Performance of Geopolymer Insulation Bricks Synthesized from Industrial Waste

Abstract

In the current work, aluminum dross from the aluminum industry and fired-clay brick waste (Homra) were combined with alkaline activators (Na₂SiO₃/NaOH) with varying molarities of NaOH to create insulating geopolymer bricks. An alkaline activator with an (S/L) ratio of 3/1 was combined with Homra to replace it partly. The mixture was then molded into 50 × 50 × 50 mm³ stainless steel molds. Water absorption, compressive strength, bulk density, and apparent porosity of the produced geopolymer were measured at various curing durations and NaOH molarities. The findings showed that adding more aluminum dross waste replacement leads to a decrease in bulk density and compressive strength and an increase in the creation of pores that improve the produced bricks’ ability to insulate against heat. Using 8 M of NaOH after 28 days, the findings revealed that 3% aluminum dross waste replacement has a compressive strength of about 8.6 MPa, water absorption of 14.6%, bulk density of 1.307 g/cm³, and thermal conductivity of 0.32 W/m·K. The Egyptian standard, ASTM C62, satisfies these outcomes. Using DesignBuilder energy simulation software version 6.1.06, the constructed insulating brick under ideal circumstances was utilized to calculate its contribution performance in energy consumption. The annual energy consumption was reduced by about 21% compared to traditional hollow cement bricks.

1. Introduction

Masonry fired-clay brick is one of the earliest known building and construction materials, and its demand has grown in tandem with the world’s population [1]. Global manufacturing produces approximately 1.5 trillion fired-clay bricks annually [2]. An energy-intensive method is used worldwide to produce bricks at high temperatures. Burnt bricks release NO_x and CO₂ into the atmosphere due to their heavy reliance on fossil fuels. However, the depletion of clay resources might result from the use of clay as a raw material for brickmaking [3]. This is why it is not ecologically advantageous to manufacture burned bricks from clay resources, despite the high energy demand and resource use involved. An economically and environmentally beneficial alternative might be to use garbage as a raw material for brick production [4]. It encourages environmental sustainability in addition to reducing disposal costs at landfills [5,6]. Environmental issues will worsen as waste and industrial byproduct volumes increase, and disposal space becomes scarce [7]. As a result, recycling materials and byproducts from solid waste management has become a preferred option over disposal [8]. Reusing this industrial waste by converting it into fresh building materials is considered a practical solution to reduce environmental pollution problems [9]. However, in order to recycle this waste, its environmental traits and behaviors must meet certain requirements and adhere to relevant environmental guidelines [10].

Ever since the geopolymerization process was discovered by Davidovits [11]. The production of several types of bricks using this novel substance is beginning to pose a serious threat to conventional cement-based civil building methods [12,13,14]. These materials include asphalt, roofing slates, concrete, and aircraft pavement [15,16,17,18,19,20,21,22,23]. The primary step in the creation of geopolymer structures is the reaction of an alkaline activator with a binding material. Fly ash, or metakaolin, which is created by heating kaolin over 600 °C, is often used to make the former material. The binder group includes iron slag, red mud, and silica fume [24,25,26,27,28,29]. Recently, researchers have used other binding materials like leftover clay bricks or ceramic tile dust [30,31]. On the other hand, alkaline activators typically combine sodium silicate and sodium hydroxide in varying amounts to maintain a specific level of total sodium oxide. Zhuang et al. [32] and Degirmenci [33] have reported on the relationship between a stronger final geopolymer and a larger ratio of silicate to caustic soda. Caustic soda, the primary activator, typically combines with additional activators like slaked lime [34,35].

