Case Studies in Construction Materials 17 (2022) e01298
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Case Studies in Construction Materials
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Case study
The performance effect of PEG-silica fume as shape-stabilized
phase change materials on mechanical and thermal properties of
lightweight concrete panels
Sahar Mokhtari, Morteza Madhkhan *
Department of Civil Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Shape-stabilized phase change material
(SSPCM)
Lightweight concrete
Thermal energy storage
Thermal performance
Sustainable construction is of paramount importance and one of the methods is deployment of
phase change materials (PCMs) in construction materials due to their large latent heat capacity.
This study aims at investigating the mechanical and thermal properties of non-structural light­
weight concrete incorporating shape-stabilized phase change materials (SSPCMs). Toward this
purpose, silica fume and polyethylene glycol 600 (PEG 600) composite was prepared with direct
absorption method and no leakage was observed. Wallboards with two thicknesses including 10
% and 18 % SSPCM and macroencapsulated PCM layers were built and tested under two ambient
weather conditions. Results stated that the application of SSPCM in specimens would reduce the
inner-side temperature of wallboards to a great extent and the panels with less thickness had a
better thermal performance. Regarding mechanical properties, tensile and compressive strength
of concrete at early ages decreased by the growth of SSPCM content, while this reduction was
compensated by the passage of time.
1. Introduction
Nowadays, energy and the environment are two major issues with which mankind is faced. In recent years, population growth and
industrial development have caused an upsurge in energy demand and building is one of the main contributors in energy consumption.
In 2009, almost 40 % of fossil fuel consumption in the United Stated and European Union was used in the building sector; therefore, it
seems crucial to innovate novel systems for thermal energy storage in order to reduce fossil fuel consumption [1]. In recent years,
phase change materials (PCMs) have been introduced as the most influential technique for latent heat energy storage owing to their
isothermal characteristics. As a result, applying PCMs in both residential and commercial buildings is beneficial for enhancing the
thermal storage as well as reducing carbon footprint [2,3]. During daytime, when the temperature is high, PCM melts and stores the
energy as latent heat and releases this energy at night while it solidifies. Such behavior reduces the maintaining costs of buildings due
to a decrease in the need for air-conditioning facilities, since PCMs are able to shift the peak load and reduce the inner temperature of
the buildings to a great extent [4].
Liu et al. used different lightweight building wallboard models each of which having different positions and thicknesses for a layer
of PCM and conducted a numerical method called enthalpy to assess the wallboards’ performance. Results demonstrated that the
fluctuation of temperature on the inner surface of models plunged and integrating PCM caused at least 1 ℃ reduction in the peak
* Corresponding author.
E-mail addresses: sahar.mokhtari992@gmail.com (S. Mokhtari), madhkhan@cc.iut.ac.ir (M. Madhkhan).
https://doi.org/10.1016/j.cscm.2022.e01298
Received 18 January 2022; Received in revised form 26 June 2022; Accepted 3 July 2022
Available online 4 July 2022
2214-5095/© 2022 Published by Elsevier Ltd.
This is an open access
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
article
under
the
CC
BY-NC-ND
license
Case Studies in Construction Materials 17 (2022) e01298
S. Mokhtari and M. Madhkhan
temperature [5]. Al-Absi et al. decided to apply PCM to the exterior of the walls in form of different composites rather than using it
inside walls and investigate physical and thermal properties of cement plaster as well as foam concrete incorporating PCM. Results
revealed that integration of PCM curtailed workability to the extent that the need for superplasticizers seemed necessary, but the
reduction of at least 52.6 % in the thermal conductivity of PCM composites compared to the specimens with sand only can lead to a
more thermal insulation [6]. Sukontasukkul et al. used polyethylene glycol (PEG) and paraffin powder with different mass fractions to
make plastering mortars and investigated their performance under several tests, namely compression, water retention and thermal
behavior. Concrete specimens’ surfaces were covered by aforementioned mortars and the outer surface of them was heated up to 60 ℃
to assess the impact of PCM on concrete blocks. Experiment results demonstrated that not only did the room temperature fall by using
both PCM types, but it also delayed the time of reaching peak temperature and PEG was more efficacious in this case compared to
paraffin [7]. Various and sundry methods of PCM incorporation into concrete have been suggested which are encapsulation, porous
inclusion, and shape-stabilized PCMs (SSPCMs). Microencapsulation has been a widely common approach for retaining various
commercial PCMs in mortar, concrete, and plaster SSPCM, however, has proved to be an effective method among the aforementioned
ones [8–11].
