https://doi.org/10.14311/APP.2022.33.0200 Acta Polytechnica CTU Proceedings 33:200–206, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague TECHNICAL EFFECTIVENESS OF CEMENT-BASED MORTAR FOR HIGH-REFLECTIVE BUILDING ENVELOPE THROUGH BUILDING ENERGY SIMULATIONS: PRELIMINARY RESULTS Alessandra Gugliandoloa,∗, Enrico Genovaa, Francesco Baldia, Claudia Caponeb, Maria Anna Segretoa a ENEA, Energy Efficiency Unit Department (DUEE), via Martiri di Monte Sole 4, 40129 Bologna, Italy b Italcementi SpA, via Stezzano 87, 24126 Bergamo, Italy ∗ corresponding author: alessandra.gugliandolo@enea.it Abstract. In areas with high levels of solar radiation, decreasing the amount of solar energy absorbed by the building envelope is useful to reduce the need of air conditioning and "heat island" effects. Most high-reflective products, however, suffer from low durability. The COOL-IT project is developing an innovative high-reflective cement based mortar for precast products to be used as outer layer in buildings for both vertical and horizontal surfaces, or for road pavement. The mix design is aimed at increasing the durability of this cement-based component while retaining high reflectance to solar radiation. This paper presents the preliminary results of the project, based on the simulation of the energy demand of a residential building, intended as a support to optimize the proposed mixes. The model is analysed in three different locations in Italy, for one year of operation. This allows evaluating the trade-off of the energy demand between the winter increase and the summer reduction. Keywords: Building energy demand, building envelope, cool material, energy efficiency, high re- flectance. 1. Introduction Urban areas are characterized by high density of built surfaces, which generally are low reflective and low permeable. This modification of land surface results in higher air temperature compared to the surround- ing rural areas, with differences up to 3◦C. This phe- nomenon, normally referred to as Urban Heat Is- land (UHI), increases the energy needs and consump- tion for summer cooling, thus contributing to global warming. The UHI can be countered through the im- plementation of so-called "cool" technologies. A "cool material" is characterized by high solar reflectance (thereafter denoted also as "albedo") and high in- frared emissivity, where the first is the ability to re- flect incident solar radiation, and the second allows to return most of the fraction to the atmosphere so- lar absorbed by thermal radiation. Several technical solutions have been developed for the construction sector, especially in outdoor pavements but also in the building envelope [1–3]. Also in the latter, the most common application field of cool materials is in horizontal surfaces, where the solar radiation peak is reached in the summer season. 2. State of Art The large use of cement-based pavement products highlights that increasing their solar reflectance may result in a significant mitigation of the heat island ef- fect and in the improvement of urban comfort. Fur- thermore, the diffusion of cement-based products in building roofs and walls, namely tiles and concrete façades, suggests that the solar reflectance of cement- based components can play a relevant role in reduc- ing the energy demand for building cooling. Mainly related to pavements, researches in this field charac- terize the albedo of concretes and their constituents or focus on developing methods and technologies to obtain high reflective cement-based products. Solar reflectance values of 0.35 − 0.40 and 0.25 − 0.30 are considered typical for new and weathered concretes respectively, if conventional mixes are used [4]. Levinson and Akbari [5] studied the effects that composition and exposure of Portland cement con- crete have on its solar reflectance. They noticed that concrete albedo grows during the cement hydration reaction but stabilizes after six weeks from casting. The concretes were on average more reflective if pro- duced by means of white than grey cement. The au- thors also observed a correlation between the compo- sition of concrete and its solar reflectance, which was influenced by cement albedo, fine aggregate albedo and, after abrasion, also by the solar reflectance of coarse aggregates. Marceau and VanGeem [6] widened the base of available data by measuring the solar reflectance of concrete constituent materials (Portland and slag ce- ments, fly ash, fine aggregates) and concrete speci- mens comparable to those commonly used in exterior flatwork. Designed with combinations expected to re- sult in low albedo, the concrete specimens showed a minimum solar reflectance of 0.34 and a maximum 200 https://doi.org/10.14311/APP.2022.33.0200 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 re-ASR Behaviour of Recycled Concrete of 0.64 up to 0.69, with an average value of 0.47. The authors highlight that the influence on concrete albedo is more relevant for cement than for the other constituents and confirm that the higher the solar re- flectance of cement, the higher that of concrete. Since