Waste Glass as Partial Cement Replacement in Sustainable Concrete: Mechanical and Fresh Properties Review

Abstract

The significant anthropogenic carbon dioxide (CO₂) emissions from cement production and the disposal of the majority of post-consumer waste glass into landfill sites have increased environmental pollution. In order to reduce the environmental impact, ground glass pozzolan (GGP) as a partial cement replacement has drawn interest from the concrete industry. This review examines the potential of GGP as a supplementary cementitious material (SCM), exploring the chemical composition of pozzolans, the different types of glass used for GGP, and the impact of glass color on pozzolanic reactivity. In addition, this study gathers the most recent research articles on the fresh and mechanical properties of concrete incorporating GGP. Key findings show that the incorporation of GGP in concrete improves the modulus of elasticity and the compressive, tensile, flexural, and punching strengths due to the pozzolanic reactions. The results indicate that GGP, made from waste glass, has pozzolanic properties that form additional strength-enhancing calcium silicate hydrate (C-S-H) gel and densify the concrete matrix. Additionally, the life cycle assessments of GGP-incorporated concrete demonstrate reductions in energy consumption and CO₂ emissions compared to conventional concrete, supporting a circular economy and sustainable construction practices.

1. Introduction

Cement, the primary binding material in concrete, consumes about 10–15% of industrial energy and emits approximately 5–9% of the world’s anthropogenic CO₂ during its production [1,2,3]. Approximately 1 ton of CO₂ is produced during the production of 1 ton of cement, which is environmentally taxing [1,4,5,6]. Furthermore, concrete is a widely used construction material due to its economic and durability benefits, and its demand is increasing every year due to its need for infrastructure construction [2,4,7]. Around 4.1 billion metric tons of cement were produced in the world in 2022, where the United States alone accounted for 93 million metric tons [8]. The production of cement is expected to increase to 5.0 billion metric tons by 2030 worldwide [7]. On the other hand, in 2018, 12.25 million tons of municipal glass waste (WG) were generated in the United States. However, according to EPA, only 25% of them were recycled and the remaining were either combusted (13.4%) or land-filled (61.6%) [9]. Additionally, the glass takes a very long time (approximately 4000 years) to degrade naturally, which is another concern for a sustainable environment. Figure 1 shows the recycling rates for discarded glass in different nations. Notably, of the countries examined, the USA and Australia have the lowest recycling rates, while European countries have higher recycling rates.

The use of cement and the effect of global warming can be reduced to a certain extent by incorporating supplementary cementitious materials (SCMs) such as fly ash (FA), ground granulated blast furnace slag (GGBFS), ground glass pozzolan (GGP), silica fume (SF), natural pozzolans, rice husk ash (RHA), etc. as a partial replacement of cement [4,7]. The availability of the most commonly used SCM, fly ash (FA), has sharply decreased due to the rapid decommissioning of coal-powered plants in the USA in recent years [1,12]. Around 25% of coal-based power plants have shut down while many are converting to cheaper and cleaner alternatives such as renewable and natural gas, leading to the shortage of FA [1,4]. The intermittent supply of FA, comparatively high cost of slag, and the sustainability challenges due to the allocation of colossal landfill sites to waste glass leads the concrete industry in the USA to explore for other alternatives of SCMs [4]. The artificial pozzolans like fine GGP and natural pozzolans like pumice, perlite, volcanic ash, etc. contain amorphous alumino-silicate to form C-S-H gel during the reaction with calcium hydroxide (Ca(OH)₂) in the presence of water, which are potential alternatives to FA [13]. Natural pozzolans have the benefits of improving the durability and sustainability of concrete; however, they do have a few drawbacks, such as variability in their chemical and physical properties, limited availability and supply, slow pozzolanic activity, possible contamination and processing challenges, and transportation costs. Glass has a lower variation of material chemical composition, greater purity, and lower water demand when compared to FA and traditional SCMs [14].

Studies [1,4,7,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] have demonstrated that inert waste glass when crushed and milled to a very fine powder level of 300 µm particle size shows the behavior of pozzolanic material enhancing the mechanical properties of concrete due to its high silicon content. Furthermore, studies [16,35] have concluded that GPP finer than 100 µm can exhibit better pozzolanic reactivity than FA after 90 days of curing at a low cement replacement percentage. This could lower approximately 1/2 pound of CO₂ emission for every pound of glass used [1,36]. Additionally, Guignone et al. [37] conducted a life cycle assessment (LCA) on the utilization of GGP in a case study of bridge retrofit. They found that using 20% GGP instead of cement in the production of concrete decreased greenhouse gas emissions by 1.23 kg of CO₂ equivalent and energy consumption by 0.008 GJ per kilogram of GGP-incorporated concrete. Thus, waste glass has a high potential to tackle the problems faced by the concrete industry—due to widespread sourcing, unlike natural pozzolans, and the sharp decrease in FA production—to be used as SCMs in the USA [1,38].

Glass powder used as a partial replacement of cement in concrete can be obtained from two main sources: commercially produced glass powder and post-consumer waste glass. The former is produced in a controlled environment, ensuring uniform particle size and chemical composition, and free from contamination. The latter, on the other hand, is initially collected from consumers and subsequently taken to a recycling facility for further processing. They are cleaned, crushed, and optically sorted for potential reuse in the glass bottle manufacturing industry [39]. Crushed waste glass (≤10 mm in size) that is unable to be optically sorted can be grounded into fine powder as an SCM in the concrete industry [39]. The Figure 2 illustrates the comprehensive process of converting waste glass into fine glass powder for utilization in the concrete industry.

This study aims to explore recent studies on the application of GGP in concrete production and its benefits as a sustainable alternative for reducing carbon footprint and promoting a circular economy. This study emphasizes the potential of GGP as an efficient SCM by focusing on the mechanical and fresh properties of GGP-incorporated concrete, including workability, density, setting time, compressive strength, tensile strength, flexural strength, and punching strength. The pressing need to lessen the environmental effects of cement production—a significant source of carbon emissions worldwide—is what spurred this research. This research highlights the importance of GGP in promoting sustainable building practices in addition to analyzing its technical feasibility as a pozzolanic material.

2. Pozzolanic Materials: Composition and Characteristics

2.1. Types of Pozzolans and Reaction Mechanism

“A material comprised of silica or silica along with alumina, which inherently lacks significant cementitious properties but can chemically react with calcium hydroxide (Ca(OH)₂) under typical temperatures when finely divided and in the presence of moisture, resulting in the formation of compounds (C-S-H gel) with cementitious characteristics is called Pozzolan” according to ASTM-C125 [45]. SCMs are frequently employed in concrete mix as partial cement replacements; the quantity of SCMs used varies based on the desired concrete properties, as shown in Figure 3. The compounds in cement, tricalcium silicate (3 CaO·SiO₂ (C₃S)) and dicalcium silicate (2 CaO·SiO₂ (C₂S)), react with water (hydration of cement) to form 3 CaO·2 SiO₂·3H₂O (C-S-H) gel and Ca(OH)₂ as byproducts. This calcium silicate hydrate (C-S-H gel) is highly adhesive in nature with a very low solubility that binds the aggregates and provides strength to the concrete. On the other hand, the byproduct Ca(OH)₂ is soluble in water and gets leeched out of the concrete. This makes concrete more porous and reduces its strength. In a pozzolanic reaction, SCM reacts with byproduct Ca(OH)₂ to form an additional C-S-H gel contributing to a decrease in the total amount of cement required to form the same amount of C-S-H gel. This benefits the environment by lowering carbon emissions from cement manufacture. SCMs can also improve the concrete’s long-term durability by decreasing permeability, strengthening resistance to chemical deterioration, and boosting resistance to sulfate assault. The chemical reaction involved in the pozzolanic reaction is illustrated below:

$$\begin{array}{c}2C_{3}S+6H\to C_3{S}_2{H}_3+3{Ca\left(OH\right)}_2\\ 2C_{2}S+4H\to C_3{S}_2{H}_3+{Ca\left(OH\right)}_2\\ \mathrm{Pozzolan}+{Ca\left(OH\right)}_2+H_{2}O\to \mathit{\text{C-S-H}}\left(\mathrm{Gel}\right)\end{array}$$