Selecting the suitable supporting material and method for fabricating SSPCMs is of remarkable importance. These methods include
vacuum impregnation, direct absorption, and sol-gel methods [12]. Pongsopha et al. applied impregnation method and optimized heat
and pressure in order to use burnt clay coarse aggregates as a carrier for paraffin and utilized the aggregates to prepare a type of
concrete incorporating PCM. Concrete specimens with different sizes and thicknesses were then molded to be evaluated under heat
insulation and compressive strength tests. Experimental results illustrated that the highest percentage of paraffin impregnation in
burnt clay was 16.1 % and this amount could lead to time difference in reaching peak temperature and lowering the peak temperature
as well [13]. Imani and Madhkhan tested gypsum sandwich panels with thicknesses of 7 cm and 10 cm under daily temperatures of two
cities to investigate the effect of adding PEG 600 as a phase change material under the thermal behavior of such wallboards. Direct
absorption method was applied in order to add PCM to the mixture of panels and the results clearly showed a maximum daily tem­
perature reduction of 5.7 ℃ and a better performance was observed in the wallboards with lower thickness [14]. Li et al. used PEG and
silica to obtain a shape-stabilized phase change material (SSPCM) for storing energy. The thermal properties of the composite were
then assessed using various techniques. Thermal reliability of the composite was proven after cycles of heating and cooling.
Furthermore, latent heat of PRG-SiO2 was also remarkable, with being 146.9 J/g [15]. Moshtaghi Jafarabad et al. applied PEG and salt
hydrate to concrete via lightweight aggregate of scoria and conducted a test by a homemade apparatus to investigate the thermal
performance of concrete and realized that PCM usage, especially PEG, had an effective impact on thermal properties of concrete [16].
This paper investigates the thermal and mechanical behavior of non-structural lightweight concrete mixed with SSPCM considering
various parameters such as SSPCM proportion, specimen shape, specimen thickness, and temperature conditions. Since exterior walls
transfer both hot and cold weather into and outside the building, PCM has to be incorporated in these walls specifically to reduce the
difference between maximum and minimum temperatures and lightweight concrete has been selected for this purpose because there is
no need for such walls to bear the loads of the building. Cubic and cylindrical specimens were prepared to study the compressive and
tensile strength of the concrete as well as water absorption in different ages. Furthermore, filled and hollow-core wallboard panels
were used to investigate the thermal properties of the fabricated panels due to the fact that hollow core walls are commonly used in
building construction for their lower weight and insulation properties. Most of the studies so far have been focused on the incorpo­
ration of PCM in construction materials while leakage is a serious issue that has to be addressed in case practical use of PCM is to be
feasible in buildings. Application of SSPCM instead of mixing PCM directly with the concrete for the prevention of leakage is the major
aspect of this article.
2. Experimental methods
2.1. SSPCM preparation
Polyethylene glycol (PEG), as an organic PCM, has become one of the most popular PCMs because of its particular features
including high latent heat capacity, low volume change during phase change, proper phase change temperature, and no or little super
cooling [17]. Moreover, PEG 600, a 600 molecular mass PEG, is one of the only PCMs that can be found in massive proportions for
purchasing in Iran, while other PCMs can only be found in laboratory scale; hence, they cannot be considered as a suitable option for
being used in building materials. The physical properties of PEG 600 used in this study are depicted in Table 1. This study used various
materials such as bentonite powder (BP), Tuma stone powder (TSP), Khash pozzolan (KhP), expanded perlite (EP), and silica fume as
the candidates for the supporting material.
Silica fume is a porous mineral material which can be considered as a suitable candidate as the carrier of PCM because of enhancing
the mechanical characteristics of concrete and also performing as a supporting material for embedding PCM so as to forestall leakage
Table 1
Physical properties of PEG 600.
