# Comprehensive Characterization of Ferrochrome Slag and Ferrochrome Ash as Sustainable Materials in Construction

Authors: Batna Anantha Venkata Ram Kumar; Lakshmi Keshav; Panneerselvam Arul Sivanantham; George Gabriel Vimal Arokiaraj; Dhaleelur Rahman Zunaithur Rahman; Puchhakatla Madan Kumar; Dasappa Somashekar
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Ferrochrome slag (FCS) and ferrochrome ash (FCA) are by-products generated during the production of ferrochrome alloy in the ferrochrome industry. The use of these by-products as construction materials appears to be an innovative strategy that could provide numerous environmental and socio-economic benefits. However, the residual chromium present in ferrochrome by-products may have some negative effects on the surrounding environment also. In a nutshell, this study provides a thorough and critical examination of ferrochrome slag and ferrochrome ash’s suitability for construction, as well as a list of the major shortcomings that must be addressed to accomplish construction sustainability. A detailed summary of the physical, chemical, and mechanical characteristics of ferrochrome slag and ferrochrome ash was presented in the study. Ferrochrome slag from previous studies is said to exhibit better mechanical properties compared to conventional coarse aggregates which contributed to better mechanical properties of concrete. The application of ferrochrome slag as a substitute for natural sand, on the other hand, is considered to have a detrimental impact. As a reason, further research is necessary to determine the impact of replacing conventional fine aggregate with ferrochrome slag on the various mechanical and durability properties of concrete. Ferrochrome ash from previous studies can be used as a partial replacement for cement and unlike FCS, FCA is nonhazardous since no residual chromium traces were present in FCA. Furthermore, the protracted safety and effect on the surrounding environment of ferrochrome slag containing concrete in a variety of exposure conditions have to be fully examined in the near future.


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## Body

## 1. Introduction

For decades, the world’s population has been quickly increasing due to rapid development and broad urbanization. This fast expansion is putting tremendous pressure on natural resources and overexploitation of natural resources has a large effect on the environment. Recently, there has been a heavy focus on finding a solution to man-made environmental challenges [1, 2]. Concrete is the most used material for construction. It is the most widely utilized commodity in the world, second only to water [3]. The utilization of wastes from industries and by-products in construction has been a topic of research for many years [4]. The utilization of by-products and wastes addresses the disposal problems associated with such wastes and also aids in sustainability [1, 5]. Faster depletion of natural resources to meet the requirements of the present is a serious threat to the concept of sustainability [2, 6]. Sustainability lies in protecting and preserving resources for the future while meeting the present requirements [2, 7, 8]. To address these issues, use of alternative construction materials in place of conventional materials sounds like a great solution. Many by-products and wastes were explored in the recent past for replacing the conventional construction materials. Ferrochrome (FeCr) is a chromium and iron alloy. By smelting chromite mineral, which contains chrome and iron oxides, in an electric arc furnace, ferrochrome is produced. Ferrochrome slag is a by-product generated during the extraction of ferrochrome whereas ferrochrome ash is a by-product of the ferroalloy industry. FCA is the dust produced by gas cleaning operations, which are used to remove harmful flue gas from smelting furnaces. According to reports, 1000–1200 kg of ferrochrome slag and 20000–30000 gm of FCA are generated for every tonne of ferrochrome produced [8]. Ferrochrome slag is a mechanically competent and eco-friendly waste material that may be utilized in concrete as fine and coarse aggregate [9, 10]. The two forms of FCS, i.e., as fine and coarse aggregates, are shown in Figure 1.Figure 1
Ferrochrome slag coarse and fine aggregate.


## 2. Research Significance

The current report’s major goal is to comprehensively evaluate the available published studies on the use of ferrochrome slag and ferrochrome ash in concrete. The fresh properties and mechanical and durability properties of concrete containing FCS and FCA were critically analyzed in the study. This will help the future researchers to find the outcomes of using FCS and FCA in a single document.


### 2.1. Manufacturing of Ferrochrome Slag

The waste product generated by the stainless-steel production industry is ferrochrome slag [11]. This slag is generated as a liquid at temperatures above 1600° C. Owing to the variation in densities, the denser ferrochrome metal settles down while thinner slag film remains on the molten metal during the process of smelting chromite ores [12]. After that, the slag layers are separated and annealed. In India, roughly 3.36 metric tonnes of ferrochrome slag is generated from 118 units out of a total of 229 furnaces in operation [13, 14]. The enormous production of ferrochrome slag results in a major disposal challenge and has an environmental impact [15]. FS offers good potential for usage as coarse aggregate because of its mineralogical composition and mechanical qualities [16]. Under varied cooling temperatures, different sizes of ferrochrome slag aggregates are formed. After alloy recovery, air-cooled slag of 10–20 mm size is produced, which has been utilized as coarse aggregate by numerous researchers [17]. Ferrochrome slag that has been water-cooled is present in granulated form with grain sizes that are similar to natural sand. Ferrochrome slag is a derivative of the manufacturing of ferrochrome, an alloy of Cr (chromium) and Fe (iron) with chromium by 51–69 percent of the mass and varying proportions of Fe (iron), C (carbon), and others [18–20]. In 2017, the worldwide production of FC was predicted to be approximately 12 million tonnes. It is also predicted to grow by about 3.6 percent per year to catch up with the increasing demand [21–23]. South Africa, which has the most chromite deposits, is the world’s leading producer of FC, followed by China [24]. They contribute to more than 66 percent of overall FC output worldwide. The stainless-steel sector consumes the largest chunk of FC generated (over 80%). It is typically formed in immersed electric arc furnaces at temperatures ranging from 1500 to 1700 degrees Celsius [25]. The manufacture of FC is linked to the production of a considerable volume of slag, known as ferrochrome slag (FCS).The characteristics of the produced ferrochrome slag are significantly influenced by the FCS cooling system [26]. The cooling system has a big impact on the rate of crystallization and the size of the generated slag [27]. Rapid water cooling generates finer (5 mm) dark-colored granules with a crystalline nature. This slag is often referred to as water-cooled slag. Air-cooled slag (ACS) is typically made in bigger sizes, ranging from 8 to 20 mm [28]. ACS has a lumpy texture and is gray in appearance. Although both types of slag have a partly crystalline and partly glassy structure, the air-cooled slag has a higher crystalline nature than the water-cooled slag [29]. The majority of FCS is said to be generated utilizing the air-dependent cooling method. The dust obtained from ferrochrome factory gas cleaning units is referred to as ferrochrome ash (FCA) [30–35]. Dust, grime, and partially combusted wood, coal, and coke are all present in the gas released by ferroalloy smelting furnaces [32]. Sulfurous, poisonous metal oxide fumes, carbon monoxide in the form of unburned hydrocarbons, and other organic gases are all present in this gas. The off-gases are cleaned in the gas cleaning facility, and a large amount of FA is obtained in the process [34]. FA has the characteristics of a slag material used in the manufacture of Portland cement; it is being tested as a substitute for cement together with lime in the production of energy-efficient, low-cost concrete [36]. FCA is manufactured in large quantities and discharged without any pollution control, or remediation mechanisms in place [37–40]. It not only uses up hectares of precious land but also poses a threat to underground and surface water resources [41]. FCA meets the slag specifications and can be utilized as a cementitious material in combination with lime to replace cement [42, 43]. Various applications of FCA and FCS are depicted in Figure 2.Figure 2
Various applications of ferrochrome slag and ash.


