Recent research in mechanical properties of geopolymer-based ultrahigh-performance concrete: A review

G.Murali

Division of Research & Innovation, Uttaranchal University, Dehradun 248007, India

Keywords:Mechanical properties Blast Projectile impact Fibre Geopolymer Silica fume Alkaline activators

ABSTRACT Due to the growing need for sustainable and ultra-high-strength construction materials, scientists have created an innovative ultra-high-performance concrete called Geopolymer based ultra-highperformance concrete (GUHPC).Besides, in the last few decades, there have been a lot of explosions and ballistic attacks around the world, which have killed many civilians and fighters in border areas.In this context, this article reviews the fresh state and mechanical properties of GUHPC.Firstly, the ingredients of GUHPC and fresh properties such as setting time and flowability are briefly covered.Secondly,the review of compressive strength,flexure strength,tensile strength and modulus of elasticity of fibrous GUHPC.Thirdly,the blast and projectile impact resistance performance was reviewed.Finally,the microstructural characteristics were reviewed using the scanning electron microscope and X-ray Powder Diffraction.The review outcome reveals that the mechanical properties were increased when 30% silica fume was added to a higher dose of steel fibre to improve the microstructure of GUHPC.It is hypothesized that the brittleness of GUHPC was mitigated by adding 1.5%steel fibre reinforcement,which played a role in the decrease of contact explosion cratering and spalling.Removing the need for cement in GUHPC was a key factor in the review, indicating a promising potential for lowering carbon emissions.However,GUHPC research is still in its early stages,so more study is required before its full potential can be utilized.

1.Introduction

Because of the constant increase in the world population and the construction of tall structures equipped with appropriate support, the most extensively utilized building material worldwide is Portland cement concrete (PCC) [1].According to environmentalists, PCC manufacturing is responsible for 30% of worldwide CO2emissions[1,2].A substantial quantity of CO2gas is released into the atmosphere while producing Ordinary Portland Cement (OPC),which harms individuals and the natural world.Consequently,there has been a surge in research into developing environmentally beneficial and less toxic alternatives to cement and other binders[3,4].Thus,the advancement and research of a geopolymer binder as an alternative to OPC have received considerable focus in the last few years.Aluminosilicate minerals such as Ground-granulated blast-furnace slag (GGBFS), fly ash (FA), metakaolin (MK), and alkaline solution (AS) undergo a chemical reaction to produce geopolymer binders[5].Glassy phases of slag can be dissolved with the help of AS, forming many different solid phases (C-A-S-H, for example), significantly affecting its mechanical, microstructural,and durability properties [6].

Three decades of research and development have resulted in ultra-high-performance concrete(UHPC),one of the most promising building materials for future sustainable and resilient infrastructure.UHPC has been a topic of growing interest in various countries over the last twenty years due to its environmentally friendly, excellent mechanical properties, with applications ranging from off-shore structures, bridges, hydraulic structures, building components,repair and rehabilitation,windmill towers and architectural features[7-16].Given its high particle packing density ranging from 0.525 to 0.855, the water-to-binder ratio varies from 0.15 to 0.25,with more than 120 MPa compressive strength and higher steel fiber dosage(>2% by volume).Because of its higher dosage of fibres, Ultra high performance fibre reinforced concrete (UHPFRC) displays strainhardening characteristics upon breaking [17,18].UHPC matrices have a higher density.This calls for removing coarse aggregates and the microstructural distribution of fine particles to fill the spaces left by the former [19].Enhancing the UHPC packing mechanism will undoubtedly result in exceptional strength, improvement in durability, and the complete efficiency of UHPC.However, consuming more cement to produce UHPC is one of the drawbacks from an environmental point of view.Concurrently, CO2emissions from Portland cement manufacture have been rising at an alarming rate in recent years [20].For this reason, scientists looked for alternate materials and aimed to limit cement usage[21,22].

Abbreviations used legends GUHPC Geopolymer based ultra-high-performance concrete PCC Portland cement concrete OPC Ordinary portland cement GGBFS Ground-granulated blast-furnace slag FA Fly ash MK Metakaolin AS Alkaline solution UHPC Ultra-high-performance concrete SLF Silica fume QZ Quartz MS Micro silica SP Superplasticizer PF Polypropylene fibre PVA Polyvinyl alcohol SF Steel fibre W/B Water to binder ratio S/B Sand to binder ratio MOR Modulus of elasticity BF Basalt fibre SWM Steel wire mesh TNT Trinitrotoluene SVM Support vector machine ITZ Interfacial transition zone CO2 Carbon dioxide GUHPFRC Geopolymer ultra high performance fibre reinforced concrete UHPFRC Ultra high performance fibre reinforced concrete DOP Depth of penetration RPC Reactive powder concrete DCE Dynamic cavity expansion ECD Equivalent crater diameter HPFRCC High-performance fiber-reinforced cement composite

The cement usage in UHPC is over two times greater than the conventional concrete, which leads to higher CO2emissions in UHPC.As a result,many scientists are concerned about the damage they cause to the ecosystem.Lessening these effects can be as simple as switching out the Portland cement in UHPC for a more sustainable one.The researchers adopted two approaches; (1)Utilization of GGBFS, FA and MK for the cement substitution in UHPC and (2) the usage of geopolymers as a binding agent in concrete has led to the creation of geopolymer concrete.According to Wu et al.[23],the flexural strengths of UHPC might be improved by substituting FA for 20% of the cement and GGBFS for 40%.Yu et al.[22]reported that by using a 650 kg/m3cement concentration to create a UHPC composite,CO2emissions might be cut by 30%.In addition, Amin et al.[21] found that the strength of UHPC made from ceramic wastes was increased when metakaolin and silica fume were substituted with a small proportion of cement.Using byproducts with a higher GGBFS and FA substitution level,Tahwia et al.[24] examined the sustainability of UHPC.Superior performance over conventional concrete can be attributed to geopolymer concrete[25],which has the characteristics of quick hardening and early strength development, exhibits excellent fire and corrosion resistance,and low permeability[26,27].Researchers have recently employed geopolymer as a binder to be more eco-friendly to create geopolymer-based UHPC (GUHPC).As GUPHC combines geopolymers with UHPC, it presents a promising option for the construction sector in the future.

Only a few studies have compared GUHPC to UHPC,revealing that cement and micro silica are the main components of UHPC binders.GUHPC binders, in the meantime, are predominantly made up of aluminosilicates and alkaline activators.Besides,the microstructure of cement paste is highly dense.In this way, UHPC is perfect for durability and mechanical strength [28,29].Ambily and Ravisankar[30]produced a GUHPC comprised silica fume(SLF),GGBFS and FA.A study [30] reported that the material had the highest compressive strength of 175 MPa.Wetzel and Middendorf[5]studied the results of using SLF as a partial replacement for slag in manufacturing GUHPC with a consistent amount of MK.Specimens with 15%SLF had a lower compressive strength than those with 12.5% slag replacement, and the highest compressive strength of 178.6 MPa was reached when 12.5% slag was replaced.Table 1 presents the many formulations of mixtures that various researchers have created.