Test
Properties
Appearance at 25 ℃
Density (g/cm3)
Water percent
Transparent liquid
1.125 ± 0.02
2
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S. Mokhtari and M. Madhkhan
[18]. Furthermore, it is an economical and abundantly available substance in Iran which is increasingly used to improve the me­
chanical strength of lightweight concrete. Hence, it was tested as one of the supporting materials and not only showed a satisfactory
PEG
permeability (silica
fume = 1.85), but was also in line with the mechanical properties of concrete. Silica fume was proved to be the mere
feasible option, since the prepared specimens with other materials either illustrated a much lower compressive strength or had a low
absorption rate. The appearance of PCM and SSPCM are demonstrated in Fig. 1.
The vacuum impregnation method guarantees that PCM particles are completely absorbed in supporting material, but it has been
extensively used in SSPCM preparation only and not in building concrete or masonry walls with large scales [18–20]. In this study, two
kinds of SSPCMs were prepared using silica fume; one with direct absorption method and the other with vacuum impregnation. The
proportion of silica fume and PEG in both SSPCMs was exactly the same. Then, three 7 × 7 × 7 cm cubic concrete specimens incor­
porating vacuum-impregnated SSPCM and three others with direct-absorbed SSPCM were prepared to compare SSPCM preparation
approaches. All of the specimens were kept in an oven with the temperature of more than 60ᵒC. After 10 days of monitoring, no leakage
was observed in both kinds. In this regard, SSPCM was kept in an oven with the cyclic temperature of 10–40 ℃ (chosen based on the
melting and freezing point of SSPCM) for two consecutive days on a tissue paper to control any possible leakage. Furthermore, the
three cubic specimens whose SSPCM was prepared by direct absorption were tested under repeated melting and freezing cycles be­
tween the temperatures of 10ᵒC and 40ᵒC to test whether any leakage will happen or not. Since no difference was observed in both
SSPCM preparation approaches in terms of leakage, in this study, direct absorption was chosen for the two final mixtures as a practical
method for large scale preparation in real projects to fabricate SSPCM to be used later in concrete wallboards. On the contrary, as it is
observed in Fig. 2, other mixtures were prepared by vacuum impregnation approach to increase the absorption percentage of PCM into
supporting material pores as much as possible.
2.2. Differential scanning calorimetry (DSC)
Thermal properties of both PEG and SSPCM were ascertained by DSC conducted at the laboratory of Iran Aircraft Manufacturing
Industrial Company (HESA). Melting temperature and latent heat capacity are among the properties that can be established using DSC.
For doing so, heating and cooling cycles were applied over the temperature ranges of − 10–50 ◦ C and 50 to − 10 ◦ C at a 5 ◦ C/min rate.
Numerical integration of the entire area under the peaks divided by temperature change can be used for latent heat calculation;
therefore, universal analysis software and cp-calculation software were used to determine specific heat values.
2.3. Concrete mixing
The important factor in mix design of the control concrete was the hardened concrete density since it had to be less than
1000kg/m3 in lightweight non-structural concrete (LW). Ordinary Portland type I cement with the density of 3.15g/cm3 was used in
all mixes. Non-structural lightweight expanded clay aggregate (Leca) and Sandstorm (SS) were used as coarse and fine aggregates
respectively. The density and fineness modulus of fine aggregate were found 2.48g/cm3 and 1.73 mm respectively, while the density of
Leca was 0.576g/cm3 . Size distribution of Leca is presented in Table 2, and the chemical properties of silica fume, cement, and Khash
Pozzolan (KhP) are presented in Table 3. Water to cement ratio was considered to be 0.51 in order to achieve required workability
without adding any superplasticizer for economic reasons. The lightweight aggregates of Leca were used in SSD condition in all of
concrete mixtures and water absorption of them was 18 %. Table 4 represents the concrete mix proportioning with trial and error using
different materials. In Table 4, C, 10-SSPCM, and 18-SSPCM are the final mixtures with no SSPCM, 10 wt % SSPCM, and 18 wt %
SSPCM respectively. Furthermore, B-SSPCM, T-SSPCM, Kh-SSPCM, and P-SSPCM are the mixtures which corporate the mixture of PEG
with bentonite powder, Tuma stone powder, Khash pozzolan, and expanded perlite respectively. Su, SS, BP, TSP and EP also stand for
superplasticizer, sandstorm, bentonite powder, Tuma stone powder and expanded perlite respectively.
