Modified lime-cement plasters with enhanced thermal and hygric storage capacity for moderation of interior climate

Energy and Buildings 126 (2016) 113–127

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Modified lime-cement plasters with enhanced thermal and hygric storage capacity for moderation of interior climate Zbyˇsek Pavlík a,∗ , Jan Foˇrt a , Milena Pavlíková a , Jaroslav Pokorny´ a , Anton Trník a,b , ˇ Robert Cern y´ a a Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 166 29 Prague 6, Czech Republic b Department of Physics, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, Tr. A. Hlinku 1, 949 74 Nitra, Slovak Republic

a r t i c l e

i n f o

Article history: Received 26 October 2015 Received in revised form 27 April 2016 Accepted 3 May 2016 Available online 12 May 2016 Keywords: Lime-cement plasters Interior climate Heat storage capacity Moisture buffer value Phase change materials

a b s t r a c t The absorption or release of heat and moisture by interior surface layers of building envelopes under specific conditions can set effective limits to the extreme levels of indoor air parameters. Moderation of interior climate achieved in this way can contribute to a reduction of energy demands for building conditioning and overall operation cost using a straightforward material solution. In this paper, lime-cement plasters modified by two types of PCM based admixtures are designed specifically for the moderation of relative humidity and temperature fluctuations of the interior environment. The plasters are subjected to a detailed characterization procedure including the assessment of a complex set of basic physical, hygric, thermal and mechanical properties. Experimental results show that the application of small encapsulated PCM particles leads to an up to 10% increase of open porosity, as compared with the reference plaster. Contrary to this fact, the water vapor transport is slightly decelerated, what is attributed to the encapsulating polymer shell which creates impermeable barrier for water vapor transmission. Two contradictory factors affect the liquid water transport, namely the higher porosity of PCM modified plasters and increasing content of not-wettable polymer shells. The moisture storage capacity increases with the increasing amount of PCM in the mix. The moisture buffer value is improved due to the utilization of both PCM admixtures, the developed plasters can be classified as good moisture buffering materials. The thermal conductivity of modified plasters is greatly improved, as well as the heat storage capacity, the additional phase change enthalpy being up to 13 J/g. Although the inclusion of PCM into the lime-cement matrix decreases the mechanical strength, the achieved values of tested mechanical parameters are still satisfactory for the application of the designed plasters in building practice. In summary, PCM modified plasters can be considered a prospective solution for the moderation of interior climate in contemporary buildings. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Indoor air temperature and relative humidity represent important parameters affecting the internal microclimate of contemporary buildings. They may also induce processes which have serious impact on both the working efficiency and health of occupants such as mold growth, respiratory and skin diseases, etc. [1]. Nowadays, it is generally accepted that indoor environment quality has a significant effect on modern life around the globe [2]. For

∗ Corresponding author. E-mail addresses: [email protected] (Z. Pavlík), [email protected] (J. Foˇrt), [email protected] (M. Pavlíková), [email protected] ˇ ´ [email protected] (A. Trník), [email protected] (R. Cern ´ (J. Pokorny), y). http://dx.doi.org/10.1016/j.enbuild.2016.05.004 0378-7788/© 2016 Elsevier B.V. All rights reserved.

example, the Americans spend approximately 90% of their time indoors [3], the French population spends at home 67% of the total time indoor [4]. Therefore, it is necessary to ensure high indoor air quality to guarantee the appropriate conditions for building occupants. Due to the varying climatic loading, the indoor temperature and humidity exhibit significant daily or seasonal variation which can lead to overheating and extreme values of humidity, causing possibly even microbial growth on surface structures. In this way, the quality of interior climate, thus living conditions, can be negatively affected. The practical solutions to this problem are, however, rather energy demanding. A growing number of heating, ventilating and air conditioning (HVAC) systems are being installed in buildings to provide thermal comfort and improve indoor air quality

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for occupants. In the developed countries, energy consumption in both residential and commercial buildings is dominated by space heating, cooling, air conditioning and lighting. As reported by Song et al. [5], in 2010HVAC systems consumed in the developed countries ∼43% of the total energy use in residential buildings and ∼40% in commercial buildings. The energy consumption of buildings in the developed countries comprises 20–40% of total energy use; it is above industry and transport figures in EU and USA [6]. With the consolidation of the demand for thermal comfort, HVAC (Heating, Ventilation and Air Conditioning) systems and their associated energy consumption became an unavoidable asset, accounting for almost half the energy consumed in buildings and around 10–20% of total energy consumption in the developed countries [7]. Therefore, the integrated designs are required in active systems such as renewable energy facilities (i.e., photovoltaic, solar thermal) or energy efficiency HVAC systems [8,9]. Several studies have been focused on improving the efficiency of these technologies by incorporating thermal energy storage systems that implies an additional storage volume [10]. Nevertheless, also the other building components can find use in order to decrease the high amount of energy used by HVAC systems. One of the possibilities represents optimization of building envelope systems from the point of view of the dynamic thermal performance [11]. Fantozzi et al. [12] studied winter and summer performance of lightweight prefabricated building system and concluded that it is possible to improve the dynamic thermal performance of the outer walls by using an optimized stratigraphy characterized by an opportune sequence of resistive and capacitive layers. Conceptually different solution of improvement of buildings dynamic hygrothermal performance represents application of advanced building materials allowing absorption or release of heat and moisture what can be used to moderate the extreme levels of temperature and humidity in the building interior. This energy storage (and/or release) concept can be classified as passive system compared to the active HVAC devices [13]. In late 90’s of the twentieth century, the idea of improving indoor humidity conditions by using highly hygroscopic building materials was firstly introduced. Since the time, the moisture buffering properties of porous building materials became an important topic of building research [14]. According to the definition given in [15], the moisture buffer performance of a building is the ability of the inbuilt mat s to moderate variations in the relative humidity, whereas these variations can be seasonal or diurnal. The moisture buffer performance depends on the moisture buffer capacities of each material combination and furniture in the room, together with the moisture production and air change rate and ratio between the material surface area and the air volume. The moisture buffering capacity of building materials is increasingly recognized for its beneficial influence on the indoor environment [16], which has associated benefits of material durability, occupant health and comfort, and also the whole-building energy performance [17,18]. Presently, the application of sustainability principles in construction encourages the development of new materials and products with new functionalities and applications, able to improve hygrothermal, thus environmental performance of buildings [19]. In a society with a high growth rate and increased standards of comfort arises the need to minimize the current high energy consumption by taking advantage of renewable energy sources [20]. The materials of surface layers, i.e., plasters [21], facing slabs [22,23], or plasterboards [24] with incorporated phase change materials (PCM) have the ability to regulate the temperature inside buildings, contributing to the thermal comfort and reduction in the use of heating and cooling equipment, using only the energy supplied by the sun. PCMs provide a large heat capacity over a limited temperature range and they could act like an almost isothermal

