# Evaluation of Mechanical and Durability Properties of Eco-Friendly Concrete Containing Silica Fume, Waste Glass Powder, and Ground Granulated Blast Furnace Slag

Authors: Mahdi Bameri; Soroush Rashidi; Mohammad Mohammadhasani; Mohammad Maghsoudi; Hesam Madani; Fereydoun Rahmani
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

By considering the adverse environmental impacts of the cement manufacturing process, there have been many efforts for cement replacement by supplementary cementitious materials (SCMs), which can enhance the produced concrete performance while reducing cement consumption. This study evaluated the effects of various proportions of silica fume (SF), waste glass powder (WGP), and ground granulated blast furnace slag (GGBFS) on the mechanical and durability properties of concrete. The properties evaluated in this study include compressive, tensile, and flexural strength, magnesium sulfate and sulfuric acid attack, surface resistivity, rapid chloride penetrability test (RCPT), water absorption, depth of penetration of water, and microstructure analysis by scanning electron microscopy (SEM). The results of compressive, tensile, and flexural strength, chloride ion penetrability, and water absorption tests showed that adding 5% of SF to mixtures containing 10% WGP or 10% GGBFS improved concrete performance significantly due to packing density and synergistic effect; however, adding 5% of SF to concrete mixtures decreased the resistance against the magnesium sulfate and sulfuric acid attack. The binary mixture of 15% of WGP showed appropriate performance against the magnesium sulfate and sulfuric acid attack, which may be due to the sacrificial nature of WGP. In addition, the binary mixtures of 15% of WGP and 15% of GGBFS reduced the depth of penetration of water by 45%. Microstructure analysis by SEM showed that the presence of SF, along with WGP and GGBFS, improves the packing density. Finally, adding 5% of SF is suggested to improve the properties of concrete mixtures containing WGP and GGBFS.


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## Body

## 1. Introduction

Cement is becoming one of the most widely used worldwide structural materials because of the development of the construction industry. The annual cement production worldwide is reached more than 4.2 billion tonnes and is anticipated to grow continuously [1]. The cement production processes use a large volume of raw materials and energy, and a significant amount of carbon dioxide is released into the atmosphere. According to the type of fuel used, about 0.9–1.0 metric tonnes of CO2 are released into the atmosphere for every tonne of clinker; if the amount of cement used is reduced, CO2 emissions will also be reduced [2].Concrete is the most common construction material in the world, and it is the second most consumed product on the planet after water [3]. Although the concrete industry has destructive effects on the environment and sustainability, it is one of the basic materials for developing the industry, infrastructure, and housing. Portland cement is one of the main components of concrete that reacts with water and produces hydration products through the hydration process. Calcium hydroxide (CaOH2) forms a considerable part of cement hydration products, which is a major drawback in terms of durability. Supplementary cementitious materials (SCMs) reduce the quantity of calcium hydroxide due to the pozzolanic reaction and improve the properties of the concrete which can replace a portion of the cement in the concrete mixture. SCMs are from industrial byproducts or natural materials; these materials react with hydrated cement paste both hydraulically or pozzolanic, which can lead to the production of durable and economical concretes [4–8]. Engineering, economic, and ecological advantages can be achieved by replacing a portion of cement with SCMs. The engineering advantage can be attributed to the improvement potential of the fresh and hardened concrete properties by adding an appropriate portion of SCMs, such as silica fume, to the concrete mixture [9]. Moreover, replacing some amount of the cement with cheaper options, such as waste glass powder and ground granulated blast furnace slag (GGBFS), can also obtain economic advantages [10–12]. Also, due to the consumption of less cement in the concrete mixture, environmental pollution can be prevented by reducing CO2 emissions [13].For decades, the focus has been fully developed on SCMs. Many studies have been carried out on their utilization in concrete production and evaluating their effects on hardened concrete properties [14–16]. SCMs are considered options for Portland cement in the concrete mixture for economic and ecological reasons, improving the mechanical and durability properties and minimizing the penetrability of concrete. Nevertheless, few studies have been carried out on industrial byproducts with cementitious or pozzolanic properties as replacements for cement [17].Studies have demonstrated that pozzolanic materials have a significant amount of amorphous silica (silicon dioxide) in their chemical composition [17, 18]. By adding pozzolanic materials to the concrete mix proportion, a reaction takes place between silicon dioxide (SiO2) and CaOH2, which is called the pozzolanic reaction [19]. The cement hydration process produces CaOH2, and it causes adverse effects on concrete durability due to higher solubility in acids and sulfates compared to other products. The affection of SCMs in the pozzolanic reaction and the CaOH2 conversion to calcium silicate hydrate (C-S-H) leads to an increase in the durability of the concrete exposed to the penetration of acids, sulfates, and chloride ions, also increasing the concrete density [21]. The main advantages of SCMs are attributed to the three properties of the pozzolanic reaction. First, the rate of reaction is reduced, which leads to a decreased heat of hydration and reduces the rate of strength development of concrete [19]. Second, the SCMs’ reaction with CaOH2 leads to a decreased amount of this unfavorable product in the concrete, which significantly affects its durability and penetrability [21–23]. Third, they reduce the capillary pores, increasing the concrete impermeability and strength [24]. Nonetheless, the use of SCMs has limitations. That is, the reactivity of SCMs is generally lower than cement [25, 26].Investigations show that the worldwide production of GGBFS is 300–360 million tonnes annually, so only part of the demand for cement can be fulfilled by replacing GGBFS [25, 27, 28]. Accordingly, the focus of many researchers is the identification of alternative SCMs and the evaluation of their performance in concrete.The waste glass powder is one of the widely available materials that can be utilized as a replacement for cement. A small amount of waste glass is recycled, and the remainder of it is disposed of due to impurities available in it or color or cost. Due to the different properties of waste glass powder, recycling any type of waste glass is impossible. There is a need to create new options for recycling waste glass. One important option is to use waste glass in building materials [25, 29]. The glass powder contains 73% SiO2, 13% Na2O, and 10% CaO, making it a pozzolanic material, and can be utilized in concrete [25, 30]. In 1963, the first study on the usage of glass for building materials was done. The authors used waste glass aggregates to produce a wall panel [31]. Later, due to the hardness of glass, many studies were conducted to use waste glass as aggregate in concrete and mortar [32–34]. Since glass has alkali content, replacing glass as aggregate in concrete is vulnerable to the alkali-silica reaction. The performance of concrete and the alkali-silica reaction depends on the size of the glass particles. The alkali-silica reaction in concrete can be reduced by adding silica fume, GGBFS, fly ash, and metakaolin [25]. Utilizing glass powder as SCM in the studies of Islam et al. [35], He et al. [36], Patel et al. [37], Mehta and Ashish [25], and Ibrahim [38] was investigated [25, 35–38]. According to the literature reviewed, 10–20 wt% of waste glass powder can be utilized as a replacement for cement. Mehta and Ashish [25] evaluated the effects of silica fume and waste glass on the workability, strength, durability properties, and microstructural analysis of concrete. They remarked that the optimal replacement percentage of glass powder by cement is 10%, which can significantly improve concrete performance [25].The SCMs have low rates of lime consumption at early ages [39, 40]. Although improving the mechanical and durability properties of concrete in the long term, these materials could not improve the mechanical and durability properties of concrete in the short term [41]. Among SCMs, fly ash and GGBFS are the most common types that have been utilized at high replacement levels. However, silica fume and metakaolin cannot be added to the concrete at a high replacement level because these materials significantly reduce the workability of concrete due to the very high surface area [41, 42]. The pozzolanic properties of silica fume and its filling effect have made it an appropriate SCM [25, 43]. Adding silica fume to concrete can reduce bleeding, porosity, and penetrability [44]. The fine particle size of silica fume allows it to act as a filler and improve the packing density by being placed among the cement particles [41, 45].According to the advantages and disadvantages of GGBFS, the above-mentioned waste glass powder and silica fume may have synergistic effects to improve concrete properties. Although silica fume can improve the cement hydration and durability characteristics of concrete, it reduces the workability of concrete, and adding high amounts of silica fume in concrete is not applicable. Hence, combining silica fume with low surface area SCMs such as glass powder and GGBFS can increase the workability (flowability) of concrete, so ternary concrete mixtures can be an effective solution to utilizing higher volumes of SCMs in sustainable concrete. Although the low reactivity of GGBFS and glass powder due to their particle size reduces the strength of concrete at early ages, silica fume increases the strength of concrete because of its finer and more reactive particles. In ternary concrete mixtures, adding silica fume improves the density as they fill the spaces between the cement, GGBFS, and glass powder particles, reducing the number of voids in the packing and consequently increasing the packing density. Furthermore, the risk of alkali-silica reaction of concrete containing glass powder can be reduced by adding silica fume. The ternary concrete mixtures can exhibit acceptable durability and strength performances at different ages and decrease vulnerability under aggressive environmental conditions; consequently, using ternary mixtures can be an appropriate option [25, 41].Several studies have been conducted on the properties of ternary concrete mixtures containing silica fume. Yazıcı [46], Bagheri et al. [47], and Wongkeo et al. [48] stated that ternary concrete mixtures containing fly ash and silica fume increased chloride resistance compared to binary concrete mixtures containing fly ash without silica fume [46–48]. Wongkeo et al. [48] and Li and Kwan [49] reported that the addition of silica fume to concrete mixtures containing fly ash might improve the mechanical properties of concrete [48, 49]. Ibrahim [38] reported the results of three groups of mixture designs, including plain concrete, concrete containing silica fume, and concrete containing fly ash. The cement content was replaced by 5%, 10%, 15%, and 20% waste glass powder. The results showed that 5% of waste glass powder could replace cement without reducing the compressive and tensile strength. In addition, replacing 20% of waste glass powder in concrete containing silica fume and fly ash reduced the compressive and tensile strength by 13–14%, respectively [38].Regarding sustainable development, the current study evaluated the effect of GGBFS and waste glass powder with/without silica fume on the mechanical and durability properties of eco-friendly concrete. To accurately evaluate the synergistic effects of concrete mixtures, tests of compressive strength, tensile strength, flexural strength, magnesium sulfate attack, sulfuric acid attack, surface resistivity, rapid chloride penetrability test (RCPT), water absorption, depth of penetration of water, and microstructure analysis of concrete by SEM were performed.


## 2. Materials and Methods

### 2.1. Materials

#### 2.1.1. Supplementary Cementitious Materials (SCMs)

Ordinary Portland Cement (OPC)-Type II [50], produced by Kerman Momtazan Cement Co., (KMCC) in Iran, Kerman, and three types of SCMs, including silica fume (SF), ground granulated blast furnace slag (GGBFS), and waste glass powder (WGP), were used in this study for concrete production. Iran Ferrosilice Co., Esfahan Steel Co., and Baztab Rah Co., supply SF, GGBFS, and WGP are used in the current study, respectively. The chemical, physical, and mechanical properties of the intended materials are demonstrated in Table 1.Table 1
Chemical, physical, and mechanical properties of cement, SF, GGBFS, and WGP.
PropertiesCement (%)SF (%)GGBFS (%)WGP (%)Silicon dioxide (SiO2)21.13Min 85.0036.5071.00Aluminium oxide (Al2O3)4.55Max 1.0011.003.23Ferrous oxide (Fe2O3)3.63Max 2.000.700.19Calcium oxide (CaO)63.72Max 1.5038.509.20Magnesium oxide (MgO)1.27Max 1.509.201.49Sulphur tri oxide (SO3)2.58—0.300.19Insoluble residue (IR)0.49—0.40100.00Sodium oxide (Na2O)0.16—0.5513.80Potassium oxide (K2O)0.59—0.600.29Loss on ignition (LOI)1.67Max 3.500.500.67Tricalcium silicate (C3S)55.71———Dicalcium silicate (C2S)18.61———Tricalcium aluminate (C3A)5.92———Tetra-calcium aluminoferrite (C4AF)11.01———Physical propertiesMedian diameter (D50) (μm)214∗1630Specification surface (cm2/gr)3110150,000–200,000040002900SoundnessAutoclave expansion, (%)0.11———Le Chatelier expansion, (mm)1.00———Setting time (min)Initial-135———Final-185———Mechanical propertiesMortar compressive strength (MPa)3 days27.6———7 days36.3———28 days47.3———∗Median diameter of agglomerated silica fume.SF added in concrete mixtures is characterized by light grey color and is composed of at least 85% SiO2 based on ASTM C1240 [51]. According to ASTM C989, the activity index of GGBFS lies between grades 80 and 100 [52]. WGP consists of 71% SiO2, 3.2% Al2O3, and 0.19% Fe2O3, and it can be treated as pozzolanic material concerning chemical requirements presented in ASTM C618 despite the alkali content [53]. The laser particle size analyzer (LPSA) is employed to determine the particle size distribution of cement and other SCMs, as shown in Figure 1. It must be noted that the particle size distribution of the SF in Figure 1 shows the size of agglomerates and does not show the particle size [43].Figure 1
The particle size distribution of cement, SF, GGBFS, and WGP.


#### 2.1.2. Aggregates

The aggregates used in this study include natural sand with a maximum nominal size of 4.75 mm, saturated surface dry (SSD) density of 2630 kg/m3 and SSD water absorption of 2.07%, and coarse aggregate with a nominal maximum size of 19 mm, SSD density of 2690 kg/m3, and SSD water absorption of 0.9% [54, 55]. It is to be noted that the gradation of the used aggregates meets the requirements of ASTM C33 [56]. Mechanical sieve shakers are used to determine the size distribution of aggregates, and analysis results can be seen in Figure 2.Figure 2
Particle size distribution curves of aggregates.


#### 2.1.3. Superplasticizer

The polycarboxylate ether-based superplasticizer (PES) with a specific gravity of 1.1 g/cm3 and pH value of 6.2 was added to all concrete mixtures according to ASTM C494 [57].


### 2.2. Mix Proportions

In this experimental study to evaluate the mechanical and durability properties of concrete, six mixture proportions with SCMs described in the previous section were considered. The binder content and water/binder ratio were kept constant in all the mixtures at 400 kg/m3 and 0.36, respectively. The concrete mixture proportions are presented in Table 2.Table 2
Proportions of the concrete mixtures.
MixMix codeW/BUnit content (kg/m3)WaterCementSFWGPGGBFSNatural sandCoarse aggregateControlCtrl0.36144400———930907SF (5%)SF50.3614438020——925.2902.1WGP (15%)WGP150.36144340—60—923.8900.6GGBFS (15%)GGBFS150.36144340——60933.2910.3SF (5%) + WGP (10%)SF5+WGP100.361443402040—921.1897.9SF (5%) + GGBFS (10%)SF5+GGBFS100.3614434020—40927.4904SF: silica fume, GGBFS: ground granulated blast furnace slag, and WGP: waste glass powder.


#### 2.2.1. Production Method and Curing Conditions for Concrete Mixtures

Materials were mixed in the following order. First, coarse aggregate and a half of sand were drily mixed for 1 min; then, the rest of the sand, cement, SCMs, and 2/3 of mixing water was added, and the mixing process was resumed for 3 min further; finally, the rest of the water along with the superplasticizer was also added and was mixed for another 8 min. After 24 h casting, all the specimens were demolded, and according to BS EN 12390-2, they were cured in water at 20 ± 2°C until the time of testing [58].


### 2.3. Test Methods

#### 2.3.1. Compressive Strength

According to BS EN 12390-3, the compressive strength tests were carried out on cubic specimens with dimensions of 150 × 150 × 150 mm at the ages of 7, 28, 56, and 90 days [59].


#### 2.3.2. Tensile Strength

According to ASTM C496, the tensile strength tests were carried out on cylindrical specimens with a diameter of 150 mm and a height of 300 mm at the ages of 7, 28, 56, and 90 days. The loading rate was 103 kg/s [60].


#### 2.3.3. Flexural Strength

According to ASTM C293, the flexural strength tests were carried out on beam specimens with dimensions of 100 × 100 × 500 mm at the ages of 7, 28, 56, and 90 days [61].


#### 2.3.4. Magnesium Sulfate and Sulfuric Acid Attack

The magnesium sulfate attack tests were performed at the ages of 7, 28, 56, and 90 days. This test method evaluates the chemical resistance of concrete under anticipated service conditions for which 1.5% magnesium sulfate was used to stimulate the magnesium sulfate environment. The magnesium sulfate solution pH was kept in the range from 5 to 7 by adding sulfuric acid throughout the test duration. Oven-dried concrete cubes (150 × 150 × 150 mm) were weighed first and then completely submerged in the magnesium sulfate solution. Three cubes of each mixture were tested after each exposure period. Weight change was compared with the initially measured weight. Compressive strength changes of concrete specimens exposed to magnesium sulfate were also performed at mentioned ages. Finally, these results were compared with the compressive strength of 28 days of water-cured concrete specimens. The sulfuric acid attack tests were also performed in a similar method with the only difference that thepH value was equal to 1.0 [62–72].


#### 2.3.5. Surface Resistivity

According to AASHTO-T 358, the surface resistivity was measured in kΩ·cm by placing the four-point Wenner array probe on cylindrical specimens with dimensions of 100 × 200 mm [73].