### 2.2. Mineral Composition

The mineralogy of ferrochrome slag is influenced by the chemical composition as well as the cooling method adopted for slag manufacturing [8, 14]. The strength, density, environmental robustness, and refractory characteristics of FCS are all influenced by the mineralogical phases [44]. FCS is dominated by forsterite and spinal phases. Further forsterite and spinal phases account for more than 80 percent of the total volume of ferrochrome slag mineralogy [45]. They are accountable for the FCS matrix’s better mechanical characteristics, more relative density, and latent chromium endurance [46–50]. The greater breaking temperature of mineral phases, as shown in Table 1, suggests that FCS could be used to make refractory materials and fire-resistant concrete [51]. Dissimilar to the spinal mineralogical phases, such as MgAl2O4 and MgCr2O4, the observed glassy phases, such as enstatite and anorthite, have far less mechanical characteristics in FCS [52]. As a result, the occurrence of a greater volume of spinal phases over glassy phases improves FCS mechanical characteristics. Moreover, remnant chromium in the spinal phase is extremely stable, hence improving FCS’s environmental compatibility [53].Table 1
Mineralogy of ferrochrome slag.
Mineral phasesFormulaMelting temperature (C)Magnesio-aluminateMgA2O42135Magnesio-chromiteMgCr2O2300Magnesio alumino chromiteMg(Al, Cr)2SiO4—Chromo-ferideFe, Cr—Forsterite(Mg Fe)2SiO41880EnstatiteMgSiO31560Cordierite2Mgo-2Al2O3-5SiO21500AnortheriteCaAl2Si2O81450FayaliteFe2SiO4—QuartzSiO4—OlivineMg1.784Fe0.216SiO4—SpinelMg,2(Al, Cr)—Calcite(Mg0.03Ca0.97)(CO3)—


### 2.3. Chemical Composition

The chemical constitution of FCS is primarily determined by the ore type and is represented in Figure3 [54]. Fluxing elements, high-temperature residence period, oxygen pressure, and melt temperature are all elements that influence the chemical structure of FCS [55]. Interestingly, the chemical structure of the FCS is unaffected by the cooling mechanism used. The molecular structure of ferrochrome slag and ash as summarized in Table 2 and Table 3, respectively, suggests that they may have pozzolanic characteristics. On average, the sum of SiO2, Al2O3, and Fe2O3 makes up more than half of the total elements [56]. The remnant chromium in ferrochrome slag is usually observed in the kind of Cr2O3. The proportion pertaining to Cr (chromium) in ferrochrome slag differs significantly [57]. The proportion of residual chromium is mostly determined by the grade of the ore and the effectiveness of the smelting process. Aside from the aforementioned compounds, FCS contains trace amounts of Sulfur Trioxide, Sodium Oxide, and Potassium Oxide [49, 58]. Other elements, together with some potentially harmful substances like Mn2O3, TiO2, and Strontium Oxide (SrO), were available in different proportions, according to some researchers [59]. The chemical composition of ferrochrome ash by various researchers is represented in Figure 4.Figure 3
Chemical constituents of ferrochrome slag by various researchers.Table 2
Chemical composition of ferrochrome slag (by mass).
S.NoColling systemSiO2%Al2O3%Fe2O3%CaO%Mgo%Cr2O3%SO3Na2OK2OK2O + Na2%Summation of SiO2,Al2O3, and Fe2O3 %References1—21.2821.285.894.0724.9419.46———48.97[7]2Air27.723.73.63.5123.79.160.210.15—55[45]3Water28.8722.842.852.9630.328.32————54.56[45]4—302642238———60[48]5—27.524.73.819.0622.59.34———0.456.01[49]6—30.779.031.3047.356.904.15————41.10[54]7Water:air70%:30%29.1423.214.016.1320.8612.590.04——0.1856.36[60]829.1423.214.016.1320.8612.590.0390.1456.01[60]9—27.3319.754.122.4932.2810.37————50.83[61]10—30.4731.090.690.4923.663.40————62.25[62]11Air24.738.721.9342.5513.013.56————35.41[62]12Air25.698.671.8344.5112.033.59———0.2336.38[62]13—28.8529.531.390.4832.115.30————58.83[63]14—29.2223.241.430.8937.415.23———0.3255.65[64]15—33.8025.480.611.1035.882.12————59.89[65–67]16—37.2922.501.911.4531.104.640.50——0.2761.70[68]17—35.4020.952.211.4728.778.90————57.65[58]18—30.1026.204.102.1023.1015.20————60.80[69–72]19—21.2821.585.894.0724.9419.46————48.75[71]20—26.9823.695.634.7524.069.360.68——0.5756.30[72]21Air33.3415.232.452.1233.3213.460.55———52.20[73]22—34.6318.709.342.4916.8614.860.86——0.2562.67[10]23—25.0016.0010.003.0026.0010.00————51.00[74]24—32.1126.452.473.1123.1212.78————61.56[29]25—27.2223.343.543.4523.349.21———0.3455.43[75–77]26—27.1422.214.015.1324.8812.57————53.36[78]27—29.9119.052.793.725.5833.383.68———51.75[79]28Air27.5024.703.819.0622.509.36———0.4056.01[41, 80, 81]29Air27.7026.052.2012.5525.304.15————55.95[82]30—59.707.901.4023.205.70—0.30——0.1469.00[83]3130.4320.823.338.6824.129.761.02——0.4154.57AverageTable 3
Chemical composition of ferrochrome ash.
S.NoMaterialSiO2%CaOAl2O3%Mgo%Fe2O3K2O%Na2%P2O5%TiO2%SO3Cr2O3%CL%LOIReference1Ferrochrome ash (FA)19.64.2211.1015.606.0614.501.30——1.9212.409.40—[32–35]19.64.2211.1015.606.06————1.9212.409.40—[34]319.64.211.215.66.114.51.3———12.40——[36]419.103.1410.9123.607.8411.422.460.07N/A—9.892——[37–39]519.64.2211.115.66.060.461.30.062.1961.92———[38]619.604.2211.1015.606.0614.501.30——1.92———[41, 42]719.64.2211.115.66.060.461.30.062.1961.92———[31]819.64.2211.115.66.06————1.92———[31]919.64.2211.1015.606.0614.501.30——1.9212.409.40—[33]Figure 4
Chemical constituents of ferrochrome ash by various researchers.


