Smart reversible thermochromic mortar for improvement of energy efficiency in buildings

Construction and Building Materials 186 (2018) 884–891

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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 Otifa, Alcorcón, Spain c Institute of Optics – CSIC, C/Serrano, 144, 28006 Madrid, Spain b

h i g h l i g h t s  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.

a r t i c l e

i n f o

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

a b s t r a c t 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 associated 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 suitability of this innovative material as a dynamic building coating for improvement of energy efficiency. Ó 2018 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/).

1. Introduction Important efforts are devoted nowadays in different research fields to the development of smart materials with properties controlled 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. ⇑ Corresponding author. E-mail address: [email protected] (G. Perez).

Chromogenic materials with optical properties reversibly changing upon changes of external parameters are also of interest as smart construction materials. In fact, electrochromic and thermochromic 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 development has been achieved in glazing with devices based on vanadium 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 VO2 nanoparticles 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/Ó 2018 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/).

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Recent publications demonstrate the feasible implementation of thermochromic properties in different types of construction materials, as asphalts for roads construction [4], bricks [5], elastomeric 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 specific base material. Their optical properties change from a coloured state for temperatures below a transition temperature to colourless when heated beyond this value and reversibly to coloured state when cooled again. This translates into a high solar absorption (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 interesting for application of thermochromic materials in buildings envelopes, 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 spectrum 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 thermochromic 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 temperature 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 characteristics 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 degradation 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 feasibility of application of this innovative material as a dynamic building 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 composition 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 pigment 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 electron 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 application as building coating. A combination of three different calcareous sands with different particle sizes was used for the mortar formulation, in order to

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.

Table 1 Composition of white Portland cement.

WC

MgO

Al2O3

SiO2

SO3

K2O

CaO

Fe2O3

0.35

2.94

17.45

2.35

0.29

61.96

0.27

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G. Perez et al. / Construction and Building Materials 186 (2018) 884–891 Table 2 Composition of the raw mortar (WM) based on white Portland cement. Product

Kg/m3

Water BL II/A-L 42.5R Calcareous sand 0.1–0.8 mm Calcareous sand < 0.6 mm Limestone filler 0.01–0.9 mm Siliceous sand 0.1–0.6 mm Water repellent Water retainer Resin Fibre Pozzolan

294.18 238.52 322.80 143.11 731.47 79.51 1.91 2.07 42.93 3.98 23.85

obtain a well-compensated granulometric curve and allow a relatively 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 calcareous 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 mechanical 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.

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 vacuum 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 consistency 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  4 cm  16 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 thermochromic 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 wavelength range from 300 nm to 2500 nm corresponding to solar radiation. 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 temperature 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%.

2.2. Characterization techniques Optical characterization of the thermochromic pigment and mortar was performed by measuring the spectral diffuse reflectance (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 samples, 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 standard [13]. The powder was sieved through the meshes defined in

Fig. 2. Reflectance spectra of the reversibly thermochromic pigment and mortar in the solar range.

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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 temperature 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 temperature (50 °C in the specimen at the top position and the underlying 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 radiation incident in its surface may be either reflected back to the incidence medium or absorbed by the material. The absorbed fraction gives rise to an increase of the surface temperature of the mortar 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 wavelength 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.

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.

3.2. Chemical and morphological characterization of reversibly thermochromic mortar The compatibility between the reversibly thermochromic pigments 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 pigments when added to WPC paste was assessed and related to the highly alkaline environment of the cementitious matrix [12]. As a 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 pigments were studied after 28 days of curing. Fig. 4 shows the infrared transmittance spectrum of the thermochromic 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 vibrations in CO23 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 vibrations 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

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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 pigment 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 spectra 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 intensity 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 spectra 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 microcapsules 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 composition 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.

Table 3 Granulometry of mortar powder. POWDER Granulometry Sieve mesh (mm) 2 1 0.5 0.25 0.125 0.063 left

% 0 0 11.7 24.9 29.8 24.3 9.3

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.

