Self-compacting concrete (SCC) is a unique kind of concrete that is flowable, non-segregating, and expands into the formwork by its own mass without the aid of external vibrators, even when there is heavy reinforcement present. SCC is becoming more popular in civil engineering. SCC continues to explore a variety of applications and characteristics [1]. Four parameters, including flowability, viscosity, passing ability, and segregation resistance, may be used to differentiate SCC from conventional concrete. If the concrete has all four of the aforementioned traits, it is referred to as SCC [2]. In terms of history, the SSC idea was initially established in 1986 [3]. However, in 1988, Japan was the first nation to successfully create a prototype of SCC. Similar to ordinary concrete, the SCC building is unsustainable due to its high natural resource use.

According to the idea of environmental development, environmental resources should be preserved as limited commodities and wastes should be adequately controlled ^[4][5][6][7][4,5,6,7]. The growing quantities of composed trash, up to 2500 million tons annually around the globe, encouraged the development of a contemporary method of dumping [8]. There are various options to use leftover raw materials in concrete in the cement manufacturing sector ^{[5][9][10][11][12]}[5,9,10,11,12]. Waste may be used in concrete as cement, or fine or coarse aggregate.

In the construction industry, concrete is the most often used man-made construction resource, and hydraulic cement is an essential component of this material ^[13][14][15][13,14,15]. Nearly 30 billion tons of concrete were produced in 2015, using approximately four enormous volumes of hydraulic cement annually [16]. The findings presented at the World Cement Association Conference demonstrate that the yearly rate of global cement production has increased [17]. There has been significant industrial expansion as a result of the expanding need for cement in modern structures and infrastructure, notably in developing nations like China, Russia, and Japan [18]. The capacity to manufacture 59.5 million tons of cement per year was built due to the projection of future demand for expanding infrastructure. The cost of cement has increased by roughly 150% in only 10 years [19]. Therefore, it’s crucial to use alternative materials wherever feasible, instead of cement [20].

The use of a mineral additive is one of the ways SCC differs from conventional concrete. Viscosity-increasing additives, or fillers, are used in SCC to prevent the separation of big particles. When casting concrete underwater, or for SCC in tunnels, an additive to improve viscosity is often employed. To enhance the viscosity of SCC, mineral admixture fillers may be added to the slurry [11]. The performance of SCC with various pozzolanic materials substituted for cement has been studied by researchers ^[21][22][23][21,22,23]. According to Siddique et al. [24], the use of mineral admixtures raises the slump without raising the cost of the mixture, and lowers the amount of superplasticizer required. According to earlier research, fly ash-containing mixtures had less compressive capacity values at early ages. The sluggish pozzolanic reaction between the binder (OPC) and fly ash (FA) was the root cause of this issue [25]. Furthermore, the rate of pozzolanic reaction of secondary cementitious material also depends on the temperature.

Silica fume (SF) is a poisonous material that harms the environment and its surroundings. Nearly all SF was discharged into the environment up until the mid-1970s. There were many uses for SF even as environmental concerns about it increased.

Silica fume’s amorphous structure makes it very reactive; they are round and have a sizable surface. SF particles are packed densely with cement grains because they are 100 times smaller than OPC grains, which allows calcium hydroxide and SiO₂ to react to produce more CSH and earlier strength ^[26][30]. SF increases the concrete’s packing density because of its microscopic size. SF concrete has been applied extensively in high-strength concrete for highway bridges, marine constructions, and parking decks, because of its outstanding performance ^[27][31].

