A critical review on mechanical, durability, and microstructural properties of industrial by-product-based geopolymer composites

4.2.1 Influence of calcination temperature

Using varying percentage contents of Na₂O by weight of GWP, Tchadjié et al. [70] examined the prospective impact of 2 h long alkali fusion at 550°C. The process mainly comprised the addition of NaOH pallets to GWP in a dry state and mixing them for 5 min, followed by 2 h of calcination. Subsequently, the resulting product was pulverized and sieved at 90 µm to synthesize the GPM. Increasing the Na₂O (%) to 60 from 0, increased the reactive phases of GWP powders to 54% from 5.70% as presented in Figure 3(a). Although higher content of alkali resulted in more reactive phases in GPM, excessive alkali beyond 40% had a negative impact on the water resistance and 38% of CS decreased after reaching 40.50 MPa as presented in Figure 3(b).

Figure 3

(a) Amount of reactive phase in GWP and fused materials and (b) CS of GPM pastes at 28 days [70].

4.2.2 Influence of alkali activators

A previous study [71] investigated the effect of Ca-enriched substances such as ground granulated blast furnace slag (GGBFS) and OPC on the initial strength gain of entirely decomposed granite (CDG) sourced GPM. With long-term weathering and chemical attacks, the authors entirely decomposed the GWP. CDG-OPC and CDG-GGBFS were the two polymers synthesized using Na₂SiO₃ solution and 14 M NaOH. GGBFS-CDG GPM mix comprising 30% GGBFS with the same molarity of activator exhibited better CS at 7 days with 20.34 MPa, whereas the CS gain of the CDG-OPC mix was comparatively lesser at 19.25 MPa. It was concluded that a high concentration of CaO in OPC and GGBFS favored the production of additional hydrated Ca₂SiO₄ composite, which contributed to the initial strength of CDG-made GPM paste. Tchadjié et al. [70] proposed using the GWP-sourced GPM for low-strength construction materials. To synthesize GWP-sourced GPM, 40% metakaolin (MK), alkali-pretreated GWP, and Na₂SiO₃ solution were utilized, whereas GWP was preheated up to a temperature of 550°C for 2 h. Not only did reactive amorphous phases increase but also newly formed crystalline phases of Na₂SiO₃ were formed because of alkali pretreatment. GWP-MK activated by 40% of Na₂O showed better CS with 40.50 MPa. However, after 60 days of immersion, resistance against water was compromised due to the large formation of cracks. Other mixtures with 20–30% Na₂O demonstrated superior water confrontation, with no cracking appearing after 60 days of immersion. Also, the resulting CS was better and ranged from 10.94 to 21.90 MPa. A previous study [72] utilized alkali-activated slag-granite waste comprising a Na₂SiO₃ mixture (5% Na₂O), NaOH, and SiO₂/Na₂O ratio of 1.40. The slag in the mixture contained 0, 5, 10, 15, and 20% of GWP. The GPM mix with the addition of 0–20% GWP reduced the CS gradually from 69.60 to 41.90 MPa. Table 2 presents the strength obtained for GWP-sourced GPM synthesized with different binders.

4.2.3 Influence of molar concentrations

Tchadjié et al. [70] observed that by increasing the alkali amount from optimum concentration, the CS and water confrontation of GWP-MK-based GPM paste are reduced. With the increase in the alkali content (%) from 10 to 40, of Na₂O by weight of GWP, the number of reactive stages and CS of GWP-MK polymers is enhanced. Also, the authors attained the highest CS by using optimal alkali content of 40% of Na₂O, and with a further increase in Na₂O, it decreased to 21.80 MPa. Reactive phases also improved to 46.40% at 40% of Na₂O. However, with time, these results remained stable. Eroshkina and Korovkin [69] noticed that an appropriate amount of NaOH and Na₂SiO₃ is required for proper workability of slag-sourced GPM and by further increasing the molar concentration beyond the optimum content, workability is lost.

4.2.4 Influence of LBR

GWP-GPM was mostly acquired at an LBR of 0.5–1.2. Tchadjié et al. [70] synthesized such GPM using an LBR of 1.20 that showed an encouraging CS value of 40.50 MPa. Gao et al. [72] considered the CS characteristics of alkali-activated slag composites using different percentages of granite waste with LBR ranging from 0.50 to 0.55. Concerning the influence of GWP, the substitution of up to 15% results in a gradual reduction of CS from 69.6 to 57.1 MPa (by 18%) after 28 days of curing. Further, an increase of up to 20% leads to a more obvious reduction of CS to 41.9 MPa (Figure 4).

Figure 4

CS of alkali-activated composites with BA and/or GWP with (a) LBR = 0.5 and (b) LBR = 0.55 [72].

4.2.5 Influence of raw materials

For well cementing, Alvi et al. [73] synthesized many mixes of GWP-sourced GPM by the addition of an appropriate amount of nanomaterials such as the liquid form of MWCNT-OH (hydroxyl-functionalized nanotubes) and alumina-based AL-0450. Similar to those of the reference paste (as presented in Figure 5), an increase in the nanoparticles resulted in significant improvement in both tensile and CS of the GPM. However, compared to that 7 days’ strength, 28 days' tensile strength of composites was influenced by the nanoparticles [74–83]. Therefore, the long duration of curing affected the mechanical strength and flexibility of GPM when the nanomaterials were added to the mix.

Figure 5

Mechanical properties of the GWP-GPM paste: (a) CS and (b) tensile strength [73].

4.2.6 Influence of the sand-to-binder ratio

GWP-MK GPM mortars synthesized by Tchadjié et al. [70] had an AGBR of 2.75:1 and an activator modulus of 3.22. Their test sample obtained the highest CS value of 40.50 MPa. On the other hand, Eroshkina and Korovkin [69] noticed the highest CS of 30 MPa for GPM mortars with a slag-GWP binder having a fine aggregate-to-binder ratio of 2:1.

4.2.7 Influence of curing

The improvement in both CS and flexural strength of GPM was achieved by Eroshkina and Korovkin [69] with a curing time kept from 20 to 60 h and the temperature raised to 80 from 60 (before heat treatment of the mortar as shown in Figure 6). Authors noticed that both, the initial curing period and curing temperature, have a great impact on the flexural and CS of the GPM mortar comprising binders with magnetic source, e.g., granite waste. Zha and Zhu [71] synthesized CDG-GGBS and CDG-OPC GPM with a curing temperature of 60 and 7 days of curing duration. Conforming to these test conditions, the CDG-GGBS GPM mix attained a better CS in compression with 20.34 MPa, which was comparatively higher than that obtained from CDG-OPC GPM subjected to 7 days of thermal treatment.

Figure 6

Mechanical properties of the slag-GWP-GPM paste at different curing temperatures: (a) CS and (b) flexural strength [69].

