Exploring the Potential of Using Waste Clay Brick Powder in Geopolymer Applications: A Comprehensive Review

Abstract

The application of geopolymers has recently been given significant attention to address climate change and the growing scarcity of construction materials in the 21st century. Researchers have utilized industrial waste or supplementary cementitious materials containing high levels of silica and alumina as precursors along with different alkaline activators. Furthermore, the technical challenges associated with waste brick management or recycling include both land use changes and financial implications. The existence of amorphous aluminosilicates in waste clay bricks, which can be used as geopolymer binders, has drawn attention recently. This paper reviews the recent advancements of the integration of clay brick wastes in geopolymer applications, individually as well as its use with other alternative materials. Prior studies suggest that waste clay bricks can effectively serve as the primary source material in geopolymer applications. This review covers various aspects, including the assessment of fresh, mechanical, microstructure, and durability-related properties. It specifically focused on enhancing these properties of waste clay bricks through mechanical and thermal treatments, through varying curing conditions, utilizing different types of alkaline activators, and considering their properties and corresponding ratios in the development of geopolymer products using waste brick powder. Furthermore, this paper portrays a critical review of the sustainability implications of the utilization of clay brick waste in geopolymer applications. Conclusively, this review provided the lessons learnt, research gaps, and the future direction for investigation into the feasibility of geopolymers derived from waste clay brick powder.

1. Introduction

Rapid urbanization worldwide is generating a substantial volume of construction and demolition waste (CDW), a trend which is expected to persist in the coming years [1]. The United States, the European Union, Australia, and China are among the largest contributors to this waste stream, collectively producing millions of tons annually [2,3,4]. However, conventional disposal methods like dumping and landfilling pose significant social and environmental challenges, including safety hazards and pollution. Recycling technologies for CDW have progressed. Thus, concrete along with brick waste are commonly recycled as recycled concrete aggregates. CDW mainly consists of concrete and masonry waste. While concrete recycling is common, masonry waste, including brick waste, is often overlooked [5]. However, with the annual production of bricks on the rise, the need for sustainable disposal methods becomes imperative. Reusing brick waste not only diminishes the amount of waste in landfills but also reduces the need for extracting natural resources. Bricks are a common material used for construction purposes in Australia, where efforts to recycle masonry waste have seen progress but still experience challenges [6].

Brick waste offers several sustainable solutions for construction applications, contributing to resource conservation and waste reduction. One significant application involves using this as a substitute for conventional aggregates in concrete and as crushed brick in asphalt mixtures. This practice not only reduces the demand for natural aggregates but also helps mitigate the environmental impact associated with their extraction and transportation. Multiple research efforts have explored the use of crushed clay brick as an aggregate in concrete production [7,8,9,10,11,12,13,14,15]. Crushed clay brick was first used with Portland cement in Germany in 1860 for concrete product manufacturing, but its significant use as aggregates in new concrete emerged during the post-Second World War reconstruction period [7]. In 1983, it was initially discovered that concrete incorporating crushed bricks exhibited a modulus of elasticity lower than 30% and a tensile strength roughly 11% greater than that of standard concrete [10]. Adamson et al. [7] found that replacing natural coarse aggregates with crushed clay bricks does not significantly affect the durability of concrete in the absence of steel reinforcement, but it is not recommended when concrete is reinforced with steel. Zhao et al. [13] found that utilizing waste clay bricks to prepare lightweight aggregate concrete, using them as both coarse and fine aggregates, is a promising approach with potential applications. Yang et al. [14] found that a 50% replacement ratio of crushed clay brick resulted in low workability, making it difficult to compact and finish the fresh concrete. However, crushed clay brick can partially replace natural aggregates (at a 15% replacement rate) in concrete without reducing concrete mechanical properties [15].

Furthermore, brick dust, a byproduct of grinding or crushing clay brick waste, can be used as a supplementary cementitious material in concrete production. By incorporating brick dust in cement, construction projects can achieve improved durability and strength, while also reducing the overall carbon footprint of the concrete. Previous studies have studied the use of clay brick dust as a substitute in cement [16,17,18,19,20,21,22,23]. Adding crushed brick powder reduced the workability of concrete at higher replacement levels but had less effect at 10%, while maintaining similar density and achieving comparable strength to regular concrete over time, with good resistance to chloride ion penetration and minimal strength loss after freezing–thawing cycles [20,23]. However, another study revealed that replacing 20% with brick waste powder had no adverse effect on mortar compressive strength, only a partial influence on mortar shrinkage, and concurrently enhanced the mortar’s resistance to freeze–thaw cycles [21]. Additionally, when utilizing clean fired clay brick waste, the optimal percentage for replacement can be increased to 40%, while still achieving higher compressive strength than the control concrete [16].

The construction industry’s carbon dioxide emissions reduction target prompts the exploration of sustainable alternatives to Ordinary Portland Cement (OPC). Geopolymer binders, derived from industrial wastes and byproducts, have been studied as a promising solution by significantly reducing greenhouse gas emissions in concrete production [3]. Brick waste, rich in silica and alumina, presents a potential precursor for geopolymer binders [4]. In recent years, the utilization of waste clay brick powder (WCBP) in the production of geopolymer binders has emerged as a promising avenue in sustainable construction practices, addressing critical concerns in waste management. Spearheaded by the groundbreaking research conducted by scholars like Fořt et al. [24,25,26], Tuyan et al. [27], Migunthanna et al. [5,28,29,30,31], Mahmoodi et al. [32,33,34,35], Shen et al. [36,37], and Silva et al. [38,39], this field has witnessed remarkable advancements in understanding the mechanical properties and potential applications of WCBP-based geopolymers. By repurposing WCBP into high-performance construction materials, land utilization both at the local fill and quarry site can be avoided. These endeavors have uncovered desirable compressive strengths, typically ranging from 30 MPa to 100 MPa, underscoring the inherent strength and versatility of WCBP as a constituent in geopolymer compositions. Furthermore, researchers have meticulously investigated various production parameters, including curing conditions and alkali activator types, elucidating their profound impact on the performance and characteristics of WCBP geopolymers. Despite these significant strides, however, critical gaps remain regarding the long-term durability, sustainability aspects, and practical applicability of WCBP-based geopolymers in real-world construction settings. Thus, ongoing, and comprehensive research efforts are indispensable to fully explore the potential of WCBP as a sustainable and efficacious alternative for modern construction materials and methodologies, thereby contributing to sustainable waste management practices.

Tang et al. [1] investigated of new concrete by integrating recycled concrete waste powder and recycled clay brick waste powder, revealing that using materials with a median diameter below 30 μm and a replacement ratio of 30% enhances concrete durability, alongside providing economic and environmental advantages, thereby endorsing their continued utilization in concrete. Dadsetan et al. [40] provided a summary of the literature concerning the utilization of CDW like concrete, brick, and ceramic as source materials in geopolymer technology, either independently or in conjunction with supplementary cementitious materials (SCMs), indicating that incorporating CDW in geopolymers can be a cost-effective and eco-friendly method to reduce carbon dioxide emissions by decreasing the demand for Portland cement and aggregates, while providing a suitable solution for CDW disposal. Additionally, Ye et al. [41] presented an overview of published articles over the last decade (2012–2021) regarding geopolymers containing recycled brick powder and recycled concrete powder (RCP) by calculating carbon emissions of geopolymers exceeding 40 MPa, confirming the viability of RBP and/or RCP as precursors, advocating for pozzolanic materials or Portland cement. Migunthanna et al. [42] conducted a review study on WCBP-based geopolymers and revealed that binary and ternary blends of WCBP with precursors like fly ash, metakaolin, slag, and OPC enhance strength, with slag or OPC favored for pavement concretes for their high early strength, superior long-term development, and low-porosity microstructure, while fly ash helps reduce shrinkage. However, 50% of the total research on WCBP-based geopolymers has been published since their review paper was released. Furthermore, that paper [42] did not discuss sustainability-related aspects or outcomes in the reviewed studies.