#### 2.3.6. Rapid Chloride Penetrability Test (RCPT)

As seen in Figure3, the RCPT was performed according to ASTM C1202. At the age of 28, 56, 90, and 120 days, three 50 mm thick slices were cut from the middle part of 100 × 200 mm cylindrical specimens of each mixture. Finally, direct current (DC) by a constant 60-volt potential difference was applied to them for 6 hours [74].Figure 3
Rapid chloride penetrability test (RCPT).


#### 2.3.7. Water Absorption

According to BS 1881-122, the water absorption tests were carried out on cubic specimens of dimensions 100 × 100 × 100 mm at the ages of 28, 56, 90, and 120 days. The specimens were removed from the water and subsequently were dried at 105°C for 72 h in an oven until they reached a constant weight. The weighted specimens were then immersed in the water for 30 minutes and 24 hours. The amount of water absorbed by each specimen was determined by weighting each specimen again [75].


#### 2.3.8. Depth of Penetration of Water under Pressure

As can be observed from Figure4, this test is carried out according to BS EN 12390–8 to measure the depth of penetration of water in cubic specimens of dimensions 150 × 150 × 150 mm under 500 kPa pressure during 72 h [76].Figure 4
Depth of penetration test of water under pressure.


#### 2.3.9. Scanning Electron Microscopy (SEM)

The SEM allows the chemical analysis and examination of the composition, surface, and concrete microstructure at the micro- and nano-scale. In the current study, SEM of voltage 10 kV and magnification 10.0000x examined the microstructure of concrete specimens. The concrete specimens of tests had the age of 90 days with dimensions of 10 × 10 × 10 mm, and a gold layer coated them before analysis.


## 2.1. Materials

### 2.1.1. Supplementary Cementitious Materials (SCMs)

Ordinary Portland Cement (OPC)-Type II [50], produced by Kerman Momtazan Cement Co., (KMCC) in Iran, Kerman, and three types of SCMs, including silica fume (SF), ground granulated blast furnace slag (GGBFS), and waste glass powder (WGP), were used in this study for concrete production. Iran Ferrosilice Co., Esfahan Steel Co., and Baztab Rah Co., supply SF, GGBFS, and WGP are used in the current study, respectively. The chemical, physical, and mechanical properties of the intended materials are demonstrated in Table 1.Table 1
Chemical, physical, and mechanical properties of cement, SF, GGBFS, and WGP.
PropertiesCement (%)SF (%)GGBFS (%)WGP (%)Silicon dioxide (SiO2)21.13Min 85.0036.5071.00Aluminium oxide (Al2O3)4.55Max 1.0011.003.23Ferrous oxide (Fe2O3)3.63Max 2.000.700.19Calcium oxide (CaO)63.72Max 1.5038.509.20Magnesium oxide (MgO)1.27Max 1.509.201.49Sulphur tri oxide (SO3)2.58—0.300.19Insoluble residue (IR)0.49—0.40100.00Sodium oxide (Na2O)0.16—0.5513.80Potassium oxide (K2O)0.59—0.600.29Loss on ignition (LOI)1.67Max 3.500.500.67Tricalcium silicate (C3S)55.71———Dicalcium silicate (C2S)18.61———Tricalcium aluminate (C3A)5.92———Tetra-calcium aluminoferrite (C4AF)11.01———Physical propertiesMedian diameter (D50) (μm)214∗1630Specification surface (cm2/gr)3110150,000–200,000040002900SoundnessAutoclave expansion, (%)0.11———Le Chatelier expansion, (mm)1.00———Setting time (min)Initial-135———Final-185———Mechanical propertiesMortar compressive strength (MPa)3 days27.6———7 days36.3———28 days47.3———∗Median diameter of agglomerated silica fume.SF added in concrete mixtures is characterized by light grey color and is composed of at least 85% SiO2 based on ASTM C1240 [51]. According to ASTM C989, the activity index of GGBFS lies between grades 80 and 100 [52]. WGP consists of 71% SiO2, 3.2% Al2O3, and 0.19% Fe2O3, and it can be treated as pozzolanic material concerning chemical requirements presented in ASTM C618 despite the alkali content [53]. The laser particle size analyzer (LPSA) is employed to determine the particle size distribution of cement and other SCMs, as shown in Figure 1. It must be noted that the particle size distribution of the SF in Figure 1 shows the size of agglomerates and does not show the particle size [43].Figure 1
The particle size distribution of cement, SF, GGBFS, and WGP.


### 2.1.2. Aggregates

The aggregates used in this study include natural sand with a maximum nominal size of 4.75 mm, saturated surface dry (SSD) density of 2630 kg/m3 and SSD water absorption of 2.07%, and coarse aggregate with a nominal maximum size of 19 mm, SSD density of 2690 kg/m3, and SSD water absorption of 0.9% [54, 55]. It is to be noted that the gradation of the used aggregates meets the requirements of ASTM C33 [56]. Mechanical sieve shakers are used to determine the size distribution of aggregates, and analysis results can be seen in Figure 2.Figure 2
Particle size distribution curves of aggregates.


### 2.1.3. Superplasticizer

The polycarboxylate ether-based superplasticizer (PES) with a specific gravity of 1.1 g/cm3 and pH value of 6.2 was added to all concrete mixtures according to ASTM C494 [57].


## 2.1.1. Supplementary Cementitious Materials (SCMs)

Ordinary Portland Cement (OPC)-Type II [50], produced by Kerman Momtazan Cement Co., (KMCC) in Iran, Kerman, and three types of SCMs, including silica fume (SF), ground granulated blast furnace slag (GGBFS), and waste glass powder (WGP), were used in this study for concrete production. Iran Ferrosilice Co., Esfahan Steel Co., and Baztab Rah Co., supply SF, GGBFS, and WGP are used in the current study, respectively. The chemical, physical, and mechanical properties of the intended materials are demonstrated in Table 1.Table 1
Chemical, physical, and mechanical properties of cement, SF, GGBFS, and WGP.
PropertiesCement (%)SF (%)GGBFS (%)WGP (%)Silicon dioxide (SiO2)21.13Min 85.0036.5071.00Aluminium oxide (Al2O3)4.55Max 1.0011.003.23Ferrous oxide (Fe2O3)3.63Max 2.000.700.19Calcium oxide (CaO)63.72Max 1.5038.509.20Magnesium oxide (MgO)1.27Max 1.509.201.49Sulphur tri oxide (SO3)2.58—0.300.19Insoluble residue (IR)0.49—0.40100.00Sodium oxide (Na2O)0.16—0.5513.80Potassium oxide (K2O)0.59—0.600.29Loss on ignition (LOI)1.67Max 3.500.500.67Tricalcium silicate (C3S)55.71———Dicalcium silicate (C2S)18.61———Tricalcium aluminate (C3A)5.92———Tetra-calcium aluminoferrite (C4AF)11.01———Physical propertiesMedian diameter (D50) (μm)214∗1630Specification surface (cm2/gr)3110150,000–200,000040002900SoundnessAutoclave expansion, (%)0.11———Le Chatelier expansion, (mm)1.00———Setting time (min)Initial-135———Final-185———Mechanical propertiesMortar compressive strength (MPa)3 days27.6———7 days36.3———28 days47.3———∗Median diameter of agglomerated silica fume.SF added in concrete mixtures is characterized by light grey color and is composed of at least 85% SiO2 based on ASTM C1240 [51]. According to ASTM C989, the activity index of GGBFS lies between grades 80 and 100 [52]. WGP consists of 71% SiO2, 3.2% Al2O3, and 0.19% Fe2O3, and it can be treated as pozzolanic material concerning chemical requirements presented in ASTM C618 despite the alkali content [53]. The laser particle size analyzer (LPSA) is employed to determine the particle size distribution of cement and other SCMs, as shown in Figure 1. It must be noted that the particle size distribution of the SF in Figure 1 shows the size of agglomerates and does not show the particle size [43].Figure 1
The particle size distribution of cement, SF, GGBFS, and WGP.


## 2.1.2. Aggregates

The aggregates used in this study include natural sand with a maximum nominal size of 4.75 mm, saturated surface dry (SSD) density of 2630 kg/m3 and SSD water absorption of 2.07%, and coarse aggregate with a nominal maximum size of 19 mm, SSD density of 2690 kg/m3, and SSD water absorption of 0.9% [54, 55]. It is to be noted that the gradation of the used aggregates meets the requirements of ASTM C33 [56]. Mechanical sieve shakers are used to determine the size distribution of aggregates, and analysis results can be seen in Figure 2.Figure 2
Particle size distribution curves of aggregates.


## 2.1.3. Superplasticizer

The polycarboxylate ether-based superplasticizer (PES) with a specific gravity of 1.1 g/cm3 and pH value of 6.2 was added to all concrete mixtures according to ASTM C494 [57].


## 2.2. Mix Proportions

In this experimental study to evaluate the mechanical and durability properties of concrete, six mixture proportions with SCMs described in the previous section were considered. The binder content and water/binder ratio were kept constant in all the mixtures at 400 kg/m3 and 0.36, respectively. The concrete mixture proportions are presented in Table 2.Table 2
Proportions of the concrete mixtures.
MixMix codeW/BUnit content (kg/m3)WaterCementSFWGPGGBFSNatural sandCoarse aggregateControlCtrl0.36144400———930907SF (5%)SF50.3614438020——925.2902.1WGP (15%)WGP150.36144340—60—923.8900.6GGBFS (15%)GGBFS150.36144340——60933.2910.3SF (5%) + WGP (10%)SF5+WGP100.361443402040—921.1897.9SF (5%) + GGBFS (10%)SF5+GGBFS100.3614434020—40927.4904SF: silica fume, GGBFS: ground granulated blast furnace slag, and WGP: waste glass powder.


### 2.2.1. Production Method and Curing Conditions for Concrete Mixtures

Materials were mixed in the following order. First, coarse aggregate and a half of sand were drily mixed for 1 min; then, the rest of the sand, cement, SCMs, and 2/3 of mixing water was added, and the mixing process was resumed for 3 min further; finally, the rest of the water along with the superplasticizer was also added and was mixed for another 8 min. After 24 h casting, all the specimens were demolded, and according to BS EN 12390-2, they were cured in water at 20 ± 2°C until the time of testing [58].


## 2.2.1. Production Method and Curing Conditions for Concrete Mixtures

Materials were mixed in the following order. First, coarse aggregate and a half of sand were drily mixed for 1 min; then, the rest of the sand, cement, SCMs, and 2/3 of mixing water was added, and the mixing process was resumed for 3 min further; finally, the rest of the water along with the superplasticizer was also added and was mixed for another 8 min. After 24 h casting, all the specimens were demolded, and according to BS EN 12390-2, they were cured in water at 20 ± 2°C until the time of testing [58].


## 2.3. Test Methods

### 2.3.1. Compressive Strength

According to BS EN 12390-3, the compressive strength tests were carried out on cubic specimens with dimensions of 150 × 150 × 150 mm at the ages of 7, 28, 56, and 90 days [59].


### 2.3.2. Tensile Strength

According to ASTM C496, the tensile strength tests were carried out on cylindrical specimens with a diameter of 150 mm and a height of 300 mm at the ages of 7, 28, 56, and 90 days. The loading rate was 103 kg/s [60].


### 2.3.3. Flexural Strength

According to ASTM C293, the flexural strength tests were carried out on beam specimens with dimensions of 100 × 100 × 500 mm at the ages of 7, 28, 56, and 90 days [61].


### 2.3.4. Magnesium Sulfate and Sulfuric Acid Attack

The magnesium sulfate attack tests were performed at the ages of 7, 28, 56, and 90 days. This test method evaluates the chemical resistance of concrete under anticipated service conditions for which 1.5% magnesium sulfate was used to stimulate the magnesium sulfate environment. The magnesium sulfate solution pH was kept in the range from 5 to 7 by adding sulfuric acid throughout the test duration. Oven-dried concrete cubes (150 × 150 × 150 mm) were weighed first and then completely submerged in the magnesium sulfate solution. Three cubes of each mixture were tested after each exposure period. Weight change was compared with the initially measured weight. Compressive strength changes of concrete specimens exposed to magnesium sulfate were also performed at mentioned ages. Finally, these results were compared with the compressive strength of 28 days of water-cured concrete specimens. The sulfuric acid attack tests were also performed in a similar method with the only difference that thepH value was equal to 1.0 [62–72].


### 2.3.5. Surface Resistivity

According to AASHTO-T 358, the surface resistivity was measured in kΩ·cm by placing the four-point Wenner array probe on cylindrical specimens with dimensions of 100 × 200 mm [73].


### 2.3.6. Rapid Chloride Penetrability Test (RCPT)

As seen in Figure3, the RCPT was performed according to ASTM C1202. At the age of 28, 56, 90, and 120 days, three 50 mm thick slices were cut from the middle part of 100 × 200 mm cylindrical specimens of each mixture. Finally, direct current (DC) by a constant 60-volt potential difference was applied to them for 6 hours [74].Figure 3
Rapid chloride penetrability test (RCPT).


### 2.3.7. Water Absorption

According to BS 1881-122, the water absorption tests were carried out on cubic specimens of dimensions 100 × 100 × 100 mm at the ages of 28, 56, 90, and 120 days. The specimens were removed from the water and subsequently were dried at 105°C for 72 h in an oven until they reached a constant weight. The weighted specimens were then immersed in the water for 30 minutes and 24 hours. The amount of water absorbed by each specimen was determined by weighting each specimen again [75].


### 2.3.8. Depth of Penetration of Water under Pressure

As can be observed from Figure4, this test is carried out according to BS EN 12390–8 to measure the depth of penetration of water in cubic specimens of dimensions 150 × 150 × 150 mm under 500 kPa pressure during 72 h [76].Figure 4
Depth of penetration test of water under pressure.


### 2.3.9. Scanning Electron Microscopy (SEM)

The SEM allows the chemical analysis and examination of the composition, surface, and concrete microstructure at the micro- and nano-scale. In the current study, SEM of voltage 10 kV and magnification 10.0000x examined the microstructure of concrete specimens. The concrete specimens of tests had the age of 90 days with dimensions of 10 × 10 × 10 mm, and a gold layer coated them before analysis.


## 2.3.1. Compressive Strength

According to BS EN 12390-3, the compressive strength tests were carried out on cubic specimens with dimensions of 150 × 150 × 150 mm at the ages of 7, 28, 56, and 90 days [59].


## 2.3.2. Tensile Strength

According to ASTM C496, the tensile strength tests were carried out on cylindrical specimens with a diameter of 150 mm and a height of 300 mm at the ages of 7, 28, 56, and 90 days. The loading rate was 103 kg/s [60].


## 2.3.3. Flexural Strength

According to ASTM C293, the flexural strength tests were carried out on beam specimens with dimensions of 100 × 100 × 500 mm at the ages of 7, 28, 56, and 90 days [61].


## 2.3.4. Magnesium Sulfate and Sulfuric Acid Attack

The magnesium sulfate attack tests were performed at the ages of 7, 28, 56, and 90 days. This test method evaluates the chemical resistance of concrete under anticipated service conditions for which 1.5% magnesium sulfate was used to stimulate the magnesium sulfate environment. The magnesium sulfate solution pH was kept in the range from 5 to 7 by adding sulfuric acid throughout the test duration. Oven-dried concrete cubes (150 × 150 × 150 mm) were weighed first and then completely submerged in the magnesium sulfate solution. Three cubes of each mixture were tested after each exposure period. Weight change was compared with the initially measured weight. Compressive strength changes of concrete specimens exposed to magnesium sulfate were also performed at mentioned ages. Finally, these results were compared with the compressive strength of 28 days of water-cured concrete specimens. The sulfuric acid attack tests were also performed in a similar method with the only difference that thepH value was equal to 1.0 [62–72].


## 2.3.5. Surface Resistivity

According to AASHTO-T 358, the surface resistivity was measured in kΩ·cm by placing the four-point Wenner array probe on cylindrical specimens with dimensions of 100 × 200 mm [73].


## 2.3.6. Rapid Chloride Penetrability Test (RCPT)

As seen in Figure3, the RCPT was performed according to ASTM C1202. At the age of 28, 56, 90, and 120 days, three 50 mm thick slices were cut from the middle part of 100 × 200 mm cylindrical specimens of each mixture. Finally, direct current (DC) by a constant 60-volt potential difference was applied to them for 6 hours [74].Figure 3
Rapid chloride penetrability test (RCPT).


## 2.3.7. Water Absorption

According to BS 1881-122, the water absorption tests were carried out on cubic specimens of dimensions 100 × 100 × 100 mm at the ages of 28, 56, 90, and 120 days. The specimens were removed from the water and subsequently were dried at 105°C for 72 h in an oven until they reached a constant weight. The weighted specimens were then immersed in the water for 30 minutes and 24 hours. The amount of water absorbed by each specimen was determined by weighting each specimen again [75].


## 2.3.8. Depth of Penetration of Water under Pressure

As can be observed from Figure4, this test is carried out according to BS EN 12390–8 to measure the depth of penetration of water in cubic specimens of dimensions 150 × 150 × 150 mm under 500 kPa pressure during 72 h [76].Figure 4
Depth of penetration test of water under pressure.