## 2.1. Manufacturing of Ferrochrome Slag

The waste product generated by the stainless-steel production industry is ferrochrome slag [11]. This slag is generated as a liquid at temperatures above 1600° C. Owing to the variation in densities, the denser ferrochrome metal settles down while thinner slag film remains on the molten metal during the process of smelting chromite ores [12]. After that, the slag layers are separated and annealed. In India, roughly 3.36 metric tonnes of ferrochrome slag is generated from 118 units out of a total of 229 furnaces in operation [13, 14]. The enormous production of ferrochrome slag results in a major disposal challenge and has an environmental impact [15]. FS offers good potential for usage as coarse aggregate because of its mineralogical composition and mechanical qualities [16]. Under varied cooling temperatures, different sizes of ferrochrome slag aggregates are formed. After alloy recovery, air-cooled slag of 10–20 mm size is produced, which has been utilized as coarse aggregate by numerous researchers [17]. Ferrochrome slag that has been water-cooled is present in granulated form with grain sizes that are similar to natural sand. Ferrochrome slag is a derivative of the manufacturing of ferrochrome, an alloy of Cr (chromium) and Fe (iron) with chromium by 51–69 percent of the mass and varying proportions of Fe (iron), C (carbon), and others [18–20]. In 2017, the worldwide production of FC was predicted to be approximately 12 million tonnes. It is also predicted to grow by about 3.6 percent per year to catch up with the increasing demand [21–23]. South Africa, which has the most chromite deposits, is the world’s leading producer of FC, followed by China [24]. They contribute to more than 66 percent of overall FC output worldwide. The stainless-steel sector consumes the largest chunk of FC generated (over 80%). It is typically formed in immersed electric arc furnaces at temperatures ranging from 1500 to 1700 degrees Celsius [25]. The manufacture of FC is linked to the production of a considerable volume of slag, known as ferrochrome slag (FCS).The characteristics of the produced ferrochrome slag are significantly influenced by the FCS cooling system [26]. The cooling system has a big impact on the rate of crystallization and the size of the generated slag [27]. Rapid water cooling generates finer (5 mm) dark-colored granules with a crystalline nature. This slag is often referred to as water-cooled slag. Air-cooled slag (ACS) is typically made in bigger sizes, ranging from 8 to 20 mm [28]. ACS has a lumpy texture and is gray in appearance. Although both types of slag have a partly crystalline and partly glassy structure, the air-cooled slag has a higher crystalline nature than the water-cooled slag [29]. The majority of FCS is said to be generated utilizing the air-dependent cooling method. The dust obtained from ferrochrome factory gas cleaning units is referred to as ferrochrome ash (FCA) [30–35]. Dust, grime, and partially combusted wood, coal, and coke are all present in the gas released by ferroalloy smelting furnaces [32]. Sulfurous, poisonous metal oxide fumes, carbon monoxide in the form of unburned hydrocarbons, and other organic gases are all present in this gas. The off-gases are cleaned in the gas cleaning facility, and a large amount of FA is obtained in the process [34]. FA has the characteristics of a slag material used in the manufacture of Portland cement; it is being tested as a substitute for cement together with lime in the production of energy-efficient, low-cost concrete [36]. FCA is manufactured in large quantities and discharged without any pollution control, or remediation mechanisms in place [37–40]. It not only uses up hectares of precious land but also poses a threat to underground and surface water resources [41]. FCA meets the slag specifications and can be utilized as a cementitious material in combination with lime to replace cement [42, 43]. Various applications of FCA and FCS are depicted in Figure 2.Figure 2
Various applications of ferrochrome slag and ash.


