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Article

Eco-Efficient Geopolymer Bricks Without Firing and Mechanical Pressing

by
Muhammad Hassan Javed
1,2,
Qasim Shaukat Khan
1,
Asad Ullah Qazi
1,
Syed Minhaj Saleem Kazmi
3 and
Muhammad Junaid Munir
3,*
1
Civil Engineering Department, University of Engineering and Technology, Lahore 54890, Pakistan
2
Communication and Works Department Punjab, Lahore 54000, Pakistan
3
School of Mechanics and Construction Engineering, Jinan University, Guangzhou 510632, China
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(2), 762; https://doi.org/10.3390/su18020762
Submission received: 5 December 2025 / Revised: 6 January 2026 / Accepted: 8 January 2026 / Published: 12 January 2026
(This article belongs to the Special Issue Advancements in Sustainable Composite Materials: From Innovative Technologies to Eco-Friendly Design—2nd Edition)
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Abstract

Kiln-fired clay bricks are energy-intensive and carbon-heavy. This study develops and validates kiln-free, pressure-free, and ambient-cured geopolymer (GPM) bricks made from uncalcined clay and Class F fly ash. A two-stage experimental program screened 33 mixes (12–16 M NaOH and 396 cubes tested at 14–90 days) and then scaled six optimized mixes to 90 full-size bricks for mechanical, durability, and microstructural evaluation. Bricks with an optimal mix of 20–30% clay and 70–80% fly ash achieved a compressive strength of up to 32.5 MPa, satisfying ASTM C62 (for severe weathering) requirements. Relative to fired clay units, GPM bricks delivered +61% average compressive strength (up to +91%), +56.5% average modulus of rupture (up to +103%), 6–29% lower water absorption, and 42–84% higher UPV while their strength losses after 28-day immersion in 5% H2SO4 or 3.5% NaCl were only ~3–5%. SEM confirmed a dense N-A-S-H gel matrix with reduced porosity. Eco-efficiency analysis showed ~95% lower embodied CO2 (0.26–0.31 vs. 5.5 kg eCO2 per brick) and ~35% lower cost per MPa of strength than fired clay bricks. The findings demonstrate a practical, low-carbon brick manufactured without mechanical pressing or heat curing, delivering verified performance and durability under ambient conditions.

1. Introduction

In the last two decades, climate action due to rising greenhouse gas emissions (GHG) has attracted global attention due to the increasing surface temperatures of Earth. GHG emissions are the primary reason behind climate change due to global warming, resulting in rapidly melting glaciers, flash flooding, sporadic heat waves, acute food shortages, and the spread of harmful diseases. In developing countries, brick production is one of the major causes of GHG emissions. Clay bricks also stand as the oldest and most widely utilized construction material on earth since ancient times [1,2], with global production exceeding 1.3 trillion units annually [3]. Pakistan also emerges as a prominent contender in South Asia, securing the third position in regional output. The scale of Pakistan’s contribution to this industry is substantial, with an annual production of approximately 45 billion units, facilitated by around 18,000 brick kilns [4]. Despite its economic significance, the exponential rise in brick production has raised concerns over the depletion of the natural fertile clayey soil layer, making the brick an unsustainable construction material [5].
The production of clay bricks involves mixing clay with water to form a viscous clay mix and poured into molds, to attain the desired shape and size of wet bricks. These wet bricks are burnt at temperatures of around 900–1100 °C in brick kilns to bind the natural oxides of silica and alumina, which imparts strength to the brick [6]. The addition of wastes in the brick mix, i.e., waste glass, rice husk ash, wood ash, and olive mill reduced the firing temperatures up to 850 °C [7,8]. The strength of burnt clay bricks mainly depends on oxides of alumina and silica in precursor, mixing process and firing temperatures [9]. Increasing firing temperatures enhances the mechanical properties of the burnt clay bricks. However, firing clay bricks at high temperatures emits harmful GHG such as carbon dioxide (CO2), carbon monoxide (CO) and particulate matter (PM 2.5) in the environment. The presence of GHG in the environment deteriorates the Air Quality Index (AQI) and causes air pollution [10,11,12].
In South Asia, clay brick production releases staggering 127 million tons of CO2 annually, with Pakistani brick kilns standing out as a major source of CO2 emissions, each emitting approximately 14,000 tons of CO2 each year [13].
Previous studies investigated the mechanical properties of burnt clay bricks and reported that compressive strength (CS) increases with the firing temperature. Tsega et al. [14] reported that burnt clay bricks fired at 1200 °C for 8 h achieved a CS of about 12.5 MPa. Charai et al. [15] reported that the firing temperature of 880 °C for 24 h was required to attain thermal stability of burnt clay bricks, and the firing process enhances their mechanical strength.
In the last decade, geopolymer, a green and sustainable building material, has been utilized in manufacturing bricks. Geopolymerization is initiated by an alkaline activator (AA) solution comprising different quantities of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solutions, and strength is gained through geopolymerization, resulting in the development of sodium-alumino-silicate hydrate (NASH) gel [16,17,18]. Ayeni et al. [19] developed metakaolin-based geopolymer mortar (GPM) bricks using a 10 M NaOH solution cured at varying temperatures (25 °C, 40 °C, and 60 °C). The CS of developed bricks increased from 5.6 MPa to 11.1 MPa with an increasing curing temperature from 25 °C to 60 °C, resulting in increased dissolution of alumina and silica ions and formation of dense geopolymer gel. Adam et al. [20] noted an optimum CS of 32.5 MPa, achieved at heat curing of 120 °C for 20 h. The rate of the geopolymerization reaction increased with higher curing temperature and enhanced the compressive strength (CS) [21,22]. Madani et al. [23] reported a 75 MPa strength using industrial residue of washed sand and gravel with 8 M NaOH and 30% calcium hydroxide. Singh et al. [24] utilized red mud and fly ash, achieving 11.1 MPa with 30% red mud and 10 M NaOH, noting improvements when replacing sand with GGBS. Amin et al. [25] attained 9 MPa using ceramic dust waste with 1% NaOH and 10% calcium hydroxide. Ibrahim et al. [26] synthesized geopolymers using a combination of sodium hydroxide with potassium hydroxide, achieving 5–15 MPa strength after 24 h of curing.
Kaur et al. [27] reported a 56.9% increase in the CS (25.6 MPa to 40.4 MPa) of heat-cured GPM with NaOH molarity increment from 12 M to 16 M. Sukmak et al. [28] investigated a GPM brick comprising 70% clay (CL) and 30% FA, heat-cured at 75 °C for 48 h and achieved a CS of 12.5 MPa.
Liu et al. [29] reported that FA rich in silica exhibited higher reactivity and achieved the desired CS without heat curing. Mahdi et al. [30] prepared geopolymer paver blocks using rice husk ash and fly ash. A CS of about 44.3 MPa was achieved, using 5% brick kiln rice husk ash and 12 M NaOH solution. Chindaprasirt & Rattanasak [31] investigated the use of high calcium fly ash in the production of fire-resistant geopolymer bricks. A CS of 47 MPa was achieved after curing at 40 °C.
In the existing investigations, several studies have extensively reported the mechanical and durability properties of GPM bricks, typically manufactured by applying molding pressures or extended heat curing periods of approximately 24 h. It is pertinent to note that both molding pressure and heat curing pose significant challenges, making brick production economically and environmentally unviable and operationally complex, particularly in on-field settings. To address these issues and pave the way for more sustainable practices, this experimental investigation develops an eco-friendly GPM brick utilizing industrial waste, i.e., fly ash while eliminating the need for molding pressure and the traditional heat-curing process. Accordingly, this approach is a more practical and feasible manufacturing process that can be readily implemented in various construction contexts, as full-scale bricks are being studied in this research work.