Fig. 1. The appearance of applied PCM and SSPCM; (a) PEG sample, and (b) PEG-silica fume sample.
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S. Mokhtari and M. Madhkhan
Fig. 2. Instruments and tools for vacuum impregnation method including tubes, vacuum, containers and filter papers.
Table 2
Size distribution of Leca in concrete mixture.
Size of sieve (µm)
15
30
60
118
236
475
Percentage retaining on the sieve ( %)
Accumulative percentage retaining on the sieve (%)
Percentage passing the sieve (%)
19
100
0
8
81
19
11
74
26
18
62
38
44
44
56
0
0
100
Table 3
Chemical compositions of ingredients.
component
Silica fume (%)
Cement (%)
Khash Pozzolan (%)
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
Cl
LOI
C3S
C2S
C3A
C4AF
Insoluble residue
95.00
1.20
0.90
1.00
1.20
0.40
0.30
−
−
0.40
−
−
−
−
−
20.70
5.20
4.60
65.00
1.80
0.15
0.50
2.20
−
0.50
59.47
14.68
6.00
14.00
0.50
62.00
18.50
5.50
7.10
2.30
−
2.00
0.20
2.40
−
−
−
−
−
−
− = not measured items
Table 4
Concrete mixtures.
Mixture’s name
C
10-SSPCM
18-SSPCM
B-SSPCM
T-SSPCM
Kh-SSPCM
P-SSPCM
W/C
0.51
0.58
0.64
0.44
0.5
0.5
0.59
Kg/m3
Leca
SS
348
308
286
419
325
247
297
209
123
251
195
99
PEG 600
BP
83.1
153.2
55.6
34.73
66.2
135
69.4
TSP
KhP
EP
Silica fume
44.9
82.8
105.26
143.8
45
Su (%)
2
3
2.4. Instrumentation
The thermal performance of PCM-containing specimens was assessed using an innovative apparatus. Marani and Madhkhan [21]
made an apparatus which is shown in Fig. 3 for investigating a few characteristics such as the amount of reduction of maximum indoor
temperature and time delay in reaching peak indoor temperature. Two specimens can be tested simultaneously by being fixed in their
position. The 8 implemented sensors in different parts of the apparatus are able to record the temperature of different parts such as
both sides of the wallboards (sensors 2, 3, 5, and 6). Daily temperature simulation test (DTST) was carried out in this study to evaluate
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S. Mokhtari and M. Madhkhan
Fig. 3. Test setup [27]; (a) Longitudinal section, and (b) Daily temperature simulator apparatus.
Fig. 4. Sample images related to hollow-core panels; (a) Fabricated mold, and (b) Hollow-core wallboard panel.
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S. Mokhtari and M. Madhkhan
the properties of various wallboard panels in two different climates.
400 mm × 400 mm light weight wallboard panels with the thicknesses of 100 mm and 150 mm and the SSPCM percentages of
10 wt% and 18 wt% were cast with regard to the investigation of thermal properties. 6 out of 12 panels were filled and the other 6 were
hollow-core in order to be used as the external, non-structural walls with the least dead-load possible. Making special molds for the
hollow-core specimens was time-consuming and expensive; consequently, high pressure pipes with the diameters of 63 mm and
75 mm were used to form holes in the panels with the thicknesses of 100 mm and 150 mm, respectively. Plates were used to fix the
pipes to the mold (Fig. 4(a)). As it can be seen in Fig. 4(b), the pipes were pulled out of the wallboards after 24 h, when the concrete was
hardened.
Four other filled wallboard panels were also prepared with a layer of PEG in the middle of the control concrete in order to compare
the difference between using macrocapsule and SSPCM. The thickness of PCM layer was chosen in a way that its weight percentage in
each wallboard was just as much as 10 % and 18 %. Macrocapsules were made by freezing PEG in special molds with the same width
and length as wallboards. Sheets of PEG were then brought out of molds and placed in aluminum wraps to prevent leakage. Fig. 5
shows the PEG molds and sheets that are ready to be placed into wallboard specimens. Moreover, the dimensions of all prepared
wallboard panels are depicted in Fig. 6.
Daily temperatures of an average day of summer in Isfahan as a hot city and Ardebil as a moderately cool one based on their
minimum and maximum daily temperatures were chosen as the ambient temperature for the DTST. Fig. 7. demonstrates the hourly
temperature of these cities in a summer day. DTST was conducted for all specimens under each weather condition for two consecutive
days (48 h).