Hence, this paper provides a comprehensive review of the latest developments in the incorporation of clay brick waste into geopolymer applications, both individually and in conjunction with other alternative materials. The focus is particularly on the characterization of brick waste utilized in geopolymer production, evaluating mechanical, microstructural, and durability-related properties. It examines the influence of different precursors and activators and varying curing conditions and includes an analysis of the sustainability implications of the produced geopolymer binders.

2. Methodology

This study employed peer-reviewed journal articles to examine current factors including chemical and mineralogical characteristics and mechanical and microstructural properties, as well as durability and sustainability aspects, of geopolymer binders derived from waste clay brick powder. Google scholar, Scopus, and Web of science databases were utilized to search for the existing literature using these keywords, “waste clay brick” and “geopolymer”, to find relevant articles. Forty-seven peer-reviewed journal articles specifically focused on WCBP-based geopolymers were discovered, all published within the past 10 years. Figure 1 illustrates the yearly distribution of publications, indicating that 83% of them were released within the last five years. This highlights the present enthusiasm for investigations integrating WCBP into geopolymer synthesis.

A bibliometric investigation was carried out within the Scopus database, employing the search criteria “geopolymer” AND “waste” AND “clay brick” AND “powder”. Keyword co-occurrence analysis was conducted utilizing VOS viewer bibliometric analysis software (version 1.6.20). The network and density visualization diagrams resulting from the analysis are depicted in Figure 2A,B, respectively. The network visualization revealed 22 items, 3 clusters, and 219 links, highlighting geopolymers, compressive strength, geopolymer, and brick as significant keywords. Modifying the search string to include “AND durability” led to the discovery of only 10 journal articles, suggesting a paucity of research on the durability properties of WCBP-based geopolymer binder, while adding “AND environmental impact analysis” narrowed the results down to just 2 articles, underscoring the limited investigation into the sustainability aspects of this binder.

3. Characterizing WCBP as a Precursor in Geopolymer Binder

3.1. Source, Treatment, and Particle Size Analysis of WCBP

A comprehensive overview of research on the synthesis of geopolymers utilizing WCBP has been conducted (Table 1), considering the origin of the source material, treatment techniques applied to WCBP, incorporation of additional precursors, analytical methodologies employed, curing conditions, and mechanical characteristics. The utilization of waste brick powder in geopolymers could increase the utilization of CDW materials. This strategic emphasis on CDW aligns with broader objectives aimed at improving waste management practices within the construction sector. By repurposing end-of-life bricks for geopolymer production, researchers not only mitigate the burden on landfills but also generate value-added materials by avoiding producing virgin construction materials. This sustainable approach addresses environmental concerns associated with waste disposal while meeting the demand for alternative construction materials.

To achieve finer particle sizes in WCBP, researchers commonly employ mechanical treatments such as crushing and grinding. These processes break down larger brick particles into smaller, more uniform particles, enhancing their reactivity and ensuring better dispersion within the geopolymer matrix [3,4,5,24,43,44,45,46,47,48]. Additionally, some studies incorporate thermal treatment in an oven as a pre-processing step to remove moisture and organic contaminants from WCBP [4,24,46,49]. This drying process helps to ensure a more consistent response to subsequent mechanical treatments, resulting in finer and more homogenous particle sizes [4]. Ultimately, these combined treatments optimize the particle size distribution of WCBP, improving its suitability for geopolymerization, contributing to the development of geopolymers with enhanced mechanical properties [4].

The range of median particle sizes for WCBP reported in previous studies vary widely, from 4.7 µm to 125 µm. The mechanical properties of WCBP-based geopolymers are significantly influenced by the median particle size of WCBP. Finer WCBP particles offer a larger surface area, which accelerates the rate of geopolymerization, resulting in denser and stronger aluminosilicate networks and enhanced strength [4]. When synthesizing geopolymer binders, larger WCBP particles lead to incomplete reactions, thereby diminishing the mechanical strength of the resulting material by producing a heterogeneous microstructure. Prior research indicates that a 150% increase in particle size resulted in a 26% reduction in compressive strength [50]. In contrast, Bumanis et al. [51] reported no impact on compressive strength when decreasing the particle size from 52.62 to 24.64 μm by tripling the grinding time. Both used WCBP as a sole precursor to synthesize geopolymers. However, when WCBP was used with other precursors in geopolymer production, smaller particle sizes exhibited high compressive strength [52].

In conclusion, the particle size of WCBP plays a crucial role in geopolymer production, with smaller particles showing higher compressive strength when used alongside other precursors. However, when solely used, smaller particle size did not affect the compressive strength under ambient curing conditions. This is because the addition of WCBP to GGBFS and fly ash increased the compressive strength due to improved reactivity and reduced voids. However, when only WCBP is used, the reactivity remains unchanged due to poor packing efficiency, and it did not increase compressive strength.

3.2. Chemical Composition

The process of brick manufacturing influences the physio-chemical properties of WCBP. During the brick production process, clay or clay-like materials are typically heated to high temperatures in a kiln, a process known as calcination. After calcination, the bricks are gradually cooled down [53]. This slow cooling process in brick production allows ample time for the formation of a predominantly crystalline structure in clay brick material, decreasing the degree of amorphousness in WCBP [42]. Despite the low amorphousness of WCBP compared to other geopolymer precursors like fly ash or GGBFS, WCBP-based geopolymer binders exhibit high mechanical properties [4,24]. Fort et al. [24] identified that WCBP obtained from brick grinding contained approximately 27.8% amorphous phase and Sharmin et al. [4] noted a 32.31% amorphous content in WCBP samples processed through brick grinding with a ball mill, resulting in compressive strengths of 100 MPa and 92 MPa, respectively. Furthermore, Rovnaník et al. [54,55] reported amorphous contents of 18.49% and 21.75% in WCBP samples derived from the grinding of hollow red clay bricks, which resulted in a geopolymer with a compressive strength of 57.8 MPa, the highest strength reported in their study. This outcome was attributed to the chemical composition of WCBP, which mainly consists of silicon (Si), aluminum (Al), iron (Fe), and calcium (Ca). Figure 3 presents an analysis of the ternary silicon (Si) and aluminum (Al) and calcium (Ca) oxide composition of WCBP based on existing research studies [4,24,29,32,44,45,48,52,56,57,58,59,60,61,62,63,64]. From this figure, it is evident that most of the studies found a high presence of Si and Al oxides in WCBP. Because of this characteristic, WCBP demonstrates favorable potential for geopolymerization when contrasted with other CDW or industrial waste materials. All WCBP analyzed in the previous study had an aggregate amount of alumina, silica, and iron oxide exceeding 70% and also had a reasonable amount of CaO, satisfying the requirements for categorization as a mineral admixture under ASTM C618 standards [65].