albedo is related to the product surface, a way to increase this property in cement-based com- ponents consists in surface treatments of conventional mix designs, generally by the addition of pigments or the application of coatings. Several solutions have been developed for the building sector, generally re- lated to roof tiles and shingles [7, 8]. In the refurbish- ment of existing constructions, acting on the finishing is appropriate to reduce costs and technical barriers to the boost of albedo. However, the enhancement of solar reflectance can be irreversibly decreased by aging processes, especially in case of abrasion and surface damages. The effects of ageing on the solar reflectance of building envelope materials are object of investigation [9–11]. On the other hand, different studies aim at in- creasing the solar reflectance of concrete by acting on its composition. For this purpose, the partial substitution of the binder, the use of white cement and the incorporation of lightly coloured aggregates are mainly investigated. It has been observed that fly ash reduces albedo with respect to conventional mixes, without a consistent trend [4]. On the con- trary, the solar reflectance of concrete increases con- sistently with the content of ground-granulated blast furnace slag, which appears to be even more effective than the use of white sand and latex [4]. Besides the ratio of binder replacement, the type of aggregates and the surface finish, also curing condi- tions influence the solar reflectance of hardened con- cretes. Recent research has observed that solar re- flectance increases if the moist level of curing con- ditions is higher, with more marked difference if the water-to-cement ratio increases [12]. 3. Objective The research herewith discussed investigates the use of building and urban simulations as a support tool to design innovative high-reflective cementitious mor- tars to be used on the building envelope. More in de- tail, the aim is optimizing the use of "cool materials" as constituents of cementitious mixtures. Different from surface treatments and curing treatments, this solution affects the building product, namely a pre- cast tile or panel, in all its thickness. Consequently, the solar reflectance, which is a surface property, is kept in the long term. Optimization is necessary because not only the "cool material" but also the other constituents of the mix influence the albedo. Furthermore, combination with the effects of mix design strategies described in the literature should be explored, in order to amplify the enhancement of solar reflectance. The focus of the preliminary investigation pre- sented in this paper, based on building simulations, is on assessing benefits and criticalities of the developed products - whose albedo varies in a range - in terms of building energy demand for heating and cooling, de- pending on geographic location, climatic conditions and urban density. 4. Materials and method 4.1. Design and characterization of cementitious "cool" mortars Innovative precast tiles (40 × 40 or 40 × 60 cm, 2 or 3 cm thick), suitable for both horizontal and vertical application on the building envelope, were produced using high-reflective cementitious mortars. The mixes included "cool" binders or "cool" additions, both based on commercial static cool materials (CM). These CMs increase the attitude of the mortars to reflect radiations in the total solar spectrum, with high efficacy in near-infrared region (NIR, with wave- lengths between 700 and 2500nm), where about 50 % of solar radiation falls. The compatibility of these inorganic CMs with the alkaline pH of cementitious matrix was verified. White and coloured traditional mortar tiles sur- faces were prepared by means of a high-fluidity ma- trix, made of white cement CEM I 52.5R, calcareous sands 0 − 2 mm, superplasticizer, shrinking-reducing and water-proofing agents. For the cool cementi- tious tiles manufacture, a cool binder, made up of se- lected cement and cool materials proportions, is pre- mixed, before the addition to the other recipe ingre- dients. After mixing in a planetary−type mixer, the cementitious mortar is cast in vertical moulds. Af- ter 24h−demoulding, the tile is kept in a conditioned room at 20◦C and 55 % RH. At fresh state, rheological behaviour of different preparations was mainly monitored through the flu- idity characterization over time (EN 7044 standard), e.g. higher than 280 mm up to 30 min. At hard- ened state, compressive (> 54 MPa at 28 days) and flexural strengths (> 9 MPa at 28 days) were charac- terized according to EN 196-1. Hydraulic shrinkage resulted in − 500 µm/m after 28 days (EN 12617 stan- dard). Preliminary promising durability tests were performed in compliance with EN 12380-8, EN 13295, EN 11164, EN 13687-1 standards. Solar reflectance was measured according to the standards ASTM E903-12 and ASTM G173-03, by means of a UV-Vis-NIR spectrophotometer with 150 mm integrating sphere. Improved optical proper- ties were assessed calculating the increase in so- lar reflectance of cool cementitious tiles