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Smart reversible thermochromic mortar for improvement of energy
efficiency in buildings
G. Perez
a,
⇑
, V.R. Allegro
a
, M. Corroto
b
, A. Pons
c
, A. Guerrero
a
a
Institute of Construction Science Eduardo Torroja – CSIC, C/Serrano Galvache, 4, 28033 Madrid, Spain
b
Otifa, Alcorcón, Spain
c
Institute of Optics – CSIC, C/Serrano, 144, 28006 Madrid, Spain
highlights
First smart reversible thermochromic mortar (SRTM) for dynamic building coating.
Mortar composition favours stability and physical/organic activity of pigments.
Solar absorptance decreases a 19% upon heating SRTM beyond its transition temperature.
Suitable optical, physical, mechanical properties for energy efficiency improvement.
article info
Article history:
Received 8 February 2018
Received in revised form 4 June 2018
Accepted 30 July 2018
Keywords:
Thermochromic materials
Mortar coating
Building façade
Energy efficiency
Solar optical properties
abstract
A smart reversible thermochromic mortar based on ordinary white Portland cement and organic
microencapsulated thermochromic pigments is presented in this work. Chemical and morphological
composition of the mortar assures chemical stability and proper physical and organic activity of the
pigments within the cementitious matrix. The optical properties of the mortar change with temperature
at the transition value of the pigments (31 °C). For higher temperatures, the material shows a light colour
and high reflectance in the visible range, while for lower temperatures it shows a dark grey colour asso-
ciated to a low reflectance. Infrared spectroscopy and electron scanning microscopy results confirm the
chemical and morphological stability of the microencapsulated pigments within mortar samples cured
for 28 days. Finally, physic-mechanical properties of fresh and hardened mortar demonstrate the suitabil-
ity of this innovative material as a dynamic building coating for improvement of energy efficiency.
Ó2018 The Authors. Published by Elsevier Ltd. This is an open accessarticle under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Important efforts are devoted nowadays in different research
fields to the development of smart materials with properties con-
trolled by external stimuli. Specifically in the field of construction
materials, smart self-healing concretes obtained with different
strategies show the ability to seal by themselves cracks appearing
at the microscopic level with no human external intervention [1].
As another example, self-sensing concrete can monitor stress,
strain, crack and damage by itself through the measurement of
electrical resistance without the need of embedded, attached or
remote sensors [2]. In both cases, the smart construction materials
increase durability and service life of infrastructures, improving
safety and reducing economic and social costs associated to
failures and repair actions.
Chromogenic materials with optical properties reversibly
changing upon changes of external parameters are also of interest
as smart construction materials. In fact, electrochromic and ther-
mochromic glazing are under development with variable response
to solar radiation controlled by changes in an externally applied
voltage and in external temperature, respectively. This type of
dynamic materials may improve energy efficiency and reduce
environmental impact of buildings through a proper control of
the flow of visible light and solar energy through the envelope [3].
In the case of thermochromic materials, important develop-
ment has been achieved in glazing with devices based on vana-
dium dioxide thin films that allow less solar energy passing
through the glazing at high temperature than at low temperature,
thus reaching indoor comfort with lower energy consumption.
Most recent studies suggest that the use of VO
2
nanoparti-
cles may significantly improve the performance of thermochromic
glazing and give rise to their practical implementation in
buildings [3].
https://doi.org/10.1016/j.conbuildmat.2018.07.246
0950-0618/Ó2018 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑
Corresponding author.
E-mail address: gperezaq@ietcc.csic.es (G. Perez).
Construction and Building Materials 186 (2018) 884–891
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Recent publications demonstrate the feasible implementation
of thermochromic properties in different types of construction
materials, as asphalts for roads construction [4], bricks [5], elas-
tomeric roof coatings [6] and coatings for buildings’ envelopes [7].
In most cases, the thermochromic behaviour is based on organic
pigments in powder or slurry form, which are encapsulated in
organic microcapsules with diameter around 15 mm or lower in
order to protect them from the surrounding chemicals in the speci-
fic base material. Their optical properties change from a coloured
state for temperatures below a transition temperature to colour-
less when heated beyond this value and reversibly to coloured
state when cooled again. This translates into a high solar absorp-
tion (low reflectance) in the material for cold conditions giving rise
to an increase of its surface temperature. On the contrary, the
material shows a low solar absorption (high reflectance) for warm
conditions that avoids a high increase of its surface temperature.