reservoir of heat [25]. As the temperature increases, PCMs change phase from solid to liquid. Since this reaction is endothermic, they absorb heat. When the temperature decreases, PCMs change phase from liquid to solid. This time they release heat as this reaction is exothermic. A number of PCMs was directly designed for quite specific applications, respecting their phase change temperature, enthalpy, specific heat capacity, and maximum operation temperature. Here, PCMs are usually applied as components of composite materials possessing latent heat storage capacity. For a use in building materials exposed to common climatic loading, paraffins were found to have several advantages as non-corrosiveness, chemical stability, no phase segregation, congruent phase change, recyclability, and low cost [26,27]. Therefore, combination of the above given beneficial thermal properties of parrafins with hygroscopic matrix of building materials represents a prospective way to moderation of interior climate, i.e., moderation of interior temperature and relative humidity. Among the commonly used building materials wood [28], gypsum board [29], brick [30], calcium silicate [31], or highly porous plasters [32,33] can be considered hygroscopic and/or capillary active. On the other hand, insulation materials such as XPS, EPS, PUR, PIR, or foam glass are non-hygroscopic and not capillary active [34]. In testing the hygroscopic properties of materials, the microstructure, specific surface area, adsorption/desorption isotherm, maximum adsorption value, sorption velocities, and moisture buffer value (MBV) belong to the most important parameters [35]. In general, the hygroscopic materials are characteristic by their high moisture buffering capacity that enables them to dampen humidity variations. Additionally, in the case of capillary active materials, occasional interstitial condensation can be redistributed and transported out of the material due to the high capillary activity. In this paper, we introduce several composite materials with enhanced thermal and hygric storage capacity which can be applied as surface layers for the moderation of interior climate. In order to keep the cost within reasonable limits, the designed solution is based on a commercially available lime-cement plaster and paraffinic PCMs.

2. Materials and mix design The commercial lime-cement dry plaster mixture Manu 1 (Baumit) consisting of hydrated lime, cement, sand and additives, which is frequently used in the Czech Republic for both interior and exterior renders, was used as the basis for the plaster mix design. In order to enhance the thermal storage capacity, two different types of phase change materials (PCM) were added to the mixes. The first was the polymethyl methacrylate microencapsulated paraffin mixture Micronal DS 5040 (BASF) which is a fine powdered material having the solid content 97–100% (according to DIN EN ISO 3251) and water content ≤3%. As specified in BASF product data sheet, its phase change temperature is around 22 ◦ C (melting 23 ◦ C, crystallization 22 ◦ C) and enthalpy of fusion 96 J/g [36]. The second PCM applied was Rubitherm RT 22HC (Rubitherm). It is based also on paraffin mixture encapsulated in polymer shell but it is delivered in the form of water dispersion, with the solid content of about 35%. The enthalpy of fusion of this material is 200 J/g, the melting and crystallization temperatures vary from 20 to 23 ◦ C with the maxima at 22 ◦ C [37]. The particle size distribution (PSD) of the lime-cement dry plaster mixture, Micronal DS 5040 and Rubitherm RT 22HC, as measured by an Analysette 22 Micro Tec plus device (Fritsch), is presented in Fig. 1. Apparently, the PSD curves of all raw materials had a bimodal shape but their maxima were at rather different

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Fig. 1. Particle size distribution of raw materials.

positions. The dry mortar mixture was the coarsest, with the maximum at 550 ␮m, Rubitherm with 8.5 ␮m the finest, and Micronal with its maximum at 35 ␮m was in between. The composition of the designed plasters is given in Table 1. Both PCMs were used in three different dosages, 8, 16, and 24% of mass of the original dry plaster mixture for Micronal, 4, 8, and 12% for RT 22HC. Because of almost two-times lower enthalpy of fusion of Micronal compared to Rubitherm material, its dosage was doubled in order to get materials with similar thermal performance and latent heat storage. Since the applied PCMs influenced the workability of the fresh mixtures, the water amount was adjusted to obtain the flow diameter of 220 mm in all cases. 3. Material characterization techniques 3.1. Microstructure and pore size distribution The microstructure was analyzed by scanning electron microscopy (SEM), using a JSM 6510 LV device (Jeol) that allows magnification from 5 to 300 000. The pore size distribution was determined by mercury intrusion porosimetry (MIP). A combination of porosimeters Pascal 140 and Pascal 440 (Thermo Scientific) was used for that purpose.

was accessed from the sample volume including pores, whereas the matrix density was calculated from sample volume of pure material matrix exclusive pores. The open porosity was calculated from the known values of bulk density and matrix density.