compared with white/coloured control samples, ranging from 0.10 − 0.40(grey surfaces) to 0.60 − 0.81 (white sur- faces). By way of demonstration, Figure 1 shows typ- ical solar reflectance spectrum of experimental cool mortars compared with relative white and grey ref- 201 A. Gugliandolo, E. Genova, F. Baldi et al. Acta Polytechnica CTU Proceedings Reflectance Cool Surface Neighbouring buildings 0.30 − 0.80 Walls No 0.30 − 0.80 Walls Yes 0.30 − 0.80 Roof No 0.30 − 0.80 Roof Yes 0.30 − 0.80 Walls + Roof No 0.30 − 0.80 Walls + Roof Yes 0.40 Walls + Roof Yes 0.50 Walls + Roof Yes 0.72 Walls + Roof Yes 0.76 Walls + Roof Yes Table 1. List of building simulations for each location. erences w/wo CM. It is worth noting that the ce- mentitious solutions with cool materials enlarge the current possibilities of application for coloured/grey surfaces more significantly than for the white ones: the former improve the solar reflectance of the rel- evant reference in the entire wavelength range, with high benefits in NIR region; the latter show more lim- ited enhancement margins, despite the CM addition, being themselves high-reflective surfaces. Accelerated and natural aging tests are in progress. )LJXUH� ��� H[SHULPHQWDO�ZKLWH�DQG�FRORXUH KLJK�UHIOHFWLYH�PRUWDUV�FRPSDUHG� ZLWK� WKH� UHOHYDQW�PDWUL[�ZLWKRX FRRO�PDWHULDOV� ��� ��� ��� ��� ��� ��� ��� ��� ��� ���� ���� ���� ���� ���� 6R OD U�5 HI OH FW DQ FH Ȝ��QP� :KLWH�FRQWURO ([S�ZKLWH PRUWDU�� ([S�ZKLWH PRUWDU�� *UH\�FRQWURO ([S��JUH\ PRUWDU�� ([S��JUH\ PRUWDU�� Figure 1. Solar reflectance of experimental white and coloured high-reflective mortars compared with the relevant matrix without cool materials. 4.2. Methodological approach The analysis of the effects that the use of the proposed cementitious products has on the energy performance of residential buildings is based on the numerical modelling and simulation of a reference building. The use of a numerical approach allowed performing an in- depth study of the different performance indicators of interest for the subject, and applying it to different locations, and to different material properties. The performance of the material is assessed based on the following performance indicators: • increase in winter energy demand; • decrease in summer energy demand; • total energy balance, after conversion to tonnes of oil equivalent (toe); • difference in operational costs, which also takes into account the different tariffs for electricity (main en- ergy vector for summer cooling systems) and nat- ural gas (energy vector for winter heating systems in the reference building). The present study is focused on the impact of the technology at building level, hence excluding the im- pact of the proposed material on the local climate. As the shading effect generated by neighbouring build- ings can have a relevant size [9], the difference be- tween the cases with, and without, neighbouring buildings is considered. The following procedure was designed: first, the performance of the building was simulated with generic, increasing values for the surface reflectance of the building surface material, allowing to get a better understanding of the effects of surface reflectance (r) on the building’s energy performance. To this scope, different simulations were performed to consider the effects of applying the material on the roof only, on vertical opaque surfaces only, or on both. Then, the analysis focused on two main scenarios: the use of a relatively dark material (r = 0.4), and of a white ma- terial (r = 0.72); in both scenarios, the performance of the "standard" material was compared to that of the proposed one, characterized by a higher surface reflectance (r = 0.5 for the dark material, r = 0.76 for the white one). These values were provided by the material manufacturer, based on internal labora- tory tests. The full list of simulations performed for each location considered in this study is provided in Table 1. The study was performed using EnergyPlus [13] to- gether with DesignBuilder [14] as graphical user inter- face (Figure 2). The description of the building and of the inputs to the model is provided in the following section. The main parameter that varied across the different simulations is the "solar absorbance", which controls the share of solar radiation that is absorbed by the material surface. As the material under study is opaque, it was assumed that r + a = 1. 