## 2.3.9. Scanning Electron Microscopy (SEM)

The SEM allows the chemical analysis and examination of the composition, surface, and concrete microstructure at the micro- and nano-scale. In the current study, SEM of voltage 10 kV and magnification 10.0000x examined the microstructure of concrete specimens. The concrete specimens of tests had the age of 90 days with dimensions of 10 × 10 × 10 mm, and a gold layer coated them before analysis.


## 3. Test Results and Discussion

### 3.1. Mechanical Tests

#### 3.1.1. Compressive Strength

The compressive strength test usually provides an overview of concrete quality because the compressive strength of concrete is directly related to the structure of the hydrated cement paste [38]. Figure 5 shows the results of compressive strength. According to that, the highest compressive strength is related to the binary specimen of SF5 because compared to the Ctrl specimen, at the ages of 7, 28, 56, and 90 days, 17%, 27.6%, 28.5%, and 25.8% increased compressive strength, respectively. SF particles with an average diameter of 150 nm are significantly smaller than cement particles with an average diameter of 15 μm, so they can fill the pores and improve the characteristics of concrete; this is known as particle packing or space-filling. In addition, the pozzolanic reaction of SF with CaOH2 in cement paste causes the formation of C-S-H, which fills the interfacial transition zone (ITZ) and improves compressive strength [41, 43, 77–85].Figure 5
Compressive strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10, at the age of 7 days, a 7.3% reduction in compressive strength compared to the Ctrl specimen can be seen; the reason is the slow rate of pozzolanic reactions of WGP and GGBFS at early ages [86–88]. But, at the ages of 28, 56, and 90 days, the ternary specimen of SF5+WGP10 obtained 17%, 12.5%, and 10.3% increases in compressive strength, respectively, compared to the Ctrl specimen because the dissolution of glass particles leads to the creation of free alkali Na+ and Si+ ions [89, 90]. Na+ and Si+ released from glass powder and hydroxyl ions from cement hydration maintain the pH of the pore solution between 13 and 14. The high pH of the pore solution increases the solubility of amorphous silicates [91]. Therefore, the rate of the SF reaction increases, and as a result, denser C-S-H is formed.Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 10.6%, 10.7%, and 8.6% increase in compressive strength compared to the Ctrl specimen, respectively. The increase in compressive strength is due to very fine and reactive particles of SF. The C-S-H gel is produced during the reaction with the SF adsorbed on the GGBFS surface; as a result, the unreacted GGBFS particles connect with the gel around it tightly. The unreacted GGBFS particles are also tightly connected to the surroundingCaOH2 crystals due to the reaction between the CaOH2 crystal and the SF adsorbed on the GGBFS surface [87]. Generally, the presence of SF, along with WGP and GGBFS, provides a sufficient amount of silica, sodium, and calcium to react with CaOH2 and accelerate the rate of C-S-H formation [92]. In truth, the increases in compressive strength in SF5 + WGP10 and SF5 + GGBFS10 specimens indicate the synergistic effect of ternary mixtures [93].In the binary specimen of WGP15, a decrease in compressive strength was observed at all ages compared to the Ctrl specimen. The reduction in compressive strength at the ages of 7, 28, 56, and 90 days was 12.2%, 8.5%, 7.1%, and 8.6%, respectively. Ibrahim [38] and Aliabdo et al. [94] confirmed these results; they stated that at WGP15%, the compressive strength decreased [38, 94]. In truth, the pozzolanic reaction of glass powder is directly related to its particle size. Finer particle size produces a higher pozzolanic reaction. Therefore, the strength increase rate is affected by the particle size distribution of the glass powder [95].The compressive strength results in the binary specimen of GGBFS15 are similar to the binary specimen of WGP15. The hydration activity of GGBFS is significantly lower than that of cement at early ages. Moreover, GGBFS has a retarding effect on cement hydration at early ages [87]. Therefore, the initial compressive strength of concrete containing GGBFS is significantly lower than that of plain concrete [96].


#### 3.1.2. Splitting Tensile Strength

Figure6 shows the tensile strength test results of cylindrical concrete at the ages of 7, 28, 56, and 90 days. Similar to the compressive strength results, the highest tensile strength is related to the binary specimen of SF5 because, at the ages of 7, 28, 56, and 90 days, 19.4%, 13%, 27.6%, and 16.9% of strength increased, compared to the Ctrl specimen, respectively. SF particles are 100 times smaller than cement particles, so they can be well placed among the cement particles and cause more densification in the cement paste matrix. In addition, the pozzolanic reaction of SF with CaOH2 induces the formation of C-S-H. Hence, the compact microstructure of the cement paste, especially in the ITZ, leads to an increase in tensile strength [78, 97].Figure 6
Tensile strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 at the age of 7 days, 30.5% and 33.3% reduction in tensile strength was observed compared to the Ctrl specimen, respectively. The reason for this is the slow reactivity of WGP and GGBFS at early ages to cement [86–88]. Nevertheless, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + WGP10 obtained 8.7%, 23.4%, and 11.3% increase in tensile strength compared to the Ctrl specimen, respectively. The increase is due to the pozzolanic reaction of SF and WGP with CaOH2, which leads to the formation of C-S-H [88]. Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 2.1%, 14.9%, and 3.7% increase in tensile strength compared to the Ctrl specimen, respectively. The increase in tensile strength in the ternary specimen of SF5 + GGBFS10 is due to the presence of SF, which leads to a significant reduction of CaOH2 and improvement of the ITZ of concrete. In addition, GGBFS particles are attached to the surrounding CaOH2 crystals due to the pozzolanic reaction of SF [87].In the binary specimens of WGP15 and GGBFS15, a slight decrease in tensile strength was observed at the final ages compared to the Ctrl specimen. This issue can be attributed to the decrease in the amount of cement in these specimens and the low reactivity rate of these two materials at these ages [38, 87, 88].


#### 3.1.3. Flexural Strength

According to Figure7, the flexural strength of the binary specimen of SF5 at the ages of 7, 28, 56, and 90 days increased by 12.3%, 11.7%, 10.3%, and 10.2%, compared to the Ctrl specimen, respectively. An increase in flexural strength was observed for ternary specimens of all ages. The increase in strength at the ages of 7, 28, 56, and 90 days in the SF5 + WGP10 specimen was 1.2%, 11.7%, 18.3%, and 19.3%, respectively, and in the SF5+GGBFS10 specimen, it was 1.2%, 9.4%, 9.1%, and 9%, compared to the Ctrl specimen, respectively.Figure 7
Flexural strength results of concrete mixtures.The reaction of SiO2 in SF with CaOH2 causes the formation of C-S-H gel. Increasing C-S-H gel and reducing capillary pores in cement paste is one of the main factors in increasing the strength and reducing the porosity of concrete [78, 85, 87, 88]. Similar to compressive strength and tensile strength results, for binary specimens of WGP15 and GGBFS15, a slight decrease in flexural strength was observed at the final ages.


### 3.2. Durability Tests

#### 3.2.1. Magnesium Sulfate Attack

Magnesium sulfate is often found in groundwater, seawater, and some industrial effluents. Magnesium solutions easily react withCaOH2 in Portland cement paste to form soluble salts of calcium. Magnesium sulfate solution is the most aggressive because the sulfate ions can damage the alumina-bearing hydrates in the Portland cement paste. The characteristic feature of magnesium ion attack on cement paste is that the attack is extended to calcium silicate hydrate, which is the main component of cement. In prolonged contact with magnesium solutions, the C-S-H in Portland cement paste gradually loses its calcium ions, which are replaced by magnesium ions. The ultimate product of this substitution reaction is a magnesium silicate hydrate, the formation of which is associated with the loss of the cementitious characteristic. Due to the presence of CaOH2 in the hydrated Portland cement paste, when the cement paste comes into contact with sulfate ions, alumina-containing hydrates are converted to the high-sulfate form (i.e., ettringite), which are shown in the following equations [19]:(1)C3A⋅CH⋅H18+2CH+3S¯+11H⟶C3A⋅3CS¯⋅H32,(2)C3A⋅CS¯⋅H18+2CH+2S¯+12H⟶C3A⋅3CS¯⋅H32.Gypsum formation as a result of cation-exchange reactions is also capable of causing expansion. However, it has been observed that deterioration of hardened Portland cement paste by gypsum formation goes through a process that first leads to a reduction of pH of the system and loss in stiffness and strength, followed by expansion and cracking, and finally, the transformation of the concrete into a mushy or noncohesive mass, which are shown in the following equations:(3)MgSO4+CaOH2+2H2O⟶CaSO4⋅2H2O+MgOH2,(4)3MgSO4+3CaO.2SiO2.3H2O+8H2O⟶3CaSO4.2H2O+3MgOH2+2SiO2.H2O.In the attack of magnesium sulfate and the conversion ofCaOH2 to gypsum, it is accompanied by the simultaneous formation of MgOH2 (brucite), which is insoluble and reduces the alkalinity of the system. In the absence of hydroxyl ions in the pore solution, the stability of C-S-H in the system is reduced and attacked by the sulfate solution (4).(1) Changes in Compressive Strength. In Figure 8(a), the changes in the compressive strength of the specimens exposed to magnesium sulfate are observed. Up to the age of 90 days, the compressive strength of all specimens exposed to magnesium sulfate increased compared to the initial compressive strength (i.e., 28 days cured in water).Figure 8
(a) Compressive strength changes of concrete after exposure to magnesium sulfate. (b) Weight changes of concrete after exposure to magnesium sulfate.
(a)(b)Binary specimens of WGP15 and GGBFS15 show the highest compressive strength increase at 90 days compared to other specimens. The improvement of magnesium sulfate resistance in the binary specimen of WGP15 may be due to the sacrificial behavior of WGP. WGP produces compounds that are not as harmful as those formed by the decomposition of cement hydration products exposed to the magnesium sulfate environment [62, 98]. WGP leaches alkali ions of Na+ and Si+ into the solution, which leads to the neutralization of magnesium sulfate. In addition, the pozzolanic reaction of WGP increases the alkaline concentration of the pore solution [89, 99], which prevents the decomposition of hydration products by magnesium sulfate. Mostofinejad et al. [63] reported that the incorporation of recycled concrete aggregates with WGP leads to significant improvements in the development of concrete compressive strength in the magnesium sulfate environment with saturated concentration (i.e., 14.7%) [63]. Liu et al. [100] also reported that replacing fine aggregate with a liquid crystal display (LCD) improves the performance of concrete against sulfate attacks. They pointed out that glass powder has low water absorption, which leads to an increase in the effective water-cement ratio of the concrete mixture, which causes increased porosity and provides sufficient space for salt to crystallize before damaging the cement matrix [100].In the binary specimen of GGBFS15, the resistance to magnesium sulfate was increased due to the decrease in the content of C3A, CaOH2, and the penetrability of concrete. GGBFS consumes less CaOH2 than other SCMs and usually reduces the penetrability of concrete due to improved pore structure [101, 102]. In addition, the behavior of GGBFS depends on the replacement level and its chemical composition; in concrete mixtures, good behavior is usually observed when the replacement level of GGBFS is high (i.e., 70%). This behavior is amplified when the GGBFS has a lower content of (Al2O3). Hooton and Emery [103], Gollop and Taylor [104, 105], Locher [106], Higgins [107], Ogawa et al. [108], Whittaker and Black [109], and Mostofinejad et al. [67] reported that GGBFS could contribute to the resistance of concrete against sulfate attack [67, 103–109].According to the changes in compressive strength at the age of 90 days, it was observed that SF5, SF5+WGP10, and SF5+GGBFS10 specimens have less resistance to magnesium sulfate attack than the Ctrl specimen. Partial replacement of cement by SF reduces the availability ofCaOH2 due to the pozzolanic reaction and allows magnesium sulfate to more easily attack C-S-H, leading to decalcification, formation of M-S-H, and destruction of the cement bond [110–113]. Similar studies by Baghabra Al-Amoudi [111], Ganjian and Pouya [112], Lee et al. [113], Mostofinejad et al. [67], and Ortega et al. [114] confirm the significant decrease in compressive strength of concretes containing SF against magnesium sulfate attack [67, 111–114].(2) Changes in Weight. The results of weight changes of concrete specimens exposed to magnesium sulfate solution are presented in Figure 8(b). The presented results show that all the specimens gained weight for up to 90 days. The increase in weight in concrete specimens is due to the formation of gypsum and ettringite, which is the result of the reaction of magnesium sulfate with CaOH2, C3A, and C4AF [77, 114].In general, the specimens containing SF had the lowest weight gain. As we know, SF pozzolanic reactions lead to a decrease inCaOH2 content, and this issue decreases the potential of gypsum and ettringite formation. Based on the results of other mechanical and durability tests in this study, the positive effect of particle packing or space-filling of SF was observed [79, 80, 82]. Therefore, the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 containing SF have lower penetrability than the binary specimens of WGP15 and GGBFS15 without SF. By reducing the penetrability, the diffusion of sulfate ions in concrete is decreased [77, 111, 114]. However, reducing the pore volume does not always guarantee acceptable performance against magnesium sulfate attacks because the arrangement of these pores is also important [64]. The destructive effect of magnesium sulfate in concrete containing SF is related to the difference in the reaction mechanisms and the attack of magnesium sulfate, which causes a further reduction of alkalinity by the pozzolanic reactions of SF, which eventually leads to the decomposition of C-S-H and excessive reduction of strength [77]. Banar et al. [77], Hendi et al. [64], Hekal et al. [115], Baghabra Al-Amoudi [111], and Torii and Kawamura [116] observed a similar effect of SF on pore refinement, but they believed that there is no clear justification for the positive effect of SF on resistance to magnesium sulfate, and using the weight change factor to predict resistance to magnesium sulfate attack is not an accurate method [64, 77, 111, 115, 116].The lower weight gain of the binary specimen of WGP15 compared to the Ctrl specimen is related to the reduction of C3A due to the decrease in the extent of cement and the consumption of CaOH2 due to the pozzolanic reaction, which ultimately reduces the formation of gypsum and ettringite. In addition, the reaction of magnesium sulfate with WGP produces compounds with a lower density than the reactants, which might be why the binary specimen of WGP15 does not show the same weight gain shown by the other specimens [62]. Mostofinejad et al. [63] evaluated the durability of concrete containing recycled concrete aggregates with WGP in the magnesium sulfate environment. They reported that adding 30% WGP to concrete containing recycled concrete aggregates improved the performance of concrete in the magnesium sulfate environment [63]. Hendi et al. [64], in another study, investigated the performance of concrete containing 6%, 13%, and 20% WGP in the magnesium sulfate environment with a fully saturated concentration. They reported that after 150 days of exposure to magnesium sulfate, all specimens containing WGP showed less weight gain than the Ctrl specimen [64].The binary specimen of GGBFS15 showed a lower weight gain than the Ctrl specimen after 90 days of exposure to magnesium sulfate. Nevertheless, compared to other specimens, it showed more weight gain. SCMs are usually deficient in calcium compared to cement [102]. Hydrated SCMs further decrease the CaOH2 content. GGBFS may also consume CaOH2 but to a lesser value [109]. Due to the lower consumption of CaOH2 by GGBFS, the potential of gypsum and ettringite formation in concrete containing GGBFS is higher than that of concrete containing WGP and SF. Therefore, the higher weight gain of the binary specimen of GGBFS15 than other specimens containing WGP and SF can be predicted. In an investigation, Durgun and Sevinç [117] determined the effectiveness of pumice, waste glass powder, GGBFS, and colemanite waste against sodium and magnesium sulfate attacks; they reported that the most effective mineral additive to reduce the weight loss of concrete specimens is GGBFS, which has an optimal usage rate of 10%, and the worst case is when GGBFS is not used in the mix design. Finally, the results showed that the ideal mixing ratio to minimize weight loss due to sulfate attack is 5% pumice, 5% waste glass powder, 10% GGBFS, and 1% colemanite waste [117].