Three cylindrical specimens were tested for each mix design according to ASTM C39 [22]. It should be noted that compressive
strength was measured after 7, 28, and 90 days of curing at room temperature under water and the average value of the results were
reported. The compressive strength of specimens was then compared to that of required for non-load bearing concrete units according
to ASTM C129–17 [23].
Tensile strength of the cylindrical specimens of 100 mm diameter and 200 mm height was assessed in accordance with ASTM C496
standard [24]. Eq. (1) was adopted to calculate the tensile strength from the ultimate load, F, in splitting tensile strength test, where L
and d are the length and diameter of the specimens, respectively.
(1)
Tensile strength = 2F/(d × L × π)
According to ASTM C642, specimens at the age of 28-day were prepared to evaluate the water absorption of different mixtures. The
specimens are submerged in water for 48 h, then the excessive water is removed from the surface of the specimens and they are
weighed. As for the next step, the specimens are completely dried in oven and weighed one more time. Eq. (2) is used to calculate the
water absorption. In this equation, B and A are the weight of saturated and dried specimen, respectively [25].
Water
absorption =
B− A
× 100
A
(2)
3. Test results and discussion
3.1. Thermal properties of PCM and proposed SSPCM
Heating and freezing curves of PEG and silica fume-PEG composite that are derived from DSC measurements are presented in Fig. 8.
As it is observed, PCM and SSPCM melt at 18.51 ℃ and 19.60 ᵒC respectively; plus they freeze at 15.60 ℃ and 14.69 ᵒC respectively.
Both of the specimens have wide melting and freezing temperature ranges which are shown in Table 5. The latent heat capacities of
Fig. 5. Tools used to prepare PEG macrocapsules for applying to concrete; (a) Mold to make PEG sheets, and (b) Aluminum wrap for encapsu­
lating PEG.
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S. Mokhtari and M. Madhkhan
Fig. 6. Cross section and dimensions; (a) Hollow-core specimens, and (b) Wallboards with PEG macrocapsule.
Fig. 7. Hourly temperature of a considered day in Isfahan and Ardebil.
Fig. 8. DSC analysis of PEG and PEG-silica fume.
PCM and SSPCM are 118.22 J/g and 71.55 J/g during solid-liquid phase change and 136.42 J/g and 74.14 J/g during liquid-solid
phase change. Melting and freezing temperatures of PEG-silica fume approaches those of PEG which is due to the fact that the pro­
posed SSPCM was merely produced by impregnating PEG with silica fume, and there are not any chemical reactions between them.
Notwithstanding the heat capacity reduction in SSPCM compared to PCM, its amount is still adequate for applying to construction
materials such as concrete.
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Table 5
Properties of PEG 600 and PEG-silica fume composite.
Name
Density (gr/cm3)
ΔTMELT (℃)
ΔTFREEZE (℃)
TMELT (℃)
TFREEZE (℃)
PCM
SSPCM
1.105
1.45
1.58–23.25
-1.86–24.48
4.02–19–10
1.88–15.66
18.51
19.60
15.60
14.69
3.2. Compressive strength
Compressive strength was measured at 3 ages for all of the mixtures, including control (C), 10-SSPCM, 18-SSPCM, B-SSPCM, TSSPCM, Kh-SSPCM, and P-SSPCM and the amounts are presented in Fig. 9.
The compressive strength of the final mixtures at the age of 7-days which consist of silica fume decreased by the increase in PEG
amount. This reduction is due to both polymer nature of PEG and a drop in the mass fraction of load-bearing components of concrete
mixture, namely clay aggregates and sandstorm. However, the 28-day compressive strength of 18-SSPCM increased in 3.6 % increment
in comparison with 10-SSPCM. Although the amount of PCM has risen and it causes a reduction in compressive strength, the growth in
the amount of silica fume compensates for this reduction and it can even affect a 9.6 % increase compared with the controlled mixture.
