Enhancing Sustainability and Economics of Concrete Production through Silica Fume: A Systematic Review

This review article addresses the problems associated with the carbon footprint of the cement industry. The PRISMA framework methodology was for data extraction from published studies. In-depth research has been done in the literature on using silica fume as a cement replacement in concrete production, considering environmental, engineering, and economic (EEE) factors. The strength, durability, and economic parameters results revealed a positive variation of up to 5–20% substitution of silica fume. However, most past studies reported the threshold at a 10% replacement ratio. A novel benefit-cost ratio analysis was also done in this review study. The benefit-cost ratio analysis reveals the economically beneficial effects that can be achieved in sustainable silica fume-based concrete with a (5–20%) silica fume combination. The benefit-cost ratio showed positive effects, up to 20% cement replacement with silica fume. Hence, the higher cement replacement with silica fume is also beneficial in terms of the benefit-cost ratio. Further research has been proposed based on the findings of this review study.


Introduction
The handcrafted economy of the past has given way to a mechanized industry today.Another significant change brought on by the Industrial Revolution was that the world's population had crossed one billion at the beginning of the 19 th century [1].According to the United Nations (UN) latest demographic projections, the world population will exceed 9 billion by 2045 DeSA [2].The World Urbanization Index states that 55% of the population lives in urbanized areas, projected to escalate to 68% by 2050 Bongaarts [3].Rapid urbanization needs infrastructure, ultimately increasing the demand for construction materials [4][5][6].
The consequences of rapid economic growth led by the Industrial Revolution have raised CO 2 emissions on a colossal scale [7][8][9][10].CO2 emissions have increased to 386 ppm, exceeding natural limits since the beginning of the 21 st century due to world urbanization [11][12][13][14].The world continues to urbanize and is required to develop infrastructure for its inhabitants.The fundamental construction materials for this development are natural aggregates, river sand, and cement.About 40% of CO2 emissions in cement kilns are contributed by burning fossil fuels such as coal [15][16][17].The UN's 11 th Sustainable Development Goal (SDG) has set targets to make cities resilient, inclusive, sustainable, green, and safe by 2030 [7].Therefore, environmental scientists and climate change committees have set stringent goals to lower CO2 emissions from all sectors to maintain the temperature below 1.5°C under the Paris Agreement of 2015 [18].

Carbon Footprint
During the pre-industrial period, the levels of CO2 were around 280 ppm, but at the beginning of the 21 st century, they rose to 386 ppm.The rising CO2 concentrations in the atmosphere make it a greenhouse heat-trapping gas.The earth's naturally rising greenhouse effect keeps its temperature at 1.5°C on average, whereas anthropogenic activities incorporate excessive CO2 into our atmosphere, increasing its natural optimum temperature [36,37].A report discussed by the Inter-Governmental Panel on Climate Change (IPCC) in the Paris Climate Agreement states that any rise in global temperature is more than 1.5°C (1.5°C is the pre-industrial temperature).It is intolerable and keeps us at high risk of natural calamities such as frequent cyclones and droughts, resulting in famine and ultimately bringing many diseases.

Figure 1. Data extraction methodology
The first conference of parties (COPs) in Rio de Janeiro, Brazil, in 1992, called COP-1, was hosted by the United Nations Framework Convention on Climate Change (UNFCCC).A treaty was signed to cut global emissions of CO2 by 5% compared to 1990s levels by the end of 2008-2012.The COP-15 held in Copenhagen, Denmark, in 2009 declared that the increase in average global temperature should be less than 2°C.In Doha, Qatar, COP-18 was held in 2012; the deadline was extended to 2020.In Paris, COP-21 was accommodated by France in 2015.One hundred ninety-six countries, almost the entire international community, pledge to maintain the increase in average global temperatures to less than 1.5°C.The COP-23 was held in Bonn, Germany, in 2017, where attempts were made to continue implementing and improving CO2 reduction promises.The COP-25 took place in Madrid, Spain, in 2019 but ended up leaving a void that will only be filled if policymakers sincerely deal with climate change issues.After reviewing them avidly, the COPs failed to enforce the policies with no positive outcomes.The most recent COP-26 in Glasgow, UK, in 2021, focuses on tangible goals to reduce CO2 emissions from different sectors [38][39][40].

