Crushed Bricks: Demolition Waste as a Sustainable Raw Material for Geopolymers

: Demolition activity plays an important role in the total energy consumption of the construction industry in the European Union. The indiscriminate use of non-renewable raw materials, energy consumption, and unsustainable design has led to a redeﬁnition of the criteria to ensure environmental protection. This article introduces an experimental plan that determines the viability of a new type of construction material, obtained from crushed brick waste, to be introduced into the construction market. The potential of crushed brick waste as a raw material in the production of building precast products, obtained by curing a geopolymeric blend at 60 ◦ C for 3 days, has been exploited. Geopolymers represent an important alternative in reducing emissions and energy consumption, whilst, at the same time, achieving a considerable mechanical performance. The results obtained from this study show that the geopolymers produced from crushed brick were characterized by good properties in terms of open porosity, water absorption, mechanical strength, and surface resistance values when compared to building materials produced using traditional technologies.


Introduction
The construction industry occupies an important position in the production of waste materials from construction and demolition activities, with around one-third of the European Union's total waste represented by this so-called Construction and Demolition Waste (CDW). Known materials range from plastics to wood and metals, but it is the inert materials category that has the highest percentage weight, reaching 75-80% of the total CDW weight [1,2]. For this reason, the EU has considered the flow of CDW as a priority stream for action. In fact, over the last decade, intense activity in the construction field in Europe has generated around 827 million tonnes of CDW on average per year, and yet only 50% of this CDW was recycled [3,4]. Moreover, climate change and environmental issues are a wake-up call to the impoverishment of resources due, above all, to the exploitation of raw materials. In this sense, the quantity of second-hand materials coming from construction and demolition operations represents a source of great value. The possibility of including CDW in building materials is, therefore, an interesting alternative.
Recently, the recycling of CDW was extensively studied and reviewed [5][6][7][8]. The data reported indicate that CDW can be successfully used in construction material production to obtain products comparable with those produced using natural raw materials. Approximately 45% of CDW is attributed to ceramic products such as bricks, tiles, and porcelain. It is crucial to evaluate alternative applications of this waste considering the large amounts produced per year (yearly production from the EU is estimated at 855 million tonnes) [9].
Clay brick wastes (CBW) represent a valuable secondary resource for concrete manufacturing, either as a coarse or fine aggregate in concrete production, or as a supplementary cementitious material [10,11]. Recently, an interesting alternative has been reported: the development of alternative low-carbon binders (alkali-activated materials, geopolymers, and calcium sulfoaluminate cement) [12,13]. Geopolymers are obtained through a chemical reaction, starting with an alumino-silicate powder with a highly concentrated aqueous alkali hydroxide and/or silicate solution, to the production of a synthetic amorphous-tosemicrystalline alkali alumino-silicate new phase [14,15]. Alkali-activated materials (AAM) exhibit excellent properties in terms of mechanical performance [16], thermal stability [17], and durability [18].
Furthermore, geopolymers are of great interest because of their high sustainability, and reduced energy requirement in their production. In addition, the production of geopolymers allows for an 80% reduction in emissions compared to Portland cement manufacturing [14]. It has been demonstrated that any natural or synthetic material containing proper amounts of silica and alumina can be used as a precursor for the geopolymerization process [15].
Chemically bonded ceramics (CBC), belonging to the AAM class, are a group of inorganic materials that share properties with both cements and ceramics [19]. The formation of CBCs occurs via the chemical route, much like with conventional cement, while the chemical bonds and mechanical properties resemble those of ceramics. These materials allow for an alternative avenue in ceramic processing, where high temperatures are not required, and solid structures are formed in ambient temperatures in the presence of mineral impurities.
The present research aims to determine the feasibility and potential of crushed brick wastes in the production of building precast components from alkali-activation with sodium hydroxide and silicate solutions. Firstly, a complete characterization of the crushed brick waste was carried out to verify its intrinsic reactivity as a raw material for geopolymerization. Then, a ceramic product was obtained from alkali-activation of the crushed brick waste, and finally, its physical and mechanical properties were evaluated.

