M. SALVADOR NORIEGA et al.: COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS 439–445 COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS CENENI IN U^INKOVITI TOPLOTNO IZOLATIVNI GRADBENI MATERIALI Salvador Noriega Morales 1 , Adán Valles Chávez 2 , Vianey Torres-Argüelles 1* , Mario Castillo Venegas 2 , Andrés Hernández Gómez 1 , Daniel Alaniz-Lumbreras 3 , Victor Castaño Meneses 4 1 Department of Industrial and Manufacturing Engineering, Institute of Engineering and Technology, Autonomous University of Ciudad Juárez, Av. Del Charro 450 N, Ciudad Juárez, Chih., México 2 Division of Graduate Studies and Research, Technological Institute of Ciudad Juárez, Av. Tecnológico No. 1340, Ciudad Juárez, Chih., México 3 Engineering Electric Faculty, Autonomous University of Zacatecas, Carretera a la Bufa, 98000 Zacatecas, Zac., México 4 Center for Applied Physics and Advanced Technology, Autonomous University of Ciudad Juárez, Av. Del Charro 450 N, Ciudad Juárez, Chih., México Prejem rokopisa – received: 2019-05-14; sprejem za objavo – accepted for publication: 2020-03-29 doi:10.17222/mit.2019.100 We report on the development of a formula for an adequate mix of aggregates to produce concrete blocks, which, while preserving high mechanical resistance, show good thermal insulation and reduced production costs. Three aggregates with different proportions were compared, consisting of pumice, wood shavings and basalt; the amounts of each material were 1800, 1970 and 2150 kg. A three-factor experimental design was applied to statistically determine the best factors for the response variables, namely, a high compression resistance and low thermal conductivity. The best mix obtained is composed of 250 kg of cement, 1970 kg of pumice and three amounts of sand with minor differences. Keywords: thermal conductivity, thermal resistance, concrete block, aggregates Avtorji poro~ajo o razvoju formule primerne za proizvodnjo betonskih blokov iz me{anice agregatov. Izdelani betonski bloki so poceni in imajo dobre mehanske ter termi~ne izolativne lastnosti. Med seboj so primerjali tri vrste agregatov (lehnjak, lesne ostru`ke in bazalt), v razli~nih dele`ih, vsakega po (1800, 1970 in 2150) kg. Izvedli so trofaktorski eksperimentalni dizajn, da bi lahko statisti~no ovrednotili najbolj{e faktorje za izbrani optimalni vrednosti obeh spremenljivk; ti sta: najvi{ja tla~na trdnost in najni`ja toplotna prevodnost. Na osnovi analize so ugotovili, da je najbolj{a me{anica sestavljena iz 250 kg cementa in 1970 kg lehnjaka. Pri tem pa so minimalne razlike v izbranih dele`ih peska (bazalta). Klju~ne besede: toplotna prevodnost, termi~na upornost, betonski blok, izolacijski materiali 1 INTRODUCTION It is well known that the average temperature on the planet surface is rising. Indeed, between 1901 and 2012, the global average surface temperature increased by about 0.89 °C, 1 and it is projected to rise by an additional 1.4 °C to 5.8 °C over the 21st century; this increase mainly depends on the emission trends of the greenhouse gases. 2–4 These changes in the temperature are occurring worldwide and they also affect regional weather because the heat in the atmosphere drives the climate system. 5 In the desert and high desert regions of the northern hemisphere, winters are cold and summers are hot. In such climates, the extreme temperatures, fluctuating in a range of –10–40 °C, increase the demand for thermal comfort, which brings, in turn, an increase in the energy consumption for heating, ventilation and air-conditioning systems. Therefore, there is a growing concern about the energy consumption in buildings needed for air condi- tioning and the likely adverse impacts on the environ- ment. 6,7 In 2018, the U.S. Energy Information Adminis- tration reported a production of 106,877.162 PJ, and a consumption of 7,279.88 PJ only by the building sector in this country. 8 While in Mexico, in 2017, the con- sumption of the residential, commercial and public sectors was 74.89 GJ per capita, i.e., 5,498.89 PJ, of which 944.09 PJ was the consumption only in the residential sector. 9 Nowadays, the International Energy Agency 10 is focusing on the efficient use of energy. In fact, buildings currently use up to 40 % of the primary energy con- sumption in most countries. 11–13 Also, 79 % of the world energy consumption comes from fossil fuel, which is a finite and non-renewable source. 14 Therefore, in recent years, both energy and environment have become high-priority areas for the developed and developing countries. The United Nations Industrial Development Organization promotes energy efficiency, with the aim of mitigating climate change and making industry environ- mentally sustainable. 