This smart variation of the optical response are especially interest-
ing for application of thermochromic materials in buildings envel-
opes, as in both external conditions, cold or warm, the properties of
the material help improving energy efficiency of the building.
There are in fact several recent works demonstrating the energy
savings associated to the use of such dynamical building coatings
as compared to common coatings and to the widely accepted cool
materials of similar colours. These cool materials are characterized
by a high reflectance in the near infrared range of the solar spec-
trum thus giving rise to reduced heating of the envelopes due to
radiation absorption [8]. This optical response makes them useful
for warm climates, while dynamic optical behaviour characteristic
of smart thermochromic coatings prove to be especially beneficial
in the case of climates with cold winters and warm summers
[7,9,10].
For the case of façades, it is more interesting to achieve the ther-
mochromic behaviour in a mortar finishing coating to avoid the
need for an additional external coating on top of the mortar. Up
to the author’s knowledge, only a preliminary work dealing with
thermochromic cement-based materials has been published [11].
In that work [11], the authors assess the colour change with tem-
perature of white Portland cement (WPC) pastes with addition of
thermochromic pigments synthesized in their laboratory. Thermal
tests in a self-made insulated box indicate that, at cold conditions
of 10 °C, a temperature 3 °C higher is achieved in a paste with a
10% of black thermochromic pigment and transition temperature
about 24 °C than in a paste of raw WPC.
The present work describes the composition and main charac-
teristics of a smart reversible thermochromic mortar based on an
ordinary white Portland cement (WPC). Compatibility between
the reversibly thermochromic pigments and the cementitious
matrix was identified as an important issue in the first stages of
the development of this innovative material. In fact, the degrada-
tion of three different commercial pigments when added to WPC
paste was assessed and the resulting pastes did not show
thermochromic behaviour. The highly alkaline environment of
the cementitious matrix was identified as the leading degradation
factor [12].
An optimized composition is proposed in the present work for
the reversible thermochromic mortar that assures the chemical
stability of organic encapsulated thermochromic pigments in the
mortar matrix. Moreover, the morphological mortar composition
is structured to asses a proper physical and organic activity of
the pigments within the cementitious matrix, while preserving
the necessary physical properties for a final application as building
external coating.
Variation of optical properties of the mortar in the solar range
upon temperature change, chemical and morphological properties
of the hardened material and physic-mechanical properties of
fresh and hardened mortar are presented to demonstrate the feasi-
bility of application of this innovative material as a dynamic build-
ing coating for improvement of energy efficiency.
2. Materials and methods
2.1. Mortar components and preparation conditions
The thermochromic mortar was based on an ordinary BLII/A-L
42.5 R white Portland cement (WPC), produced in the facilities of
El Alto of the Spanish company Portland Valderribas. Cement com-
position is collected in Table 1 in terms of the main oxides
(concentration > 0.2%) as determined by X-ray fluorescence.
A commercial thermochromic pigment Chromazone Slurry
Black 31 from LCR Hallcrest Ltd was used for the preparation of
the mortars. This is a black coloured reversible thermochromic pig-
ment with transition temperature of 31 °C. Fig. 1 shows the aspect
of the slurry changing from a black colour for temperatures below
31 °C to light grey for temperatures beyond this transition value.
The pigment is formed by three components: a pH-sensitive
colour former, which determines the colour and donates an elec-
tron upon the thermochromic reaction, an electron-accepting
colour developer and a hydrophobic non-volatile solvent with a
low melting point that defines the transition temperature for the
thermochromic reaction. The pigment employed is enclosed in
melamine formaldehyde microcapsules as a protection from
aggressive environments and the slurry contains a 50% of capsules
in aqueous solution.
The composition of the raw mortar (protected under Spanish
Application Patent number 201731186) is shown in Table 2.It
was structured to asses a proper physical and organic activity of
the thermochromic pigment within the cementitious matrix, while
preserving the necessary physical properties for the final applica-
tion as building coating.
A combination of three different calcareous sands with different
particle sizes was used for the mortar formulation, in order to
Table 1
Composition of white Portland cement.