Materials and Methods
This research was divided into two experimental phases. The first phase consisted of evaluating the effective geopolymerization of the selected mixture, based on the type of waste material chosen and its percentage, through to the production of small cylindrical specimens. The second phase consisted of a scale-up of the specimens that had effectively passed the first phase, followed by the production of samples on which physical, chemical, and mineralogical analyses and mechanical tests were carried out, in order to determine the characteristics of the new material and hypothesize possible future applications.

Preparation of the Clay Brick Waste-Based Geopolymer
CBW was used as the raw material to carry out the experimental development of the research. The CBW came from the demolition site of an old building situated near the city of Naples, Italy. The original waste material (Figure 1a) was grounded to a fineness of 0.125-0.150 mm (Figure 1b). A sodium silicate solution (SS) (Na 2 O, 8.15%; SiO 2 , 27.40%), provided by Prochin Italia S.r.L. (Caserta, Italy), and a 10 M sodium hydroxide solution, prepared by dissolving NaOH in pellets (NaOH 98%, J.T. Baker) in bi-distilled water, were used as the alkali activators. provided by Prochin Italia S.r.L. (Caserta, Italy), and a 10 M sodium hydroxide solution, prepared by dissolving NaOH in pellets (NaOH 98%, J.T. Baker) in bi-distilled water, were used as the alkali activators.
(a) (b) Based on the clayey nature of the waste [15], the preparation of the geopolymer was carried out following the prescription reported in our previous studies [20,21]. Accordingly, powdered materials were previously dry-mixed and homogenized, and then the activator solution was added.
The alkaline activator solution was prepared by mixing sodium silicate solution (SS) with 10M sodium hydroxide solution. The weight ratio SS/N/binder was 1:1:3, and the activator/binder ratio was 0.66. Finally, the mixture was cast into cylindrical polyethylene molds (diameter 30 mm; height 70 mm) and the samples were sealed and cured for 3 days at 60 °C in an oven. At the end of the curing, all the specimens (Geo_CBW) were removed from their molds and stored at room temperature until reaching a 28-day aging period before being run for experimental analysis. A qualitative evaluation of the degree of geopolymerization was carried out by the immersion of the samples in bi-distilled water for 24 h at room temperature. Importantly, if the geo-composites were intact after the immersion period, then the geopolymer gel formation can be considered successful ( Figure 2) because water absorption can be used as an indicator of the extent of the geopolymeric reaction [22].

Mineralogical and Chemical Characterization
The chemical composition was obtained according to the following procedure. Firstly, a weighted sample was calcined at 950 °C for 2 h, then a weighted amount of the obtained sample was subjected to digestion, under microwave-induced heating (Perkin-Elmer Multiwave 3000 oven) in a standard solution obtained by mixing hydrochloric acid (37%, w/w), nitric acid (65%, w/w) and hydrofluoric acid (39.5%, wt/wt). Next, a boric acid solution was used to achieve fluoride complexation and the resulting solution was analyzed by an ICP-OES (Optima 2100 DV ICP-OES Inductively Coupled Plasma Spectrometer, Perkin-Elmer). Based on the clayey nature of the waste [15], the preparation of the geopolymer was carried out following the prescription reported in our previous studies [20,21]. Accordingly, powdered materials were previously dry-mixed and homogenized, and then the activator solution was added.
The alkaline activator solution was prepared by mixing sodium silicate solution (SS) with 10M sodium hydroxide solution. The weight ratio SS/N/binder was 1:1:3, and the activator/binder ratio was 0.66. Finally, the mixture was cast into cylindrical polyethylene molds (diameter 30 mm; height 70 mm) and the samples were sealed and cured for 3 days at 60 • C in an oven. At the end of the curing, all the specimens (Geo_CBW) were removed from their molds and stored at room temperature until reaching a 28-day aging period before being run for experimental analysis. A qualitative evaluation of the degree of geopolymerization was carried out by the immersion of the samples in bi-distilled water for 24 h at room temperature. Importantly, if the geo-composites were intact after the immersion period, then the geopolymer gel formation can be considered successful ( Figure 2) because water absorption can be used as an indicator of the extent of the geopolymeric reaction [22]. Based on the clayey nature of the waste [15], the preparation of the geopolymer was carried out following the prescription reported in our previous studies [20,21]. Accordingly, powdered materials were previously dry-mixed and homogenized, and then the activator solution was added.
The alkaline activator solution was prepared by mixing sodium silicate solution (SS) with 10M sodium hydroxide solution. The weight ratio SS/N/binder was 1:1:3, and the activator/binder ratio was 0.66. Finally, the mixture was cast into cylindrical polyethylene molds (diameter 30 mm; height 70 mm) and the samples were sealed and cured for 3 days at 60 °C in an oven. At the end of the curing, all the specimens (Geo_CBW) were removed from their molds and stored at room temperature until reaching a 28-day aging period before being run for experimental analysis. A qualitative evaluation of the degree of geopolymerization was carried out by the immersion of the samples in bi-distilled water for 24 h at room temperature. Importantly, if the geo-composites were intact after the immersion period, then the geopolymer gel formation can be considered successful ( Figure 2) because water absorption can be used as an indicator of the extent of the geopolymeric reaction [22].