(1)

The key differences between the cement hydration (primary hydration) reaction and the pozzolanic reaction (secondary hydration) are the reactants and the reaction rate. The primary hydration starts immediately after the addition of water to the cement and contributes primarily to the early strength development of concrete. However, the pozzolanic reaction starts after the formation of the cement hydration byproduct Ca(OH)₂, which is a key ingredient for the secondary hydration process. The pozzolanic reaction is a slower process and continues over time, depending upon the amount of Ca(OH)₂ from primary hydration.

The methods required for selecting materials as pozzolans are largely determined by the amorphous contents of minerals found in raw materials and/or the industrial processes that produce by-products with pozzolanic reactivity [46]. The pozzolans are divided into three categories based on their production methods: (i) Natural Pozzolan: The materials such as volcanic ash, pumice, and certain types of clay minerals that are produced through the grinding of raw natural materials into fine particles, without the need for calcination or heat treatment to alter their chemical and/or structural properties to enhance pozzolanic reactivity [47]. (ii) Processed pozzolans: Materials made from clays and shales. These are produced by grinding raw natural materials into fine particles, without requiring calcination or heat treatment to modify their chemical and/or structural properties for improving pozzolanic reactivity. When calcined at high temperatures, they become pozzolanic and contain a significant proportion of crystalline silica and alumina in their raw form. Fly ash (FA), rice husk ash (RHA), and silica fume (SF) are considered processed pozzolans, as they are industrial by-products resulting from the combustion of coal, rice husks, and silica, respectively. (iii) Manufactured Pozzolans: The materials such as ground granulated blast furnace slag (GGBFS) and GGP that acquire pozzolanic characteristics after grinding the refined substances into fine powder [46].

2.2. Chemical Composition of Pozzolans

The origin of the raw materials or refined materials used in the production process affects the elemental and mineral composition of pozzolans [46]. Silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) are among the major elements that make up this composition. When utilized in cementitious systems, the pozzolanic activity and performance are directly impacted by the amounts of these components, which differ based on the materials’ source. The Si and Al content dominate in all pozzolans except for silica fume, which contains a very high amount of Si.

Table 1. Chemical composition (mass (%)) of cementitious materials. Source: [48,49,50,51].

Material	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O + K2O	LOI
Portland Cement	19.7–21.4	4.7–5.5	2.2–3.9	63.6–65	1.19–1.5	0.1–0.97	0.16–2.3
Blast Furnace Slag	32–42	6–19.3	0–2.5	30–48	3–14	0–1.4	-
Fly Ash—A	56.7	27.6	4.4	3.6	1.2	0.5	1.8
Fly Ash—B	73.8	17.9	3.3	1	1.2	0.4	3.25
Glass HA	71	0.8	0.3	9.4	3.1	13	1.1
Glass LA	53.6	13.7	0.4	21.9	1.1	0.5	0.6
Ground Glass	53.6–71.5	0.8–13.7	<0.1–0.4	9.4–21.9	1.1–3.1	0.1–13.3	0.2–1.1
Ground Quartz	98.5	0.5	<0.1	<0.1	<0.1	0.2	0.5
Lassenite	64.2	13.7	5.7	1.6	0.8	1.6	8.8
Metakaolin	51.5–51.17	40.2–44.5	0.45–4.64	0.01–2.0	0–0.2	0–0.2	0.4
Perlite	73.9	12.7	1.1	0.8	0.1	6	3.6
Pumice 1	76.3	12.1	1.7	0.4	-	0.6	-
Pumice 2	69	10.9	1.3	0.8	0.46	2.4	3.4
RHA	82.13–87.3	0.09–24.1	0.09–15.7	0.5–16.1	0.3–8.65	0.09–2.1	-
Silica Fume—A	96.5	0.5	0.1	0.4	0.4	-	2.89
Silica Fume—B	89.3	0.3	1	0.7	-	0.4	1.21

displays the elemental composition, which is typically represented by the oxides; silicon dioxide (SiO₂), aluminum oxide ( Al₂O₃), ferric oxide (Fe₂O₃), calcium oxide (CaO), magnesium oxide (MgO), potassium oxide (K₂O), and sodium oxide (Na₂O).

An efficient analytical technique for assessing and contrasting the pozzolanic reactivity of different processed and produced pozzolans is a ternary diagram, illustrated in Figure 4, normalized to the combined proportions of SiO₂, Al₂O₃, and CaO. One of the most reactive pozzolanic materials is silica fume (SF), which is mostly made of SiO₂. Its ultra-fine particle size and unusually high silica content enable a quick reaction with Ca(OH)₂ that produces more calcium silicate hydrate (C-S-H) gel, improving the durability and performance of concrete [52,53].

GGP has a lower SiO₂ concentration than SF [54]; nevertheless, it outperforms other widely used pozzolans in this area. GGP’s pozzolanic reactivity may surpass that of pozzolans such as fly ash, metakaolin, and slag due to its comparatively high silica concentration. Thus, the higher SiO₂ content in GGP provides more reactive silica to produce additional (C-S-H) gel during the secondary hydration process. When GGP is utilized as an additional cementitious material, its reactivity may result in better mechanical properties in cementitious systems. Furthermore, the performance of GGP is significantly influenced by other parameters, including particle size, specific surface area, curing temperatures, and the total chemical composition, even though it has a comparatively high SiO₂ concentration [55,56,57,58,59]. The pozzolanic reaction’s kinetics and the material’s capacity to interact with Ca(OH)₂ during hydration are controlled by these characteristics [60]. Glass’s reactivity and usefulness as an additional cementitious material varies greatly depending on the type (such as soda–lime, borosilicate, or tempered glass) [61]. For consistent performance, the underlying material must be thoroughly characterized. The sections that follow offer more information on the characteristics and advantages of GGP in improving the strength and durability of concrete.

2.3. Ground Glass Pozzolan (GGP)

GGP, obtained from recycled waste glass, can be categorized based on glass types, glass colors, and application. This classification provides important information about GGP characteristics and their uses in concrete mix design optimization. Lead glass, soda–lime glass, borosilicate glass, aluminosilicate glass, barium glass, electrical glass, E glass, and other substances are among the many different forms of glass. These glasses can be crushed and ground to a finer powder size using a ball mill to partially replace cement in concrete. The average ground time and particle size distribution are shown in Figure 5. Additionally, recycled waste glass has an amorphous structure in X-ray diffraction (XRD) patterns, as depicted in Figure 6a. This amorphous silica has a higher number of reactive sites than crystalline silica due to its less ordered structure, which speeds up the hydration in the early phases of cementitious reactions [1]. The micromorphology of GGP, as illustrated in Figure 6b, shows a smooth surface and a dense microstructure that can be used for self-compacting concrete due to low water absorption capacity [62].