According to a review of the relevant Ref.[44], the existing methods for creating mixtures used in GPCs may be broken down into three distinct categories:the target strength,the performancebased, and the statistical factorial model.The following are the steps involved in the target strength technique of mixture design:(1) Determine alkaline liquid-to-binder ratio or water-to-binder based on the compressive strength; (2) assess the workability and strength of the mixture to establish the ideal water or binder ratio;(3)calculate the amount of fine aggregate needed depending on the ratio of cement and coarse aggregate; (4) calculate the required amount of coarse aggregate depending on the sand ratio for optimal workability; and (5) change the ratio of ingredients to achieve the desired effect.The performance-based approach offers a great deal of subtlety in the production of geopolymer concrete,as it considers its whole strength and durability rather than sticking to the conventional view of water to binder and mixture design.The literature would provide further information [45].While compressive strength was the primary emphasis of this research[46], practical applications will also need to account for other strength characteristics and workability.The workability of geopolymer concrete is measured not only by its flowability but also by another crucial statistic,its setting time.Xu et al.[46]found that the initial setting time for GUHPC ranged from 25 mm to 55 min,while the final setting time was between 32.5 min and 62.5 min.Increasing the Al2O3and sodium silicate modulus led to a longer final setting time while increasing the CaO dose led to a shorter final setting time.As a result, the setting time of GUHPC can be adjusted to meet the requirements of the standard (initial setting time 25 min and ultimate setting time 180 min).

For most studies on GUHPC, GGBS has been employed as an aluminosilicate,activated by a solution of Na2SiO3and NaOH.These studies employ GGBFS,SLF and FA to enhance the microstructure of GUHPC.A combined alkaline solution is used to catalyze the activation of the raw ingredients rich in silicon and aluminum.Monomers are formed when dissolving SiO4and AlO4tetrahedra interact to share an oxygen molecule.The material microstructuresignificantly impacts the durability, dimensional stability and mechanical characteristics of GUHPC.Geopolymer microstructure provides insight into the formation mechanism of its macroscopic features.Hence,it is possible to adapt the mechanical properties of GHUPC by suitably adjusting its microstructure.However,research and reviews have been conducted on the mechanical properties of GUHPC with limited Refs.[47-49].However, the influence of activators and aluminosilicates on the mechanical and microstructure of GUHPC has not been thoroughly studied with up-to-date literatures.However, there is no comprehensive analysis contrasting GUHPC regarding fresh and hardened properties, performance against blast and projectile impact and microstructural characteristics.Because of this,the forementioned properties of GUHPC have all been thoroughly reviewed in this article.

Table 1 Summary of the mix proportion used by the earlier research (kg/m3).

2.Fresh properties of GUHPC

2.1.Setting time

The setting time of GUHPC containing different waste materials is depicted in Fig.1.Compared with the reference mixture, it was found by Tahwia et al.(2022)[2]that the GUHPC comprised GGBFS(647 kg/m3), SLF (216 kg/m3), sand (915 kg/m3) crushed glass(260 kg/m3, which is the 22.5% replacement of sand) typically increases the initial and final setting by 15%and by 14%,respectively(Fig.1(a)).Additionally, crushed ceramic in GUHPC exhibited minimal initial and final setting times and drastically reduced flowability, as shown in Fig.1(b).The research demonstrated that the incorporation of ceramic particles has a beneficial influence on speeding up the geopolymerization procedure.The hydrophobic properties of rubber and significant friction coefficient lead to a poor bond between the rubber particle and geopolymer gel.Hence,using crumb rubber causes a decline in flowability and the most extended setting durations (Fig.1(c)).According to Kathirvel and Sreekumaran [42], the mixture setting time is strongly influenced by the homogeneity; furthermore, it was investigated to lengthen with higher GGBFS replacement with SLF(Fig.1(d)).The extended setting time is primarily attributable to the enhanced waterholding potential of SLF, but the fact that it is non-reactive, like GGBFS, also plays a role.

2.2.Flowability

In Fig.2,a comparison is made between the effects of various fiber inclusions,GGBFS and SLF concentrations on the workability of fresh geopolymer ultra high performance fibre reinforced concrete(GUHPFRC).It is clear from Fig.2(a) that the incorporation of fibres into GUHPC dramatically modifies its characteristics in its "fresh state".[34].It has been determined that there was a rise in polypropylene fibre(PF)volume fraction[50-52],and the flow diameter was reduced.In addition,it is worth mentioning that incorporating a greater volume fraction of PF,such as 1.75%and 2.75%,resulted in the production of bit harsh mixtures in the fresh condition when the mode was static [34,53,54].Meanwhile, the highest flow diameter was recorded at the mixtures containing 15% of micro silica, irrespective of the fibre dosage used (Fig.2(a)) [34].A higher calcium concentration and its rapid reactivity with the alkaline activator reduce mixture workability, including fly ash or slag, where the coagulation rate and the rate at which dissolved species precipitated from fly ash were both affected by the presence of additional calcium[37,55].It was found by Aisheh et al.that the flow diameter reduced proportionally with increasing SF volume fraction[37](Fig.2(b)).It is also worth noting that more than 1.75%steel fibre(SF)added to fresh mixtures in the static mode resulted in slightly harsh mixes.Moreover,hydrophobicity is a key feature of high-strength PF.Due to the wetness of the geopolymer binder,PF is less affected and less likely to experience fibre clustering during mixing.

Fig.1.Setting time of GUHPC (a)-(c) Tahwia et al.(2022) [2]; (d) Kathirvel and Sreekumaran (2021) [42].

Fig.2.Flowability of GUHPC: (a) Tayeh et al.[34]; (b) Aisheh et al.[37] (SS/SH: ratio of sodium silicate to sodium hydroxide); (c) Kathirvel and Sreekumaran (2021) [42].

According to Shi et al.[38] the GUHPFRC has a flowability of 271 mm when no SF are added.Compared to the absence of SF,flowability decreases to 189 mm when the SF level is increased to 3%.When fibres are added, they establish a framework that impedes the movement of new mixtures.As the fibre concentration rises,the total specific surface area of the fibres rises along with it,occupying a greater percentage of the solution [38].Based on the prior studies [23,56], the researchers concluded that the UHPFRC flowability ranged from 160 mm to 230 mm.A greater thickness of liquid coating on solid particles is made possible by the larger water to binder(W/B)ratio(=0.32),which explains this phenomenon.As a result, reducing friction between the aggregate and fibres [38].Furthermore, the flowability improved from 234 mm to 249 mm when the SLF concentration was raised from 5%to 10%.At 20%SLF content, meanwhile, that flowability value drops to 221 mm.Possible explanation: when SLF content is below 10%, lubricating properties of SLF make the material more fluid.On the other hand,increasing the contents of SLF could cause a rise in the need for a solution because of the large specific surface area of SLF particles;thereby, the flowability is impaired.

Kathirvel and Sreekumaran [42] found that the flow decreased with increasing SLF concentration and Quartz(QZ)%.The degree of workability is affected by the shape and size of the particles and the filling effect.The SLF filling a GGBFS matrix pores enhances particle interaction and flow resistance at higher replacement levels.Adding 1% SF to the mix did not change the flow characteristics(Fig.2(c))[42].The flow value of a mixture comprised of 30%SF and 20%QZ is 157 mm when no fibre is present and 147 mm when fibre content is 1%.This may be because of the capacity of SLF to fill in the spaces around the fibres, preventing free water from the matrices,and the binder overall improved particle size distribution [42].According to Abbas et al.[57]the flow was unaffected by adding 2%of SF throughout the formation of UHPFRC.Tahwia et al.[39] reported that the maximum flow diameter (269 mm) for fibre-free GUHPC was achieved at 10% SLF.The slump value was decreased by about 3.4% and 6.7% when 20% and 25% SLF were included.By comparing the GUHPC with 10% SLF without steel fibre to the GUHPFRC with 25% SF containing 1, 2%, and 3% SF, flowability was lowered by 4.1%,8.6%,and 17.5%,respectively.According to Liu et al.[41]the flowability of GUHPFRC drops by 1.2%,5.4%,and 6.9%when 1%, 2%, and 3% fibre are added.The skeleton is formed from the random arrangement of steel fibres, which undoubtedly impedes the movement of the mixes [56].Tahwia et al.[2] reported that adding crushed glass improved the flowability and that increasing the crushed glass quantity improved the flowability even more.Compared to GUHPC without any waste material, the flowability with 7.5%,15%,and 22.5%waste glass in GUHPC mixes increased by 1.4%, 2.40%, and 3.75%, respectively.When increasing the Na2SiO3modulus from 0.85 to 1.83, the flowability was seen to decrease from 365 to 286 mm (a 21.6% decrease) reported by Xu et al.[46].Additionally,the FA content increased from 0.127 to 0.829(ratio of FA to slag + SLF + QZ) improving flowability by 11%.In a nutshell,the maximum slump value attained in GUHPC comprising a 10%SF and increasing beyond this percentage resulted in decrease in slump value.Furthermore, the flowability is decreased while the dosage of SF increased.