15 Nevertheless, the economic/in- Materiali in tehnologije / Materials and technology 54 (2020) 4, 439–445 439 UDK 67.017:332.812:69:621.186.4 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(4)439(2020) *Corresponding author's e-mail: vianey.torres@uacj.mx (Vianey Torres-Argüelles) dustrial development and population boom in the last few centuries resulted in a huge increase in the energy demand with an annual incremental trend of about 2.3 %. 16 An alternative to reduce the energy consumption is the use of building envelopes. However, the use of building envelopes increases the thickness of the walls and requires a higher cost. Hence, another possible alternative to reduce this high energy demand is the use of thermal building materials. 17 Generally speaking, the main properties of a thermal material is a low thermal conductivity and high thermal resistance. The thermal conductivity ( )i so n e characteristic of concrete that can be changed when combined with other materials. It is defined as the quantity of the heat transmitted through a thickness unit in the direction normal to the surface of the unit area due to the unit temperature gradient under steady state conditions. 18 The thermal resistance of building materials is an important property while defining the total energy consumption of heating and cooling systems and achieving the optimal thermal comfort for the occu- pants. 19,20 Meanwhile, the thermal resistance is propor- tional to the thickness of a layer of the construction and inversely proportional to its conductivity: 21 R e = (1) where e is the thickness of the layer (m) and is the thermal conductivity (W m –1 K –1 ). The combined impact of the climate variables, such as the temperature, solar irradiation, wind and humidity on the energy balance of a building depends on the characteristics of the building, such as its design, orien- tation, mode of operation, maintenance, and construction materials. Concrete is the most widely used material in the world with an annual consumption of around five billion tons, 22 and the most used material in the con- struction industry is a concrete block, although its highest quality leads to higher housing prices, even more so when the heating and cooling costs are taken into account. Therefore, it is very important to develop better thermal qualities of concrete blocks, searching for adequate materials and the best mix for the improvement of the thermal insulation and acceptable strength. Cement is the binding phase in concrete and it usually constitutes about 15–25 % of the concrete weight, 22 while the aggregates can constitute between 70 % and 80 %. One of the means to increase the efficiency of energy consumption in the case of building materials and, specifically, blocks of concrete, is based on the reduction of the cement percentage 23 as well as adding other materials, which may result in improved properties, such as the thermal resistance and compres- sive resistance. 24 The common aggregates for the rein- forced concrete are plastics, clays, volcanic slag like basalt and pumice stone, rubber, cork and wood. Furthermore, one of the effects of swelling clay additions to concretes is an increase in the compressive yield stresses. 25 Basalt is the most frequent rock in the earth crust. It has a vesicular texture, with traces of the bubbles pro- duced by water vapor during the cooling of lava. Basalt is an inert, naturally occurring volcanic rock, with advantages in terms of cost 26 and because it can be aggregated without other additives. In addition, it is well known that less energy is needed for the production because of its production simplicity. Also, adding basalt to concrete increases the critical compressive strain and, consequently, the deformation capacity of geopolymer concrete can notably be improved. 27 Regarding wood shavings, they are easily acquirable, low-priced and re- newable. When wood is added to concrete, a stable, compact, resistant and durable structure is obtained; at the same time, the alveolar structure causes a good ther- mal behavior and lowers the weight. 28 It exhibits an unlimited durability, without a chemical or biological degradation, and this is why it is considered one of the best ecological materials. Pumice stone is a natural material of volcanic origin; it is light and resistant; due to its properties, it helps to reduce the weight of con- crete. 