4.2.8 Influence of molar ratios

Better strength properties are obtained by a geopolymerization reaction of the activator modulus or silicate ratio (SiO₂/Na₂O), which can be achieved with the addition of an appropriate amount of NaOH/Na₂SiO₃ solution. The highest CS of 20.34 MPa was attained by Zha and Zhu [71] by using a SiO₂/Na₂O ratio of 1.50 for CDG-GGBS GPM. Likewise, Tchadjié et al. [70] synthesized a new type of GPM comprising SiO₂/Na₂O ratio of 3.2 with 10–60 (%) changing Na₂O content for GPM. It comprised MK and granite waste and gained the highest CS of 40.50 MPa. GWP-GGBS-sourced GPM was synthesized by Eroshkina and Korovkin [69] with a SiO₂/Na₂O ratio of 2.70. The resulting product showed the highest CS value of 30 MPa. Gao et al. [72] observed that by reducing the SiO₂/Na₂O ratio to as low as 1.40 for slag-GWP-GPM, CS increases to 41.90 MPa.

4.2.9 Influence of plasticizers

In the early stages, GWP was used as a cement substitute but it caused a dispersing problem during mixing, and due to the presence of free water content, its usage was limited. Gao et al. [72] deliberated the workability features of alkaline-activated slag composites. They utilized GWP to replace the cement and assessed its impact on workability. In this study, waste BA was dried to eliminate the free water to secure a constant starting condition. The sticky GWP was first dispersed in water with the accumulation of a superplasticizer, and this pre-dispersion procedure led to a well-dispersed GWP in water, which can be used directly in mortar production.

5.2.1 Influence of the calcination temperature

Being poorly reactive, BR is frequently utilized after the calcination process at higher temperatures. The process ultimately dissolves the SiO₂ with Al₂O₃ significantly by enhancing the Si and Al in BR to accomplish higher levels of CS. Calcination decomposes the three crystalline phases of Al, Ca, and Fe by calcination via dehydration, dehydroxylation, and carbonation in different ranges of temperatures. Transformation of these phases converts the crystalline-based Al, Ca, and Fe to other phases such as aluminum oxide phases, AlNa₁₂SiO₅ phases, gehlenite crystalline phases, and alkali-enriched nepheline stages [89]. The addition of the alkali or thermal pretreatment for the alkali activation before calcination makes BR more reactive and enhances the formation of glassy and amorphous stages.

Hu et al. [12] developed BR-based GPM utilizing FA, pretreated BR specimen with alkali-thermal pre-activation along with a solution of Na₂SiO₃ activators. Researchers emphasized the encouraging aspect of thermally pretreated alkali or BR’s calcination to geopolymerize and strengthen the BR-dependent GPM. The BR has no active participation in the geopolymerization process and comprises Si and Al traces in crystalline forms. Pretreating the alkali with thermal pre-activation or calcination of alkali-treated BR at elevated temperatures ranging from 600 to 900°C makes the BR reactive and dissolves the SiO₂ and Al₂O₃. Soluble, alkaline, and sodium-rich nepheline and peralkaline aluminosilicate formation in alkali thermal pretreating stages of BR increase the Na₂O by 5–10%. Being an important hydraulic component of BR-dependent GPM, the peralkaline phase improves the strength properties. Figure 12 highlights the dissolution efficacy of SiO₂ and Al₂O₃ in alkali mixtures subjected to various ranges of calcination temperatures. The optimum dissolution efficacy was obtained at 800°C. Ye et al. [93] observed the impact of temperature for the calcination process on the CS of GPM comprising BR and GBFS with a 5:5 wt ratio. Increasing the temperature from 100 to 800°C leads to an increase in the CS from 17.50 to 49.20 MPa and thus influences the conversion of mineral phases of BR by forming latest developed phases with higher solubility levels within the alkaline atmosphere. Calcination of BR enhances the development of good soluble aluminosilicates leading to a dynamic suspension in an alkali environment and ultimately improves the geopolymerization process.

Figure 12

Influence of calcination temperature for dissolution efficacy of (a) Al₂O₃ and (b) SiO₂ in an alkali solution [12].

Ye et al. [94] established OGPs using alkali pre-thermal pre-activated BR as well as SF. After drying at 105°C and mixing the BR with NaOH pellets, the calcination process was carried out at 800°C for 1 h duration. Pretreating the BR not only contributed to achieving appropriate hardening of the design mix after 24 h but was also found effective in developing the early (3 days) CS of 7.5 MPa. GPM mixtures without alkali pretreatment in BR exhibited no strength after 24 h of casting. Twenty-eight days of curing of BR-dependent OPG portrayed a CS of 31.5 MPa when 25% of SF was added to the mix. A previous study [62] developed BR-MK-derived GPM and emphasized the favorable impact of the calcination process to dissolve the aluminosilicates effectively. After drying the as-received BR at 100°C for almost 1 h, calcination was then carried out for 3 h at 800°C. The 24 h cured GPM comprising 20% of calcined BR at 60°C curing temperature followed by 25°C for 2 months demonstrated a higher CS with 14.20 MPa. To effectively dissolve the alumina and silica traces in an alkaline atmosphere, pretreating the alkali with thermal pretreatment appeared preferable in establishing higher levels of CS.

5.2.2 Influence of alkali activators

The popular alkaline activators employed in the polymerization process include alkali silicates and alkali hydroxides [95] comprising KOH or NaOH and Na₂SiO₃ or K₂SiO₃, which are the most commonly utilized in GPM. Si and Al types from aluminosilicates’ precursors actively dissolve in the alkali mixture. He et al. [96] investigated the structural behavior and microstructure of BR-FA derived GPM (BR-GPM), activated with Na₂O⋅3SiO₂ powder having a molarity of 1.5 M with varying contents of BR and FA weight ratio in the GPM. They also compared it with MK-based GPM (MK-GPM), which was activated with NaOH and Na₂SiO₃ solution. MK-GPM exhibited relatively a greater CS of 31 MPa. This increase in CS may be attributed to its higher pH value, which was even higher than 14. This pH was mainly subsidized by the addition of the NaOH mixture of 6.7–7.8 M, whereas BR-GPM (pH of 11.9) showed the largest CS of 13 MPa. Interestingly, this strength was obtained without the addition of extra NaOH and by only employing the remaining NaOH (20.2 wt%) available in raw BR. Likewise, He and Zhang [97] reported the structural behavior of BR-FA-GPM with two diverse silicate solutions, such as Na₂SiO₃ and Na₂O⋅3SiO₂ solutions. GPM activated using trisilicate solution showed more stiffness, more strain, and also improved CS. The availability of higher reactive silica in the solution supported the geopolymerization process better. Similarly, He et al. [8] developed the GPM using BR and rice husk ash and further activated it with NaOH to investigate the mechanical properties. They found that without Na₂SiO₃ solution, GPM showed a maximum CS of 20.5 MPa. Moreover, because RHA contains rich traces of reactive amorphous silica, Na₂SiO₃ or Na₂O⋅3SiO₂ solutions for BR-RHA GPM are irrelevant.