## 2.2. Mineral Composition

The mineralogy of ferrochrome slag is influenced by the chemical composition as well as the cooling method adopted for slag manufacturing [8, 14]. The strength, density, environmental robustness, and refractory characteristics of FCS are all influenced by the mineralogical phases [44]. FCS is dominated by forsterite and spinal phases. Further forsterite and spinal phases account for more than 80 percent of the total volume of ferrochrome slag mineralogy [45]. They are accountable for the FCS matrix’s better mechanical characteristics, more relative density, and latent chromium endurance [46–50]. The greater breaking temperature of mineral phases, as shown in Table 1, suggests that FCS could be used to make refractory materials and fire-resistant concrete [51]. Dissimilar to the spinal mineralogical phases, such as MgAl2O4 and MgCr2O4, the observed glassy phases, such as enstatite and anorthite, have far less mechanical characteristics in FCS [52]. As a result, the occurrence of a greater volume of spinal phases over glassy phases improves FCS mechanical characteristics. Moreover, remnant chromium in the spinal phase is extremely stable, hence improving FCS’s environmental compatibility [53].Table 1
Mineralogy of ferrochrome slag.
Mineral phasesFormulaMelting temperature (C)Magnesio-aluminateMgA2O42135Magnesio-chromiteMgCr2O2300Magnesio alumino chromiteMg(Al, Cr)2SiO4—Chromo-ferideFe, Cr—Forsterite(Mg Fe)2SiO41880EnstatiteMgSiO31560Cordierite2Mgo-2Al2O3-5SiO21500AnortheriteCaAl2Si2O81450FayaliteFe2SiO4—QuartzSiO4—OlivineMg1.784Fe0.216SiO4—SpinelMg,2(Al, Cr)—Calcite(Mg0.03Ca0.97)(CO3)—


## 2.3. Chemical Composition

The chemical constitution of FCS is primarily determined by the ore type and is represented in Figure3 [54]. Fluxing elements, high-temperature residence period, oxygen pressure, and melt temperature are all elements that influence the chemical structure of FCS [55]. Interestingly, the chemical structure of the FCS is unaffected by the cooling mechanism used. The molecular structure of ferrochrome slag and ash as summarized in Table 2 and Table 3, respectively, suggests that they may have pozzolanic characteristics. On average, the sum of SiO2, Al2O3, and Fe2O3 makes up more than half of the total elements [56]. The remnant chromium in ferrochrome slag is usually observed in the kind of Cr2O3. The proportion pertaining to Cr (chromium) in ferrochrome slag differs significantly [57]. The proportion of residual chromium is mostly determined by the grade of the ore and the effectiveness of the smelting process. Aside from the aforementioned compounds, FCS contains trace amounts of Sulfur Trioxide, Sodium Oxide, and Potassium Oxide [49, 58]. Other elements, together with some potentially harmful substances like Mn2O3, TiO2, and Strontium Oxide (SrO), were available in different proportions, according to some researchers [59]. The chemical composition of ferrochrome ash by various researchers is represented in Figure 4.Figure 3
Chemical constituents of ferrochrome slag by various researchers.Table 2
Chemical composition of ferrochrome slag (by mass).
S.NoColling systemSiO2%Al2O3%Fe2O3%CaO%Mgo%Cr2O3%SO3Na2OK2OK2O + Na2%Summation of SiO2,Al2O3, and Fe2O3 %References1—21.2821.285.894.0724.9419.46———48.97[7]2Air27.723.73.63.5123.79.160.210.15—55[45]3Water28.8722.842.852.9630.328.32————54.56[45]4—302642238———60[48]5—27.524.73.819.0622.59.34———0.456.01[49]6—30.779.031.3047.356.904.15————41.10[54]7Water:air70%:30%29.1423.214.016.1320.8612.590.04——0.1856.36[60]829.1423.214.016.1320.8612.590.0390.1456.01[60]9—27.3319.754.122.4932.2810.37————50.83[61]10—30.4731.090.690.4923.663.40————62.25[62]11Air24.738.721.9342.5513.013.56————35.41[62]12Air25.698.671.8344.5112.033.59———0.2336.38[62]13—28.8529.531.390.4832.115.30————58.83[63]14—29.2223.241.430.8937.415.23———0.3255.65[64]15—33.8025.480.611.1035.882.12————59.89[65–67]16—37.2922.501.911.4531.104.640.50——0.2761.70[68]17—35.4020.952.211.4728.778.90————57.65[58]18—30.1026.204.102.1023.1015.20————60.80[69–72]19—21.2821.585.894.0724.9419.46————48.75[71]20—26.9823.695.634.7524.069.360.68——0.5756.30[72]21Air33.3415.232.452.1233.3213.460.55———52.20[73]22—34.6318.709.342.4916.8614.860.86——0.2562.67[10]23—25.0016.0010.003.0026.0010.00————51.00[74]24—32.1126.452.473.1123.1212.78————61.56[29]25—27.2223.343.543.4523.349.21———0.3455.43[75–77]26—27.1422.214.015.1324.8812.57————53.36[78]27—29.9119.052.793.725.5833.383.68———51.75[79]28Air27.5024.703.819.0622.509.36———0.4056.01[41, 80, 81]29Air27.7026.052.2012.5525.304.15————55.95[82]30—59.707.901.4023.205.70—0.30——0.1469.00[83]3130.4320.823.338.6824.129.761.02——0.4154.57AverageTable 3
Chemical composition of ferrochrome ash.
S.NoMaterialSiO2%CaOAl2O3%Mgo%Fe2O3K2O%Na2%P2O5%TiO2%SO3Cr2O3%CL%LOIReference1Ferrochrome ash (FA)19.64.2211.1015.606.0614.501.30——1.9212.409.40—[32–35]19.64.2211.1015.606.06————1.9212.409.40—[34]319.64.211.215.66.114.51.3———12.40——[36]419.103.1410.9123.607.8411.422.460.07N/A—9.892——[37–39]519.64.2211.115.66.060.461.30.062.1961.92———[38]619.604.2211.1015.606.0614.501.30——1.92———[41, 42]719.64.2211.115.66.060.461.30.062.1961.92———[31]819.64.2211.115.66.06————1.92———[31]919.64.2211.1015.606.0614.501.30——1.9212.409.40—[33]Figure 4
Chemical constituents of ferrochrome ash by various researchers.