2. Fresh Properties

Mortars constructed exclusively with Portland cement are substantially less cohesive than secondary cementitious materials (SCMs) including 6% SF. The flowability of mortars with SF is not as excellent as that of mortars with fly ash (FA) because there is greater contact between the higher cohesiveness (FA) ^[29][39]. Due to its lower particle size compared to cement, SF loses workability ^[30][40]. Since SF has a greater specific surface area than cement, a substantial volume of water is required to fuel hydration processes ^[31][41]. To attain the requisite workability in all SF concrete combinations, the superplasticizer dosage was raised. It should be noticed that concrete’s flowability declined as the SF substation increased. Because silica interacts with calcium hydroxide when it is in its finely divided state, secondary cementitious calcium silicate hydrate is created, which makes the concrete stiffer and more cohesive and contributes to the reduction in flowability with SF substitution ^[32][42]. However, all SCC combinations have outstanding flowability and are within the confines of the SCC technical standard ^[28][38]. Due to the SF and FA particles’ spherical form, which creates a ball-bearing action, the packing effect was more severe. With less intra-particle friction, the lubricated spherical particles provided effective rolling. Since the amounts of water and superplasticizer were maintained throughout the formulations, the rolling action could be produced with up to 20% replacement level lubrication. Following that, the overall powder surface area increased, requiring more water and superplasticizer to reach the same workability level. This result causes the overall spread to steadily diminish after the replacement level of 20%. The L-box and U-box tests were applied to evaluate SCC’s capacity for filling and passing. In both procedures, the concrete to be tested was placed in a closed, vertical chamber to provide a hydrostatic pressure head. The concrete must level out across the vertical or horizontal flow obstructions once a slide is opened. The likelihood of blocking was influenced by the level discrepancy. When the blocking ratio (H₂/H₁) was between 0.8 and 1.0 and the L-box blocking ratio was less than 0.8, the EFNARC guideline ^[28][38] states that a combination provide a blocking risk. SF was added in varying binder concentrations of 5, 10, 15, 20, 25, and 30%, respectively. According to the findings, increasing the amount of SF in the mixes caused a decrease in the slump flow, while simultaneously increasing the amount of time the mixtures spent flowing through the V-funnel ^[33][45]. In contrast, the research said that the V-duration funnel steadily reduces with an increase in SF concentration, suggesting that this improves the vertical passage of fragments by reducing air bubbles that obstruct the vertical flow of the paste ^[34][49]. However, research ^[35][29] found that 5% SF results in an L-Box test ratio of 0.78, which is beyond the acceptable limit for SCC as stipulated by technical criteria. Although research ^[36][50] found that blocking ratios up to 0.60 had a high passing ability, the blocking ratio value in the L box test must be greater than 0.8 for the SCC to have a satisfactory passing ability, according to the technical standard ^[37][51].