A previous experimental investigation produced an ASBC comprising 70% slag and 30% BR, activated with solid water glass (8% by weight of binders) and sodium aluminate clinker (8% by weight of binders) [90]. They evaluated the strength characteristics of ASBC and its resistance against chemical attacks concerning a 525 OPC (as per GB 175–1992). The CS obtained with ASBC was comparable with the strength developed with OPC. The ASBC showed a higher CS of 56 MPa with 28 days of curing period. Over the same period, the material’s bending strength improved to 8.4 MPa exhibiting inferior density, greater water requirement, and shorter ST. Its resistance against corrosion attacks, free–thaw cycles, and carbonation remained in comparison to that of 525 OPC. Hu et al. [12] prepared BR-GPM using alkali-thermal pretreated BR and FA by mixing BRs with dry alkali or solid NaOH pills resulting in calcination at 800°C for a 1 h duration. The pretreatment significantly enhanced the reactivity of Si and Al in BR. This further improved the dissolution efficiency in an alkaline solution resulting in maximum CS up to 23.8 MPa at 5% Na₂O, whereas BR-FA-GPM without thermal pretreatment showed much lower strength, i.e., 5 MPa.

One-part GPM (OPG) includes “just add water” GPM. These are developed by mixing the water right into the ready-mix GPM binders [98]. OPG utilizes different solid alkaline activators, for instance, hydroxides of calcium and lithium, Na₂SiO₃, Na₂O, K₂CO₃, and their mixtures [99]. Ke et al. [98] pretreated the alkali of BR with powdered alkali activators to obtain BR-based OPG with 10, 15, and 5% of Na₂O at 800°C for 1 h. With thermal treatment, silicates and aluminosilicates decomposed efficiently improving the reactive phase formation and resulting in peak 7 day CS of up to 9.8 MPa at 10% Na₂O accumulation. On the other hand, alkali addition adversely influenced the long-term strength of purely BR-based OPG. Figure 13 portrays the CS of BR-GPM at different curing periods with varying Na₂O contents in alkali [12,98]. This strength decreased due to excessive alkali hindering the formation of the Si–O–Si bond for both the GPM once an optimal amount of alkali was obtained.

Figure 13

CS of BR-GPM with varying amounts of Na₂O [12,98].

Lemougna et al. [7] produced BR-slag GPM in 75:25, 50:50, and 25:75 mixing ratios of BR and slag. They developed all the mixes with solid NaOH and Na₂SiO₃ solution and investigated their use in lightweight materials and mortars. The GPM’s CS of 50:50 mix samples tested at an ambient curing temperature had an upward-growing effect. This pattern of increasing strength was found to vary from 65 to 85 MPa and was associated with increments in silica modulus (SM) of the activator mixture ranging from 1.60 to 2.00. Beyond this optimum level of silica, the strength decreased to 60 MPa. This decrease in strength may be associated with the excessive silicates hindering the evaporation of water and breaking the GPM chain into different monomers, accordingly decreasing the strength. Zhang et al. [100] synthesized BR-FA-GPM using a 1.5 M Na₂O⋅3SiO₂ solution without NaOH and assessed the GPM’s potential feasibility for its application in pavement materials because of its strength developed to 13 MPa. Li et al. [101] investigated the strength parameters of BR-sourced composite materials (BCM) with AS. They concluded that the activator lime significantly affects the BCM unconfined CS (UCS). Because the AS predominantly comprises calcium sulfate, it promotes hydration products. Also, when BR reacts with lime, the sludge activates the BR. Binders of 28 days without activators showed a UCS of only 5.3 MPa. Due to the utilization of the lime in the test mix, amorphous ettringite under ambient circumstances was developed with a binder/lime ratio of 94:6, which led to an ultimate increase in UCS of up to a maximum of 12.05. Guidelines followed for BCM strength standards were in line with those suggested by MU10 FA brick (JC239-2001). A previous study [102] developed FA-derived inorganic OPG by utilizing BR as a source of NaOH or an alkaline activator. Comparing the CS of BR-dependent inorganic OGP with that of FA-based OGP activated with NaOH pallets, researchers found that the addition of 3–5% of NaOH in FA-based OGP exhibited almost the same pattern in comparison to those obtained by the addition of 40–60% BR in FA-based inorganic GPM.

Ye et al. [94] innovatively established OGPs with SF and BR by alkali-thermal pretreatment at 800°C for 60 min and then subjected the samples to cool at normal room temperature. A number of these BR specimens were obtained by pretreating the alkali with 10 and 15% of Na₂O. GPM comprising 75% of alkali-pretreated BR activated with 10% of Na₂O showed higher strength in compression; 20 days’ strength was a maximum of 31.5 MPa. Mucsi et al. [103] investigated 0.476 mol·l⁻¹ of alkali content in BR and observed its impact as an alkali and FA substitute in GPM derived from FA. The FA-based GPM with zero proportion of BR, activated with 6 M NaOH solution, showed a relatively lower strength of almost 2.6 MPa. It was found that the alkali content finally decreased by various percentages as FA was substituted by BR, which was gradually added to the GPM in different increments from 5 to 30% with increments of 5%. FA was substituted with 5% BR, and a 40% NaOH activator solution was added. The amount of NaOH added was decreased to 35% by the consequent accumulation of 10, 15, and 20% BR. Only 30% of NaOH was added to the GPM mixes containing 25 and 30% BR. Substituting the FA with 15% BR and curing the samples at 90°C for 3,600 min resulted in 7 days’ CS of 5.5 MPa.

Zhou et al. [104] developed GPM using waste mudstones, BR, and GGBS activated by the mix synthesized from NaOH and Na₂SiO₃ solutions. This GPM was prepared to utilize the refuse mudstones and those of alkali activators in different contents (%), e.g., refuse mudstones were in the range of 30, 50, and 70, whereas the latter constitutes varied from 10 to 30 with increments of 5; however, GGBS/BR ratio was kept to 7:3. Experimentation revealed that in comparison to the GPM strength obtained from varying percentages of alkali activator, variation in the refuse mudstones significantly influenced the strength. Alkali activators affected by the refuse mudstone influenced geopolymerization. Refuse mudstone is an inactive substance with crystalline mineralogy that severely influences both the mechanical properties and the geopolymerization process. Nevertheless, GPM with 20% alkali and 30% refuse mudstone content showed the highest strength value of 23.86 MPa.