Mineralogical and Chemical Characterization
The chemical composition was obtained according to the following procedure. Firstly, a weighted sample was calcined at 950 °C for 2 h, then a weighted amount of the obtained sample was subjected to digestion, under microwave-induced heating (Perkin-Elmer Multiwave 3000 oven) in a standard solution obtained by mixing hydrochloric acid (37%, w/w), nitric acid (65%, w/w) and hydrofluoric acid (39.5%, wt/wt). Next, a boric acid solution was used to achieve fluoride complexation and the resulting solution was analyzed by an ICP-OES (Optima 2100 DV ICP-OES Inductively Coupled Plasma Spectrometer, Perkin-Elmer).

Mineralogical and Chemical Characterization
The chemical composition was obtained according to the following procedure. Firstly, a weighted sample was calcined at 950 • C for 2 h, then a weighted amount of the obtained sample was subjected to digestion, under microwave-induced heating (Perkin-Elmer Multiwave 3000 oven) in a standard solution obtained by mixing hydrochloric acid (37%, w/w), nitric acid (65%, w/w) and hydrofluoric acid (39.5%, wt/wt). Next, a boric acid solution was used to achieve fluoride complexation and the resulting solution was analyzed by an ICP-OES (Optima 2100 DV ICP-OES Inductively Coupled Plasma Spectrometer, Perkin-Elmer).
The mineralogical composition of the CBW was evaluated by XRD analysis of the powdered samples using a Panalytical X'Pert Pro diffractometer equipped with a PixCel 1D detector (operative conditions: CuKα1/Kα2 radiation, 40 kV, 40 mA, 2Q ranging from 5 to 80 • C, step size 0.0131 • 2Φ, counting time 40 s per step).
In order to further investigate the degree of geopolymerization, FTIR and thermal analysis were carried out. FTIR tests were performed using a Nexus-Nicolet apparatus and selecting a wavenumber resolution of 4 cm −1 for 32 scans ranging from 4000 to 400 cm −1 . The FTIR spectra were collected in absorbance mode from the transparent pellet obtained by dispersing the sample powders in KBr (2% wt/wt) both with the CBW and with the geopolymeric samples produced. Thermal characterization was performed by TGA/DTGA analysis (Netzsch, STA 409 PC Luxx) in temperatures ranging from 20 to 1200 • C, with a heating rate of 10 • C/min, in a nitrogen atmosphere.
Finally, morphological analyses were carried out by SEM (SEM, Cambridge S440).

