Acta Polytechnica https://doi.org/10.14311/AP.2024.64.0322 Acta Polytechnica 64(4):322–335, 2024 © 2024 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague HYDROTHERMAL AND MECHANICAL PERFORMANCE OF MORTARS CONTAINING WASTE BRICK POWDER Abderrahim Brixia,∗, Moulay Smaïne Ghembazab, Abderrahim Boudennec a Tlemcen University, Faculty of Technology, Department of Civil Engineering, Eau et Ouvrages dans Leur Environnement (EOLE) Laboratory, PB 230, 13000 Tlemcen, Algeria b University of Sidi-Bel-Abbes, Civil Engineering and Environment Laboratory, PB 89, 22000 Sidi-Bel-Abbes, Algeria c University Paris Est Creteil, CERTES, F-94010 Creteil, France ∗ corresponding author: abdoubx88@gmail.com Abstract. It is widely recognised that green building, environmental sustainability, and technical performance have recently become requirements in the field of civil engineering. For this reason, the new trend in research is to focus on the recycling and recovery of materials, even if some of them such as industrial waste are still underexplored. In this perspective, the main objective of this work is to study the influence of brick powder on the thermophysical and mechanical properties of mortars containing this type of waste, in different environments, and within the temperature range between ambient temperature and 50 °C. To this end, a number of mortar mixtures, in which cement was replaced by brick powder in different proportions, were studied and characterised according to technical standards in order to define the optimal substitution percentage. Three batches of samples were examined at different ages, i.e., 3, 7, 28, and 90 days. The samples of the first batch were kept in water at a temperature of 20 ± 2 °C with a relative humidity RH = 100 %, while those of the second batch were immersed and stored in water at 50 °C in order to simulate a hot and humid climate. As for the samples of the third batch, they were kept in a dry oven at 50 °C in order to investigate the effect of the hot and dry climate. The results obtained revealed that the partial replacement of cement by brick powder makes it possible to improve the thermal insulation characteristics but reduce the mechanical strength of the mortar. In addition, it was shown that in a hot and dry environment, the mechanical characteristics of the different mortars decrease as the rate of weight substitution of cement by brick powder rises. However, in a hot and humid environment, a reverse trend is observed. The findings also suggested that the optimal recommended rate of substitution of cement by brick powder is 20 %. Keywords: Cement mortar, brick powder, mechanical strength, conductivity, temperature. 1. Introduction It is widely known that mortar is a workable paste that hardens in order to bind and join building blocks such as stones, bricks, and concrete masonry units and to fill and seal the irregular interstices between them. Mortar generally includes cement, sand, water, and some admixtures. It should also be noted that the manufacture of cement, which is the main component of mortars, requires considerable amounts of energy and engenders the emission of significant quantities of greenhouse gases into the atmosphere. These gases have been shown to be primarily responsible for global warming [1]. It has been revealed that a quantity of CO2, varying between 0.9 and 1 tonnes is produced for 1 tonne of clinker, which corresponds to approximately 900 g of CO2 that is released into the environment per day, and per inhabitant. In addition, the current high demand for cement contributes to the progressive and rapid depletion of natural resources, and therefore leads to the production of different types of waste prod- ucts, not to mention the accompanying costs. There- fore, it is crucial to integrate the approach of sustain- able development in the manufacturing process of es- sential building materials since this will certainly help to ensure a balance between the constraints of environ- mental protection and economic and social considera- tions [2]. It is worth indicating that the production of cements containing pozzolanic additives is increasingly growing throughout the world. Moreover, in order to meet the rising demands for construction materials, re- searchers have recently become increasingly interested in recycling and recovering various types of waste ma- terials that are widely abundant in nature or even those resulting from the demolition of old buildings. It was found that incorporating industrial waste in the form of additions of very fine crumbs during the manufacture of cement could constitute a com- plementary and effective solution that can help to significantly reduce the amount of CO2 released to the atmosphere. Today, this alternative caught only limited attention from professionals, who are faced with regulatory restrictions, and with the absence of 322 https://doi.org/10.14311/AP.2024.64.0322 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . in-depth studies on this subject [3]. It is worth indi- cating that since the 1970s, a large number of research studies have been conducted on the possibility of us- ing cementitious additives as a partial replacement of Portland cement. These additives can come either from natural sources or may be a by-product from various industries [4]. It is interesting to mention that additives have a binding activity that depends on the fineness and mineralogical composition as well as the quantity used [5–7]. Hence, over the past few years, several re- searchers [8–16] have become increasingly interested in recycling and recovering various industrial waste materials, such as marble, brick, glass, plastic, ash, slag from blast furnaces, and others, in order to use them in the manufacturing process of building materi- als and to respond to ecological concerns as well. One of the solutions proposed consists of recovering waste in order to use it in the field of public construction (in- frastructure, backfilling materials, subgrade, and