29 This paper reports about a search for a formulation of a cost-effective mixture of materials, including three aggregates, for the production of concrete blocks with a high compressive resistance and low thermal conducti- vity. This is very important because the sustainable world’s economic growth greatly depends on the use of new products in the construction industry. 2 MATERIALS AND METHODS The manufacturing technology for concrete blocks is based on the Mexican Official Norms NMX-C-404- ONNCCE-2005 and NMX-C-441-ONNCCE-2005, 30,31 which have equivalents in other countries, specifically in the US and Europe. By definition, a concrete block is a prefabricated concrete piece, prism-shaped and with one or more vertical openings; it is used in masonry systems or simple structures, which opens the possibility of having strengthening pieces in both directions of its plane. 30 According to the Mexican Official Norm NMX- C-038, 32 for manufacturing concrete blocks, the thick- ness of each block must be at least 15 mm 2 and the dimensions of the blocks are (20 × 20 × 40) cm, based on the standard NMX-C-441-ONNCCE-2005. 32 Also, the compressive resistance was based on the standard NMX-C-036, 33 and the standard values are 40 kg f cm –2 for the standard line and 60 kg f cm –2 for the structural line. In general, the properties of concrete are mainly determined by the quality of aggregates as they are the major constituents of concrete, typically occupying bet- ween 60 % and 80 % of the concrete volume. 34,35 The materials used in this study include ordinary Portland M. SALVADOR NORIEGA et al.: COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS 440 Materiali in tehnologije / Materials and technology 54 (2020) 4, 439–445 cement complying with the ASTM Type I standards, at an average percentage of 8.8 %, sand at an average per- centage of 22 %, and the studied aggregates including pumice, basalt and wood shavings at an average percen- tage of 69 %. 36 The block production process consists of vibration/ compression of the mix, together with a conveyor feed of the materials into a mold, vibration and compression to displace air and enhance cohesion. After that, the blocks are placed in a cure chamber with controlled water vapor to obtain ready-to-use high-quality blocks within 24 h. The process involves molding in a Besser machine block, model Dynapac. The measured variables were: the ther- mal conductivity (R factor), compressive resistance and total thermal resistance (R T ). The thermal conductivity was obtained through the Netzsch equipment, model 2300 Lambda, which measures the heat flow in Wm –1 K –1 . The compressive resistance was measured based on the ASTM C 31, C 39, C 617, C 1077 and C 1231 norms, 36 which are applied when testing the com- pressive resistance of concrete. Finally, the total thermal resistance is the sum of the partial thermal resistances and was obtained with Equation (2), which is based on Equation (1). RrRRRr h LLL h si se i ne T =++++= =++++ 123 12 11 (2) Here, L is the thickness of the material layer of the component (m); is the thermal conductivity of the material obtained with measurements (W m –1 K –1 ); h i is the conductance in the inner surface (W m –1 K –1 ), the value used in this study was 8.1 W m –1 K –1 based on the standard NOM-008-ENER-2001; 37 h e is the conductance is the external surface (W m –1 K –1 ) and its standard value is 13 W m –1 K –1 ; n is the number of terms of the evolving portion. Figure 1 presents a schematic representation of the partial compressive resistance. 2.1 Experimental set-up The experimental work was performed based on a mixed factorial design of experiments. Three factors were selected: cement content, sand content, and the content of aggregates. For the first factor, we assigned two levels, 250 kg and 300 kg of cement. For the second and third factor, we assigned three levels, (550, 630 and 710) kg of sand, and (1800, 1970 and 2150) kg for each aggregate. Thirty-six samples were analysed and the resistance was measured for the blocks made of different mixtures of aggregates, obtaining twelve values for each aggregate: six values for the combination of cement, sand and aggregate, and two values for each combination of aggregate and sand. 2.2 Statistical analysis A statistical analysis was performed using a Student’s t-test and ANOVA. The former is used to compare the results of the mean compressive resistance for each aggregate, considering the levels of the sand and cement contents. Additionally, in order to determine if there is a significant difference in the average compressive resistance between the aggregates, a statistical analysis was applied involving an analysis of variance (ANOVA), using the SPSS statistical software, version 17. 