2.3.1. Types of Glass

Various types of glass include soda–lime glass, borosilicate glass, lead glass, aluminosilicate glass, and E-glass, among others, as depicted in Figure 7. The chemical composition of different types of glass is shown in

Table 2. Composition of different types of glass. Source: [10,39].

Glass Type	SiO2 (%)	Na2O + K2O (%)	CaO (%)	Al2O3 (%)	MgO (%)	B2O3 (%)
Electric glass	52.0–56.0	0.0–2.0	16.0–25.0	12.0–16.0	–	–
Borosilicate glass	70.0–80.0	4.0–8.0	–	7.0	–	11.0–15.0
Lead glass	54.0–65.0	13.0–15.0	–	–	–	–
Soda–lime glass	71.0–75.0	12.0–16.0	10.0–15.0	–	0.1–4.0	–
E-glass	59.9–61.3	0.77–0.81	21.4–21.9	12.5–12.64	2.69	–
Barium glass	36.0–35.0	7.0	2.0	2.0–4.0	9.0	10.0
Aluminosilicate glasses	57.0–64.5	0.5–1	8.0–10.0	17.0–24.5	7.0–10.5	5

. Among these, soda–lime glass is the most widely used, especially in windows, jars, and bottles used in homes. The USA produces approximately 11.5 tons of post-consumer waste glass, where 90% of glasses are soda–lime [65]. A typical example of soda–lime glass is its use in bottles, as illustrated in Figure 7a. Soda–lime glass is composed of SiO₄ tetrahedral units connected at the oxygen atoms [66]. Based on its intended function, soda–lime glass is further divided into two primary types: plate glass, which is frequently used in windows and flat panels, and container glass, which is used for objects like bottles and jars [4,65]. Plate glass is produced by floating on a molten tin. It is available in both clear or tinted color varieties and is utilized as a glazing material, providing transparency in automobiles and buildings. Similarly, Container glass is produced through a molding process utilizing air pressure and is available in a range of colors, including clear, green, blue, and amber. They are usually used for packaging since they are highly durable, chemically resistive, and have the ability to preserve the integrity of stored products [39]. Lead glass is similar to soda-lime glass but contains a higher concentration of lead oxide as depicted in Figure 7b. Another common type of widely used glass is borosilicate glass. This type of glass primarily consists of silicon and boron oxide. The usage of borosilicate glass ranges from cookware to lab equipment. For instance, beakers used as laboratory equipment in laboratories are made up of borosilicate glass [67], as depicted in Figure 7c. Likewise, the structure of aluminosilicate glass [68], which consists of a three-dimensional framework of tetrahedral silicon and aluminum cations surrounded by oxygen anions, is illustrated in Figure 7d. E-glass is renowned for its high electrical resistance and mechanical properties with low alkali content. This glass is produced through an extrusion process and is primarily used as a reinforcement in fiber-reinforced polymers [69,70].

While other glass types may possess a higher silica content suitable for pozzolanic reactions, the uniformity and stability of soda–lime glass in a variety of applications are facilitated by the minimal variance in its silica concentration [66]. Because of this property, soda–lime glass is a good pozzolan to be used as an SCM in the concrete industry. Furthermore, as indicated in

Table 3. Chemical composition requirements for glass Types GS and GE. Source: [71].

Type	SiO2	Al2O3	CaO	Fe2O3	SO3	Na2Oeq	Moisture	LOI
GS	≥60.0	≤5.0	≤15.0	≤1.0	≤1.0	≤15.0	≤0.5	≤0.5
GE	≥55.0	≤15.0	≤25.0	≤1.0	≤1.0	≤4.0	≤0.5	≤0.5

, ASTM [71] also specifies the minimum and maximum ranges of the component elements in the chemical composition of both soda–lime and E glasses. “Type GS” and “Type GE” are shorthands for ground soda–lime glass and ground E-glass, respectively.

Figure 7. Different types of glass: (a) Soda–lime glass; (b) Lead glass; (c) Borosilicate glass; (d) Aluminosilicate glass. Source: [10,68,72,73].

Figure 7. Different types of glass: (a) Soda–lime glass; (b) Lead glass; (c) Borosilicate glass; (d) Aluminosilicate glass. Source: [10,68,72,73].

2.3.2. Glass Color

Glass can be divided into clear glass, amber glass, brown glass, green glass, or white glass based on color composition [10,62]. For instance, New York City waste glass contained 62% of clear glass, 19% of green glass, 14% of amber glass, and 5% other mixtures of colors [74]. Several studies have been conducted to evaluate the effect of glass color on cement and cementitious mixes [10,62,74,75,76,77,78]. These studies mentioned that the color of the glass is typically based on the chemical composition of the glass. In this regard, the properties of concrete can vary depending upon the color of glass used. The chemical compositions of amber, green, brown, white, and clear glass (representing different colors of the same type of glass) are shown in

Table 4. Chemical composition of different colors of glass. Source: [10,62].

Component	SiO2 (%)	CaO (%)	Na2O (%)	Al2O3 (%)	Fe2O3 (%)	MgO (%)	Cr2O3
Clear Glass	73.2–73.5	-	13.6–14.1	1.7–1.9	0.04–0.05	-	-
Amber Glass	70.66	9.12	8.32	6.53	2.52	1.45	0.01
Green Glass	72.25	12.35	10.54	2.54	-	1.18	0.43–0.44
Brown Glass	72.1	-	-	1.74	0.31	-	0.01
White Glass	69.82	8.76	8.42	1.02	0.55	3.43	-

. It is observed that colored glass generally has a high amount of Fe₂2O₃ and Cr₂O₃. Studies have shown that the presence of Cr₂O₃, especially in green glass, causes a reduction in the reactivity of sand and suppresses the alkali–silica reaction (ASR) expansion [10,74]. On the other hand, some studies suggest that the color of the glass does not have a significant effect on the mechanical properties of GGP-incorporated concrete [76]. In recycled glass, the acceptable limits for color contamination are as follows: 4–6% for clear glass, 5–15% for amber glass, and 5–30% for green glass alone [62]. To effectively separate and sort various colors for additional processing, mixed-color glass is separated and sorted at recycling plants utilizing optical sorting technology. Since around one-third of the mixed-color glass contains particles smaller than 3/8 inch (9.51 mm), optical color separation is not cost-effective because of the lower sorting efficiency and greater processing expenses for finer particles [65]. Additionally, chemical incompatibility and temperature fluctuations, which can impact the quality and structural integrity of recovered glass when melted and reprocessed, make it difficult to reuse mixed-color glass. A small quantity of 5 g of non-recyclable glass is adequate to contaminate an entire ton of recyclable glass [65]. However, color separation of the glass is not required when used as a pozzolan. Because color has no effect on the glass’s pozzolanic qualities; such as its capacity to react with Ca(OH)₂, mixed-color glass can be used without separation [39]. Through the reduction in carbon emissions, the partial substitution of glass as a pozzolan for cement would make a substantial contribution to sustainable construction methods.