#### 3.2.2. Sulfuric Acid Attack

Effluents from furnaces that use high-sulfur fuels and effluents from chemical industries may contain sulfuric acid. The loss of organic matter in marshes, shallow lakes, mining pits, and sewage pipes often results in the formation of H2S, which can be converted to sulfuric acid by bacterial action. Therefore, it is necessary to investigate the performance of concrete in acidic environments.In a well-hydrated concrete, the cement paste phase, comprised of moderately insoluble hydrates of calcium (C-S-H,CaOH2, and the AFt and AFm phases), exists in a state of stable balance with a high-pH pore solution. Depending on the Na+, K+, and OH− ions concentration, the pH value ranges from 12.5 to 13.5. Less than 12.5 pH theoretically leads to instability of the cementitious products of hydration; this means that most natural waters and urban wastewater are damaging to concrete. The rate of chemical attack depends on the pH and penetrability of concrete, so in concrete with low penetrability and pH above 6, the rate of chemical attack is too slow [19, 118].On the other hand, by reducing the concentration of Ca2+ ions in the pore solution, the chemical balance of the cement paste is disturbed, which first damages the CaOH2 and then the C-S-H gel [119]. The reaction of sulfuric acid with components of hydrated cement paste is shown in (5) and (6). In the next step, ettringite could be formed through the chemical reaction between gypsum and some hydration products such as monosulfate, calcium aluminate hydrate, and unhydrated tricalcium aluminate (C3A). These reactions are sulfate attacks (equations (7)–(9)). It should be noted that the reactions of ettringite production increase the volume, which creates cracks in concrete, and the damage caused by the sulfuric acid attack is intensified [19, 119].(5)H2SO4+CaOH2⟶CaSO4+2H2O,(6)H2SO4+CaO.SiO2.2H2O⟶CaSO4+SiOH4+H2O,(7)3CaO.Al2O3.CaSO4.12H2O+2CaSO4.2H2O+16H2O⟶3CaO.Al2O3.3CaSO4.32H2O,(8)3CaO.Al2O3.CaOH2.12H2O+3CaSO4.2H2O+13H2O⟶3CaO.Al2O3.3CaSO4.32H2O+CaOH2,(9)3CaO.Al2O3+3CaSO4+32H2O⟶3CaO.Al2O3.3CaSO4.32H2O.(1) Changes in Compressive Strength. Figure 9(a) shows the changes in the compressive strength of the specimens after exposure to sulfuric acid. The results show that all specimens decreased compressive strength up to 90 days. Changes in compressive strength depend on the formation of chemical products in exposure to the sulfuric acid solution. The decrease in compressive strength in all specimens containing SF, WGP, and GGBFS was higher than in the Ctrl specimen. The Ctrl specimen has more potential in gypsum and ettringite formation than other specimens due to the higher content of CaOH2 and C3A. Filling concrete pores with gypsum and ettringite improves compressive strength in the short term [98].Figure 9
(a) Compressive strength changes in concrete after exposure to sulfuric acid. (b) Changes in concrete weight after exposure to sulfuric acid.
(a)(b)The binary specimen of WGP15, after 90 days of exposure to the sulfuric acid solution, showed the best performance compared to other binary and ternary specimens. The lower reduction of the compressive strength of the binary specimen of WGP15 can be related to the high porosity of the cement paste, which has more pores for the formation of gypsum and ettringite because these products act as fillers at early ages and improve the compressive strength of the concrete specimens.In addition, the lower reduction of the compressive strength of the binary specimen of WGP15 can be due to the sacrificial nature of the WGP, which produces chemical compounds that are nonexpandable in nature and have less damage than ettringite. Due to the sacrificial nature of WGP, the rate of ettringite formation is reduced. By acting as a sacrificial material, WGP prevented the disintegration of C-S-H and C-A-S-H compounds, thus reducing the intensity of loss in compressive strength [62, 98]. Bisht et al. [98] also reported that when 24% of waste glass (150–600 μm) was replaced by river sand, the resistance of concrete against sulfuric acid attack is improved. Even after 90 days of exposure to sulfuric acid, the compressive strength was 54% higher than plain concrete [98]. Jain et al. [120] also stated that adding glass powder up to 15% to plain concrete improved the resistance against sulfuric acid attack. Concrete containing 15% glass powder showed the best performance after 84 days of exposure to a 3% sulfuric acid solution [120].The binary specimen of GGBFS15 showed acceptable performance at early ages. GGBFS creates the least pore ratio due to improving the pore structure of concrete [101, 102]. Therefore, any slight formation of gypsum and ettringite contributes to the easy filling of the pores, which leads to an increase in compressive strength; nevertheless, after 28 days of exposure to sulfuric acid, loss of compressive strength was observed; because the formed products need high volume, it leads to simultaneous expansion and cracking, and it facilitates the ingress of sulfuric acid; as a result, the compressive strength of concrete loses over time [62, 121]. The study of Sturm et al. [121] evaluated the resistance of one-part geopolymer mortars against sulfuric acid. They reported that adding 25 wt% of GGBFS reduced the resistance against sulfuric acid by forming expanded calcium sulfate phases [121].As mentioned earlier, SF significantly improved the mechanical properties and reduced the porosity of concrete. However, specimens containing SF showed a weaker performance than other specimens after 90 days of exposure to the sulfuric acid solution. Thus, it can be argued that convertingCaOH2 to C-S-H and refining the porosity of concrete are not effective in preventing the loss of compressive strength against sulfuric acid attack [122]. Due to the reduction of concrete porosity, any slight formation of gypsum and ettringite leads to internal stresses and premature cracking. In the next step, sulfuric acid attacks C-S-H easier and faster due to the reduction of the availability of CaOH2, which causes the instability of the cement paste and loss of strength [123].The results of the current study showed that higher compressive strength does not necessarily guarantee the durability of concrete against the attack of magnesium sulfate and sulfuric acid. The results of this study are in accordance with the findings of Senhadji et al. [122]. They found that cement mortar and mortars containing natural pozzolan and limestone showed better performance against sulfuric acid attack compared to mortar containing SF [122]. Torii and Kawamura [116] also reported that the partial replacement of Ordinary Portland Cement by SF and fly ash could not effectively prevent the destruction of concrete by sulfuric acid involving the scaling and softening [116].(2). Changes in Weight. Figure 9(b) shows the changes in the weight of the specimens after exposure to sulfuric acid. Based on the results, all specimens lost weight after 90 days. The ternary specimen of SF5 + GGBFS10 showed the lowest weight loss compared to other specimens, which may be due to the pozzolanic reaction of SF, which reduced the CaOH2 content and formed more C-S-H. But the binary specimen of GGBFS15 has a higher potential for gypsum and ettringite formation due to the extent of consumption of less CaOH2, so expansion and cracking lead to rupture and weight loss.The binary specimen of WGP15 also showed a lower weight loss than the Ctrl specimen, which may be related to the higher porosity of the cement paste and the sacrificial nature of the WGP. As mentioned earlier, WGP produces compounds that are nonexpansive in nature and are not as damaging as ettringite, consequently reducing internal stresses that lead to cracking and preventing rupture and weight loss [62, 98].The negative effect of using SF on the weight changes of SF5 + WGP10 and SF5 specimens was observed. This behavior may be attributed to the lower porosity of the cement paste because the pore refinement by SF reduces the concrete pore volume. Undoubtedly, the pore volume is one of the most effective parameters in improving the resistance of concrete against sulfuric acid attacks. Because the higher pore volume provides more accessible spaces for the formation of gypsum and ettringite, and as a result, internal stresses and intense damage are prevented [123].According to the results of the current study, the results reported by Hendi et al. [123], Bassuoni and Nehdi [124], Chang et al. [125], and De Belie et al. [126] found that there is no direct correlation between compressive strength changes and weight changes after exposure to the sulfuric acid solution [123–126].


#### 3.2.3. Surface Resistivity

The resistivity of concrete is an essential parameter in the corrosion of reinforced concrete structures. High-resistivity concrete has little possibility of developing reinforcement corrosion. In the field, the electrical resistivity is determined by measuring the potential differences at the concrete surface caused by injecting a small current at the surface [19]. Electrical resistivity indicates the mobility of ions throughout the concrete matrix [127]. The higher the penetrability of the concrete, the easier and faster the ions penetrate into the concrete. If the concrete has higher electrical resistance and lower penetrability, it has a better resistance against destructive ions such as chloride [128, 129].According to Figure10, the surface resistivity for all specimens containing SF increased higher than 200% in the early ages and up to 300% in the final ages compared to the Ctrl specimen; the positive effect of using SF in increasing the surface resistivity was confirmed by the authors of [82, 83, 127–130]. According to Table 3, presented by AASHTO-T 358, the chloride ion penetration for specimens of SF5, SF5 + WGP10, and SF5 + GGBFS10 was in the very low range.Figure 10
Surface resistivity results of concrete mixtures.Table 3
Chloride ion penetration based on AASHTO-T 358.
Chloride ion penetrationSurface resistivity (KΩ·cm)High<12Moderate12–21Low21–37Very low37–254Negligible>254At the age of 120 days, the surface resistivity of the binary specimens of WGP15 and GGBFS15 was increased by 94.7% and 73.6% compared to the Ctrl specimen, respectively. The chloride ion penetration was in the low range, while in the Ctrl specimen, the chloride ion penetration remained in the moderate range for 120 days. This matter shows the positive effect of SCMs in improving the density of cement paste structure and reducing the volume of capillary pores due to the pozzolanic reaction.Mehta and Monteiro [19] stated that the risk of reinforcement corrosion increases with the reduction of the electrical resistance of concrete. Nevertheless, as long as the electrical resistance of concrete is above 50 to 70 KΩ·cm, significant corrosion is not observed [19].The risk of reinforcement corrosion was significantly reduced in the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 due to the electrical resistance above 70 KΩ·cm.


#### 3.2.4. Rapid Chloride Penetrability Test (RCPT)

Fluctuations in temperature and humidity, short-term and long-term internal chemical reactions, and pores geometry in concrete all contribute to chloride ion penetration rates [131]. The RCPT result is interpreted as an indicator of electrical conductivity [86]. In this test, the accumulated charges passing through the specimens are measured, which determines the chloride ion penetrability of concrete. Electrical conductivity is sensitive to the flow of ions released in the cementitious products of hydration through the water in the pores. The lower the electrical conductivity of the concrete, the lower the current flowing between the anodic and the cathodic areas, and thus, the lower rate of corrosion. Hence, the risk of reinforcement corrosion can be evaluated by measuring the electrical conductivity of concrete [132, 133].RCPT results are shown in Figure11. The overall results are similar to surface resistivity. As can be seen, due to the completion of hydration, the amount of charges passing decreases at the final age. According to the classification presented in the ASTM C1202 standard (Table 4), it can be stated that the chloride ion penetrability in the specimens of SF5, SF5+WGP10, and SF5+GGBFS10 after 56 days is very low. The very fine particles of SF, the high rate of pozzolanic reaction, and the filling effect can reduce the pore volume and increase the homogeneity of concrete, thus hindering the mobility and interaction of ions [127, 128, 134].Figure 11
Rapid chloride penetrability test (RCPT) results of concrete mixtures.Table 4
Chloride ion penetrability based on ASTM C1202.
Chloride ion penetrabilityCharge passed (coulombs)High>4000Moderate2000–4000Low1000–2000Very low100–1000Negligible<100The high amount of charges passing at the age of 28 days in the binary specimen of GGBFS15 is probably due to the low rate of pozzolanic reaction. Because after the age of 56 days, due to the progress of the pozzolanic reaction that leads to the densification of the pore structure, the amount of charges passing shows a significant decrease [135]. Finally, at the age of 120 days, the chloride ion penetrability was in the low range.The highest amount of charges passing is at the age of 28 days in the binary specimen of WGP15. It is well known that RCPT is basically an electrical conductivity or resistivity test, and the conductivity of the pore solution also affects the results of this test. The presence of a pore solution with higher conductivity leads to a higher apparent amount of charges passing even if the microstructure is the same. Based on the studies of Du and Tan [86], Zheng [89], Jain and Neithalath [136], it was observed that due to its high Na2O and SiO2, glass powder releases a large amount of alkaline content in the pore solution and thus increases the conductivity of the pore solution [86, 89, 136]. But at the ages of 56, 90, and 120 days, a significant decrease in charges passing was observed; because of the cement hydration reaction, a pore solution rich in Ca2+, SiO42−, and OH− ions is produced, and the amorphous silica present in (Si-O-Si bond) glass powder can dissolve under this high pH solution and form a layer rich in Si+ on the surface of the glass powder. This layer reacts with Ca2+ ions in the solution and turns into C-S-H; this formed C-S-H has a lower Ca/Si ratio and thus absorbs more Na+ released from the glass powder [86, 137]. Consequently, the improved microstructure and the decrease in the concentration of ions in the pore solution lead to a decrease in electrical conductivity [86]. Eventually, at the age of 120 days, the chloride ion penetrability in the binary specimen of WGP15 was in the low range.The investigation of Yeau and Kim [135], Kayali et al. [134], Pilvar et al. [127], Du and Tan [86], and Delnavaz et al. [128] confirmed that the addition of FS, WGP, and GGBFS to concrete is effective in reducing chloride ion penetrability and increases the durability of concrete in chloride corrosive environments [86, 127, 128, 134, 135].


#### 3.2.5. Water Absorption

The water absorption test provides information on capillarity suction and is a standard indicator for measuring the resistance of concrete when exposed to aggressive environments. The durability of concrete is significantly affected by water movement in concrete [138]. Water absorption is generally affected by pore structure, porous paste, and ITZ, and these factors are essential at early ages [19].Figure12 shows the results of water absorption after 30 minutes (initial absorption) and 24 hours (final absorption) at the ages of 28, 56, 90, and 120 days. According to the results, it can be seen that the initial and final water absorption at the age of 120 days is lower for SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens than other specimens. The decrease in water absorption can be attributed to the pozzolanic reaction of SF and converted interconnected pores to unconnected pores. As a result, the porosity of cement paste decreases. The results obtained by Yusuf et al. [10], Gupta et al. [139], Khaloo et al. [140], Madani et al. [41], and Sabet et al. [141] confirmed that the use of SF in concrete reduces water absorption [10, 41, 139–141].Figure 12
Water absorption test results of concrete mixtures.At the ages of 56, 90, and 120 days, the binary specimens of WGP15 and GGBFS15 showed good performance compared to the Ctrl specimen; due to the pozzolanic reaction and particle packing, the porosity decreased and led to low water absorption.CEB [142] recommends water absorption after 30 minutes (initial absorption). According to CEB recommended limits, water absorption of all concrete specimens was less than 3%, indicating good concrete durability [142]. Shetty and Jain [143] believe that high-quality concrete has a final absorption of less than 5% [143]. Based on the results, all mixtures can be considered high-quality concrete in water absorption.


#### 3.2.6. Depth of Penetration of Water under Pressure

Another method of measuring concrete penetrability is the depth of penetration of water. Water penetration into concrete can cause physical and chemical damages. Water as a solvent can dissolve many cement components [144]. Many ions cause damage to concrete by water penetration. Therefore, water penetration in concrete can be used as an indicator of concrete durability [82, 145]. The penetrability of concrete is influenced by the total volume, shape, and connectivity of the pores [146].In this study, the average depth of penetration measure was reported. Motahari Karein et al. [129] and Banar et al. [77] reported that using the average depth of penetration of water is an acceptable measurement and illustrates more accurate results. During the test, it was observed that the depth of penetration of water could be artificially increased locally due to the presence of aggregates near the surface of the specimen under water pressure [77, 129]. According to Mindess et al. [147], the depth of penetration of water indicates the total volume of open pores larger than 100 nm. Open pores are pathways for water penetration and ions in the concrete system [147].The results of the depth of penetration of water test at the ages of 28, 56, 90, and 120 days are shown in Figure13. The depth of penetration of water for all specimens containing SF decreased compared to the Ctrl specimen. This decrease in penetration depth at the age of 120 days in the SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens is 29%, 26%, and 27.5%, respectively. The reduction of depth of penetration of water at early ages in ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 is due to the presence of SF. Due to its very fine particles and the high rate of pozzolanic reaction at early ages, SF improves the microstructure and densification of the ITZ, which partially blocks the paths of water penetration [139]. Based on the results obtained from Ahmad et al. [148], Ince et al. [149], and Gupta et al. [139] researches, the positive effect of using SF in reducing the depth of penetration of water was confirmed [139, 148, 149].Figure 13
Depth of penetration of water under pressure results of concrete mixtures.The binary specimens of WGP15 and GGBFS15 exhibited weak performance at early ages, but at the age of 120 days, 43% and 44.2% reduction in the depth of penetration of water was observed compared to the Ctrl specimen, respectively, which shows the best performance among all specimens. According to the results of other researchers, the use of WGP and GGBFS has positive effects on the mechanical properties and durability of concrete. Because of the pozzolanic reaction, the porosity decreases, and a smaller pore structure is created in the concrete nucleus, which leads to a decrease in the depth of penetration of water [86, 150, 151].


