/S WCP components (wt%) Raw material combination for solidification (wt
%)
(WCP), the discharge of which is expected to increase, and analyzed the
performances of these methods by applying them to actual production.
CaOWCP SiO2WCP WCP BFS FA
The results of this study are summarized as follows:
0.4 18.6 46.4 15.2 84.8 –
0.5 21.7 43.3 18.8 81.3 –
1) Recycled cement was manufactured through calcination using WCP
0.6 24.4 40.6 23.5 76.5 –
0.7 26.8 38.2 30.4 69.6 – as a raw material and evaluated in terms of f.CaO content, minerals
0.8 28.9 36.1 40.9 59.1 – generated by calcination, density, color, fluidity, and strength. As a
0.9 30.8 34.2 59.4 40.6 – result, the generation of cement-constituting minerals was
1.0 32.5 32.5 100.0 – – confirmed, and the effects of f.CaO on stability were determined.
1.1 34.0 31.0 95.1 4.9
Recycled cement exhibited low fluidity due to the presence of large
–

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

Fig. 13. Producible solidification output as a function of (a) WCP generation rate and (b) concrete recycling rate for recycled aggregate.

Fig. 14. Proposed system of demolished concrete recycling.

particles and a narrow particle size distribution. This problem, expected to resolve the environmental issues such as resource
caused by in-laboratory manufacturing, can be solved if pulveriza­ depletion, CO2 emission and concrete waste generation.
tion time and fineness are increased. The strength of recycled cement
was low at early ages but was almost similar to that of OPC at 28 In the future, additional studies on the energy and CO2 emission
days. Thus, recycled cement manufactured using WCP as a raw evaluation of the two different recycling materials are required. More­
material had the same performance as OPC and could be used as a over, further studies are required on the use of CaO- and SiO2-containing
building material. inorganic building materials, other than WCP, BFS, and FA, as raw
2) Solidification was manufactured through hydrothermal synthesis materials to reduce the final amount of disposed waste.
from WCP containing CaO and SiO2, the chemical components of
tobermorite. XRD analysis and compressive strength testing of the Credit author statement
manufactured solidification showed that the crystal structure dif­
ferences between the raw materials (WCP and FA) did not affect the Dayoung OH, Takafumi NOGUCHI and Ryoma KITAGAKI conceived
generation of tobermorite. Thus, it is possible to generate high- of the presented idea. Dayoung OH and Hyeonggil CHOI developed the
crystallinity tobermorite from amorphous silica if long curing times theory. All authors discussed the results and contributed to the final
(12 h) are used. Based on the above results, it is judged that CaO- and manuscript.
SiO2-containing inorganic building materials, other than WCP, can
also be used as raw materials for reducing the final amount of Declaration of competing interest
disposed waste. The produced solidification can be used as high-
strength blocks because it features a high compressive strength of None.
~60 MPa, while low-strength solidification can be used as sidewalk
blocks. Acknowledgment
3) Finally, an effective system of demolished concrete recycling, in
which demolished concrete is employed as a resource for recycled This work was supported by the National Research Foundation of
cement or solidification production (depending on the purpose and Korea (NRF) grant funded by the Republic of Korea government (MSIT)
the condition of waste), was proposed. This proposed system is [grant number 2018R1A5A1025137].

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D. Oh et al. Renewable and Sustainable Energy Reviews 135 (2021) 110147

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