2.3.Contradiction and gaps in the research

Regarding the qualities of fresh GUHPC, to date, studies have ignored the potential impact of activator type and dose.As an additional perk, it has been overlooked that the aggregate type utilized in GUHPC could significantly impact the qualities of newly placed concrete.Also, not enough research has been done to fully understand how fibre dosage, aspect ratio and fibre type (polypropylene, steel, carbon, basalt, glass, polyvinyl alcohol, natural fibres and hybrid combinations) influence the fresh qualities of GUHPC.Considering the research done on silicious materials and their impact on the fresh properties of GUHPC, it is clear that silicious materials play an important role in this material.The setting time of GUHPC may be predicted to reduce if slag is used to substitute silicious materials, though this hypothesis has not been verified.Finally, the effect of molarity, Na2SiO3/NaOH ratio, precursor proportions and superplasticizer dosage on fresh GUHPC has not been studied extensively and need special attention.

3.Statical mechanical properties

3.1.Compressive strength

Compressive strength data at 28 days of testing from prior research is depicted in Fig.3(a)and(b).Aisheh et al.[36]reported reduced compressive strength when 0.50 vol% SF was replaced with PF in composite mixes; however, it was raised when PF was added to mixes that already contained SF, demonstrating that PF positively affected composite mixes.A steady increase can be seen in the compressive strength of the SF2.25 mix over time, from 123 MPa at 7 days to 155 MPa at 28 days to 170 MPa at 90 days.Improvements of 25%,37%,and 43%were seen when SF was added at a 2%by volume with respect to reference mix for the same ages.Additionally,the rising trend in compressive strength benefits from the decreased water/binder ratio brought forth by the increased Na2SiO3/NaOH ratio.Furthermore,a comparison between the ages of 28 and 90 days reveals that the rates of strength development slow down with age,suggesting that the microstructure of concrete improves over time [36].The rate at which aluminosilicates dissolve in solutions of increasing concentrations of NaOH is directly proportional to the value of the compressive strength gains attributable to this reaction[30,58].Aisheh et al.[37]reported that the average compressive strength is 102 MPa without adding SF.Compressive strength increases from 110 to 129-156 MPa for 1%,2%, and 3% SF content, respectively.On the other side, while using only 5% micro silica, the compressive strength of materials was measured at 128 MPa; however, using 10% caused a 19% decrease.However,when the percentage of micro silica was raised to 15%and 25%, notable gains in compressive strength were seen [37].This suggests that the steel fibre quantities can be reduced without compromising the compressive behavior of materials by increasing the micro silica content [59].

Liu et al.[38] reported that adding 3% of SF increases the GUHPFRC compressive strength to 154.9 MPa,53%greater than the value achieved without SF.When SLF is added to GUHPC, the compressive strength changes drastically.Compressive strength is measured at 128 MPa when 5%SLF is utilized; however, this value drops by 18.4% when the SLF concentration is increased to 10%(Fig.3(a)).In contrast,when the percentage of SLF is raised to 20%and 30%, the compressive strengths demonstrate substantial enhancements [38].According to Tahwia et al.[39] 10% SLF was utilized to achieve the lowest compressive strength of GUHPC,which ranged from 94 to 123 MPa.The activator existence primarily causes the GUHPC brittleness.Consequently, the activator properties can be modified with the help of SLF inclusion.Furthermore,increasing the SLF activator content to 10% has little impact on its qualities,and the compressive strength of GUHPC has significantly diminished.An interesting side effect of 25%SLF is that the activator silicate minerals concentration increases considerably due to its high specific surface area and activity.Thus, 25% SLF addition yielded the best compressive strength (124-152 MPa).Also,compared to a mix containing 10% SLF and no steel fibre, the strengths of the ones with 25%SLF combined with 1%,2%,and 3%SF were enhanced by roughly 43.6%, 58.4%, and 74.7%, respectively.The compressive strength of UHPC at 28 days was lowered from 124 to 120 MPa due to the addition of 22.5%waste glass[38].The same trend was reported when adding 3% of SF and strength lowered from 152 MPa to 148 MPa.It is difficult to bond the glass aggregate to the geopolymer matrix because the glossy surface of glass is the reason for this phenomenon.In addition, the rising trend of compressive strength may be negatively impacted by the interior gaps that glass particle angularity.Compressive strength has followed a similar pattern for aggregate glass combinations in previous research[62,63].

Fig.3.Compressive strength of GUHPC at 28 days [36-41,43,60,61].

Lao et al.[61] stated that the compressive strength of GUHPC varied according to the FA:GGBFS ratio, with the highest strength achieved at an FA:GGBFS ratio of 0.25,which allowed for values of compressive strength to exceed 210 MPa(Fig.3(b)).This is because GGBFS is more reactive to alkali activation than FA.Nonetheless,it was hypothesized that the high modulus of steel fibres was responsible for the 11.2 MPa and 22.8 MPa increases in compressive strength when the straight SF dose increased from 2%to 3%and 4%[61].Mousavinejad [40] reported that the compressive strength decreases when 0.25% of SF by volume is substituted with PF in composite designs.Compressive strength was improved by including PF in SF-containing designs, demonstrating the material usefulness in composite construction.Both the normal and steam curing processes result in compressive strengths of GUHPC that are 100.6 and 101.9 MPa, respectively, when SF is not included in the formulation[41].The compressive strengths of the material under both of the curing conditions increased up to 157.7 and 170.3 MPa,respectively when 3%SF are added to the mix.Increasing SF content would decrease the average distances between the fibres, making the material stronger and better able to support load [56,64,65].Kim et al.[43]stated that the maximum compressive strength was recorded up to 160.7 MPa with the corresponding sand to binder(S/B) ratio of 0.8, which was 14.7% higher than the weakest value measured at an S/B ratio of 0.16(Fig.3(b)).Because of the numerous constituents effect on particle packing, the packing density of GUHPC improved when the ratio of S/B was raised [66].After the silica sand occupied the volume,the micropores were occupied by GGBFS particles and very minute SLF particles.