38 3 RESULTS AND DISCUSSION This section presents the findings of the study. Table 1 shows the results obtained with the study and we can see in the second column that the thermal conductivity is lower than the standard value for concrete, with values of 0.265 W m –2 K –1 for pumice, 0.274 W m –2 K –1 for basalt and 0.229 W m –2 K –1 for wood shavings. When com- pared to the standard value obtained for the concrete without an aggregate, there are differences of (37, 35 and 45) %, respectively, given that according to F. M. Díez Ramírez et al. 21 the coefficient of thermal conductivity of dry concrete fluctuates in a range of 0.09–2.30 W m –2 K –1 , and depends on the type of aggregate, its composition and air content. In the case of this study, the standard M. SALVADOR NORIEGA et al.: COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS Materiali in tehnologije / Materials and technology 54 (2020) 4, 439–445 441 Table 1: Thermal conductivity, thermal resistance, total thermal resistance and mean compressive resistance for each aggregate Material Thermal conductivity (W m –2 K –1 ) Thermal resistance (m 2 KW –1 ) Total thermal resistance (m 2 KW –1 ) Mean compressive resistance (kg f cm –2 ) Pumice 0.265 0.755 1.036 57.50 Basalt 0.274 0.730 1.011 56.00 Wood shavings 0.229 0.875 1.156 46.00 Concrete 0.420 0.390 0.400 40.00 Figure 1: Schematic representation of the partial compressive resist- ance value of the concrete was 0.42 W m –2 K –1 . These results are better than the ones reported by L. Gündüz, 39 who says that by adding pumice to a mix of cement and aggregate, a thermal conductivity of 0.34 W m –2 K –1 was obtained. In addition, our results are better than the ones from S. A. Marcott et al. 4 who reported a mean thermal conductivity of 1.25 W m –2 K –1 . According to the results, in the case of the thermal resistance, the difference is significant and represents an increase of 93.6 % for pumice, 87.1 % for basalt and 124 % for wood shavings, in relation to the concrete standard value. At the same time, the mean compressive resist- ance measured for the blocks with added materials shows an important increase of (43.75, 40 and 15) % for pumice, basalt and wood shavings, respectively. The mix with pumice exhibits the best properties among the studied mixes. The results described are presented in Figure 2, which also gives the differences in the be- haviour of the variables measured for the blocks with the concrete mix, sand and aggregates, in relation to the standard values for concrete blocks. As mentioned above, the average percentages of the contents of the materials used are 8.8 % for cement, 22 % for sand and 69 % for the aggregates for each formula. Table 2 shows the calculated compressive resistance. The minimum compressive strength was 19 kg f cm –2 , obtained by mixing 250 kg of cement, 550 kg of sand and 2150 kg of wood shavings. The maxi- M. SALVADOR NORIEGA et al.: COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS 442 Materiali in tehnologije / Materials and technology 54 (2020) 4, 439–445 Figure 2: Comparison of the results for pumice, basalt, wood shavings (ws) and concrete, for four measured variables: a) thermal conductivity, b) thermal resistance, c) total thermal resistance, and d) mean compressive resistance Table 2: Descriptive statistics for block compressive resistance: mean, standard deviation, minimum and maximum values Cement (kg) Sand (kg) Pumice (kg) Basalt (kg) Wood shavings (kg) 1800 1970 2150 1800 1970 2150 1800 1970 2150 250 550 37 38 25 39 32 22 28 29 19 33 39 28 35 32 29 25 24 22 630 46 39 44 42 35 46 33 23 38 42 44 47 46 47 43 38 31 38 710 36 50 34 38 53 37 25 47 22 37 52 33 39 56 36 21 40 20 300 550 57 51 51 55 55 49 52 45 39 56 56 46 56 53 47 50 47 34 630 54 58 50 56 57 52 41 45 45 51 57 49 52 55 46 43 47 46 710 54 51 42 55 54 39 46 45 31 57 54 46 59 57 41 51 42 39 Mean 46.67 49.08 41.25 47.67 48.83 40.58 37.75 38.75 32.75 Sd 9.23 7.28 8.94 8.70 9.93 8.65 11.11 9.31 9.75 Min 33 38 25 35 32 22 21 23 19 Max 57 58 51 59 57 52 52 47 46 mum compressive strength was 59 kg f cm –2 , obtained by mixing 300 kg cement, 710 kg of sand and 1800 kg of basalt. According to the average and standard deviation values of 49.08 kg f cm –2 and 7.28 kg f cm –2 , respect- ively, the best mixture was obtained by adding 1970 kg of pumice. The lower mean value of compressive strength was obtained by adding wood shavings, the value being 32.75. However, with this material, we obtained a lower thermal conductivity and higher ther- mal resistance. Our results are similar to those reported by D. K. Panesar and B. Shindman 40 who obtained a compressive resistance of 79.5 kg f cm –2 , after 28 days of drying, of a mixture of 