sur- facing), or to develop new eco-construction materials and use them as aggregates in concrete or as additives, such as fillers, binders, or stabilisers, in cementitious matrix of mortars. Unfortunately, it turned out that some of these applications present some limitations that are mainly attributed to the lack of clear regula- tions and sufficient technical data on their rheological, thermal, mechanical performance, durability, etc. It has been revealed that the development of new mortars based on brick powder could present an in- teresting alternative. Several research works have recently been carried out in order to evaluate the po- tential use of crushed bricks as coarse and fine aggre- gates in the production of building materials [17–20]. Thus, a number of feasibility studies [8, 21–24] have fo- cused on the manufacture of composite cement based on finely ground brick waste. The above-mentioned research [8, 21–24] comprehensively examines the be- haviour of brick crumb mortars under normal con- ditions, but does not address the effect of crucial variables, such as temperature and environment, par- ticularly in hot and dry or hot and humid climates. Although these studies provide valuable insights into the properties and performance of mortars, the lack of consideration of these environmental factors may limit the applicability of their findings. Understanding how these mortars react in different climatic condi- tions is essential to assess their durability and adapt- ability in regions subject to extreme temperatures or high humidity. Therefore, future research that integrates these aspects could provide a more holis- tic view of the behaviour of brick crumb mortars and their suitability in different environmental con- texts. It should be noted that most researchers [25– 27] have focused on the recycling and valorisation of crushed bricks in the form of coarse aggregates to be used in concrete. The major problem of not us- ing waste brick powder (WBP) from finely crushed bricks is probably since the preparation of mortar requires large amounts of water. In this regard, sev- eral authors asserted that the compressive strength of cement-brick powder mortars at 28 days [28, 29] decreases due to the absorption of a large amount of water by the brick aggregates, which is not the case for natural aggregates. Therefore, in order to overcome this compressive strength reduction, some researchers found out that the incorporation of micro- silica into mortar as a replacement for cement should significantly improve the mechanical properties of the material, because micro-silica acts as a filler mate- rial, and therefore reduces porosity and helps to make the cementitious mixture denser [30–33]. Other stud- ies have focused on the water retention capacity of hydrated lime mortars modified by brick dust and the effect of the latter on the pozzolanic behaviour of mortars [34]. Furthermore, it should be noted that the quality of the prepared mortars or concrete depends mainly on their composition, but also on the drying and hardening conditions during the first hours, or even during the first days, of their life [35]. It has indeed been shown that the mechanical per- formance, physical and thermal properties of mortar during the hardening period, in a hot and dry or hot and humid climate, can be different. However, some researchers found that, under harsh conditions, certain properties can sometimes be improved by in- corporating mineral additives [36, 37]. It has also been reported that under similar climatic conditions, concreting without effective curing can lead to sig- nificant disadvantageous effects on the rheological properties and long-term durability of the mortar due to the acceleration of the hydration process. It has also been shown that cement hydration on the sur- face of an exposed element can cease if the relative humidity (RH) of the surrounding air is sufficiently low [38]. For this, Powers et al. [39] suggested that cement hydration practically ceases when the relative water vapour pressure in the capillaries is less than 0.8. Spears [40] asserted that continuous hardening, be- low 80 % relative humidity, does not contribute to increasing the hydration of the cement that is nec- essary in order to further improve the quality of the concrete. It is worth emphasising that, in practice, construction site concrete is daily subjected to hu- midity cycles corresponding to seasonal variations and active hardening may stop before the cement is fully hydrated [38]. Finally, limited studies have been carried out on the effect of temperature in different environments on the mechanical behaviour of mor- tars based on brick powder as a partial substitute for cement by weight, however, additional research is needed to adequately study the thermophysical and thermomechanical properties of mortars containing a certain percentage of brick powder as a replacement for cement by weight. Additionally, brick crumb mortars offer a combi- nation of properties that make them suitable for a range of applications in wet and dry, ambient and 323 A. Brixi, M. S. Ghembaza, A. Boudenne Acta Polytechnica elevated temperature environments, whilst providing additional ecological benefits including: (1.) Resistance to frequent humidity (bathrooms, kitchens or outdoor areas), (2.) dimensional stability (mortars do not contract or expand excessively in response to variations in humidity or temperature), (3.) adhesion of mortars based on brick crumb to covering surfaces where humidity is a constant chal- lenge [41], (4.) resistance to corrosion (coastal areas or industrial installations) (5.) thermal insulation in a variable high temperature environment, (6.) increased durability: Due to their resistance to humidity and corrosion, brick crumb mortars tend to be more durable in environments where these fac- tors are predominant, reducing maintenance needs and long-term replacement. The main objective of this research is to investi- gate the possibility of valorising