3. Fresh Concrete Properties

3.1. Rheological Behavior

The rheological parameters play a significant role in determining the flowability, workability, and overall performance of concrete. Several studies [79,80,81] have assessed the rheological behavior of concrete incorporated with GGP. Li et al. [79] reported that the plastic viscosity of fresh concrete increases with higher cement replacement by GGP, while the yield stress gradually decreases, exhibiting rheological behavior approaching close to that of a Newtonian fluid. Similarly, Khudair et al. [80] reported an insignificant change in the flow diameter of GGP-incorporated self-compacting concrete (SCC) and a gradual decrease in the segregation index with increasing replacement levels. Furthermore, Bouty and Homsi [81] reported an increase in the flow percentage on the flow table, as the cement replacement level by GGP increases, compared to the control mix.

In conclusion, the studies demonstrate the improvement in the rheological behavior of concrete with varying cement replacement levels by GGP.

3.2. Slump

The ease with which fresh concrete may be mixed, transported, and laid without segregation or excessive bleeding is known as concrete workability, and it is commonly assessed by a slump. It offers a straightforward but efficient way to evaluate the fluidity and consistency of the mix by measuring the vertical settlement of a concrete sample following the removal of a conical mold. It is crucial to remember that slump by itself does not fully represent workability. Therefore, slump needs to be considered in conjunction with other elements to make sure that concrete satisfies the performance and durability standards required for the intended use.

Figure 8 and Figure 9 summarize the slump flow with varying percentages of waste glass (0–30% in increments of 5%) in relation to partially replacing cement with GPP. Overall, the findings from the previous experimental data present significant discrepancies. Abdulazeez et al. [82], K.I.M. Ibrahim [83], Fattouh et al. [84], and Balasubramanian et al. [63] reported increases of approximately 83%, 57%, 37%, and 36%, respectively, in the slump value with 20% cement replacement by GGP. This is explained by waste glass’s poor water absorption and the availability of free water to reduce the internal friction between aggregates. Additionally, the strong pozzolanic activity of GGP strengthens the matrix, and its tiny particles fill the porosity of the coarse aggregates and lower internal friction, making the mix more flowable and workable. However, Khan et al. [85], Ani et al. [86], and Sayeeduddin and Chavan [87] demonstrated a decrease of approximately 30%, 22%, and 19%, respectively, in the slump value with 20% cement replacement by GGP. This decrease in slump is explained due to the waste glass particles’ higher surface area and angular form, which necessitated more cement paste for coating and consequently decreasing the amount of cement paste available for lubrication. On the other hand, Gupta et al. [88] reported that the value of the slump gradually increased to approximately 18% for a 20% replacement level, and then gradually decreased to the slump value of control mix until 40% replacement of cement by GGP. According to Ahmad et al., an experimental study [7] reported a rise in slump value, while Ahmad et al. [89] reported a fall in slump value, with higher levels of cement replacement by GGP.

In conclusion, these studies demonstrate the complex relationship of GGP and its effects on the workability of concrete, indicating that further research is required to provide precise recommendations for the ideal replacement amounts.

3.3. Setting Time

The setting time is an important factor that affects concrete workability, placing, and finishing. In order to maximize the curing process and ensure the required strength and longevity of the hardened concrete, it is crucial to comprehend the starting and final setting time frames.

Previous studies [85,90,91] have reported an increase in the initial setting time of concrete, as illustrated in Figure 10. This phenomenon is explained by GGP’s reduced water absorption properties, which raise the amount of free water in the concrete matrix and reduce the rate at which the OPC paste condenses [90]. However, a gradual increase in the final setting time was reported by Khan et al. [85] for the increasing replacement levels. Jiang et al. [90] reported a gradual increase in the initial setting time of concrete incorporated with increasing cement replacement levels by GGP. However, the final setting time exhibited a minimal increase for 30% and insignificant reduction for 10% and 20% cement replacement levels. The study concluded that GGP hindered the condensation of cement hydration products in the initial stage. However, GGP’s retarding effect diminished over time and became negligible for the final setting time. Similar results were reported by Aliabdo et al. [91] for 5–20% cement replacement levels by GGP. On the other hand, Wattanapornprom and Stitmannaithum [92] demonstrated a delay in the initial and final setting time of GGP-incorporated concrete. The initial setting time was decreased by 12% and 35%, while the final setting time was reduced by 14% and 32% for a replacement level of 10% and 20%, compared to the control mix. This effect is due to the finer particle size of GGP compared to cement, promoting an enhanced hydration process and thickness of paste [93,94,95].

Overall, the setting time increases with the addition of GGP in concrete as a partial cement replacement. However, in some cases, a reduction in setting time can be attributed to the finer particle size of GGP compared to cement.

4. Physical Properties

Density

Concrete density, which typically ranges from 2200 to 2500 kg/m³, depends on a number of variables, including the type and size of the aggregate, the cement content, and the air content. By adjusting the aggregate type, particle size distribution, cement concentration, and air content, concrete density can be tailored to improve durability and structural performance under particular environmental circumstances. Studies [83,85,88,96] have demonstrated a decrease in concrete density with increasing levels of cement replacement by GGP, as depicted in Figure 11. Ibrahim [83], Khan et al. [85], Gupta el al. [88], and Tamanna and Tuladhar [96] reported a decrease of approximately 3.3%, 1.4%, 2.6%, and 2.9%, respectively, in the concrete density with 20% cement replacement by GGP. This decrease is attributed to the lower specific gravity of GGP in comparison to cement and the development of micropores due to incomplete pozzolanic reactions. Meanwhile, several other studies [63,82,84] reported an initial increase in concrete density to a certain replacement level, followed by a subsequent decline as the replacement percentage is further increased, as illustrated in Figure 12. It is concluded that this phenomenon is due to an increased C-S-H gel created by the increased pozzolanic activity of GGP with Ca(OH)₂. Thus, the matrix is densified and strengthened as a whole by filling the interfacial transition zone (ITZ) and reducing the weakest regions. Up to a specific replacement level, the formation of C-S-H gel is enhanced by both primary hydration and the pozzolanic reaction. This helps to fill the voids, leading to the formation of a denser matrix microstructure and packing density. However, beyond a certain replacement level, the Ca(OH)₂ availability becomes insufficient to aid the pozzolanic reaction, resulting in a reduction in the density of concrete. Similarly, other studies demonstrate [86,97] an increase in concrete density with the incorporation of a 20% GGP replacement level. Ani et al. [86] and Yavuz et al. [97] reported an increase of approximately 5.3% and 19%, respectively, in the concrete density with 20% cement replacement by GGP.

In summary, these results demonstrate the varying relationship of GGP and its effect on the density of concrete, indicating that further research is required to provide precise recommendations for the ideal replacement amounts.

5. Mechanical Properties

5.1. Compressive Strength

The compressive strength of concrete, which indicates its capacity to sustain structural loads, is essential to the design and quality assurance of concrete infrastructures. Several studies [7,64,78,96,98,99,100,101,102] have reported that, in comparison to the control mix, the addition of GGP to concrete mix designs affects the compressive strength at different curing ages. The pozzolanic process that GGP starts inside the concrete matrix is largely responsible for this effect, which over time promotes improved strength and durability. The compressive strength results for various GGP replacement levels and curing durations are presented in Figure 13.