In their study, Tayeh et al.[34] found that 15% micro silica produced the lowest compressive strength in GUHPC.The inclusion of activators is the primary cause of GUHPC brittleness.Furthermore, micro silica helps modify the activator features [67-69].When micro silica (high effective surface area and activity) was added at a concentration of 35%, the activator produced greater(SiO4)4-, leading to increased activity.Therefore, the highest compressive strength was measured after using 35% micro silica[34].Compared to the control mixture that did not contain micro silica, the compressive strength was enhanced by approximately 20%,19%,15%,and 17%when 35%micro silica was included with 0,0.75%, 1.75%, and 2.75% PF (Fig.4(a)).Because of its bridging properties, PF increased both the aggregate-to-paste cohesion and the concrete tensile strength; this restricts lateral expansion and enhances the compressive performance of PF-based GUHPFRC.Mousavinejad[40]reported that the significant rise of compressive strength had been aided by the decreasing trend of the W/B ratio,which has resulted from the rising Na2SiO3/NaOH ratio.Compared to Na2SiO3/NaOH=1 with 8 M NaOH concentration,the proportion of Na2SiO3/NaOH increased from 1 to 3, while the ratio of W/B reduced by 5.5% and 8.5%, and compressive strength improved by 6.4 and 11.4%,respectively(Fig.4(b)).When compared to the ratio of 2, these shifts are easier to spot when observed in the Na2SiO3/NaOH ratio of 3.A study showed that without changing the proportions of QZ and fibres, and the results showed that increasing the SLF content improved compressive strength [42].The highest compressive strength of mixtures containing 15%and 30%SLF was 13.8%and 33.7%greater than those containing no SLF,respectively,while using a 20%QZ content.The same findings were obtained for the mixtures with a 30% QZ content, which peaked at 20.3% and 34.6%, respectively.Maximum compressive strength was 10.54%higher in mixtures containing 1% SF volume and 20.25% higher in mixtures containing 2% SF volume compared to the fibre free mixtures.The highest compressive strength was reported for the GUHPFRC containing 40%QZ with 2%fibres volume(Fig.4(c)).The strength capability can be reduced from 152 MPa to 131 MPa by adding 7.5% crushed ceramic as a substitution for fine aggregate(Fig.4(d))[2].Crushed glass can be used in place of fine aggregate,and the strength of the concrete gets better as the percentage of crushed glass increases.The reactive (amorphous) silicon dioxide present in the glass shows to mix with the geopolymer composites,leading to a high-density microstructure that displays large phases of C-A-S-H products and hence, enhanced compression performance for the crushed glass [2].

3.2.Splitting tensile strength of GUHPC

The addition of SLF to steel-fiber-reinforced geopolymer concrete has been shown to strengthen the matrix hold on the reinforcements[70].The alkalinity was nevertheless decreased, and the activating modulus was raised [71,72].The detrimental effects of SLF on the activator must be considered while selecting the ideal concentration of SLF in the GUHPC to produce maximum tensile strength.It is reported that the splitting tensile strength is increased by 33%when 2.25%SF by volume is added to conventional concrete[36].The results indicated that adding 2%PF to GUHPFRC was beneficial of attained the highest tensile strength(8.6 MPa),as shown in Fig.5.As a result of the bridging effect on cracks, the tensile strength of GUHPFRC is significantly greater than that of non-fiber GUHPC.According to Mousavinejad and Sammak[40]findings,the tensile strength of conventional concrete is increased by 30.17%when a 2%dosage of SF is added.The GUHPFRC contained 1.75% SF and 0.25% PF reduced the tensile strength by 4.0% compared to the GUPHFRC containing 2% SF.Kim et al.[43]stated that the maximum tensile strength was recorded up to 10.38 MPa with the corresponding S/B ratio of 0.8,which was 44.5%higher than the weakest value measured at an S/B ratio of 0.16.A study reported that the maximum tensile strength of 16.4 MPa was measured in GUHPFRC specimens reinforced with SF, and this was achieved when 30% of the SLF was substituted with slag [38].The denser microstructure that resulted was able to counteract the activators detriments.Also,the hydration products and substrate density have both grown as the amount of micro silica substitution has risen.The 3%SF volume produced the highest tensile strength in GUHPFRC.Hardening strain behavior was induced in UHPC samples using polyethylene fibres,while the same fibres induced the same behavior in GUHPFRC [73-75].Tensile strength was noticed to rise between 16% and 64% when using PF ranging from 1% to 3% in place of slag when no SLF was utilized as a replacement for slag.Tensile strength decreased when 10% SLF was used instead of slag and adding 3% PF with 30%SLF enhanced the tensile strength by around 5 MPa.This can be explained by the increase in tensile strength of the cementitious matrix that occurs with high SLF levels.Indeed, 30% SLF could improve the microstructure of GUHPFRC together with higher dosage of SF,which led to greater tensile strength.

3.3.Flexural strength of GUHPC

The results of flexural strength obtained by several researchers are depicted in Fig.6.According to the Aisheh et al.[36]results,the flexural strength of GUHPFRC with 2.5%SF was 13.7 MPa,which is 91% greater than the strength of the control mix.Compared to GUHPFRC, which had 2.25% of SF, the GUHPFRC strength was reduced by 1.75% due to the inclusion of 0.25% PF and 2% SF.Compared to the GUHPFRC that included 0.25% SF and 0.25% PF,adding 0.5%PF resulted in a 3.65%increase in strength.According to Abbas et al.[57], the fibre pull-out process generates numerous microscopic cracks in the contact region due to the adhesive and significant mortar stiffness and SF in UHPFRC.Whereas PF fibres are more easily broken than SF ones,in some cases,it has been possible to extricate them from mortar without damaging the surface contact.The benefits of fiber addition are increased flexural strength and ductility in GUHPFRC [30].However, the amount of growth is influenced by many parameters, including the type, material, size,and proportion of fibres employed.According to Mousavinejad and Sammak[40],2%SF designs have the maximum flexural strength of GUHPFRC (12.80 MPa); furthermore, increasing the volume of SF from 0%to 2%increases the strength by 88.47%in comparison to the fibre-free control specimen.Compared to the GUHPFRC with 2%SF,the flexural strength GUHPFRC was decreased by 1.41% due to adding 0.25%PF and 1.75%SF.A study reported that the steam and conventional cured GUHPFRC see substantial gains in their ultimate flexural strength as the fibre dosage of the material is increased[41].For the flexural strength of GUHPC, 28 days of conventional curing yields 4.6 MPa, whereas 1 day of steam curing yields 8.7 MPa.Adding 3% SF volume fraction, flexural strengths at both the standard and steam curing conditions increased correspondingly by 434.1%and 189.4%.The fibre bridging effect brought about by adding SF greatly improves the ultimate flexural strength.The bridging effect is triggered once the initial break appears, and a portion of the load moves from the matrix to the fibre.A higher flexural strength would result from the load being distributed along both fibres and the interface.Flexural strength can also be improved by raising the fibre content, significantly increasing the overall contact surface between the fibres and matrix [76,77].PF strengthens and enriches the geopolymer cement paste matrix,stops microscopic cracks from releasing,and modifies when cracks spread [78].Surface hardness tests show that SF is superior to aggregate; also, this fiber helps decrease pores and improves bonding in the mortar transition zone where it contacts the SF[79].The developed geopolymer gel, non-reacted alumino-silicate particles, and any voids present all contribute to the overall geopolymer structure [78,80].By surrounding the geopolymer matrix at both ends, fibre can act as a bridge across cracks and voids, increasing the geopolymer matrix strength and stiffness.Hence, flexural strengths in fibre designs have been superior to those in non-fiber designs [78].In addition to increasing flexural strength, increasing the amount of slag used to substitute SLF positively affected the GUHPC microstructure.Moreover, studies reveal that a flexural strength of roughly 23 MPa was reached by replacing 30% of the SLF with slag [38].To be more precise, it has been found that increasing the proportion of micro silica substitution up to 30%results in a binder matrix with higher density and better microstructure overall [81-84].

Fig.4.Compressive strength of GUHPC: (a) Tayeh et al.[34]; (b) Aisheh et al.[36]; (c) Kathirvel and Sreekumaran (2021) [42]; (d) Tahwia et al.(2022) [2].

Fig.5.Tensile strength of GUHPC recorded by various research [36,40,43].

Fig.6.Tensile strength of GUHPC recorded by various research [36,40,41].