Hu et al. [105] recommended not to use only NaOH but they suggested the use of NaOH and Na₂SiO₃ solution for maximum attainment of CS. BR being alkaline in nature improves the geopolymerization process; however, addition of additional NaOH is always recommended for higher strength levels. Therefore, based on the published work as cited, it is obvious that a mix comprising Na₂SiO₃ and NaOH solution frequently forms BR-dependent GPM. Being alkaline in nature and utilizing the available residual NaOH to synthesize the GPM, NaOH consumption is always less in BR-dependent GPM binders. This drop is much lower when compared with those of the binders derived from FA, slag, MK, etc. Interestingly, in situations where BR comprises more than 20% residual NaOH, Na₂SiO₃ solution is sufficient to synthesize the GPM without extra sodium hydroxide.

5.2.3 Influence of binder/sand ratio

Lemougna et al. [7] synthesized GPM mortars using ¼, ½, and ¾ sand proportions for slag-BR mud systems. Importantly, due to the weak interfacial connection between GPM’s internal structure and sand particles, 28 days’ strength of the mortar decreased with an increase in the sand content for all the slag-sourced mixes comprising 25, 50, and 75% BR proportion at a curing temperature of 25°C. Figure 14 shows the CS of the slag-BR-GPM paste with different sand contents.

Figure 14

CS of the slag-BR-GPM paste with different sand contents [7].

Nevertheless, the addition of a higher content of slag amounting to 75% significantly impacts the mortars’ strength. It was recommended to use the slag-BR-GPM mortar in the construction industry, particularly for low-budget units. As it is known that increasing the sand decreases the strength of the mortar, Singh et al. [106] prepared a new type of GPM bricks by partial utilization of the GBFS in place of sand and also by FA and BR. Figure 15 shows the resulting strength (both dry and wet) developed with varying ratios of binder to aggregates (BTA).

Figure 15

Influence of the ratio of binder/fine aggregate on CS of PM bricks: (a) 1:1 and (b) 1:2 [106].

Considering the BTA of 1:1 and 1:2, a higher concentration (%) of GBFS with 0, 20, 30, 40, and 50 was utilized in replacement of sand. With the increase in the BTA, GPM with BR content (%) of 10, 30, and 50 showed decreased strength. To achieve the maximum wet and dry CS of 9.50 and 11.10 MPa, respectively, 30% BR and 1:1 BTA was required. The CS was further increased to 15.50 MPa by using 50% of BR and 1:1 BTA with 50% GBFS. The highest CS of 23.86 MPa was achieved by introducing the GPM composite mortar, which was made up of BR, GGBS, and standard sand as aggregate with 1:1 BTA. Also, refuse mudstone and alkali activators were added to act as varying parameters in the mix [104].

5.2.4 Influence of curing

Singh et al. [19] investigated the strength of BR-sourced FA-GPM with two different BRs, i.e., unprocessed and processed BR. Both GPMs were subjected to ambient and thermal curing conditions. Compared to thermal curing conditions, ambient curing of BR-FA geomaterials comprising 30% BR showed a higher CS of 40.0 MPa with 6.0 M NaOH. Because thermally cured specimens experienced several crack formations, researchers suggested that GPM synthesized with pulverized BR should not be exposed to thermal curing. Likewise, to develop the highest CS with 38 MPa via thermal curing, unprocessed BR-sourced GPM needed greater concentration with up to a value of 12 M NaOH. Hajjaji et al. [61] observed BR-MK GPM’s mechanical properties using a thermal curing approach by keeping the temperature at 50°C for a whole day after maintaining the room curing temperature for the upcoming 24 h followed by 28 days of water curing. To investigate the impact of curing duration, GPM was tested for CS development at both stages of 24 h curing and 28 days curing. The GPM samples comprising maximum BR concentration (MK/BR ratio of 1:12 and 1:10) exhibited 25% higher strength after 28 days in comparison to that obtained with 24 h of testing. However, other mixtures having an MK/BR ratio of 1:6 and 1:4 resulted in a negligible increase of strength (with only a 10% increase in strength) from 1 to 28 days.

Pan et al. [90] found an increasing trend for the two types of strengths, i.e., in compression and in bending for ASBC subjected to a long-term period of 180 days of curing. Figure 16 presents the formation of the ASBC strength in bending and in compression. Zhang et al. [107] examined the durability of BR-FA-GPM (RFFG) subjected to H₂SO₄ and deionized water (DW) in varying conditions of curing temperatures, i.e., under an elevated temperature of 80°C and at normal room temperature. Researchers carried out different tests to investigate the leaching potential of GPM soaked in DW and SA for 128 days. With the leaching experiments, it was observed that the identified heavy metals were within the acceptable standards set by US EPA. GPM subjected to curing at 80°C performed comparably to that of GPM in ambient curing conditions derived from concentrated heavy metals, e.g., Cd, Cr, Cu, and As found in leachates. Leaching in BR-sourced GPM remained independent of thermal curing. Zhao et al. [108] investigated the impact of varying curing environments on the durability and mechanical characteristics of GPM developed from BR slurry and class F FA (RMSFFA). Exposing the 50°C cured test specimen to freezing–thawing cycles had improved durability and strength properties in comparison to those of room temperature and 80°C cured specimen. On the other hand, RMSFFA-GPM was more resistant to freezing–thawing cycles and exhibited the highest CS at a curing temperature of 50°C. Likewise, in comparison to those of 28 days cured specimen, 14 days cured GPM showed higher resistance against the freeze–thaw environment (of 50 cycles) without any notable reduction in strength (Figure 17).

Figure 16

Mechanical properties of the ASBC composite: (a) flexural strength and (b) CS [90].

Figure 17

Effect of curing conditions on UCS of RMSFFA composites [108].

Panias et al. [109] investigated the influence of curing conditions on CS of BR-MK-sourced GPM. At all curing temperatures between 80 and 40°C, it was revealed that GPM dried more rapidly during the first 3 h of curing. After 96 h of curing, the drying rate entirely decreased. Increasing the temperature of curing to 80°C from 60°C resulted in more notable cracks. At an optimal curing temperature of 60°C, 96 h of cured BR-MK GPM exhibited better strength in reduction with minor cracks at a moderate rate of drying. Considerable shrinkage was found at almost all the ranges of temperature during the first phases of curing where the drying rate was comparatively higher. The authors examined the mechanical characteristics of a GPM derived from MK and BR at various curing conditions, including room temperature, 50°C for 24 h, and 80°C for 5 h. Increasing the curing temperature resulted in a drop in both, i.e., CS and bulk density. This loss in CS and rapid hardening with the dissolution of GPM may be associated with lower ratios of silica/alumina, and the development of notable cracks in the mix due to considerable loss of water along with the development of voids at elevated curing temperatures. Furthermore, the addition of inert fillers such as sand enhances the amount of other inert fillers available in the GPM BR ultimately resulting in debonding and reduction in strength.