3.3. Water vapor transmission properties Water vapor transmission properties were measured using the cup method under isothermal conditions. The samples had circular cross section having the diameter of 110 mm, whereas the samples thickness was 30 mm. The cup containing silica gel and having the analyzed specimen sealed on its top by technical plasticine was placed in a controlled climatic chamber, where constant relative humidity of 50% and constant temperature of 21 ◦ C was maintained. Then, it was weighed periodically until the steady state values of mass gain per unit time were obtained [38]. The water vapor diffusion permeability ı [s] was determined according to Eq. (1),

␦=

m × d , S × ␶ × Pp

(1)

3.2. Basic physical properties The bulk density was determined on cubic samples of 50 mm side, by measuring the sample dimensions and its dry mass. The matrix density was measured by helium pycnometry, using a Pycnomatic ATC apparatus (Thermo Scientific). Here, the bulk density

where m is the mass of water vapor diffused through the sample, d (m) the sample thickness, S (m2 ) the specimen surface,  (s) the period of time corresponding to the transport of mass m of water vapor, and pp (Pa) is the difference between partial water vapor pressure in the air under and above specific specimen surface.

Table 1 Composition of studied plasters. Mixture

Water [kg]

Dry plaster [kg]

PCM [kg]

Reference plaster (RP) Plaster with 8 mass% of Micronal (PM8) Plaster with 16 mass% of Micronal (PM16) Plaster with 24 mass% of Micronal (PM24) Plaster with 4 mass% of Rubitherm (PR4) Plaster with 8 mass% of Rubitherm (PR8) Plaster with 12 mass% of Rubitherm (PR12)

1.50 1.95 2.15 2.35 1.18 0.87 0.63

6.3 6.3 6.3 6.3 6.3 6.3 6.3

– 0.50 1.00 1.51 0.83 1.69 2.52

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Using the ı values from Eq. (1), the water vapor diffusion resistance factor ␮ [−], which is for its simplicity most frequently used in the practice, was calculated as ␮=

Da . D [m2 /s]

(3)

where T (K) is temperature, R [8.314 J/K mol] the universal gas constant, M [18.02 g/mol] the molar mass of water vapor, and Da [m2 /s] is the water vapor diffusion coefficient in the air. 3.4. Water transport properties The liquid water transport properties were characterized using the water sorptivity concept [39]. Cubic samples with 50 mm edge were used, having the lateral sides insulated by epoxy resin. An experimental setup based on automatic data acquisition was used [40]. The water absorption coefficient A [kg/m2 s1/2 ] and sorptivity S [m/s1/2 ] were determined from the straight line obtained by plotting the cumulative mass of water absorbed per unit area against the square root of time. The apparent moisture diffusivity, app [m2 /s], was then calculated according to Eq. (4) [41],

 A 2 wsat

,

(4)

where wsat [kg/m3 ] is the saturated moisture content of the analyzed material. 3.5. Sorption and desorption isotherms A dynamic vapor sorption device DVS Advantage (Surface Measurement Systems) was used to determine sorption and desorption isotherms at 21 ◦ C. The instrument measures the uptake and loss of vapor gravimetrically, using highly precise balances with a resolution of 1.0 ␮g. Such a high resolution is obtained by hanging samples on the end of a beam where the position of the beam is measured by an optical sensor. The partial vapor pressure around the sample is generated by mixing the saturated and dry carrier gas streams, using electronic mass flow controllers. The humidity range is 0–98% with the accuracy ±0.5%. The DVS Advantage instrument is designed to make measurements isothermally over the temperature range of 5–60 ◦ C with the accuracy ± 0.1 ◦ C. The temperature is controlled in the measurement unit using a Pt100 temperature sensor. Before the measurements, samples of the studied materials were dried at first in a vacuum drier and then, during the cooling, they were kept in desiccators. After drying, they were put into the climatic chamber of the DVS Advantage and exposed to the 0, 20, 40, 60, 80, and 98% relative humidity. Each step in RH during the DVS measurement was incremented either when a stable mass was achieved with mass change less than 0.00004%/min or a maximum time interval of 400 min was reached. Because reaching of sample mass equilibrium at high RH was problematic, the maximum time interval of sample exposure to 80% was 4000 min, and for 98% it was prolonged up to 7000 min. 3.6. Moisture buffer capacity The theoretical description of moisture buffer capacity on the material level is based on the heat-mass transfer analogy. Well known from the heat transport theory is the thermal effusivity which expresses the rate of heat transfer over the surface of a material when the surface temperature changes. By introducing the

ıp × 0 × ∂u

∂

bm =

is the water vapor diffusion coefficient,

RT , M

app ≈



(2)

In Eq. (2), D D=␦

moisture effusivity bm [kg/m2 Pa s½ ] in a similar way to the definition of thermal effusivity (Eq. (5)), we can describe the ability of a material to absorb or release moisture [17]

ps

.

(5)

In Eq. (5) ıp [kg/m s Pa] is the water vapor permeability, 0 [kg/m3 ] the dry density of the material, u [kg/kg] the moisture content,  [−] the relative humidity, and ps [Pa] the saturation vapor pressure at the temperature of the experiment. For the determination of moisture buffer capacity we used a similar procedure as proposed by McGregor et al. [16]. The experiment was based on a step-response method. This method recorded the mass variation during relative humidity (RH) cycles of a specimen with a known exposed surface area. The particular specimens were vapor proof insulated on lateral sides in order to get an accurate information on the exposed surface area. A DVS Advantage device was used to set cycles of 8 h at high RH (70%) and 16 h at low RH (30%). The sample mass variation during adsorption and desorption phases was continuously monitored during 4 cycles in order to reach dynamic equilibrium where the final mass at the end of the cycle and initial mass varied by less than 5%. The practical Moisture Buffer Value (MBVpractical ) was calculated using the maximum moisture uptake (g/m2 ) after 8 h of adsorption phase divided by the RH interval, which in this case was 40%. The Ideal Moisture Buffer Value (MBVideal ) was determined according to the definition introduced by Rhode in [42]. The accumulated moisture uptake G(t) [kg/m2 ] and moisture release that both happen within the time period tp is found by integrating the moisture flux over the surface, g(t), as in Eq. (6)



t G(t) =

g(t)dt = bm × p × h(˛)

tp ,

, (6)

0

where 2  sin2 (n ˛) ≈ 2.252[˛(1 − ˛)]0.535 . n3/2 ∞

h(˛) =

(7)

n=1

␣ [−] is the fraction of the time period where the humidity level is high. For the 8/16 h scheme ␣ = 1/3, which makes h(˛) = 1.007, and the accumulated moisture uptake can be expressed in a simpler form, G(t) ≈ 0.568 × bm × p ×



tp .