3.4.Modulus of elasticity of GUHPC

The results of the modulus of elasticity (MOE) recorded by the researchers are depicted in Fig.7.Earlier studies have demonstrated that geopolymer concrete exhibits a lower MOE than conventional concrete [85-87].Both UHPC and GUHPC experienced the same behavior.When comparing materials with identical compressive strength,the MOE provided by GUHPC specimens is lower than that of UHPC[47].Aisheh et al.[36]found that an increase of 2.25 vol%in the SF content yields a 131%improvement over the reference mix without fibre.Aisheh et al.[37]stated that the MOE of 27 GPa was attained when SF was omitted from the GUHPFRC mix, while the MOE value was 28 GPa when the SF content was 1%.When the volume of the SF reaches 2%and 3%,the MOE values increase to 30 GPa and 32 GPa,respectively.Since the mean space between fibres reduces as SF content rises, cracks in the matrix have a more challenging time starting and spreading.The MOE of specimens can also be improved with the help of SF with a greater elastic modulus.Liu et al.[38] reported that the recorded MOE values of GUHPFRC were 25.8 GPa,27.6 GPa and 31.5 GPa,corresponding to the 1%,2%and 3%dosages of SF,which are higher than the GUHPC without SF.Similar patterns of MOR fluctuation have been observed in previous research on geopolymer composites reinforced with SF[88,89]and UHPC [90,91].When SLF is included in GUHPC, the MOE changes drastically.The MOE value was measured to be 29 GPa when 5%SLF was utilized; however, once the SLF content was increased to 10%,the values dropped by 21.2%,accordingly[38].However,as the SLF concentration is increased to 20% and 30%, the MOE drastically improves.

As a result of its potential to reduce GUHPC expenses, PF has been a topic of discussion amongst researchers.However, its modulus of elasticity can be decreased if SF is used instead.The MOE can be decreased by 1.11% when SF is replaced with PF at a 0.25% substitution level [40].While the MOE of UHPFRC was maximized with the optimal percentage of SF alone,it was further improved by adding PF.Based on the correlation between MOE and compressive strength found in the literature, researchers may extrapolate how adding fibres to GUHPFRC will affect the MOE[3,93-95].The highest MOE in the UHPFRC was achieved using PF at a concentration of 3%[3].Because of this,the cement matrix and aggregates were strengthened,and cracks were stationary in their tracks.Optimal MOE was achieved by replacing 30%of the slag with SLF, while results were comparable to those obtained by replacing 30%of the slag with SLF while using 20%SLF[3].Complicated micro silica impacts are sensitive to the right combination of activator modification and microstructure improvement.The fibres also contributed to a greater MOE for the fiber-reinforced concrete specimens than the paste produced [96].

3.5.Contradiction and gaps in the research

Fig.8.Failure modes of commonly encountered concrete slabs [92].

Fig.7.Modulus of elasticity of GUHPC [36-38,60].

Although an increase in fibre percentage will likely enhance mechanical properties of GUHPFRC, this section could use some clarification.Also, since PF naturally leads to increased porosity in concrete, it stands to reason that as the PF content in GUHPFRC rises, it also affects concrete ability to withstand compression.Furthermore, the SF in conventional concrete is restricted to 2%because fibre balling leads to holes, lessens density when compacted, and creates defects that lead to reduced mechanical properties.However,it is reported that 3%of SF in GUHPFRC resulted in increased mechanical properties, which contradicts the literature.The findings of the study that has been carried out up to this point in the field of GUHPC conflict with this issue.This section disregards how the selection of fibre type and aspect ratio, the hybrid combination, can affect the mechanical properties of GUHPFRC.There has not been a complete analysis of how various silicious materials affect the mechanical properties of GUHPC under different curing circumstances (normal and accelerated).The optimum percentage of SLF in GUHPC is still unclear and requires further investigation.Furthermore, it has not been well explored how aggregate and activator types affect the features of hardened GUHPC.Finally, the other GUHPC properties, such as creep,shrinkage,impact strength,fatigue,abrasion and fire,have not been investigated by any researcher till now.

4.Performance of GUHPC against explosion

4.1.Damage at the top and rear side of the slab

Destruction to buildings and casualties to people in the area immediately surrounding an explosion can be caused by the blast loads.However, further injuries and damage are caused by the high-velocity waves and pieces left behind from the explosions that hit the already damaged structures.Inadequate structural resistance or a lack of preventative measures can have terrible results,including massive deaths and injuries and expensive property loss.Also,in the previous decades,there has been a worldwide increase in terrorist threats and unintentional explosives such as incident gas explosions and blasts.Very localized damage is typical of the brittle failure response of high-strength concrete buildings to contact explosions; even when reinforced with steel rebars, the shear and tensile strengths are inadequate [97,98].The structural components are suspectable to fragmentation, damage, cracking,shattering and penetration in case of explosive attack.As a result,concrete fragments can fly at tremendous speeds, inflicting considerable collateral damage on nearby buildings, people, and machinery.Many researchers have considered varying approaches to this problem, including the use of fibres to increase crack resistance[99],moderating how materials absorb and use energy[100],enhancing ductility [101] and slowing the rate at which peeling debris is produced[102].Hooked end fibres added to high strength concrete alter the blast reaction and the nature of damage and failure,protecting the structural integrity of the slabs [103].Fibres sewn into their lips assist in preventing flexural cracks.The primary factor for this improvement is the bridging effect,which reinforces the concrete after fiber addition[104].The mechanical properties of steel fibres,precisely their tensile strength,are a significant factor.Many scientists have looked into the unique material because of its superior blast resistance.Li et al.studied the behavior of UHPC slab during contact explosions using experimental and numerical methods [105,106].Using UHPC in place of ordinary strength concrete showed promise in increasing the blast-resistant ability of the reinforced concrete slabs during contact explosions.This indicates the excellent blast performance of UHPC; however, the performance of GUHPC is similar to the UHPC.The summary of the key findings regarding the GUHPC against the explosion is demonstrated in Table 2.Cratering,cratering and spalling,perforating,and punching were the four most common types of damage seen;

(1) Crater damage: a crater is created on the front side of the slab, cracks begin to radiate outward and form circles, and the rear face does not exhibit any signs of spalling(Fig.8(a)).

(2) Cratering and spalling damage:As concrete spalling occurs on the rear,it causes a crater to appear on the front;but still,the crater and spalling do not appear to be merging(Fig.8(b)).

(3) Perforating damage:The front end suffers a crater while the back end spalls.At the same time, these grow in size until they merge, and the diameter of the crater is far more significant than the breachs(Fig.8(c)).

(4) Concrete-spalling is avoided by punching out the concrete region that was hit most by the contact detonation in advance (Fig.8(d)).

Xu et al.[92] investigated the effects of contact explosions on innovative GUHPC slabs made with multilayer.Three different GUHPC slabs were designed (Fig.9); (1) Slab-I with 10 layers of steel wire mesh were introduced in GHPC,(2) Slab-II introduced a 8 mm diameter rebar, 4 layers of steel wire mesh and four polyurethane foam plate and (3) Slab-III of 8 mm diameter rebar, 4 layers of steel wire mesh, two polyurethane foam plate and two honeycomb aluminum plate were introduced.The typical explosion test setup for the slab used by the researcher [92,107,108] is shown in Fig.10.Results revealed that the GUHPC with 10 layers of steel wire mesh exhibits a diameter of crater and spall 372.5 and 675 mm, respectively; the crater had a total depth of 30 spalling total of 10 mm.Besides, spalling and crater diameters on slab-II measured 377.5 mm and 311.3 mm, respectively, and the total depth of spalling and crater measured 80 mm and 120 mm,respectively.Further, comparing slab-II, there was no evidence of spalling failure in slab-III,and the crater depth was decreased from 120 to 55 mm.The crater depth improved by 50%,going from 30 to 55 mm, compared to slab-I.Compared to the other slabs, slab-III displayed the highest level of blast resistance overall.This may be because the energy-absorbent compressibility of material and increased acoustic impedance decreased the"shock improvement"in the slab-III scenario.The layers of aluminum honeycomb panels,on the other hand,were able to absorb and diffuse the blast energy.