In a previous study by Moshtaghi Jafarabad et al. [26], silica fume is used only as a substitute for cement, not as a supporting material
for phase change materials. In this research, adding silica fume increases the concrete strength. But the difference between present
article and the previous one is that silica fume is used as a carrier of phase change materials to prevent the leakage. The method of
mixing of materials is also different. At first, silica fume is mixed with phase change materials and a gel is formed and the phase change
materials are filled the pores between the silica fume powder and after usage, its leakage is prevented. On the other hand, the phase
change material prevents complete contact with the cement paste and its pozzolanic properties are not fully applied. Hence, a decrease
occurs at early age in mechanical strengths, but with more contact of silica fume and cement paste over time, the mechanical strengths
are likely to increase. All of the compressive strengths ranging from 10.9 to 11.7 MPa are far higher than the minimum amount
required for non-load-bearing concrete units which has satisfied a minimum of 4.14 MPa at the time of delivery to purchaser according
to ASTM C129–17 [23].
As it is shown in Fig. 9(a), the compressive strength of most of the concretes containing other types of SSPCM showed a low amount
in all ages. For instance, the compressive strengths of B-SSPCM, T-SSPCM, and P-SSPCM at the age of 28 days were 2.76 MPa,
Fig. 9. Compressive strength of all mixtures; (a) Mixtures with non-used types of SSPCM, and (b) Mixtures with 10 % and 18 % PEG-silica fume.
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S. Mokhtari and M. Madhkhan
7.48 MPa, and 5.12 MPa respectively. Although the compressive strength of Kh-SSPCM is 11.9 MPa which is enough for non-structural
elements of the building, the amount of PEG 600 was very low due to the low permeability of Khash Pozzolan (PEG/Kh = 0.46);
therefore, increasing the percentage of Pozzolan was needed to raise the amount of PCM which is not economical.
3.3. Tensile strength
However, the non-bearing walls must be able to withstand the loads due to their weight and lateral loads such as wind on their
surface and the seismic force due to the weight of the wall. Therefore, evaluation of mechanical strengths such as compressive and
tensile strengths of concrete is important. In this section, the effect of SSPCM on the tensile strength of concrete has been investigated.
The Brazilian tensile strength was only conducted for mixtures incorporating silica fume and is depicted in Fig. 10. As is observed, the
7-day tensile strength declined with PCM increase to the extent that the tensile strength of 10-SSPCM and 18-SSPCM was 13 % and 31
% lower than specimens without PEG 600 respectively. As was expected, PEG 600 is debilitative to the mechanical properties of
concrete such as tensile strength and this can also be observed in Fig. 10. The reason for such behavior is the oily nature of PCM which
can weaken the bonds among concrete components. However, silica fume makes up for a part of the decline in 28 and 90-day
specimens. In 28-day, for instance, the tensile strength of 18-SSPCM is roughly 5 % more than that of 10-SSPCM. Nevertheless, the
tensile strength of both mixtures containing SSPCM was less than the control mixture.
3.4. Water absorption
The test results of water absorption measurements are presented in Fig. 11 according to ASTM C 642–13 standard [25]. The more
SSPCM is applied to the concrete, the less water absorption will be. This fact is due to the reduction of Leca, an extremely porous
aggregate, in the mixtures consisted of SSPCM. Furthermore, silica fume particles are completely saturated with PEG and are not able
to add any void to concrete matrix.
3.5. Daily temperature simulation test
Fig. 12 and Fig. 13 depict the inner side temperature of all specimens under the ambient weather of Isfahan and Ardebil on the left
and right, respectively. Table 6 gives information about the names of different specimens. The number before SSPCM and Sheet stands
for the wt. percentage of silica fume-PEG composite and weight percentage of PEG respectively. The number after SSPCM, Sheet or C
shows the thickness of the wallboard and H in the name of the specimens represents the fact that specimen is hollow core. As is
observed, the temperature of all specimens incorporating PCM is less than that of control wallboards and this difference is more
significant in the wallboards with the thickness of 10 cm. Table 7 represents maximum amount of reduction in daily temperature
(ΔTf ), the reduction of the maximum temperature (ΔTmax ), and time delay in reaching maximum temperature at the end of the first day
in comparison with control specimen (td ) with respect to wallboards without SSPCM.
The thermal performance of almost all of the specimens showed better result in the first day compared to the second one (ΔTmax ,
ΔTf ) and this is due to the fact that PEG’s melt and freeze cycle has not been completed. This is down to the hot weather in summer in
both of the cities and the freezing point of PCM which is around 14 ℃− 19ᵒC.