Nie et al. [4] noticed that increasing the curing age resulted in a consistent pattern in the strength of BR-FA-GPM sourced from both the novel and desulfurized BR. With the increase in the curing duration to 28 from just 3 days, BR-GPM and desulfurized-BR-GPM CS gradually increased to 15.20 from 6.00 MPa and 23.20 from 7.00 MPa, respectively. According to He et al. [8], BR-RHA GPM needed a prolonged period, i.e., 35 days for complete curing to develop the highest CS of 11.82 MPa. This outcome supports the findings of Sekou et al. [110] that deal with 33 days of cured BR-supported GPM with rice husk ash acting as an additive to gain the highest strength of 11.0 MPa. To attain maximal stiffness and strength, GPM with raw materials containing significant quantities of unreacted amorphous stages and non-reactive crystalline segments must be allowed to cure for longer periods. Lemougna et al. [7] obtained reasonably comparable CS ranging from 55 to 60 MPa using 7 days cured BR-slag GPM comprising 50% BR at standard curing conditions. Increasing the BR to 75% resulted in a significant decrease in CS of 10 MPa at 7 days, whereas, with the same content of BR, 28 days BR-slag GPM showed a maximum CS of 30 MPa. Therefore, it was concluded that to attain this strength with more than 50% BR, a minimum of 28 days of curing period is mandatory. Likewise, curing temperature ranging from 25 to 60°C also affects the GPM’s strength. GPM comprising BR up to 50% exhibited the highest CS of 85 MPa at 40°C, but with additional BR content of more than 50 strength was finally decreased. Commonly, heat curing yields rapid dissolution of silica–alumina composites with faster development of the GPM composite. Nevertheless, the source materials determine the threshold limits of the temperature, beyond which GPM is set rapidly due to inadequate dissolution and quick geopolymerization reaction that ultimately reduces the final strength. Bădănoiu et al. [111] examined the early age of 3 days CS of GPM comprising BR and waste glass (WG) powder activated with 5.0 M NaOH at 60°C curing temperature. The CS of composite materials, comprising 25% of BR instead of glass powder, improved as the curing duration increased. GPM, which was cured at 60°C and was 3 days aged, attained the highest CS of 16 MPa. However, with 1, 2, and 3 days of curing at 60°C, with subsequent curing up to 20°C for 7 days, the CS of the composites reached 15.90, 18.50, and 20.50 MPa, respectively.

Hoang and Do [112] investigated the advantageous influence of autoclaving with extreme pressure and higher temperatures on the dissolution efficiency of aluminosilicate composites. Instead of high-temperature curing under atmospheric pressure, this approach resulted in better strength of BR-FA-GPM. Without any additional NaOH,10 h of autoclaving at 201°C, under atmospheric pressure of 1.60 exhibited a better CS of 14.20 MPa. Additionally, with a pressure of 1.22 and 16 h of curing at 188°C, the CS increased to 20.10 MPa when 1 M NaOH was added. Researchers suggested autoclaved BR-FA-GPM for the masonry construction. Figure 18 represents the impact of curing conditions on the 7-day UCS of BR-FA sourced GPM’s base materials [113]. The mechanical behavior of these materials was assessed in comparison with those of FA-derived GPM and conservative lime-FA and cement-FA sourced stabilizing composites. Increasing the temperature to 38°C from 20°C resulted in better CS of BR-FA-GPM ranging from 10.50 to 16.10 MPa. The reported strength was much higher than that obtained for the mix prepared by utilizing conventional resources. However, decreasing the humidity levels (%) to 45 from 98 adversely influenced the CS.

Figure 18

Impact of curing conditions on the 7-day UCS of BR-FA sourced GPM composites [113].

5.2.5 Influence of plasticizers

To synthesize OGPs from SF and BR, Ye et al. [94] adopted a WSR in the range of 0.55–0.65. When the water retarder, 0.50% (by wt) sodium lignosulfonate (SNF),was added to regulate the mix’s workability, WSR decreased to 0.55, 0.50, and 0.45. As shown in Figure 19, the addition of 0.50% (by wt) dispersant SNF aided in the reduction of WSR to 0.45, to gain the highest 28 days CS of 31.50 MPa, and to impact the pH of binders that was reported to be higher in comparison to that of the GPM binders having no SNF.

Figure 19

Effect of 0.5% SNF on WSR and CS of BR-GPM pastes [94].

Li et al. [114] examined the BR-slag GPM grouting substances with several types of plasticizers including naphthalene-based superplasticizers (SPN), aliphatic-superplasticizers (SPA), and polycarboxylate-superplasticizers (SPC), and investigated their mechanical characteristics along with workability properties. Compared to SPC, SPN and SPA remained highly stable when subjected to alkaline conditions. Researchers found that the geopolymeric grouting substances showed improved fluidity when superplasticizers in various ratios of water to binders were added. SPN and SPA were more workable with better strength when the water-to-binder ratio was decreased. However, SPC performance remained poor due to less stability with an adverse impact on the CS.

5.2.6 Influence of molar concentrations

Concentrated alkali activators, e.g., NaOH, play a vital role in the rapid dissolution of aluminosilicate traces to expedite the process of geopolymerization and thus yield binders with better stiffness and higher strength [96]. Among these activators, utilizing 0–16 M of NaOH is considered an effective and frequently adopted approach for the geopolymerization of various aluminosilicate traces in different molar ratios [8,98,102]. Kim et al. [5] utilized NaOH mixture in 5, 3, and 1 wt% in BR-CNF (a combination of Na₂CO₃, Ca(OH)₂, and FA) amalgamated mixture, which led to pore size improvement, higher flowability, enhanced CS, better geopolymerization process, and development of CSH stages. Instead of increasing the water-to-binder ratio, increasing the amount of NaOH showed better mixing ability and initial-stage strength attainment in GPM.

Hu et al. [105] studied the impact of different concentrations of NaOH on the CS of BR-class C FA-GPM. The results obtained were compared with the findings derived from the experimentations of BR-class F FA-GPM. In comparison to another FA-derived GPM (up to 10 M NaOH), BR-based FA-GPM was developed with considerably lower NaOH concentration, ranging from 2.50to 7.50 M. Higher the amount of CaO in C class FA, the higher the CS obtained with 15.20 MPa at relatively lower content of 2.50 M NaOH mixture, even at normal curing temperature in the laboratory. Replacing NaOH with the solution of Na₂SiO₃ and NaOH led to a considerable enhancement in CS of all the test samples of GPM. Nonetheless, this increase was insignificant at higher curing temperatures. Ke et al. [98] developed BR OGP using alkaline activators with 5, 10, and 15% of Na₂O. They observed that the test samples gained maximum strength after 7 days of curing. However, at later stages of the curing period, the strength decreased reasonably. Because alkalinity yields rapid hardening at earlier stages and GPM develops early strength even after 24 h, the tested GPM comprising 10% of Na₂O showed a maximum CS of 9.80 MPa. Nie et al. [4] investigated GPM produced from received and desulfurized BR (after FGD) subjected to varying molar concentrations of NaOH (0–6.50 M) under normal curing environments (Figure 20).