MgO Al
2
O
3
SiO
2
SO
3
K
2
O CaO Fe
2
O
3
WC 0.35 2.94 17.45 2.35 0.29 61.96 0.27
Fig. 1. Aspect of the reversibly thermochromic slurry at a temperature lower (left,
at 8 °C) and higher (right, at 50 °C) than the transition temperature of 31 °C.
G. Perez et al./ Construction and Building Materials 186 (2018) 884–891 885
obtain a well-compensated granulometric curve and allow a rela-
tively low cement content. Two of them, with particle sizes in
the range of 0.1–0.8 mm (Granicarb) and <0.6 mm (Betocarb
P1DA), were provided by the Spanish company Omya Clariana
and had calcite as the main mineralogical phase. The third calcare-
ous sand was a limestone filler predominantly formed by aragonite
phase. A small percentage of siliceous sand with rounded particles
of size between 0.1 and 0.6 mm and light colour was also added to
improve distribution of water and additives and increase mechan-
ical resistances.
A set of additives was added to the blend, namely sodium oleate
as water repellent additive, a methyl-hydroxyethyl-cellulose water
retainer additive, an organic resin with ethylene, vinyl laureate and
vinyl chloride in its composition, calcined aluminium silicate as
pozzolanic addition and cellulosic white fibres with mean length
of 600 mm (see Table 2).
For the preparation of mortars, the solid blend was prepared
with the proportions collected in Table 2 and 18.5% of weight of
solid of water was added (water/cement ratio of 1.2). In order to
obtain the reversibly thermochromic mortars (TWM) a 3% of
weight of solid of pigment was mixed with the water and stirred
before pouring into the solid. The pigment and water proportions
in the Chromazone Slurry Black 31 were always taken into account
in the formulations.
The mixing of the mortars was performed with the following
sequence: 45 s at 140 rpm, then 15 s at rest and finally 1 min at
285 rpm. A four-minute’s resting time was respected in all cases
to assure a complete chemical reaction of the mortar components.
2.2. Characterization techniques
Optical characterization of the thermochromic pigment and
mortar was performed by measuring the spectral diffuse reflec-
tance (8°:di) at different temperatures in a Perkin Elmer high
performance Lambda 900 spectrophotometer provided with an
integrating sphere of 150 mm in diameter. Measurements have
been made in the 300 nm–2500 nm wavelength interval at steps
of 10 nm.
Infrared transmittance spectra of the pigment and mortars were
measured at room temperature in vacuum conditions, using a
Bruker iFS 66v/S Fourier Transform Infrared spectrophotometer.
To complete morphological and chemical analysis of mortar sam-
ples, a scanning electron microscope (SEM) Hitachi S-4800
equipped with an energy dispersive X-ray (EDX) analyser BRUKER
5030 was used. Samples were previously coated with a gold
conducting layer.
Granulometry of raw mortar in powder form was analysed
following the dried methodology of UNE-EN-1015-1:1999 stan-
dard [13]. The powder was sieved through the meshes defined in
the standard in an automatic vibrator during 20 min to define
the particle size distribution.
To characterize fresh mortar, water retention was measured
according to the methodology described in Cahiers CSTB 2669
[14]. The weight of water obtained from the fresh mortar by vac-
uum filtering during 5 min was calculated relative to the initial
water content of the sample which was estimated considering
the water to solid ratio used in the mortar preparation. The consis-
tency of the fresh mortar was measured according to UNE-EN
1015-3 standard [15].
Regarding the properties of hardened mortar, the bulk density
was obtained from the mean weight of three prismatic specimens
of 4 cm 4cm16 cm. The specimens were demoulded after
2 days in plastic bags, kept for a total curing time of 28 days in a
climatic chamber at 20 °C and 60% relative humidity. The same
type of specimens were used to test mechanical resistance, both
flexural and compressive, as described in UNE-EN 1015-11:1999
standard [16] and to calculate the coefficient of capillary water
absorption, as defined in UNE-EN 1015-18:2002 standard [17].