(8)

MBV [g/m2 %RH] is expressed based on the moisture exchange from Eq. (8) normalized with the change in surface relative humidity, RH. MBV is proportional to the moisture effusivity bm times the square root of the time period, tp ½ [s½ ]. Thus, the defined theoretical, or ideal, value of MBVideal is given by Eq. (9), MBVideal ≈

 G(t) ≈ 0.00568 × ps × bm × tp . RH

(9)

3.7. Heat transport parameters Thermal conductivity and thermal diffusivity were measured by a portable hand-held instrument ISOMET 2114 (Applied Precision) using surface cylindrical probe. The thermal conductivity measurement accuracy given by producer is 5% of reading +0.001 W/mK in the measuring range 0.015–0.70 W/m Km K. The device operates on a dynamic measurement principle, which enables to reduce the measurement time in a comparison with the steady state measurement methods. The experiments were done under constant

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Fig. 2. SEM image of the reference plaster.

Fig. 3. SEM image of plaster PM16.

laboratory conditions at (22 ± 1) ◦ C and (30 ± 1) % RH. For the measurement, 5 pre-dried cubic samples having side of 70 mm were used.

For the measurement of phase change enthalpies and their temperatures, a Difference Scanning Calorimetry (DSC) analysis was done. A DSC 822e apparatus (Mettler Toledo) was employed, together with a Julabo FT 900 cooling device. During the measurement, the following temperature regimes were applied: 5 min isothermal regime, cooling at 10 ◦ C/min from 40 ◦ C to −20 ◦ C, 5 min isothermal regime, heating at 10 ◦ C/min from −20 ◦ C to 40 ◦ C, 5 min isothermal regime. On the basis of the measured heat flow data, the specific heat capacity as a function of temperature was calculated.

The MIP data are summarized in Table 2. Both PCM additions led to the increase of cumulative pore volume. This data clearly supports SEM images presented in Figs. 3 and 4. One can observe PCM capsules distributed in plaster matrix without tight binding and interaction with hydrated products that increased the cumulative pore volume, as compared to the reference material. From the quantitative point of view, plasters modified by Rubitherm addition exhibited a lower porosity increase, whereas both average and threshold pore diameters decreased. This could be explained by partial filling of bigger pores of the basic plaster matrix by small Rubitherm particles as can be observed in Fig. 4. Here, better crosslink between the PCM capsules and hydrated products is visible than in the case of Micronal imbibition. Application of Micronal led to a systematical increase of all MIP characteristics what was beneficial for the MBV improvement.

3.9. Mechanical properties

4.2. Basic physical properties

The flexural and compressive strengths were determined according to the European Standard EN 1015–11 after 28 days of wet curing. Flexural strength was measured in a three point bending test arrangement on three prismatic samples having a size of 40 × 40 × 160 mm. The compressive strength was determined on the fragments of specimens left over after the bending test.

The basic physical properties of studied materials are shown in Table 3. The presented data represent average values of 5 independent measurements. Micronal 5040 having quite low powder density (361 kg/m3 ) significantly decreased the bulk density and matrix density of the developed plasters. On the other hand, Rubitherm dispersion affected plasters’ basic physical properties in a smaller extent. In

3.8. Heat storage parameters

4. Results and discussion 4.1. Microstructure and pore size distribution The SEM images of tested plasters are given in Figs. 2–4 . According to the literature [43], C–S–H gel can exhibit three morphologies: fibrous-acicular form (type I), reticule or honeycomb form (type II) and denser-almost spheres form (type III). In Fig. 2, SEM image of the reference lime-cement plaster is presented. One can distinguish following features in the material microstructure and characterize its morphology: the characteristic hexagonal plates belonging to calcium hydroxide, the morphology of C-S-H gels that appears mainly as type III and partially as type I, the ettringite needless and small aggregate particles. Similar data were observed by Gleize et al. [44] for the silica fume-cement-lime mortar. In SEM images in Figs. 3 and 4, inert spherical particles of PCMs are found incorporated into the lime-cement matrix. Nevertheless, polymer encapsulation blocked partially the paraffin wax imbibition into the hydrated products matrix. The black areas between the hydrated products, PCM capsules and aggregates represent pores.

Fig. 4. SEM image of plaster PR12.

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Table 2 Pore size distribution parameters. Material

Cumulative pore volume [cm3 /g]

Average pore diameter [␮m]

Threshold pore diameter [␮m]

Total porosity [%]

RP PM8 PM16 PM24 PR4 PR8 PR12

0.231 0.256 0.274 0.294 0.247 0.258 0.280

1.91 2.85 3.61 3.92 2.49 2.03 1.41

3.91 4.61 5.36 5.87 3.66 2.98 2.14

35.2 37.6 38.6 40.4 38.1 38.2 38.4

Table 3 Basic physical properties. Material

Bulk density [kg/m3 ]

Matrix density [kg/m3 ]

Total open porosity [%]

RP PM8 PM16 PM24 PR4 PR8 PR12

1572 1330 1142 1025 1474 1429 1369

2415 2128 1851 1667 2375 2257 2120

34.9 37.5 38.3 38.5 36.9 37.7 38.4

general, for all plaster mixtures a distinct increase of total open porosity was identified which was, apparently, due to the change of their inner structure caused by the inclusion of small encapsulated PCM particles. A comparison of the open porosity results in Table 3 with the MIP data (Table 2) showed only small differences, which were within the error range of the applied methods.