It is of note that the thermal performance of almost all specimens with the thickness of 10 cm was better than that of ones with
15 cm thickness. In 10 cm specimens incorporating SSPCM, the maximum temperature reduction and the reduction of maximum
temperature increases with the rise in the mass fraction of SSPCM; on the other hand, these temperature reductions decreased sur­
prisingly in 15 cm wallboards with more SSPCM percentage. This might be owing to the fact that the thermal resistance of thicker
specimens leads to less heat transfer and debilitates the process. Therefore, a considerable amount of phase change material does not
melt during heating process and does not contribute to thermal storage. This trend is also observable in wallboards with PEG mac­
rocapsule and hollow core ones. In Isfahan, for example, the 10 cm wallboard experienced an 18 % and 100 % increment in ΔTf and
ΔTmax respectively with 8 % increase in SSPCM proportion. Generally, the best performance among the filled and hollow core
Fig. 10. Tensile strength of specimens with 0 %, 10 %, and 18 % silica fume-PEG SSPCM.
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Fig. 11. Water absorption of specimens with and without PEG-silica fume SSPCM.
Fig. 12. Diagrams of inner-side temperature of wallboards under Ardebil weather condition.
specimens with SSPCM was observed in the 10 cm wallboard with 18 % SSPCM. No observable leakage was seen in SSPCM wallboards.
Wallboards with PEG sheet showed a better improvement compared to the ones with SSPCM with the highest amount of maximum
temperature reduction and reduction of maximum temperature. 10-cm wallboard with 18 % PEG in the one in which best performance
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Fig. 13. Diagrams of inner-side temperature of wallboards under Isfahan weather condition.
Table 6
The description of specimen names.
specimens
Thickness (cm)
Weight percentage of PEG 600 (%)
SSPCM
10SSPCM-10
18SSPCM-10
10SSPCM-15
18SSPCM-15
10HSSPCM-10
18HSSPCM-10
10HSSPCM-15
18HSSPCM-15
10Sheet-10
18Sheet-10
10Sheet-15
18Sheet-15
10
10
15
15
10
10
15
15
10
10
15
15
10
18
10
18
10
18
10
18
10
18
10
18
*
*
*
*
*
*
*
*
Microcapsule of PEG 600
*
*
*
*
Hollow core
*
*
*
*
was observed. When macrocapsules are incorporated in the wallboards, heat has to pass only half of the specimen’s thickness to reach
PEG and melt it, so it takes a shorter time and all heat capacity of PEG will contribute to thermal performance of panel compared to
using SSPCM in the mixture of concrete.
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Case Studies in Construction Materials 17 (2022) e01298
S. Mokhtari and M. Madhkhan
Table 7
Maximum temperature reduction, reduction of the maximum temperature, and time delay in reaching the maximum inner-side temperature of the
wallboards compared to control specimens.
Name
10SSPCM-10
18SSPCM-10
10SSPCM-15
18SSPCM-15
10HSSPCM-10
18HSSPCM-10
10HSSPCM-15
18HSSPCM-15
10Sheet-10
18Sheet-10
10Sheet-15
18Sheet-15
ΔTf
(ᵒC)
ΔTmax
(ᵒC)
td (hr)
Isfahan
Ardebil
Isfahan
Ardebil
Isfahan
Ardebil
4.9
5.8
1.4
0.9
1.2
5.1
1.6
1.5
6.8
7.2
5.9
6.1
4.5
5.1
1.2
0.4
0.8
5.6
1
0.6
6.3
6.9
5.6
5.7
1.9
3.8
0.7
0.6
0.4
3.1
1.9
2.1
6.3
6.3
5.8
4.8
2.8
3.9
0.7
0.3
0.3
4.3
0.9
0.5
5.5
6.4
5.4
5.6
4
5
2
1
2
8
3.5
3
4
5.5
2.5
1.5
4
4
1.5
1
2
6
2
1.5
4
5
2
1
The amounts of time delay which are presented in Table 7 were obtained by the difference between the time that specimens and
their control wallboard reached their maximum temperature during the first 24 h of the experiment. The aforementioned trends for
ΔTf and ΔTmax can be seen in time delay as well. For instance, time delay in hollow core wallboards under Ardebil temperature,
increased from 2 to 6 h for 10 cm panels and decreased from 2 to 1.5 h for 15 cm ones. The maximum amount of time delay in both
weather conditions is related to 18HSSPCM with the thickness of 10 cm. It was rather predictable that hollow core panels would
perform better than filled ones with regard to time delay, since the air which exits in the hollow parts of the wallboard performs as a
natural heat insulator and postpones reaching maximum temperature.