For the determination of dynamic elastic modulus of the mortar,
the forced resonance test described at ASTM C-215 standard [18]
was used to measure the resonant longitudinal frequency of
2.5 cm 2.5 cm 28 cm prismatic specimens cured in the
previously described conditions. Finally, both concrete and ceramic
substrates were used to measure the adherence of the ther-
mochromic mortar following the UNE-EN 1015-12:2016 [19].
3. Results
3.1. Optical properties of reversibly thermochromic mortar
Fig. 2 shows the reflectance spectra of both the thermochromic
pigment and the mortar incorporating this pigment in the wave-
length range from 300 nm to 2500 nm corresponding to solar radi-
ation. The spectra taken at two different temperatures are
included: one temperature (20 °C) clearly lower than the transition
temperature value of the thermochromic pigment (31 °C) and the
other clearly higher (40 °C).
The spectra in Fig. 2 show differences in reflectance with tem-
perature in the visible range that are directly related to the change
in reflectance of the pigment. As it can be seen, same as in the
pigment, the reflectance of TWM is higher at 40 °C than at 20 °C,
with percent reflectance increases as large as 50% in the spectral
interval from 430 nm to 650 nm, reaching the 84% at a wavelength
of 590 nm at which reflectance increases from 16.5% up to 30.5%.
Table 2
Composition of the raw mortar (WM) based on white Portland cement.
Product Kg/m
3
Water 294.18
BL II/A-L 42.5R 238.52
Calcareous sand 0.1–0.8 mm 322.80
Calcareous sand < 0.6 mm 143.11
Limestone filler 0.01–0.9 mm 731.47
Siliceous sand 0.1–0.6 mm 79.51
Water repellent 1.91
Water retainer 2.07
Resin 42.93
Fibre 3.98
Pozzolan 23.85
Fig. 2. Reflectance spectra of the reversibly thermochromic pigment and mortar in
the solar range.
886 G. Perez et al. / Construction and Building Materials 186 (2018) 884–891
On the contrary, the mortar shows the same reflectance at both
temperatures from 800 nm along the near infrared range up to
2500 nm, in spite of the fact that the pigment reflectance is lower
for higher temperature in this spectral range.
The observed change in the TWM mortar reflectance with tem-
perature in the visible range has two important effects. On the one
hand, it implies a significant modification of the material’s aspect
[7,20]. As observed in Fig. 3, the specimens of TWM mortar cooled
down to 8 °C (bottom position in Fig. 3) show a dark grey colour,
while the specimens of the same mortar heated to a higher tem-
perature (50 °C in the specimen at the top position and the under-
lying base mortar in Fig. 3) show a clearly lighter grey colour.
The second important effect of the change in the optical
response of the mortars with temperature is important in outdoor
constructions and relates to thermal effects produced by incident
solar radiation. As the mortar is an opaque material, the solar radi-
ation incident in its surface may be either reflected back to the
incidence medium or absorbed by the material. The absorbed frac-
tion gives rise to an increase of the surface temperature of the mor-
tar that will also increase the temperature in the surrounding
media. The higher the absorption (the lower the reflection), the
higher will be the temperature increase.
Although the thermochromic mortar reflectance changes with
temperature only in the visible range, the thermal effect of the
radiation absorption may be significant due to the higher relative
intensity of solar radiation at terrestrial surface in this specific
wavelength range. In fact, around a 50% of the solar energy
corresponds to the relatively narrow UV–VIS range, from 300 nm
to 780 nm. Considering this, solar reflectance and absorptance
are calculated from the two reflectance spectra in Fig. 2 according
to UNE-EN 410:2011 standard [21]. These parameters take into
account the relative distribution of solar energy within the wave-
length range from 300 nm to 2500 nm to obtain the global solar
response of materials useful to assess the impact of thermal effects.
The value of solar reflectance of the thermochromic mortar is
0.32 at 20 °C and 0.38 at 40 °C. This means a 19% increase in solar
energy reflected upon heating the mortar beyond its transition
temperature.
3.2. Chemical and morphological characterization of reversibly
thermochromic mortar
The compatibility between the reversibly thermochromic pig-
ments and the cementitious matrix is an important issue in the
development of the thermochromic mortar. In fact, in the first
stage studies, the degradation of three different commercial pig-
ments when added to WPC paste was assessed and related to the
highly alkaline environment of the cementitious matrix [12].Asa
result from this study, the conditions for assuring the stability of
thermochromic pigments in the mortar matrix were established
and considered for the mortar composition of Table 2.