#### 3.2.7. SEM Analysis

Figure14 shows the images of SEM. To understand the cause, rate, and mechanism of destruction, or how to improve some properties of concrete, complete knowledge of the microstructure of hardened concrete is necessary. The mechanical properties of concrete often depend on its microstructure. Cement paste contains products of cement hydration, which is 50–60% of the solid volume of cement paste containing C-S-H, 20–25% including CaOH2 crystals, and the rest including AFt (i.e., ettringite), AFm (i.e., monosulfate), unhydrated cement, and the porosity of cement paste [152]. As shown in Figure 14, the presence of SF has a positive effect on the microstructure of concrete and has helped to improve the packing density and structure of the cement paste [82].Figure 14
SEM images of the concrete specimens after 90 days of curing. (a) Ctrl. (b) SF5. (c) WGP15. (d) GGBFS15. (e) SF5 + WGP10. (f) SF5 + GGBFS10.
(a)(b)(c)(d)(e)(f)


## 3.1. Mechanical Tests

### 3.1.1. Compressive Strength

The compressive strength test usually provides an overview of concrete quality because the compressive strength of concrete is directly related to the structure of the hydrated cement paste [38]. Figure 5 shows the results of compressive strength. According to that, the highest compressive strength is related to the binary specimen of SF5 because compared to the Ctrl specimen, at the ages of 7, 28, 56, and 90 days, 17%, 27.6%, 28.5%, and 25.8% increased compressive strength, respectively. SF particles with an average diameter of 150 nm are significantly smaller than cement particles with an average diameter of 15 μm, so they can fill the pores and improve the characteristics of concrete; this is known as particle packing or space-filling. In addition, the pozzolanic reaction of SF with CaOH2 in cement paste causes the formation of C-S-H, which fills the interfacial transition zone (ITZ) and improves compressive strength [41, 43, 77–85].Figure 5
Compressive strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10, at the age of 7 days, a 7.3% reduction in compressive strength compared to the Ctrl specimen can be seen; the reason is the slow rate of pozzolanic reactions of WGP and GGBFS at early ages [86–88]. But, at the ages of 28, 56, and 90 days, the ternary specimen of SF5+WGP10 obtained 17%, 12.5%, and 10.3% increases in compressive strength, respectively, compared to the Ctrl specimen because the dissolution of glass particles leads to the creation of free alkali Na+ and Si+ ions [89, 90]. Na+ and Si+ released from glass powder and hydroxyl ions from cement hydration maintain the pH of the pore solution between 13 and 14. The high pH of the pore solution increases the solubility of amorphous silicates [91]. Therefore, the rate of the SF reaction increases, and as a result, denser C-S-H is formed.Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 10.6%, 10.7%, and 8.6% increase in compressive strength compared to the Ctrl specimen, respectively. The increase in compressive strength is due to very fine and reactive particles of SF. The C-S-H gel is produced during the reaction with the SF adsorbed on the GGBFS surface; as a result, the unreacted GGBFS particles connect with the gel around it tightly. The unreacted GGBFS particles are also tightly connected to the surroundingCaOH2 crystals due to the reaction between the CaOH2 crystal and the SF adsorbed on the GGBFS surface [87]. Generally, the presence of SF, along with WGP and GGBFS, provides a sufficient amount of silica, sodium, and calcium to react with CaOH2 and accelerate the rate of C-S-H formation [92]. In truth, the increases in compressive strength in SF5 + WGP10 and SF5 + GGBFS10 specimens indicate the synergistic effect of ternary mixtures [93].In the binary specimen of WGP15, a decrease in compressive strength was observed at all ages compared to the Ctrl specimen. The reduction in compressive strength at the ages of 7, 28, 56, and 90 days was 12.2%, 8.5%, 7.1%, and 8.6%, respectively. Ibrahim [38] and Aliabdo et al. [94] confirmed these results; they stated that at WGP15%, the compressive strength decreased [38, 94]. In truth, the pozzolanic reaction of glass powder is directly related to its particle size. Finer particle size produces a higher pozzolanic reaction. Therefore, the strength increase rate is affected by the particle size distribution of the glass powder [95].The compressive strength results in the binary specimen of GGBFS15 are similar to the binary specimen of WGP15. The hydration activity of GGBFS is significantly lower than that of cement at early ages. Moreover, GGBFS has a retarding effect on cement hydration at early ages [87]. Therefore, the initial compressive strength of concrete containing GGBFS is significantly lower than that of plain concrete [96].


### 3.1.2. Splitting Tensile Strength

Figure6 shows the tensile strength test results of cylindrical concrete at the ages of 7, 28, 56, and 90 days. Similar to the compressive strength results, the highest tensile strength is related to the binary specimen of SF5 because, at the ages of 7, 28, 56, and 90 days, 19.4%, 13%, 27.6%, and 16.9% of strength increased, compared to the Ctrl specimen, respectively. SF particles are 100 times smaller than cement particles, so they can be well placed among the cement particles and cause more densification in the cement paste matrix. In addition, the pozzolanic reaction of SF with CaOH2 induces the formation of C-S-H. Hence, the compact microstructure of the cement paste, especially in the ITZ, leads to an increase in tensile strength [78, 97].Figure 6
Tensile strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 at the age of 7 days, 30.5% and 33.3% reduction in tensile strength was observed compared to the Ctrl specimen, respectively. The reason for this is the slow reactivity of WGP and GGBFS at early ages to cement [86–88]. Nevertheless, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + WGP10 obtained 8.7%, 23.4%, and 11.3% increase in tensile strength compared to the Ctrl specimen, respectively. The increase is due to the pozzolanic reaction of SF and WGP with CaOH2, which leads to the formation of C-S-H [88]. Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 2.1%, 14.9%, and 3.7% increase in tensile strength compared to the Ctrl specimen, respectively. The increase in tensile strength in the ternary specimen of SF5 + GGBFS10 is due to the presence of SF, which leads to a significant reduction of CaOH2 and improvement of the ITZ of concrete. In addition, GGBFS particles are attached to the surrounding CaOH2 crystals due to the pozzolanic reaction of SF [87].In the binary specimens of WGP15 and GGBFS15, a slight decrease in tensile strength was observed at the final ages compared to the Ctrl specimen. This issue can be attributed to the decrease in the amount of cement in these specimens and the low reactivity rate of these two materials at these ages [38, 87, 88].


### 3.1.3. Flexural Strength

According to Figure7, the flexural strength of the binary specimen of SF5 at the ages of 7, 28, 56, and 90 days increased by 12.3%, 11.7%, 10.3%, and 10.2%, compared to the Ctrl specimen, respectively. An increase in flexural strength was observed for ternary specimens of all ages. The increase in strength at the ages of 7, 28, 56, and 90 days in the SF5 + WGP10 specimen was 1.2%, 11.7%, 18.3%, and 19.3%, respectively, and in the SF5+GGBFS10 specimen, it was 1.2%, 9.4%, 9.1%, and 9%, compared to the Ctrl specimen, respectively.Figure 7
Flexural strength results of concrete mixtures.The reaction of SiO2 in SF with CaOH2 causes the formation of C-S-H gel. Increasing C-S-H gel and reducing capillary pores in cement paste is one of the main factors in increasing the strength and reducing the porosity of concrete [78, 85, 87, 88]. Similar to compressive strength and tensile strength results, for binary specimens of WGP15 and GGBFS15, a slight decrease in flexural strength was observed at the final ages.


## 3.1.1. Compressive Strength

The compressive strength test usually provides an overview of concrete quality because the compressive strength of concrete is directly related to the structure of the hydrated cement paste [38]. Figure 5 shows the results of compressive strength. According to that, the highest compressive strength is related to the binary specimen of SF5 because compared to the Ctrl specimen, at the ages of 7, 28, 56, and 90 days, 17%, 27.6%, 28.5%, and 25.8% increased compressive strength, respectively. SF particles with an average diameter of 150 nm are significantly smaller than cement particles with an average diameter of 15 μm, so they can fill the pores and improve the characteristics of concrete; this is known as particle packing or space-filling. In addition, the pozzolanic reaction of SF with CaOH2 in cement paste causes the formation of C-S-H, which fills the interfacial transition zone (ITZ) and improves compressive strength [41, 43, 77–85].Figure 5
Compressive strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10, at the age of 7 days, a 7.3% reduction in compressive strength compared to the Ctrl specimen can be seen; the reason is the slow rate of pozzolanic reactions of WGP and GGBFS at early ages [86–88]. But, at the ages of 28, 56, and 90 days, the ternary specimen of SF5+WGP10 obtained 17%, 12.5%, and 10.3% increases in compressive strength, respectively, compared to the Ctrl specimen because the dissolution of glass particles leads to the creation of free alkali Na+ and Si+ ions [89, 90]. Na+ and Si+ released from glass powder and hydroxyl ions from cement hydration maintain the pH of the pore solution between 13 and 14. The high pH of the pore solution increases the solubility of amorphous silicates [91]. Therefore, the rate of the SF reaction increases, and as a result, denser C-S-H is formed.Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 10.6%, 10.7%, and 8.6% increase in compressive strength compared to the Ctrl specimen, respectively. The increase in compressive strength is due to very fine and reactive particles of SF. The C-S-H gel is produced during the reaction with the SF adsorbed on the GGBFS surface; as a result, the unreacted GGBFS particles connect with the gel around it tightly. The unreacted GGBFS particles are also tightly connected to the surroundingCaOH2 crystals due to the reaction between the CaOH2 crystal and the SF adsorbed on the GGBFS surface [87]. Generally, the presence of SF, along with WGP and GGBFS, provides a sufficient amount of silica, sodium, and calcium to react with CaOH2 and accelerate the rate of C-S-H formation [92]. In truth, the increases in compressive strength in SF5 + WGP10 and SF5 + GGBFS10 specimens indicate the synergistic effect of ternary mixtures [93].In the binary specimen of WGP15, a decrease in compressive strength was observed at all ages compared to the Ctrl specimen. The reduction in compressive strength at the ages of 7, 28, 56, and 90 days was 12.2%, 8.5%, 7.1%, and 8.6%, respectively. Ibrahim [38] and Aliabdo et al. [94] confirmed these results; they stated that at WGP15%, the compressive strength decreased [38, 94]. In truth, the pozzolanic reaction of glass powder is directly related to its particle size. Finer particle size produces a higher pozzolanic reaction. Therefore, the strength increase rate is affected by the particle size distribution of the glass powder [95].The compressive strength results in the binary specimen of GGBFS15 are similar to the binary specimen of WGP15. The hydration activity of GGBFS is significantly lower than that of cement at early ages. Moreover, GGBFS has a retarding effect on cement hydration at early ages [87]. Therefore, the initial compressive strength of concrete containing GGBFS is significantly lower than that of plain concrete [96].


## 3.1.2. Splitting Tensile Strength

Figure6 shows the tensile strength test results of cylindrical concrete at the ages of 7, 28, 56, and 90 days. Similar to the compressive strength results, the highest tensile strength is related to the binary specimen of SF5 because, at the ages of 7, 28, 56, and 90 days, 19.4%, 13%, 27.6%, and 16.9% of strength increased, compared to the Ctrl specimen, respectively. SF particles are 100 times smaller than cement particles, so they can be well placed among the cement particles and cause more densification in the cement paste matrix. In addition, the pozzolanic reaction of SF with CaOH2 induces the formation of C-S-H. Hence, the compact microstructure of the cement paste, especially in the ITZ, leads to an increase in tensile strength [78, 97].Figure 6
Tensile strength results of concrete mixtures.In the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 at the age of 7 days, 30.5% and 33.3% reduction in tensile strength was observed compared to the Ctrl specimen, respectively. The reason for this is the slow reactivity of WGP and GGBFS at early ages to cement [86–88]. Nevertheless, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + WGP10 obtained 8.7%, 23.4%, and 11.3% increase in tensile strength compared to the Ctrl specimen, respectively. The increase is due to the pozzolanic reaction of SF and WGP with CaOH2, which leads to the formation of C-S-H [88]. Also, at the ages of 28, 56, and 90 days, the ternary specimen of SF5 + GGBFS10 obtained 2.1%, 14.9%, and 3.7% increase in tensile strength compared to the Ctrl specimen, respectively. The increase in tensile strength in the ternary specimen of SF5 + GGBFS10 is due to the presence of SF, which leads to a significant reduction of CaOH2 and improvement of the ITZ of concrete. In addition, GGBFS particles are attached to the surrounding CaOH2 crystals due to the pozzolanic reaction of SF [87].In the binary specimens of WGP15 and GGBFS15, a slight decrease in tensile strength was observed at the final ages compared to the Ctrl specimen. This issue can be attributed to the decrease in the amount of cement in these specimens and the low reactivity rate of these two materials at these ages [38, 87, 88].


## 3.1.3. Flexural Strength

According to Figure7, the flexural strength of the binary specimen of SF5 at the ages of 7, 28, 56, and 90 days increased by 12.3%, 11.7%, 10.3%, and 10.2%, compared to the Ctrl specimen, respectively. An increase in flexural strength was observed for ternary specimens of all ages. The increase in strength at the ages of 7, 28, 56, and 90 days in the SF5 + WGP10 specimen was 1.2%, 11.7%, 18.3%, and 19.3%, respectively, and in the SF5+GGBFS10 specimen, it was 1.2%, 9.4%, 9.1%, and 9%, compared to the Ctrl specimen, respectively.Figure 7
Flexural strength results of concrete mixtures.The reaction of SiO2 in SF with CaOH2 causes the formation of C-S-H gel. Increasing C-S-H gel and reducing capillary pores in cement paste is one of the main factors in increasing the strength and reducing the porosity of concrete [78, 85, 87, 88]. Similar to compressive strength and tensile strength results, for binary specimens of WGP15 and GGBFS15, a slight decrease in flexural strength was observed at the final ages.


## 3.2. Durability Tests

### 3.2.1. Magnesium Sulfate Attack

Magnesium sulfate is often found in groundwater, seawater, and some industrial effluents. Magnesium solutions easily react withCaOH2 in Portland cement paste to form soluble salts of calcium. Magnesium sulfate solution is the most aggressive because the sulfate ions can damage the alumina-bearing hydrates in the Portland cement paste. The characteristic feature of magnesium ion attack on cement paste is that the attack is extended to calcium silicate hydrate, which is the main component of cement. In prolonged contact with magnesium solutions, the C-S-H in Portland cement paste gradually loses its calcium ions, which are replaced by magnesium ions. The ultimate product of this substitution reaction is a magnesium silicate hydrate, the formation of which is associated with the loss of the cementitious characteristic. Due to the presence of CaOH2 in the hydrated Portland cement paste, when the cement paste comes into contact with sulfate ions, alumina-containing hydrates are converted to the high-sulfate form (i.e., ettringite), which are shown in the following equations [19]:(1)C3A⋅CH⋅H18+2CH+3S¯+11H⟶C3A⋅3CS¯⋅H32,(2)C3A⋅CS¯⋅H18+2CH+2S¯+12H⟶C3A⋅3CS¯⋅H32.Gypsum formation as a result of cation-exchange reactions is also capable of causing expansion. However, it has been observed that deterioration of hardened Portland cement paste by gypsum formation goes through a process that first leads to a reduction of pH of the system and loss in stiffness and strength, followed by expansion and cracking, and finally, the transformation of the concrete into a mushy or noncohesive mass, which are shown in the following equations:(3)MgSO4+CaOH2+2H2O⟶CaSO4⋅2H2O+MgOH2,(4)3MgSO4+3CaO.2SiO2.3H2O+8H2O⟶3CaSO4.2H2O+3MgOH2+2SiO2.H2O.In the attack of magnesium sulfate and the conversion ofCaOH2 to gypsum, it is accompanied by the simultaneous formation of MgOH2 (brucite), which is insoluble and reduces the alkalinity of the system. In the absence of hydroxyl ions in the pore solution, the stability of C-S-H in the system is reduced and attacked by the sulfate solution (4).(1) Changes in Compressive Strength. In Figure 8(a), the changes in the compressive strength of the specimens exposed to magnesium sulfate are observed. Up to the age of 90 days, the compressive strength of all specimens exposed to magnesium sulfate increased compared to the initial compressive strength (i.e., 28 days cured in water).Figure 8
(a) Compressive strength changes of concrete after exposure to magnesium sulfate. (b) Weight changes of concrete after exposure to magnesium sulfate.
(a)(b)Binary specimens of WGP15 and GGBFS15 show the highest compressive strength increase at 90 days compared to other specimens. The improvement of magnesium sulfate resistance in the binary specimen of WGP15 may be due to the sacrificial behavior of WGP. WGP produces compounds that are not as harmful as those formed by the decomposition of cement hydration products exposed to the magnesium sulfate environment [62, 98]. WGP leaches alkali ions of Na+ and Si+ into the solution, which leads to the neutralization of magnesium sulfate. In addition, the pozzolanic reaction of WGP increases the alkaline concentration of the pore solution [89, 99], which prevents the decomposition of hydration products by magnesium sulfate. Mostofinejad et al. [63] reported that the incorporation of recycled concrete aggregates with WGP leads to significant improvements in the development of concrete compressive strength in the magnesium sulfate environment with saturated concentration (i.e., 14.7%) [63]. Liu et al. [100] also reported that replacing fine aggregate with a liquid crystal display (LCD) improves the performance of concrete against sulfate attacks. They pointed out that glass powder has low water absorption, which leads to an increase in the effective water-cement ratio of the concrete mixture, which causes increased porosity and provides sufficient space for salt to crystallize before damaging the cement matrix [100].In the binary specimen of GGBFS15, the resistance to magnesium sulfate was increased due to the decrease in the content of C3A, CaOH2, and the penetrability of concrete. GGBFS consumes less CaOH2 than other SCMs and usually reduces the penetrability of concrete due to improved pore structure [101, 102]. In addition, the behavior of GGBFS depends on the replacement level and its chemical composition; in concrete mixtures, good behavior is usually observed when the replacement level of GGBFS is high (i.e., 70%). This behavior is amplified when the GGBFS has a lower content of (Al2O3). Hooton and Emery [103], Gollop and Taylor [104, 105], Locher [106], Higgins [107], Ogawa et al. [108], Whittaker and Black [109], and Mostofinejad et al. [67] reported that GGBFS could contribute to the resistance of concrete against sulfate attack [67, 103–109].According to the changes in compressive strength at the age of 90 days, it was observed that SF5, SF5+WGP10, and SF5+GGBFS10 specimens have less resistance to magnesium sulfate attack than the Ctrl specimen. Partial replacement of cement by SF reduces the availability ofCaOH2 due to the pozzolanic reaction and allows magnesium sulfate to more easily attack C-S-H, leading to decalcification, formation of M-S-H, and destruction of the cement bond [110–113]. Similar studies by Baghabra Al-Amoudi [111], Ganjian and Pouya [112], Lee et al. [113], Mostofinejad et al. [67], and Ortega et al. [114] confirm the significant decrease in compressive strength of concretes containing SF against magnesium sulfate attack [67, 111–114].(2) Changes in Weight. The results of weight changes of concrete specimens exposed to magnesium sulfate solution are presented in Figure 8(b). The presented results show that all the specimens gained weight for up to 90 days. The increase in weight in concrete specimens is due to the formation of gypsum and ettringite, which is the result of the reaction of magnesium sulfate with CaOH2, C3A, and C4AF [77, 114].In general, the specimens containing SF had the lowest weight gain. As we know, SF pozzolanic reactions lead to a decrease inCaOH2 content, and this issue decreases the potential of gypsum and ettringite formation. Based on the results of other mechanical and durability tests in this study, the positive effect of particle packing or space-filling of SF was observed [79, 80, 82]. Therefore, the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 containing SF have lower penetrability than the binary specimens of WGP15 and GGBFS15 without SF. By reducing the penetrability, the diffusion of sulfate ions in concrete is decreased [77, 111, 114]. However, reducing the pore volume does not always guarantee acceptable performance against magnesium sulfate attacks because the arrangement of these pores is also important [64]. The destructive effect of magnesium sulfate in concrete containing SF is related to the difference in the reaction mechanisms and the attack of magnesium sulfate, which causes a further reduction of alkalinity by the pozzolanic reactions of SF, which eventually leads to the decomposition of C-S-H and excessive reduction of strength [77]. Banar et al. [77], Hendi et al. [64], Hekal et al. [115], Baghabra Al-Amoudi [111], and Torii and Kawamura [116] observed a similar effect of SF on pore refinement, but they believed that there is no clear justification for the positive effect of SF on resistance to magnesium sulfate, and using the weight change factor to predict resistance to magnesium sulfate attack is not an accurate method [64, 77, 111, 115, 116].The lower weight gain of the binary specimen of WGP15 compared to the Ctrl specimen is related to the reduction of C3A due to the decrease in the extent of cement and the consumption of CaOH2 due to the pozzolanic reaction, which ultimately reduces the formation of gypsum and ettringite. In addition, the reaction of magnesium sulfate with WGP produces compounds with a lower density than the reactants, which might be why the binary specimen of WGP15 does not show the same weight gain shown by the other specimens [62]. Mostofinejad et al. [63] evaluated the durability of concrete containing recycled concrete aggregates with WGP in the magnesium sulfate environment. They reported that adding 30% WGP to concrete containing recycled concrete aggregates improved the performance of concrete in the magnesium sulfate environment [63]. Hendi et al. [64], in another study, investigated the performance of concrete containing 6%, 13%, and 20% WGP in the magnesium sulfate environment with a fully saturated concentration. They reported that after 150 days of exposure to magnesium sulfate, all specimens containing WGP showed less weight gain than the Ctrl specimen [64].The binary specimen of GGBFS15 showed a lower weight gain than the Ctrl specimen after 90 days of exposure to magnesium sulfate. Nevertheless, compared to other specimens, it showed more weight gain. SCMs are usually deficient in calcium compared to cement [102]. Hydrated SCMs further decrease the CaOH2 content. GGBFS may also consume CaOH2 but to a lesser value [109]. Due to the lower consumption of CaOH2 by GGBFS, the potential of gypsum and ettringite formation in concrete containing GGBFS is higher than that of concrete containing WGP and SF. Therefore, the higher weight gain of the binary specimen of GGBFS15 than other specimens containing WGP and SF can be predicted. In an investigation, Durgun and Sevinç [117] determined the effectiveness of pumice, waste glass powder, GGBFS, and colemanite waste against sodium and magnesium sulfate attacks; they reported that the most effective mineral additive to reduce the weight loss of concrete specimens is GGBFS, which has an optimal usage rate of 10%, and the worst case is when GGBFS is not used in the mix design. Finally, the results showed that the ideal mixing ratio to minimize weight loss due to sulfate attack is 5% pumice, 5% waste glass powder, 10% GGBFS, and 1% colemanite waste [117].