Physical Characterization of the Geopolymers
Capillary rise can be considered as one of the most significant mechanisms of water penetration into building materials, and is one of the main mechanisms responsible for building decay. For this reason, capillarity tests were performed according to the European Standard UNI EN 15801 [23], with an evaluation of the amount of water absorbed (Q) per surface unit as a function of time. Tests were performed in triplicate on cubic samples (5 cm-side). At the end of these tests, the mean value of the capillary absorption coefficient (CA, mg/cm 2 s −1/2 ) was determined. Since there is a quite linear relationship between the water adsorbed (Q) and the square root of time during shorter times, the CA value can be evaluated as the slope of the straight line in the first step (30 min) of the capillarity test [23].
The open porosity and water absorption were evaluated according to the European Standard [24]. Firstly, the specimens, after being dried at 60 ± 5 • C until a constant mass was reached, were immersed in water under vacuum at room temperature and left for two hours. Then, each sample was weighted (bulk and hydrostatic weight). The pycnometer method was used after grinding the sample to a dimension of 0.063 mm in order to evaluate the closed porosity. Each test was performed in triplicate and the results were calculated as the average values. Moreover, the water uptake from immersion, without placing samples under a vacuum, was evaluated by immersing the prismatic samples (4 × 4 × 16 cm 3 ) in a water tank for 48 h. Samples were first dried at 60 ± 5 • C until a constant mass was reached, then weighted before and after the immersion, and, finally, the amount of water absorbed was deduced from the weight difference.

Mechanical Characterization
The surface hardness was determined according to the standard EN 13279-2 [25]. The Shore D hardness test was carried out by establishing the indent left by an exerted force on each test specimen, measured in Shore D units varying in range from 0 (softest) to 100 (hardest).
In particular, the experimental tests were performed on the two longitudinal sides of the prismatic samples (160 × 40 mm 2 ) (see Figure 3).
The mineralogical composition of the CBW was evaluated by XRD analysis of the powdered samples using a Panalytical X'Pert Pro diffractometer equipped with a PixCel 1D detector (operative conditions: CuKα1/Kα2 radiation, 40 kV, 40 mA, 2Q ranging from 5 to 80 °C, step size 0.0131° 2Φ, counting time 40 s per step).
In order to further investigate the degree of geopolymerization, FTIR and thermal analysis were carried out. FTIR tests were performed using a Nexus-Nicolet apparatus and selecting a wavenumber resolution of 4 cm −-1 for 32 scans ranging from 4000 to 400 cm −1 . The FTIR spectra were collected in absorbance mode from the transparent pellet obtained by dispersing the sample powders in KBr (2% wt/wt) both with the CBW and with the geopolymeric samples produced. Thermal characterization was performed by TGA/DTGA analysis (Netzsch, STA 409 PC Luxx) in temperatures ranging from 20 to 1200 °C, with a heating rate of 10 °C/min, in a nitrogen atmosphere.
Finally, morphological analyses were carried out by SEM (SEM, Cambridge S440).

Physical Characterization of the Geopolymers
Capillary rise can be considered as one of the most significant mechanisms of water penetration into building materials, and is one of the main mechanisms responsible for building decay. For this reason, capillarity tests were performed according to the European Standard UNI EN 15801 [23], with an evaluation of the amount of water absorbed (Q) per surface unit as a function of time. Tests were performed in triplicate on cubic samples (5 cm-side). At the end of these tests, the mean value of the capillary absorption coefficient (CA, mg/cm 2 s −1/2 ) was determined. Since there is a quite linear relationship between the water adsorbed (Q) and the square root of time during shorter times, the CA value can be evaluated as the slope of the straight line in the first step (30 min) of the capillarity test [23].
The open porosity and water absorption were evaluated according to the European Standard [24]. Firstly, the specimens, after being dried at 60 ± 5 °C until a constant mass was reached, were immersed in water under vacuum at room temperature and left for two hours. Then, each sample was weighted (bulk and hydrostatic weight). The pycnometer method was used after grinding the sample to a dimension of 0.063 mm in order to evaluate the closed porosity. Each test was performed in triplicate and the results were calculated as the average values. Moreover, the water uptake from immersion, without placing samples under a vacuum, was evaluated by immersing the prismatic samples (4 × 4 × 16 cm 3 ) in a water tank for 48 h. Samples were first dried at 60 ± 5 °C until a constant mass was reached, then weighted before and after the immersion, and, finally, the amount of water absorbed was deduced from the weight difference.