## 3. Physical and Mechanical Properties

Compared to standard limestone aggregate, ferrochrome slag as coarse aggregate exhibits more relative density [21]. FCS has a specific gravity spanning between 2.64 to 3.20. Because of the presence of Fe and Mg, FCS exhibits relatively better specific gravity [45]. FCS also exhibits better bulk density than standard limestone and granite aggregates. As compared to standard limestone aggregate, coarse FCS aggregate has greater water absorption, i.e., sometimes higher than 2 percent, possibly be linked to FCS’s porous physical form [46]. It has been established that the strength characteristics of ferrochrome slag coarse aggregates merely match but also surpass those of traditional aggregates like limestone. The existence of various metals, like iron and magnesium, and their considerable crystallinity are responsible for FCS’s remarkable mechanical characteristics [52]. Throughout many studies, the aggregate impact, aggregate crushing, and Los angle’s abrasion values were testified to be less than 20% [60]. When concrete is intended to resist wear, such as on pavement structures and flooring, coarse FCS aggregate is a preferable choice [62]. FCS fragments were employed as coarse concrete aggregates without subsequent processing in some investigations. Their average size ranged between 10 and 20 mm, according to reports [74]. FCS fragments were crushed in certain cases to get the necessary particle sizes and grain size pattern [81]. The physical properties of ferrochrome ash and slag are presented in Tables 4 and 5, respectively.Table 4
Physical properties of ferrochrome ash.
Physical propertiesSpecific gravityFinenessColorpHDensity (g/cm3)Particles retained on 45-micron sieve (wet sieving) (%)ReferenceFerrochrome ash (FA)2.24571Gray9.792.24—[33]—571——2.246.50[42]Table 5
Coarse ferrochrome slag physical and mechanical characteristics.
S.NoName of materialBulk density (kg/m3)Specific gravityAggregate impact value (%)Aggregate crushing value (%)Water absorption (%)Flakiness index (%)Los Angles abrasion value (%)Elongation indexFineness modulusReference1Lime stone—2.50-2.8015.0-20.019.0-31.00.50-0.53—22.0-3.00——[21]2FCS—2.8411.0017.890.639.8318.1910.50—[32–35]31480(L)2.6318.2522.531.1214.3021.83——[45]41480(L)2.6318.2522.531.1214.3121.83——[46]5—2.8423.2025.100.42—19.20——[47]61480(L)3.0513.35—0.5215.24—10.236.73[48]71164(L)2.52——1.89———2.69[49]8—2.9413.7—0.5————[50]9—2.3318.7323.721.2214.9522.4110.50—[51]10—2.77——1.70—23.00——[52]111783(C)36.6311.692.03———5.12[53]12—2.85——0.74————[60]131930(C)3.01——2.22—18.58——[62]141785(C)3.17———————[64]151785(C)3.218.6121.640.809.2825.6414.44—[71]16—3.1518.7018.100.3014.3013.9014.506.97[74]171480(L)2.6218.3222.531.2114.2321.72——[79]18—2.3318.7323.721.2214.95222.4110.5—[81]19Granite—2.8315.320.10.218.5118.613.5[81]20FCS1880(C)3.16——2.23—18.44——[84]212182(C)2.1416200.8132525—[85]


### 3.1. Workability

According to the studies, the addition of ferrochrome slag as coarse aggregate to concrete would result in the same or slightly reduced workability as the conventional concrete made with natural limestone aggregate. The reduction may be attributed to more water absorption of ferrochrome slag aggregates which will increase the water demand. This could also be due to the fact that ferrochrome slag coarse aggregate exhibits more angularity and ferrochrome fine aggregate has a rough texture than natural aggregate. Nevertheless, few authors found that when natural coarse aggregate was replaced with FCS, the slump increased by over 100 percent. This could be due to the fact that FCS absorbs more water when soaked in water. On the other hand, increase in workability was also observed in cases where fully saturated ferrochrome slag was used. Physical and thermal characteristics of ferrochrome fine aggregate are presented in the following Table6.Table 6
Fine ferrochrome slag physical and thermal characteristics.
S.NoMaterialSpecific gravityWater absorption (%)GradingPHFineness modulusBulk density (kg/m3)Thermal conductivity (W/m. K)Reference12.87——————[7]22.72—Zone I—4.80——[45]3FCS2.852.01—9.79———[60]43.270.80———1870(C)—[71]52.52—Zone II—2.691164.55(L)1411.82(C)—[71, 72]62.850.63————0.96[86]72.850.63————0.96[87]82.32—Zone I—2.32——[88]9Sand2.60-2.701.10-3.15—————[21]10——————0.28[89]11Specification2.50-2.90—2.0-3.10—2.0-3.10——[85]