The low alumina and calcium contents of the waste foundry sand necessitated the addition of various amounts of fly ash and electric arc furnace slag to improve the alumina and calcium ratios and, consequently, the compressive strength. This was the main material composition used to fabricate geopolymer bricks illuminated by Suchanya Apithanyasai et al. [8] for pavement applications. In a similar vein, a secondary supply of alumina was used to create a geopolymer based on rice husk ash with enhanced characteristics [36,37,38,39,40,41]. Ehsan Mohseni et al. [42] nano-Al₂O₃ is added to rice husk ash to make up for the absence of Al₂O₃ in the rice husk ash (RHA) and produces a rice-husk-ash-based geopolymer. Producing a geopolymer gel, calcium silicate hydrate, calcium aluminosilicate hydrate, or sodium aluminosilicate hydrate in the alkali-activated matrix further enhanced the mechanical characteristics [43]. So, alumina must be added to low-alumina materials [44]. Similar to glass or ceramic foams, which are made at temperatures exceeding 900 °C, geopolymer foams may be produced at low temperatures (below 100 °C) [45,46]. High-temperature applications, such as wall panels, thermal insulation, and fire-resistant coatings, utilize geopolymer foams [45,47,48,49]. Most of these foams are lightweight, making them suitable for applications requiring thermoacoustic insulation and fire resistance [45,50]. They depend on the essential use of foaming agents as materials that generate pores.

Ibrahim et al. [51] produced fly-ash-based geopolymer bricks using sodium silicate and caustic soda as an activating solution, along with an unidentified foaming component at levels ranging from 5 to 10%. Their samples revealed densities between 1400 and 1500 kg/m³, which were associated with compressive strengths ranging from 5 to 10 MPa, depending on the amount of foaming agent used. Despite this, these densities are too high for insulating components. Risdanareni et al. [52] also obtained similar results using Styrofoam as a pore-generating material at levels as high as 0.9%. On the other hand, Roviello et al. [53] created hybrid geopolymer-based foams that had lower densities (250–850 kg/m³) but great mechanical properties, fire resistance, and low thermal conductivity. The manufacture of geopolymers also used hydrogen peroxide as a foaming agent to create porosity [54,55,56,57]. Ahmed et al. [58] reported on the fabrication of thermally insulating geopolymer bricks using ferrosilicon slag and alumina waste. They achieved the maximum compressive strength at a concentration of 8 M NaOH. Furthermore, the inclusion of alumina waste decreased the heat conductivity of the resultant geopolymer bricks. El-Naggar et al. created geopolymer-insulating bricks using waste materials that had a bulk density of around 1000 kg/m³, low heat conductivity, and good mechanical strength [59].

Exposure to aluminum dross waste is considered dangerous to human health and the environment, and since 2000, the EU has classified it as such. Specifically, discarded aluminum dross is considered to be carcinogenic as well as a source of skin irritation and corrosion. The non-metallic product (NMP) component of aluminum dross waste also functions as a sensitizer and irritant when it comes into extended or regular contact with the skin or mucous membranes [60]. Dust is created during the processing of aluminum dross waste, and inhaling or consuming it presents a major danger. Moreover, the active contaminants in aluminum dross waste react rapidly with moisture to produce gasses that are poisonous, explosive, and offensively odorous [61]. In the past, landfills disposed of about 95% of the industrially generated aluminum dross waste annually [62]. However, due to the constant aluminum production, the remaining 5% has been steadily increasing. To make geopolymer concrete, Migunthanna et al. employed fly ash, slag in the precursor, and one-part binders made from leftover clay brick powder [63]. Another study by Tazune et al. [64] looked at how different Fe₂O₃/SiO₂ molar ratios in Fe-silica affected the properties of geopolymer materials made from metakaolin and waste-fired-clay brick.

According to prior research, no study has employed aluminum melting waste dross to create lightweight geopolymer concrete or geopolymer bricks that insulate against heat. Thus, the significance of this experimental study lies in its potential use in the geopolymer brick industry to produce lightweight thermal insulation bricks from leftover aluminum dross. This work will open doors and encourage the construction industry to employ aluminum dross from industrial waste in geopolymer. This study suggests using wasted bricks as a solid starter and a solution of sodium hydroxide and sodium silicate as an alkaline activator to make inexpensive low-density insulating geopolymer bricks. Porosity forms when waste aluminum dross from various factories melts and combines with an alkaline activator. The importance of this study lies in the fabrication of a lightweight, room-temperature geopolymer brick that can replace burned bricks.