brick waste for the manufacture of cementitious eco-composites which, in this case, can be repair and facade mortars, and used to cover and protect, such as when applied as screeds, renders and plasters. On account of the above, a number of mortar formulations in which cement was replaced with powdered bricks at various rates, namely 10 %, 20 %, and 30 %, was prepared. In addition, the effect of mortar curing conditions for hot and humid or hot and dry natural climate was analysed. An experimental study was also carried out on the ther- mophysical and mechanical properties of the different mortar mixtures in the hardened state under different conditions of temperature and relative humidity. It is useful to remember that one of the major objectives of the study is to assess the satisfactory improvement in the mechanical and thermal behaviour of mortar following the incorporation of brick powder in order to develop a construction material that is, at the same time, economical, efficient, and ecological. 2. Experimental study 2.1. Materials used 2.1.1. Cement The cement CEM I 52.5N was employed to create the mortars in this study. This cement is from the Lafarge Cement Production Plant at La Malle in France and is compliant with Standard EN 197-1 [42]. It should be noted that this cement contains practically only clinker (95 %), with about 5 % gypsum, which is used to regulate the setting. Its Blaine surface is estimated at 3 480 cm2 g−1, and its absolute density is 3.15 g cm−3. The initial and final setting times of this cement are 200 and 260 minutes, respectively. Moreover, the preliminary chemical analysis showed that this clinker complies with Standard NF P 15-301 [43] and contains Constituents Cement [%] SiO2 19.61 Al2O3 4.71 Fe2O3 3.15 CaO 63.45 MgO 3.4 SO3 2.24 K2O 0.73 Na2O 0.21 Cl 0.03 Table 1. Chemical composition of cement. Constituents Value [%] Rwp Alite M3 5.632 Alite M1 35.19 Alite Sum 23.17 FractionM1 39.71 Alite CS 175.95 mn Belite_beta 12.54 Alum_cubic 2.71 Alum_ortho 5.07 Alum_Sum 7.78 Ferrite 8.73 Lime 0.08 Portlandite 1.37 fCaO_XRD 1.11 Periclase 2.49 Quartz 0.18 Arcanite 1.14 Calcite 4.16 SO3_XRD 2.25 Gypsum 0.14 Hemi-hydrat 2.96 CO2_XRD 1.83 Aphthitalite 0.07 Table 2. Mineralogical composition of cement. less than 5 % of the mixture (MgO + CaO (free)). Its potential mineralogical composition can be calculated according to the empirical formula of Bogue [44]. The chemical and mineralogical compositions are shown in Tables 1 and 2, respectively. 2.1.2. Sand The sand used in the mortar mixtures is a quartz quarry sand with a maximum grain size range of 0–2 mm. This sand is characterised by a fineness modulus of 2.40 with a specific and apparent density of 2.72 g cm−3 and 1.71 g cm−3, respectively. It is dried and sifted in a laboratory providing absolute consistency. 2.1.3. Brick powder Brick Waste (BW) for this study was obtained from a brick manufacturing plant located in Henaya munic- ipality (department of Tlemcen, Ouesthern Algeria). 324 vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . (a). Raw materials. (b). Grinding at the factory. (c). Materials after 1st grinding. (d). Grinder in the laboratory. (e). Materials after 2nd grinding. (f). Blaine’s surface control. Figure 1. Preparation process of waste brick powder. Brick powder is obtained by crushing and grinding waste clay bricks. The grinding is usually carried out using a conventional ball mill. In this study, the com- position and quantity of the crushed materials as well as the grinding times are kept constant. Afterwards, after a given settling time, the resulting material is sifted through sieves with diameters less than 63 µm, as illustrated in Figure 1. It was found that the ab- solute density of the material obtained was between 2.60 and 2.70 g cm−3, with a specific surface equal to 2 700 cm2 g−1. As for the chemical composition of the brick powder used in this study, it is shown in Table 3. Environmental pollution can be reduced by using recycled powder as a cementing material. 2.2. Preparation of mortar specimens The experimental study was designed to investigate the influence of incorporating the brick powder into the mixture on the thermophysical and mechanical properties of different mortar mixtures, in the fresh and hardened state, and under different temperature and relative humidity conditions. It should equally be noted that the seasonal climatic conditions can be con- sidered as simulating the Saharan and coastal regions. Regarding the composition of the mortar mixtures, they were all prepared with a ratio Water Cement+Addition constant and equal to 0.58. This was done for the purpose of highlighting the effect of the nature of the additions used on the properties of the prepared mortars and also to obtain better mechanical strength with constant workability, according to the standard EN 196-1 [45]. It should also be noted that three brick powder contents, i.e. 10 %, 20 % and 30 %, were selected to replace the dry cement in the mixes. In Constituents Brick powder [%] SiO2 43.84 Al2O3 13.06 Fe2O3 5.09 CaO 10.99 MgO 3.45 SO3 0.43 K2O 0.96 Na2O 0.45 Cl 0.02 P2O5 0.19 TiO2 0.69 AM 2.57 SM 2.42 LSF 7.77 Table 3. Chemical composition of brick powder. addition, before the preparation of the various speci- mens, the cement (C) and the brick powder (BP) were mixed well in order to obtain a completely homoge- neous mixture (C-BP). It is important to note that the base mortar includes CEM I cement only (rated Mi-0 %). It contains three parts of sand (1 350 g), one part of cement (450 g), and a half-part of water (225 g), in accordance with Standard EN 196-1 [45]. Note that all mixes contain the same proportions of sand and water as the base mortar (Mi-0 %). These mixtures are denoted Mi- BPX %, with X % representing the mass percentage of substitution of cement by brick powder. An electronic scale with an accuracy of 0.1 g was used to weigh 325 A. Brixi, M. S. Ghembaza, A. Boudenne Acta Polytechnica N° Cement [g] Brick powder (BP) [g] Quarry sand [g] Water [g] M0-0 450 0 1 350 261 M1PB10 405 45 1 350 261 M2PB20 360 90 1 350 261 M3PB30 315 135 1 350 261 Table 4. Proportions of mixtures for one batch. 