In summary, despite having low levels of amorphousness, WCBP demonstrates strong mechanical properties in geopolymer binders due to its composition primarily consisting of silicon, aluminum, iron, and calcium oxides, making it a viable mineral additive for geopolymerization.

3.3. X-ray Diffraction and Rietveld Analysis

The X-ray diffraction phases present in both WCBP and WCBP-based geopolymer binder were gathered (Figure 4) from the previous studies. The phases mainly include quartz, albite, anorthite, hematite, mullite, muscovite, calcite, microcline, orthoclase, dolomite, berlinite, sanidine, akermanite, aragonite, and pirssonite. Among these phases, quartz emerges as the most prevalent, with albite being the second most frequently mentioned phase.

After synthesizing geopolymers, certain phases may vanish, while others remain, or new phases may emerge. Quartz is generally unaffected and remains stable during geopolymerization, alongside mica, albite, mullite, hematite, muscovite, calcite, orthoclase, illite, and akermanite, which also maintain stability throughout the process without diminishing [3,56,57,58]. Conversely, phases such as gismondine, cristobalite, zeolite, C–S–H, C–A–S–H, and N–A–S–H gels are expected to form during geopolymerization [32,44,59,61]. Additionally, new phases such as coesite, perovskite double, akermanite, diopside, illite, sodalite, chabazite, merwinite, aragonite, montmorillonite, prissonite, calcium silicate, and hydrotalcite may emerge during the geopolymerization process [24,52].

In the study by Liang et al. [52], quartz, illite, and microcline were initially identified, but it was observed that illite and microcline disappeared after the synthesis of geopolymers. Interestingly, calcium silicate hydrate emerged as a new phase after geopolymer production. Guo et al. [61] also discovered the presence of quartz, calcite, dolomite, and berlinite, but observed the disappearance of calcite, dolomite, and berlinite post synthesis. Conversely, gismondine was newly identified following the synthesis process in their study. Hwang et al. [59] identified quartz, albite, anorthite, calcite, hematite, and mullite, yet they noted the disappearance of albite, anorthite, calcite, hematite, and mullite post synthesis. Conversely, they observed the emergence of calcium silicate hydrate (CSH) and sodium calcium aluminate silicate hydrate (NCASH) as new phases following the synthesis process. In the case of Yehualaw et al. [56], albite, anorthite, and hematite vanished, while zeolite, C–S–H, and C–A–S–H appeared as new phases in their study.

The phases in WCBP and its geopolymer binders are crucial in production, with some stable and others changing or emerging. Quartz remains stable, while phases like gismondine, cristobalite, zeolite, C–S–H, C–A–S–H, and N–A–S–H gels significantly affect mechanical strength [32,44,52,56,59,61]. These phases contribute to the binder’s structure, influencing strength and durability [68]. C–S–H and C–A–S–H gels provide cohesion and strength, while zeolite phases enhance mechanical properties [33].

Table 1. Research undertaken regarding the synthesis of geopolymers using WCBP at a glance.

Research Reference	Brick Powder Source	Mechanical and Thermal Treatment	Median Particle Size	Characterization Techniques					Mechanical Properties		Curing Condition	Other Precursors Used in the Study
				XRD	XRF	SEM-EDS	FTIR	TGA	Compressive Strength	Flexural Strength
Cong et al. [44]	CDW, China	Crushing and grinding	-	+	+	+	+	+	7 and 28 days	-	Heat curing (50, 75, and 100 °C), ambient	CCR, SF
Migunthanna et al. [5,29]	CDW, Australia	Oven-dried at 100 °C for 48 h and then grinding	46 µm	+	+	+	+	-	1, 3, 5, 7, 14, 28, 60, and 90 days	-	Ambient	FA, GGBFS
Vafaei et al. [69,70]	Red clay brick	Crushing and grinding	Less than 70 µm	+	+	+	+	-	28 days	-	Heat curing (95 °C) for 20 h, ambient	PHS
Guo et al. [61]	CDW, China	-	-	+	+	+	+	-	7 and 28 days	-	Ambient	FA
Wong et al. [71]	CDW	Crushing and grinding	-	+	+	+	-	-	1 and 28 days	-	Ambient	FA
Mahmoodi et al. [32,33,34,35,72]	CDW, Canada	Grinding	Less than 75 µm	+	+	+	+	-	7 and 28 days	-	Heat curing (50, 75, and 100 °C), ambient	RCT, RWT, RCW, MK, GGBFS, FA
Robayo-Salazar et al. [62,66]	CDW, Colombia	Grinding	24.25 µm	+	+	+	-	-	7 and 28 days	-	Heat curing (70 °C) for 24 h, ambient	OPC
Ouda et al. [48]	CDW, Egypt	Crushing and grinding	davg = 90 µm	+	-	+	+	+	1, 7, 14, and 28 days	-	Heat curing (80 °C) for 24 h	DCP
Rovnaník et al. [54,55]	Waste hollow red clay brick	Grinding	8.5 µm	+	+	+	+	-	7, 14, 28, and 90 days	7, 14, 28, and 90 days	Heat curing (40 °C), ambient	MK, FA
Tuyan et al. [27]	Waste clay brick powder, Izmir	-	davg = 100 µm	+	+	+	+	+	1, 3, 5, 7, 28, and 90 days	-	Heat curing (80 and 90 °C), ambient
Sassoni et al. [73]	Brick waste	Grinding	davg = 10 µm	+	+	+	-	-	7, 28, and 90 days	7, 28, and 90 days	Heat curing (50 °C), ambient	-
Fort et al. [24,25,26]	Waste brick, Czech Republic	Crushing, grinding, and oven-dried at 80 °C for 24 h	50 µm	+	+	+	-	+	28 days	-	Heat curing (60 and 80 °C), ambient	GGBFS
Pasupathy et al. [3,43]	Waste brick, Australia	Oven-dried at 105 °C for 24 h and then grinding	20.5 µm	+	+	+	+	-	7 and 28 days	-	Heat curing (60 °C) for 24 h	FA, GGBFS
Ahmed et al. [48,57,67,74]	Hollow clay brick, Baghdad	Crushing and grinding	-	-		+	-	-	7 and 28 days	7 and 28 days	Heat curing (50, 60, 70, 80, and 100 °C), ambient	MK
Al-Noaimat et al. [58]	End-of-life brick, London	Crushing and grinding	Less than 125 µm	+	+	+	-	-	3 days	3 days	Heat curing (60 °C) for 24 h	FA
Alzeebaree et al. [46]	Clay brick, Iraq	Crushing and grinding	Less than 300 µm	+	+	+	-	-	28, 56, 90, and 120 days	-	Heat curing (45 °C) for 24 h	Glass powder, Fine soil
Bumanis et al. [51]	Waste red brick	Grinding	24.64 µm	+	+	+	+	+	1, 28, and 35 days	-		Silica gel, Clay
Kul et al. [75]	CDW	Crushing and grinding	8.5 µm	+	+	+	-	-	-	-	Heat curing	RT
Yehualaw et al. [56]	Waste brick powder	-	10.32 µm	+	+	+	+	-	3, 7, and 28 days	-	Ambient	GGBFS
Dang et al. [76]	Masonry waste	Grinding	-	-	+	-	-	-	7, 28 days	7 and 28 days	Ambient	GGBFS
D’Angelo et al. [63]	CDW, Italy	Grinding	Less than 0.125 mm	+	+	+	+	+	28 days	28 days	Heat curing (60 °C) for 3D
Li et al. [64]	Waste brick	Grinding	4.34 µm	+	+	+	-	+	7, 28, and 90 days	7, 28, and 90 days	Heat curing (70 °C) for 3D
Liu et al. [77]	CDW treatment plant	Oven-dried at 105 °C for 24 h and grinding	Less than 0.075 mm	+	-	+	+	+	3, 7, and 28 days	-	Steam curing (80 °C), ambient	SS, FA
Liang et al. [52]	Waste red brick	Oven-dried at 80 °C and Grinding	4.7 µm	+	+	+	+	+	7 and 28 days	-	Ambient	GGBFS, FA
Hwang et al. [59]	CDW, Taiwan	Grinding	10.32 µm	+	+	+	+	-	3, 7, 28, and 56 days	-	Ambient	WCP, GGBFS, FA
Silva et al. [38,39]	Fired clay brick	Crushing and grinding	7.39 µm	+	+	+	+	-	7 days	-	Heat curing (65, 80, and 90 °C) for 1,3,7 D, ambient
Tazune et al. [78]	Fired clay brick	Grinding	125 µm	+	+	+	+		28 days	-	Ambient	MK, RHA
Shen et al. [36]	Waste brick	Grinding	-	+	+	+	+	+	3 days	-	Heat curing (70 °C) for 3D
Kaze Rodrigue et al. [79]	Waste Fired clay brick	Oven-dried for 48 h and grinding	Less than 80 µm	+	+	+	-	-	-	-	Ambient	Volcanic scoria (Zg)
Rakhimova et al. [80]	CDW	-	-	-	-	-	-		1, 3, and 7 days	-	Ambient and steam curing	GGBFS
Sharmin et al. [4]	Brick pellets, Australia	Oven-dried at 105 °C for 24 h and grinding	11.91 µm	+	+	+	-	-	7, 28, 56, and 90 days	-	Heat curing (40 and 60 °C) for 10 h, ambient	GGBFS, FA