Carbon Footprint from the Cement Industry
The cement sector generated 4.3 gigatons of greenhouse gases globally, making it the top generator among all other industries [37,[41][42][43].The CO2 emitted can be reduced by reusing and recycling SCMs.The cement industry produces 2.4% of CO2 emissions, on which modern construction depends firmly.When calcium carbonate is heated during calcination, it is described in Equation 1.
The IPCC states that Equation 2 can be used to determine the CO2 emissions from clinker formation during calcination, as shown in Equation 2.

Production data × CKD correction factors = CO2 emission from clinker
(2) The dust released from the cement kiln during the annual cement production in CKD [44].Table 1 displays the information on CO2 emissions gathered by the world scenario [45][46][47][48][49][50].The worldwide CO2 emissions in metric tons are presented in Table   production.Burning fossil fuels like coal is also responsible for high CO2 emissions from cement kilns [15,16,52,53].Scientists and concrete designers have suggested a better and more sustainable substitute, SF, to replace OPC.The fineness of SF helps to fill the voids, and a more compact microstructure is formed by using SF in place of OPC, enhancing the concrete's strength and durability [54][55][56][57][58].

Silicon Production
Silicon, after oxygen, is the earth's most abundant element.It combines with oxygen to form natural and pure silicon dioxide and silicates.Silicon as an element was recognized in the early 20 th century, and its production started at industrial levels.Due to the onset of the IT sector, the demand for calculators, computers, and everyday electronic devices has increased, whose essential component is silicon chips.To produce chips, extremely pure silicon was produced by the metallurgical extraction of ferro-silicate ores and pure quartz [59].Figure 2 shows the silicon production worldwide in thousands of metric tons.It can be seen from Figure 2 that the USA produced 310 thousand metric tons, and Norway, Brazil, and Russia produced 350, 390, and 580 thousand metric tons, respectively.China produces the world's largest industrial silicon, around 6,000 metric tons annually.China is the largest producer of silicon because most developed nations have outsourced the production and manufacturing of their electronic/silicon devices to China.

Silica Fume
When silicon is extracted and refined metallurgically, silica fume (SF) is produced, as explained by Williams & Partheeban [21].Non-biodegradable silicone goods used in electronics, medications, and cosmetics are discarded in open landfills.The disposal of SF in public areas has adverse health issues.Therefore, recycling such waste is a crucial concern.
The construction industry faces numerous challenges today, primarily due to rapid urbanization, depleting natural resources, and its adverse impact on climate change.The construction industry can utilize massive amounts of industrial solid waste in construction activities; one such waste is SF.The primary reason for choosing this waste is its acceptance and widespread practical implementation.The American Concrete Institute (ACI) describes SF as an ultra-minute amorphous fume generated industrially by reducing silicon dioxide with carbon in electric furnaces.The quartz of high purity, or ferro-silicate ores, was smelted at a temperature of up to 2000°C.After extracting the metal, the condensed amorphous silicon fumes or its alloys having silicon were collected, and silica fume was obtained by Singh et al. [59].Equation 3 shows the breakdown of silicon dioxide during the production of SF in the furnace.SiO 2 +2C=Si+2CO (3) The complete stepwise industrial metallurgical extraction of silicon is graphically shown in Figure 3.The condensed silica dust's main component is more than 90% amorphous SiO2 with a particle size of around 0.1 µm Mohyiddeen & Maya [60].