50 °C Under water M0-0 M1PB10 M2PB10 M3PB10 50 °C Dry M1PB10 M0-0 M3PB10 20 °C Under water M0-0 M3PB10 M1PB10 M2PB10 M2PB 0 Figure 2. Storage of specimens in different media. the ingredients during the preparation of the different mortar mixtures. Table 4 shows the proportions of the materials used in the composition of the mortars examined in this study. Several mortar specimens, measuring (4 × 4 × 16) cm3, containing different concentrations of brick powder, were prepared. Four cement mortar mixtures were prepared with different BP amounts. The resulting mixutres differed in the amount of BP addition, which was 0 %, 10 %, 20 % and 30 %, replac- ing the weight of the cement. They were removed from their moulds 24 hours after their manufacture. In addition, three media simulating different hard- ening states, which in the present case are critical climatic conditions that are typical of Saharan and coastal regions, were tested: • The first state corresponds to a hot and dry envi- ronment. In this case, the samples were introduced into a temperature-controlled oven set at 50 °C, and dry. • For the second state, the samples were directly immersed in tanks filled with tap water and kept in an oven in a warm environment at a temperature T = (50 ± 2) °C. • For the third and last state, the samples were di- rectly immersed in tanks filled with tap water at temperature T = (20 ± 2) °C, as shown in Fig- ure 2. It is worth emphasising that all samples were kept as expected until the test deadlines, in accordance with Standard BS EN 12390-2 [46]. In addition, the age of the specimens was calculated from time zero, which corresponds to the first moment from the start of mixing. Note also that, in this study, mortar M0- 0% is taken as the reference mixture when different properties were measured. Figure 3. Ultrasonic pulse velocity test method for mortar. Among the wide range of tests that can be carried out on the various mortars prepared, the choice was made according to the most essential characteristics, namely the compressive strength test, bending tensile test, modulus deformation by ultrasonic testing, and tests to measure the thermophysical properties, such as the thermal conductivity λ, the thermal diffusiv- ity α, and the specific heat CP. It is worth specifying that the samples used for the thermophysical measure- ments are those that were immersed and kept in water tanks at T = 20 °C during the hardening period. 2.3. Measurement of mechanical properties 2.3.1. Deformation modulus This modulus can be determined by means of non- destructive ultrasonic testing which consists of sending ultrasonic waves through the mortar via a transmitting transducer. For this, an ultrasonic pulse is sent across the mortar specimen to be tested, and the time taken by the pulse through the sample is measured in order to calculate the speed of pulse propagation inside the material, at 28 and 90 days as per to ASTM C642 [47] as depicted in Figure 3. Afterwards, the speed thus calculated allows evaluating the elastic modulus given that the Poisson’s ratio of the tested material is already known. The measuring device used in this work is an ul- trasound generator that is connected to two wave 326 vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . transducers, i.e. a transmitter and a receiver, which are coupled to each other. They are aligned on two parallel surfaces of the sample to be tested. The acous- tic contact between the transducer and the sample is ensured by means of petroleum jelly; the transducer is then pressed against the surface of the mortar spec- imen. Afterwards, a longitudinal ultrasonic pulse of approximately 20 kHz is emitted by the transmitter and is then received by the receiver that is placed on the opposite surface. This device is intended to measure the transit time of the pulse. For an elas- tic material, the relationship between the speed of the pulse, the elastic constants, and the density of the mortar tested is used to determine the dynamic modulus of elasticity that is given below: E = ( L t )2 ρ (1 + ν) (1 − 2ν) 1 − ν , (1) where E is the elastic modulus, L is the distance separating the two transducers, t is the pulse time, ρ is the actual density of the eco-mortar used, ν is the Poisson’s ratio. We set ν equal to 0.3, the usual value for mortars. 2.3.2. Compression and tensile strength The compressive strength test and bending tensile tests of all the mixtures were carried out according to the Standard EN 197-1 [42]. Three specimens were examined in each test. It should be noted that the half-samples resulting from the bending tensile test were crushed by compression (six specimens per formulation and per term) using an automated mortar press, at a speed of 2.4 kN s−1, as shown in Figure 4. 2.3.3. Measuring the thermophysical properties of mortar The thermophysical properties of the different brick powder-based mortars were determined using the Hot Disk (HD) transient method using the international standard ISO-22007-2 [48]. The HD analyser uses a sensor element in the shape of a double hairspring, as shown in Figure 5. Its working principle is based on the transient plane source (TPS) method. This sensor acts both as a heat source for increasing the tempera- ture of the sample and as a resistance thermometer for recording the time-dependent temperature increase. In addition, the spiral is supported by an unfilled poly- imide thermoplastic material (Kapton insulator) for protection and electrical insulation. Figure 5 clearly shows that the sensor is sandwiched between two halves of the sample. The hot disk (HD) technique is employed for si- multaneously evaluating the thermal conductivity λ [W m−1 K−1] and the thermal diffusivity α [m2 s−1] in a rapid, accurate, and non-destructive way [2]. Figure 4. Tensile and compression testing machine. Figure 5. Apparatus for measuring the