Muhedin and Ibrahim [98] examined the effect of the partial replacement of cement with GGP at a water-to-cement (W/C) ratio of 0.48 with different GGP replacement levels (5%, 10%, 15%, and 20%) to a control mix without GGP at 28, 56, and 90 days. The findings showed that for all evaluated curing ages, concrete containing GGP had greater compressive strength than the control mix up to a 15% replacement level. The compressive strength increased maximum at a 5% replacement level compared to the control mix with approximately 14%, 23%, and 18% for 28, 56, and 90 days, respectively. The main reasons for the 15% improvement in compressive strength when GGP is used in place of cement are the material’s pozzolanic activity and filler effect, which improve the microstructure and longevity of the concrete. Similar findings were reported by Ahmad et al. [7] at 0.53 W/C with GGP replaced at 0% to 30% levels. Regardless of the replacement levels, the GGP-modified mixes continuously showed higher compressive strength values than those of the control mix at later curing days, although the 7-day strengths were lower. The compressive strength was approximately 19% and 23% higher on 28 days and 56 days, respectively, for 20% replacement of cement by GGP. This resulted from the well-known fact that pozzolanic reactions are slow and do not provide strength at a young age [7]. On later curing stages (28 and 56 days), additional calcium silicate hydrate (C-S-H) gel is formed when the amorphous silica in GGP combines with Ca(OH)₂, produced by cement hydration. Thus, by strengthening the internal link structure, filling capillary holes, and densifying the cement matrix, this secondary C-S-H formation increases the GGP concrete compressive strength. Li et al. [64] explored GGP replacement levels from 0% to 25% at a 0.55 W/C ratio and reported that the 20% replacement level surpassed the compressive strength of the control mix by 5% at 90 days. This is due to the formation of optimized C-S-H gel at a 20% replacement level, facilitated by primary hydration and pozzolanic reaction leading to denser matrix in concrete. Similarly, Barkauskas et al. [99] studied replacement levels from 0% to 30% at a 0.46 W/C ratio, showing a relative increase in compressive strength at later curing days due to the pozzolanic reaction of GGP. Barkauskas et al. [99] reported that the 5% replacement level consistently surpassed the compressive strength of the control mix by approximately 8%, 13%, and 8% at 7, 28, and 56 days, respectively. However, Tamanna and Tuladhar [96] experimental results reported that none of the replacement levels exceeded the compressive strength of the control mix on 7, 28, and 56 days of curing at 0.535 W/C. The compressive strength of 10% replacement level was the highest among all and was 8%, 16%, and 2% lower than the control mix at 7, 28, and 56 days, respectively. In addition,

Table 5. Compressive strength of GGP-incorporated concrete from different literature.

Author	Mean Size	W/C Ratio	Replacement	Compressive Strength (MPa)					Optimum Replacement
	(µm)		Level (%)	7 D	14 D	28 D	56 D	90 D	Level and Curing Days
Gupta et al. [88]	≈75	0.45	5	↑	-	↓	-	-	35% at 28 days
			10	↑	-	↑	-	-
			15	↑	-	↑	-	-
			20	↑	-	↑	-	-
			25	↑	-	↑	-	-
			30	↑	-	↑	-	-
			35	↑	-	↓	-	-
Ikotun et al. [103]	≈75	0.47	5	↑	-	↓	↓	-	5% at 56 days
			15	↓	-	↓	↓	-
			25	↓	-	↓	↓	-
OAMA Qasem [104]	≈75	0.45	10	-	-	↓	-	-	25% at 28 days
			15	-	-	↑	-	-
			20	-	-	↑	-	-
			25	-	-	↑	-	-
			30	-	-	↑	-	-
Herki. B MA [105]	≈75	0.50	10	↓	-	↓	-	-	10% at 28 days
			20	↓	-	↓	-	-
Baikerikar et al. [106]	≈50	0.42	5	↑	-	↑	-	-	25% at 28 days
			10	↓	-	↓	-	-
			15	↓	-	↓	-	-
			20	↓	-	↓	-	-
			25	↓	-	↓	-	-
Zhu et al. [107]	≈40	0.46	10	↓	↓	↓	↓	↓	10% at 56 days
			20	↓	↓	↓	↓	↓
			30	↓	↓	↓	↓	↓
Naaamandadin et al. [108]	≈30	0.45	4	↑	-	↑	-	-	8% at 28 days
			8	↑	-	↑	-	-
			12	↓	-	↓	-	-
Moreira et al. [109]		0.40	50	↑	↑	↑	-	-	50% at 28 days
		0.35	50	↑	↑	↑	-	-	W/C = 0.4
Paul et al. [110]	≈20	0.40	10	↓	-	↓	-	↑	40% at 90 days
			20	↓	-	↓	-	↑
			30	↑	-	↑	-	↑
			40	↓	-	↓	-	↓
Chen et al. [111]	≈20	0.50	5	↓	↓	↓	-	↓	10% at 56 days
			10	↓	↓	↓	-	↓
			15	↓	↓	↓	-	↓
Paul et al. [110]	≈8	0.48	10	↓	-	↓	-	↑	30% at 90 days
			20	↑	-	↑	-	↑
			30	↑	-	↑	-	↑
			40	↑	-	↓	-	↑

Note: “↑” increase, “↓” decrease, “-” data not available.

shows the summary of the compressive strength development with GGP-incorporated concrete and optimum GGP replacement level.

The particle size of GGP at the same cement replacement level has shown a considerable effect on the compressive strength of the concrete incorporated with GGP in previous studies [100,101]. Letelier et al. [100] studied the effect of three different particle sizes (74 µm, 45 µm, and 38 µm) of GGP on the compressive strength of concrete incorporated with GGP as a partial replacement of cement, as shown in Figure 14. The result showed that the compressive strength of GGP-incorporated concrete increases as the particle size of the GGP becomes finer. At 14, 28, and 90 days of curing, the GGP, which had a particle size of 38 µm, exceeded the control mix’s compressive strength by approximately 7%, 19%, and 30%, respectively, at a 20% replacement level. This suggests that GGP can be used as a replacement of traditional pozzolans when subjected to prolonged curing durations. The finer particle size, which raises the surface area accessible for pozzolanic reactions, is responsible for the improved performance. Because there is more contact between the reactive particles and the accessible Ca(OH)₂ in the mix, a larger surface area promotes a faster rate of reaction [112]. Tamanna et al. [101] reported similar findings on the effect of 0–40% cement replacement levels with various GGP sizes (75 µm, 75–38 µm, <38 µm) on the compressive strength of concrete at a 0.45 W/C ratio, as illustrated in Figure 15. The compressive strength of 10% GGP-incorporated concrete with a particle size < 38 µm shows comparable strength to that of the control mix at 28 days of curing. Furthermore, for particle size < 38 µm and 10% replacement level, the compressive strength was reported to be approximately 14% and 25% higher than that of GGP with particle sizes 75–38 µm and 75 µm, respectively.

In conclusion, the incorporation of GGP as a partial replacement shows positive results on the compressive strength of concrete at later curing days due to microstructural densification. While some studies initially reported a decrease in strength, longer curing days resulted in increased concrete strength compared to the control mix. Additionally, finer GGP has higher pozzolanic reactivity and increases the compressive strength in concrete.

5.2. Strength Activity Index (SAI)

The strength activity index (SAI) serves as an important parameter that quantifies the pozzolanic reactivity of supplementary cementitious materials by measuring their influence on the compressive strength development when used as a partial replacement for cement in concrete. It is the ratio of a concrete mix’s compressive strength with a certain amount of SCM substituted to that of a control mix that does not contain SCM on identical testing and curing conditions. According to ASTM [113,114], any GGP to be used as pozzolan at 7 and 28 days must have an SAI value of at least 75%. However, ASTM-C1866 [71] requires GGP-incorporated concrete to have a minimum of 85% SAI at 28 days.