TGA—Thermogravimetric analysis, FA—Fly ash, SEM-EDS—Scanning electron microscopy with energy-dispersive X-ray spectroscopy, CCR—Calcium carbide slag, SF—Silica fume, PHS—Phosphorous slag, RCT—Recycled ceramic tile, RWT—Recycled ceramic wall tile, RCW—Recycled concrete waste, MK—Metakaolin, DCP—Dolomite-Concrete powder; RT—Roof tile, SS—Steel slag; RHA—Rice husk ash.

Table 1. Research undertaken regarding the synthesis of geopolymers using WCBP at a glance.

Research Reference	Brick Powder Source	Mechanical and Thermal Treatment	Median Particle Size	Characterization Techniques					Mechanical Properties		Curing Condition	Other Precursors Used in the Study
				XRD	XRF	SEM-EDS	FTIR	TGA	Compressive Strength	Flexural Strength
Cong et al. [44]	CDW, China	Crushing and grinding	-	+	+	+	+	+	7 and 28 days	-	Heat curing (50, 75, and 100 °C), ambient	CCR, SF
Migunthanna et al. [5,29]	CDW, Australia	Oven-dried at 100 °C for 48 h and then grinding	46 µm	+	+	+	+	-	1, 3, 5, 7, 14, 28, 60, and 90 days	-	Ambient	FA, GGBFS
Vafaei et al. [69,70]	Red clay brick	Crushing and grinding	Less than 70 µm	+	+	+	+	-	28 days	-	Heat curing (95 °C) for 20 h, ambient	PHS
Guo et al. [61]	CDW, China	-	-	+	+	+	+	-	7 and 28 days	-	Ambient	FA
Wong et al. [71]	CDW	Crushing and grinding	-	+	+	+	-	-	1 and 28 days	-	Ambient	FA
Mahmoodi et al. [32,33,34,35,72]	CDW, Canada	Grinding	Less than 75 µm	+	+	+	+	-	7 and 28 days	-	Heat curing (50, 75, and 100 °C), ambient	RCT, RWT, RCW, MK, GGBFS, FA
Robayo-Salazar et al. [62,66]	CDW, Colombia	Grinding	24.25 µm	+	+	+	-	-	7 and 28 days	-	Heat curing (70 °C) for 24 h, ambient	OPC
Ouda et al. [48]	CDW, Egypt	Crushing and grinding	davg = 90 µm	+	-	+	+	+	1, 7, 14, and 28 days	-	Heat curing (80 °C) for 24 h	DCP
Rovnaník et al. [54,55]	Waste hollow red clay brick	Grinding	8.5 µm	+	+	+	+	-	7, 14, 28, and 90 days	7, 14, 28, and 90 days	Heat curing (40 °C), ambient	MK, FA
Tuyan et al. [27]	Waste clay brick powder, Izmir	-	davg = 100 µm	+	+	+	+	+	1, 3, 5, 7, 28, and 90 days	-	Heat curing (80 and 90 °C), ambient
Sassoni et al. [73]	Brick waste	Grinding	davg = 10 µm	+	+	+	-	-	7, 28, and 90 days	7, 28, and 90 days	Heat curing (50 °C), ambient	-
Fort et al. [24,25,26]	Waste brick, Czech Republic	Crushing, grinding, and oven-dried at 80 °C for 24 h	50 µm	+	+	+	-	+	28 days	-	Heat curing (60 and 80 °C), ambient	GGBFS
Pasupathy et al. [3,43]	Waste brick, Australia	Oven-dried at 105 °C for 24 h and then grinding	20.5 µm	+	+	+	+	-	7 and 28 days	-	Heat curing (60 °C) for 24 h	FA, GGBFS
Ahmed et al. [48,57,67,74]	Hollow clay brick, Baghdad	Crushing and grinding	-	-		+	-	-	7 and 28 days	7 and 28 days	Heat curing (50, 60, 70, 80, and 100 °C), ambient	MK
Al-Noaimat et al. [58]	End-of-life brick, London	Crushing and grinding	Less than 125 µm	+	+	+	-	-	3 days	3 days	Heat curing (60 °C) for 24 h	FA
Alzeebaree et al. [46]	Clay brick, Iraq	Crushing and grinding	Less than 300 µm	+	+	+	-	-	28, 56, 90, and 120 days	-	Heat curing (45 °C) for 24 h	Glass powder, Fine soil
Bumanis et al. [51]	Waste red brick	Grinding	24.64 µm	+	+	+	+	+	1, 28, and 35 days	-		Silica gel, Clay
Kul et al. [75]	CDW	Crushing and grinding	8.5 µm	+	+	+	-	-	-	-	Heat curing	RT
Yehualaw et al. [56]	Waste brick powder	-	10.32 µm	+	+	+	+	-	3, 7, and 28 days	-	Ambient	GGBFS
Dang et al. [76]	Masonry waste	Grinding	-	-	+	-	-	-	7, 28 days	7 and 28 days	Ambient	GGBFS
D’Angelo et al. [63]	CDW, Italy	Grinding	Less than 0.125 mm	+	+	+	+	+	28 days	28 days	Heat curing (60 °C) for 3D
Li et al. [64]	Waste brick	Grinding	4.34 µm	+	+	+	-	+	7, 28, and 90 days	7, 28, and 90 days	Heat curing (70 °C) for 3D
Liu et al. [77]	CDW treatment plant	Oven-dried at 105 °C for 24 h and grinding	Less than 0.075 mm	+	-	+	+	+	3, 7, and 28 days	-	Steam curing (80 °C), ambient	SS, FA
Liang et al. [52]	Waste red brick	Oven-dried at 80 °C and Grinding	4.7 µm	+	+	+	+	+	7 and 28 days	-	Ambient	GGBFS, FA
Hwang et al. [59]	CDW, Taiwan	Grinding	10.32 µm	+	+	+	+	-	3, 7, 28, and 56 days	-	Ambient	WCP, GGBFS, FA
Silva et al. [38,39]	Fired clay brick	Crushing and grinding	7.39 µm	+	+	+	+	-	7 days	-	Heat curing (65, 80, and 90 °C) for 1,3,7 D, ambient
Tazune et al. [78]	Fired clay brick	Grinding	125 µm	+	+	+	+		28 days	-	Ambient	MK, RHA
Shen et al. [36]	Waste brick	Grinding	-	+	+	+	+	+	3 days	-	Heat curing (70 °C) for 3D
Kaze Rodrigue et al. [79]	Waste Fired clay brick	Oven-dried for 48 h and grinding	Less than 80 µm	+	+	+	-	-	-	-	Ambient	Volcanic scoria (Zg)
Rakhimova et al. [80]	CDW	-	-	-	-	-	-		1, 3, and 7 days	-	Ambient and steam curing	GGBFS
Sharmin et al. [4]	Brick pellets, Australia	Oven-dried at 105 °C for 24 h and grinding	11.91 µm	+	+	+	-	-	7, 28, 56, and 90 days	-	Heat curing (40 and 60 °C) for 10 h, ambient	GGBFS, FA