In order to confirm the chemical and morphological stability of
the thermochromic pigments within the cementitious matrix of
the developed mortar, samples of mortar with and without pig-
ments were studied after 28 days of curing.
Fig. 4 shows the infrared transmittance spectrum of the ther-
mochromic mortar (TWM), together with that of the mortar with
no pigments (WM). The two spectra show common features as
the wide band at 3438 cm
1
related OAH bond stretching in
hydrated phases and the strong band at 1424 cm
1
that, together
with the sharp peaks at 875 cm
1
and 712 cm
1
, are due to vibra-
tions in CO
3
2
groups of calcareous aggregates [22]. In addition, the
sharp peak at 320 cm
1
related to CaAO bond stretching and
weaker bands at 984 cm
1
and 817 cm
1
, corresponding to vibra-
tions of SiAO and AlAO bonds of siliceous aggregates and cement,
appear in the spectra of both mortars. Moreover, weak peaks at
2982 cm
1
, 2919 cm
1
, 2585 cm
1
, 2516 cm
1
and 1799 cm
1
are
related to organic additives in the mortar formulation (see Table 2).
The main effect of adding the thermochromic pigments to the
mortar is the increased absorption at 2919cm
1
and 2851cm
1
,
the broadening of the band at 1424cm
1
and the appearance of a
weak peak at 1741cm
1
. These features correspond to peaks
clearly observed in the pigment’s spectrum included in Fig. 4 and
Fig. 3. Aspect of the reversibly thermochromic mortar TWM at a temperature lower
(in the specimen at the bottom, 8 °C) and higher (in the specimen at the top and in
the base, 50 °C) than the transition temperature of 31 °C.
Fig. 4. Infrared transmittance spectra of reversibly thermochromic mortar (TWM)
and WPC mortar without pigments (WM). The spectrum of the raw pigment is
included for interpretation.
G. Perez et al. / Construction and Building Materials 186 (2018) 884–891 887
are characteristic of methyl stearate [23], probably used as solvent
in the Chromazone Slurry Black 31. Although absorption bands of
the mortar hinder other absorption features of the pigment, the
presence of the absorption peaks of the solvent in the spectrum
of the TWM sample suggests the stability of the pigment upon
the mixing and curing processes of the mortar.
In fact, the presence and physical–chemical stability of the pig-
ment in a TWM sample cured for 28 days is confirmed by electron
microscopy in the image at top left position in Fig. 5 and in more
detail in the zoom of this image shown in top right position of
the same Figure 5. Microcapsules are clearly identified embedded
in the matrix of TWM mortar as compared to a similar sample of
the mortar WM, with no pigment addition, shown in the down
left image of Fig. 5. This result confirms the integrity of the
microcapsules in the mortar cured for 28 days and suggests their
proper role as protection of the thermochromic pigment from the
cementitious matrix.
The qualitative results from the compositional analysis of the
TWM mortar sample obtained by EDX are represented by the spec-
tra collected in Fig. 5 (down right position). The high signal from
Au atoms in all the spectra relates to the metallic coating deposited
on the samples to avoid isolation problems.
Spectrum marked as 1, in a position of the TWM cementitious
matrix separated from the microcapsules, shows significant inten-
sity in peaks corresponding to Ca, Cl, Al, Si, O and C, coherently
with the composition indicated in Table 2, with several organic
components, together with cement and aggregates. The EDX spec-
tra marked as 2 and 3 show the composition in the area of the
microcapsules, which is similar to that of spectrum 1. This suggests
that the cementitious matrix has developed on top of the micro-
capsules surface or has reacted with this surface. Nevertheless,
the contribution from the microcapsules to the spectra 2 and 3 is
clearly identified by the slight contribution from N atoms and
the higher intensity related to C atoms, coherent with the organic
nature of the pigments and the melamine–formaldehyde composi-
tion of the capsules.
3.3. Properties of thermochromic mortar for application as building
external coating
Table 3 collects the granulometry of the raw mortar in powder
form as determined by sieve, while Table 4 summarizes the results
of tests performed on TWM, both in the fresh and the hardened
states.