Figure 3. Silica fume production process
Adding SF to concrete enhances durability and internal micro pore spaces.It minimizes the permeability, ion diffusion, and amount of calcium hydroxide, subsequently providing exceptional resistance toward the chemical attack of sulfate.The amorphous and excellent physical properties of SiO2 make this pozzolanic material very reactive.When the OPC within the concrete reacts, it liberates calcium hydroxide, where OH -ions tend to diffuse as free radicals.In contrast, SF bonds with calcium hydroxide and makes a gel-like substance known as "calcium silicate hydrate," which also forms during the production of OPC, as shown in Equation 4.
Pozzolanic materials absorb calcium oxide and make the C-S-H gel phase, as seen in Equation 5.
This C-S-H binder gel packed the concrete's microstructure pores, which dense the internal structure, providing additional strength and enhancing the concrete's strength and durability, as shown in Figure 4. Luo et al. [61] suggested that replacing the cement paste with industrial SF (0-10%) of approximately 2.9-11.5 kg of cement in concrete production could save per cubic meter of concrete.In 2020, the global SF market reached USD 480.9 million, projected to reach USD 333.6 million by 2028 [45,46].According to the data, scientists promoted the utilization of SF waste.The new artificially produced resource will be more environmentally friendly, replacing OPC in the construction industry.

Physical Properties of Silica Fume
The SF particles are primarily white and grey in color and spherical in shape, and more than 95% are less than one micrometer.The surface area of particles ranges from 13,000 to 30,000 m 2 /kg [62][63][64].The ordinary Portland cement (OPC) particles are roughly 100 times larger than the SF particles reported by Khan and Siddique [63].The physical characteristics are shown in Table 2.

Chemical Properties of Silica Fume
Table 3 demonstrates trace amounts of iron, magnesium, and alkali oxides.The silicon oxide improves SF's cementitious properties and pozzolanic reactivity, which makes it suitable for replacing OPC in concrete.The chemical properties are shown in Table 3.

Results and Discussion
This review paper utilized the SLR methodology to evaluate SF-based concrete environmental, engineering, and economic (EEE) management.The methodology part discussed the SLR methodology in detail.The discussion is presented based on the findings and interpretations of the previously published studies.

Silica Fume-based Concrete
In some countries, it's mandatory to utilize flyash up to certain limits in cement and concrete production.India announced that any coal-based thermal power plant must use 100% flyash in construction activities within 300 km of the thermal power plant [9,13,[76][77][78][79][80][81].SF can also be replaced by SCM in concrete manufacturing.Table 4 shows the key findings of different percentages of OPC replacement with SF (5-20%).Some recent studies have also investigated combinations of silica fume with desert sand and recycled crushed sand from demolished concrete [24,82].However, the performance of SF with sustainable desert and recycled crushed sand was highly remarkable, but the studies are scanty.Therefore, more research can be conducted in this area.1. OPC used SF as an additive material for up to 10% of these studies.
2. The highest compressive strength was observed at 20% RFA and 10% SF combination.
3. Using SF with RFA in concrete intensifies the compressive strength compared to the reference mixes.
4. During the experiments, SF successfully controlled chloride ion penetration and the solution temperature, indicating that ion movement in the concrete was controlled.
6.The research shows that the higher SF content at 15% in the marine environment reduced the chloride diffusion coefficient in concrete.

7.
The experiments projected valuable information on SF addition in 15%.
8. A positive increment was observed in splitting tensile and FS by adding SF at 15%. 9.The study results revealed that adding SF had improved the durability against chemical deterioration.
11. Up to 20%, SF replacement by weight of OPC shows a slight increment in compressive strength to control mixes.
12. The study's results revealed that a higher percentage of SF, such as 20%, did not affect compressive strength.
13.The replacement of up to 20% of SF by OPC made concrete more economical and sustainable.
14.It has been concluded from the studies that more than 20% of SF is not recommended in concrete production.