thermophysi- cal properties of mortar. Moreover, since the thermophysical properties of porous building materials can depend on several fac- tors other than temperature and moisture content, it is highly recommended to perform the measurements in a well-controlled environment. It is worth noting that all samples investigated in this study were conditioned to the temperature of 20 °C and 50 % of relative humidity in a climatic chamber, for three weeks, until weight stabilisation. This procedure allows monitoring the humidity of sam- ples because moisture can have a significant effect on the measurement of mortar thermophysical proper- ties. To do this, several measurements were performed at room temperature for each surface of the sample, while changing the probe position and the sample side. In the end, the mean values of the thermal conductiv- ity, diffusivity, specific heat measurements as well as their uncertainties for mortar and brick powder-based mortar composites were calculated. Furthermore, for the purpose of studying the effect of moisture on the thermophysical properties, the same mortar samples were then kept in an oven at 50 °C for a period of one week in order to achieve dry state before measuring the thermophysical properties. Finally, to perform measurements in the moist state, the samples were placed once again in a climatic room at 100 % relative humidity for a period of one month before performing the measurements. 327 A. Brixi, M. S. Ghembaza, A. Boudenne Acta Polytechnica Figure 6. Thermal conductivity of brick powder based mortar composites at different moisture con- tents. 3. Results and discussion 3.1. Hydrothermal properties of brick powder-based mortar composites With the aim of examining the effect of moisture on the thermophysical properties of the mortar speci- mens, while the brick powder content varied, several measurements were performed in a climatic chamber with 50 % RH. The moist state and dry state are shown in Figure 5. Figures 6 and 7 show both the thermal conductivity and the thermal diffusivity behaviour and their un- certainties for the three investigated states. One can easily see that when the brick powder content in the mixture increases, both the thermal conductivity and the thermal diffusivity decrease. This may certainly be attributed to the difference between the thermal conductivity of a brick (0.85 W m−1 K−1) and that of mortar (1.78 W m−1 K−1). In fact, at 50 % RH, we notice that the thermal conductivity decreased by 12 % and the thermal diffusivity decreased by 17 % for the formulation M3PB30 compared to the neat mortar. Furthermore, both Figures 6 and 7 explicitly show the difference in thermophysical behaviour when the relative humidity conditions of the samples are changed (dry and moist states). This difference sug- gests that the impact on the thermophysical properties of materials is higher in the moist state than in the dry state, due to the water content. The increase in the thermal conductivity and diffusivity remains constant as the brick powder content changes. This increase was estimated at 9 % for the thermal conductivity and 11 % for the thermal diffusivity. 3.2. Thermomechanical behaviour of brick powder based mortars The obtained results of compressive and flexural strength tests for the different mixtures were discussed and compared with those of the reference mortar, according to the different dosages and curing times Figure 7. Thermal diffusivity of brick powder based mortar composites at different moisture contents. Figure 8. Evolution of the compressive strength of the different mortars in an ambient environment. (3, 7, 28 and 90 days), and under different climatic. With regard to the formulation of the mortars studied in this work, mixtures were prepared with identical consistencies, i.e. spreading on the vibrating table of about (18 ± 2) cm. This was done to produce mortars that are easy to apply. Figures 8, 9, 10, and 11 show the evolution of the compressive and tensile strength at ambient tempera- ture (20 °C) and at high temperature (50 °C) in hot and humid environments, respectively, for the refer- ence mortar as well as for the different brick powder- based mortars, and for different ages. According to Figure 8, the compressive strength of the reference mortar increased from 36 to 65 MPa for the age in the range of 3 to 90 days. The same observations are made for the other brick powder-based mortars (M, % PB). It was also observed that the compressive strength of the mortar containing 10 % brick powder is higher than that of mortars containing 20 % or 30 % BP, this is true for all ages. It can clearly be seen that the strength increase as a function of cure time follows a nonlinear trend; 328 vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . 0 20 40 60 80 100 Age (days) 10 20 30 40 50 60 70 15 25 35 45 55 65 C o m p re ss io n S tr en g th , C S ( M P a) Water / Binder = 0.58 ; T = 20 ° C Reference mortar Mortar/10 % brick composite Mortar/20% brick composite Mortar/30% brick composite Water / Binder = 0.58 ; T = 50 ° C  -  w e t Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite C S (MPa) 3 days 7 days 28 days 90 days T = 50 °C wet, S = 33.8 t 0.13 T = 50 °C wet, S = 34.25 t 0.10 T = 50 °C wet, S = 32.3 t 0.08 T = 50 °C wet, S = 23.6 t 0.12 R2 = 0.70 R2 = 0.50 R2 = 0.36 R2 = 0.38 (a). Hot and humid environment. 