absorption coefficient A = 0.209 kg/m2 s1/2 . The water absorption coefficient of lime-pozzolan mortar with 34.6% open porosity reported by Vejmelková et al. [50] had a slightly higher value, A = 0.215 kg/m2 s1/2 . Evaluating the effect of PCM additions on the liquid water transport properties, two distinct features were revealed. All the studied plasters with PCM addition had higher water absorption coefficient than the reference lime-cement plaster. At a first glance, this could be simply explained by the higher porosity of these materials (Tables 2 and 3). However, the highest water absorption coefficient, thus water sorptivity and apparent moisture diffusivity, exhibited the plasters PM8 and PR4 with the lowest amount of PCM. Here, the incorporation of PCMs led to a change in microstructure characterized by higher porosity, whereas the inert polymer shell had no obvious effect on the deceleration of water transport. For plasters containing higher amount of PCMs, the polymer microencapsulation of both paraffinic waxes decreased the water transport through the examined materials.

4.3. Water vapor transmission properties 4.5. Sorption and desorption isotherms The water vapor transmission properties calculated as average values from 5 measurements are given in Table 4. Apparently, the PCM addition led in both cases to an increase of the water vapor diffusion resistance factor and decrease of the water vapor diffusion permeability despite the higher open porosity. The explanation could be found in the effect of the PCM polymer shell, which created an impermeable barrier for water vapor transmission. However, it should be noted that the worsening of water vapor transmission capability caused by the PCM addition was not very significant; the modified plasters retained their favorable open character for water vapor transport. The obtained results were in a good agreement with the measurements reported for similar types of plasters by other investigators [45–48]. 4.4. Water transport properties The results of free water intake experiment which were calculated as average values from 5 measurements are shown in Table 5. The lowest water absorption coefficient A = 0.203 kg/m2 s1/2 was measured for the reference lime-cement plaster RP which was in accordance with its lowest porosity (Table 3). This finding is in agreement with the data published by Veiga et al. [49] who obtained for lime-metakaolin mortar (L:P = 1:0.5) the water Table 4 Water vapor transmission properties.

The sorption and desorption isotherms presented in Figs. 5 and 6 show that the addition of PCMs into the lime-cement matrix led to an increase of equilibrium moisture content (EMC) values, as compared to the reference material RP. This feature was observed for all studied plasters; it was more distinct for RH levels higher than 40%. These RH levels are characterized by capillary condensation that fills the micropores and mesopores [51]. The increase in moisture storage capacity can be assigned especially to the higher porosity of PCM modified plasters and also to the different pore size distribution that allowed higher adsorption of water molecules. The highest water vapor storage capacity exhibited PM24 with the maximum hygroscopic EMC of 5.1%. The EMC measured for sorption and desorption process for plasters with Rubitherm addition was lower than for the Micronal modified plasters. In this case, the maximum hygroscopic values of EMC varied from 3.0 to 3.4%. Similar data were obtained by McGregor et al. [16] who studied MBV of unfired clay masonry and found that materials with similar moisture storage capacity exhibited good or excellent MBV. From a practical point of view, the enhanced hygric storage capacity can be useful for the cyclic moderation of indoor RH level.

Table 5 Water transport properties.

Material

ı [s]

[−]

Material

A [kg/m2 s1/2 ]

S [m/s1/2 ]

 app [m2 /s]

RP PM8 PM16 PM24 PR4 PR8 PR12

2.20E-11 1.97E-11 1.86E-11 1.79E-11 2.12E-11 1.99E-11 1.94E-11

8.3 9.2 9.8 10.1 8.6 9.1 9.4

RP PM8 PM16 PM24 PR4 PR8 PR12

0.203 0.262 0.249 0.221 0.265 0.242 0.215

2.034E-04 2.625E-04 2.495E-04 2.214E-04 2.655E-04 2.425E-04 2.154E-04

5.66E-07 9.96E-07 7.28E-07 7.10E-07 8.55E-07 6.80E-07 5.08E-07

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Fig. 5. Sorption and desorption isotherms of the reference and Micronal modified plasters.

Fig. 6. Sorption and desorption isotherms of the reference and Rubitherm modified plasters.

4.6. Moisture buffer capacity Fig. 7 shows an example of the dynamic moisture buffer experiment. The calculated values of MBVideal and MBVpractical are summarized in Table 6. MBVpractical was for all studied materials higher, as compared to MBVideal , the differences were about 20%. A similar comparison of ideal and practical data was done, e.g., in [52], where the authors referred on differences between the data calculated using steady state effusivity values and data measured in a dynamic experiment. In our case MBVideal calculated according to [15] for an idealized case of moisture uptake underestimated the dynamic behavior of materials exposed to RH cycles. All the studied plasters having MBVpractical in the range of 1–2 g/m2 %RH for 8/16 h cycles could be classified as good moisture buffering materials [15]. As the good moisture buffering capacity of hygroscopic

Table 6 Moisture buffer values. Material

MBVideal [g/m2 %RH]

MBVpractical [g/m2 %RH]

RP PM8 PM16 PM24 PR4 PR8 PR12

1.01 1.03 1.10 1.14 1.07 1.08 1.09

1.19 1.22 1.29 1.36 1.28 1.32 1.36

materials can reduce heating and cooling energy consumption in buildings [53], the application of PCM modified plasters designed in this paper can help to decrease the overall energy consumption by decreasing the requirements for the indoor air quality control.

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Fig. 7. Example of the dynamic experiment for determination of MBV.

Table 7 Thermal conductivity and thermal diffusivity measured at 22 ± 1 ◦ C.

Table 8 Thermal conductivity and thermal diffusivity measured at 5 ± 1 ◦ C.