The significant value of particles not passing through the
0.5 mm sieve in the mortar powder (11.7%) assess a proper surface
finish of the mortar external coating and adequate mechanical
resistances. Medium size particles in the mortar are present in a
well-compensated proportion, while the low size fraction (33.6%
lower than 0.063 mm) is adjusted to reduce the risk of cracks.
Fig. 5. SEM-EDX analysis of the studied mortars cured for 28 days. Top left SEM image of reversibly thermochromic mortar (TWM); Top right - Zoom of image at top left;
Down left - SEM image of mortar without pigments (WM); Down right - EDX spectra in different positions of image at top right.
Table 3
Granulometry of mortar powder.
POWDER Granulometry
Sieve mesh (mm) %
20
10
0.5 11.7
0.25 24.9
0.125 29.8
0.063 24.3
left 9.3
888 G. Perez et al. / Construction and Building Materials 186 (2018) 884–891
Water retention is one of the most important mortar properties
in the fresh state. In the case of TWM, a 100% is obtained for this
parameter, measured according to Cahiers CSTB 2669 [14], as the
weight of the sample does not vary during the 5-minute test. This
result indicates that water is totally retained in the mortar.
Consistence of fresh mortar relates to its fluidity and is mea-
sured by its spread ability in flow table as defined in UNE-EN
1015-3:2000 [15]. A mean diameter of 140 mm was obtained for
TWM spread mortar, slightly higher than the value of 126 mm
obtained for WM mortar. A low bulk density value of
1448 kg/cm
3
is estimated for the thermochromic mortar in hard-
ened state and proper values are obtained for the mechanical
resistance of TWM, both in flexural (4.2 MPa) and compressive
(9.7 MPa) strengths. A slight decrease in mechanical strength is
observed with respect to WM mortar that shows a mean compres-
sive strength of 12.2 MPa and a mean flexural strength of 5.0 MPa.
Regarding the dynamic elastic modulus, the value of 3662 MPa
corresponding to TWM mortar is slightly lower than the value
obtained for the mortar with no pigments (4726 MPa). Finally,
the adherence of the thermochromic mortar to concrete substrates
takes a value of 0.61 MPa, with adhesive fracture, which is higher
that the value of 0.36 MPa obtained for WM mortar with the same
fracture pattern. In the case of mortar with pigment addition on
ceramic substrates, the measured adherence value is 0.77 MPa
with cohesive fracture at the mortar, meaning that this is a lower
limit for the mortar adherence. This value is also higher than that
of plain mortar (0.54 MPa).
4. Discussion
The optical properties of the reversibly thermochromic mortar
presented in this work are different from those of the ther-
mochromic pigment used in its composition. In fact, the reflectance
of the mortar is higher for each temperature in the entire spectral
range of Fig. 2. This is due to the higher reflectance of all the light
coloured mortar components with respect to the darker ther-
mochromic pigment. More interesting is to observe that the
change in reflectance of the mortar upon temperature changes is
qualitatively similar to that of the pigment in the visible range.
On the contrary, the reflectance of the mortar does not change with
temperature in spite of the higher reflectance of the pigment for
lower temperatures.
The optical response of the TWM mortar for different tempera-
tures is qualitatively similar to that presented in other works
describing the behaviour of thermochromic building coatings
based on organic encapsulated pigments [7,20]. For instance, in
reference [20] the reflectance of a black thermochromic coating
with transition temperature of 30 °C, similar to that described in
the present work, is observed to present high values in the near
infrared range that do not change upon temperature changes. Sig-
nificant increase in reflectance is observed between coloured and
uncoloured phases, from a value in the order of 5% to reflectance
higher than 20%, but same as in Fig. 2 these variations are
restricted to wavelength values lower than 800 nm.
As indicated previously, the higher the absorption (the lower
the reflection) of solar radiation by the external material at build-
ing façade, the higher will be its surface temperature. Taking this
into account, in warm conditions, the higher reflectance (lower
absorptance) shown in Fig. 2 for the reversibly thermochromic
mortars at temperatures higher than the pigment transition tem-
perature (Tc = 31 °C) will reduce heating of the surface and sur-
rounding media. On the contrary, in cold conditions, the lower
reflectance (higher absorptance) will increase heating of the mor-
tar and media. In both cases, the optical response of the mortar
mitigates the effect of outdoor temperature. This behaviour is
especially interesting in regions with cold winters and hot sum-
mers [7,9,10].