### 3.2.2. Sulfuric Acid Attack

Effluents from furnaces that use high-sulfur fuels and effluents from chemical industries may contain sulfuric acid. The loss of organic matter in marshes, shallow lakes, mining pits, and sewage pipes often results in the formation of H2S, which can be converted to sulfuric acid by bacterial action. Therefore, it is necessary to investigate the performance of concrete in acidic environments.In a well-hydrated concrete, the cement paste phase, comprised of moderately insoluble hydrates of calcium (C-S-H,CaOH2, and the AFt and AFm phases), exists in a state of stable balance with a high-pH pore solution. Depending on the Na+, K+, and OH− ions concentration, the pH value ranges from 12.5 to 13.5. Less than 12.5 pH theoretically leads to instability of the cementitious products of hydration; this means that most natural waters and urban wastewater are damaging to concrete. The rate of chemical attack depends on the pH and penetrability of concrete, so in concrete with low penetrability and pH above 6, the rate of chemical attack is too slow [19, 118].On the other hand, by reducing the concentration of Ca2+ ions in the pore solution, the chemical balance of the cement paste is disturbed, which first damages the CaOH2 and then the C-S-H gel [119]. The reaction of sulfuric acid with components of hydrated cement paste is shown in (5) and (6). In the next step, ettringite could be formed through the chemical reaction between gypsum and some hydration products such as monosulfate, calcium aluminate hydrate, and unhydrated tricalcium aluminate (C3A). These reactions are sulfate attacks (equations (7)–(9)). It should be noted that the reactions of ettringite production increase the volume, which creates cracks in concrete, and the damage caused by the sulfuric acid attack is intensified [19, 119].(5)H2SO4+CaOH2⟶CaSO4+2H2O,(6)H2SO4+CaO.SiO2.2H2O⟶CaSO4+SiOH4+H2O,(7)3CaO.Al2O3.CaSO4.12H2O+2CaSO4.2H2O+16H2O⟶3CaO.Al2O3.3CaSO4.32H2O,(8)3CaO.Al2O3.CaOH2.12H2O+3CaSO4.2H2O+13H2O⟶3CaO.Al2O3.3CaSO4.32H2O+CaOH2,(9)3CaO.Al2O3+3CaSO4+32H2O⟶3CaO.Al2O3.3CaSO4.32H2O.(1) Changes in Compressive Strength. Figure 9(a) shows the changes in the compressive strength of the specimens after exposure to sulfuric acid. The results show that all specimens decreased compressive strength up to 90 days. Changes in compressive strength depend on the formation of chemical products in exposure to the sulfuric acid solution. The decrease in compressive strength in all specimens containing SF, WGP, and GGBFS was higher than in the Ctrl specimen. The Ctrl specimen has more potential in gypsum and ettringite formation than other specimens due to the higher content of CaOH2 and C3A. Filling concrete pores with gypsum and ettringite improves compressive strength in the short term [98].Figure 9
(a) Compressive strength changes in concrete after exposure to sulfuric acid. (b) Changes in concrete weight after exposure to sulfuric acid.
(a)(b)The binary specimen of WGP15, after 90 days of exposure to the sulfuric acid solution, showed the best performance compared to other binary and ternary specimens. The lower reduction of the compressive strength of the binary specimen of WGP15 can be related to the high porosity of the cement paste, which has more pores for the formation of gypsum and ettringite because these products act as fillers at early ages and improve the compressive strength of the concrete specimens.In addition, the lower reduction of the compressive strength of the binary specimen of WGP15 can be due to the sacrificial nature of the WGP, which produces chemical compounds that are nonexpandable in nature and have less damage than ettringite. Due to the sacrificial nature of WGP, the rate of ettringite formation is reduced. By acting as a sacrificial material, WGP prevented the disintegration of C-S-H and C-A-S-H compounds, thus reducing the intensity of loss in compressive strength [62, 98]. Bisht et al. [98] also reported that when 24% of waste glass (150–600 μm) was replaced by river sand, the resistance of concrete against sulfuric acid attack is improved. Even after 90 days of exposure to sulfuric acid, the compressive strength was 54% higher than plain concrete [98]. Jain et al. [120] also stated that adding glass powder up to 15% to plain concrete improved the resistance against sulfuric acid attack. Concrete containing 15% glass powder showed the best performance after 84 days of exposure to a 3% sulfuric acid solution [120].The binary specimen of GGBFS15 showed acceptable performance at early ages. GGBFS creates the least pore ratio due to improving the pore structure of concrete [101, 102]. Therefore, any slight formation of gypsum and ettringite contributes to the easy filling of the pores, which leads to an increase in compressive strength; nevertheless, after 28 days of exposure to sulfuric acid, loss of compressive strength was observed; because the formed products need high volume, it leads to simultaneous expansion and cracking, and it facilitates the ingress of sulfuric acid; as a result, the compressive strength of concrete loses over time [62, 121]. The study of Sturm et al. [121] evaluated the resistance of one-part geopolymer mortars against sulfuric acid. They reported that adding 25 wt% of GGBFS reduced the resistance against sulfuric acid by forming expanded calcium sulfate phases [121].As mentioned earlier, SF significantly improved the mechanical properties and reduced the porosity of concrete. However, specimens containing SF showed a weaker performance than other specimens after 90 days of exposure to the sulfuric acid solution. Thus, it can be argued that convertingCaOH2 to C-S-H and refining the porosity of concrete are not effective in preventing the loss of compressive strength against sulfuric acid attack [122]. Due to the reduction of concrete porosity, any slight formation of gypsum and ettringite leads to internal stresses and premature cracking. In the next step, sulfuric acid attacks C-S-H easier and faster due to the reduction of the availability of CaOH2, which causes the instability of the cement paste and loss of strength [123].The results of the current study showed that higher compressive strength does not necessarily guarantee the durability of concrete against the attack of magnesium sulfate and sulfuric acid. The results of this study are in accordance with the findings of Senhadji et al. [122]. They found that cement mortar and mortars containing natural pozzolan and limestone showed better performance against sulfuric acid attack compared to mortar containing SF [122]. Torii and Kawamura [116] also reported that the partial replacement of Ordinary Portland Cement by SF and fly ash could not effectively prevent the destruction of concrete by sulfuric acid involving the scaling and softening [116].(2). Changes in Weight. Figure 9(b) shows the changes in the weight of the specimens after exposure to sulfuric acid. Based on the results, all specimens lost weight after 90 days. The ternary specimen of SF5 + GGBFS10 showed the lowest weight loss compared to other specimens, which may be due to the pozzolanic reaction of SF, which reduced the CaOH2 content and formed more C-S-H. But the binary specimen of GGBFS15 has a higher potential for gypsum and ettringite formation due to the extent of consumption of less CaOH2, so expansion and cracking lead to rupture and weight loss.The binary specimen of WGP15 also showed a lower weight loss than the Ctrl specimen, which may be related to the higher porosity of the cement paste and the sacrificial nature of the WGP. As mentioned earlier, WGP produces compounds that are nonexpansive in nature and are not as damaging as ettringite, consequently reducing internal stresses that lead to cracking and preventing rupture and weight loss [62, 98].The negative effect of using SF on the weight changes of SF5 + WGP10 and SF5 specimens was observed. This behavior may be attributed to the lower porosity of the cement paste because the pore refinement by SF reduces the concrete pore volume. Undoubtedly, the pore volume is one of the most effective parameters in improving the resistance of concrete against sulfuric acid attacks. Because the higher pore volume provides more accessible spaces for the formation of gypsum and ettringite, and as a result, internal stresses and intense damage are prevented [123].According to the results of the current study, the results reported by Hendi et al. [123], Bassuoni and Nehdi [124], Chang et al. [125], and De Belie et al. [126] found that there is no direct correlation between compressive strength changes and weight changes after exposure to the sulfuric acid solution [123–126].


### 3.2.3. Surface Resistivity

The resistivity of concrete is an essential parameter in the corrosion of reinforced concrete structures. High-resistivity concrete has little possibility of developing reinforcement corrosion. In the field, the electrical resistivity is determined by measuring the potential differences at the concrete surface caused by injecting a small current at the surface [19]. Electrical resistivity indicates the mobility of ions throughout the concrete matrix [127]. The higher the penetrability of the concrete, the easier and faster the ions penetrate into the concrete. If the concrete has higher electrical resistance and lower penetrability, it has a better resistance against destructive ions such as chloride [128, 129].According to Figure10, the surface resistivity for all specimens containing SF increased higher than 200% in the early ages and up to 300% in the final ages compared to the Ctrl specimen; the positive effect of using SF in increasing the surface resistivity was confirmed by the authors of [82, 83, 127–130]. According to Table 3, presented by AASHTO-T 358, the chloride ion penetration for specimens of SF5, SF5 + WGP10, and SF5 + GGBFS10 was in the very low range.Figure 10
Surface resistivity results of concrete mixtures.Table 3
Chloride ion penetration based on AASHTO-T 358.
Chloride ion penetrationSurface resistivity (KΩ·cm)High<12Moderate12–21Low21–37Very low37–254Negligible>254At the age of 120 days, the surface resistivity of the binary specimens of WGP15 and GGBFS15 was increased by 94.7% and 73.6% compared to the Ctrl specimen, respectively. The chloride ion penetration was in the low range, while in the Ctrl specimen, the chloride ion penetration remained in the moderate range for 120 days. This matter shows the positive effect of SCMs in improving the density of cement paste structure and reducing the volume of capillary pores due to the pozzolanic reaction.Mehta and Monteiro [19] stated that the risk of reinforcement corrosion increases with the reduction of the electrical resistance of concrete. Nevertheless, as long as the electrical resistance of concrete is above 50 to 70 KΩ·cm, significant corrosion is not observed [19].The risk of reinforcement corrosion was significantly reduced in the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 due to the electrical resistance above 70 KΩ·cm.


### 3.2.4. Rapid Chloride Penetrability Test (RCPT)

Fluctuations in temperature and humidity, short-term and long-term internal chemical reactions, and pores geometry in concrete all contribute to chloride ion penetration rates [131]. The RCPT result is interpreted as an indicator of electrical conductivity [86]. In this test, the accumulated charges passing through the specimens are measured, which determines the chloride ion penetrability of concrete. Electrical conductivity is sensitive to the flow of ions released in the cementitious products of hydration through the water in the pores. The lower the electrical conductivity of the concrete, the lower the current flowing between the anodic and the cathodic areas, and thus, the lower rate of corrosion. Hence, the risk of reinforcement corrosion can be evaluated by measuring the electrical conductivity of concrete [132, 133].RCPT results are shown in Figure11. The overall results are similar to surface resistivity. As can be seen, due to the completion of hydration, the amount of charges passing decreases at the final age. According to the classification presented in the ASTM C1202 standard (Table 4), it can be stated that the chloride ion penetrability in the specimens of SF5, SF5+WGP10, and SF5+GGBFS10 after 56 days is very low. The very fine particles of SF, the high rate of pozzolanic reaction, and the filling effect can reduce the pore volume and increase the homogeneity of concrete, thus hindering the mobility and interaction of ions [127, 128, 134].Figure 11
Rapid chloride penetrability test (RCPT) results of concrete mixtures.Table 4
Chloride ion penetrability based on ASTM C1202.
Chloride ion penetrabilityCharge passed (coulombs)High>4000Moderate2000–4000Low1000–2000Very low100–1000Negligible<100The high amount of charges passing at the age of 28 days in the binary specimen of GGBFS15 is probably due to the low rate of pozzolanic reaction. Because after the age of 56 days, due to the progress of the pozzolanic reaction that leads to the densification of the pore structure, the amount of charges passing shows a significant decrease [135]. Finally, at the age of 120 days, the chloride ion penetrability was in the low range.The highest amount of charges passing is at the age of 28 days in the binary specimen of WGP15. It is well known that RCPT is basically an electrical conductivity or resistivity test, and the conductivity of the pore solution also affects the results of this test. The presence of a pore solution with higher conductivity leads to a higher apparent amount of charges passing even if the microstructure is the same. Based on the studies of Du and Tan [86], Zheng [89], Jain and Neithalath [136], it was observed that due to its high Na2O and SiO2, glass powder releases a large amount of alkaline content in the pore solution and thus increases the conductivity of the pore solution [86, 89, 136]. But at the ages of 56, 90, and 120 days, a significant decrease in charges passing was observed; because of the cement hydration reaction, a pore solution rich in Ca2+, SiO42−, and OH− ions is produced, and the amorphous silica present in (Si-O-Si bond) glass powder can dissolve under this high pH solution and form a layer rich in Si+ on the surface of the glass powder. This layer reacts with Ca2+ ions in the solution and turns into C-S-H; this formed C-S-H has a lower Ca/Si ratio and thus absorbs more Na+ released from the glass powder [86, 137]. Consequently, the improved microstructure and the decrease in the concentration of ions in the pore solution lead to a decrease in electrical conductivity [86]. Eventually, at the age of 120 days, the chloride ion penetrability in the binary specimen of WGP15 was in the low range.The investigation of Yeau and Kim [135], Kayali et al. [134], Pilvar et al. [127], Du and Tan [86], and Delnavaz et al. [128] confirmed that the addition of FS, WGP, and GGBFS to concrete is effective in reducing chloride ion penetrability and increases the durability of concrete in chloride corrosive environments [86, 127, 128, 134, 135].