2. Materials and Methods

The experimental work of this research consisted of two parts, i.e., a pilot study and main testing. The mix designs comprised 80% solids (clay and fly ash) by mass and 20% liquids (alkaline activator solutions) by mass. The pilot study was designed to determine the optimum molarity of alkaline activator (AA) solution and percentage replacement of clay (CL) with fly ash (FA) to achieve the minimum required dry compressive strengths (CS) of geopolymer mortar (GPM) bricks of 14 MPa. The pilot study comprised 33 GPM mixes, classified into three groups. Group I comprised 11 GPM mixes (xx% CL and (100-xx)% FA, xx = 0, 10, … 100), i.e., 100% CL, 90% CL + 10% FA, …, 10% CL + 90% FA, and 100% FA, prepared using 12 M AA solution. Similarly, Group II comprised 11 GPM mixes prepared with 14 M AA solution, and Group III comprised 11 GPM mixes prepared with 16 M AA solution. In this study, the molarity of AA ranging from 12 M to 16 M was selected based on the previous study [21], which reported that the CS was significantly increased with increasing molarity from 12M to 16M. The mix design of cubes for a cubic meter (1 m3) comprising 80% solids, i.e., CL and FA contents, and 20% liquids, i.e., sodium hydroxide (NaOH) solution and sodium silicate (Na2SiO3) solution, has been presented in Table 1. It is noted that GPM bricks prepared using a 10 M NaOH solution and Sahiwal CPP FA crumbled due to the reduced NaOH molarity. In this experimental research, three NaOH molarities, i.e., 12 M, 14 M, and 16 M, were selected. Moreover, no molding pressure and heat curing were utilized. Each GPM mix comprised 12 cubes of 50 × 50 × 50 mm. A total of 11 mixes × 3 AA concentration × 12 specimens = 396 specimens was made. From each mix, three cubes were tested in compression at 14, 28, 56, and 90 days.
The main testing comprised six GPM mixes selected from the pilot testing. Each mix comprised fifteen GPM bricks of 228 × 114 × 75 (6 mixes × 15 bricks = 90 specimens). GPM bricks from each mix were tested for weight per unit area, water absorption, efflorescence, ultrasonic pulse velocity (UPV), CS, modulus of rupture, and acid and saline attacks.

2.1. Raw Materials

The raw materials of the GPM brick comprised CL, FA, and AA solution. CL was procured from a site situated in Lahore, Pakistan. The wet sieve analysis of CL was carried out to determine particle size distribution (PSD) as per ASTM D6913/D6913M (2017) [32]. The PSD showed that more than 70% of the particles were finer than 0.01 mm. The liquid limit (LL) and plastic limit (PL) of CL were determined as per ASTM D 4318 (2018) [33]. The LL of CL was 27, the PL of CL was 19, and the plasticity index of CL was 8. The sieve analysis results exhibited that CL was non-expansive in nature and was graded as silty clay. The chemical analysis of CL was conducted, and it showed that clay consists of CaO (6.7%), MgO (1.2%), SiO2 (74.9%), SO3 (2.4%), Al2O3 (5.4%), and Fe2O3 (2.2%). The loss on ignition (LOI) was 7.2%. The XRD analysis exhibited that silty clay used in this experimental work mainly comprises quartz (silica), followed by feldspar, anorthite, and calcite. The SEM and XRD of silty clay are presented in Figure 1 and Figure 2, respectively.
The non-expansive soil was used to avoid expansion and shrinkage cracks in the produced bricks. The clay was uncalcined to reduce the carbon footprint and cost of producing GPM brick. In the existing literature, Iftikhar et al. [6] prepared GPM bricks using uncalcined clay with FA and achieved a CS of 17.1 MPa. Similarly, Sukmak et al. [28] used uncalcined clay and achieved desired strengths. In general, clay is enriched with oxides of silica and alumina and can effectively be used in geopolymerization [6]. The clay clusters and clay particles easily slide over each other, which increases the workability of the mix [28].
FA was sourced from Sahiwal Coal Power Plant, Punjab, Pakistan. FA was grayish in color. The chemical composition of FA. was determined, as per ASTM C311/C311M (2019) [34]. FA consists of CaO (3.9%), MgO (1.9%), SiO2 (80.7%), SO3 (1.1%), Al2O3 (5.5%), and Fe2O3 (3.1%). The LOI was 3.8%. FA. was categorized as Class F as per ASTM C 618 (2019) [35]. The XRD analysis exhibited that FA. particles primarily comprised quartz (silica), hematite (iron), and mullite (alumina) (Figure 2). The SEM analysis showed that FA particles were spherical in shape with smooth surfaces (Figure 3).
In this research investigation, the range of NaOH molarities from 12 M to 16 M was chosen founded on the broad review of research sources and earlier published experiments. To prepare the AA solution, solid flakes of NaOH were obtained from a local vendor having 98% purity. The 12 M, 14 M, and 16 M solutions were prepared by mixing 36.6%, 42.75% and 48.85% NaOH solids in 1000 mL (1 L) of water. Sodium silicate (Na2SiO3) was obtained in the liquid form and comprised 46% solids and 54% liquids. In this study, the AA solution comprised Na2SiO3 solution and NaOH solution ratio of 1.5 to 1.0. Ghafoor et al. [33] reported that Na2SiO3/NaOH of 1.5 exhibited the optimum CS of GPC.
To prepare a GPM mix, required quantities of FA and CL were dry mixed for 60 s in a pan mixer. Next, the AA sol. was poured over the dry mix. In addition to AA, tap water (4% of fly ash by weight) was poured into the mix to obtain a homogenous and workable mix; the additional water acted as a lubricant. The mix with 16 M NaOH solution exhibited reduced workability; however, 16 M mix exhibited adequate workability to fill the molds without segregation or honey combing issues [21,22]. The prepared mix was poured into the molds and compacted using a vibrating table for 30–60 s. Moreover, the tamping rod was also used to aid in filling the molds and provide proper filling and compaction. The tamping rod and vibration were used to remove the air voids and have better mold filling with GPM. No molding pressure was used in this study to reduce the CO2 footprints of brick production. Afterwards, the molds were placed in the Concrete Laboratory at room temperature (23 + 2 °C) for 24 h. The samples were demolded after 24 h and were ambient cured for the required number of days.