It is interesting to note that the change in optical properties of
the pigment in the near infrared range, with higher reflectance for
lower temperatures would not be favourable in terms of mitigation
of outdoor conditions. It is consequently important the fact that
the reflectance of the mortar in this range is predominantly defined
by the mortar matrix, thus constant with temperature.
The thermal effects are measured or evaluated in the works
analysing thermochromic building coatings [7,10,20] through
energetic simulations, which in most cases consider solar absorp-
tance to define the global solar optical response of façade materi-
als. In reference [20] differences in surface temperatures between
concrete tiles coated with a thermochromic black coating with
transition temperature of 30 °C and a common black coating are
monitored during summertime in Athens. The change in solar
reflectance of the thermochromic coating between the colour and
the colourless phases is a 17.5% (from 0.47 to 0.40). Mean daily sur-
face temperature during the month of August is 9.1 °C lower in the
thermochromic coated tile (38.4 °C) than in the common one
(47.5 °C) for a mean ambient temperature of 29.2 °C. The difference
increases up to 16.5 °C in the maximum daily surface temperature
for the same period, from 51.5 °C in the thermochromic element to
68.0 °C in the common one, with a maximum ambient temperature
of 35.1 °C during this measuring time. No significant differences
are found between the different coatings in the case of nocturnal
surface temperatures. A similar, or even better, potential benefit
may be expected from the measured optical properties of TWM
mortar in Fig. 2, showing a slightly higher change in solar reflec-
tance of 19% (from 0.38 in the colour phase to 0.32 in the colourless
one).
The chemical compatibility between the thermochromic pig-
ments and the cementitious matrix is of primary importance to
assure preservation of the thermochromic behaviour of the pig-
ments within the mortar coating. Wei et al. [24] recently reported
durability problems of phase change materials added to cement
based materials within melamine formaldehyde microcapsules,
as those used to encapsulate the thermochromic compound in
the present work. The authors relate degradation of PCM behaviour
with the presence of sulphate ions in the cementitious matrix that
give rise to formation of a melamine-sulphate supramolecular
crystal after hydrolysis of the melamine formaldehyde microcap-
sules. The results presented in Section 3.2. confirm the chemical
and morphological stability of the thermochromic pigments within
the cementitious matrix, so that this type of durability problems
are not likely to occur in TWM mortar.
Different properties define the suitability of the thermochromic
mortar for application as building external coating. Among them,
the high value of water retention assures that water is available
for a proper setting and hardening of the cement contained in
the mortar composition and for the subsequent development of
mechanical resistances. The results from the spread test, with a
Table 4
Main properties of the mortar in fresh and hardened
states.
FRESH STATE
Water retention (%) 100
Consistency (mm) 140
HARDENED STATE
Bulk density (kg/m
3
) 1448
Flexural strength (MPa) 4.2
Compressive strength (MPa) 9.7
Dynamic elastic modulus (MPa) 3662
Adherence to concrete substrate (MPa) 0.61
Adherence to ceramic substrate (MPa) 0.77
Capillarity coefficient (kg/m
2
min
1/2
) 0.108
G. Perez et al. / Construction and Building Materials 186 (2018) 884–891 889
mean spread diameter of 140 mm, indicates a plastic consistency
for TWM according to EN 1015-6:1999 [25], with a water/powder
ratio adequate for application as building external coating. In addi-
tion, the low bulk density value estimated for the thermochromic
mortar in hardened state (1448 kg/cm
3
) is positive to avoid charge
excess in the building structure when applied on the building
façade.
Regarding mechanical strength, TWM mortar shows values of
flexural/compressive resistance (4.2 MPa/9.7 MPa) slightly lower
than WM mortar (5.0 MPa/12.2 MPa). This effect may be related
to the incorporation of pigment microcapsules that may act as
weaker points in the mortar matrix [26]. In spite of this effect,
the thermochromic TWM mortar, having a compressive resistance
higher than 6 MPa, may be classified as CS IV according to EN 998-
1 standard [27], which is suitable for the application as external
building coating. Moreover, the low difference between flexural
and compressive resistance reduces fatigue in the mortar, thus
improving its durability.