2. Experimental Methodology

2.1. Raw Materials

The aluminum factory in Naga Hamad, Egypt, supplied the aluminum dross for this work, while a ceramics brick manufacturing company provided the fired-clay bricks waste (Homra). Geopolymer was prepared using NaOH and sodium silicate solution, two alkaline activators. The El Shark El-Awsat company in Cairo, Egypt, supplied NaOH (99% pure), while the El Morgan company for chemicals in Cairo, Egypt, supplied sodium silicate solution. The mineralogical analysis, chemical composition a of the raw materials (Homra and aluminum dross) were determined using X-ray diffraction (Brukur D8 advanced computerized apparatus, with a step size of 0.05 and 1 s duration for each step), the X-ray fluorescence technique (Axios, panalytical 2005, wavelength dispersive sequential spectrometer machine), respectively placed in national research centre (NRC), Cairo, Egypt. Sieve analysis of the raw materials was determined using the standard sieving procedure described by ASTM D 422 (NRC, Cairo, Egypt) [65].

2.2. Treatment of Aluminum Dross

Aluminum dross was crushed and ground. The ground aluminum dross was washed with boiled water with (S/L) of 1:10 at a temperature of 80 °C for 1 h. This step effectively dissolved and removed any salt residue in the aluminum dross. Next, the mixture was filtrated to separate the aluminum dross from the solution. The washed aluminum dross underwent filtration and drying at a temperature of 105 °C until the weight reached a constant value [66,67].

2.3. Preparation of Alkaline Activator

The alkaline activator consisted of a combination of sodium hydroxide and sodium silicate solutions. Pellets of sodium hydroxide were dissolved in distilled water to prepare a solution of sodium hydroxide with varying concentrations (8, 10, and 12 M). The sodium silicate solution consisted of 30.3% SiO₂, 12.7% NaOH, and 57% H₂O. The sodium silicate solution was combined with a sodium hydroxide solution (Na₂SiO₃/NaOH) with a mass ratio of 2.5 to formulate the alkaline activator solution. Before preparing the geopolymer bricks, the solution was mixed for 24 h to ensure a uniform mixture [9,10].

2.4. Preparation of Geopolymer Bricks

Table 1. Composition of the proportions of geopolymer bricks.

Mix	Homra (%)	Alumina Dross (%)	Molarity of NaOH	Alkaline Activator (%)
8M1	75	0	8	25
8M2	74.5	0.5	8	25
8M3	74	1	8	25
8M4	73.5	1.5	8	25
8M5	73	2	8	25
8M6	72.5	2.5	8	25
8M7	72	3	8	25
8M8	70	5	8	25
10M1	75	0	10	25
10M2	74.5	0.5	10	25
10M3	74	1	10	25
10M4	73.5	1.5	10	25
10M5	73	2	10	25
10M6	72.5	2.5	10	25
10M7	72	3	10	25
10M8	70	5	10	25
12M1	75	0	12	25
12M2	74.5	0.5	12	25
12M3	74	1	12	25
12M4	73.5	1.5	12	25
12M5	73	2	12	25
12M6	72.5	2.5	12	25
12M7	72	3	12	25
12M8	70	5	12	25

describes the mixture proportions required for the fabrication of the thermal insulation of geopolymer bricks. Homra and aluminum dross powders were mixed on a dry basis for 5 min to ensure the homogeneity of the mixture. The prepared alkaline activator solution was added slowly to the mixed powder with (S/L) ratio of 3, then the mixture continued mixed for 4 min to prepare a paste of geopolymer. The prepared geopolymer paste was cast into an iron mold with dimensions of 50 × 50 × 50 mm³, dried at room temperature for 24 h before removing from the molds, and left at room temperature for different curing times (3, 7, 14, and 28 days). The compressive strength of the different curing bricks was determined. The thermal conductivity and water absorptions were estimated for the most compressive strength. Figure 1 shows the steps of the preparation of the lightweight geopolymer.

2.5. Characterization of the Geopolymer

2.5.1. Physical Properties

Bulk Density

The bulk density of the prepared geopolymer was determined with sample dimensions of 50 × 50 × 50 mm³. Three replicates of each mixture were used to determine the bulk density after 3, 7, 14, and 28 days. The weight and volume of each sample were determined, and the bulk density was estimated according to ISO 5017 [68]. The following equation was used to determine the bulk density:

$${\mathsf{\rho }}_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}={\displaystyle \frac{\mathrm{W}}{\mathrm{V}}}$$

(1)

where ρ is the bulk density in (g·cm⁻³), W is the weight of the dry sample in (g), and V is the volume of the sample in (cm³).