TGA—Thermogravimetric analysis, FA—Fly ash, SEM-EDS—Scanning electron microscopy with energy-dispersive X-ray spectroscopy, CCR—Calcium carbide slag, SF—Silica fume, PHS—Phosphorous slag, RCT—Recycled ceramic tile, RWT—Recycled ceramic wall tile, RCW—Recycled concrete waste, MK—Metakaolin, DCP—Dolomite-Concrete powder; RT—Roof tile, SS—Steel slag; RHA—Rice husk ash.

4. Fresh Properties, Strength, and Microstructure of WCBP-Based Geopolymers

4.1. Effect of WCBP on Setting Time

The initial and final setting times of WCBP-based geopolymers from previous studies [3,33,43,52,57,64] were reviewed (Figure 5) to understand how the setting time is affected by precursor materials and WCBP sources. The initial setting times of WCBP-based geopolymer binder could range from 125 to 315 min and final setting times can range from 168 to 355 min by utilizing different precursor combinations, including WCBP, sourced from waste brick materials with ground granulated blast furnace slag (GGBFS) and FA [3,43]. The setting time was found to be considerably longer, with initial setting times ranging from 780 to 950 min, due to use of WCBP sourced from CDW and fired clay brick wastes [57]. This longer setting time is due to having 56% less silica compared to the geopolymers using WCBP in the other study [3,43]. Li et al. [64] recorded shorter initial and final setting times of 110 and 120 min, respectively, employing a combination of WCBP sourced from waste brick materials, Metakaolin, GGBFS, and FA. Mahmoodi et al. [33] and Liang et al. [52] achieved initial setting times of 52 and 73 min, respectively, and final setting times of 180 and 92 min, respectively, with various combinations of WCBP sourced from CDW, Metakaolin, GGBFS, and FA. From these findings, it is evident that the choice of precursor materials and the source of WCBP significantly affect the setting times of geopolymer binders. Specifically, geopolymer binders made solely from WCBP tend to have longer setting times compared to those incorporating additional precursors like GGBFS or FA. This variation in setting times highlights the importance of considering both the chemical compositions of the WCBP source and the combinations of precursors used in geopolymerization processes.

4.2. Effect of WCBP on Workability

Various studies have investigated the workability of WCBP-based geopolymer binders and identified factors influencing their slump flow or flowability due to variations in WCBP-based geopolymers [32,43,56,57,58,59,76]. Figure 6 showcases differences in slump values across various research reports. These findings highlight the impact of precursor materials and WCBP sources on geopolymers’ workability, aiding in formulation selection for diverse applications. Due to their spherical shapes and smooth surfaces, fly ash particles act as ball bearings, reducing internal friction between particles in a geopolymer mix and resulting in greater slump values and improved workability [4]. For example, the addition of fly ash in a geopolymer mix with waste clay brick powder and waste ceramic powder resulted in a slump value of 285 mm [59]. Additionally, Mahmoodi et al. [32] found that the slump value range for the geopolymer mix was highest at 240 mm in their study. On the other hand, when irregularly shaped and rough-surfaced particles such as GGBFS or WCBP are used in higher percentages in the geopolymer mix, the slump flow decreases. Pasupathy et al. [43] attributed a decrease in slump flow to the loss of the ball-bearing effect and high-water absorption characteristics of GGBFS, with a slump range of 152.5 to 135 mm. Ahmed et al. [57] pointed out that the irregular shape and rough surface of GGBFS particles contribute to reduced flowability, achieving a flowability range of 73 to 80%. Al-Noaimat et al. [58] noted that the outer surface, interlayer, and inner pores of recycled GGBFS can attract an alkaline solution, leading to a reduction in free water in the slurry, with a flowability range of 65 to 85%. Additionally, Dang et al. [76] highlighted that GGBFS’s high surface area and irregular surface mixture increase the absorption of mixing water, resulting in a slump flow range of 150 to 240 mm. Similarly, an increase in the concentration of alkaline activators in geopolymer mixes results in a reduction in slump values [56]. For example, Yehualaw et al. [56] observed that increasing the concentration of alkali activator solutions negatively affects the slump flow of fresh AAM mixtures, resulting in a slump range of 115 to 195 mm. These findings demonstrate that the variation in workability values of WCBP-based geopolymers is attributed to factors such as the concentration of alkali activator solutions, characteristics of precursor materials, and properties of recycled materials.

4.3. Effect of Curing, Activators, and Additives on Compressive Strength

Figure 7 illustrates the impact of curing methods, activators, and additives on WCBP-based geopolymer binders. The curing conditions significantly impact the geopolymerization process and mechanical properties of WCBP geopolymers. Elevated temperatures generally lead to better geopolymerization rates and higher compressive strengths [24,57]. However, increasing the curing temperature can result in a higher rate of water evaporation and greater matrix density, ultimately resulting in decreased mechanical strength [63]. Optimal curing conditions for WCBP geopolymers blended with other precursors typically involve longer periods at temperatures between 65 °C and 80 °C. While ambient curing is feasible, it results in lower early strength development compared to elevated temperatures [4]. Studies suggest that curing temperatures around 80 °C yield optimal results for WCBP geopolymers, with compressive strengths reaching up to 59 MPa [42]. Overall, relying solely on WCBP as a precursor may not be a feasible approach to achieve early high strength, as it would necessitate curing at elevated temperatures.