Figure 20

Influence of NaOH concentration on 28-day CS [4].

Compared to desulfurized BR, original BR-based GPM containing residual NaOH requires a lower concentration of NaOH (2.50 M) to obtain the maximum CS of 15.20 MPa. However, BR after the FGD does not contain any residual NaOH; therefore, being lesser alkaline in nature, desulfurized BR-based GPM requires a higher level of NaOH concentration (3.50 M) to develop a maximum CS of 20.30 MPa. Even after 28 days of the curing period, FA-based GPM activated with 2.50 M NaOH solution failed to gain any strength in compression. In addition to the original BR, desulfurized BR proved to be a potential candidate in the synthesis of GPM. Alkaline activators in BR-dependent GPM containing the residual NaOH are generally much lower than that needed to produce the FA-derived GPM.

Singh et al. [19] studied the application of industrial products to activate the BR via slag/micro-silica and FA with the NaOH flake and Na₂SiO₃ solution. GPM under ambient curing conditions was prepared by using 10 wt% slag, whereas GPM subjected to thermal curing was developed utilizing 10 wt% micro-silica as an additive in the mix. An increase in the NaOH solution (M) from 6 to 10 resulted in a reduction in CS of BR FA comprising 30% BR and 10% of added slag. The specimen was subjected to curing under ambient conditions. Test samples prepared with 6.0 M NaOH solution gained a maximum strength of 40.50 MPa, whereas GPM with thermal curing having 10% microscopic and prepared with 12.0 M NaOH showed a maximum CS of 38.0 MPa. Figure 21 highlights the impact of the increase in NaOH concentration on CS of BR-based GPM [8,19].

Figure 21

Influence of NaOH molarity on the CS of BR-GPM [8,19].

Alam et al. [115] developed GGBS-stabilized BRs with the Na₂SiO₃ solution in different concentrations (M), e.g., 0.25, 0.50, and 1.00. With the increase in the concentration of Na₂SiO₃, both BR, i.e., one in slurry form NALCO red mud (NRM) and the remaining in a drier form (called HINDALCO red mud (HRM)) showed a substantial enhancement in the UCS. HRM-25% GGBS with 1 M Na₂SiO₃ exhibited 8.30 MPa of CS, whereas utilizing the same concentration of Na₂SiO₃, NRM GPM mix having 25%GGBS showed higher CS of 11.20 MPa. BR-dependent GPMs having different concentrations of alkaline activators with their corresponding CS are presented in Table 4.

5.2.7 Influence of the NaOH/Na₂SiO₃ ratio

Due to the corrosive properties of alkali hydroxide-based activating agents and their higher thermal emissions in aluminosilicates’ dissolution, a blend of alkali hydroxide and alkali silicate is recommended. To facilitate the reactivity between raw agents and alkali solution, Na₂SiO₃ along with NaOH is generally utilized. Cheng et al. [116] synthesized NaOH and Na₂SiO₃ slag-BR-based geomaterial known as slag-sourced cementitious materials (SRCM). Considering a NaOH/Na₂SiO₃ ratio of 1:3 for SRCM comprising 30 and 70% of BR and slag, respectively, they achieved 28 days’ maximum CS of 16.70 MPa. This strength gain was associated with alumina-enhanced dissolution. Likewise, Zhang et al. [107] developed BR-FA-GPM (RFFG) with NaOH, NaO₉Si₃, and DW with a proportion of 3:7:3. They also maintained Si/Al and Na/Al ratios of 2 and 0.6. Exposing the RFFG specimen to different conditions of DW and H₂SO₄ resulted in a significant impact on the durability of the tested samples. Soaking the RFFG specimen in acid resulted in a reduction of UCS from 10 to 6 MPa, whereas UCS decreased to 7.30 after immersing the samples in water for 120 days. Long-term exposure of the GPM to an aggressive environment yields comparable durability and strength with those obtained from OPC specimens subjected to a similar environment.

Hu et al. [105] investigated NaOH-activated BR-FA-GPM’s strength at concentrations of 2.50, 5.00, 7.50, and 10.00 M. The specimen was compared with the GPM activated with a composite solution of NaOH and Na₂SiO₃ with a ratio of 1.0:2.50 and concentrations of 2.50, 5.00, 7.50, and 10.00 M. Tests were conducted in severe and ambient circumstances. GPM derived from NaOH and Na₂SiO₃ composites led to enhanced CS at both elevated and ambient temperatures. NaOH-activated BR-class C FA (BR-FC) GPM exposed to thermal curing showed the highest CS of 18.30 MPa. On the other hand, GPM activated via the composite solution exhibited comparable results under both conditions, i.e., ambient and severe environments. Likewise, under similar temperature conditions, the BR-FC specimen with a composite solution achieved the highest CS of 30.30 and 34.50 MPa, respectively. However, NaOH-activated BR-FC was inadequate to achieve notable strength under an ambient environment. Also, necessary thermal curing was needed for the highest CS gain of 20.40 MPa. Ambient-cured BR-FF GPM with a composite solution showed a strength of 24.60 MPa. Hu et al. [113] synthesized a stabilizing composite BR-FA-GPM with a NaOH/Na₂SiO₃ ratio of 1:2.5 for pavement constructions and tested its feasibility for application in aggregate base concerning lime-FA and cementitious-FA agents. BR-FA-GPM base composites showed minor evidence of drying shrinkage with comparable UCS to those obtained from lime-FA and cementitious-FA derived composites, respectively. Singh et al. [117] synthesized BR-FA-derived GPM coatings with a NaOH/Na₂SiO₃ ratio of 1:2.5 while varying the BR (in %) with 10, 30, 50, 70, 90, and 100 M a NaOH ranging from 6 to 12. 28 days GPM comprising 30% BR activated with 12.0 M NaOH showed improved CS with 38 MPa.