Water Absorption

The water absorption test was performed on the three replicate samples to determine the average water absorption. The water absorption test was conducted according to the ISO 9652 standard [69]. The weight of samples was measured, and then the samples were soaked in water for 24 h at room temperature. The samples were removed from the water, and the excess water was wiped off, followed by measurement of the weight of the wet samples. The water absorption was determined according to the following equation:

$$\mathrm{W}\mathrm{A},\left(\%\right)={\displaystyle \frac{{\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{W}}_{\mathrm{d}}}}\times 100$$

(2)

where WA is the percentage of water absorption, W_s is the weight of the saturated sample in (g), and W_d is the weight of the dry sample in (g).

Apparent Porosity

The apparent porosity is directly proportional to the water absorption. The prepared lightweight geopolymer with high porosity and high water absorption has a low thermal conductivity that is considered a good thermal insulation property. The apparent porosity is estimated according to ASTM C20 [70]. The percentage of apparent porosity is the ratio between the volume of open pores to the exterior volume of the geopolymer bricks using the following equation:

$$\mathrm{A}\mathrm{p}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{P}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y},\left(\%\right)={\displaystyle \frac{{\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{V}}_{\mathrm{d}}}}\times 100$$

(3)

where W_s is the weight of the saturated sample, W_d is the weight of the dry sample, and V_d is the volume of the dry sample.

2.5.2. Mechanical Properties

The compressive strength of the lightweight geopolymer bricks was determined according to ISO 9652 standard [69]. A specimen of dimensions 50 × 50 × 50 mm³ was used for this test. Three specimens of geopolymer bricks were used to determine the average compressive strength for each mixture. The compressive strength was estimated using the following equation:

$$\mathrm{C}\mathrm{S}={\displaystyle \frac{\mathrm{F}}{\mathrm{A}}}$$

(4)

where CS is the compressive strength in MPa, F is the applied force on the specimen in (N), and A is the area perpendicular to the force in (mm²).

2.5.3. Thermal Properties

Thermal conductivity is the amount of heat (watts) flowing through the unit area (m²) per unit temperature that is perpendicular to the direction of the isothermal surface (W/m·K). The thermal conductivity of prepared lightweight geopolymer is determined using a hot wire method ranging from 0.023 to 12 W/m·K using the KD2 Pro Thermal Properties Analyzer at the Housing & Building National Research Center (HBRC, Cairo, Egypt).

3. Results and Discussion

3.1. Characterization of Raw Materials

3.1.1. Mineralogical Analysis of Raw Materials

An X-ray diffractometer (XRD) device was utilized to evaluate the phase composition of solid phases. Quartz is the primary phase found in the fired-clay brick wastes, according to XRD analysis. This is because burned clay brick wastes are often composed primarily of quartz and amorphous meta-kaolin, as they are fired at temperatures lower than 900 °C. Moreover, quartz (JCPDS#46-1045) predominated in aluminum dross wastes both before and after treatment. The following primary phases were found in the wastes from aluminum dross: corundum (Al₂O₃) (JCPDS#46-1212), halite (NaCl) (JCPDS#77-2064), aluminum magnesium oxide (Al₂MgO₄) (JCPDS#021-1152), and aluminum nitride (AlN) (JCPDS#65-0831) as shown in Figure 2. Moreover, aluminum nitride and fluorite (CaF₂) are present in trace amounts. The molten salt flux, which was utilized to shield the metal from the reactive environment during the melting process, is the source of NaCl and CaF₂. The corundum, also called α-alumina, is produced when high-temperature molten metallic aluminum reacts with oxygen from the atmosphere [60]. Aluminum nitride is created when aluminum melts in the air and combines with nitrogen in addition to oxygen. On the other side, halite (NaCl) has disappeared due to its high water solubility in the treated aluminum dross wastes, but on the contrary fluorite (CaF₂) (JCPDS#88-2301) appears in a higher percentage after treatment because it is sparingly soluble in water [71].