3. Mechanical Properties

3.1. Compressive Strength (CS)

The substitution of SF enhanced the CS of concrete. The CS improved as the substitute percentages of SF enhanced, but the CS decreased when FA concentration increased. The SCC mixes with the best CS included 15% SF instead of cement. The results of tests on both freshly poured and cured concrete shown that it is feasible to create SCC using SF and FA with usable SCC qualities ^[38][26]. Over 130 days, SCC with 15% SF exhibited the greatest compressive and tensile capacity, measuring 73.87 and 5.489 MPa, respectively. When the FA/SF level in SCC mixes rose, the UPV values of SCC specimens with FA/SF generally decreased for all curing durations. However, at 28 and 130 days, normal concrete (NC) and SCC samples with SF exhibited the maximum UPV values, followed by SCC samples with FA ^[39][27]. The reinforced self-compacting concrete with recycled steel fiber of 0.75% has the greatest rate of increasing CS as a result of substituting cement with SF ^[40][43]. The research studied how SF and FA affected the concrete’s fresh and hardened qualities, alone and in combination. Results showed that SF increases elastic dynamic modulus, pulse velocity, and CS ^[41][52]. Due to the sluggish pozzolanic activity and diluting impact of FA, the CS of SCC comprising 25% FA and 0% SF, and is 35% lower than that of the reference. However, the inclusion of SF increased the CS of concrete made with cement ^[42][28]. Concretes containing SF perform better in hardened conditions than concretes with viscosity-modifying admixtures (VMA). Unusually, SF10 and VMA0.10 have almost the same mechanical attributes (compressive flexural and modulus of elasticity) ^[33][45]. The impact of several curing regimes, air, water, and steam, on the compressive characteristics of SCC with various ratios of SF and FA substitutions, were investigated. They emphasized that standard cured samples had the greatest CS results (cured in water for 28 days). The compressive capacity increased as the water curing period lengthened. Compression strength decreased during air curing, and air-cured specimens had the lowest strength values across all groups ^[38][26]. The compressive capacity of cement mortars including FA and SF during autoclaved curing was studied. They demonstrated that the FA blended cement mortar’s compressive capacity tended to be lower than the control mortar, owing to greater FA replacement. However, SF enhanced the compressive capacity of the binary mixed cement mortar, making it stronger than the reference sample. The increase in CS was due to the rise in SF. However, the opposite was true when it came to the rise in FA content. CS increased owing to a rise in the SF ratio; an increase in FA content had the opposite effect ^[43][53]. By adding SF to concrete, it becomes more durable and has a higher early compressive, tensile, and flexural strength. It also has a substantial influence on the elastic modulus ^[44][54]. Strength is one characteristic, particularly in design mixtures that include 30% CR substitution. When 30% of CR and 5% of SF were used, the minimum 28-day CS of 5.29 MPa was obtained. Additionally, model equations for predicting the compressive, flexural, and splitting tensile strengths were effectively created, and optimization was carried out ^[45][47]. The compressive capacity of all SCM samples, excluding 15% of the SF, tends to decline after 90 days of immersion, except for those samples that were aged for 90 days. Binary mixtures of v displayed superior sulfate resistance even when exposed for longer periods ^[46][55]. Similar findings to those found in this inquiry have been found in other investigations. Sasanipour and Aslani also noted a reduction in CS in SSC, with 50% cement substitution for SF ^[47][36]. Due to a poor interfacial transition zone, CS decreases when silica fumes are partially substituted for cement. The application of SF has a poor transition zone that harms the strength qualities of curing ^[48][56]. Higher CS than the strength of the control sample was produced by the application of SF. Increases in the amount of SF and the use of metakaolin, which account for up to 15% of the total cement mass, both boosted compressive strengths. The increased compressive and tensile capacity was achieved by employing the recommended optimal proportion of replacement, 15 % mixed pozzolan (7.5 % metakaolin and 7.5 % of SF) ^[35][29]. The CS and structural durability of concrete that is environmentally friendly, and environmentally friendly for construction, are only slightly impacted by the addition of 20% SF. The findings of this reshearch show that using SF in lieu of cement, and adding steel fiber, resulted in a more durable and economically viable SCC ^[49][57].

3.2. Tensile Strength (TS)

As with the compressive capacity, it can be shown that SF replacement increased the flexural capacity of concrete. Due to pozzolanic and filling voids, the substitution of SF has improved the TS of the material. The TS increased gradually when FA and SF were substituted. The percentage of SF increased the Portland-fly ash-silica fume concrete mixture’s TS ^[50][60]. For all curing regimens at the age of 28 days, binary mixtures of SF and ternary mixtures of SF and FA yielded the greatest tensile capacity. Binary blends of SF, except SF (14%), and ternary blends of FA (10%), and SF (6%), performed better for self-curing at the age of 180 days compared to other curing regimens ^[43][53]. The reinforced SSC with recycled steel fiber of 0.75% has the greatest rate of increasing compressive, tensile, and flexural capacity as a result of substituting cement with SF ^[40][43]. Askari et al. ^[51][61] examined the compressive, tensile capacity, fresh qualities, and mechanical characteristics of SCC incorporating FA and SF. The findings showed that between 28 and 120 days of curing, a large volume of FA content increases the CS. This result has demonstrated that FA’s pozzolanic activity has persisted throughout time, however, FA concentration increases as tensile strength declines. However, the tensile strength of SCC having a large volume FA is improved and maintained with a 10% SF substitution of cement ^[51][61]. In a similar study, with SF added at a constant rate of 10% and constant water to a binder ratio of 0.28, Yazici et al. ^[52][62] studied the impact of substitution of cement with class C FA at different substitute levels from 30 to 60% on the fresh assets, compressive, tensile capacity, modulus of elasticity, and durability aspects. They discovered that using class C FA in place of cement raised the compressive aspect. The results showed that increasing the SF content in SCC increased its overall tensile strength for the FA replacement levels ^[52][62]. At 28 days, the splitting tensile capacity of SCC improves from 4.84 to 5.86 MPa. The tensile capacity of concrete increased gradually when FA and SF were added to the concrete mixtures. The splitting tensile capacity is improved by the addition of 10% SF ^[42][28]. The splitting tensile capacity is improved by 27% with the addition of 5% SF to the reference concrete, and it lowers by 14% with the addition of 10% SF. Crumb rubber considerably lowers the concrete splitting tensile to roughly half of its control mixture ^[53][63]. The usage of metakaolin and SF had a large and modest impact on tensile strength, respectively. The increased compressive and tensile capacity was achieved by employing the recommended optimal proportion of replacement, 15% mixed pozzolan (7.5% metakaolin and 7.5% SF) ^[35][29]. The silicon carbide whiskers increased the cement-based materials’ tensile strength, and the ideal dose was 0.1% by weight ^[54][64]. Another similar study shows a 12% increase in tensile capacity compared to the reference sample, which included 10% SF and 1% steel fibers ^[55][65]. By adding SF to concrete, CS, splitting tensile strength, and resistance to freezing and thawing, are all increased ^[56][66]. The compressive and splitting tensile capacity were unchanged when brick debris made up to 25% of the sand. Therefore, using this replacement % in RCC is simple. However, the compressive and splitting tensile capacity was decreased by utilizing this replacement ratio ^[57][59]. While only SF was employed as the pozzolanic material, as opposed to FA, the CS, as well as TS, progressively rose ^[32][42]. The behavior of the tensile strength of concrete changes dramatically when SF or micro silica is substituted for M40 or M50 grade concrete by 0, 5, 7.5, 10, or 15%. Beyond 28 days, the maximum tensile capacity of concrete containing SF was achieved; after that point, it began to decline ^[58][67]. According to research, the M35 grade of concrete exhibits an amazing improvement in the tensile characteristics of concrete when SF is substituted by 0, 5, 10, 15, or 20. With split tensile strength, 10% was found to be the greatest improvement ^[59][68].