2.2. Testing Procedures

The physical and mechanical properties of GPM bricks, i.e., weight per unit area, efflorescence, compressive strength, and modulus of rupture (MOR) tests, were performed according to ASTM C67 (2021) [36]. The water absorption test was conducted as per C830-16 (2016) [37]. The ultrasonic-pulse-velocity (UPV) test was conducted as per the guidelines of ASTM C 597 (2016) [38]. The acid attack test and salt attack test on GPM bricks were carried out according to the procedures outlined in Shakir et al. [39]. To determine weight per unit area, the length and width (measured both at the top face and the bottom face) of bricks were measured using vernier calipers (least count 0.1 mm). The weight of the brick was determined using a weighing balance having a least count of 5 g.
To conduct the water absorption of GPM bricks, five bricks from each mix were immersed in a bucket filled with water for 24 h so that the majority of permeable pores in the bricks were filled with water, which gives indirect information about the porosity in the bricks. After 24 h, bricks were taken out of the bucket, and the water on the surface of the brick was cleaned off with a cloth, and the wet weight of the bricks was noted. Afterwards, bricks were placed in an electric oven at a temperature of 110 °C for 24 h. After 24 h, the dry weight of the bricks was measured and documented. The water absorption was determined as a ratio of the difference in wet and dry weights to the dry weight of the brick.
The efflorescence of GPM bricks was determined as per ASTM C67-21 (ASTM 2021) [36]; two pairs of five GPM bricks from each mix were taken. One pair of bricks was placed in a dry place (controlled bricks). The other pair of bricks was dipped in water up to 25 mm depth in a stainless-steel pan for 7 days. After 7 days, the immersed bricks were dried in an oven at 60 °C for a duration of 24 h. Afterwards, salt deposition on the immersed bricks was compared with the controlled pair of bricks to examine the efflorescence.
The UPV test was conducted to determine the uniformity and homogeneity of GPM bricks. To measure UPV, five bricks from each mix were taken. First, the side faces of the brick were cleaned, and gel was applied. Two transducers were attached to the opposite sides of a brick (Figure 4). The straight distance between the two sides of the brick was measured with a vernier caliper, and the time in microseconds, that is, the required time for the sound wave to travel from one end to the other, was noted.
A total of five bricks from each mix were tested in compression in 100 tons Shimadzu Universal-Testing-Machine (UTM), having the lowest count of 0.1 ton to determine the CS of bricks (Figure 5). The crushing load was noted, and the CS was derived.
To determine the MOR of GPM bricks, five bricks from each mix were tested in UTM at a loading rate of 1 mm/min in three-point loading. A brick was placed on two rollers, reproducing a simply supported condition, with a span of 114 mm (Figure 6). The loading was applied at the midspan via a steel roller on the top of the brick. The MOR of the brick was determined using the failure load.
For acid attack and salt attack tests, the dimensions (length, width, and height) of GPM bricks were measured using a vernier caliper having a least count of 0.05 mm, and the weight of the bricks was determined using a weighing balance in grams. Afterwards, one set of the brick samples was immersed in 5% sulfuric acid solution for 28 days, and the other set of bricks was immersed in a 3.5% sodium chloride solution for 28 days. After 28 days, the dimensions and weight of bricks were remeasured, and the loss in volume and weight was determined. The CS was also determined to observe the loss in compressive strength of GPM bricks.

3. Results and Discussion

3.1. Pilot Study Results

The average CS of the pilot study at 14, 28, 56, and 90 days of Group-I, Group-II, and Group-III is presented in Figure 7, Figure 8, and Figure 9, respectively. As shown in Figure 7, the CS of specimens in Group I increased with the percentage replacements of CL with FA up to 70%. Among those, Mix 30% CL-70% FA exhibited the highest CS of 36.1 MPa and 42 MPa at 14 day and 28 day, respectively. Mix 20% CL-80% FA achieved the highest CS of 49.2 MPa and 49.7 MPa at 56 day and 90 day, respectively. This observation indicates that the optimal mix consists of about 20–30% CL and 80–70% FA. The key governing factors in the strength development of geopolymers are FA content and molarity of the AA.
The improved compressive strength of Mix 20% CL-80% FA over Mix 30% CL-70% FA at later ages can be attributed to the slow geopolymerization process under ambient conditions, where the first one might require more time to achieve its full strength due to a higher content of FA. Sukmak et al. [28] also reported that a lower FA/CL ratio in GPM mix requires a longer duration to achieve a higher strength. The FA requires a higher time to dissolve, which is responsible for the low early strength development of FA-based geopolymers cured at ambient temperature [40]. Therefore, geopolymers can gain sufficient strength without significant energy requirements [41].
It was observed that the average CS of GPM cubes increased by 36.8%, 14% and 2%, respectively, as curing duration was increased from 14 to 28 days, 28 to 56 days, and 56 to 90 days. This observation indicates that these bricks could gain about 16% additional CS after the standard duration of 28 days, and its CS almost stabilized at 90 days.
Similarly to Group-I, the CS of specimens in Group-II also increased with the percentage replacements of CL with FA up to 80% (Figure 8). Among those, Mix 20% CL-80% FA exhibited the highest CS of 49.4 MPa, 52.3 MPa, 53.2 MPa and 52.8 MPa at 14 days, 28 days, 56 days and 90 days, respectively. It was observed that the average CS of GPM bricks increased by 17.8%, 9.9% and 7.1%, respectively, as curing duration increased from 14–28 days, 28–56 days and 56–90 days. The CS of GPM bricks in Group-II increased by about 17% after 28-days, which was marginally higher than that of Group-I. Particularly, these bricks could gain nearly 8% after 56 days. This observation is very different from Group-I. The higher concentration of the AA could have contributed to strength gain at a later age.
Similarly, the CS of specimens in Group-III increased with the percentage replacements of CL with FA up to 70–80%, as shown in Figure 9. Mix 20% CL-80% FA exhibited the highest CS of 33.2 MPa and 52.2 MPa at 14 days and 28 days, respectively. Mix 30% CL-70% FA achieved the highest CS of 65.6 MPa and 68 MPa at 56 days and 90 days, respectively. It was observed that the average CS was increased by 49.6%, 18.3% and 2.3%, respectively, as the curing duration was increased from 14–28 days, 28–56 days, and 56–90 days. For this group, the strength gain after 28 days was about 21%, which is higher than the other two groups. The fastest strength gain (18.3%) was observed during the period between 28 and 56 days, and this phenomenon stabilized after 56 days. The experimental results confirm that clay-only systems have limited geopolymerization and exhibit low compressive strength because only silica and alumina ions on the surface of the CL participate in the geopolymerization reaction.
It is pertinent to note that GPM mixes with high clay content (60–80%) exhibited lower compressive strengths as compared to GPM mixes with low clay content (20–30%), as the contribution of uncalcined clay in the geopolymerization process at ambient curing is low.
To illustrate the strength development over time, Figure 10, Figure 11 and Figure 12 present the CS from 14 to 90 days, considering different clay replacements with FA and various concentrations of AA (12–16 M). It was observed that the CS progressively increased, almost linearly, up to 56 days and slowed down till 90 days. This trend was observed for various clay replacements with FA and different concentrations of AA (12–16 M). The Mix 30% CL-70% FA achieved the highest strength at 56 and 90 days, while the Mix 20% CL-80% FA consistently demonstrated strong performance across all durations, indicating stability in strength development over time. Higher CS is achieved for those geopolymer mixes that have a higher ratio of Si/Al. The mix having more FA contains a higher Si/Al ratio. This was due to the more readily available silica and alumina in FA compared to clay and higher leaching capacity [28].
The obtained experimental results exhibited that the CS of GPM increased with the molarity of the NaOH solution, and the CS of Groups II and III were higher than that of Group I. In general, Group-III exhibited the highest strength gain over time. The higher molarity of the AA solution is the least environmentally friendly, has higher associated costs, and is more suitable for industrial applications as compared to manual production, as worker safety and handling is a concern with higher molarity. The selection of the mix design for the main test took into account both mechanical properties and environmental impact.