### 3.2. Compressive Strength

In comparison with the original concrete mix containing natural coarse aggregate, the concrete mix utilizing ferrochrome slag as coarse aggregate displayed equivalent or superior compressive strength. However, the optimum percentage of ferrochrome that can replace natural coarse aggregate was not uniform by the previous researchers. The majority of the research reported that the complete replacement of traditional coarse aggregate with ferrochrome aggregate resulted in a 20% increase in compressive strength. Some of the other authors reported a maximum increase in compressive strength up to 40% at 75 percent replacement. Various factors like mix design considerations and properties of slag based on source were expected to influence the variation in compressive strength improvement. When compared to sand, the FCS aggregate’s surface roughness is an excellent property for improving the mechanical bond seen between slag grains and cement matrix. The inherent mechanical characteristics of ferrochrome aggregate could be the cause that contributes to concrete’s enhanced compressive strength. Very few authors used ferrochrome slag as fine aggregate and observed that varied results were obtained. Increased FCS content to 50% resulted in a continual drop in concrete compressive strength according to Patro [72]. Panda et al. [45] found that replacing natural sand with ferrochrome fine aggregate reduced compressive strength by 3.6 percent. Besides, Kopuri and Ramesh [89] found that using ferrochrome slag as a partial substitute for natural sand improved the compressive strength. By replacing forty percent of sand with ferrochrome slag fine aggregate, an enhancement of 36% in compressive strength was observed. A few of the explanations for these contradictory test results could be the grain size variation of FCS. The changes in phases of crystallinity in ferrochrome fine aggregates may have an impact on the bond between FCS and cement matrix.The outcomes of tests indicate that when fly ash in the range of 10 to 30% and ferrochrome ash by 3% were added to replace cement, the compressive strength was enhanced during all curing ages on par with conventional concrete Tribikram Mohanty [31]. The concentration of silica and alumina present in ferrochrome ash is accountable for the formation of dicalcium silicate that provides greater strength at longer curing ages. In another study when 40% ferrochrome ash and 7% lime were used, compressive strength got enhanced by 40% compared to conventional concrete at 7 days of curing. However, at 28 days, the increase was only confined to one percent compared to normal concrete Prasanna K Acharya [31]. The inclusion of ferrochrome ash and powdered gypsum considerably enhanced the compressive strength. The concrete mix with 10% powdered gypsum and 30% ferrochrome ash exhibited maximum strength. When ferrochrome ash was used in geopolymer concrete, there was an improvement in compressive strength observed up to 80% replacement Jyotirmoy Mishra [37–39]. Relative change of compressive strength for various percentages of FCS is represented in Figure 5.Figure 5
Relative change in compressive strength for different percentage of fine and coarse FCS.


### 3.3. Flexural Strength

Modulus of rupture was also observed to be enhanced by using ferrochrome slag as a substitute for natural coarse aggregate, identical to compressive strength. Zelic [73] observed that replacing natural limestone aggregate with FCS increased flexural strength by 4.5 percent and 12.9 percent, respectively. Likewise, Sathwik [52] and Rajasekhar [46] demonstrated a 25 percent and 17 percent rise at full replacement. Because of the inadequate research data and the wide range of results, no firm conclusions about utilizing ferrochrome aggregates as a partial substitute for sand on fracture strength could be established. The modulus of rupture of the concrete containing 40% ferrochrome fine aggregate decreased by 9.91 percent compared to reference concrete with natural sand. Conversely, Kopuri and Ramesh [89] found that using FCS as a replacement for sand had a favorable effect with an improvement of flexural strength by 5%. With reference to the conventional mortar with natural sand, Al-Jabri [87] observed a 40% growth in bending strength with 30 percent ferrochrome substitution. The contradictory conclusions may be attributed to the dispersion of grain size of ferrochrome fines, like that of coarse aggregate. Additionally, the betterment in fracture strength is similar to concrete’s compressive strength while using ferrochrome slag as fine aggregate.The outcomes of tests indicate that when fly ash in the range of 10 to 30% and ferrochrome ash by 3% were added to replace cement, the flexural strength was enhanced at 28 and 56 days on par with conventional concrete. The interfacial transitions zone (ITZ) connecting cement matrix and aggregate gets denser during the chemical reaction, and the size of nanopores reduces, resulting in an increase in flexural strength [90]. The modulus of rupture of prisms containing 40% FCA and 7% lime as a partial replacement for cement is similar to that of conventional concrete beams. Beams containing 40% ferrochrome ash and 7% lime as a partial replacement for cement exhibited an 8.33 percent higher ultimate load-carrying capacity over ordinary concrete beams [33].


### 3.4. Split Tensile Strength

An increasing pattern for concrete tensile strength was detected, which was similar to that of using ferrochrome coarse aggregates on compressive strength. At 100 percent substitute for natural limestone aggregate with FCS, all published studies demonstrated better performance, i.e., in the range of 9.58 percent to 31.44 percent. Various researches, however, indicated different substitution percentages as optimal. Susheel et al. [83] as well as Reddy et al. [46] through their research established that 75 percent substitution of natural limestone aggregate with ferrochrome slag results in the largest increases in tensile strength. Besides, Sathwik et al. [52] discovered the significant enhancement at a 50% level of replacement. Mixed results were obtained while ferrochrome was used as fine aggregate. Few authors, for instance, reported an increase in 35 percent tensile strength at forty percent substitution, while on the contrary others reported a decline in 12.6 percent at fifty percent substitution.The findings of the previous research demonstrated that when fly ash in the range of 10-30% and 3% ferrochrome ash were used to substitute cement, the split tensile strength increased relative to standard concrete [31]. The inclusion of ferrochrome ash and powdered gypsum resulted in a constant rise in split tensile strength. According to Shrinu Mohanty [31], the maximum strength was acquired by a concrete mix comprising 10% powdered gypsum and 30% ferrochrome ash FA [90].


### 3.5. Chloride, Sulfate, and Acid Resistance

Resistance to aggressive environments is a desirable property for concrete used in construction. Resistance to chloride penetration was assessed by a chloride penetration test. However, no data was available from past literature related to chloride penetration when ferrochrome aggregates were used [77]. Just a few researchers have looked into the impact of ferrochrome slag aggregate on concrete’s sulfate resistance. Concrete specimens after casting using ferrochrome slag aggregates were soaked in 5% magnesium sulfate solution for 180 days and tested for compressive strength [84]. At all curing periods, the decrease in comparative compressive strength was discovered to be more with the increase in ferrochrome coarse aggregates proportion. Relatively more moisture absorbing nature of ferrochrome coarse aggregate and more porosity in concrete were established as potential reasons for reduced sulfate resistance [88]. When concrete containing ferrochrome slag as a replacement for natural fine aggregate was immersed in a 10% magnesium sulfate solution, the results were completely different. At all curing periods, concrete specimens with various ferrochrome slag substitution percentages, i.e., 10 to 50%, were noted to improve strength. With a prolonged magnesium sulfate treatment, concrete specimens obtained much more strength. The rapid synthesis of C-S-H, such as ettringite, is attributed to the increase in compressive strength [89]. The additional pozzolanic activity which might be a result of fine ferrochrome particles and less soluble magnesium sulfate is said to prevent the penetration of sulfate ions.. In terms of mass loss and compressive strength loss, concrete using ferrochrome slag coarse aggregate performed marginally superior than conventional concrete. The acid tolerance of concrete specimens containing ferrochrome fine aggregate was marginally superior to conventional concrete [80]. Furthermore, the addition of ferrochrome slag as a replacement for natural fine aggregate in concrete treated with 3 percent H2So4 resulted in lower mass loss and compressive strength losses. The alkaline nature of ferrochrome fine aggregate might be the cause for improved durability against acid attack [65].