5.2.8 Influence of the molar ratio

A few of the most important variables to formulate the geopolymerization include but are not limited to alkalinity (M₂O/H₂O) and silica modulus (SiO₂/M₂O), whereas M = K, Na, or Ca. An increase in alkalinity enhances the dissolution potential of solid aluminosilicate by leaching the alkali elements. Likewise, better strength characteristics can be achieved by catalyzing the geopolymerization with enhancements in SM [118]. Also, an increase in SiO₂/M₂O to 1.70 from 1.30 in BR-slag-derived GPM yielded superior strength in compression, and it dropped beyond the identified optimum range of SM. This drop may be associated with the establishment of cyclic silicates due to excessively concentrated silica species hindering the reaction between the tetrahedral monomers [93]. According to Hajjaji et al. [61], Na₂O/SiO₂ increases with an increase in BR concentration in MK-BR derived GPM at different ratios of MK/BR such as 0.25, 0.17, 0.125, 0.10, and 0.08. Increasing the Na₂O/SiO₂ to 0.10 in the GPM comprising MK/BR led to a better CS of 10.8 MPa at 28 days.

Lemougna et al. [119] synthesized BR-dependent GPM and activated them using a solid form of NaOH along with sodium of liquid glasses species using SiO₂/Na₂O ratios of 1.60, 1.80, 2.00, and 2.20. GPM comprising SiO₂/M₂O ratios of 1.80 and 2.0 cured at 60°C showed an increase in dry CS to 45 MPa when compared with wet CS of 25 MPa. This strength reduction (%) from 85 and 65 may be associated with the SiO₂/M₂O ratio ranging from 1.6 to 2.2, respectively. Lemougna et al. [7] witnessed strength gain from 65 to 85 MPa under the ambient environment of curing with the increase in SiO₂/M₂O ratio from 1.60 to 2.00 after incorporating the slag into BR-GPM comprising 50:50 ratio of the mix. At an ideal value of 2.0, SM supports rapid reactivity during the early phases of hydration and develops higher strength. Zhang et al. [120] studied the BR-class F FA-GPM using Na/Al ratio varying from 0.60, 0.70, 0.80, and 1.00 by changing the concentration of NaOH/Na-trisilicate solution from 0.2 to 0.4. BR is the primary raw material, and FA was sourced from three different locations. Initially, Si/Al was taken as 2 for all the test specimens. Both Si/Al and Na/Al play an important role in geopolymerization needed to activate the aluminosilicates’ dissolution. Using energy-dispersive X-ray spectroscopy (EDX), the authors observed that in comparison to those of the starting material, results obtained for Si/Al and Na/Al of the GPM composite were much better with the highest CS of 13.50 MPa in compression with maximum EDX as presented in Figure 22.

Figure 22

SEM-EDX results of GPM specimens: (a) B2-T23, (b) B2-T50, and (c) B2-T80 [120].

Figure 23 presents the impact of Na/Al on the CS of BR-coal MK GPM composites [121]. Test samples were developed with the consistent use of Si/Al of 1.20 and varying Na/Al ratios of 0.80, 0.90, 1.00, 1.10, 1.20, and 1.30 with different concentrations of NaOH. Additionally, the strength increased with an increase in the Na/Al ratio to 1 but it decreased afterward. Microstructural tests revealed that the GPM sample with a Na/Al ratio of 1 produced greater composites without unreacted BR. Surplus alkalinity above the optimal concentration had a harmful impact on microstructure and CS by preventing the development of GPM composites.

Figure 23

Influence of the Na/Al ratio on CS of the BR-coal MK GPM paste [121].

GPM strength mainly depends on the ratio of silica to alumina (SiO₂/Al₂O₃). Ye et al. [94] developed OPG by pretreatment of alkali-thermal BR and SF. They observed that BR-GPM without SF demonstrated low strength because of lower SiO₂/Al₂O₃ in BR. This ratio was significantly lower than that of the acceptable range of 3.30–4.50 [122]. The results demonstrated that the addition of 20% SF in OGP BR-GPM with alkali pretreated BR was stable in the long term when compared with strength in their initial stages. Also, the test samples attained the maximum 28 days’ strength of 13.50 MPa. The addition of SF ranging from 20 to 30% enhanced the GPM’s molar ratio to acceptable limits, i.e., H₂O/M₂O 10–25, SiO₂/Al₂O₃ 3.30–4.50, and M₂O/SiO₂ 0.20–0.48 [122]. Li et al. [123] prepared BR-sourced GPM with solid waste incineration FA (MSWIFA). The binding agents were automatically activated with a solution of Na₂SiO₃ and NaOH. Varying mass ratios of binders to alkali activators in the range of 100:8 to 100:16 were examined. As the concentration of Na₂SiO₃ increased to 14% from 8%, UCS increased to 10 MPa for BR-sourced GPM comprising 30% of MSWIFA and 70% BR with maximum strength observed for the mix developed from SiO₂/Al₂O₃ of 2.08.

Singh et al. [19] investigated the impact of processing the BR subjected to varying ratios of alumina to silica on the CS of BR-FA-GPM materials subjected to ambient curing and elevated temperature curing conditions. Mechanically activated BR was pulverized to improve the BR-sourced GPM’s strength by making the unreactive silica reactive in nature. BR was processed to a size even lower than 5 µm. Depending upon the processed BR, SiO₂/Al₂O₃ ratios for GPM subjected to ambient curing were in the range of 2.20–4.70 against the processed BRs (in %) added at 90, 70, 50, 30, and 0. GPM under 7 days of ambient curing conditions, depending upon the processing levels of BRs, comprising 30% BR and SiO₂/Al₂O₃ ratio of 4 resulted in the CS gain of up to 40 MPa. On the contrary, for GPM comprising varying incorporation of raw BR (%) from 90 to 0, SiO₂/Al₂O₃ varied to 5.50 from 3.0. Also, experimental results confirmed that the 7 day’s strength of BR-FA-GPM subjected to thermal curing increased by increasing the SiO₂/Al₂O₃ ratio. The highest CS was achieved at a SiO₂/Al₂O₃ ratio of 5.1 and it dropped thereafter. Pulverized BRs comprising reactive silica of 5.12% with BR size smaller than 5 µm reacted better than those of unprocessed BR having a reactive silica content of 1.62%. Bobirica et al. [124] studied BA-sourced GPM comprising 10% WG and 10–30% of BR. By the addition of the BR in the mix, the SiO₂/Al₂O₃ ratio of GPM increased significantly. Increasing the SiO₂/Al₂O₃ ratio to 5.98 increased the CS of GPM comprising 10% BR up to 14.50 MPa. On the other hand, the addition of BR from 20 to 30% increased the SiO₂/Al₂O₃ ratio to 10.40 from 8.23, which ultimately resulted in a strength reduction from 10.40 to 8.50 MPa. Ahmed et al. [89] investigated the impact of the increase in the SiO₂/Al₂O₃ ratio on the strength of alkali-pretreated BR-based GPM under varying curing duration. An increase in the SiO₂/Al₂O₃ ratio from 3.60 to 4.40 MPa showed a notable increase in the CS at both the 7 and 28 days of cured samples. For a SiO₂/Al₂O₃ ratio of 4.40, the 28 days’ maximum strength was 37 MPa. This increase in the strength may be associated with a higher concentration of silica that is contributed by the potential source of the aluminosilicate composite developed from the solution of Na₂SiO₃.