3.2. Main Testing Results

Established on the pilot test results, six GPM mixes were chosen to undergo the main testing. These selections were made considering different aspects. From each of the three groups (Groups I, II and III), one GPM mix was identified based on its optimum compressive strength, while another GPM mix was chosen for its economical attributes, maintaining a comparable CS to the desired target of 14 MPa at 28 days (Table 2).
The properties of conventional fired clay (control bricks) and GPM bricks were compared.
Accordingly, for their remarkable CS performance, Mix P70F12M (consisting of 30% CL-70% FA) from Group-I, Mix P80F14M (with 20% CL and 80% FA) from Group-II, and Mix P80F16M (containing 20% CL and 80% FA) from Group-III were selected for the main testing.
On the other hand, in terms of the economic perspective, Mix E60F12M (comprising 40% CL and 60% FA) from Group-I, Mix E70F14M (containing 30% CL and 70% FA) from Group-II, and Mix E60F16M (with 40% CL and 60% FA) from Group-III were selected to undergo main testing. The details of these six mixes are given in Table 2.

3.2.1. Weight per Unit Area

The weight per unit area (WPA) of GPM bricks exhibited a slight reduction compared to that of the control clay bricks. Specifically, the WPA of the GPM bricks of Mix P70F12M (131.3 kg/m2), Mix P80F14M (130 kg/m2), Mix P80F16M (130.1 kg/m2), Mix E60F12M (132.1 kg/m2), Mix E70F14M (131 kg/m2), and Mix E60F16M (131.3 kg/m2) were 1%, 2%, 1.9%, 0.4%, 1.2%, and 1% smaller, respectively, than the control bricks (Figure 13). It is worth noting that the variance in WPA between the GPM bricks and the control clay bricks is minimal and can be considered negligible.

3.2.2. Water Absorption (WA)

The water absorption (WA) characteristics of GPM bricks outperformed those of fired clay bricks. The WA of Mix P70F12M (10.47%), Mix P80F14M (10.16%), Mix P80F16M (7.90%), Mix E60F12M (11.79%), Mix E70F14M (9.18%), and Mix E60F16M (9.63%) GPM bricks were 6.4%, 9.2%, 29.4%, 3.6%, 18%, and 13.9% lower, respectively, compared to conventional clay bricks (Figure 14). A similar trend was observed by Aiken et al. [42], who reported that FA bricks are structurally dense, impermeable, and chemically stable, while ASTM C62 (2021) [43] does not provide a specific upper limit of WA of bricks. Meanwhile, both IS 1077 (1992) [44] and CNS382 (2007) [45] specify the upper limit of WA of bricks as 15%. Notably, all the tested GPM bricks in this study exhibited significantly lower WA than the 15% threshold, underscoring their favorable performance in this regard. The GPM bricks exhibited the lowest water absorption as compared to other FA-based bricks.
It was evident that GPM bricks having higher FA contents exhibited lower WA. Sukmak et al. [28] previously highlighted this relationship, explained that the addition of FA to the clay mix decreased the LL. This decrease in LL contributed to enhancing compaction and subsequently decreased the WA of the CL-FA mix.
The GPM bricks having higher molarity exhibited a clear reduction in WA. The increase in molarity of NaOH solution from 12 M to 16 M resulted in about 21.3% reduction in WA. An increase in the molarity of NaOH solution resulted in increased dissolution of alumina and silica ions, which fostered the geopolymerization phenomenon and hence increased geopolymerization, i.e., sodium alumino silicate hydrate (NASH) gel formation.

3.2.3. Efflorescence

During efflorescence, the white-salts appeared on the brick surface, comprising unreacted carbonates and hydroxide (OH−) ions. The unreacted OH− ions migrate towards the surface through internal pore channels, facilitated by the evaporation of surface water. Moreover, OH− ions on the surface react with atmospheric CO2 and form whitish carbonate precipitation on the surface. The degree of efflorescence is closely related to the silica to alumina (Si/Al) ratio and internal pore structure [46].
The efflorescence observed on all six mixes of GPM bricks was lower than 10% of the brick area (Figure 14). Hence, GPM bricks can be categorized as lightly efflorescent bricks. Notably, GPM bricks that have a higher-molarity of NaOH solution, i.e., 16 M, exhibited relatively more efflorescence than GPM bricks with a lower-molarity NaOH solution, i.e., 12 M. A comparison of efflorescence bricks with the control bricks is presented in Figure 15.

3.2.4. Ultra Sonic Pulse Velocity (UPV)

The UPV of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M GPM bricks was 57.9%, 68.4%, 84.2%, 42.1%, 47.3%, and 47.4% higher than that of conventional clay bricks, respectively (Figure 16). The UPV enhancement can be attributed to the augmented content of FA within GPM bricks. The spherical and round shape of the FA particles contributes to the enhanced fluidity and compactness of the mix, and geopolymerization [47].
The velocity of the stress wave in mortar can be calculated as V = E c ρ , where ρ is the density of mortar. When the density of mortar remains almost unchanged, the UPV is proportional to the elastic modulus, which is related to the compressive strength of GPM. This correlation is valid for similar material systems, mix design, same curing conditions, similar procedures and assumptions of material homogeneity and constant density are applied during making these correlations.
Among these GPM bricks, the Mix P80F16M brick exhibited the highest UPV of 3.5 km/s, corresponding to its highest strength. This measure also falls in a reasonable range, e.g., the modulus of these bricks was about 22 GPa, and their density was approximately 1900 kg/m3. Accordingly, the stress wave velocity can be estimated using the equation above V = 22 ×   10 9 1900 =   3402   m / s , which is very close to the measured value of 3.5 km/s (3500 m/s). The higher UPV of GPM bricks than control bricks is attributed to the increased geopolymerization process, which resulted in a dense microstructure of the brick specimens.