Relative Compressive Strength
The SF percentages against relative compressive strength are compared to previously published studies shown in Figure 5.The SLR of this study disclosed the replacement percentages of OPC with SF of 5-20%.The fluctuation in compressive strength against SF percentage was reported by published studies in Figure 5.A significant increase was revealed, up to 15% incorporation of SF in compressive strength.The maximum relative compressive strength at 5, 10, 12.5, 15, and 20% replacement of OPC with SF was 16, 29, 27, 16, and 2% [100,106,107].The replacement of OPC with SF at 5% compressive strength was increased further; at 10%, more increments were observed.Up to 15% increments were maintained in SF-based concrete.When the replacement ratio reached 20%, a reduction in increment was shown in Figure 5. Compressive strength was reported to rise as and when the substitution of OPC with SF increased up to 10%.Compressive strength increased by about 23% at a 7% replacement ratio.The increase in compressive strength at a 10% replacement ratio was 29, 27, 27, 23, and 15%, as shown in Figure 5.The research findings [85,108] indicate an exact 27% increase in compressive strength at a 10% replacement ratio of OPC with SF.The highest 29% increment in compressive strength was reported again at 10% replacement of OPC with SF, as studied by Katkhuda et al. [106].Different design mixes and the use of many SCMs included in the same mix are the causes of strength variation at the same replacement ratio.At 15%, the results of studies [97,109,110] increased the compressive strength by approximately 18, 27, and 23%, respectively.The lowest increment in compression strength was seen at the higher % replacement ratio of 20%, as studied by Saba et al. [100].

Figure 5. Comparison of the ratio of relative compressive strength of SF-based concrete
As a result of the discussion, most studies [95,98,99,101,103,105] did not recommend the higher replacement (≥ 20%) of OPC with SF.An excellent improvement in the compressive strength was observed when the replacement ratios were between (7 to 15%).A threshold of compressive strength improvement was recorded at a 10% replacement ratio.It was discovered that adding SF at lower concentrations, between (7 to 15%) results in denser microstructure and an enhancement in compressive strength (i.e., additional C-S-H gel produced).Adding the right amount of SF will also cause compaction, and the fineness of the SF will create an enormous surface area that will result in a solid bond with the cement.As a result, the mix's compressive strength will rise relative to all control mixes.

Relative Splitting Tensile Strength
The relative splitting tensile strength (STS) values at various SF percentages of 5%, 7.5%, 10%, 12.5%, 15%, and 20%, according to published studies [33,98,101,102,111,112] are presented in Figure 6.The highest increment in STS results was achieved at 33%, 39%, and 39% by replacing 7.5%, 10%, and 12.5% of OPC with SF in SF-based concrete.At 15% incorporation of SF in SF-based concrete, 29%, 28%, and 26% increases were seen in STS values studied by [33,106,107].A higher replacement ratio of 20% of OPC with SF has also been implemented in recent research by [100,101], which found almost 21% and 29% increments in STS values.The concrete's STS results estimate the sample's maximum load without cracking.Concrete samples' paste quality and interfacial transition zone depend on fine aggregate quality.Incorporating SF percentages improves the paste quality and enhances the bond at the interface, ultimately increasing the STS values.

Relative Flexural Strength
Figure 7 displays the relative flexural strength (FS) versus various SF percentages.In the study [111], the FS at 7.5% replacement of OPC with SF showed a significant increase of about 35% in FS value.At 10% incorporation of SF, the highest gain of 37% and 40% in FS was reported by [111,113].Not many studies have seen a 15% substitution of SF; a study by Othuman Mydin [33] achieved a 28% increment in FS.Even a higher replacement ratio of 20% OPC with SF revealed significant improvement if FS values of 30, 26, and 28% were studied by [100,102,111], respectively.Hence, replacing OPC with SF by up to 20% dramatically impacts the FS values of SF-based concrete.

. Durability Characteristics of Silica Fume-based Concrete
The durability characteristics of hardened concrete, such as density, porosity, water absorption, effect on flammability, and acid attack, were examined.The behavior of hardened concrete at high temperatures, its ability to absorb water, its density, and its voids have all been examined.The results of the studies [32,72,82,[114][115][116][117][118] show the most significant outcomes at 10% OPC replacement with SF.After carefully examining the durability behaviors of SF-based concrete, it was discovered that 10% of OPC replacement with SF revealed the best outcomes.The results of earlier research show that up to 20% utilization of SF also has positive durability characteristics.However, the increment rate decreased when the replacement ratio increased above 10% in SF-based concrete.The heat of hydration is decreased, and the setting time is increased when OPC is partially substituted with SF without affecting the strength.The replacement of OPC with SF in new concrete is positive up to optimized values since it has a higher density, less water absorption, and porosity [24,116].Table 5 shows SF-based concrete durability characteristics with 5-20% SF incorporation in SF-based concrete.