0 20 40 60 80 100 Age (days) 0 20 40 60 10 30 50 70 C o m p re ss io n S tr en g th , C S ( M P a) Eau/Liant = 0.58 , T = 20 ° C Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite Water / Binder = 0.58 ; T = 50 °C (dry) Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite C S (MPa) 3 days 7 days 28 days 90 days T = 50 °C dry, S = 20.67 t 0.08 T = 50 °C dry, S = 21.1 t 0.03 T = 50 °C dry, S = 20.6 t 0.001 T = 50 °C dry, S = 17.83 t 0.02 (b). Hot and dry environment. Figure 9. Evolution of the compressive strength of different mortars in a hot and humid environment and a hot and dry environment. it can be expressed using a power function of the form: CS = AtB, where S is the strength, t the time (days), and A and B are constants. This is true regardless of the percentage of brick powder (BP) used. The strength values of mortar samples including 10 % of BP vary between 35 and 54.90 MPa, which represents an improvement rate of 85 % compared to the reference mortar, while those of samples containing 20 % and 30 % BP, vary between 30.14 and 52 MPa, and between 23 and 41.35 MPa, respectively. Which translates to an improvement rate of 80 % for the case of 20 % brick powder and 64 % for the case of 30 % brick powder. This can be attributed to the high- water absorption capacity of the brick waste (BW) aggregate (16 %). This high hygric characteristic is caused by the affinity of BW to water as well as its porosity (50 %), as indicated by Boukour et al. [49]. These strength values are considered as acceptable for mortars intended for filling or repair. It should be noted that the brick waste used in the mortars is recovered from bricks fired at 900 °C, this causes a slightly lower resistance than that of the reference mortar. This phenomenon of decrease in resistance was also explained by Dos Reis et al. [50], where they present results showing that CS of a brick depends on the firing temperature. Indeed, in the work of Dos Reis et al. [50], the CS values of brick samples fired at 800 °C reached higher CS values than those of fired at 1 000 °C, hence the reduction in resistance brick-based mortars when the rate of replacement by bricks exceeds 10 % (low pozzolanic effect and the specific surface area of brick crumb is lower than that of cement). Regarding the case with the temperature set at 50 °C with humid climate (100 % RH), it can be noted that the strength increase at young age is similar to that of the different mixtures prepared and cured in an ambient environment. It should be noted that the compressive strength continued to increase up to 90 days (Figure 9a). This is primarily due to the continued hydration of cement and the presence of sufficient moisture in the cementitious matrix of the various mixtures as well. However, after 28 days, the compressive strength in a hot and humid environment (Figure 9a) is always lower than that of the mixtures cured in an ambi- ent environment (20 °C). Furthermore, it was noted that after 7 days, the strength of the reference mortar increased from 51.3 to 57.40 MPa, whereas that of mor- tars containing 10 % of brick powder increased from 47.2 to 54.55 MPa, this means a resistance rate varying from 92 % to 95 % compared to the reference mortar. However, for mortars with 20 % BP, this strength in- creased from 42.90 to 50.66 MPa, i.e. a rate of 84 % to 88.3 % compared to the reference mortar, while for mortars with 30 % BP, the strength ranged between 32.80 and 42.63 MPa, which represents a rate of 64 % to 74.3 %, compared to the reference mortar. For the mortar with 30 % BWP, we observe a clear improve- ment in resistance at a very young age, this is due to the acceleration of hydration under the effect of tem- perature, which leads to the formation of an additional hydrated calcium silicate, CSH [51, 52]. This gain in resistance is also observed in the other mixtures of 10 % and 20 % brick powder at a very young age. Figure 9b shows the results related to the com- pressive strengths of cured mortars in a hot and dry environment, i.e. 50 °C and 0 % RH. It can be seen that the strength increase was interrupted due to the lack of water by capillary rise during the hydration pro- cess. Consequently, in a hot environment and in the presence of a low humidity rate, very low resistances 329 A. Brixi, M. S. Ghembaza, A. Boudenne Acta Polytechnica 0 10 20 30 40 Brick powder dosage, BP (%) 20 30 40 50 60 70 25 35 45 55 65 C o m p re ss iv e st re n g th , C S ( M P a) Water / Binder = 0.58 ; T = 20 °C 3 days 7 days 28 days 90 days Rc = - 0.77 BP+ 56.6 Rc = - 0.41 BP + 37.4 BP (%) C S (MPa) Rc = - 0.61 BP + 46.95 Rc = - 0.88 PB + 65.4 (a). Ambient environment. 0 10 20 30 40 Brick powder dosage, BP (%) 20 30 40 50 60 70 25 35 45 55 65 C o m p re ss iv e st re n g th , C S ( M P a) Water / Binder = 0.58 ; T = 50 °C wet 7 days 28 days 90 days C S = - 0.5 BP + 57.42 BP (%) C S (MPa) C S = - 0.6 BP + 52.52 C S = - 0.48 PB + 58.54 (b). Hot and humid environment. Figure 10. Evolution of the compressive strength as a function of brick powder dosage for different mortars in an ambient environment and a hot and humid environment. were observed for the various mixtures regardless of the brick powder percentage. That is, in such an en- vironment, the compressive strength practically does not change. These results are in good agreement with those reported by Spears [40]. Similarly, Figures 10a and 10b show the evolution of the compressive strength of mortars as a function of the brick powder (BP) dosage, for standard hardening under different temperatures (20 °C and 50 °C) for humid climate. For example, at an advanced age, as at 90 days (Figure 10a), the compressive strength is estimated at 52 MPa which is lower than that of the reference mortar (around 65 MPa). The same obser- vations were noted for the other mortars containing different BP percentages. As such, the findings are as follows: Rc90 (M1-PB10) = 54.90 MPa > Rc90 (M2-PB20) > Rc90 (M3-PB30) = 41.35 MPa, which shows a slight decrease in performance of around 15 %. It is also observed that the values obtained for the mechanical strength are acceptable without, however, reaching that of the reference mortar. It is noted that the compressive strength decreases as the brick powder content increases (Figure 10b). In other words, the substitution of cement by brick powder is accompanied by a drop in performance, especially at young ages. This compressive strength drop is quite significant, for brick powder dosages greater than or equal to 20 %, in comparison with the values recommended by the French Association of Civil Engineering (FACE). Furthermore, one may say that the decrease in strength is