3.2.5. CS of GPM Brick

The CS of bricks was determined by testing the full brick lengthwise. The full-scale GPM bricks achieved up to 95% of cube strength at 28 days. The CS of all GPM bricks was significantly higher than that of conventional clay bricks. The CS of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M GPM bricks were 62.4%, 71.2%, 91.2%, 38.2%, 42.9% and 61.8% higher, respectively, than control bricks (Figure 17). Among these different GPM mixes, Mix P80F16M bricks stood out with the highest CS of 32.5 MPa, representing the optimal performance of the tested GPM bricks. Notably, all GPM bricks surpassed the minimum requirements for CS in severe weathering (SW) as specified by ASTM C62 (2021) [43], i.e., 21.4 MPa, and in normal weathering conditions (NW), i.e., 10.5 MPa.
Moreover, all these GPM bricks also met the lower limit of CS (13.4 MPa) as specified by CNS382 (2007) [45], ensuring their suitability for practical applications. On the other hand, the conventional clay bricks failed to meet the minimum CS requirement for SW, although they fulfilled the minimum requirement for MW as specified by ASTM C62 (2021) [43].
The observed increase in the CS of GPM bricks with the inclusion of FA can be attributed to the enhanced dissolution capability of FA as compared to CL. The clay’s layered structure of alumina and silica contains both internal and external negative charges. These negatively charged particles function as anions, surrounding the positively charged Na ions. Reducing the clay content decreases the number of these negatively charged particles [48]. Geopolymers require a significant amount of SiO2 and Al2O3 in amorphous form for geopolymerization. Fly ash contains more readily available silica and alumina compared to clay and has a higher leaching capacity. Consequently, mixes with higher FA content exhibit greater strength [28].
It is reported that during the geopolymerization process, only silica and alumina ions on the surface of the CL actively participated in the geopolymerization, which resulted in the enhancement of the CS of GPM bricks at higher FA content [6].
In addition, the CS of GPM bricks increased by 28.9% with increasing molarity of NaOH solution. This is because at higher molarity, pH, and dissolution of SiO2 and Al2O3 in the solution is increased, and consequently geopolymerization process is increased as observed in the pilot testing and also reported in the previous study [49].

3.2.6. MOR of GPM Brick

The MOR of the GPM bricks was calculated as per ASTM C67-21 [36] for mid-point loading. Similarly to the observation from the CS, the MOR of GPM bricks was substantially higher than that of conventional burnt clay bricks. For example, the MOR of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M bricks were 3.1%, 68.9%, 92.3%, 27.0%, 102.6% and 45.4%, respectively, higher than that of conventional clay bricks (Figure 18). The MOR can be increased by adjusting the molarity of the NaOH solution used in the brick-making process. As the molarity of the NaOH solution increases, the dissolution of silica and alumina ions in the matrix also increases. This leads to an increased formation of N-A-S-H polymers, which condense to form a 3-dimensional geopolymer network. The enhanced formation of this geopolymer network results in improvements in both the compressive strength and the MOR. These results also align well with those documented in the literature since they varied in a wide range. For example, Noor-E-Khuda & Albermani [50] reported that the average MOR of conventional clay bricks ranged between 2.95 MPa and 5.81 MPa.
In general, the MOR increased with an increasing molarity of NaOH solution, for example, the MOR increased by 64% from 2.02 MPa (Mix P-A) to 3.31 MPa (Mix P-B), 13.9% from 3.31 MPa (Mix P-B) to 3.77 MPa (Mix P80F16M), and 59.4% from 2.49 MPa (Mix E60F12M) to 3.97 MPa (Mix E70F14M). This is because FA particles are amorphous in nature and exhibited greater leaching potential than clay, which resulted in the increased rate of geopolymerization as observed in the CS above.
However, the MOR does not always increase with the molarity, but it is also affected by other constituents in the matrix. For instance, the MOR of 16 M GPM bricks decreased by 28.2% from 3.97 MPa (Mix E70F14M) to 2.85 MPa (Mix E60F16M) with an increasing CL content. This was because the charged aluminosilicate layers in the clay structure act as an anion and form an interlayer of negatively charged particles, reducing the force required to split layers; consequently, the MOR is reduced with an increasing CL content [28].

3.2.7. Resistance to Acid Attack

The resistance to acid attack was determined by immersing GPM cubes (50 mm × 50 mm × 50 mm) in 5% sulfuric acid solution for 28 days. Cube specimens were employed to have uniform acidic exposure conditions and minimize geometric effects, and provided better control conditions. Immersed samples in acidic solution were compared with non-immersed samples (control group). Afterwards, the dimensions of the test specimens were measured, and the weight loss of the specimens was determined.
The GPM samples of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M exhibited weight loss of 9.1%, 4.3%, 12.5%, 4.5%, 9.1% and 4.5%, respectively (Figure 19). Similarly, Bakharev [51] reported that minor deterioration in aluminosilicate structure occurred due to depolymerization and dealumination of aluminosilicate gel when immersing bricks in sulfuric acid solution. Aiken et al. [42] also reported that low calcium content bricks exhibited low to no deterioration when immersed in an acidic environment. In general, GPM samples exhibited a reduction in weight after immersion in the acidic solution for one month. The reported acid resistance results are short-term and are based on 28 days of immersion. Long-term durability assessment was out of the scope of this research study.
Similarly, the loss in the CS of GPM cubes was determined after re-measuring the specimens after taking them out of the acidic solution. The CS of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M exhibited a reduction of 2.7%, 4.9%, 4.5%, 4.6%, 1.4% and 1.5%, respectively (Figure 20). The loss in the CS was attributed to the ingress of sulfuric acid solution in the porous microstructure, which causes the release of silica and alumina ions, which led to the depolymerization and dealumination of the NASH gel as explained in a previous study [52]. In general, the GPM samples exhibited an average reduction of 3.3% in the CS when being immersed in the acidic environment for 28 days.

3.2.8. Resistance to Salt Attack

The resistance to salt attack was determined by immersion of test cubes (50 × 50 × 50 mm) in 3.5% sodium-chloride (NaCl) solutions for 28-days. Afterwards, the immersed samples were compared with the non-immersed samples that were not subjected to the salt attack (control samples). In general, GPM samples remained intact after the salt attack. The weight loss of GPM samples of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M was measured at 4.5%, 4.3%, 8.3%, 4.5%, 4.5% and 4.5%, respectively (Figure 19).
The GPM samples exhibited high resistance against the salt attack. The CS was determined after re-measuring the specimens after taking them out of the salt solution. The GPM samples of P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M exhibited a reduction in the CS of 3.7%, 4.6%, 2.8%, 3.6%, 1.4% and 1.5%, respectively (Figure 20). This reduction in the CS may be due to the presence of excessive amounts of OH− ions, which hinders the formation of NASH gel, as also supported by Sukmak et al. [28]. GPM samples exhibited an average loss of 2.9% and the maximum reduction of 4.6% in (Mix P-B) in the CS. The comparison of the samples immersed in acidic solution (acid attack) and samples immersed in salt solution (salt attack) with the samples not immersed in any type of solution (control samples) is exhibited in Figure 21. The reported salt resistance results are short-term and are based on 28 days of immersion.