Material

␭ [W/m K]

a [10−6 m2 /s]

Material

␭ [W/m K]

a [10−6 m2 /s]

RP PM8 PM16 PM24 PR4 PR8 PR12

0.54 0.39 0.24 0.19 0.50 0.46 0.37

0.36 0.29 0.25 0.20 0.32 0.30 0.26

RP PM8 PM16 PM24 PR4 PR8 PR12

0.53 0.37 0.23 0.17 0.48 0.43 0.31

0.35 0.27 0.23 0.19 0.31 0.29 0.25

4.7. Heat transport parameters The thermal conductivity and thermal diffusivity data measured by the pulse method at 22 ± 1 ◦ C is shown in Table 7 in a form of average values from 5 experiments. For the plasters with Micronal addition, thermal conductivity decreased up to three times which was positive from the point of view of thermal performance of the wall. The thermal diffusivity decreased up to two times which was also significant. The remarkable improvements in heat transport parameters could be assigned to the higher porosity of Micronal modified plasters and also to the low thermal conductivity of PCM additive itself (∼0.08 W/m K). On the other hand, plaster mixtures with Rubitherm were not influenced by the PCM admixture in such extent, apparently due to the higher thermal conductivity of the Rubitherm dispersion (∼0.44 W/m K). In this case, the measured values of thermal conductivity resulted from combination of several factors, such as increased plasters porosity in comparison with reference material, chemical binding of water forming the water dispersion of Rubitherm PCM in hydration products and its partial evaporation within the hydration process. Since the thermal conductivity of water is at 298 K approx. 0.61 W/m K [54], even a slight loss of water from PCM dispersion affects its thermal conductivity. In this way, the primarily high thermal conductivity of Rubitherm dispersion significantly decreased. It should be noted that the above given heat transport parameters were obtained for plasters with PCMs in the liquid phase, since the applied transient impulse method heated the test specimens by approximately 10 ◦ C. Because of the estimated effect of the phase transition on thermal parameters of the developed plasters, the additional tests of thermal conductivity and thermal diffusivity were done at 5 ◦ C to determine thermal properties of the studied

materials with incorporated PCMs in the solid phase. The data is presented in Table 8. Both the thermal conductivity and thermal diffusivity decreased for the lower temperature measurement, as compared with the results obtained for plasters having PCMs in the liquid phase. However, the differences were very low, within the range of measuring accuracy. Therefore, there was a good reason for a conclusion that also other material parameters, such as mechanical and hygric properties, were unaffected by the phase transition, in particular because of the effect of the inner polymer shell.

Table 9 Phase change temperature and enthalpy. Material

Process

Phase change temperature (main peaks) [◦ C]

PR4

cooling

PR8

heating cooling

PR12

heating cooling

9.20 17.62 22.47 9.15 19.65 23.74 9.04 19.03 23.82 20.30 26.54 21.76 27.70 21.90 27.78

PM8 PM16 PM24

heating cooling heating cooling heating cooling heating

Phase change enthalpy [J/g] 4.55 5.01 7.40 7.62 13.49

4.88 4.37 8.20 7.92 13.15 12.81

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Fig. 8. DSC analysis of the reference plaster and plasters with Micronal addition – heating/cooling rate 10 ◦ C/min.

Fig. 9. DSC analysis of the reference plaster and plasters with Rubitherm addition – heating/cooling rate 10 ◦ C/min.

4.8. Heat storage parameters Figs. 8 and 9 show the heat fluxes measured at the DSC analysis. The phase change temperatures (corresponding to main peaks of DSC plots) and enthalpies are summarized in Table 9. The heat flux curves had a unimodal character for the plaster with Micronal during both heating and cooling phases. For the plasters containing Rubitherm the unimodal shapes of heat flux curves were observed at heating only, in the cooling phase they were bimodal. There can be found several examples of bimodal shape of DSC curve in literature. Jeon et al. [55] reported on two phase change peaks of paraffin. The first phase change peak in the cooling process was high, corresponding to the liquid-solid phase change and the second peak was much lower, corresponding to the solid-solid phase transi-

tion of the paraffin. In heating process, the DSC curve was almost unimodal. Bimodal shape of DSC plots observed also Jeong et al. [56] who prepared PCM/diatomite composites. In case of paraffin wax/diatomite composite they observed distinct bimodal DSC curves for both heating and cooling processes. On the other hand, for n-octadecane/diatomite composite they monitored bimodal shape of DSC plot for cooling process only, similarly as in our case. Similar results published also Yang et al. [57] who developed paraffin/Palygorskite composite PCM for thermal energy storage. Considering the above given data we can assign the performance of Rubitherm modified plasters to two main effects. First, the paraffin underwent two phase transitions as reported in [55–57]. Second, due to the high cooling rate adopted for DSC tests, the super-cooling phenomena contributed to the bimodal shape of DSC plot. In order

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Fig. 10. DSC analysis of the pure Rubitherm addition – cooling process – cooling rate 1 ◦ C/min.

Fig. 11. DSC analysis of the pure Rubitherm addition – heating process – heating rate 1 ◦ C/min.

to support this statement, we realized additional DSC test of pure Rubitherm admixture with heating/cooling rate of 1 ◦ C/min. Looking at Fig. 10, one can observe low temperature peak at DSC curve (approx. at 11.6 ◦ C) corresponding to the solid-solid phase transition and high solidification peak having maximum at 19.4 ◦ C. Within the heating process, one phase-change transition was found only as displayed in Fig. 11. The phase change temperatures during the heating phase were for both types of plaster higher than in the cooling phase which was due to the dynamic character of the experiment. Obviously, the phase change enthalpies increased up to three times with the increasing PCM dosage in the mixes. It should be noted that the measured phase change temperatures were slightly higher in a

comparison with the producer’s data [36,37]. This was caused, apparently, by the relatively low thermal conductivity of the limecement matrix which partially delayed the heat transport into the PCM additives in the plaster specimens. The specific heat capacity corresponding to the heating process is presented in Figs. 12 and 13. The effect of both PCMs was quite apparent. While the reference plaster RP had in the range of 23–27 ◦ C the specific heat capacity ∼0.45 J/g ◦ C, the specific heat capacity of plasters with Micronal varied from 1.13 to 2.12 J/g ◦ C at 27 ◦ C and for the plasters containing Rubitherm it was 1.45–2.48 J/g ◦ C at 23 ◦ C. It is quite apparent that the heating and cooling rate applied for DSC tests affects its results.