attributed, to the specific surface of the brick powder, which is smaller than that of pure ce- ment (2 690 cm2 g−1 < 3 480 cm2 g−1), and to the in- crease in porosity due to the larger quantity of water absorbed by the brick powder. The porosity increase engenders a slower hydration of cement. This strength is due to the hydration of several hydraulic compounds that form hydrated calcium silicates (commonly CSH phases) as the main compounds [53]. Figure 11a indicates that the flexural tensile strength values of the different mortars made by par- tially replacing cement by the brick powder represent increasing functions, of the form Fs = CtD, similar to those found for the compressive strengths. Here C and D are constants that depend on the curing times (3, 7, 28, and 90 days) and the ambient envi- ronment (20 °C, 90 % RH), regardless of the addition rate. Moreover, the flexural tensile strengths are close to each other for a given age. Nevertheless, the bend- ing tensile strengths values of the reference mortars remain higher than those of the other mortars. The re- sults show a rapid evolution between day 1 and day 28, with a significant increase in resistance. After 28 days, the rate of change in tensile strength decreased, re- gardless of the formulation of the mortars produced. Note as well that the tensile strength decreases as the percentage of substituted cement increases. As clearly shown in Figure 11b, the curves corre- sponding to each addition percentage are positioned in a tight spindle at advanced ages, under ambient (20 °C) which explains the fact that the effect of min- eral additions is mainly observed at young ages. These findings and those reported by Husson [54] are con- tradictory. The replacement of brick powder led to a reduction in the flexural strength of the mortar specimens (M10 %, M20 % and M30 %) by 13 %, 22 %, and 40 %, respectively. The reduction in resistance is significant above 30 % replacement of brick powder. Figure 12 presents the results of the flexural resis- tance tests of the different mortars stored underwater (humid environment) in a hot environment (at 50 °C). 330 vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . 0 20 40 60 80 100 Age (days) 4 6 8 10 5 7 9 11 F le xu ra l s tr en g th , F S ( M P a) Water/binder = 0.58 ; T = 20 °C Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite F S (MPa) 3 days 7 days 28 days 90 days A R2 = 0.81 R2 = 0.97 R2 = 0.85 R2 = 0,92 (a). Flexural strength as a function of age. 0 10 20 30 40 Brick powder dosage, BP (%) 4 6 8 10 5 7 9 11 F le xu ra l S tr en g th , F S ( M P a) Water / Binder = 0.58 ; T = 20 °C 3 days 7 days 28 days 90 days FS = - 0.09 BP + 8.3 FS = - 0.06 BP + 6.71 BP (%) F S (MPa) FS = - 0.11 BP + 8.34 FS = - 0.13 PB+ 10.2 B (b). Flexural strength as a function of brick powder dosage. Figure 11. Evolution of the flexural tensile strength of mixtures in an ambient environment. 0 20 40 60 80 100 Age (days) 4 6 8 10 5 7 9 11 F le xu ra l s tr en g th , F S ( M P a) Water/binder = 0.58 ; T =20 °C Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite Water / binder = 0.58 ; T = 50 °C - wet Reference mortar Mortar/10% brick composite Mortar/20% brick composite Mortar/30% brick composite F S (MPa) 3 days 7 days 28 days 90 days Figure 12. Evolution of the compressive strength of different mortars in a hot and humid environment. It can be seen that the flexural strengths increase with curing time for the different mixtures and are positioned in a narrow pin. We note a rapid evolu- tion between 3 and 28 days, this evolution is more pronounced in the case of mortars based on brick pow- der. This observation is the opposite of that made on flexural tensile strengths in an ambient hardening environment. After 28 days, the increase in bending resistance tends to stabilise. It appears that the flexural strength of the speci- mens stored at 50 °C has increased, and even more sig- nificantly when the dosage of brick powder increases, an optimum was found to be 10 % replacement by brick powder, without however, achieving the bending 0 20 40 60 8010 30 50 70 Compressive strength, C S (MPa) 0 10000 20000 30000 40000 Y o u n g 's m o d u lu s, E ( M P a) Brick powder_Brixi_20°C , 28 day Brick powder_Brixi_20°C , 90 day Brick powder_Brixi_50°C_wet , 28 day Brick powder_Brixi_50°C_wet , 90 day Brick powder_Brixi_50°C_dry , 28 day Brick powder_Brixi_50°C_dry , 90 day E (MPa) C S (MPa) Hot dry environment Hot wet environment ? E = 3800*Cs1/2 E = 3400*Cs1/2 E = 4600*Cs1/2 Figure 13. Correlation between the compressive strength and modulus of elasticity in a hot and humid environment. strength of the specimens stored at room tempera- ture. These results can be explained by the fact that the hydration process of the binders is accelerated faster and more strongly as the temperature rises from 20 to 50 °C. It can therefore be concluded that the brick powder in the samples immersed in water at a temperature of 50 °C reacts in the long term. Figure 13 illustrates the results of the ultrasonic pulse velocity (UPV) tests translated into defor- mation modulus of the different mortars stored at 28 and 90 days under different environments (ambient, hot/humid and hot/dry). The results indicate that the moduli of mortars containing the different per- centages of brick powder waste at 28 days of curing 331 A. Brixi, M. S. Ghembaza, A. Boudenne Acta Polytechnica Samples Curing conditions α β R2∗ 50 °C Hot and immersed in water Ed = αCSβ 20 °C Immersed in water 3 400 to 4 600 0.5 0.83 to 0.98 50 °C Hot and dry ∗ Coefficient of determination. Table 5. Power function parameters: Ed = αCSβ . increase in a hot and humid environment compared to the control mortar. However, these mortars show a slight increase in moduli when stored for 90 days in the same environment. The trend is similar for mortars containing the same dosages of brick pow- der but in a hot and dry environment. The results revealed a slight to moderate decrease for different percentages of these additives which require