G. Perez et al. / Construction and Building Materials 186 (2018) 884–891 Table 4 Main properties of the mortar in fresh and hardened states. FRESH STATE Water retention (%) Consistency (mm)

100 140

HARDENED STATE Bulk density (kg/m3) Flexural strength (MPa) Compressive strength (MPa) Dynamic elastic modulus (MPa) Adherence to concrete substrate (MPa) Adherence to ceramic substrate (MPa) Capillarity coefficient (kg/m2min1/2)

1448 4.2 9.7 3662 0.61 0.77 0.108

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 measured 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/cm3 is estimated for the thermochromic mortar in hardened 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 compressive 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 thermochromic 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 thermochromic 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 temperatures 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. Significant increase in reflectance is observed between coloured and

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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 building 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 temperature (Tc = 31 °C) will reduce heating of the surface and surrounding media. On the contrary, in cold conditions, the lower reflectance (higher absorptance) will increase heating of the mortar 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 summers [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 absorptance to define the global solar optical response of façade materials. 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 surface 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 reflectance of 19% (from 0.38 in the colour phase to 0.32 in the colourless one). The chemical compatibility between the thermochromic pigments and the cementitious matrix is of primary importance to assure preservation of the thermochromic behaviour of the pigments 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 microcapsules. 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

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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 addition, the low bulk density value estimated for the thermochromic mortar in hardened state (1448 kg/cm3) 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 9981 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 thermochromic 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/m2min1/2 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 thermochromic 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 improvement 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 pigment 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 thermochromic 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 behaviour in the complete solar radiation range, a 19% decrease in solar absorptance is evaluated upon heating the mortar beyond its transition temperature. This optical response is positive to reduce surface 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/cm3, 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/ m2min1/2 for the coefficient of capillary water absorption. These experimental characteristics indicate that the optimized composition of the mortar is adequate for the intended application as external building coating. Future work in the development of this smart reversible thermochromic 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, thermal 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 Spanish Ministry of Economy and Competitiveness under the Project BIA2014-56827R. References [1] H. Huang, G. Ye, C. Qian, E. Schlangen, Self-healing in cementitious material: materials, methods and service conditions, Mater. Des. 92 (2016) 499–511. [2] B. Han, S. Ding, X. Yu, Intrinsic self-sensing concrete and structures: a review, Measurement 59 (2015) 110–128. [3] C.G. Granqvist, Recent progress in thermochromics and electrochromics: a brief survey, Thin Solid Films 614 (2016) 90–96. [4] H. Zhang, Z. Chen, L. Li, Ch. Zhu, Evaluation of aging behaviors of asphalt with different thermochromic powders, Constr. Build. Mater. 155 (2017) 1198–1205. [5] Y.H. Chang, P.H. Huang, B.Y. Wu, S.W. Chang, A study on the color change benefits of sustainable green building materials, Constr. Build. Mater. 83 (2015) 1–6. [6] M. Sharma, M. Whaley, J. Chamberlain, T. Oswald, R. Schroden, A. Graham, M. Barger, B. Richey, Evaluation of thermochromic elastomeric roof coatings for low-slope roofs, Energy Build. 155 (2017) 459–466. [7] S. Zheng, Y. Xu, Q. Shen, H. Yang, Preparation of thermochromic coatings and their energy saving analysis, Sol. Energy 112 (2015) 263–271. [8] A.L. Pisello, State of the art on the development of cool coatings for buildings and cities, Sol. Energy 144 (2017) 660–680. [9] Y. Ma, Y. Li, B. Zhu, Analysis of the thermal properties of air-conditioning-type building materials, Sol. Energy 86 (2012) 2967–2974. [10] B. Park, M. Krarti, Energy performance analysis of variable reflectivity envelope systems for commercial buildings, Energy Build. 124 (2016) 88–98. [11] Y. Ma, B. Zhu, Research on the preparation of reversibly thermochromic cement based materials at normal temperature, Cem. Concr. Res. 39 (2009) 90–94. [12] G. Perez, A. Guerrero, M.C. Alonso, A. Pons, First approach to thermochromic mortars: compatibility between thermochromic pigments and cement, in: J.C. Galvez, A. Aguado de Cea, D. Fernandez-Ordoñez, K. Sakai, E. Reyes, M.J. Casati, A. Enfedaque, M.G. Alberti, A. de la Fuente (Eds.), ICCS16 Concrete

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