Liu et al.[108]investigated the GUHPFRC reinforced with 20 layers of BFM and SWM and 1.5% dosage of SF with the slab coated with polyurethane and tested against the contact explosion.The results show that compared to the BFM, the GUHPFRC significantly outperformed it regarding slab fragments, local damage area, and integrity of structure during the contact explosion.By utilizing its extremely high tensile strength and ductility,it can absorb the energy caused by the blast.The rear side of the slab coated with polyurethane can reduce the damage from scabbing and stop the pieces from flying everywhere during contact explosions.Additionally,substantial spall damage was seen on the rear portion of the 200 mm thick GUHPFRC slab reinforced with 1.5% SF under 1 kg of TNT during the contact explosion activity.To overcome the brittleness of GUHPC, Liu et al.[107] suggested that 1.5% SF reinforcement was successful, hence assisting in reducing contactexplosion cratering and spalling.Further,the localized damage categorization of GUHPFRC slabs containing 1.5% SF exposed to contact explosions was accomplished using a machine learning approach,support vector machine(SVM)[113,114],rooted in the statistical learning theory.If onlya small quantity of data was provided,this technique excelled above other machine learning algorithms because of its superior categorization,regression analysis,and pattern recognition capabilities.Peng et al.[115] effectively detected and forecasted the local damage caused to fibre-based UHPC slabs of varying thicknesses and TNT charge weights.Multi-classifier damage detection based on SVM accurately assessed the local damage GUHPFRC slabs with 1.5% SF subjected to contact explosions.Varying TNT charge weights from 0 kg to 1.1 kg and thicknesses ranging from 60 mm to 320 mm were examined.Fig.11 present the resultsof the calculations according tothetraining and tested samples usedfortheSupportvectormachine(SVM)algorithm.Theyellowarearepresents the perforation,the crater and spall are represented by the green area and the purple region represents only the crater in this diagram.Due to the absence of test data,itis essential to point out that the precision of the damage detection multi-classifier was still severely constrained.As a result, it was determined that additional experimental research was required to improve the accuracy of the damage detection multi-classifier.

Table 2 Summary of explosion test results on GUHPC slab.

Fig.9.The layout of the novel GUHPC slabs [92].

4.2.Damage pattern of the slab under explosion

Fig.10.Testing arrangement for explosion test [92].

The fibre-free GUHPC slab (150 mm thick) and the 2% BFreinforced GUHPFRC slab (200 mm thick) subjected to 0.4 kg and 1.0 kg TNT suffered total fracturing into multiple fragments(Figs.12(a) and (b)).Crater and scabbing damage pattern was observed in a 1.5% SF reinforced GUHPFRC slab that was 150 mm thick when subjected to 0.4 kg of TNT (Figs.12(c) and (d)).It was revealed that the fibre-free GUHPC slab under 0.4 kg TNT suffered greater brittle destruction(full fracturing)than the normal strength concrete slab with steel rebar (perforation).This is ascribed to the concrete made with geopolymers being more brittle than concrete made with Portland cement, which led to their unfavorable performance and the enhanced brittleness of fibre-free GUHPC resulting from the improved strength of concrete [116].Flexural displacement of the slab was blamed alongside blast stress waves for the catastrophic failure behavior exhibited in GUHPC.Blast stress waves and reflection propagation produced radial cracks across the crater and spall regions.The reported failure process was consistent with the concept of stress wave propagation.Specifically,the contact detonation caused the high compressive stress wave was found to have surpassed the compressive strength (dynamic)of GUHPC,which led to the crater failing(Fig.13(a))[92].After that,the bottom surface mirrored the compressive stress wave as a tensile stress wave;GUHPC has a poor tensile strength,which led to the spalling failure.The GUHPC slab comprised a 10 layers of steel wire mesh and displayed a crating and scabbing failure(Fig.13(b)).Spalling was found on the rear side, and several radial cracks extended outward from that area.At the same time,a major version and a rip in two layers of steel wire mesh were spotted.GUHPC slab reinforced with 4 layers of steel wire mesh and four layers of four polyurethane foam plates exhibited a punching failure (Fig.13(c)).GUHPC slab with 8 mm diameter rebar,4 layers of steel wire mesh,two polyurethane foam plates and two honeycomb aluminum plates results in no spalling produced at the rear of slab, and the slab cracked crater-like (Fig.13(d)).

4.3.Gaps in the research

The performance of GUHPC slabs against the contact explosion of 0.4 kg-1.0 kg TNT were presented in this review.However, the GUHPC slab with varying thickness, length, width, steel ratio,varying space between the reinforcement,diameter of bar different explosive charge, and explosion with stand-off distance has not been investigated by any of the researchers in GUHPC.A comparison of the damaged crater and spalling parameters (length, width and depth or diameter) under the contact and stand-off distance explosion is not studied before for any composite and a huge research gap is left in this research topic.Additionally,several vital parameters,including explosion pressure,time,and wave velocity,could be the scope of future study.Finally, the explosion test can also be conducted for the research gaps mentioned in subsections 2.3 and 3.5, which also could be considered for future studies.

5.Dynamic behavior

5.1.Projectile impact

Explosion and impact loading hazards may likely cause damage to structures during their design life.Due to its high strength,great ductility,and outstanding toughness,UHPC made from cement has been created in response to the rising need for the structural safety necessary to withstand such devastating heavy loadings.Due to its high strength [117], great ductility, and outstanding toughness,UHPC made from cement has been created in the past times in response to the rising need for the structural safety necessary to withstand such devastating heavy loadings or projectile impacts[118-123].Research into the mechanical properties of UHPC has increased dramatically in the last few years.A scant study has been dedicated to improving the GUHPC resistance against projectile impacts in the design phase [124-126].Recently, Liu et al.[126]provided a methodical validation of the Holmquist Johnson Cook model for ordinary and fiber-reinforced GUHPC.The validation focused on the equation of state, damage progression, strain rate effect and strength surface.The proposed model was shown to be entirely accurate when comparing the predicted and measured penetration depth for GUHPC objects after projectile impact.Liu et al.[124] investigate the impact damage on various GUHPC targets,including volume loss,crater diameter and penetration depth,and their results are shown in Fig.14.Also, with GUHPFRC reinforced with a 2%volume of 15 mm SF and 1%volume of 15 mm BF,and 1%volume of 15 mm SF,the results of earlier research[125]are also presented in Fig.14.By contrasting conventional GUHPC and ceramic ball based GUHPC, it was seen that the latter significantly improved upon the former in terms of reducing penetration depth.This behavior is due to the presence of coarse aggregates with high strength,the elastic modulus and compressive strength of concrete were improved, and projectile trajectories were altered and mass erosion occurred.Furthermore, ceramic balls could serve as barriers and generate friction to perform better over projectile penetration, decreasing penetration depth.In addition, the presence of coarse aggregates assisted in the dissipation of a greater amount of fracture energy throughout the penetrate process [127].Considering that a greater number of cracks developed within the coarse particles as opposed to moving anywhere along interfaces between the concrete matrix and coarse aggregate [123].The addition of hybrid combination of SF and BF in GUHPFRC resulted in a great reduction in the depth of penetration (DOP) (Fig.14).The hybrid combination of SF and BF based GUHPFRC significantly reduced the crater diameter to 135 mm and 140 mm under the velocity of 550 m/s and 800 m/s, respectively.Adding SF to the GUHPFRC containing ceramic balls resulted in an even greater reduction of volume loss.These results showed that the anti-penetration output of GUHPFRC could be further enhanced by adding ceramic balls as coarse aggregates.