202 vol. 33/2022 re-ASR Behaviour of Recycled Concrete 4.3. Reference building Morphology, size and construction features of the ref- erence residential building were based on the 2011 Italian general census on population and housing [15]. From this data, it emerges that the most represen- tative period of the national residential stock is the decade 1971 − 1980. In the top ten Italian towns for population, the census shows that residential build- ings are generally made of at least four storeys and sixteen dwellings. To define the building model, therefore, it was assumed that: • the building has 6 floors and is equipped with a sin- gle staircase, serving three building units per floor; • the dimensions of the building are 30 m × 12 m; • each housing unit has a usable area of 90 m2, while the stairwell has an area of 30 m2; including walls, the gross floor area of each storey is 360 m2; the internal height is 2.70 m; • the roof is flat. It was assumed that the two minor façades of the building face North and South respectively; the stair- well faces East and is considered to be an "uncon- ditioned" thermal zone (Z1). For each floor, every apartment is considered as a single thermal zone (Z2, Z3, Z4). The external walls don’t have overhangs and shielding. According to the UNI 10339 standard, evalua- tions were conducted to ensure appropriate thermo- hygrometric conditions, air changes and comfortable occupational conditions. Two density values of dif- ferent occupations were assigned between the apart- ments and the stairwell. From the point of view of the model construc- tion, the building is composed of 6 overlapping blocks (30 m × 12 m). Each block is divided into four parts, representing the three floor housing units and the stairwell. The building model is referred to the building tech- niques commonly used in Italy in the 1971 − 1980 decade (UNI/TR 11552:2014). It was assumed that the building structure is a reinforced concrete frame, that is, the most widespread solution in Italian res- idential construction after the Second World War. The external walls of the reference building consist of a single layer of perforated bricks (medium perfo- ration), plastered on both sides, with a total thickness of 35 cm. The floor slabs, made of concrete beams and clay blocks, are 30 cm thick. The windows have an aluminum frame without thermal break and a 4−mm double glass with 12−mm air gap. Roller shutters (with a box emerging from the inner side of the wall) are used as shielding sys- tem. For each building unit, the window area is equal to 1/8 of the floor area, which is the minimum allowed by the Italian building regulation. The energy systems of the reference building cho- sen for this study were defined with reference to a "standard" building built in the 1970s. The systems in the building are, therefore, the following: • centralized heating system (natural gas boiler) with radiators; • centralized hot water generation system (DHW, natural gas boiler); • decentralized systems for summer air conditioning, based on a compression refrigeration cycle. The energy performance of the reference building was simulated in dynamic conditions in three Ital- ian cities, located in the Northern (Bologna), Central (Rome) and Southern (Palermo) part of the country. The energy demand of the building was simulated over the time of one year. The weather data (in the form of an .EPW file) used in the simulation was a representative average of climatic data gathered in the 2003 − 2017 period. Figure 2. Building model and neighbouring build- ings in the graphical user interface. Orientation is specified on the lower left. 5. Results and discussion The results of the parametric analysis carried out in this research, collected in Table, show the influ- ence that increasing the solar reflectance of differ- ent envelope components has on the overall build- ing energy demand for heating and cooling. From this energy perspective, only the application to the case of Palermo appears to be beneficial, as it is the only one where the benefits during the cooling sea- son outweigh the losses during the heating season. The reason behind this effect can be better under- stood looking at Figure 3, where the yearly energy demand for winter heating and summer cooling is shown for the three locations: both in Bologna and in Rome, the gas demand for heating is much higher than the electricity demand for cooling in summer, while difference shrinks in the case of Palermo. The tendency for the costs is similar to that for energy demand, as expected; as a general trend, the cost per toe is lower for electricity than for natural gas (840 EUR/toe and 1030 EUR/toe, respectively). This works against the use of high-reflectance surface ma- terials, as it increases the weight of the natural gas 203 A. Gugliandolo, E. Genova, F. Baldi et al. Acta Polytechnica CTU Proceedings Reflectance NG (winter) EL (summer) Total [-] [kWh/y] [toe/y] [€/y] [kWh/y] [toe/y] [€/y] [toe/y] [€/y] Bologna 0.40 173208 12.58 12962 5823 1.46 1223 14.04 14185 0.50 175466 12.75 13131 5270 1.32 1107 14.06 14238 0.72 180548 13.11 13512 4043 1.01 849 14.13 14361 0.76 181505 13.18 13583 3832 0.96 805 14.14 14388 Roma 0.40 114342 8.31 8557 5155 1.29 1083 9.59 9640 0.50 116872 8.49 8746 4528 1.13 951 9.62 9697 0.72 122660 8.91 9179 3182 0.80 668 9.71 9848 0.76 123747 8.99 9261 2949 0.74 619 9.73 9880 Palermo 0.40 44164 3.21 3305 6994 1.75 1469 4.96 4774 0.50 45569 3.31 3410 6102 1.53 1281 4.84 4692 0.72 48858 3.55 3656 4230 1.06 888 4.61 4545 0.76 49485 3.59 3703 3906 0.98 820 4.57 4524 Table 2. Results of simulation: primary energy demand (NG = natural gas; EL = electricity). Figure 3. Annual energy demand for winter heating and summer cooling of the reference building, with- out "cool" cementitious