Durability parameters
Important conclusions Properties Description [71,72,82,95,[114][115][116]119] Density, water absorption, and porosity OPC is substituted with SF at 5, 7.5, 10, 12, 15, and 20% 1.The experimental findings showed a decline in the void ratio, which decreased water absorption.The SF fills the holes with smaller particle sizes, enhancing density and, eventually, durability.2. Replacing 10% of OPC with SF had the best water absorption, voids, and density results in cured concrete.3. Results from earlier studies show that the characteristics of the concrete are diminished when SF is replaced with more than 20% of the cement's weight.4. The SF particle's tiny size fills the gaps in cement mortar.More than 20% of SF increases cement mortar's number of voids, which makes these particles more brittle.5. Adding (more than 20%) SF will increase the volume of voids, decreasing the solid concentration rather than filling the gaps and boosting packing density.As a result, the concrete's resilience will be reduced.[113,120,121] Concrete hardened properties at ambient temperature 100, 200, 300, 400, 500, and 600⁰C 6. Normal concrete strength and silica-based concrete do not degrade as the temperature rises to 200°C.7. Regular concrete behaved the same way as SF-based concrete up to 300°C regarding its toughened qualities.8.The compressive strength of SF-based concrete rapidly declines after 400°C.9. Above 400°C, both standard and SF-based concrete experience significant weight loss.10.SF-based concrete had more muscular strength and weight loss reduction rates at 600°C than ordinary concrete.The findings of the studies [24,113,120,121] show that the performance of SF-based concrete at high temperatures was also examined as a durability factor.Up to 300°C, SF-based concrete exhibited minimal strength loss compared to standard concrete.The compressive strength of standard and SF-based concrete gradually reduces at 400°C.At 600°C, SF-based concrete's strength and weight loss is significantly lower than standard concrete's.
The results of durability characteristics are satisfactory in all replacement ratios (5-20%).However, earlier research was used to find future studies' optimal parameters.This rule of thumb will alter the durability of SF-based concrete by employing new materials rather than recycled ones.The discussion concludes that each design combination must optimize its unique SF combination.As a result, before using the SF percentages for commercial reasons, optimizing individually for each project is recommended.

4.2.1.Production Cost of Concrete with Silica Fume Percentages
The amount of OPC used in the concrete depends on the type of grade and design mixes.Approximately 0.3 tons of OPC per ton of concrete is used in the ready-mix concrete plant, as disclosed by Akhtar et al. [24].IEA [41] reported that 0.5-0.6 tons of CO2 are emitted per ton of OPC production; OPC companies' reports indicate a value of around (0.6-0.7) tons of CO2 per ton of OPC.For the calculation of CO2 emission, the average value of 0.6 was taken by multiplying the average value of 0.6 by the total OPC production to obtain the total CO2 emission from the cement industry annually.For the present study, results and interpretation of the selected articles [83,101,107,122] at 0%SF*, 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF were utilized in place of OPC to reduce CO2 emissions from the cement industry.The control mix was taken at 0%SF* for comparison with other studies.
Based on the international market price, the 1 m 3 production cost of the concrete mixes with 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF is estimated in Table 6.The cost of developed concrete mixes with and without silica fume has been calculated based on the rate at which their ingredients were purchased and the cost of the materials used in the United States (US) ($/kg).The production cost of 1 m 3 of developed design mix concrete was estimated by multiplying the quantity of each ingredient in kg/m 3 by the cost of the ingredients in US dollars ($/kg).The final cost of each mix is computed in US dollars.The total cost of the mixes at 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF with and without SF is estimated in Table 5.The control mix at 0%SF* cost was approximately 57.47 dollars, and all-natural ingredients were mixed without SF content.The cost of the mixes at 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF was estimated at 60.56 $, 61.16 $, 67.72$, 70.02 $, and 71.82 $, respectively.It has been observed that the 1 m 3 production cost of the newly developed mixes at 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF is higher than the control mix (0%SF* without SF content).