Fig.11.Multi-classifier damage detection system [107].

Fig.12.Damage pattern of GUHPC reported by Liu et al.[107].

Fig.13.Damage pattern of novel GUHPC reported by Liu et al.[92].

Table 3 summarizes the projectile impact test results of the UHPC and RPC specimens.Zhang and Zhong[128]reported that no specimens showed perforation or scabbing when struck at velocities between 100 and 1000 m/s.As the speed at which a bullet hits an object rises, so does the DOP.Consistent with those described in other investigations [120,129], the DOP increases more rapidly with the striking velocity after a certain point.Under conditions with a striking velocity of about 850 m/s,the residual velocity reduces as the fibre volume ratio increases [130].A 40 mm thick panel with a 2%fibre volume ratio has a striking velocity of 901 m/s,which is higher than the target value of 850 m/s and obviously impacts the residual velocity [130].However, similar to Almansa conclusion [131], steel fiber can only insignificantly improve compressive strength.Therefore,it has limited compressive strength and contributes little to the residual velocity.Feng et al.conducted an investigation into the resistance of UHPC comprised hybrid fibres(steel,PP,and PVA)at a dosage of 2%-14.5 mm 72.5 mm steel sharp projectiles at three striking velocities of 800 m/s,950 m/s,and 1050 m/s.While using a combination of steel and PP or PVA fibres can assist in mitigating UHPC crater damage, using steel fibres alone is more effective for mitigating DOP.Lee et al.[132] reported that testing the tensile performance of high-performance fiber-reinforced cement composite(HPFRCC)at the comparatively low impact velocity of 170 m/s is challenging.However,at an impact velocity of 330 m/s,the material fails with a perforation and the scab depth is roughly 70% of the specimen thickness.This reveals that the tensile behaviour of HPFRCC is more important than its compressive behaviour in determining structural failure.It was established that adding steel fibres improved the penetration resistance of UHPFRC [133,134].Nonetheless,this effect was typically portrayed as limitations on the diameters of the cratering and scabbing instead of on the DOP[135,136].Since UHPC has much higher compressive strength and elastic modulus than normal concrete, it has unquestionably fared better in penetration testing[133,134].

5.2.Failure mechanism

Fig.15 depicts the localized damage of GUHPC against the different velocities.The fracture process of concrete objects can be described by ideas of wave propagation when they are exposed to bullet impacts at high velocities [127,140].Strong compression waves are generated in the zone of impact whenever a bullet strikes the front face of concrete.It initially damages the UHPC by creating a crater since this projectile compressive stress exceeds the UHPC dynamic compressive strength.The concrete targets are simultaneously subjected to extreme tensile waves and shear loads brought on by projectile penetration,leading to the crack beginning and spreading[141].After that,a tunnel is formed in the concrete as compressive stress waves migrate radially inward from the path of the projectile motion.When incident compressive waves encounter a variety of mediums in the target,they undergo partial reflection and transform into tensile waves.A portion of the tensile waves that were reflected into the concrete propagates through the material until they encounter new media, at which point they transform back into compressive waves.At the time when the initial compressive waves and reflect tensile waves are traveling across space,they are superimposed,which causes the compressive waves to become less intense and the tensile waves to become more intense [127].Tensile waves may cause damage to the concretes front face if they travel at high speeds and hit the material head-on; expansion of the initial crater and the development of accompanying cracks are both possible outcomes.Because, at this point, the concretes tensile wave amplitude could be greater than its dynamic tensile strength.Because of the generated stress waves,the rear surface may suffer scabbing.The concretes dynamic tensile strength can be exceeded,causing scabbing if the stress waves that arise are intense enough [127,140-142].

Fig.14.Impact damage of GUHPC [124,125].

Table 3 Summary of the projectile impact test results.

5.3.Gaps in the research

The research gaps in GUHPC include different geometry of target, impact energy absorption method, front and rear spalling diameter,projectiles with different caliber sizes,projectile impacts at different velocities, the effect of reinforcement, volume loss,ejected mass at the front and rear face, resistance to bullet penetration, both deforming and non-deforming, intrinsic mechanism analysis and failure mode should be examined, and it could be considered as scope for future.Besides introducing a protective cover, fibre-reinforced polymer bars, functionally graded GUHPC,long projectiles,and rubber insertion between the layers in GUHPC could be explored.Finally, the research gaps mentioned in subsections 2.3 and 3.5 also apply to projectile impact on GUHPC.

6.Microstructure

When it comes to GUHPC, research on its microstructural characteristics, there is a noteworthy dearth.Researchers in the GUHPC have only done a small amount of work thus far.This section describes the microstructure of GUHPC.

Fig.15.Damage of GUHPC after the projectile impact[125]:(a)OPC concrete at 554 m/s;(b)G-UHPC-1 at 563 m/s;(c)G-UHPC-2 at 561 m/s;(d)G-UHPC-3 at 568 m/s;(e)G-UHPC-4 at 557 m/s; (f) OPC concrete at 798 m/s; (g) G-UHPC-1 at 793 m/s; (h) G-UHPC-2 at 819 m/s; (i) G-UHPC-3 at 788 m/s.

6.1.SEM analysis of GUHPC

Fig.16(a)demonstrates that nearly no undesired reactants were found, suggesting that a sizable fraction of the GGBFS and fly ash were successfully dissolved and got involved in the subsequent polymerization [37].However, the system also contained silicates,aluminates,and N-A-S-H.The presence of highly crystalline hydroaluminates and hydro-silicates among the aluminates and silicates and must be underlined (Fig.16(b)).The binding phase-aggregate interaction transition zone (ITZ) was also much denser and less apparent.This enhanced bonding property with the aggregate was due to the much larger ITZ and microstructure,as seen in Fig.16(c).The same trend was observed by the Xie et al.[143].The ITZ between GUHPFRC and SF is shown in Fig.16(d).The GUHPFRC and SF have a strong bond because their ITZ was practically flawless.As a result, GUHPC mechanical properties were enhanced due to the enhanced steel fiber-matrix synergy[36].According to Xu et al.[46]a considerably thick microstructure was seen after a higher slag dose was incorporated.To be more precise, flaws like holes and micro-cracks were spotted during the binding stage.Similarly,with a slag dosage of 0.193, the partially polymerized FA was detected.Because of these causes,the resulting microstructure is slightly less dense.According to Wang et al.[144],the microstructure of SLF free mixtures was characterized by spherical FA particles; the cracks and pores were easily visible in Fig.17.The morphology of mixtures containing 10%SLF has been shown to alter drastically,revealing a dense microstructure with a diminished number of non-reacted particles.This is why these mixtures have greater compressive strength than those without SLF.In response to a rise in SLF concentration,particle agglomeration was observed.