sufficient quantity of water, hence the interruption of hydration, resulting in an increase in porosity and the void ratio in hardened mortar compared to mortars stored in a hot and humid environment. Figure 13 shows the results relating to the modulus of elasticity of the different brick powder based mor- tars for different compressive strength values, under different curing conditions, and for different relative humidity levels. One can easily see that the modulus of elasticity increases as a power function with com- pressive strength, regardless of the curing time and under a conditioned environment. This power function has a lower limit and an upper limit. It is in the form: Ed = αCSβ , where α and β are constants, Ed is the dynamic modulus of elasticity, and CS is the compres- sive strength, as shown in Table 5. For a hot and dry environment, the values of Young’s modulus for this brick powder-based mortar are less than 21 GPa with strength values not exceeding 30 MPa. In a hot and hu- mid environment, the modulus of elasticity values are greater than 21 GPa with strength exceeding 32 MPa. 4. Conclusion This main aim of this research was to study the char- acteristics of cement-brick powder mortars. For this, several tests, such as the compressive strength test, the flexural tensile strength test, the ultrasonic test, the thermal conductivity test, and the thermal diffu- sivity test, were conducted on mortar samples in the hardened state, at different curing ages, and under dif- ferent environmental conditions. The mortar samples under study were made by partially replacing the mass of cement with finely ground brick waste at different percentages (0, 10 %, 20 % and 30 %). The results obtained for the different mixtures were discussed and compared with those of the reference mortar at different curing ages (3, 7, 28 and 90 days). The experimental study led to the following conclu- sions: • It should be noted that the amount of energy re- quired for heating and cooling buildings strongly depends on the thermal properties of the building material (thermal conductivity, specific heat capac- ity, and thermal diffusivity of the building envelope). Therefore, it was found that using brick powder at different percentages (0 %, 10 %, 20 % and 30 %) in the formulation of mortar improves the thermal insulation characteristics of this material. It can be said that the thermal conductivity and ther- mal diffusivity coefficients are strongly influenced by the brick powder content. Thermal properties as a function of brick powder dosage give a linear relationship. • The partial substitution of cement by powdered bricks decreased the compressive strength of mor- tars, especially at a young age. This was due, on the one hand, to the fact that the specific surface of waste bricks is lower than that of the cement used and, on the other hand, to the increase in porosity due to the decrease in hydrates. This compressive strength decrease was more significant for a brick powder percentages greater than 20 %. The replace- ment ratio of 10 % shows the best performance of mortar compared with another replacement ratio at ambient temperature. Furthermore, the specimens cured for 28 days in a hot and humid environment presented resistances almost equal to the control mortar. However, at higher curing times, these re- sistances decrease slightly compared to the control mortar. In the case of mortars stored in a hot and dry environment, there is a remarkable reduction in compressive strength with higher curing times. • The compressive strength was lower for samples stored in a dry environment at a temperature of 50 °C compared to those obtained for samples stored in water at 50 °C. This was certainly due to the different hydration rates in the different curing en- vironments. • Concerning the flexural resistance of different mor- tars stored in different climates, we note a decrease in this resistance for higher percentages of brick powder at room temperatures. On the other hand, an increase in bending strength is observed when the specimens are stored in a hot and humid cli- mate. The increase is more significant as the per- centage of brick powder increases and we find that the optimum percentage is 10 % of brick powder, without however reaching the bending resistance of the specimens stored at room temperature. These results can be explained by the fact that the hydra- 332 vol. 64 no. 4/2024 Hydrothermal and Mechanical Performance of Mortars . . . tion process of the binders is accelerated faster and more strongly as the temperature increases from 20 to 50 °C. • The desert and Mediterranean climates of North Africa are characterised by hot-dry and hot-humid climates, respectively, where the temperature can reach up to 50 °C and the relative humidity can drop to 8 % [4]. Mortars tested under similar cli- matic conditions showed a reduction in compressive strength and an increase in flexural strength for ages up to 28 days. 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Ph.D. thesis, Ecole Nationale Supérieure des Mines de Saint-Etienne, 1991. 335 https://doi.org/10.1520/C0642-13 https://doi.org/10.1016/j.conbuildmat.2016.05.119 https://doi.org/10.1016/j.conbuildmat.2016.05.119 https://doi.org/10.1016/j.jclepro.2020.120733 https://doi.org/10.1016/j.cemconcomp.2020.103758 https://doi.org/10.1016/j.cemconcomp.2020.103758 https://doi.org/10.1016/j.conbuildmat.2019.03.307 https://doi.org/10.1016/j.conbuildmat.2019.03.307 https://doi.org/10.3390/fractalfract5020047 Acta Polytechnica 64(4):322–335, 2024 1 Introduction 2 Experimental study 2.1 Materials used 2.1.1 Cement 2.1.2 Sand 2.1.3 Brick powder 2.2 Preparation of mortar specimens 2.3 Measurement of mechanical properties 2.3.1 Deformation modulus 2.3.2 Compression and tensile strength 2.3.3 Measuring the thermophysical properties of mortar 3 Results and discussion 3.1 Hydrothermal properties of brick powder-based mortar composites 3.2 Thermomechanical behaviour of brick powder based mortars 4 Conclusion References