3.1.2. Chemical Analysis of Raw Materials

The grade of the Al scrap processed, the operational conditions, the type of technology used, and the furnace used for the Al metal production are the key factors influencing the chemical and mineralogical composition of aluminum dross [61,72,73,74,75].

Table 2. Chemical analysis of raw materials.

Component	Aluminum Dross, wt. (%)	Treated Aluminum Dross, wt. (%)	Homra, wt. (%)
SiO2	3.52	8.47	59.57
Al2O3	57.20	68.54	16.78
Fe2O3	0.72	1.89	10.41
MgO	5.25	2.08	1.23
CaO	4.43	6.43	4.37
Na2O	2.84	0.42	1.12
K2O	0.52	0.36	1.45
SO3	20.30	1.66	1.70
TiO2	0.46	0.54	1.25
P2O5	0.50	0.20	0.17
Cr2O3	0.01	0.06	0.04
SrO	0.02	0.04	0.06
ZrO2	0.01	0.02	0.06
BaO	0.11	0.19	0.04
MnO	0.15	0.15	0.05
Cl−	3.94	0.17	0.25
F−	0.001	0.94	-
Nd2O3	0.001	0.06	-
V2O5	0.00	0.13	-
NiO	0.01	0.03	-
ZnO	0.02	0.20	-
CuO	0.06	0.18	-
LOI	-	7.21	1.44
Total	100.06	99.99	99.99

displays the chemical composition of the as-received dross. It is made up of aluminum scraps, soluble salts (sodium, potassium, and chloride), and alloying elements (magnesium, silicon iron, titanium, etc.). As for the treated aluminum, some changes occurred in the proportions of its constituent elements as a result of the solubility of some salts, such as sulfur, magnesium, and sodium salts, while others are sparingly soluble in water, such as calcium and titanium salts. The primary oxides that constitute the fired-clay brick waste are SiO₂, Al₂O₃, and Fe oxides, according to an XRF analysis of the waste. Assuming logically that all alumina exists as metakaolinite (Al₂O₇Si₂), the appropriate percentage of coupled silica was computed to be approximately 20%, with the remaining 39% constituting free silica. Due to the moisture content of dross waste that remains after washing, its loss on ignition can reach over 7% in the case of treated aluminum dross waste. In contrast, that of the fired-clay bricks waste and untreated aluminum dross wastes is negligible. Fired-clay brick waste is classified as a pozzolanic material by ASTM C 618-22 [76] owing to the high amount of silica, along with alumina and iron oxides, which together make up a percentage of over 85%.

3.1.3. Sieve Analysis

The spherical lump size of the aluminum dross was found to be 28 mm. A representative sample of dross was crushed and ground. The grinding metallic aluminum was gathered, weighed, and sieved. The mean particle size of the dross was about 0.204 mm taking into account that the non-metallic product that passed through was present. The mean particle size of fired-clay waste (Homra) was determined to be 0.13 mm as shown in Figure 3.

3.2. Mechanical Properties of Prepared Geopolymer Bricks

The prepared geopolymer made from aluminum dross waste was tested for compressive strength at various concentrations of sodium hydroxide (8–12 M). The compressive strength results for different curing times with various sodium hydroxide concentrations are displayed in Figure 4. Academic research indicates that a sodium hydroxide concentration ranging from 5 M to 15 M is considered appropriate [77,78,79,80,81,82]. Throughout all curing times, the samples with a NaOH content of up to 8 M exhibited superior compressive strength. The higher amount of Na₂O in the mixture made it easier for Si and Al species to dissolve, which helped geopolymeric gels form [83]. There was a lack of hydroxyl groups (OH⁻) available for dissolving Si and Al, resulting in weakened strength. As the alkalinity increased beyond 8 M NaOH, there was a noticeable decrease in strength. The rapid and high dissolution of the aluminosilicate species may be the cause of this phenomenon. This could lead to an excessive crystallization of the aluminosilicate gel and insufficient packing of the specimens, thereby reducing the compressive strength of the bricks [77]. Mohseni et al. [42] prepared lightweight geopolymer composites using polypropylene, nano-alumina, rice husk ash, and scoria particles. The introduction of nano alumina as an additive compensated for the insufficient alumina content in the rice husk ash, potentially enhancing its mechanical properties through the formation of specific compounds and gels within the alkali-activated matrix. The addition of scoria in varying amounts from 0% to 20% leads to a decrease in compressive strength and density [84]. A high alumina content can cause a less uniform microstructure and cracks due to the presence of undissolved particles within the material’s gelatinous matrix. This, in turn, can have a negative impact on the compressive strength. High aluminum content leads to the formation of an octahedral aluminum structure, which the gel network cannot incorporate [85]. In a study conducted by Kouamo et al. [86], the impact of incorporating varying amounts of alumina as substitutes for kaolin on compressive strength was examined. The results revealed that a 20% alumina content yielded the highest increase in compressive strength, with an improvement of 18.1% compared to kaolin geopolymer [59].