tiles, for the three simulated locations. demand, which increases when using these materials. Consequently, the results of the simulations carried out on the reference building highlight that the use of high-reflective products, such as those developed in this research, might be more successfully imple- mented in hot-summer climate conditions. Figure 4 shows the effect of the presence of neigh- bouring buildings in the case of Palermo. As ex- pected, differences are negligible if the "cool" prod- uct is used as roof covering, while the shades casted by neighbouring buildings reduce the benefits of the high-reflective wall, thus making it less effective. Fur- thermore, in case the "cool" product is used on the vertical envelope, the overall energy demand increases in presence of neighbouring buildings. For the refer- ence building, they appear to enhance the criticalities of high reflectance during the heating period than the benefits during the cooling season. The results for the specific application to the four "realistic" sce- narios chosen for this study are shown in Figure 5, which confirms the same tendency: increasing the surface reflectance is only convenient for the case of � Figure 4. Influence of neighbouring buildings on the effects that the "cool" cementitious tiles would have on the annual energy demand of the reference building in Palermo. Palermo, where the natural gas demand for winter heating is low (close to one fourth of the demand in Bologna). Moving from a "standard" dark cover to a "high-reflective" one, the electricity demand for cool- ing is reduced by approximately 13 %, while the total energy demand (expressed in toe) is reduced by 2.5 %. 6. Conclusions and further developments In this work, dynamic energy simulations of a build- ing model were used to develop a support tool, with the aim of optimizing the design and testing of pre- cast tiles for the building envelope based on new high- reflective (i.e. with enhanced NIR-reflective charac- teristics) cementitious constituents. The effects of different tiles on the energy demand of a reference building were evaluated in three Italian locations. The preliminary results showed that the use of high- 204 vol. 33/2022 re-ASR Behaviour of Recycled Concrete � Figure 5. Annual energy demand of the reference building and related cost savings according to location (BO = Bologna, PA = Palermo, RM = Rome), treated envelope components (W if only walls, R if only roof, W+R for both) and solar reflectance of the "cool" product. In grey the range between the albedo of the high-reflective dark material (r = 0.5) and that of the white one (r = 0.76). reflective cementitious products might be more suc- cessfully implemented in hot-summer climate condi- tions, where natural gas demand in winter season does not offset the electrical energy saving in build- ing air conditioning expenditures. In the simulations performed for this work, this corresponded to the case of Palermo. Future analysis needs to be broadened in terms of location and building features. On the one side, lo- cations representative of a wider variety of climate conditions (within Italy and in other Mediterranean countries) will be considered. On the other side, building orientation will be included as a parame- ter for simulations. Furthermore, different building types must be investigated, since the building use af- fects the morphology of the envelope (especially the ratio between roof and wall surfaces) and the energy demand for heating and cooling. Improving the building model could support the cost-benefit analysis of cooling materials in order to design more performing cementitious mortars. Not carried out in this early stage of this research, the economic assessment should take into account both the investment cost of the "cool" material to be used in the mix and the variation in the operational costs for the building use. From this perspective, the para- metric curves based on the building model will help in integrating the economic evaluation of the devel- oped products with the effects of long-term aging on their solar reflectance. Acknowledgements This paper is related to the industrial research and the experimental development performed in the framework of the Italian project COOL-IT, funded by the Italian Ministry of Economic Development (Ricerca di Sistema Elettrico Nazionale). The authors thank Eng. A. Cacciatore from Ital- cementi for the valuable contribution to the research and application activities carried out in the Innovation Labo- ratories. References [1] M. Santamouris. Using cool pavements as a mitigation strategy to fight urban heat island - A review of the actual developments. Renewable and Sustainable Energy Reviews 26:224-40, 2013. https://doi.org/10.1016/j.rser.2013.05.047. [2] A. Pisello, F. Rossi, F. Cotana. Summer and Winter Effect of Innovative Cool Roof Tiles on the Dynamic Thermal Behavior of Buildings. Energies 7(4):2343-61, 2014. https://doi.org/10.3390/en7042343. [3] I. Hernández-Pérez, G. Álvarez, J. Xamán, et al. 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