10 % CSandBlend + 10 % CStoneBlend and 80 % Portland cement. It is noteworthy that our blends contain only9%ofcement and 22 % of sand. In our study, the aggregate predominates in the mixture with 69 %. F. Pelisser et al. 23 combined cement and rubber, adding 40 % of this aggregate; the average thermal conductivity was 0.737 and the thermal resistance was 0.306. In the case of the thermal conductivity, our results are, on average, by 65 % lower, and in the case of the thermal resistance, our results are better by more than 100 %. Figure 3 shows the behaviour of the compressive strength obtained with mixtures of different aggregates. We can see that, in all the cases, the compressive strength of different mixtures containing wood chips is lower than that of the mixtures containing pumice and basalt. Also, the pumice and basalt mixes show similar behaviours at all levels, including the mixes of 250 kg of cement, 630 kg and 710 kg of sand, and the mix of 300 kg of cement with 710 kg of sand. 4 STATISTICAL ANALYSIS Table 3 reports the results of ANOVA. It indicates statistically significant differences between the results for the three aggregates. For this purpose, Student’s t-test was used. Table 4 shows significant differences between the three levels of each aggregate. The statistically sig- nificant differences were found in the case of the highest level, i.e., 2150 kg of both pumice and basalt, compared to the other two levels, 1800 and 1970 kg, except for the comparison between 1970 and 2150 kg of wood shav- ings. At the same time, Table 5 shows Student’s t-test for the three levels of the aggregates. In this case, significant differences observed between the mixtures containing wood chips and those containing pumice and basalt, have a .000 P-value for the three levels of the aggregate material studied. On the other hand, the results obtained for the mixtures containing pumice and basalt show no statistically significant differences. The materials proposed in this study performed better than those reported by O. Ünal et al. 41 who used dio- M. SALVADOR NORIEGA et al.: COST-EFFECTIVE THERMAL-INSULATING BUILDING MATERIALS Materiali in tehnologije / Materials and technology 54 (2020) 4, 439–445 443 Figure 3: Comparison of the values for the total compressive resistance for three aggregates with three different levels of sand and two different levels of cement: a) 550, b) 630, c) 710 kg of sand for 250 kg of cement; d) 550, e) 630, f) 710 kg of sand for 300 kg of cement Table 3: ANOVA of the compressive resistance ANOVA aggregates Sum of squares df Mean square F Sig. Between groups 3172.241 8 396.530 4.621 .000 Within groups 8495.833 99 85.816 Total 11668.074 107 tomite as the aggregate and cement contents of 250 kg and 300 kg, obtaining average compressive strengths of 42 kg f cm –2 and 51 kg f cm –2 , respectively. In the same study, these authors reported a thermal conductivity of 0.314 W m –1 K –1 . This is lower by (15, 12 and 27) % than for pumice, basalt and wood shavings, respectively. 5 CONCLUSIONS This study verifies the adequateness of the material aggregation for the production of high-strength light- weight concrete blocks. Because of various concrete mixes and the use of standard techniques, it was possible to obtain a high-quality lightweight concrete mix, suit- able for application in reinforced-concrete structures. According to the results, the recommended mix determined during this project includes 1970 kg of pumice, with minimum and maximum values of 38 kg f cm –2 and 58 kg f cm –2 , respectively, a mean value of 49.08 and a standard deviation of 7.28. It is re- commended to continue the search for other natural materials and the best formulations to improve concrete blocks and production processes. 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(2-tailed) 95 % confidence interval of the difference Mean Std. deviation Std. error mean Lower Upper Pair 1 P_1800 – P_1970 –2.42 7.05 2.04 –6.90 2.06 –1.18 11 .260 Pair 2 P_1800 – P_2150 5.42 5.11 1.47 2.17 8.66 3.67 11 .004 Pair 3 P_1970 – P_2150 7.83 7.25 2.09 3.23 12.44 3.74 11 .003 Pair 4 B_1800 – B_1970 –1.17 7.57 2.18 –5.97 3.64 –0.53 11 .604 Pair 5 B_1800 – B_2150 7.08 6.79 1.96 2.77 11.40 3.62 11 .004 Pair 6 B_1970 – B_2150 8.25 8.23 2.37 3.02 13.48 3.47 11 .005 Pair 7 WS_1800 – WS_1970 –1.00 10.25 2.96 –7.51 5.51 –0.34 11 .742 Pair 8 WS_1800 – WS_2150 5.00 7.65 2.21 0.14 9.86 2.26 11 .045 Pair 9 WS_1970 – WS_2150 6.00 11.21 3.24 –1.12 13.12 1.85 11 .091 Table 5: Student t-test for the compressive resistance between three agreggates Paired differences td f Sig. 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