### 3.2.5. Water Absorption

The water absorption test provides information on capillarity suction and is a standard indicator for measuring the resistance of concrete when exposed to aggressive environments. The durability of concrete is significantly affected by water movement in concrete [138]. Water absorption is generally affected by pore structure, porous paste, and ITZ, and these factors are essential at early ages [19].Figure12 shows the results of water absorption after 30 minutes (initial absorption) and 24 hours (final absorption) at the ages of 28, 56, 90, and 120 days. According to the results, it can be seen that the initial and final water absorption at the age of 120 days is lower for SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens than other specimens. The decrease in water absorption can be attributed to the pozzolanic reaction of SF and converted interconnected pores to unconnected pores. As a result, the porosity of cement paste decreases. The results obtained by Yusuf et al. [10], Gupta et al. [139], Khaloo et al. [140], Madani et al. [41], and Sabet et al. [141] confirmed that the use of SF in concrete reduces water absorption [10, 41, 139–141].Figure 12
Water absorption test results of concrete mixtures.At the ages of 56, 90, and 120 days, the binary specimens of WGP15 and GGBFS15 showed good performance compared to the Ctrl specimen; due to the pozzolanic reaction and particle packing, the porosity decreased and led to low water absorption.CEB [142] recommends water absorption after 30 minutes (initial absorption). According to CEB recommended limits, water absorption of all concrete specimens was less than 3%, indicating good concrete durability [142]. Shetty and Jain [143] believe that high-quality concrete has a final absorption of less than 5% [143]. Based on the results, all mixtures can be considered high-quality concrete in water absorption.


### 3.2.6. Depth of Penetration of Water under Pressure

Another method of measuring concrete penetrability is the depth of penetration of water. Water penetration into concrete can cause physical and chemical damages. Water as a solvent can dissolve many cement components [144]. Many ions cause damage to concrete by water penetration. Therefore, water penetration in concrete can be used as an indicator of concrete durability [82, 145]. The penetrability of concrete is influenced by the total volume, shape, and connectivity of the pores [146].In this study, the average depth of penetration measure was reported. Motahari Karein et al. [129] and Banar et al. [77] reported that using the average depth of penetration of water is an acceptable measurement and illustrates more accurate results. During the test, it was observed that the depth of penetration of water could be artificially increased locally due to the presence of aggregates near the surface of the specimen under water pressure [77, 129]. According to Mindess et al. [147], the depth of penetration of water indicates the total volume of open pores larger than 100 nm. Open pores are pathways for water penetration and ions in the concrete system [147].The results of the depth of penetration of water test at the ages of 28, 56, 90, and 120 days are shown in Figure13. The depth of penetration of water for all specimens containing SF decreased compared to the Ctrl specimen. This decrease in penetration depth at the age of 120 days in the SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens is 29%, 26%, and 27.5%, respectively. The reduction of depth of penetration of water at early ages in ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 is due to the presence of SF. Due to its very fine particles and the high rate of pozzolanic reaction at early ages, SF improves the microstructure and densification of the ITZ, which partially blocks the paths of water penetration [139]. Based on the results obtained from Ahmad et al. [148], Ince et al. [149], and Gupta et al. [139] researches, the positive effect of using SF in reducing the depth of penetration of water was confirmed [139, 148, 149].Figure 13
Depth of penetration of water under pressure results of concrete mixtures.The binary specimens of WGP15 and GGBFS15 exhibited weak performance at early ages, but at the age of 120 days, 43% and 44.2% reduction in the depth of penetration of water was observed compared to the Ctrl specimen, respectively, which shows the best performance among all specimens. According to the results of other researchers, the use of WGP and GGBFS has positive effects on the mechanical properties and durability of concrete. Because of the pozzolanic reaction, the porosity decreases, and a smaller pore structure is created in the concrete nucleus, which leads to a decrease in the depth of penetration of water [86, 150, 151].


### 3.2.7. SEM Analysis

Figure14 shows the images of SEM. To understand the cause, rate, and mechanism of destruction, or how to improve some properties of concrete, complete knowledge of the microstructure of hardened concrete is necessary. The mechanical properties of concrete often depend on its microstructure. Cement paste contains products of cement hydration, which is 50–60% of the solid volume of cement paste containing C-S-H, 20–25% including CaOH2 crystals, and the rest including AFt (i.e., ettringite), AFm (i.e., monosulfate), unhydrated cement, and the porosity of cement paste [152]. As shown in Figure 14, the presence of SF has a positive effect on the microstructure of concrete and has helped to improve the packing density and structure of the cement paste [82].Figure 14
SEM images of the concrete specimens after 90 days of curing. (a) Ctrl. (b) SF5. (c) WGP15. (d) GGBFS15. (e) SF5 + WGP10. (f) SF5 + GGBFS10.
(a)(b)(c)(d)(e)(f)


## 3.2.1. Magnesium Sulfate Attack

Magnesium sulfate is often found in groundwater, seawater, and some industrial effluents. Magnesium solutions easily react withCaOH2 in Portland cement paste to form soluble salts of calcium. Magnesium sulfate solution is the most aggressive because the sulfate ions can damage the alumina-bearing hydrates in the Portland cement paste. The characteristic feature of magnesium ion attack on cement paste is that the attack is extended to calcium silicate hydrate, which is the main component of cement. In prolonged contact with magnesium solutions, the C-S-H in Portland cement paste gradually loses its calcium ions, which are replaced by magnesium ions. The ultimate product of this substitution reaction is a magnesium silicate hydrate, the formation of which is associated with the loss of the cementitious characteristic. Due to the presence of CaOH2 in the hydrated Portland cement paste, when the cement paste comes into contact with sulfate ions, alumina-containing hydrates are converted to the high-sulfate form (i.e., ettringite), which are shown in the following equations [19]:(1)C3A⋅CH⋅H18+2CH+3S¯+11H⟶C3A⋅3CS¯⋅H32,(2)C3A⋅CS¯⋅H18+2CH+2S¯+12H⟶C3A⋅3CS¯⋅H32.Gypsum formation as a result of cation-exchange reactions is also capable of causing expansion. However, it has been observed that deterioration of hardened Portland cement paste by gypsum formation goes through a process that first leads to a reduction of pH of the system and loss in stiffness and strength, followed by expansion and cracking, and finally, the transformation of the concrete into a mushy or noncohesive mass, which are shown in the following equations:(3)MgSO4+CaOH2+2H2O⟶CaSO4⋅2H2O+MgOH2,(4)3MgSO4+3CaO.2SiO2.3H2O+8H2O⟶3CaSO4.2H2O+3MgOH2+2SiO2.H2O.In the attack of magnesium sulfate and the conversion ofCaOH2 to gypsum, it is accompanied by the simultaneous formation of MgOH2 (brucite), which is insoluble and reduces the alkalinity of the system. In the absence of hydroxyl ions in the pore solution, the stability of C-S-H in the system is reduced and attacked by the sulfate solution (4).(1) Changes in Compressive Strength. In Figure 8(a), the changes in the compressive strength of the specimens exposed to magnesium sulfate are observed. Up to the age of 90 days, the compressive strength of all specimens exposed to magnesium sulfate increased compared to the initial compressive strength (i.e., 28 days cured in water).Figure 8
(a) Compressive strength changes of concrete after exposure to magnesium sulfate. (b) Weight changes of concrete after exposure to magnesium sulfate.
(a)(b)Binary specimens of WGP15 and GGBFS15 show the highest compressive strength increase at 90 days compared to other specimens. The improvement of magnesium sulfate resistance in the binary specimen of WGP15 may be due to the sacrificial behavior of WGP. WGP produces compounds that are not as harmful as those formed by the decomposition of cement hydration products exposed to the magnesium sulfate environment [62, 98]. WGP leaches alkali ions of Na+ and Si+ into the solution, which leads to the neutralization of magnesium sulfate. In addition, the pozzolanic reaction of WGP increases the alkaline concentration of the pore solution [89, 99], which prevents the decomposition of hydration products by magnesium sulfate. Mostofinejad et al. [63] reported that the incorporation of recycled concrete aggregates with WGP leads to significant improvements in the development of concrete compressive strength in the magnesium sulfate environment with saturated concentration (i.e., 14.7%) [63]. Liu et al. [100] also reported that replacing fine aggregate with a liquid crystal display (LCD) improves the performance of concrete against sulfate attacks. They pointed out that glass powder has low water absorption, which leads to an increase in the effective water-cement ratio of the concrete mixture, which causes increased porosity and provides sufficient space for salt to crystallize before damaging the cement matrix [100].In the binary specimen of GGBFS15, the resistance to magnesium sulfate was increased due to the decrease in the content of C3A, CaOH2, and the penetrability of concrete. GGBFS consumes less CaOH2 than other SCMs and usually reduces the penetrability of concrete due to improved pore structure [101, 102]. In addition, the behavior of GGBFS depends on the replacement level and its chemical composition; in concrete mixtures, good behavior is usually observed when the replacement level of GGBFS is high (i.e., 70%). This behavior is amplified when the GGBFS has a lower content of (Al2O3). Hooton and Emery [103], Gollop and Taylor [104, 105], Locher [106], Higgins [107], Ogawa et al. [108], Whittaker and Black [109], and Mostofinejad et al. [67] reported that GGBFS could contribute to the resistance of concrete against sulfate attack [67, 103–109].According to the changes in compressive strength at the age of 90 days, it was observed that SF5, SF5+WGP10, and SF5+GGBFS10 specimens have less resistance to magnesium sulfate attack than the Ctrl specimen. Partial replacement of cement by SF reduces the availability ofCaOH2 due to the pozzolanic reaction and allows magnesium sulfate to more easily attack C-S-H, leading to decalcification, formation of M-S-H, and destruction of the cement bond [110–113]. Similar studies by Baghabra Al-Amoudi [111], Ganjian and Pouya [112], Lee et al. [113], Mostofinejad et al. [67], and Ortega et al. [114] confirm the significant decrease in compressive strength of concretes containing SF against magnesium sulfate attack [67, 111–114].(2) Changes in Weight. The results of weight changes of concrete specimens exposed to magnesium sulfate solution are presented in Figure 8(b). The presented results show that all the specimens gained weight for up to 90 days. The increase in weight in concrete specimens is due to the formation of gypsum and ettringite, which is the result of the reaction of magnesium sulfate with CaOH2, C3A, and C4AF [77, 114].In general, the specimens containing SF had the lowest weight gain. As we know, SF pozzolanic reactions lead to a decrease inCaOH2 content, and this issue decreases the potential of gypsum and ettringite formation. Based on the results of other mechanical and durability tests in this study, the positive effect of particle packing or space-filling of SF was observed [79, 80, 82]. Therefore, the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 containing SF have lower penetrability than the binary specimens of WGP15 and GGBFS15 without SF. By reducing the penetrability, the diffusion of sulfate ions in concrete is decreased [77, 111, 114]. However, reducing the pore volume does not always guarantee acceptable performance against magnesium sulfate attacks because the arrangement of these pores is also important [64]. The destructive effect of magnesium sulfate in concrete containing SF is related to the difference in the reaction mechanisms and the attack of magnesium sulfate, which causes a further reduction of alkalinity by the pozzolanic reactions of SF, which eventually leads to the decomposition of C-S-H and excessive reduction of strength [77]. Banar et al. [77], Hendi et al. [64], Hekal et al. [115], Baghabra Al-Amoudi [111], and Torii and Kawamura [116] observed a similar effect of SF on pore refinement, but they believed that there is no clear justification for the positive effect of SF on resistance to magnesium sulfate, and using the weight change factor to predict resistance to magnesium sulfate attack is not an accurate method [64, 77, 111, 115, 116].The lower weight gain of the binary specimen of WGP15 compared to the Ctrl specimen is related to the reduction of C3A due to the decrease in the extent of cement and the consumption of CaOH2 due to the pozzolanic reaction, which ultimately reduces the formation of gypsum and ettringite. In addition, the reaction of magnesium sulfate with WGP produces compounds with a lower density than the reactants, which might be why the binary specimen of WGP15 does not show the same weight gain shown by the other specimens [62]. Mostofinejad et al. [63] evaluated the durability of concrete containing recycled concrete aggregates with WGP in the magnesium sulfate environment. They reported that adding 30% WGP to concrete containing recycled concrete aggregates improved the performance of concrete in the magnesium sulfate environment [63]. Hendi et al. [64], in another study, investigated the performance of concrete containing 6%, 13%, and 20% WGP in the magnesium sulfate environment with a fully saturated concentration. They reported that after 150 days of exposure to magnesium sulfate, all specimens containing WGP showed less weight gain than the Ctrl specimen [64].The binary specimen of GGBFS15 showed a lower weight gain than the Ctrl specimen after 90 days of exposure to magnesium sulfate. Nevertheless, compared to other specimens, it showed more weight gain. SCMs are usually deficient in calcium compared to cement [102]. Hydrated SCMs further decrease the CaOH2 content. GGBFS may also consume CaOH2 but to a lesser value [109]. Due to the lower consumption of CaOH2 by GGBFS, the potential of gypsum and ettringite formation in concrete containing GGBFS is higher than that of concrete containing WGP and SF. Therefore, the higher weight gain of the binary specimen of GGBFS15 than other specimens containing WGP and SF can be predicted. In an investigation, Durgun and Sevinç [117] determined the effectiveness of pumice, waste glass powder, GGBFS, and colemanite waste against sodium and magnesium sulfate attacks; they reported that the most effective mineral additive to reduce the weight loss of concrete specimens is GGBFS, which has an optimal usage rate of 10%, and the worst case is when GGBFS is not used in the mix design. Finally, the results showed that the ideal mixing ratio to minimize weight loss due to sulfate attack is 5% pumice, 5% waste glass powder, 10% GGBFS, and 1% colemanite waste [117].