3.3. Flexural Strength (FS)

As with the CS, it can be shown that SF substitute enhanced the FS of concrete. The flexural toughness factors are increased by substituting cement with SF, and the specimens reinforced with recycled steel fiber at a rate of 0.5%, and the highest substitute ratio of cement with SF showed the greatest improvement (14%). Furthermore, assuming a consistent amount of SF in the substituted cement, adding recycled fiber lowers the flexural toughness indices ^[40][43]. The FS of mixes increased by around 24 to 50% when SF was increased from 0 to 25%. The enhanced pozzolanic activity as a result of the increased development of calcium silicate hydrate (C-S-H) gels is primarily responsible for the increase in strength that results from the inclusion of SF as a supplemental cementitious ingredient. On the other hand, since SF particles are smaller in size, more gaps are filled and the porosity on the surface junction of rubber and surrounding materials is reduced, which results in the strengthening of paste ^[60][44]. Binary blends of SF and ternary combinations of SF and FA produced the highest levels of flexural capacity for all treatment regimens at the age of 28 days. In comparison to other curing regimens, binary mixes of SF, other than SF14 and ternary blends of FA10SF6, performed better for self-curing (LPWC) at the age of 180 days ^[43][53]. The addition of SF to concrete increases the concrete’s toughness, while also delivering early high levels of compressive, tensile, and flexural strength. It also has a substantial influence on the elastic modulus. Additionally, SF renders concrete resistant to chemical assaults. Because of this, high-performance concrete with SF is preferred for parking decks, overpasses, and maritime constructions ^[44][54]. The FS of the SSC was decreased during the whole curing period when cement was partially substituted with SF. Due to a poor interfacial transition zone, the CS decreases when SF are partially substituted for cement. The application of SF has a poor transition zone that has a damaging impact on the strength qualities ^[48][56]. According to research, the compressive, flexural, and modulus of elasticity may all be increased by 10% of SF without negatively affecting any of the other properties by 29%, 22%, and 14%, respectively ^[61][69]. According to one study, using FA and SF increases concrete’s flexural capacity by 10%. The mixtures containing 5% of SF and 10% of FA had the best replacement percentage in terms of flexural capacity ^[62][70]. According to Fakhri et al. ^[63][71], adding rubber particles and SF to RCC at the same time boosts the concrete’s compressive and flexural capacity, decreases water absorption, and improves density ^[63][71].