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Fig. 12. Specific heat capacity of the reference plaster and plasters with Micronal addition – heating process (10 ◦ C/min).

Fig. 13. Specific heat capacity of the reference plaster and plasters with Rubitherm addition – heating process (10 ◦ C/min).

Feng at al. [58] introduced that heating/cooling rate 5 ◦ C/min is the common choice applied in DSC tests of PCMs. It complies with the typical standards used in DSC analysis, but it did not follow the advices of Lazaro and Dumas [59,60] who recommended a slow heating rate for the PCM sample to be able to reach phase equilibrium in both thermal and chemical aspects. On the other hand, a similar heating rate as in our case, i.e., 10 ◦ C/min, was used for example by Song at al. [61] who introduced microencapsulated capric–stearic acid with silica shell as a novel phase change material for thermal energy storage. Kang et al. [62] applied heating rate of 10 ◦ C/min for TGA measurements of energy efficient bio-based PCM composites. Based on the results reported in [58], the liquid temperature and latent heat are gradually increasing with the increase of heating rate. Also the phase transition range is closely related to the heating/cooling rate.

In order to provide a detailed thermal characterization of the studied plasters with PCM addition, the supplementary DSC tests were done for heating rate of 1 ◦ C/min. The specific heat capacity data is shown in Figs. 14 and 15. In both analyzed cases, the decrease in heating rate led to a narrower temperature interval of phase change, as compared to the data measured for the heating rate of 10 ◦ C/min. For plasters with Micronal addition, the main peaks of specific heat capacity slightly shifted to lower temperatures, e.g., for the material PM24 the shift was from 27.8 ◦ C to 26.1 ◦ C. For plasters with Rubitherm, the shifts in maximum specific heat capacity peaks were not significant. Looking at the obtained specific heat capacity from a quantitative point of view, the decrease of heating rate increased its maximum value by about 16.9% for PM24. Contrary to that, the maximum value of specific heat capacity of the plaster PR decreased by about 12.0%, as compared to the data measured at the heating rate of 10 ◦ C/min.

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Fig. 14. Specific heat capacity of the reference plaster and plasters with Micronal addition – heating process (1 ◦ C/min).

Fig. 15. Specific heat capacity of the reference plaster and plasters with Rubitherm addition – heating process (1 ◦ C/min).

In order to quantify the effect of the developed plasters on building requirement for HVAC energy consumption, simple calculation of the energy saving estimation for Czech climate parameters was done. The case study was a room with dimensions of 4 × 4 × 2.8 m. The thickness of applied PCM modified plasters was 20 mm. For the calculations, the real data on temperature fluctuations inside a room measured in our previous work in the administrative building built in 1980s [63] was used. In this case, the heating system of the building in winter period of the year partially compensated the minimum interior temperatures. The monitored building was a typical reinforced concrete column structure having 3 stories. The exterior surface was constructed from the hung facade slabs formed of steel frames, aluminous window frames, glass, and insulation

boards. Part of the envelope was built from cavity brick blocks on the surface provided by cement-lime plaster. The building is located in the north-west suburb part of Prague, the capital of the Czech Republic. The monitored temperature history during the year 2012 is displayed in Fig. 16. Based on the recorded values of the temperature, estimation of number of phase change cycles was done in order to calculate amount of stored and released thermal energy. For the ideal function of the plaster with PCM, the bigger temperature differences between day and night are required in order to complete phase transition of PCM. Unfortunately, during the winter period, no phase changes occurred because of lower temperatures and the applied heating system. This limitation also decreased the func-

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Fig. 16. Interior temperature fluctuations in the studied room. Table 10 Calculated possible energy saving. Month

Number of cycles

Saved energy by PM24 [kWh]

Saved energy by PR12 [kWh]

January February March April May June July August September October November December Total

0 0 2 4 8 7 3 4 9 7 0 0 44

0.00 0.00 6.59 13.17 26.35 23.05 9.88 13.17 29.64 23.05 0.00 0.00 144.91

0.00 0.00 8.82 17.65 35.29 30.88 13.24 17.65 39.71 30.88 0.00 0.00 194.12

tionality of PCM modified plasters during the summer, when the measured temperatures were much higher, but did not decrease below the crystallization temperature. The monthly and total energy savings calculated using the number of past phase change cycles are given in Table 10. Looking at the measured data, one can see significant benefit of PCM addition into the plaster mixes on improvement of energy performance of the analyzed room, whereas the main energy savings were achieved in spring and autumn. Moreover, one can assume higher energy saving in case the heating system will not fully compensate the temperatures drops. Estimated data are similar to that published by Mazo et al. [64] that modeled thermal performance of houseshaped cubicles based on the experimental installation of the GREA research group, situated in Puigverd, Spain. Authors reported that almost 300 kWh per year can be saved in Spain region by utilization of the CSM modules with incorporated PCM produced by Rubitherm, whereas both the cooling and heating energy consumptions were quantified. 4.9. Mechanical properties The mechanical parameters are summarized in Table 11 as average values from 5 measurements for flexural strength and Table 11 Mechanical properties. Material

Flexural strength [MPa]

Compressive strength [MPa]