Mechanical Characterization
The surface hardness was determined according to the standard EN 13279-2 [25]. The Shore D hardness test was carried out by establishing the indent left by an exerted force on each test specimen, measured in Shore D units varying in range from 0 (softest) to 100 (hardest).
In particular, the experimental tests were performed on the two longitudinal sides of the prismatic samples (160 × 40 mm 2 ) (see Figure 3). porting rollers, 100 mm apart from each other. A load was applied until the specimens broke apart, while the test machine recorded the maximum load supported by the samples.
Compressive strength tests were performed following the same standard used for the flexural tests, EN 13279-2:2004 (Figure 4b). A load was then applied to the broken parts of the samples derived from the previous flexural strength tests.
The test pieces were then placed between two steel plates in a cubic geometry, with the dimensions 40 mm × 40 mm. The test specimens were loaded until rupture occurred. The compressive strength Rc was calculated using the formula: where Rc is the compressive strength, Fc is the maximum load at fracture (N); and 1600 is the load-bearing surface (mm 2 ) of the tested sample. All tests were carried out in triplicate.

Preliminary Characterization of Clay Brick Waste
The chemical and mineralogical composition of the waste, reported in Table 1, confirmed the clayey nature of the sample with a SiO2/Al2O3 ratio of 1:5 and a significant amount of alkaline and alkaline earth oxides. It is worth recalling that, when clay minerals are heated at a constant rate, two principal heat effects occur: a broad endothermic effect near 550 °C, caused by the dissociation of the clay structure, and an intense exothermic peak (between 800 and 900 °C) due to the crystallization of new crystalline phases such as aluminosilicates (diopside and leucite) [26]. In addition, the XRD data from the CBW (Table 1 and Figure 5) The test pieces were then placed between two steel plates in a cubic geometry, with the dimensions 40 mm × 40 mm. The test specimens were loaded until rupture occurred. The compressive strength Rc was calculated using the formula: where Rc is the compressive strength, Fc is the maximum load at fracture (N); and 1600 is the load-bearing surface (mm 2 ) of the tested sample. All tests were carried out in triplicate.

Preliminary Characterization of Clay Brick Waste
The chemical and mineralogical composition of the waste, reported in Table 1, confirmed the clayey nature of the sample with a SiO 2 /Al 2 O 3 ratio of 1:5 and a significant amount of alkaline and alkaline earth oxides. It is worth recalling that, when clay minerals are heated at a constant rate, two principal heat effects occur: a broad endothermic effect near 550 • C, caused by the dissociation of the clay structure, and an intense exothermic peak (between 800 and 900 • C) due to the crystallization of new crystalline phases such as aluminosilicates (diopside and leucite) [26]. In addition, the XRD data from the CBW (Table 1 and Figure 5) confirmed the presence of a major crystalline phase of sodium and potassium aluminium silicates. Quartz mineral (SiO 2 ) and calcium carbonate (CaCO 3 ) were also detected. The amorphous aluminosilicate materials formed upon the decomposition and destuction of the clay minerals during the firing could not be observed by the XRD technique due to their amorphous nature. their amorphous nature.
Moreover, from the TGA curve of the CBW (Figure 6), a weight loss at temperatures between 100 °C and 200 °C was reported, due to the evaporation of free water absorbed by the specimen. Additionally, a small amount of weight loss (≈2%) occurred at temperatures between 650 °C and 750 °C which was attributed to the decomposition of the carbonates either present in the raw material or due to the atmospheric carbonation that occurred during the preparation of the samples prior to analysis.

Characterization of Geopolymeric Samples
FTIR was used to verify the degree of geopolymerization of the geocomposites [27][28][29]. Geopolymers produced from CBW were mainly amorphous aluminosilicates themselves, and displayed FTIR spectra (Figure 7 Moreover, from the TGA curve of the CBW (Figure 6), a weight loss at temperatures between 100 • C and 200 • C was reported, due to the evaporation of free water absorbed by the specimen. Additionally, a small amount of weight loss (≈2%) occurred at temperatures between 650 • C and 750 • C which was attributed to the decomposition of the carbonates either present in the raw material or due to the atmospheric carbonation that occurred during the preparation of the samples prior to analysis.
confirmed the presence of a major crystalline phase of sodium and potassium aluminium silicates. Quartz mineral (SiO2) and calcium carbonate (CaCO3) were also detected. The amorphous aluminosilicate materials formed upon the decomposition and destuction of the clay minerals during the firing could not be observed by the XRD technique due to their amorphous nature.
Moreover, from the TGA curve of the CBW (Figure 6), a weight loss at temperatures between 100 °C and 200 °C was reported, due to the evaporation of free water absorbed by the specimen. Additionally, a small amount of weight loss (≈2%) occurred at temperatures between 650 °C and 750 °C which was attributed to the decomposition of the carbonates either present in the raw material or due to the atmospheric carbonation that occurred during the preparation of the samples prior to analysis.