6.2.XRD analysis of GUHPC

The 10%calcium aluminate cement-containing GUHPC mixtures XRD trends are shown in Fig.18(a) [144].There was a halo at a 2θ angle of 20°-40°in all three mixtures, and this effect was independent of the SLF concentration.It is assumed that the growth of strength in geopolymeric materials is due to the creation of sodium aluminosilicate gel(N-A-S-H),which is reflected by this halo[145].The existence of the CA and QZ diffraction peaks at 26.5°and 35.5°of 2θ suggests that the product of this reaction still contains some unreacted FA and calcium aluminate cement.C-S-H gel creation was linked to diffraction peaks at 2θ = 29.4°.In earlier research,C-S-H gels have been seen forming in either a calcium aluminate cement system or an alkali-activated FA system in the presence of SLF[146,147].Little amounts of calcium aluminium silicate hydrate and gismondine were also detected,in addition to C-S-H and N-AS-H gels.After SF addition, a lower quartz peak intensity indicates more Si dissolved during alkali activation [148].It is important to notice that SLF-10-10 has the greatest compressive strength and the minimum quartz peak intensity.As a result of the homogeneous distribution of unreacted SLFs in the intercellular spaces between solid grains, the resulting microstructure was dense, and the materials strength was enhanced(Fig.18(a))[46].Because of this,SLF-10-10 has a greater compressive strength than SLF-10-0.There is a large concentration of unreacted SLF because of the enormous SLF dosage.Fig.18(b)illustrates how the FA affected the phase structure of GUHPC.The amorphous aluminosilicate gel was reflected in a diffuse diffraction peak from 15°to 40°.Hence, suggesting the presence of the amorphous form in the products of the reaction[149].Quartz was found to have a significant quantity of unreacted material,as evidenced by the presence of diffraction peaks at the 2θ values of 22°,26°,and 52°.This finding suggested that the reaction products contained considerable unreacted quartz.The diffraction peaks at 2θ = 30°and 55°were used to characterize the primary reaction product, which included the C-A-S-H/C-(N)-A-S-H gel.When the amount of fly ash was increased from 0.311 to 0.829,the diffraction peak at 2θ = 30°nearly disappeared (Fig.18(b)).It demonstrated that an increase in FA percentage was not helpful in producing the C-A-S-H/C-(N)-A-S-H gel.When the FA is increased,the mechanical behaviour of GUHPC may degrade, and this phenomenon may be the primary cause [46].It is reported that boosting the slag dose increased the intensity of the diffraction peaks at 2θ = 30°and 55°, indicating that the increased calcium system was more amenable to the development of the C-A-S-H/C-(N)-A-S-H gel (Fig.18(c)).Notably, the 2θ = 27°diffraction peak detected reflecting albite(N-A-S-H)disappeared with a slag dosage of 0.731, but it was present at a slag dosage of 0.193.Albites conversion into the C-A-S-H/C-(N)-A-S-H gel in a high-Ca2+reactivity system was blamed for this phenomenon.This transformation was also seen in the zeolite.

7.Conclusions

This research has reviewed these characteristics of GUHPC by investigating the fresh and hardened properties and microstructure.Experts will be able to use the information gleaned from this review of current research to define further the parameters within which the next-generation, attainable, and long-lasting GUHPC must be designed.This will allow design engineers to make the most of the strength of UHPC and other unique characteristics,and it will allow them to create simulations that accurately predict ultimate load carrying capacity of GUHPC under a wide range of loads.This evaluation was written with the intention of assisting scientists, engineers, and technologists in expanding the use of GUHPCs for cutting-edge infrastructure needs.The following are some broad conclusions drawn from this analysis.

(1) Increasing the SLF content from 5%to 10%enhanced fluidity.However,when SLF content was increased to 20%,flowability decreased.The flowability of GUHPC mixes, including 7.5%,15%, and 22.5% waste glass, increased by 1.4%, 2.40%, and 3.75%, respectively, compared to GUHPC, which did not contain any waste material.Observations showed that the flowability decreased from 365 mm to 286 mm as the Na2SiO3modulus increased from 0.85 to 1.83, which represents a decrease of 21.6%.

(2) The compressive strength of GUHPFRC increased by adding 1%, 2% and 3% dosage of SF.Conversely, the compressive strength of the GUHPC increased noticeably when 5%, 15%,and 25% micro silica were employed but decreased when only 10% was utilized.It is also reported that the highest compressive strength was reported for the GUHPC containing 40% QZ with 2% fibres volume.

(3) Tensile strength was increased when 30%SLF was added to a higher dose of SF to improve the microstructure of GUHPFRC,resulting in a denser binder matrix and an improved microstructure.In addition, research shows that replacing 30% of the SLF with slag achieved the highest flexural strength, while the greatest modulus of elasticity was reported for the exact mixes.

(4) During the contact explosion activity, significant spall damage was observed on the back portion of the 200 mm thick GUHPFRC slab reinforced with 1.5%SF under 1 kg of TNT.It is speculated that 1.5%SF reinforcement successfully overcame the brittleness of GUHPFRC,hence aiding in the reduction of contact explosion cratering and spalling.

Fig.16.SEM analysis by Aisheh et al.[37].

Fig.17.SEM analysis by Wang et al.[144].

(5) Adding a hybrid combination of SF and BF in GUHPFRC significantly reduced penetration depth and crater diameter.Furthermore, adding SF to the GUHPFRC containing ceramic balls resulted in an even more significant reduction in volume loss.Also, they could serve as barriers and generate friction to perform better over projectile penetration,decreasing penetration depth.

(6) A significant amount of the GGBFS and fly ash were dissolved and participated in the subsequent polymerization, as SEM analysis did not reveal any undesirable reactants.N-A-S-H were present in the system in addition to the silicates and aluminates.Among the aluminates and silicates,it is vital to highlight the occurrence of highly crystalline hydroaluminates and hydro-silicates.

(7) The intensity of the diffraction peaks at 2θ = 30°and 55°reportedly increased upon increasing the slag dose,showing that the increased calcium system was more receptive to forming the C-A-S-H/C-(N)-A-S-H gel.In particular, at a slag dosage of 0.731,the 2θ=27°diffraction peak found reflecting albite (N-A-S-H) vanished, although it remained visible at a slag dosage of 0.193.This phenomenon was caused by the high Ca2+reactivity system in which albite transformed into the C-A-S-H/C-(N)-A-S-H gel.

Fig.18.XRD analysis of (a) Wang et al.[144] and (b & c) Xu et al.[46].

8.Perspectives

Further steps in the investigation have been determined according to the previously mentioned review, including the following.

(1) GUHPFRC has a high unit weight due to using steel fibre.More research is needed to minimize the GUHPC unit weight for rehabilitation, repair, and lightweight structures.

(2) Features like electromagnetic interference shielding, selfcleaning and self-sensing can be provided to GUHPC using particular nanomaterials.There is room for additional study in this area.

(3) Cementitious additives, such as ground granulated blast furnace slag,fly ash,and silica fume,play a significant role in the formulation of GUHPC.Cementitious additives are in short supply since natural gas ones are replacing coal power facilities.Thus, there is a pressing need for studies of Cementitious additive alternatives.

(4) The GUHPFRC performance can be affected in many ways depending on the type of non-metallic fibre used, such as carbon, mineral and synthetic.Using these fibres to improve GUHPFRC performance is an area worthy of research due to their unique properties.Future research strongly needs more investigations on fibre treatments, such as physical or chemical approaches,to enhance fibre-matrix bonding capabilities.

(5) Although research into hybrid fibres for GUHPFRC is limited,while hybrid fibres have been shown to have a synergistic impact, the mechanism by which this occurs is not well understood.Optimal hybrid fibre combinations that enhance UHPFRC microstructure, mechanical characteristics, blast and impact resistance require more study.

(6) The absence of globally acknowledged design specifications is one of the aspects of the large-scale implementation of GUHPC that presents a challenge.In the future, potential research areas could include looking for an efficient and rational GUHPC design process.The engineering community as a whole would benefit from a more in-depth and precise understanding of GUHPC if there were a standardized approach of evaluating it that took into account its price, qualities,and the total cost of its upkeep over its useful lifetime.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.