### 3.6. Water Absorption

Water absorption is a parameter that decides the longevity and also density of the cement concrete. Scanty publications were present related to the effect of ferrochrome slag on water absorption [72]. Water absorption of concrete is reported to increase with the use of ferrochrome slag. For example, concrete containing 50% ferrochrome slag exhibited an absorbency of roughly 3 percent, compared to around 2% for the standard concrete [72]. There was no study related to the use of coarse ferrochrome slag aggregate on the water absorption of concrete.


## 3.1. Workability

According to the studies, the addition of ferrochrome slag as coarse aggregate to concrete would result in the same or slightly reduced workability as the conventional concrete made with natural limestone aggregate. The reduction may be attributed to more water absorption of ferrochrome slag aggregates which will increase the water demand. This could also be due to the fact that ferrochrome slag coarse aggregate exhibits more angularity and ferrochrome fine aggregate has a rough texture than natural aggregate. Nevertheless, few authors found that when natural coarse aggregate was replaced with FCS, the slump increased by over 100 percent. This could be due to the fact that FCS absorbs more water when soaked in water. On the other hand, increase in workability was also observed in cases where fully saturated ferrochrome slag was used. Physical and thermal characteristics of ferrochrome fine aggregate are presented in the following Table6.Table 6
Fine ferrochrome slag physical and thermal characteristics.
S.NoMaterialSpecific gravityWater absorption (%)GradingPHFineness modulusBulk density (kg/m3)Thermal conductivity (W/m. K)Reference12.87——————[7]22.72—Zone I—4.80——[45]3FCS2.852.01—9.79———[60]43.270.80———1870(C)—[71]52.52—Zone II—2.691164.55(L)1411.82(C)—[71, 72]62.850.63————0.96[86]72.850.63————0.96[87]82.32—Zone I—2.32——[88]9Sand2.60-2.701.10-3.15—————[21]10——————0.28[89]11Specification2.50-2.90—2.0-3.10—2.0-3.10——[85]


## 3.2. Compressive Strength

In comparison with the original concrete mix containing natural coarse aggregate, the concrete mix utilizing ferrochrome slag as coarse aggregate displayed equivalent or superior compressive strength. However, the optimum percentage of ferrochrome that can replace natural coarse aggregate was not uniform by the previous researchers. The majority of the research reported that the complete replacement of traditional coarse aggregate with ferrochrome aggregate resulted in a 20% increase in compressive strength. Some of the other authors reported a maximum increase in compressive strength up to 40% at 75 percent replacement. Various factors like mix design considerations and properties of slag based on source were expected to influence the variation in compressive strength improvement. When compared to sand, the FCS aggregate’s surface roughness is an excellent property for improving the mechanical bond seen between slag grains and cement matrix. The inherent mechanical characteristics of ferrochrome aggregate could be the cause that contributes to concrete’s enhanced compressive strength. Very few authors used ferrochrome slag as fine aggregate and observed that varied results were obtained. Increased FCS content to 50% resulted in a continual drop in concrete compressive strength according to Patro [72]. Panda et al. [45] found that replacing natural sand with ferrochrome fine aggregate reduced compressive strength by 3.6 percent. Besides, Kopuri and Ramesh [89] found that using ferrochrome slag as a partial substitute for natural sand improved the compressive strength. By replacing forty percent of sand with ferrochrome slag fine aggregate, an enhancement of 36% in compressive strength was observed. A few of the explanations for these contradictory test results could be the grain size variation of FCS. The changes in phases of crystallinity in ferrochrome fine aggregates may have an impact on the bond between FCS and cement matrix.The outcomes of tests indicate that when fly ash in the range of 10 to 30% and ferrochrome ash by 3% were added to replace cement, the compressive strength was enhanced during all curing ages on par with conventional concrete Tribikram Mohanty [31]. The concentration of silica and alumina present in ferrochrome ash is accountable for the formation of dicalcium silicate that provides greater strength at longer curing ages. In another study when 40% ferrochrome ash and 7% lime were used, compressive strength got enhanced by 40% compared to conventional concrete at 7 days of curing. However, at 28 days, the increase was only confined to one percent compared to normal concrete Prasanna K Acharya [31]. The inclusion of ferrochrome ash and powdered gypsum considerably enhanced the compressive strength. The concrete mix with 10% powdered gypsum and 30% ferrochrome ash exhibited maximum strength. When ferrochrome ash was used in geopolymer concrete, there was an improvement in compressive strength observed up to 80% replacement Jyotirmoy Mishra [37–39]. Relative change of compressive strength for various percentages of FCS is represented in Figure 5.Figure 5
Relative change in compressive strength for different percentage of fine and coarse FCS.