5.2.9 Influence of LSR

To synthesize BR-sourced GPM, the majority of the researchers recommend using the LSR in the range of 0.30–1.20 due to its influencing characteristics to develop the binding properties in GPM [12,97,125–128]. Dimas et al. [126] found that an increase in LSR tends to increase the CS of BR-MK geomaterials. To ensure the workable, strong, and shrinkage-resistant BR-MK geomaterials, the optimal SLR was found to be ranging from 2.80 to 3.0. Higher solid concentration in aqueous levels showed an increasing content of Si–O–Al, and Si–O–Si, which resulted in GPM binders having better CS due to the polycondensation phenomenon. Alkali pretreated BR-GPM showed an increase in the CS from 32 to 44 MPa for LSR of 0.55 and 0.50, respectively. With the increase in LSR from 1.00 to 1.50, BR-FA-GPM comprising 20% BR also resulted in better CS ranging from 3 to 13 MPa. This encouraging impact of the increase in LSR to improve the BR-FA-GPM’s strength was mainly because of the availability of better efficiency of reactive silica to dissolve the mix of solids [97].

Geng et al. [128] synthesized GPM using a 5.0 M NaOH solution along with sodium liquid glasses with an LSR of 0.40. Raw geomaterials were used with a ratio of BR/CG of 8:2 wt. The study recommended a novel approach of co-grinding sourced activation for the geopolymeric precursors without adopting the calcination process. The 20 min long grinding of the BR mixture along with CG was carried out in a planetary ball mill for milling at 200 revolutions per minute. In comparison to GPM activated via calcination, geomaterials milled by the co-grinding technique showed higher dissolution efficacy of SiO₂ and Al₂O₃. Hu et al. [12] developed BR-sourced GPM with an LSR of 0.45, using BR with alkali treatment along with FA. They activated the mix with the Na₂SiO₃ solvent. With a BR/FX ratio of 50:50, the alkali pretreated BR-FA mix attained the highest CS of 23.80 MPa with only 5% of Na₂O. Using an LSR of 1.20, He et al. [8] synthesized BR-GPM containing different weight ratios via the BR and RHA mixture. The mix containing an RHA/BR weight ratio of 0.50 and 4 M NaOH exhibited the highest CS of 20.40 MPa.

5.2.10 Influence of additives

According to Ahmed et al. [89], BR-GPM with nano-silica in varying contents had better strength properties. Nano-silica was incorporated (wt% of BR) in 0.10, 0.20, 0.30, 0.40, and 1.0. Studies revealed the encouraging impact of nano-silica on the CS of composites. Also, the addition of nano-silica in varying percentages in BR-made GPM led to better strength when compared with that of the control mixture, excluding the mixtures comprising 0.10 and 1.0% of nano-silica. Nano-silica with Ca composites yields accelerated development of CSH and the GPM composite that finally increases the strength properties. High amounts of nano-silica result in a substantial content of unreacted silicate ions that ultimately impact the CS of binders. Li et al. [92] investigated the potential of various types of gypsums, e.g., NG, FGD gypsum, and PG on the BR-slag grouting mortar in varying amounts [92]. The strength results achieved from these types of gypsum are presented in Figure 24. The 3 and 28 days’ CS of BR-slag grout had notable improvement when NG and FGD were added into the mix. However, insignificant enhancement in the strength was reported with the BR-slag grout incorporated with PG. Also, Bošković et al. [127] investigated the positive influence of different concentrations of NaOH (8 and 4 M) in BR-MK GPM with Ca(OH)₂. Nonetheless, BR-MK GPM exhibited a lower strength in comparison to that obtained with MK GPM. Incorporating Ca(OH)₂ supported the CSH development and ultimately improved the strength in compression. However, even with Ca, the CS reduced with an increase in alkalinity. This reduction in strength may be associated with more alkaline content preventing the formation of CSH and only allowing the formation of the GPM composite. A decrease in alkalinity yields better strength and a denser microstructure with minimum cracking.

Figure 24

CS of the BR-slag grouting GPM mortar having various forms of gypsum at (a) 3 days and (b) 28 days [92].

5.2.11 CS behavior

Many studies have suggested different approaches to mechanically activate BR, such as pulverizing, grinding, alkali thermal pretreatment, or calcination, to improve the CS of BR-dependent GPM. Likewise, to pre-active the BR, Geng et al. [128] recommended a co-grinding pre-activation process (CGPP) and studied the impact of CGPP of raw composites on BR-GPM’s strength. A blending ingredient known as CG, consisting of higher Al and Si concentrations was adopted. CGPP involving mix grinding of the majority of the raw composites was performed without calcination in a mill rotating at 2,000 revolutions per minute for 20 min. The CGP of BR with CG showed an improved geopolymeric process of the raw BR and enhanced the dissolution potential of alumina and silica resulting in the highest CS gain of 25.50 MPa after 28 days of curing. Thus, CGPP entirely removes the requirement of calcination by supporting the strength gain. Li et al. [123] examined the impact of mechanically activated BR by ball milling utilizing MSWIFA at 500 revolutions per minute. The process took 30 min to grind the particles of size (µm) lower than 80. Mechanically activated BR-MSWIFA-GPM comprising 14% Na₂SiO₃ solution showed better CS with 12.7 MPa in comparison to 10 MPa resulting from the GPM sample lacking mechanical activation. Increasing the BR (%) from 40 to 90, 4.0 M NaOH-activated GPM sourced from sawdust and acid-modified BR reported higher CS ranging from 138.0 to 8.30 MPa [125].

Zhang et al. also explored the positive impact of the particle size on mechanical characteristics of BR-sourced GPM and concluded that this GPM comprising finer and coarser particles both yield improved properties [129]. Therefore, based on desired characteristics, particle sizes can be classified. Because of the greater leaching potential of Al³⁺ and Si⁴⁺ with fine particles and the micro aggregate filling impact of coarse particles, mechanical characteristics improve significantly. Also with fine BR, Al₂O₃, SiO_2, and CaO amounts decreased, while that of Na₂O and Fe₂O₃ experience a slight increase. The increase in the particle size of BR decreased strongly in the beginning but eventually it increased. Mendes et al. [20] examined the role of BR in the mechanical characteristics of MK-based GPM paste. The addition of BR improved the bulk density of the paste; nevertheless, the CS dropped. The reason may be associated with a decrease in SiO₂/Al₂O₃ due to increasing BR content and also it is less potential to react in comparison to that MK. Table 5 presents the CS of BR-sourced GPM resulting from various heat curing circumstances.