According to the results of 2022 researches, the cost of running an air conditioner is 50 cents per hour while the same figures for a
refrigerator and a washing machine is 6 cents and 23 cents respectively. The numbers clearly demonstrate that one of the highest
maintenance costs of buildings belongs to air conditioning systems which can differ from 1612 dollars to 2376 dollars per year just for
heating a home for 6 h a day for 4 months and cooling it for 6 h per day for 3 months. Therefore, decreasing the need for the number of
hours of using air conditioners can be economically helpful and compensate for the initial building costs [27].
SSPCM proved complete thermal stability over the thermal changes that were applied to the wallboards by the apparatus. Almost
no leakage was observed in any of the wallboards, except one of the specimens with microcapsule form of PEG which was due to the
fact that its aluminum cover was torn and when a replacement specimen was prepared instead of it, no leakage happened during the
tests. Both PEG 600 and SSPCM were also tested under continuous cycles of temperature change between − 10 ℃ and 50 ℃ and were
proved to be thermally stable.
4. Conclusions
In this study, the concept of improving thermal behavior of concrete with PCMs was introduced. For doing so, a novel SSPCM using
PEG-silica fume and three different kinds of wallboards namely filled with SSPCM, hollow core with SSPCM, and filled with a layer of
PEG in the middle of panel’s thickness were prepared. Mechanical and thermal tests were conducted in order to examine the char­
acteristics of specimens. The following conclusions can be drawn from the results which are achieved from experiments:
1. Among all of the tested materials, silica fume was chosen as a suitable stabilizer for retaining PEG into its pores because of the good
mechanical properties that it adds to concrete and the porous structure that helps using more proportion of PCM to improve the
thermal features of concrete.
2. Since no leakage happened in neither of vacuum-impregnated and directly-absorbed silica fume-PEG composites, direct absorption
approach was chosen in this study. It is not only a more practical way in real concrete components production, but also an easier
one.
3. Tensile and compressive strengths of concrete decreased by increasing the mass fraction of SSPCM in concrete because of the
negative impact of PEG 600 as an oily substance on concrete strength, but the reduction was almost compensated over the age of
28-day because of silica fume usage in a way that 90-day compressive strength of 18-SSPCM rose roughly 9 % in comparison with
control mixture.
4. Water absorption of mixtures decreased by around 29 % and 39 % in 10 % and 18 wt % mixtures respectively due to the fact that
silica fume pores in SSPCM do not absorb any water thanks to being completely filled with PEG 600 and the more SSPCM is used,
the less aggregates which are porous are incorporated in concrete.
5. Although all of the wallboards with SSPCM improved in terms of temperature reduction and time delay compared to the ones
without any PCM, Thermal behavior of the specimens under Isfahan weather condition showed a better result because of its hotter
temperature that leads to melting the entire incorporated PEG.
12
Case Studies in Construction Materials 17 (2022) e01298
S. Mokhtari and M. Madhkhan
6. Thinner wallboards performed better because of their less thermal resistance and their temperature reduction increased by SSPCM
rise. Maximum inner-side temperature reductions in Isfahan in 10 cm wallboards were 4.9 ℃, 5.8 ℃, 1.2 ℃, and 5.1 ℃ for 10 %
and 18 % SSPCM with filled and hollow core cross section respectively.
7. Thermal performance of wallboards with 15 cm thickness deteriorated with the increase in the amount of SSPCM, in a way that the
maximum inner-side temperatures of filled and hollow core panels incorporating 10 % and 18 % SSPCM under Isfahan daily
temperature were respectively 1.4 ℃, 0.9 ℃, 1.6 ℃, and 1.5 ℃.
8. Encapsulated specimens illustrated the best thermal performance in both cities compared to SSPCM ones.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Data Availability
No data was used for the research described in the article.
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