Figure 5 illustrates that at 8 M, using 3% aluminum dross waste as a replacement, achieved the highest level of compressive strength. In addition, it was discovered that as the curing time increased, the compressive strength also increased. The optimum sample recorded a compressive strength of 8.58 MPa after 28 days of curing, meeting the requirements of ASTM C62 for Egyptian standards [87], which specifies a minimum strength of 8.6 MPa. Figure 5, Figure 6 and Figure 7 demonstrate the significant improvement in compressive strength of geopolymer brick foams when aluminum dross waste is utilized. This improvement is achieved by extending the curing times at room temperature for different NaOH concentrations (8 M, 10 M, and 12 M). The correlation coefficient was used in the Excel program to estimate the impact of molarity, curing time, and aluminum dross waste replacement percentages on the compressive strength. The results are shown in

Table 3. Correlation of aluminum dross geopolymer.

	Aluminum Dross Waste	Molarity of NaOH	Curing Time	Compressive Strength
Aluminum Dross Waste	1	0
Molarity of NaOH	0	1
Curing Time	0	0	1
Compressive Strength	−0.596024922	−0.241183062	0.599677	1

.

The findings clearly indicate that the waste replacement percentages have a greater influence on the compressive strength than the molarity of NaOH. Extending the curing time yields favorable results for the compressive strength, whereas the concentration of NaOH and substitution of waste materials have an adverse impact on it.

3.3. Physical Properties of Prepared Geopolymer Bricks

3.3.1. Bulk Density of Prepared Geopolymer Bricks

Figure 8, Figure 9, Figure 10 and Figure 11 show how different NaOH concentrations and curing durations affected the bulk density of the geopolymer brick samples made from aluminum dross waste. At different curing durations, the highest bulk density was achieved when using 8 M NaOH. The concentration of alkaline solution plays a vital role in assessing bulk density variations due to changes in geopolymerization and porosity [31]. Figure 8 demonstrated that the bulk density rose as the concentration of sodium hydroxide increased, but it was observed that the bulk density decreased as the curing times increased. This phenomenon is caused by a cumulative reduction in the water content of the samples, which leads to a progressive creation of pores as a result of the loss of water molecules attached to the amorphous phase during the production of the geopolymer. This is confirmed by the fact that materials with a higher NaOH concentration tend to lose less water and have higher bulk density results. This yielded a maximum density of 1.89 g/cm³, which agrees with the compressive strength. The increase in alumina, which made the brick samples more compact and less porous, may be the cause of increasing the density. The bulk density of 8M, which has a replacement of 3% aluminum dross, is 1.307 g/cm³.

3.3.2. Water Absorption and Apparent Porosity

One of the most important parameters used to determine the durability of the bricks and the surface porosity is water absorption [88]. Porosity refers to the number of pores within the matrix that affect the strength, mass transport, and efficiency of thermal insulation [89]. The variation in cold and boiling water absorption and apparent porosity is illustrated in Figure 12 and Figure 13. Figure 12 shows the effect of dross addition on the water absorption with different concentrations of NaOH (8, 10, and 12) at a curing time of 28 days. Water absorption and porosity increase with the increasing percentage of aluminum dross with a linear behavior that confirms the efficiency of aluminum dross as an agent of pore-forming. Further increasing the concentration of NaOH causes a decrease in water absorption and porosity due the enhanced formation of aluminosilicates, which causes the structure of bricks to become denser and also increases the compressive strength as well as the water resistance [90]. The recommended concentration of NaOH is 8M to comply with the standard specification of brick grade.