Kalakada et al. [2] evaluated SAI at three curing ages—7 days, 28 days, and 91 days—with W/C ratios of 0.50, 0.45, 0.42, and 0.39 and reported higher SAI values at higher curing days and lower W/C ratios, as illustrated in Figure 16. This could be attributed to the combined effects of maximum pozzolanic activity on later curing days and concurrent water reduction, which occur simultaneously [2]. Notably, all replacement levels at all curing days achieved a minimum SAI of 75% in accordance with ASTM-C311 [113] and ASTM-C618 [114], with the exception of a mix containing 30% GGP. The lower SAI value for the 30% replacement level is due to insufficient availability of Ca(OH)₂ from primary cement hydration. This unavailability of Ca(OH)₂ to react with silica from GGP restricts the pozzolanic reaction and the formation of additional C-S-H gel, which is essential for obtaining a higher SAI value. Furthermore, it should be noted that GGP-incorporated mixes exhibit a decrease in the amount of initial C-S-H gel from the primary hydration process in comparison to the control mix. As a result, the pozzolanic reaction’s extra C-S-H gel is not enough to overcome this deficit. Thus, the total C-S-H gel is lower than that of the control mix and the required SAI value is not satisfied. The GGP with a W/C ratio of 0.39 continuously satisfied the necessary 85% SAI value for ASTM C1866 during the course of all curing phases. Similar results were reported by Afshinnia and Rangaraju [115], with GGP particle sizes of 17 µm and 70 µm; Shao et al. [56], with sizes of 38 µm, 75 µm, and 150 µm; and Shi et al. [116], with approximately 12 µm, 20 µm, 75 µm, and 110 µm. Afshinnia and Rangaraju [115] replaced 5%, 10%, 15%, and 20% of cement with GGP and reported that SAI values decreased with increasing replacement levels during the initial days. However, at 56 curing days, the compressive strength surpassed the control mix up to 15% replacement levels, exhibiting higher SAI values. On the other hand, coarser GGP consistently showed lower SAI values at all curing days, demonstrating reduced pozzolanic reactivity from larger particle sizes. Similarly, Patel et al. [117] demonstrated that GGP, with a mean particle size of approximately 1 µm and up to 20% replacement at a 0.4 W/C ratio, exhibits SAI values exceeding 100% at all curing days, as illustrated in Figure 17a. The 20% replacement level shows the highest SAI values, with approximately 109%, 116%, and 127% at 7, 28, and 90 days of curing period, respectively. This is due to the formation of optimized C-S-H gel from both primary hydration and pozzolanic reaction. Furthermore, Banerji et al. [102] also reported that all mixes exceeded the ASTM C1866 requirement of 85% SAI at both 7 and 28 days, as depicted in Figure 17b, highlighting its potential as an SCM in concrete.

In conclusion, SAI is a key indicator of the pozzolanic reactivity of GGP when used as a partial cement replacement in concrete. Studies indicate that GGP often satisfies or exceeds the necessary SAI values at longer curing days and lower W/C ratios due to enhanced pozzolanic activity.

5.3. Tensile Strength

Concrete’s ability to withstand brittle failure under tensile loads is evaluated by measuring its split tensile strength, which is around 10% of its compressive strength. Concrete’s low tensile strength and high brittleness make it susceptible to environmental degradation [118]. The split tensile test helps to determine how easily concrete may crack, which is important for the longevity and integrity of the structures [119]. The split tensile strength of GGP-incorporated concrete reported in recent studies is depicted in Figure 18.

Muhedin and Ibrahim [98] studied the effect of partial cement replacement with GGP at a 0.48 W/C ratio, evaluating tensile strength development over 7, 28, 56, and 90 days for mixes with GGP replacement levels of 0–30%. The highest peak was reported for a 5% replacement level of cement with GGP across all curing durations. This can be attributed to the pozzolanic activity of GGP, which enhances strength development during the secondary hydration phase. Beyond a 5% replacement level, the strength of GGP concrete was lower than that of the control mix. Another study by Ahmad et al. [7] at 0.6 W/C for 0–30% cement replacement level with a particle size lower than 38 µm at 7, 28, and 56 days reported higher tensile strength for GGP-incorporated mixes at 28 and 56 days. The tensile strength was approximately 15% and 22% at 28 and 56 days, respectively, for the 20% replacement level. However, at 7 days, the tensile strength was lower than the control mix for all the GGP-replaced concrete. This is because the pozzolanic reaction between the silica from GGP and the Ca(OH)₂ generated during cement hydration is largely responsible for the improvement in tensile strength at later curing ages. Over time, this reaction produces more calcium silicate hydrate (C-S-H) gel, which greatly increases the concrete matrix’s density and strength. Additionally, it was reported that the concrete with 30% GGP substitution level had approximately 42%, 9%, and 3% lower compressive strength than the control mix at 7, 28, and 56 days. This decrease is explained by the dilution effect, which occurs when a large amount of cement is substituted, lowering the amount of calcium hydroxide available and the total amount of binder. As a result, the pozzolanic reaction’s scope is constrained, which inhibits the growth in strength. Similar results were obtained by Yuvaz et al. [97] on the utilization of 40 µm GGP at 0.35 W/C. The GGP added as a partial cement replacement at 5%, 10%, 15%, and 20% in the mix, resulted in 7-day tensile strength values 7.7%, 15.4%, 16.2%, and 23.1% lower than the control mix, respectively. At 28 days, however, the tensile strength values were 7.5%, 14.3%, 14.9%, and 12.4% higher than the control mix, respectively. The increases were 5.7%, 9.3%, 6.7%, and 4.1% after 120 days, indicating that GGP had a beneficial long-term impact on tensile strength. The full hydration of cementitious materials, which promotes the development of calcium silicate hydrate (C-S-H) gel, is also responsible for the strength increase at later curing ages. Similar results were found in pervious concrete by Li et al. [120]. Higher tensile strength at later curing ages was reported with replacement levels ranging from 0 to 25% and W/C ratios of 0.22, 0.24, 0.26, and 0.28. The splitting tensile strength peaked at 20% GGP replacement after first declining at 28 days and then progressively increasing over the next 56 days. In contrast, in comparison to the control mix, GGP-incorporated concrete showed greater tensile strength up to a 15% replacement level after seven days, according to Abdulazeez et al. [82]. GGP concrete tensile strength at 14 days was on par with the control mix. Nevertheless, by 21 days, the tensile strength of the concrete containing GGP was either the same as or greater than that of the control mix, indicating the positive impact of GGP on the development of later-age strength.

In conclusion, concrete split tensile strength incorporated with GGP as a partial cement replacement increases, especially at lower replacement levels (5–20%) and longer curing days. Overall, the strength surpasses the control mix over time, although the initial tensile strength may decline due to slower pozzolanic reactions.

5.4. Flexural Strength

Concrete flexural strength, or modulus of rupture, is a crucial factor that indicates how resistant it is to bending stresses. It gives information on the material’s resistance to tensile forces brought on by bending, which makes it a crucial characteristic for designing structural components like pavements, slabs, and beams that are subjected to flexural stresses. The split tensile strength of GGP-incorporated concrete from recent studies is illustrated in Figure 19.