## 3.3. Flexural Strength

Modulus of rupture was also observed to be enhanced by using ferrochrome slag as a substitute for natural coarse aggregate, identical to compressive strength. Zelic [73] observed that replacing natural limestone aggregate with FCS increased flexural strength by 4.5 percent and 12.9 percent, respectively. Likewise, Sathwik [52] and Rajasekhar [46] demonstrated a 25 percent and 17 percent rise at full replacement. Because of the inadequate research data and the wide range of results, no firm conclusions about utilizing ferrochrome aggregates as a partial substitute for sand on fracture strength could be established. The modulus of rupture of the concrete containing 40% ferrochrome fine aggregate decreased by 9.91 percent compared to reference concrete with natural sand. Conversely, Kopuri and Ramesh [89] found that using FCS as a replacement for sand had a favorable effect with an improvement of flexural strength by 5%. With reference to the conventional mortar with natural sand, Al-Jabri [87] observed a 40% growth in bending strength with 30 percent ferrochrome substitution. The contradictory conclusions may be attributed to the dispersion of grain size of ferrochrome fines, like that of coarse aggregate. Additionally, the betterment in fracture strength is similar to concrete’s compressive strength while using ferrochrome slag as fine aggregate.The outcomes of tests indicate that when fly ash in the range of 10 to 30% and ferrochrome ash by 3% were added to replace cement, the flexural strength was enhanced at 28 and 56 days on par with conventional concrete. The interfacial transitions zone (ITZ) connecting cement matrix and aggregate gets denser during the chemical reaction, and the size of nanopores reduces, resulting in an increase in flexural strength [90]. The modulus of rupture of prisms containing 40% FCA and 7% lime as a partial replacement for cement is similar to that of conventional concrete beams. Beams containing 40% ferrochrome ash and 7% lime as a partial replacement for cement exhibited an 8.33 percent higher ultimate load-carrying capacity over ordinary concrete beams [33].


## 3.4. Split Tensile Strength

An increasing pattern for concrete tensile strength was detected, which was similar to that of using ferrochrome coarse aggregates on compressive strength. At 100 percent substitute for natural limestone aggregate with FCS, all published studies demonstrated better performance, i.e., in the range of 9.58 percent to 31.44 percent. Various researches, however, indicated different substitution percentages as optimal. Susheel et al. [83] as well as Reddy et al. [46] through their research established that 75 percent substitution of natural limestone aggregate with ferrochrome slag results in the largest increases in tensile strength. Besides, Sathwik et al. [52] discovered the significant enhancement at a 50% level of replacement. Mixed results were obtained while ferrochrome was used as fine aggregate. Few authors, for instance, reported an increase in 35 percent tensile strength at forty percent substitution, while on the contrary others reported a decline in 12.6 percent at fifty percent substitution.The findings of the previous research demonstrated that when fly ash in the range of 10-30% and 3% ferrochrome ash were used to substitute cement, the split tensile strength increased relative to standard concrete [31]. The inclusion of ferrochrome ash and powdered gypsum resulted in a constant rise in split tensile strength. According to Shrinu Mohanty [31], the maximum strength was acquired by a concrete mix comprising 10% powdered gypsum and 30% ferrochrome ash FA [90].


## 3.5. Chloride, Sulfate, and Acid Resistance

Resistance to aggressive environments is a desirable property for concrete used in construction. Resistance to chloride penetration was assessed by a chloride penetration test. However, no data was available from past literature related to chloride penetration when ferrochrome aggregates were used [77]. Just a few researchers have looked into the impact of ferrochrome slag aggregate on concrete’s sulfate resistance. Concrete specimens after casting using ferrochrome slag aggregates were soaked in 5% magnesium sulfate solution for 180 days and tested for compressive strength [84]. At all curing periods, the decrease in comparative compressive strength was discovered to be more with the increase in ferrochrome coarse aggregates proportion. Relatively more moisture absorbing nature of ferrochrome coarse aggregate and more porosity in concrete were established as potential reasons for reduced sulfate resistance [88]. When concrete containing ferrochrome slag as a replacement for natural fine aggregate was immersed in a 10% magnesium sulfate solution, the results were completely different. At all curing periods, concrete specimens with various ferrochrome slag substitution percentages, i.e., 10 to 50%, were noted to improve strength. With a prolonged magnesium sulfate treatment, concrete specimens obtained much more strength. The rapid synthesis of C-S-H, such as ettringite, is attributed to the increase in compressive strength [89]. The additional pozzolanic activity which might be a result of fine ferrochrome particles and less soluble magnesium sulfate is said to prevent the penetration of sulfate ions.. In terms of mass loss and compressive strength loss, concrete using ferrochrome slag coarse aggregate performed marginally superior than conventional concrete. The acid tolerance of concrete specimens containing ferrochrome fine aggregate was marginally superior to conventional concrete [80]. Furthermore, the addition of ferrochrome slag as a replacement for natural fine aggregate in concrete treated with 3 percent H2So4 resulted in lower mass loss and compressive strength losses. The alkaline nature of ferrochrome fine aggregate might be the cause for improved durability against acid attack [65].


## 3.6. Water Absorption

Water absorption is a parameter that decides the longevity and also density of the cement concrete. Scanty publications were present related to the effect of ferrochrome slag on water absorption [72]. Water absorption of concrete is reported to increase with the use of ferrochrome slag. For example, concrete containing 50% ferrochrome slag exhibited an absorbency of roughly 3 percent, compared to around 2% for the standard concrete [72]. There was no study related to the use of coarse ferrochrome slag aggregate on the water absorption of concrete.


## 4. Conclusions

(i)
Ferrochrome slag and ferrochrome ash can be effectively used as construction materials for various applications like making concrete, brick production, partial replacement to cement, lightweight aggregates, ceramics, and refractory materials manufacturing(ii)
Ferrochrome slag can be used to completely replace coarse aggregate in concrete due to its excellent mechanical characteristics compared to any conventional coarse aggregates. It is said to improve almost all mechanical characteristics of concrete-like elasticity modulus, flexural strength, tensile strength, compressive strength, and abrasion resistance(iii)
Use of FCS fine aggregate has an evidence of improving the temperature stability of concrete but no study was available related to FCS coarse aggregate(iv)
Contradictory results were obtained when FCS fine aggregate is used to partially replace conventional fine aggregates in concrete owing to variation in the grain size distribution of ferrochrome fine aggregate(v)
Leached chromium from concrete with FCS is found to be within the specified limits as perUnited States Environmental Protection Agency (USEPA); however, quantification of leached chromium under an aggressive environment has not been addressed by any researcher



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*Source: 8571055-2022-07-15.xml*