3.2.9. SEM of GPM Bricks

The SEM analysis of GPM bricks was carried out to examine their internal structures. The SEM was performed using FEI Nova 450 NanoSEM, having a resolution of up to 8 nm. The SEM samples were cleaned with distilled water. The GPM samples were dehydrated with alcohol, dried, and placed in a vacuum condition for scanning. SEM images at a magnification factor of 500X are presented in Figure 22. It was observed that with increasing molarity of NaOH solution, Mix P70F12M (12 M) to Mix P80F16M (16 M) and Mix E60F12M (12 M) to Mix E60F16M (16 M), the geopolymerization is promoted, resulting in a reduced number of unreacted FA particles and increasing formation of N-A-S-H gel (Figure 22). Due to increasing N-A-S-H gel with increasing molarity of NaOH solution, dense crystalline structures indicating a reduced number of unreacted FA particles and a reduced number of voids and cracks were observed. Mix P80F16M exhibited the densest microstructure because of a higher percentage of FA particles and 16 M NaOH solution (Figure 22c).
The GPM mixes, having a lower FA content and higher CL content, exhibited relatively larger voids in their microstructure. For example, Mix E60F12M (40% CL) exhibited the highest number of voids and unreacted FA particles, resulting in the reduced formation of NASH gel.
In the SEM pcitures, certain unreacted particles were evident, primarily related to the less reactive nature of CL particles as compared to FA. Moreover, mixes featuring higher percentages of CL displayed distinct spherical unreacted FA particles. This was particularly noticeable in Mix E70F14M, which contained 40% CL content (Figure 22e).
Meanwhile, the SEM analysis of Mix P80F16M (Figure 22c) showed the densest structure and fewer microcracks, indicative of an accelerated rate of geopolymerization. The class F fly ash promotes the formation of NASH gel in the geopolymer, which serves as a bonding gel and contributes to the strength development [53]. The binding gels compact the microstructure of the geopolymer [54]. Mixes P70F12M and P80F14M exhibited clear microcracks within their structures. This observation suggested that the compressive strength of Mix P80F16M should be higher than that of Mixes P70F12M and P80F14M. The test results align with the outcomes of the mechanical tests conducted on the GPM bricks, confirming the higher rate of geopolymerization in Mix P80F16M. The current research only covered the qualitative assessment of the microstructure of GPM, and quantitative assessment was out of the scope of this research work.

3.2.10. Environmental Impact Assessment

The main objective of this study is to produce a sustainable alternative to conventional burnt clay bricks, i.e., fly ash-based GPM bricks. This shift aims to significantly mitigate the release of embodied carbon dioxide (e-CO2) emissions. Therefore, the environmental impact assessment of GPM bricks was computed and compared with conventional burnt clay brick.
In the published studies, Qazi et al. [55], Turner and Collins [56], Park et al. [57] and Dabaieh et al. [58] carried out the detailed Life Cycle Assessment (LCA) of sodium hydroxide and sodium silicate solutions, fly ash and aggregates, and burnt clay bricks. The e-CO2 emissions of NaOH solids (1.915 kg-e-CO2/kg), Na2SiO3 solids (1.514 kg-e-CO2/kg), FA (0.009 kg-e-CO2/kg) and water (0.0003 kg-e-CO2/kg) were opted from Qazi et al. [55]. The e-CO2 emissions of 12 M NaOH solution (0.6913 kg-e-CO2/kg), 14 M NaOH solution (0.77337 kg-e-CO2/kg), 16 M NaOH solution (0.8503 kg-e-CO2/kg) and Na2SiO3 solution comprising 46% solids and 54% water (0.8503 kg-e-CO2/kg) were adopted from Qazi et al. [55], Turner and Collins (2013) [56] and Park et al. [57]. The environmental impact of clay (CL); an earthen raw material, which is locally available in abundance is minimum. In general, the materials which exist naturally and does not involve the processing/manufacturing process have very low values of e-CO2 emissions. Similarly, clay being an earther material, which exists naturally in the field has been assigned a zero e-CO2 emissions value. However, transportation of clay to the site does include a cost and e-CO2 emissions. Accordingly, the e-CO2 emissions of clay (0.00024 kg-e-CO2/kg) was opted from Dabaieh et al. [58].
To obtain the e-CO2 emission of GPM mix comprising five bricks, the FA, CL, NaOH and Na2SiO3 solution quantities in kg/m3 in Table 2 were multiplied with total volume to obtain the masses of FA, CL, NaOH and Na2SiO3 solution in kg, which are then multiplied with their respective e-CO2 emission values to get the total e-CO2 emission of the mix. The GPM mix contains 5 bricks comprising 80% solids (CL and FA) and 20% liquids (NaOH and Na2SiO3 solutions). The collective volume of these 5 GPM bricks, including 10% wastage, is 0.01072 m3. The corresponding e-CO2 emissions of Mix P70F12M, Mix P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M and Mix E60F16M are 1.321 kg, 1.438 kg, 1.545 kg, 1.308 kg, 1.425 kg and 1.519 kg, respectively. Accordingly, the e-CO2 emissions per brick of Mix P70F12M, Mix P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M and Mix E60F16M are 0.264 kg, 0.288 kg, 0.309 kg, 0.262 kg, 0.285 kg and 0.304 kg, respectively (Figure 23). On the other hand, the e-CO2 emissions per brick of conventional burnt clay brick including extraction and delivery of raw materials, firing of bricks in brick kiln and delivery to building site is 5.5 kg as adopted from Dabaieh et al. [58]. This estimation includes e-CO2 emissions from brick kiln, which uses coal as fuel, transportation costs of materials and services, firing costs as temperature in brick kiln reaches up to 1000 °C.
As can be seen that GPM bricks are greatly friendly to the environment. The e-CO2 emissions for P70F12M, P80F14M, Mix P80F16M, Mix E60F12M, Mix E70F14M, and Mix E60F16M, when compared to conventional burnt clay bricks, exhibit reductions of 95.2%, 94.8%, 94.4%, 95.2%, 94.8% and 94.4%, respectively. It is concluded that GPM mixes prepared with 12 M NaOH solutions exhibited the lowest e-CO2 emissions. In other words, the e-CO2 of the GPM brick is only about 5% of that of a conventional burnt clay brick. Further, the e-CO2 emissions per MPa of CS exhibited that GPM emits only 0.01 kg of e-CO2 per MPa compared to 0.324 kg of e-CO2 per MPa of conventional burnt clay brick (Figure 24). The data (e-CO2 emissions) of the GPM brick presented in Figure 23 were normalized against its corresponding CS.