The saturation coefficient (SC) is used to estimate the durability of the prepared bricks when they are saturated with water. SC is the ratio between the cold water absorption after immersing for 24 h and the boiling water absorption for 5 h. The range of SC is between 0.4 and 0.95, the high values of SC indicating low durability and vice versa [91]. Figure 14 illustrates the saturation coefficient of the prepared bricks at different concentrations of NaOH after 28 days. As expected, increasing the dross addition causes a decrease in the saturation coefficient due to the pore formation and decreases with the increase in the concentration of NaOH as discussed in the previous paragraph. The limit value of the saturation coefficient according to ASTM C62 is 0.9, which complies with the addition of aluminum dross by more than 2%.

3.4. Thermal Conductivity

The thermal conductivity of the prepared geopolymer bricks was determined after 28 days of curing time as illustrated in Figure 15. It can be observed that the control geopolymer brick at zero aluminum dross replacement showed the highest value of the thermal conductivity of 0.583 W/m·K; this is due to the fewer pores present in the brick, which makes the brick denser as illustrated in Figure 15. The maximum reduction in the thermal conductivity was about 48% at the maximum dross addition of 5%. The lowest thermal conductivity (0.25 W/m·K) was obtained for 5% dross addition using 8M of sodium hydroxide. Increasing the addition of aluminum dross caused a decrease in the thermal conductivity as a result of the creation of pores due to the reaction between the alkaline activator (NaOH and Na₂SiO₃) and the aluminum present in dross that releases hydrogen, which is responsible for pore formation. Thermal conduction is the process of heat transfer through the microscopic movement of matter. Atomic vibration causes the production of phonons, which are responsible for the thermal conductivity of solids due to their transportation. The scattering of phonons is highly affected by the free path that reduces the heat transfer. The main sources of scattering are pores, additives, and isotopes [92]. Increasing the dross addition causes an increase in the creation of pores that increase the phonon scattering and decrease the free path of phonons, resulting in a decrease in the thermal conductivity.

Table 4. Comparative summary with the previous studies.

Raw Material	Pore Formation	Compressive Strength, (MPa)	Thermal Conductivity, (W/m·K)	Bulk Density, (g/cm3)	Reference
Homra	Aluminum dross	2.42–14.74	0.25–0.583	1.29–1.73	Current study
Metakaolin		33	0.4	-	[93]
Coconut ash	Aluminum slag	-	0.045	0.6	[94]
Dealuminated kaolin	Aluminum trimmings	1.4	0.26	1	[59]
Ferrosilicon slag	Aluminum waste	3.8–10.9	0.28–0.59	0.71–1.5	[58]
Metakaolin	H2O2	1.2–7	0.1–0.4	0.6–1.2	[95]
Fly ash	Al	5.5–10.9	0.25–0.39	0.8–0.93	[96]

presents a comparison of the results of the prepared geopolymer bricks with the previous studies including compressive strength, bulk density, and thermal conductivity.

3.5. Microstructure Analysis and XRD of the Optimum Sample

Figure 16a,b show the microstructure characteristics of the prepared geopolymer brick using 3% and 0% aluminum dross replacement and 8 M of NaOH after 28 days, respectively. Obviously, the prepared brick that contains 3% aluminum dross has a homogenous structure and a uniform distribution of micropores throughout the matrix. There is a direct relation between the number of pores and the percentage of aluminum dross replacement. The pore formation that appears in the optimum sample is due to the reaction of aluminum dross with NaOH and the releasing water, which leaves pores after evaporation according to the following reaction [97].

$${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=2\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(5)

It is clear from the two figures that the prepared sample with the optimum conditions has a much more porous structure than that containing 0% dross waste replacement.