The synthesis of geopolymers involves the use of activators. Solid activators are used for one-part geopolymers, while liquid activators are utilized for two-part geopolymers. Sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) are commonly utilized activators for two-part geopolymers, while recent studies have investigated the utilization of Na₂SiO₃ as the exclusive activator for one-part geopolymers [29]. Studies have shown that the addition of Na₂SiO₃ to the activator mixture can lead to better compressive strengths compared to using NaOH alone, owing to its ability to accelerate the polycondensation process and enhance the formation of geopolymerization products [42]. Despite NaOH’s effectiveness, its corrosive and hazardous nature has led to reluctance in commercial applications, prompting the evaluation of Na₂SiO₃ as a potential alternative activator. Figure 7 indicates that the optimal mechanical performance, with compressive strength ranging from 92 to 105 MPa, for WCBP-based geopolymers is attained when using a combined NaOH–Na₂SiO₃ activator [4,24,32,57]. NaOH helps in initiating the geopolymerization process, while Na₂SiO₃ acts as a silica source, enhancing the reactivity of the binder system. This combination results in a more efficient geopolymerization reaction and the formation of a denser and more homogeneous geopolymer matrix, leading to higher compressive strength. Although, Na₂SiO₃ alone has shown similar mechanical performance, with a compressive strength of up to 102 MPa, in blends with other aluminosilicate precursors like fly ash, slag, or OPC [42,62]. This is because Na₂SiO₃ acts as a strong alkaline activator, initiating the geopolymerization process and contributing to the formation of a durable geopolymer matrix. When combined with other aluminosilicate precursors like fly ash, slag, or OPC, it enhances their reactivity, resulting in the formation of additional binding phases and improved mechanical properties. Other than NaOH and Na₂SiO₃, KOH, K₂SiO₃, Na₂CO₃, and Na₂CO₄ were also used as alkaline activators in a few studies [81,82]. However, NaOH and Na₂SiO₃ are considered more feasible in terms of availability and properties of the geopolymer products.

Different specimen sizes were used for compressive strength testing by previous researchers. Most of them used 50 mm cubes [4,5,32,43,57]. Robayo et al. [62] utilized 20 mm cubes, Liang et al. [52] used 40 mm cubes, and Liu et al. [17] employed specimens sized 32.5 mm × 31.5 mm × 50 mm. Fort et al. [24,25] conducted compressive strength measurements on prisms broken during flexural testing, with a loading area of 40 mm × 40 mm.

Blending WCBP with other aluminosilicate materials has emerged as a promising strategy to enhance the mechanical properties and applicability of geopolymers. Blending WCBP with GGBFS at a high percentage of GGBFS results in a geopolymer with high mechanical strength, making it a favorable option for high-strength geopolymer production (Figure 7). When GGBFS is blended with WCBP, a high-strength geopolymer is achieved [24,32,57]. Robayo et al. [62] achieved high strength by blending 20% OPC with WCBP. All these studies employed ambient curing methods, indicating that when sodium hydroxide and sodium silicate are used as activators, ambient curing is effective in producing high-strength geopolymers.

On the contrary, blending WCBP with FA or other slag alongside GGBFS resulted in a geopolymer with a strength range of 40–80 MPa. Heat or steam curing did not significantly improve the strength in these cases due to the lower reactivity of the precursors and their limited ability to form strong binding phases. This led to the formation of a less dense and less homogeneous geopolymer matrix, resulting in lower compressive strength values. The use of a high molarity activator, such as 12 M sodium hydroxide as utilized by Sharmin et al. [4], can also be an option to achieve high strength, as they obtained a strength of 92 MPa. The high molarity of NaOH provides a highly alkaline environment, which is crucial for initiating and accelerating the geopolymerization process. This results in a more rapid formation of geopolymer bonds and the development of a strong, dense geopolymer matrix. Additionally, the high alkalinity of NaOH helps in the dissolution of silica and alumina in the precursor materials, promoting the formation of geopolymer gels and ultimately leading to higher compressive strength values. Lastly, a geopolymer produced solely from WCBP is unlikely to achieve a strength greater than 42 MPa, as per previous studies [25]. These examples illustrate the effectiveness of blending WCBP with various precursors in improving the strength and performance of geopolymers. Overall, the utilization of blended WCBP-based geopolymers holds significant promise, offering enhanced mechanical properties and sustainability benefits. The mechanical properties of WCBP-based geopolymers are primarily influenced by alkaline activators like NaOH and Na₂SiO₃, which accelerate geopolymerization and enhance matrix density. Curing at elevated temperatures (around 80 °C) improves strength, while blends with GGBFS and other precursors enhance reactivity and form strong binding phases. Particle characteristics and the use of high molarity activators further impact strength, with blends typically outperforming geopolymers composed solely of WCBP in terms of mechanical performance and sustainability.

4.4. Microstructural Properties of WCBP-Based Geopolymers

The research findings suggest that WCBP-only geopolymer binder tends to be porous and less compact [25]. This indicates that the process of geopolymerization with WCBP alone does not achieve optimal particle arrangement, leading to irregularities and a higher presence of pores in the microstructure. However, blending WCBP with additional precursor materials like fly ash, slag, or OPC offers a viable solution to this issue due to the enhanced reactivity and pozzolanic properties of these composite materials [4,24,81]. Incorporating these supplementary materials improves particle packing during geopolymerization, resulting in a denser and less-porous microstructure. This enhancement contributes to improved mechanical properties and durability of the geopolymers [29]. By employing this blending approach, the inherent limitations of using WCBP alone are effectively addressed, enabling the production of geopolymers with enhanced structural integrity and performance.

Figure 8 depicts the microstructure of WCBP alone, WCBP blended with other precursors, and a geopolymer paste sample without WCBP after 28 days. The authors replicated selected mix designs from Fort et al. [24] and Sharmin et al. [4]. The WCBP-only binder exhibited more microcracks compared to others. In contrast, blends of WCBP with fly ash and GGBFS showed fewer microcracks and a denser microstructure. Geopolymer binders solely based on fly ash and GGBFS exhibited a dense microstructure with some unreacted and partially reacted fly ash particles.

Microstructural analysis has been also conducted on WCBP-based geopolymers using different precursors in previous studies [4,24,32,43,47,62]. Fort et al. [24] conducted a study where they produced geopolymer using 100% WCBP. The resulting geopolymer had a compressive strength of 25 MPa. Analysis of the microstructure showed the presence of porosity and microcracks, along with unreacted particles, leading to poor mechanical properties. In contrast, the addition of other precursors such as FA or slag improved the mechanical strength due to their pozzolanic reactivity, which enhances the formation of additional binding phases and reduces the presence of unreacted particles and voids in the microstructure. This enhancement is attributed to the improved microstructure, which showed fewer microcracks and porosity. Sharmin et al. [4] produced geopolymer from WCBP blended with FA and GGBFS, achieving a mechanical strength of 92 MPa, which can be attributed from the denser microstructural images. Pasupathy et al. [43] produced geopolymer from WCBP blended with FA, reaching a mechanical strength of 55 MPa. However, WCBP blended with steel slag (SS) or CDW did not exhibit high mechanical strength due to their lower pozzolanic reactivity compared to other precursors like fly ash or slag. This resulted in inadequate formation of binding phases and increased porosity in the microstructure, leading to reduced strength properties. On the other hand, blending WCBP with OPC, even at a 20% OPC blend, increased the strength up to 102 MPa.