3.2.11. Cost Analysis

Conventional burnt clay brick is still being widely used in many countries. To assess the wide acceptance of GPM brick as a replacement to conventional bricks, a cost analysis has been carried out. The unit costs of materials in GPM are opted based on the bulk quantities and purchased at the market rate. The unit costs of FA, CL, NaOH solids, and Na2SiO3 are 0.030 USD/kg, 0.004 USD/kg, 0.200 USD/kg, and 0.100 USD/kg, respectively [55].
The GPM mix contains 5 bricks comprising 80% solids (CL and FA) and 20% liquids (NaOH and Na2SiO3 solutions). The volume of 5 GPM bricks, including 10% wastage, is 0.01072 m3. The cost per brick of these mixes varies between USD 0.072–0.082, while the corresponding cost of burnt clay brick is USD 0.057. A GPM brick is approximately 16–32% more expensive than a conventional burnt clay brick, while the first one has a significantly higher strength than the latter one. To ensure a fair comparison, the production cost of these bricks is normalized against their strength. Accordingly, the cost in USD per MPa of CS varied from 0.0025 to 0.0032 USD per MPa, whereas a burnt clay brick has a cost in USD per MPa of CS of 0.0034 (Figure 25). To produce bricks that satisfy the minimum requirements for CS of 21.4 MPa in severe weathering (SW) as specified by ASTM C62 [43], the expected costs of a GPM brick (Mix P80F16M) and a conventional burnt clay brick are USD 0.054 and USD 0.073, respectively. The GPM brick is expectedly 35% cheaper than a conventional burnt clay brick. The cost analysis presented above is it is strongly dependent on local material prices and market conditions.
It is noted that the cost of GPM brick increases with increasing molarity of NaOH solution. Notably, among the materials used for GPM bricks, NaOH solution stands out as the most expensive. In this context, the GPM brick belonging to Mix P80F16M is the most cost-effective option.

4. Conclusions

This experimental investigation evaluates the potential of GPM bricks as a substitution to regular burnt clay bricks. The test program was divided into pilot testing and main testing. For the pilot testing, the influence of percentage replacements of Clay (CL) with fly ash (FA), NaOH molarities (12M, 14M, and 16M), and curing durations (14 days, 28 days, 56 days, and 90 days) were investigated. Based on the Pilot test results, the main testing examined both the physical and mechanical properties of GPM bricks. The environmental assessment and cost analysis of GPM bricks are also performed afterwards. The following conclusions are drawn based on the test results.
  • For the pilot study, the compressive strength (CS) of GPM cubes improved with increasing molarity of NaOH solution and curing durations as expected. The CS increased by about 34.7%, 53.5% and 60.5%, respectively, with curing duration from 14 to 28 days, 14–56 days, and 14–90 days. The optimum CS of GPM mixes was obtained at 70–80% replacements of CL with FA.
  • For the main testing phase, the average weight per unit area and water absorption of GPM bricks were 1.2% and 13.4% lower than burnt clay bricks, respectively. Mix P80F14M (20% CL-80% FA; 14 M NaOH) exhibited the lowest weight per unit area, and Mix P80F16M (20% CL-80% FA; 16 M NaOH) demonstrated the lowest water absorption amongst the investigated GPM bricks.
  • The average CS, modulus of rupture (MOR), and ultrasonic pulse velocity (UPV) of GPM bricks were 61.4%, 56.5%, and 57. 9% greater than burnt clay bricks, respectively. Mix P80F16M (20% CL-80% FA; 16 M NaOH) showed the highest CS and UPV, and Mix E70F14M (30% CL-70% FA; 14 M NaOH) exhibited the highest MOR amongst the GPM bricks.
  • The GPM bricks exhibited higher resistance to both acidic and alkaline conditions as compared to conventional burnt clay bricks, e.g., 5.9% and 2% reduction in CS after immersing in 5% sulfuric acid solution and 3.5% NaCl solution, respectively. Moreover, GPM bricks exhibited slight efflorescence.
  • The e-CO2 emissions and normalized cost of GPM brick were approximately 95% and 35% lower, respectively, than those of burnt clay bricks.
In general, the GPM brick is a viable, sustainable alternative to burnt clay brick with a significantly reduced carbon footprint. Moreover, the mechanical and durability properties of GPM bricks are either equivalent to or better than those of conventional burnt clay bricks. However, the cost of GPM brick is higher than that of burnt clay brick, which can be offset by higher mechanical properties, improved durability properties, and lower carbon footprint.