Concrete ingredient
Ingredients cost (US $/kg) Compared with the control mix of 0%SF*, the cost of developed concrete at 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF is higher.As mentioned in the study by Akhtar et al. [24], SF costs are four times higher than the OPC.Replacing 5%, 10%, 12.5%, 15%, and 20% OPC with SF increased the cost of mixes.The cost of the mixes 5%SF, 10%SF, 12.5%SF, 15%SF, and 20%SF was about 5%, 7%, 17.8%, 21%, and 25% higher than the control mix at 0%SF*.It can be concluded from the discussion that the increase in SF content in the design mix of concrete increases the cost.The higher the increment, the more the cost of concrete is revealed.

Benefit-cost Ratio of Concrete at Different Silica Fume Percentages
The cost, engineering properties, and benefit-cost ratio comparison of the mixes with different SF percentages in concrete is presented in Table 7.The prices of the materials used in concrete manufacturing are taken as per the prevailing international market rates.The cost has been converted into US dollars for comparison with the world scenario.The production cost of 1 m 3 of concrete from the selected published studies in the present study is mentioned in Table 6.Table 7 compares the parameters of the selected studies with respect to (w.r.t.) the control mix for each study, as we know that the strength characteristics are the most important characteristics to evaluate the concrete value.For this reason, the four most general and effective parameters, compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity, were selected to compare the benefit-cost ratio of the concrete at 5%, 10%, 12.5%, 15%, and 20% replacement of OPC with SF.The value of selected parameters, compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity, was considered at 28 days of curing.The negative sign represents a percentage decrease, and positive values indicate a percentage increase.It can be seen from Table 7 that all parameters show positive values, indicating that at each replacement value, increments in the selected parameters ranged from 5 to 20%.
The benefit-cost ratio of the mixes at 5%, 10%, 12.5%, 15%, and 20% SF studied by [73,101,107,122] compared with each mix control value without SF is shown in Table 7.The values of the benefit-cost ratio were computed by Equation 6 in Table 7.
The weightage factor measures the importance of specific properties of concrete to bring them to the same scale of calculations.There are no specific guidelines concerning the weightage factor for the benefit-cost ratio of concrete.Therefore, the weightage factor is taken according to the latest studies [123,124].This study considers parameters such as compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity equally important.Therefore, the highest weighting factor of 10 is given to each parameter.The benefit-cost ratio of concrete is the sum of the benefits for compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity divided by the cost.
The benefit-cost ratio in the studies [73,101,107,122] was computed (4.31, 16.94, 7.74, 19.28, and 2.36) at 5%, 10, 12.5, 15, and 20%, respectively.The highest benefit-cost ratio was observed at 15% replacement of OPC with SF, and the lowest was at 20%.However, compared with the selected parameters, the study by Khan & Ali [107] at 15% SF incorporation studied all parameters.All parameters show positive effects, with all SF percentages ranging from 5 to 20%.By this fact, we can conclude that the higher the parameters, the higher the benefit-cost ratio.Another point is that the greater the gain in the parameters, the greater the benefit-cost ratio.The research by Mahalakshmi & Khed [122] and Younis et al. [101] studied three parameters and achieved a benefit-cost ratio of 4.31 and 2.36.Akhtar et al. [18] studied only two parameters and achieved a benefit-cost ratio of 7.74.The discussion leads us to the conclusion that two factors are crucial for maximizing benefits: the number of parameters and the gain in each parameter.
The confirmation of improvement in concrete properties by adding SF percentages was revealed from published studies cited in this review article.A most comprehensive recent review by Akhtar et al. [24] shows a positive finding for the replacement of OPC by SF.The optimum level of 10% OPC replacement with SF was discovered.Incorporating the optimum level of SF in sustainable concrete has proved to increase the benefit-cost ratio in terms of enhanced parameters of prepared sustainable SF-based concrete with reference to the control mix without SF.The benefit-cost ratio is a new way of analyzing OPC replacement with SF in SF-based concrete.However, the cost of concrete containing SF is higher than that of the control mix without SF content.The benefit-cost ratio analysis reveals the economically beneficial effects that can be achieved in a developed sustainable SF-based concrete (5-20%) SF combination.This study gives a different way of analyzing the utilization of SF at higher percentages.As shown in Table 7, the benefit-cost ratio is positive, up to a 20% replacement of OPC with SF.Hence, it can be concluded from this analysis that higher replacement is also beneficial in terms of the benefit-cost ratio.