## 3.2.2. Sulfuric Acid Attack

Effluents from furnaces that use high-sulfur fuels and effluents from chemical industries may contain sulfuric acid. The loss of organic matter in marshes, shallow lakes, mining pits, and sewage pipes often results in the formation of H2S, which can be converted to sulfuric acid by bacterial action. Therefore, it is necessary to investigate the performance of concrete in acidic environments.In a well-hydrated concrete, the cement paste phase, comprised of moderately insoluble hydrates of calcium (C-S-H,CaOH2, and the AFt and AFm phases), exists in a state of stable balance with a high-pH pore solution. Depending on the Na+, K+, and OH− ions concentration, the pH value ranges from 12.5 to 13.5. Less than 12.5 pH theoretically leads to instability of the cementitious products of hydration; this means that most natural waters and urban wastewater are damaging to concrete. The rate of chemical attack depends on the pH and penetrability of concrete, so in concrete with low penetrability and pH above 6, the rate of chemical attack is too slow [19, 118].On the other hand, by reducing the concentration of Ca2+ ions in the pore solution, the chemical balance of the cement paste is disturbed, which first damages the CaOH2 and then the C-S-H gel [119]. The reaction of sulfuric acid with components of hydrated cement paste is shown in (5) and (6). In the next step, ettringite could be formed through the chemical reaction between gypsum and some hydration products such as monosulfate, calcium aluminate hydrate, and unhydrated tricalcium aluminate (C3A). These reactions are sulfate attacks (equations (7)–(9)). It should be noted that the reactions of ettringite production increase the volume, which creates cracks in concrete, and the damage caused by the sulfuric acid attack is intensified [19, 119].(5)H2SO4+CaOH2⟶CaSO4+2H2O,(6)H2SO4+CaO.SiO2.2H2O⟶CaSO4+SiOH4+H2O,(7)3CaO.Al2O3.CaSO4.12H2O+2CaSO4.2H2O+16H2O⟶3CaO.Al2O3.3CaSO4.32H2O,(8)3CaO.Al2O3.CaOH2.12H2O+3CaSO4.2H2O+13H2O⟶3CaO.Al2O3.3CaSO4.32H2O+CaOH2,(9)3CaO.Al2O3+3CaSO4+32H2O⟶3CaO.Al2O3.3CaSO4.32H2O.(1) Changes in Compressive Strength. Figure 9(a) shows the changes in the compressive strength of the specimens after exposure to sulfuric acid. The results show that all specimens decreased compressive strength up to 90 days. Changes in compressive strength depend on the formation of chemical products in exposure to the sulfuric acid solution. The decrease in compressive strength in all specimens containing SF, WGP, and GGBFS was higher than in the Ctrl specimen. The Ctrl specimen has more potential in gypsum and ettringite formation than other specimens due to the higher content of CaOH2 and C3A. Filling concrete pores with gypsum and ettringite improves compressive strength in the short term [98].Figure 9
(a) Compressive strength changes in concrete after exposure to sulfuric acid. (b) Changes in concrete weight after exposure to sulfuric acid.
(a)(b)The binary specimen of WGP15, after 90 days of exposure to the sulfuric acid solution, showed the best performance compared to other binary and ternary specimens. The lower reduction of the compressive strength of the binary specimen of WGP15 can be related to the high porosity of the cement paste, which has more pores for the formation of gypsum and ettringite because these products act as fillers at early ages and improve the compressive strength of the concrete specimens.In addition, the lower reduction of the compressive strength of the binary specimen of WGP15 can be due to the sacrificial nature of the WGP, which produces chemical compounds that are nonexpandable in nature and have less damage than ettringite. Due to the sacrificial nature of WGP, the rate of ettringite formation is reduced. By acting as a sacrificial material, WGP prevented the disintegration of C-S-H and C-A-S-H compounds, thus reducing the intensity of loss in compressive strength [62, 98]. Bisht et al. [98] also reported that when 24% of waste glass (150–600 μm) was replaced by river sand, the resistance of concrete against sulfuric acid attack is improved. Even after 90 days of exposure to sulfuric acid, the compressive strength was 54% higher than plain concrete [98]. Jain et al. [120] also stated that adding glass powder up to 15% to plain concrete improved the resistance against sulfuric acid attack. Concrete containing 15% glass powder showed the best performance after 84 days of exposure to a 3% sulfuric acid solution [120].The binary specimen of GGBFS15 showed acceptable performance at early ages. GGBFS creates the least pore ratio due to improving the pore structure of concrete [101, 102]. Therefore, any slight formation of gypsum and ettringite contributes to the easy filling of the pores, which leads to an increase in compressive strength; nevertheless, after 28 days of exposure to sulfuric acid, loss of compressive strength was observed; because the formed products need high volume, it leads to simultaneous expansion and cracking, and it facilitates the ingress of sulfuric acid; as a result, the compressive strength of concrete loses over time [62, 121]. The study of Sturm et al. [121] evaluated the resistance of one-part geopolymer mortars against sulfuric acid. They reported that adding 25 wt% of GGBFS reduced the resistance against sulfuric acid by forming expanded calcium sulfate phases [121].As mentioned earlier, SF significantly improved the mechanical properties and reduced the porosity of concrete. However, specimens containing SF showed a weaker performance than other specimens after 90 days of exposure to the sulfuric acid solution. Thus, it can be argued that convertingCaOH2 to C-S-H and refining the porosity of concrete are not effective in preventing the loss of compressive strength against sulfuric acid attack [122]. Due to the reduction of concrete porosity, any slight formation of gypsum and ettringite leads to internal stresses and premature cracking. In the next step, sulfuric acid attacks C-S-H easier and faster due to the reduction of the availability of CaOH2, which causes the instability of the cement paste and loss of strength [123].The results of the current study showed that higher compressive strength does not necessarily guarantee the durability of concrete against the attack of magnesium sulfate and sulfuric acid. The results of this study are in accordance with the findings of Senhadji et al. [122]. They found that cement mortar and mortars containing natural pozzolan and limestone showed better performance against sulfuric acid attack compared to mortar containing SF [122]. Torii and Kawamura [116] also reported that the partial replacement of Ordinary Portland Cement by SF and fly ash could not effectively prevent the destruction of concrete by sulfuric acid involving the scaling and softening [116].(2). Changes in Weight. Figure 9(b) shows the changes in the weight of the specimens after exposure to sulfuric acid. Based on the results, all specimens lost weight after 90 days. The ternary specimen of SF5 + GGBFS10 showed the lowest weight loss compared to other specimens, which may be due to the pozzolanic reaction of SF, which reduced the CaOH2 content and formed more C-S-H. But the binary specimen of GGBFS15 has a higher potential for gypsum and ettringite formation due to the extent of consumption of less CaOH2, so expansion and cracking lead to rupture and weight loss.The binary specimen of WGP15 also showed a lower weight loss than the Ctrl specimen, which may be related to the higher porosity of the cement paste and the sacrificial nature of the WGP. As mentioned earlier, WGP produces compounds that are nonexpansive in nature and are not as damaging as ettringite, consequently reducing internal stresses that lead to cracking and preventing rupture and weight loss [62, 98].The negative effect of using SF on the weight changes of SF5 + WGP10 and SF5 specimens was observed. This behavior may be attributed to the lower porosity of the cement paste because the pore refinement by SF reduces the concrete pore volume. Undoubtedly, the pore volume is one of the most effective parameters in improving the resistance of concrete against sulfuric acid attacks. Because the higher pore volume provides more accessible spaces for the formation of gypsum and ettringite, and as a result, internal stresses and intense damage are prevented [123].According to the results of the current study, the results reported by Hendi et al. [123], Bassuoni and Nehdi [124], Chang et al. [125], and De Belie et al. [126] found that there is no direct correlation between compressive strength changes and weight changes after exposure to the sulfuric acid solution [123–126].


## 3.2.3. Surface Resistivity

The resistivity of concrete is an essential parameter in the corrosion of reinforced concrete structures. High-resistivity concrete has little possibility of developing reinforcement corrosion. In the field, the electrical resistivity is determined by measuring the potential differences at the concrete surface caused by injecting a small current at the surface [19]. Electrical resistivity indicates the mobility of ions throughout the concrete matrix [127]. The higher the penetrability of the concrete, the easier and faster the ions penetrate into the concrete. If the concrete has higher electrical resistance and lower penetrability, it has a better resistance against destructive ions such as chloride [128, 129].According to Figure10, the surface resistivity for all specimens containing SF increased higher than 200% in the early ages and up to 300% in the final ages compared to the Ctrl specimen; the positive effect of using SF in increasing the surface resistivity was confirmed by the authors of [82, 83, 127–130]. According to Table 3, presented by AASHTO-T 358, the chloride ion penetration for specimens of SF5, SF5 + WGP10, and SF5 + GGBFS10 was in the very low range.Figure 10
Surface resistivity results of concrete mixtures.Table 3
Chloride ion penetration based on AASHTO-T 358.
Chloride ion penetrationSurface resistivity (KΩ·cm)High<12Moderate12–21Low21–37Very low37–254Negligible>254At the age of 120 days, the surface resistivity of the binary specimens of WGP15 and GGBFS15 was increased by 94.7% and 73.6% compared to the Ctrl specimen, respectively. The chloride ion penetration was in the low range, while in the Ctrl specimen, the chloride ion penetration remained in the moderate range for 120 days. This matter shows the positive effect of SCMs in improving the density of cement paste structure and reducing the volume of capillary pores due to the pozzolanic reaction.Mehta and Monteiro [19] stated that the risk of reinforcement corrosion increases with the reduction of the electrical resistance of concrete. Nevertheless, as long as the electrical resistance of concrete is above 50 to 70 KΩ·cm, significant corrosion is not observed [19].The risk of reinforcement corrosion was significantly reduced in the ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 due to the electrical resistance above 70 KΩ·cm.


## 3.2.4. Rapid Chloride Penetrability Test (RCPT)

Fluctuations in temperature and humidity, short-term and long-term internal chemical reactions, and pores geometry in concrete all contribute to chloride ion penetration rates [131]. The RCPT result is interpreted as an indicator of electrical conductivity [86]. In this test, the accumulated charges passing through the specimens are measured, which determines the chloride ion penetrability of concrete. Electrical conductivity is sensitive to the flow of ions released in the cementitious products of hydration through the water in the pores. The lower the electrical conductivity of the concrete, the lower the current flowing between the anodic and the cathodic areas, and thus, the lower rate of corrosion. Hence, the risk of reinforcement corrosion can be evaluated by measuring the electrical conductivity of concrete [132, 133].RCPT results are shown in Figure11. The overall results are similar to surface resistivity. As can be seen, due to the completion of hydration, the amount of charges passing decreases at the final age. According to the classification presented in the ASTM C1202 standard (Table 4), it can be stated that the chloride ion penetrability in the specimens of SF5, SF5+WGP10, and SF5+GGBFS10 after 56 days is very low. The very fine particles of SF, the high rate of pozzolanic reaction, and the filling effect can reduce the pore volume and increase the homogeneity of concrete, thus hindering the mobility and interaction of ions [127, 128, 134].Figure 11
Rapid chloride penetrability test (RCPT) results of concrete mixtures.Table 4
Chloride ion penetrability based on ASTM C1202.
Chloride ion penetrabilityCharge passed (coulombs)High>4000Moderate2000–4000Low1000–2000Very low100–1000Negligible<100The high amount of charges passing at the age of 28 days in the binary specimen of GGBFS15 is probably due to the low rate of pozzolanic reaction. Because after the age of 56 days, due to the progress of the pozzolanic reaction that leads to the densification of the pore structure, the amount of charges passing shows a significant decrease [135]. Finally, at the age of 120 days, the chloride ion penetrability was in the low range.The highest amount of charges passing is at the age of 28 days in the binary specimen of WGP15. It is well known that RCPT is basically an electrical conductivity or resistivity test, and the conductivity of the pore solution also affects the results of this test. The presence of a pore solution with higher conductivity leads to a higher apparent amount of charges passing even if the microstructure is the same. Based on the studies of Du and Tan [86], Zheng [89], Jain and Neithalath [136], it was observed that due to its high Na2O and SiO2, glass powder releases a large amount of alkaline content in the pore solution and thus increases the conductivity of the pore solution [86, 89, 136]. But at the ages of 56, 90, and 120 days, a significant decrease in charges passing was observed; because of the cement hydration reaction, a pore solution rich in Ca2+, SiO42−, and OH− ions is produced, and the amorphous silica present in (Si-O-Si bond) glass powder can dissolve under this high pH solution and form a layer rich in Si+ on the surface of the glass powder. This layer reacts with Ca2+ ions in the solution and turns into C-S-H; this formed C-S-H has a lower Ca/Si ratio and thus absorbs more Na+ released from the glass powder [86, 137]. Consequently, the improved microstructure and the decrease in the concentration of ions in the pore solution lead to a decrease in electrical conductivity [86]. Eventually, at the age of 120 days, the chloride ion penetrability in the binary specimen of WGP15 was in the low range.The investigation of Yeau and Kim [135], Kayali et al. [134], Pilvar et al. [127], Du and Tan [86], and Delnavaz et al. [128] confirmed that the addition of FS, WGP, and GGBFS to concrete is effective in reducing chloride ion penetrability and increases the durability of concrete in chloride corrosive environments [86, 127, 128, 134, 135].


## 3.2.5. Water Absorption

The water absorption test provides information on capillarity suction and is a standard indicator for measuring the resistance of concrete when exposed to aggressive environments. The durability of concrete is significantly affected by water movement in concrete [138]. Water absorption is generally affected by pore structure, porous paste, and ITZ, and these factors are essential at early ages [19].Figure12 shows the results of water absorption after 30 minutes (initial absorption) and 24 hours (final absorption) at the ages of 28, 56, 90, and 120 days. According to the results, it can be seen that the initial and final water absorption at the age of 120 days is lower for SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens than other specimens. The decrease in water absorption can be attributed to the pozzolanic reaction of SF and converted interconnected pores to unconnected pores. As a result, the porosity of cement paste decreases. The results obtained by Yusuf et al. [10], Gupta et al. [139], Khaloo et al. [140], Madani et al. [41], and Sabet et al. [141] confirmed that the use of SF in concrete reduces water absorption [10, 41, 139–141].Figure 12
Water absorption test results of concrete mixtures.At the ages of 56, 90, and 120 days, the binary specimens of WGP15 and GGBFS15 showed good performance compared to the Ctrl specimen; due to the pozzolanic reaction and particle packing, the porosity decreased and led to low water absorption.CEB [142] recommends water absorption after 30 minutes (initial absorption). According to CEB recommended limits, water absorption of all concrete specimens was less than 3%, indicating good concrete durability [142]. Shetty and Jain [143] believe that high-quality concrete has a final absorption of less than 5% [143]. Based on the results, all mixtures can be considered high-quality concrete in water absorption.


## 3.2.6. Depth of Penetration of Water under Pressure

Another method of measuring concrete penetrability is the depth of penetration of water. Water penetration into concrete can cause physical and chemical damages. Water as a solvent can dissolve many cement components [144]. Many ions cause damage to concrete by water penetration. Therefore, water penetration in concrete can be used as an indicator of concrete durability [82, 145]. The penetrability of concrete is influenced by the total volume, shape, and connectivity of the pores [146].In this study, the average depth of penetration measure was reported. Motahari Karein et al. [129] and Banar et al. [77] reported that using the average depth of penetration of water is an acceptable measurement and illustrates more accurate results. During the test, it was observed that the depth of penetration of water could be artificially increased locally due to the presence of aggregates near the surface of the specimen under water pressure [77, 129]. According to Mindess et al. [147], the depth of penetration of water indicates the total volume of open pores larger than 100 nm. Open pores are pathways for water penetration and ions in the concrete system [147].The results of the depth of penetration of water test at the ages of 28, 56, 90, and 120 days are shown in Figure13. The depth of penetration of water for all specimens containing SF decreased compared to the Ctrl specimen. This decrease in penetration depth at the age of 120 days in the SF5, SF5 + WGP10, and SF5 + GGBFS10 specimens is 29%, 26%, and 27.5%, respectively. The reduction of depth of penetration of water at early ages in ternary specimens of SF5 + WGP10 and SF5 + GGBFS10 is due to the presence of SF. Due to its very fine particles and the high rate of pozzolanic reaction at early ages, SF improves the microstructure and densification of the ITZ, which partially blocks the paths of water penetration [139]. Based on the results obtained from Ahmad et al. [148], Ince et al. [149], and Gupta et al. [139] researches, the positive effect of using SF in reducing the depth of penetration of water was confirmed [139, 148, 149].Figure 13
Depth of penetration of water under pressure results of concrete mixtures.The binary specimens of WGP15 and GGBFS15 exhibited weak performance at early ages, but at the age of 120 days, 43% and 44.2% reduction in the depth of penetration of water was observed compared to the Ctrl specimen, respectively, which shows the best performance among all specimens. According to the results of other researchers, the use of WGP and GGBFS has positive effects on the mechanical properties and durability of concrete. Because of the pozzolanic reaction, the porosity decreases, and a smaller pore structure is created in the concrete nucleus, which leads to a decrease in the depth of penetration of water [86, 150, 151].


## 3.2.7. SEM Analysis

Figure14 shows the images of SEM. To understand the cause, rate, and mechanism of destruction, or how to improve some properties of concrete, complete knowledge of the microstructure of hardened concrete is necessary. The mechanical properties of concrete often depend on its microstructure. Cement paste contains products of cement hydration, which is 50–60% of the solid volume of cement paste containing C-S-H, 20–25% including CaOH2 crystals, and the rest including AFt (i.e., ettringite), AFm (i.e., monosulfate), unhydrated cement, and the porosity of cement paste [152]. As shown in Figure 14, the presence of SF has a positive effect on the microstructure of concrete and has helped to improve the packing density and structure of the cement paste [82].Figure 14
SEM images of the concrete specimens after 90 days of curing. (a) Ctrl. (b) SF5. (c) WGP15. (d) GGBFS15. (e) SF5 + WGP10. (f) SF5 + GGBFS10.
(a)(b)(c)(d)(e)(f)


## 4. Conclusions

The current study evaluated the mechanical and durability properties of concrete containing SF, WGP, and GGBFS. The main concluded remarks are as follows:(1)
Adding 5% SF to concrete mixtures containing 10% GGBFS or 10% WGP increased the compressive, tensile, and flexural strength compared to plain concrete. Due to particle packing or space-filling, SF can fill the voids and improve the characteristics of the ternary concrete mixtures; hence, an increase in strength in the ternary concrete mixtures indicates the synergistic effect of these materials.(2)
The binary concrete mixtures containing 15% WGP or 15% GGBFS showed the best performance against the magnesium sulfate attack. Although the positive effect of adding 5% SF to concrete mixtures was observed in other mechanical and durability tests, adding SF decreased the performance of concrete against the magnesium sulfate attack.(3)
The binary concrete mixture containing 15% WGP showed good performance against the sulfuric acid attack. By acting as a sacrificial material, WGP prevented the disintegration of C-S-H and C-A-S-H compounds. All concrete mixtures containing SF showed a decrease in performance similar to the magnesium sulfate attack.(4)
Based on the surface resistivity and the RCPT results, all the binary and ternary concrete mixtures showed an increase in electrical resistivity and a decrease in electrical conductivity compared to plain concrete at the age of 120 days. Chloride ion penetrability of the ternary concrete mixtures was in the very low range, which showed the best performance. In addition, the ternary concrete mixtures are high-resistivity concrete, and there is no risk of reinforcement corrosion.(5)
The water absorption of all the binary and ternary concrete mixtures decreased compared to plain concrete because the porosity of concrete decreased due to the pozzolanic reaction and particle packing. It should be noted that the 24-hour water absorption of all concrete mixtures was less than 5%, which can be considered high-quality concrete. The depth of penetration of water under pressure has the same results. The lowest depth of penetration at the age of 120 days was related to the binary concrete mixtures of 15% WGP and 15% GGBFS, which showed a decrease of about 45% compared to plain concrete.



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*Source: 2730391-2022-12-06.xml*