Characterization of Geopolymeric Samples
FTIR was used to verify the degree of geopolymerization of the geocomposites [27][28][29]. Geopolymers produced from CBW were mainly amorphous aluminosilicates themselves, and displayed FTIR spectra (Figure 7

Characterization of Geopolymeric Samples
FTIR was used to verify the degree of geopolymerization of the geocomposites [27][28][29]. Geopolymers produced from CBW were mainly amorphous aluminosilicates themselves, and displayed FTIR spectra (Figure 7, continuous line) characterized by the typical absorption bands of Si-O-Si and Si-O-Al bonds (absorption range 600-800 cm −1 ). The bands at 3450 cm −1 and 1647 cm −1 , associated with the O-H stretching and bending, are connected to the bound water molecules, which are surface-absorbed or entrapped in the large cavities of the molecular structure [28,30]. The intensity of these bands was greater in the FTIR spectra of geopolymers, indicating both a higher degree of water molecule adsorption in their mass and the occurrence of a geopolymerization reaction of the raw materials into a geopolymer paste [31]. Further confirmation of the presence of the aluminosilicate species typical of geopolymeric composites in Geo_CBW samples can be found in the TGA curve and the XRD spectrum ( Figure 8). In particular, the XRD spectrum ( Figure 8b) indicated an appearance of peaks associated with the presence of Phillipsite ((Ca,Na 2 ,K 2 ) 3 Al 6 Si 10 O 32 ·12H 2 O), which was the only aluminosilicate crystalline phase, while the presence of other amorphous geopolymeric phases was related to the gradual weight loss detected between 250 and 600 • C in the TGA curve.
SEM investigations at different magnifications (from 50× to 3000×) of the geopolymers are presented in Figure 9. EDX spectra confirmed the silico-aluminatic nature of the geopolymeric product. The Geo_CBW sample was characterized by a porous and heterogeneous matrix with some unreacted particles (500× magnification). At higher magnifications (from 1500× to 3000×) the amorphous structure of the geopolymeric structure was evidenced. which are surface-absorbed or entrapped in the large cavities of the molecular structure [28,30]. The intensity of these bands was greater in the FTIR spectra of geopolymers, indicating both a higher degree of water molecule adsorption in their mass and the occurrence of a geopolymerization reaction of the raw materials into a geopolymer paste [31]. Further confirmation of the presence of the aluminosilicate species typical of geopolymeric composites in Geo_CBW samples can be found in the TGA curve and the XRD spectrum ( Figure 8). In particular, the XRD spectrum (Figure 8b) indicated an appearance of peaks associated with the presence of Phillipsite ((Ca,Na2,K2)3Al6Si10O32·12H2O), which was the only aluminosilicate crystalline phase, while the presence of other amorphous geopolymeric phases was related to the gradual weight loss detected between 250 and 600 °C in the TGA curve.   SEM investigations at different magnifications (from 50× to 3000×) of the geopolymers are presented in Figure 9. EDX spectra confirmed the silico-aluminatic nature of the geopolymeric product. The Geo_CBW sample was characterized by a porous and heterogeneous matrix with some unreacted particles (500× magnification). At higher magnifications (from 1500× to 3000×) the amorphous structure of the geopolymeric structure was evidenced. The capillary absorption curves are reported in Figure 10. The data from the tests indicated that the experimental method was performed with a high repeatabili that all the samples reached water saturation after 5-6 h. Moreover, the water abso rate decreased for longer times, and this could be related to the increased water c inside the specimen and to the slow progressive participation of the less-accessible [32]. The average value of the capillary absorption coefficient is reported in Tabl which all the main physical properties of the CBW geopolymer are summarize worth observing that the capillary absorption coefficient obtained from the CBW geopolymers was comparable to common values of the NHL mortars and, at the sam lower than that of the typical solid clay brick (≈26) [33].