The energy-dispersive X-ray spectroscopy (EDS) analysis of geopolymer gels revealed significant peaks for Si, Al, Ca, and Na, suggesting the presence of C-A-S-H and N-A-S-H gels [4,44,51,76]. Geopolymer samples containing 40% brick powder exhibited higher Si/Al ratios and increased silicon content from WCBP, resulting in fewer large pores and enhanced compressive strength compared to geopolymers made with fly ash and GGBFS [4]. Heat curing at 60 ℃ intensified the peaks of Si, Al, Ca, and Na [4]. Nevertheless, more research on WCBP-based geopolymers is needed to comprehensively grasp how curing methods, activators, and additives influence the mechanical and microstructural characteristics of WCBP geopolymers.

5. Durability Related Properties of WCBP-Based Geopolymers

5.1. Effect of Precursors and Curing on Water Absorption of WCBP-Based Geopolymers

Water absorption (%) in WCBP-based geopolymers could vary under various experimental conditions and precursor combinations (Figure 9). Studies found that there can be rapid water absorption of WCBP-based geopolymers over time due to capillary action as the proportion of capillary pores was positively correlated with water absorption, but the absorption decreased with higher alkali dosage due to enhanced geopolymerization [64]. Also, water absorption can be decreased with an increase in fine soil powder ratio, attributing higher absorption to less dense microstructures with higher replacement ratios [46]. The water absorption also increased with higher NaOH concentration, leading to denser structures and lower absorption over time due to ongoing alkali activation in WCBP-based geopolymers over time [71]. Additionally, higher water absorption was observed for mixes with more permeable voids, resulting in lower compressive strength while increasing WCBP concentrations in the geopolymer binder [29]. Significantly lower water absorption was observed under vacuum due to having highly porous structures in WCBP-based geopolymers [63]. Figure 9 illustrates that combining WCBP with fly ash led to lower water absorption rates compared to blends involving glass powder, fine soil, or using WCBP alone in the geopolymer binder formation. Specifically, a mixture containing 20% WCBP with fly ash showed a slight decrease in water absorption as the molarity decreased. Another study indicated that increasing molarity up to 12 M resulted in reduced water absorption, but beyond 12 M, absorption rates increased, peaking at 16 M [46]. This is because when WCBP is blended with fly ash at lower concentrations of activators, it forms a denser structure with reduced porosity. In contrast, mixing WCBP with fine soil powder and glass powder does not yield such a dense binder, resulting in higher water absorption. Overall, it was observed that geopolymers made solely from WCBP exhibited higher water absorption compared to those blended with other precursors. Furthermore, WCBP-only geopolymers cured under ambient conditions showed higher water absorption than those cured through heat curing. However, further research is necessary to fully understand the effect of precursors, particle size, and curing conditions on the water absorption of WCBP-based geopolymers.

5.2. Effect of WCBP on Other Durability Properties

WCBP-based geopolymers exhibit promising performance in terms of their durability-related properties, as summarized in

Table 2. Summary of past literature on investigations of durability aspects.

Research Reference	Durability Related Investigation Performed
D’Angelo et al. [63]	Water absorption.
Migunthanna et al. [29]	Water Absorption, absorption and volume of permeable voids, performance after long-term immersion in water, resistance to sulfate attack, cyclic wetting and drying, abrasion.
Vafaei et al. [70]	Changes in compressive strength and mass exposure to H2SO4 and HCl.
Wong et al. [71]	Water absorption, sorptivity.
Alzeebaree et al. [46]	Sorptivity, water absorption.
Ahmed et al. [67]	Drying shrinkage, water absorption.
Al-Noaimat et al. [58]	Water absorption, freeze–thaw resistance, fire resistance.
Vidak et al. [82]	Water absorption.
Li et al. [64]	Water absorption.
Komnitsas et al. [50]	Freeze–thaw resistance.

, which presents findings from prior literature investigations focusing on these geopolymers’ durability aspects [29,46,58,63,64,67,69,71,82]. Even after 150 days of sulfate exposure, mortar containing 32% WCBP demonstrates superior durability against sulfate attack, showing no severe deterioration in terms of dimensional stability, compressive strength, or microstructure when synthesized with GGBFS and fly ash [29]. Some studies compared the WCBP-based geopolymer with OPC-based mortar [69,70]. After 6 months of immersion in HCl, the WCBP-based geopolymer experienced 25% less mass loss and the same compressive strength loss as the OPC-based mortar [69]. Similarly, after 24 months in HCl, the WCBP-based geopolymer had 71.8% less mass loss and 96% less compressive strength loss compared to the OPC-based mortar [69]. In H₂SO₄, the WCBP-based geopolymer showed 41% less mass loss and 22% less compressive strength loss after 6 months and 23% less mass loss and 91% less compressive strength loss after 24 months, compared to the OPC-based mortar [69,70].

Previous studies have also compared WCBP-based geopolymer mortar with samples of CDW, slag-only, and slag–fly ash-based mortar and concluded that the geopolymer mortar containing WCBP exhibited similar water absorption, volume of permeable voids, and water absorption rate [5,50]. In terms of water resistance, a 27% strength decrease was observed for WCBP-based geopolymer mortar when 40% WCBP was used with fly ash [5]. In contrast, minimal deterioration was found for geopolymer mortar samples with less than 36% WCBP content [5].

WCBP-based geopolymer can serve as a viable alternative to OPC for high-temperature applications, demonstrating thermal stability up to 1000 °C [27]. The minimal weight loss of WCBP geopolymers under heat treatment and the improved properties with Na₂SiO₃ addition emphasize their potential [27]. Additionally, they showed satisfactory resistance to freeze–thaw cycles [50,58]. The mechanical performance of brick powder and GGBFS-based geopolymers was found to be better than the fly ash and GGBFS-based geopolymers [58]. Specifically, the fly ash–GGBFS geopolymer experienced a 55% reduction in compressive strength during freeze–thaw cycles, whereas the brick powder–GGBFS geopolymer showed only a 7% reduction [58]. Brick powder was observed to decrease pore size and overall porosity, thereby enhancing durability by reducing water infiltration and subsequent ice formation during freezing conditions and then subsequently enhancing durability and associated mechanical properties. Komnitsas et al. [50] also found that the geopolymers made from brick powder exhibited better durability against freeze–thaw cycles compared to geopolymers made from tiles and waste concrete powders [50]. The compressive strength loss for the tile waste-based geopolymer was 38% higher than those based on waste brick powder [50]. Additionally, geopolymer binders made from waste clay brick powder were found to possess higher durability against repeated cycles of wetting and drying, with a sacrifice of only 3% strength [29]. However, further comprehensive research is necessary to fully understand the durability and potential applications of WCBP-based geopolymers. Additionally, existing test methods for conventional concrete may require slight modifications to adequately evaluate the durability of geopolymers.

6. Environmental Impact Analysis of WCBP-Based Geopolymers

The use of waste byproducts in geopolymers could have economic and environmental implications, as determined by material sources, and structural properties for specific applications. This section presents the environmental impacts and outcomes of different approaches in geopolymer research. The construction industry is responsible for approximately 39% of global greenhouse gas (GHG) emissions and consumes about 36% of the world’s energy, according to the world green building council [83]. Geopolymers are being introduced by researchers to reduce GHG emissions and energy consumption, with the prior literature highlighting the considerable environmental advantages of alternative binders over OPC [3,6,24,44,72].