Yuvaz et al. [97] studied prismatic molds incorporated with GGP with dimensions of 100 × 100 × 400 mm under three-point loading for flexural strength. The findings reported that the control mix had approximately 5.3%, 6.2%, 5.7%, and 8.1% higher 7-day flexural strength than the GGP-incorporated samples of 5%, 10%, 15%, and 20%, respectively. However, the flexural strength of the samples with GGP added exceeded that of the control mix at 28 and 120 days. For 5%, 10%, 15%, and 20% replacement levels, at 120 days, the respective flexural strength reported was approximately 4.9%, 12.5%, 7.6%, and 6.1% higher than the control mix. The pozzolanic reactivity of GGP, which promotes the production of more C-S-H gel and gradually improves the concrete flexural qualities, is responsible for this improvement at later curing ages. Li et al. [120] observed similar results for pervious concrete that included GGP. The three-point loading method was used to measure the flexural strength of samples measuring 100 × 100 × 400 mm. At 56 and 112 days of curing, it was reported that concrete with 10% GGP had approximately 0.9% and 2.1%, while 20% GGP had approximately 2.8% and 11.9% higher flexural strength than the control mix, respectively. Significant amounts of SiO₂ and Al₂O₃ from GGP react with Ca(OH)₂ during the prolonged curing period, forming C-S-H and C-A-H gels, which is responsible for this improvement. A denser matrix with lower permeability is the outcome of the microstructure being refined by these hydration products [120].

Similarly, Khan et al. [85] studied flexural strength for a 152.4 mm wide × 152.4 mm deep × 762 mm long beam incorporated with GGP. The study reported an increase in flexural strength at later ages of 56 and 84 days. While a notable improvement was noted at 56 days by 12.6%, the strength gain for 25% replacement was around 2% greater than that of regular concrete after 84 days. At 28, 56, and 84 days, approximately 28%, 20%, and 27% lower flexural strength was demonstrated by beams with a 30% replacement level, respectively, which is explained by the primary hydration process producing less Ca(OH)₂. This restricted Ca(OH)₂ availability inhibits the SiO₂ pozzolanic process to form strength-imbibing C-S-H gel. Similar results were reported by Abdulazeez et al. [82] for the flexural strength of GGP-incorporated concrete. The study, conducted on beams measuring 150 mm × 150 mm × 750 mm, showed comparable trends, highlighting the influence of GGP replacement levels on the flexural performance of concrete. The study [82] reported an increase of 2.3% and 1.5% in flexural strength for 10% replacement level at 7 and 28 days, respectively. Ahmad et al. [7] examined 150 mm × 150 mm × 750 mm GGP-incorporated beam samples (#4 longitudinal reinforcement and #2@125 mm c/c stirrup) subjected to four-point loading. The results demonstrated that glass substitution could result in a 20% increase in flexural strength at 28 and 56 days. However, the strength started to decline after a 20% replacement level. Additionally, the compaction process was difficult due to decreased flowability at higher waste glass dosage (30% by weight of cement). This required more compaction, which caused pores to form in the cured concrete and ultimately decreased its flexural strength.

Previous studies [65,89] reported that partial cement replacement by GGP has an influence on the ductility of concrete beams. Christiansen and Dymond [65] examined the effects of different cement replacement levels (10%, 20%, and 30%) at 0.46 W/C on the ductility of a reinforced concrete beam measuring 15.24 cm × 25.4 cm × 1.83 m. Although the peak flexural strength variation was insignificant, the study reported that the ductility of the beam gradually decreased as the cement replacement percentage increased, as depicted in Figure 20. This shows how higher replacement levels may affect the performance of concrete structures in terms of their capacity to experience plastic deformation prior to failure. Ahmad et al. (2022) [89] conducted a study on the effect of GGP incorporation in concrete beams. The findings showed that at a 20% GGP replacement level, the ductility reached its maximum value. But, after this point, the ductility of the beams decreased as the replacement levels increased further.

In summary, due to pozzolanic reaction and the formation of additional C-S-H gel, GGP-incorporated concrete improves the flexural strength at later curing days. A considerable increase in flexural strength is reported at 28 days and beyond, particularly at lower replacement levels (5–20%), even though GGP decreases flexural strength at initial curing days.

5.5. Modulus of Elasticity

The modulus of elasticity (ME) is an important factor that indicates the concrete’s capacity to resist deformation under applied stress. Several studies [121,122,123] have been conducted to study the effect of incorporating GGP as a partial replacement for cement in the elasticity modulus of concrete, as illustrated in Figure 21.

Yassen et al. [121] studied the modulus of elasticity of concrete incorporating GGP for 10%, 15%, 20%, and 25% cement replacement levels by GGP with 0.32 W/C. The study reported an increase of 4.42%, 3.28%, −0.52%, and 1.4% in the ME for 10%, 15%, 20%, and 25% GGP replacement levels, respectively, compared to the control mix. Similarly, Mohammed and Hama [122] evaluated 15% replacement of cement by GGP with 0.32 W/C to analyze the elastic modulus of concrete. The study reported insignificant variation in elastic modulus with the incorporation of GGP as a partial replacement to cement. Furthermore, Omar and Saeed [123] investigated the effect of GGP particle size, W/C, and replacement levels on the modulus of elasticity of GGP-incorporated concrete. The study examined two GGP particle ranges (135–55 µm and ≤55 µm) at 0.49 and 0.57 W/C for 5%, 10%, 15%, and 20% cement replacement levels. The findings reported a slight reduction in the elastic modulus up to 15% replacement level regardless of W/C ratio. However, at 20% replacement, the modulus of elasticity decreased significantly by approximately 10%. This could be due to the reduction in the formation of optimized C-S-H gel, influencing the strength and stiffness of the concrete matrix. Furthermore, lower W/C in GGP-incorporated showed a higher elastic modulus than higher W/C concrete.

In conclusion, the incorporation of GGP as a partial replacement of cement has little effect on the modulus of elasticity of concrete at low replacement levels.

5.6. Punching Strength of Concrete

The punching strength of concrete is a critical factor in the structural integrity of slabs and foundations, particularly under concentrated loads. Limited studies have been performed on GGP as a partial replacement to the cement in concrete, indicating a need for further research to understand its full potential and performance characteristics. Ahmad et al. [89] performed a punching shear test on a GGP-incorporated concrete slab specimen of 500 mm × 500 mm at 7, 28, and 56 days. The authors replaced 10% and 20% cement with GGP at 0.467 W/C. They reported that the concrete punching strength increased up to 20% replacement level; after that, it fell due to more voids as a result of low workability as illustrated in Figure 22. At 56 days, the punching strength exhibited by 20% GGP replacement level was observed to be approximately 13% higher than the control mix due to microfillings from secondary hydration in the concrete matrix. A 20% waste glass substitution produced the best punching strength at 56 days, whereas 0% substitution produced the lowest strength for all curing days.

In summary, the punching strength of GGP-incorporated concrete is highest at a 20% replacement level. However, higher replacement levels show reduced workability and increased voids, ultimately decreasing the strength.

6. Life Cycle Assessment (LCA)

LCA is an essential tool to assess the comprehensive environmental impact of GGP as an SCM in the concrete industry. There are several methods to evaluate the LCA of a GGP depending on the specific field of study and defined system boundaries [124]. Studies [37,125,126] have demonstrated the environmental benefits of the utilization of GGP as a partial replacement to the cement.