The different phases present in the optimum geopolymer-prepared sample, 3% aluminum dross replacement, and 8 M of NaOH after 28 days were determined and are shown in Figure 17. The main phases were quartz (SiO₂), mullite (3Al₂O₃·2SiO₂), and magnesium aluminate (Al₂MgO₄); these phases are consistent with the reaction in Equation (5), which predicted the same results.

3.6. Energy Simulation of the Prepared Insulation Brick

The DesignBuilder simulation software is an excellent tool for assessing the thermal and energy performance of building materials. Energy simulations were carried out in this work for two model rooms with the dimensions 4 m long × 4 m wide × 3 m high illustrated in Figure 18 using DesignBuilder (V.6.1.0.6). Cairo, Egypt’s climate zone, as described in EREC [92], was taken into account; this zone is distinguished by a moderate desert climate. A room constructed with the prepared insulating bricks had its annual energy consumption measured, and the findings were compared to a room constructed using traditional/dominant cement hollow blocks. The thermal properties of the two wall structures that were assessed for this investigation are shown in

Table 5. Thermal properties of simulation bricks.

Brick/Block Type	Thickness (m)	Density, (kg/m3)	Thermal Conductivity, (W/m·K)	Thermal Resistance (R-Value), (m2K/W)	U-Value, (W/m2·K)
Hollow Cement Brick	0.2	1700	1	0.415	2.407
Prepared Insulation Brick	0.2	1307	0.32	0.882	1.134

.

A south-facing model room was used for this study. The following is a summary of the simulation setup’s primary assumptions:

• There was split air conditioning in the room (no fresh air).
• The set point, or thermal comfort, was 24 °C.
• The other kinds of equipment and the lightning were disregarded.
• The hours of operation were 6 a.m. to 7 p.m.
• There was a single transparent 6 mm glassy material, and the window-to-wall ratio was 10%.

Figure 19 displays the monthly cooling energy consumption (kWh) statistics for the two-room simulated models as well as the outdoor dry bulb temperatures (°C). The prepared insulating bricks demonstrated superior energy performance in comparison to the conventional cement hollow blocks, as predicted. The room constructed with the prepared bricks uses 475.11 kWh of cooling energy annually, which is 20% less than the room constructed with cement hollow blocks, which used 599.75 kWh. When compared to conventional clay bricks, the lightweight bricks that were constructed demonstrated superior energy efficiency [98]. This is attributed to the developed bricks’ higher thermal resistance, which is made possible by their large number of air voids. These air voids help to attenuate heat flow into the building, which lowers the indoor cooling loads needed to provide an acceptable level of thermal comfort for its occupants. This, in turn, helps to minimize CO₂ emissions and reduce energy consumption for cooling in hot climate regions [99]. These outcomes align with those of an earlier investigation that verified the rise in thermal resistance of cement blocks with different proportions of date palm ash (DPA) in place of cement. In comparison to the control block (which does not contain DPA), the blocks containing 30% DPA showed an approximate 47.4% increase in thermal resistance [98].

4. Conclusions

For insulation purposes, a geopolymer brick should ideally have a low bulk density and low thermal conductivity while maintaining a reasonable mechanical strength. In light of the experimental findings, it appears that mixing clay brick waste (Homra) with 3% aluminum dross waste replacement using 8 M of NaOH with a curing time of 28 days produces lightweight, thermally insulating geopolymer bricks with a compressive strength of 8.58 MPa, a bulk density of 1.307 g/cm³, an apparent porosity of 29.79%, and thermal conductivity of 0.32 W/m·K. The prepared bricks’ compressive strength and bulk density decrease as the amount of aluminum dross waste replacement rises, and more pores form, improving the bricks’ thermal insulation. The prepared geopolymer bricks have an economic advantage because their primary components are recyclable waste materials, specifically aluminum dross and fine clay brick waste. The prepared insulation brick with 3% aluminum dross replacement was energy-simulated using DesignBuilder (V.6.1.0.6). The simulation results indicated that the prepared insulation brick contributes to decreasing the energy consumption of the air conditioner. About 21% less energy was used. This is due to the strong heat resistance caused by the addition of aluminum dross to the prepared geopolymer. In the future, feasibility studies will be performed to evaluate the price of the prepared geopolymer against conventional bricks.