The low value of dynamic elastic modulus obtained for the ther-
mochromic mortar (3662 MPa) is also important for application as
façade coating as it assures the capacity of the mortar to deform
without cracking upon movement of the substrate [28]. Also the
adherence values obtained for the TWM mortar to concrete and
ceramic substrates are correct and it is confirmed that pigment
addition improves mortar behaviour in this aspect.
Finally, the value of 0.108 kg/m
2
min
1/2
that is obtained for the
coefficient of capillary water absorption is low as compared to
the values reported by other authors [29] and classifies the TWM
as a W2 mortar by the EN 998-1 standard [27]. This low capillary
absorption defines a compact material with small capillary
network, which is important to prevent infiltrations of water by
capillarity in the mortar that would compromise its durability.
5. Conclusions
Reversible thermochromic materials, with optical response in
the solar range variable with temperature, are of special interest
for applications in building envelopes. They show a dark colour
and low solar reflectance (high absorptance) for cold external
conditions, thus increasing surface temperature. The same material
reversibly turns to a light colour and high solar reflectance
(low absorptance) for warm conditions, thus reducing the surface
heating of the envelope. In both temperature conditions, the
dynamic optical solar response of the material is useful to improve
energy efficiency.
Several construction materials with such reversible ther-
mochromic response have been reported in the literature but, up
to the author’s knowledge, this work is the first one reporting on
a smart reversible thermochromic mortar for external building
coating. This innovative material is of special interest for improve-
ment of building energy efficiency with no need of additional
external layers in facades.
The proposed mortar is based on an ordinary white Portland
cement with different types of aggregates and additives to which
commercial reversible thermochromic pigments are added to
provide the intended dynamical optical response. The specific pig-
ment used is a black organic microencapsulated pigment with a
transition temperature of 31 °C presented in aqueous dispersion.
The chemical and morphological characterization of the ther-
mochromic mortar by infrared spectroscopy and electron scanning
microscopy confirm that the proposed composition of the mortar
favours the stability and the proper physical and organic activity
of the pigments.
The optical characterization of the smart mortar in solar range
shows a proper change in reflectance upon temperature change.
In fact, the reflectance of the mortar is higher at 40 °C than at
20 °C in the visible range, while no change with temperature is
observed in the near infrared, in spite of the higher measured
reflectance of the pigment for lower temperatures in this range.
Reflectance increases as large as 50% are obtained in the visible
spectral range and, accordingly, the material shows a significantly
darker colour for the lower temperature. Considering the beha-
viour in the complete solar radiation range, a 19% decrease in solar
absorptance is evaluated upon heating the mortar beyond its tran-
sition temperature. This optical response is positive to reduce sur-
face heating in warm conditions and increase surface temperature
in cold conditions.
Finally, a 100% water retention and a plastic consistence are
obtained for the thermochromic mortar in the fresh state. The tests
performed in the hardened state, show a low bulk density value of
1448 kg/cm
3
, a flexural/compressive resistance of 4.2/9.7 MPa, a
dynamic elastic modulus of 3662 MPa, an adherence to concrete/
ceramic substrates of 0.61/0.77 MPa, and a low value of 0.108 kg/
m
2
min
1/2
for the coefficient of capillary water absorption. These
experimental characteristics indicate that the optimized composi-
tion of the mortar is adequate for the intended application as
external building coating.
Future work in the development of this smart reversible ther-
mochromic mortar must be devoted to confirmation of long term
pigment stability and to durability issues related to its application
as external building coating. The tests defined in the standard to
evaluate transport properties of the mortar, impact resistance
and degradation under heating-ice cycles, water–ice cycles, ther-
mal shock and exposure to ultraviolet radiation are under progress.
Conflict of interest
None.
Acknowledgements
Thanks are due to Jose A. Sanchez, to the Physical-Chemical
Analysis Unit and the Innovative Products Assessment Unit of
IETcc-CSIC and to the IR Spectrometry Lab of ICMM-CSIC for their
help in experimental work. This work was supported by the Span-
ish Ministry of Economy and Competitiveness under the Project
BIA2014-56827R.
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