Regarding water absorption, it is worth noting that, as expected, the water abso for immersion was almost half when compared to the value of the water abso obtained by immersing the sample under a vacuum (see Table 2). The value o porosity reported was about 46%, which further confirmed that the CBW The capillary absorption curves are reported in Figure 10. The data from the three tests indicated that the experimental method was performed with a high repeatability and that all the samples reached water saturation after 5-6 h. Moreover, the water absorption rate decreased for longer times, and this could be related to the increased water content inside the specimen and to the slow progressive participation of the less-accessible pores [32]. The average value of the capillary absorption coefficient is reported in Table 2, in which all the main physical properties of the CBW geopolymer are summarized. It is worth observing that the capillary absorption coefficient obtained from the CBW-based geopolymers was comparable to common values of the NHL mortars and, at the same time, lower than that of the typical solid clay brick (≈26) [33].  The results obtained from the Shore D tests are shown in Table 3. The average value of the surface hardness (82.16) was reported, which is comparable to building plasters and gypsum composites [34,35]. The flexural and compression tests provided a maximum resistance value of 2.85 ± 0.73 MPa and 5.34 ± 0.66 MPa, respectively. In order to grasp the possible applications of CBW, some of the results obtained from the Geo_CBW samples were compared to those found in the literature for several typologies of building materials that were comparable in terms of their mechanical behavior (see Table 4). In particular, different traditional building materials, such as lightweight gypsum and natural the hydraulic lime (NHL),  Regarding water absorption, it is worth noting that, as expected, the water absorption for immersion was almost half when compared to the value of the water absorption obtained by immersing the sample under a vacuum (see Table 2). The value of open porosity reported was about 46%, which further confirmed that the CBW-based geopolymeric samples produced were highly porous, as is clearly visible from the microstructural characterization ( Figure 9).
The results obtained from the Shore D tests are shown in Table 3. The average value of the surface hardness (82.16) was reported, which is comparable to building plasters and gypsum composites [34,35]. The flexural and compression tests provided a maximum resistance value of 2.85 ± 0.73 MPa and 5.34 ± 0.66 MPa, respectively. In order to grasp the possible applications of CBW, some of the results obtained from the Geo_CBW samples were compared to those found in the literature for several typologies of building materials that were comparable in terms of their mechanical behavior (see Table 4). In particular, different traditional building materials, such as lightweight gypsum and natural the hydraulic lime (NHL), were selected based on comparisons to different typologies of geopolymers and sustainable building materials produced from several kinds of waste. From the data shown in Table 4, it is possible to deduce that the density and mechanical performance of the Geo_CBW samples were very similar to the gypsum samples produced from waste glass and ceramic waste made from brick similar to the CBW used in this paper.

Conclusions
Geopolymer manufacturing represents one of the most innovative ways to reuse different types of solid waste, especially when employed with aluminosilicate and clayey waste. In this paper, we investigated the potential of crushed brick waste as a raw material for the production of geopolymers. Preliminary chemical and mineralogical characterization were conducted on the CBW to evaluate if its chemical composition could be rendered suitable to undergo a geopolymerization process and to confirm its mainly clayey nature. Then, geopolymeric samples that were produced were widely characterized by means of chemical-physical, morphological, and mechanical laboratory testing. In particular, FTIR, XRD and SEM analyses confirmed that the geopolymeric reactions took place effectively. Evaluation of the physical and mechanical properties led to the conclusion that CBW can be successfully employed as a raw material in the production of geopolymer-based building precast components.
This approach of using secondary raw materials, derived from construction and demolition activities, in the production process of sustainable materials to reintroduce into the building market represents an intelligent and environmentally friendly solution to the significant environmental issues surrounding the disposal of this widely available waste.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.