RP PM8 PM16 PM24 PR4 PR8 PR12

0.90 0.75 0.72 0.65 0.79 0.58 0.53

1.91 1.88 1.78 1.40 1.60 1.02 0.90

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10 measurements for compressive strength. Apparently, the differences in pore volume affected negatively the mechanical performance of all PCM modified plasters. Another negative impact had the non-reactive polymer shell of the PCM admixtures that was only physically embedded in the hydrated lime-cement matrix. The most significant decrease in mechanical parameters was observed, obviously, for the plasters with highest PCM amounts. In the case of PM24, the flexural and compressive strengths exhibited 28% and 26% decrease in measured values, respectively, as compared to the reference plaster RP. Plaster PR12 had the flexural strength 41% lower and compressive strength 52% lower than RP. Although the application of PCM additions led to a significant decrease of mechanical performance of the studied plasters, the achieved values of mechanical parameters were still satisfactory for their application in building practice, taking into account characteristic results obtained for different plasters by other investigators. For instance, Silva at al. [65] studied the effect of hydraulic binder content on mechanical properties of mortars. They found compressive strength of lime-cement mortar ranging from 1.0 to 2.7 MPa. The flexural strength varied from 0.2 to 0.6 MPa. In another study Vejmelková et al. [49] measured mechanical properties of lime mortars with pozzolan addition having different grain fineness. Their reference lime mortar had the compressive strength of 0.90 MPa and flexural strength of 0.2 MPa. These values were comparable or even lower, as compared to the lime-cement plasters with PCM additives designed in this paper. Based on the results of detailed tests on plasters properties and performance, the PM24 and PR12 plasters were identified as the best solutions for energy savings in buildings and for the moderation of relative humidity fluctuations in the interior environment. Both these materials exhibited moisture buffer values in the range of good buffering materials whose moisture buffering capacity can help to reduce the overall buildings energy consumption by decreasing the requirements for the indoor air quality control. From the point of view of thermal performance and possible heat energy storage and release during the phase transition, these two materials exhibited the highest latent heat in both cooling and heating processes. Calculated possible energy savings proved that the application of these materials can provide substantial reduction of building operational costs.

5. Conclusions A detailed experimental analysis of hygric, thermal, mechanical and other related physical properties of newly developed limecement plasters with enhanced thermal and hygric storage capacity making possible to moderate the interior climate was presented in the paper. The mix design involved a commercially available limecement plaster and paraffinic PCM, in order to keep the cost within reasonable limits. The main experimental results can be summarized as follows: The application of small encapsulated PCM particles resulted in an up to 10% increase of open porosity, as compared with the reference lime-cement plaster. The water vapor transport was slightly decelerated in the PCM modified plasters despite their higher porosity. This was attributed to the encapsulating polymer shell, which created impermeable barrier for water vapor transmission. The liquid water absorption increased due to the PCM addition but the difference between the reference and modified materials decreased with the growing PCM dosage in the mix. This was explained by two contradictory factors affecting the liquid water transport at the same time, namely the higher porosity of PCM

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modified plasters and increasing content of not-wettable polymer shells. The moisture storage capacity increased with the increasing amount of PCM in the mix. This was a positive finding with respect to the moderation of indoor air quality, in particular taking into account that the increase was more distinct for RH levels corresponding to multi-layer adsorption and capillary condensation. The moisture buffer value was improved after adding PCM to the lime-cement mixes. This was once again a positive feature, as for the potential for interior climate moderation. The developed plasters were classified as good moisture buffering materials. The thermal conductivity of modified plasters was greatly improved; for the plasters with Micronal addition it decreased up to three times which was positive from the point of view of thermal performance. The thermal conductivity of plaster mixtures with Rubitherm was influenced in lower extent due to the higher thermal conductivity of the Rubitherm dispersion. The heat storage capacity of the lime-cement plasters containing PCM increased remarkably, the additional phase change enthalpy being up to 13 J/g. This result is of the particular importance for the moderation of interior environment since the improved heat storage capacity in the appropriate temperature range can partially limit the interior temperature fluctuations and shift thermal/cooling load of air-conditioning system. The monthly and total energy savings calculated using the number of past phase change cycles proved the significant benefit of PCM addition into the plaster mixes on improvement of energy performance of the room in simplified case study, whereas the main energy savings were achieved in spring and autumn. The inclusion of PCM into the lime-cement plasters decreased the mechanical parameters up to 40%. This was caused by the increased pore volume and the presence of non-reactive polymer shells of the PCM admixtures which were only physically embedded in the hydrated lime-cement matrix. However, the achieved values were still satisfactory for their application in building practice. It is quite apparent, with the increasing amount of PCM used, the effect of plaster application for energy saving increases. Therefore, the maximum possible dosage of PCM in optimized plaster mix design will be the subject of the future work, whereas all the aspects affecting plasters properties will be considered and evaluated. For the practical use of PCM modified plasters, there will be necessary to realize full scale test, for example on a real size room provided with PCM plasters exposed to the real environmental conditions. This will allow to accurately access the contribution of PCM addition to building hygrothermal performance and quantify possible energy savings. Acknowledgment The authors gratefully acknowledge the financial support received from the Czech Science Foundation under project No. 1422909S. References [1] L. Fang, G. Clausen, P.O. Fanger, Temperature and humidity: important factors for perception of air quality and for ventilation requirements, ASHRAE Trans. 106 (2000) 503–510. [2] W. Wei, O. Ramalho, C. Mandin, Indoor air quality requirements in green building certifications, Build. Environ. 92 (2015) 10–19. [3] US Environmental Protection Agency (EPA), EPA/400/1-89/001C, Washington, DC, 1989. [4] S. Kirchner, Quality of indoor air, quality of life, a decade of research to breathe better, breathe easier, CSTB, edition, French Indoor Air Quality Observatory, 2013. [5] S. Song, S. Wu, Y.Y. Yan, Control strategies for indoor environment quality and energy efficiency—a review, Int. J. Low Carbon Technol. 10 (2015) 305–312.

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