WCBP-based alkali-activated materials showcase an impressive decrease in energy consumption ranging from 60 to 97% and GHG emissions ranging from 53 to 97% when compared to OPC [3,44,84]. Ternary geopolymer formulations also demonstrate superiority in reducing energy consumption and GHG emissions, with reported decreases compared to OPC and mono-based geopolymers [72].

These findings highlight the potential of geopolymer technologies to significantly reduce the environmental impact of construction materials, making them a promising alternative to traditional cement-based materials. Further research and development in geopolymer technology could lead to even greater reductions in greenhouse gas emissions and energy consumption in the construction industry. Overall, the diverse range of studies as represented in

Table 3. Sustainability-related research findings from previous literature with key outcomes.

Research Reference	Sustainability Related Investigation	Major Outcome
Pasupathy et al. [43]	CO2-e emission and energy consumption analysis	GC showed a 60–80% reduction in both embodied energy and carbon footprint compared to OPC.
Robayo-Salazar et al. [81]	LCA	NaOH and Na2SiO3 exhibit higher CO2 emissions but superior mechanical performance, whereas Na2CO3 and Na2SO4 emit less CO2 but offer lower mechanical performance.
Kul et al. [75]	LCA	NaOH and grinding of CDW-based precursors had the largest environmental impact in LCA analyses, followed by transportation and heat curing.
Mahmoodi et al. [34]	CO2-e emission and energy consumption analysis	Ternary geopolymer reduced energy and CO2 compared to OPC and mono-based.
Cong et al. [44]	Environmental and life cycle cost analysis	BP-based alkali-activated materials reduced energy consumption by 97% and GHG emissions by 53% compared to OPC.
Li et al. [64]	CO2-e emission and energy consumption analysis	BP-based geopolymers reduce CO2 emissions by 40%–70% and energy consumption by 20%–50% compared to OPC.
Mir et al. [85]	LCA	GWP values for industrial large-scale scenarios are 25% lower than those for small-scale laboratory scenarios.
Liang et al. [52]	LCA	Using BP in geopolymers reduced CO2 emissions by 70% and energy consumption by 54%.
Migunthanna et al. [31]	LCA	Na2SiO3 exhibited the highest CO2 emissions (1477 kg/t) and energy consumption (4860 MJ/t), surpassing that of OPC.
Fort et al. [24]	CO2-e emission and energy consumption analysis	The embodied energy analysis highlights the significant impact of alkaline activators.
Sharmin et al. [6]	Techno-echo-efficiency analysis	Energy used in alkaline activator production at the batching plant emerged as a significant factor, followed by waste brick grinding.

LCA—Life cycle analysis, GWP—Global warming potential.

highlights the multifaceted nature of sustainability considerations in geopolymer research, emphasizing the importance of adopting environmentally responsible approaches in the development and implementation of construction materials.

Figure 10 provides a comparative analysis of the global warming potential (GWP) reported in various studies regarding geopolymer materials, along with the major impact identified. Robayo-Salazar et al. [81] reported a GWP of 220 kgCO₂/m³, with the major impact found to be related to GWP itself. Mir et al. [85] presented a notably higher GWP of 1811 kgCO₂/m³, signifying a significant contribution to climate change. However, it is important to note that Mir et al. [85] utilized heat curing in their study, which could have influenced their results compared to studies [81] employing ambient curing conditions. Sharmin et al. [6] observed a GWP of 675 kgCO₂/m³, with the major impact attributed to human toxicity. These findings underscore the diverse environmental impacts associated with geopolymer materials, ranging from direct contributions to climate change to broader considerations such as human health effects.

CO₂ emissions and energy consumption reduced by 60–80% due to replacement of OPC with geopolymer [43]. Similarly, the use of ternary geopolymers consumes significantly less energy and emits less CO₂ compared to OPC and mono-based alternatives [72]. On the other hand, Robayo-Salazar et al. [62] explored the environmental implications of different activators through LCA (life cycle analysis), highlighting the trade-offs between CO₂ emissions and mechanical performance. For instance, while NaOH and Na₂SiO₃ exhibited higher CO₂ emissions, they also offered superior mechanical properties compared to alternatives like Na₂CO₃ and Na₂SO₄, which emitted less CO₂ but provided lower mechanical performance [6,24]. NaOH and the grinding process of CDW-based precursors has the largest environmental impact in LCA [32,85]. CO₂-e emissions and energy consumption due to replacement of OPC with geopolymers using WCBP could reduce CO₂ 40–70% and energy consumption 20–50% [52,64].

Overall, there have been relatively few studies conducted on the environmental effects of WCBP-based geopolymer binders, indicating a significant opportunity to comprehensively identify the sustainability aspects associated with this material.

7. Conclusions and Recommendations

The development of WCBP-based geopolymers presents an innovative approach towards producing low-carbon binders while concurrently addressing solid waste management concerns. WCBP, sourced from construction and demolition waste or brick manufacturing processes, shows promise as an aluminosilicate precursor due to its high amorphous content of SiO₂ and Al₂O₃. Studies indicate that finer WCBP particles yield higher compressive strengths in resulting geopolymers, with NaOH and Na₂SiO₃ commonly employed as activators to facilitate geopolymerization. Curing conditions, particularly temperatures above 60 °C, significantly enhance early strength development. Hybrid geopolymer systems blending WCBP with materials like slag or OPC demonstrate superior mechanical properties compared to 100% WCBP-based geopolymers. These blends also exhibit enhanced thermal stability and reduced porosity, critical for durability. Moreover, research suggests that WCBP-based geopolymers can meet stringent requirements, showcasing potential in construction.

WCBP-based geopolymers can have potential applications in the construction industry for manufacturing concrete and mortars. The literature suggests that using WCBP geopolymers instead of conventional concrete can reduce carbon emissions. Researchers have also investigated their durability, finding that they possess properties indicating good durability. Therefore, they could also have applications in ceramics and composites. However, gaps in understanding remain, necessitating further investigation into activator alternatives, optimization of mix designs for high-strength concrete, and comprehensive field studies to assess real-world performance along with environmental analysis. Addressing these gaps will be crucial for advancing the utilization of WCBP geopolymers in sustainable construction practices.

In essence, WCBP-based geopolymers offer a promising avenue for sustainable construction materials, but further research and development are required to optimize their performance and broaden their application potential. The research gaps in the development of WCBP-based geopolymers include the following:

• Further research is needed to optimize mix designs for WCBP-based geopolymers, aiming to achieve desired properties, minimize raw material usage, and reduce environmental impact, including the production of WCBP-only geopolymers;
• Comprehensive studies are needed to assess the mechanical and microstructural properties and durability of WCBP-based geopolymers. Research should investigate the long-term performance of these materials under various environmental conditions, including exposure to moisture, temperature variations, and chemical agents, to ensure their suitability for different applications especially for ambient cured geopolymers;
• More research is required to develop simplified geopolymers based on WCBP precursors, akin to conventional cement. Investigating one-part geopolymer methods could lead to the development of simplified geopolymers that are easier to produce and use in various applications;
• There is a need for further exploration of activators alternative to NaOH. Research should focus on identifying sustainable activators with comparable performance to NaOH but lower environmental impact;
• Exploration of new applications for WCBP-based geopolymers beyond construction materials is necessary. Research could focus on potential uses in areas such as waste management, soil stabilization, or alternative building materials to expand the range of sustainable solutions offered by WCBP geopolymers.