Recommendation for Future Research
This review article presents SLR on SF-based concrete.The SLR shows several researchers have successfully utilized SF to partially replace OPC (5-20%).The studies on the higher replacement of OPC with SF in SF-based concrete are short.A benefit-cost ratio analysis was also done in this review study.It revealed the economically beneficial effects that can be achieved in SF-based concrete, up to a 20% replacement of OPC with SF.This review study suggested that the higher replacement of OPC with SF is also beneficial in terms of the benefit-cost ratio.
Insufficient data on the combined effects of SF with sustainable sand, such as recycled fine aggregate and desert sand, in making SF-based concrete is available.It has been suggested that sustainable sand, such as recycled crushed sand from demolished concrete and desert sand, can be combined with a higher replacement of SF.Therefore, a research gap has been found for future research by replacing higher percentages of OPC with SF combined with sustainable sand in new SF-based concrete.Successful utilization of higher incorporation of SF combined with sustainable sand would be a green step towards sustainability.This recommendation will open a new domain, solve the issues of excessive natural river sand mining, and help reduce CO2 emissions from the cement industry.If policymakers in every country enforce better environmental laws, they could save millions of tons of OPC annually, and utilizing SF helps to manage CO2 emissions globally.The following are recommendations for future experimental research work: • It is recommended that silica fume be replaced in design mix concrete at a higher replacement ratio of up to 20%; however, proper optimization is required.
• A study can be conducted to observe the combined effect of sustainable sand replacement with natural river sand along with an optimized silica fume percentage.
• In-depth knowledge of concrete's strength and durability parameters can be examined using sustainable sand with a higher silica fume replacement.
• It is proposed to develop a cost analysis and sustainability measurement model when higher silica fume replacement is combined with sustainable sand.

Conclusions
This review article investigates silica fume replacement with cement in SF-based concrete production.The successful utilization of silica fume can combat the issues of excessive CO2 emissions, climate change, and the ecosystem.The detailed conclusions drawn after data extraction from 124 peer-reviewed, high-quality studies are below.
• The mixes with all combinations of (5-20%) cement replacement with silica fume showed a positive increment in compressive strength.The average increment of 24% was seen at 10% incorporation of silica fume.
• A splitting tensile and flexural strength increased when silica fume was replaced between (5-20%).The highest increase of 37% in splitting tensile strength and 40% in flexural strength was seen at 10% incorporation of silica fume.Splitting tensile and flexural strength increments were also observed at higher replacement of 20% cement with silica, by 29% and 30%, respectively.
• All (5-20%) silica fume incorporation positively affected the durability of SF-based concrete.At 10%, cement replacement with silica fume revealed the best results.
• The benefit-cost ratio analysis reveals the economically beneficial effects that can be achieved in SF-based concrete, up to a 20% replacement of cement with silica fume.
Hence, it can be concluded from this review study that the higher cement replacement with silica fume is also beneficial in terms of the benefit-cost ratio.Finally, we can conclude that 10% silica fume incorporation is the best replacement ratio discovered by past studies.However, benefit-cost ratio analysis revealed that up to 20% of silica fume can be replaced with cement.Replacing more cement with silica fume produces more sustainable concrete, which would be an environmental savior.

Figure 2 .
Figure 2. Worldwide silicon production in metric tons

Figure 4 .
Figure 4.A pictorial depiction of concrete made with silica fume and OPC showing C-S-H gel binding effects in a study by Luo et al. [61]
. China reported the highest CO2 emissions since 2005, as presented in Table 1.India ranks second in the world for CO2 emissions, behind China.The calcination process significantly impacts environmental problems in cement factories.According to Mokhtar & Nasooti [51], 50-60% of CO2 emissions are released during clinker