Author Contributions

Conceptualization, Q.S.K., A.U.Q., M.J.M. and S.M.S.K.; data curation, M.H.J.; formal analysis, M.H.J.; investigations, Q.S.K., A.U.Q. and M.H.J.; methodology, M.H.J. and Q.S.K.; project administrations, A.U.Q. and Q.S.K.; resources, Q.S.K., A.U.Q., M.J.M. and S.M.S.K.; supervisions, Q.S.K. and A.U.Q.; validations, M.H.J., Q.S.K., M.J.M. and S.M.S.K.; visualizations, M.J.M., S.M.S.K. and A.U.Q.; writing—original draft, M.H.J.; writing—review and editing, Q.S.K., A.U.Q., M.J.M. and S.M.S.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by HEC Pakistan approved CPEC-CRG Research Grant [P. No. 221].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are thankful to the University of Engineering and Technology, Lahore for providing the research and testing facilities to conduct this study and HEC CPEC-CRG-2021 (Project No. 221) for providing financial assistance to successfully execute this research study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Silty clay SEM sample results.
Figure 1. Silty clay SEM sample results.
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Figure 2. Fly ash sample results of XRD.
Figure 2. Fly ash sample results of XRD.
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Figure 3. Fly ash sample results of SEM.
Figure 3. Fly ash sample results of SEM.
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Figure 4. Ultrasonic Pulse Velocity Brick Arrangement.
Figure 4. Ultrasonic Pulse Velocity Brick Arrangement.
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Figure 5. Experimental setup for compression test.
Figure 5. Experimental setup for compression test.
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Figure 6. Experimental setup for MOR test.
Figure 6. Experimental setup for MOR test.
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Figure 7. Average compressive strength of Group-I mixes with varying percentage substitutions of CL with FA at various curing durations.
Figure 7. Average compressive strength of Group-I mixes with varying percentage substitutions of CL with FA at various curing durations.
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Sustainability 18 00762 g008
Figure 8. Average compressive strength of Group-II mixes with varying percentage substitutions of CL with FA at various curing durations.
Figure 8. Average compressive strength of Group-II mixes with varying percentage substitutions of CL with FA at various curing durations.
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Sustainability 18 00762 g009
Figure 9. Average compressive strength of Group-III mixes with varying percentage substitutions of CL with FA at various curing durations.
Figure 9. Average compressive strength of Group-III mixes with varying percentage substitutions of CL with FA at various curing durations.
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Sustainability 18 00762 g010
Figure 10. The variation in average CS versus curing duration of test specimens of Group-I with varying percentage replacements of CL with FA for 12 M AA solution.
Figure 10. The variation in average CS versus curing duration of test specimens of Group-I with varying percentage replacements of CL with FA for 12 M AA solution.
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Figure 11. The variation in average CS versus curing duration of test specimens of Group-II with varying percentage replacements of CL with FA for 14 M AA solution.
Figure 11. The variation in average CS versus curing duration of test specimens of Group-II with varying percentage replacements of CL with FA for 14 M AA solution.
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Sustainability 18 00762 g012
Figure 12. The variation in average CS versus curing duration of test specimens of Group-III with varying percentage replacements of CL with FA for 16 M AA solution.
Figure 12. The variation in average CS versus curing duration of test specimens of Group-III with varying percentage replacements of CL with FA for 16 M AA solution.
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Sustainability 18 00762 g013
Figure 13. Variation in WPA of GPM bricks.
Figure 13. Variation in WPA of GPM bricks.
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Figure 14. Variation in WA of GPM bricks [44,45].
Figure 14. Variation in WA of GPM bricks [44,45].
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Figure 15. Effect of efflorescence on GPC bricks immersed in 25 mm water for 7 days with control pair (un-immersed). (a) Mix P70F12M, (b) Mix E60F16M.
Figure 15. Effect of efflorescence on GPC bricks immersed in 25 mm water for 7 days with control pair (un-immersed). (a) Mix P70F12M, (b) Mix E60F16M.
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Figure 16. UPV of GPM bricks.
Figure 16. UPV of GPM bricks.
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Figure 17. Comparison of average compressive strength of the bricks [43,45].
Figure 17. Comparison of average compressive strength of the bricks [43,45].
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Figure 18. Comparison of average MOR of GPM and conventional burnt clay bricks.
Figure 18. Comparison of average MOR of GPM and conventional burnt clay bricks.
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Sustainability 18 00762 g019
Figure 19. Comparison of reduction in average weight of GPM specimens with the control specimens.
Figure 19. Comparison of reduction in average weight of GPM specimens with the control specimens.
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Sustainability 18 00762 g020
Figure 20. Comparison of reduction in compressive strength of GPM cubes with the control specimens.
Figure 20. Comparison of reduction in compressive strength of GPM cubes with the control specimens.
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Sustainability 18 00762 g021
Figure 21. Comparison of samples immersed in acidic solution (acid attack), samples immersed in salt solution (salt attack), and samples not immersed in any type of solution (control).
Figure 21. Comparison of samples immersed in acidic solution (acid attack), samples immersed in salt solution (salt attack), and samples not immersed in any type of solution (control).
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Figure 22. SEM of GPM bricks of (a) P70F12M, (b) P80F14M, (c) Mix P80F16M, (d) Mix E60F12M, (e) Mix E70F14M, and (f) Mix E60F16M.
Figure 22. SEM of GPM bricks of (a) P70F12M, (b) P80F14M, (c) Mix P80F16M, (d) Mix E60F12M, (e) Mix E70F14M, and (f) Mix E60F16M.
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Figure 23. Embodied carbon dioxide (e-CO2) emissions per brick: burnt clay brick vs. GPM bricks.
Figure 23. Embodied carbon dioxide (e-CO2) emissions per brick: burnt clay brick vs. GPM bricks.
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Figure 24. Embodied carbon dioxide (e-CO2) emissions normalized against the compressive strength.
Figure 24. Embodied carbon dioxide (e-CO2) emissions normalized against the compressive strength.
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Figure 25. Cost normalized against the compressive strength of burnt clay brick and GPM bricks.
Figure 25. Cost normalized against the compressive strength of burnt clay brick and GPM bricks.
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Table 1. Mix proportioning of GPM cubes.
Table 1. Mix proportioning of GPM cubes.
Mix-IDFA (kg/m3)NaOH (kg/m3)Water (kg/m3)Na2SiO3 (kg/m3)
12 M100%CL0%FA0.0055.7398.67231.67
90%CL10%FA154.40
80%CL20%FA308.79
70%CL30%FA463.33
60%CL40%FA617.62
50%CL50%FA772.02
40%CL60%FA926.42
30%CL70%FA1080.82
20%CL80%FA1235.21
10%CL90%FA1389.61
0%CL100%FA1543.97
14 M100%CL0%FA0.0062.2192.17231.67
90%CL10%FA154.40
80%CL20%FA308.79
70%CL30%FA463.33
60%CL40%FA617.62
50%CL50%FA772.02
40%CL60%FA926.42
30%CL70%FA1080.82
20%CL80%FA1235.21
10%CL90%FA1389.61
0%CL100%FA1543.97
16 M100%CL0%FA0.0068.5185.85231.67
90%CL10%FA154.40
80%CL20%FA308.79
70%CL30%FA463.33
60%CL40%FA617.62
50%CL50%FA772.02
40%CL60%FA926.42
30%CL70%FA1080.82
20%CL80%FA1235.21
10%CL90%FA1389.61
0%CL100%FA1543.97
Table 2. Mix proportioning of different mixes of GPM bricks.
Table 2. Mix proportioning of different mixes of GPM bricks.
Mix IDCL-FAMolarity of NaOH SolutionFA
(kg/m3)		CL
(kg/m3)		NaOH Solution (kg/m3)Na2SiO3
(kg/m3)
SolidsWater
Mix P70F12M30%CL70%FA12118951052118255
Mix P80F14M20%CL80%FA1413593407397255
Mix P80F16M20%CL80%FA1613593408387255
Mix E60F12M40%CL60%FA12101968052118255
Mix E70F14M30%CL70%FA1411895107397255
Mix E60F16M40%CL60%FA1610196798387255
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MDPI and ACS Style

Javed, M.H.; Khan, Q.S.; Qazi, A.U.; Kazmi, S.M.S.; Munir, M.J. Eco-Efficient Geopolymer Bricks Without Firing and Mechanical Pressing. Sustainability 2026, 18, 762. https://doi.org/10.3390/su18020762

AMA Style

Javed MH, Khan QS, Qazi AU, Kazmi SMS, Munir MJ. Eco-Efficient Geopolymer Bricks Without Firing and Mechanical Pressing. Sustainability. 2026; 18(2):762. https://doi.org/10.3390/su18020762

Chicago/Turabian Style

Javed, Muhammad Hassan, Qasim Shaukat Khan, Asad Ullah Qazi, Syed Minhaj Saleem Kazmi, and Muhammad Junaid Munir. 2026. "Eco-Efficient Geopolymer Bricks Without Firing and Mechanical Pressing" Sustainability 18, no. 2: 762. https://doi.org/10.3390/su18020762

APA Style

Javed, M. H., Khan, Q. S., Qazi, A. U., Kazmi, S. M. S., & Munir, M. J. (2026). Eco-Efficient Geopolymer Bricks Without Firing and Mechanical Pressing. Sustainability, 18(2), 762. https://doi.org/10.3390/su18020762

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