Deschamps et al. [125] studied LCA on a 2 m wide by 1 m long pedestrian sidewalk for a service life of 40 years incorporating 20% cement replacement by GGP. They reported a 20% decrease in CO₂eq emissions and approximately 20% reduction in non-renewable energy consumption for concrete containing GGP compared to that of conventional concrete. Similarly, Jiang et al. [126] performed an LCA on 1 m³ concrete/mortar, incorporating GGP as a partial replacement for cement, exhibiting similar 28-day compressive strength and durability. The study reported a 19% reduction in CO₂eq emissions and a 14% decrease in energy consumption for 35 MPa GGP-incorporated concrete compared to conventional concrete. Additionally, Guignone et al. [37] studied LCA for 1 m³ prestressed concrete hollow-core slabs with 10% and 20% cement replacement by GGP for material fabrication, construction, and a 100-year maintenance period. They reported that for a maintenance horizon of 100 years, the environmental impact related to maintenance and repairs could be decreased by 50%. The study also revealed that the utilization of waste glass could reduce the manufacture of 2000 tons of type CEM III/A 42.5 cement by decreasing 972 tons of clinker in concrete production. Additionally, the study also showed that the environmental gain in global warming potential can reach 1.23 kg CO₂eq emissions for every Kg of waste glass to be used as a pozzolanic material.

In conclusion, the utilization of GGP as a partial cement replacement in concrete highlights substantial environmental benefits in terms of CO₂ equivalent emissions and energy consumption. These studies underscore effective waste glass management for different concrete applications.

7. Field Application of GGP

GGP-incorporated concrete has been applied in different civil infrastructures, including buildings, pavements, and bridges, demonstrating its feasibility as an SCM in the concrete industry [4,19,37,127,128,129,130,131].

Two 110 m² and 120 m² pedestrian sidewalks for a 20% replacement level (G20) and 40% replacement level (G40) of cement by GGP with 0.34 W/C were built in Jamaica, Queens, NY, USA by NYC-DDC on 5 May 2016 [4]. The compressive strength of field samples were approximately 13% lower than the prototype lab mixes. However, field samples demonstrated a strength gain over the period of time similar to that of the laboratory mix. Additionally, G40 concrete exhibited a lighter color compared to G20. Similarly, 1.5 × 2.5 × 0.25 m slabs with 20% and 30% replacement levels at a 0.486 W/C were cast for a car park area in Australia on September 2002 [19]. The results indicated that both GGP replacement levels attained a target strength of 35 MPa at 90 days of curing. Similarly, GGP was utilized in rigid pavement, sidewalks, curbs, and bike parking lots at Michigan State University’s (MSU) campus, USA [129]. Cement was partially replaced by 20% GGP at a W/C ratio of 0.46 and 0.38. When compared to the control mix, GGP-incorporated, lower W/C concrete mixes exhibited higher compressive strength. The field concrete cores had a comparable compressive strength to that of lab cured specimens at 156 days. Additionally, after three years of operation, GGP-incorporated concrete showed no physical deterioration or material failure.

Interior and exterior slabs and structural wall elements were constructed in Montreal, Canada with 20% cement replacement by GGP at different W/C ratios over different time frames [128]. All the mixes attained the target strength of 32 MPa at 91 curing days. Two footbridges were constructed using ultra-high-performance glass concrete (UHPGC) at the University of Sherbrooke main campus, Quebec, Canada [131]. In terms of strength and durability quality, UHPGC concrete was comparable to or higher than traditional ultra-high-performance concrete (UHPC). This enabled reducing the concrete volume by around 60% compared to conventional concrete.

8. Conclusions

This study explores the effect of using GGP as a sustainable material that serves as a supplementary cementitious material in concrete production. As GGP reduces the amount of cement required in concrete, it minimizes the possible environmental impact caused during cement production. In addition to environmental benefits, the use of GGP is also beneficial in enhancing different concrete properties due to its higher silica content. This study focuses on the effect of fresh and mechanical properties of concrete produced by using GGP as a partial cement replacement. The reviewed concrete properties include rheology, workability, density, setting time, compressive strength, tensile strength, flexural strength, modulus of elasticity, and punching strength. The major findings are listed below:

• The concrete rheological properties change with GGP incorporation as a partial cement replacement, enhancing the flowability and viscosity while reducing segregation.
• The slump value increases with a higher amount of GGP in some cases, while in other cases the slump value decreases. The increase in the slump occurs as a result of lower water absorption and reduced internal friction from the presence of GGP. However, the decrease in slump results from the angular shape of GGP particles with a high surface area, which requires more cement paste for coating. Further research is recommended to clarify these varying effects on the slump.
• An increase in the GGP content decreases the concrete density. This occurs due to the low specific gravity of GGP and incomplete pozzolanic reactions. However, other findings show an increase in concrete density due to the densification of the matrix by filling ITZ with an additional C-S-H gel. Further research is recommended to gain a deeper understanding of these contrasting results.
• The addition of GGP in concrete increases the initial setting time. This occurs because the glass particles of GGP reduce water absorption, increasing the amount of free water in the mix and delaying the initial setting time. However, some studies reported a decrease in the final setting time while others reported an increase. Further research can be performed to better understand these differences in results in a final setting time.
• The addition of GGP improves the compressive strength at later curing ages due to the pozzolanic reaction forming an additional C-S-H gel, while at early curing stages, the compressive strength remains lower compared to the control mix. Furthermore, a finer GGP particle size results in a higher compressive strength.
• The tensile strength and flexural strength of GGP-incorporated concrete improve at later curing days due to additional pozzolanic reactions and the formation of a denser concrete matrix. However, ductility decreases with an increase in the amount of GGP.
• The punching strength is enhanced due to the addition of GGP because of microfillings from secondary hydration in the concrete matrix.
• The variation in the modulus of elasticity of the GGP-incorporated concrete is minimal at lower replacement levels (≤15%). However, due to the formation of less C-S-H gel, stiffness decreases at higher replacement levels (≥20%).
• The optimum dosage of GGP depends upon different factors such as mix design; particle size; specific surface area; and glass chemical composition, type, and color.
• LCA studies demonstrate a significant reduction in global warming potential and associated energy consumption, contributing to the sustainability of GGP incorporation in concrete.
• Field application in pavements, buildings, and bridges confirms the feasibility of GGP incorporation as a partial cement replacement in the construction industry.

In conclusion, the review summarizes the impact on various concrete properties due to the addition of GGP as a partial replacement of cement. In the majority of instances, incorporation can not only contribute to sustainable construction practices but also improve the mechanical and fresh concrete properties. Although most of the literature indicates that GGP enhances these qualities, others show the contrary. These discrepancies are attributed to the complex interaction between GGP and concrete properties. In this regard, this review suggests that further research is recommended to determine the reliable correlation between GGP replacement and concrete properties.

9. Future Research Prospects

This review demonstrates a positive effect on the fresh and mechanical properties of concrete with the incorporation of GGP as a partial replacement of cement. The following areas could be further explored in the future for providing more complete insights on the application of GGP in the concrete industry:

• Further research should be focused on a detailed investigation of the key parameters such as W/C ratio and GGP particle size, type, color, chemical composition, and replacement percentage to quantify their inter-dependencies. Through experimental research, a strong correlation between these parameters could be established, allowing for the optimization of GGP-incorporated concrete.
• The application of GGP-incorporated concrete in load-bearing components including beams, columns, and foundations should be studied. Such studies provide insights into the field application of GGP-incorporated concrete and contribute to sustainable building practices.
• The thermal response of GGP-incorporated concrete should be studied to better understand the behavior of glass pozzolan under high-temperature conditions.
• The application of machine learning models could be utilized to optimize the mix design based on the currently available experimental data. This could significantly reduce the utilization of raw materials and the need for prolonged curing periods, enhancing resource-efficient concrete production.