**7. Utilization of Waste Glass Powder in 3D Printing and Geopolimerization** *7.1. 3D Printing*

Three-dimensional (3D) printing of CBMs, also referred to as additive manufacturing in the construction sector, is the process of combining CBM extrusion in layers with robotic motion control [122]. Interest in 3D printing technology has increased significantly over the last few decades, both in academia and industry [123]. This is largely because it eliminates the need for formwork and human interference due to automation, which significantly decreases the time required to construct a structure. Additionally, because this technology is capable of creating complex structures, it enables structural optimization [124]. Mostly, fine aggregates were used in the 3D printing of CBMs [125]. The aggregate size is constrained by the material delivery system. The most frequently used material in 3D printing is natural sand [122]. By substituting recycled WG for natural sand in the material, a new market for WG can be created while also reducing demand for natural sand, a finite resource. With the rapid growth in the popularity of 3D printing technology, the widespread use of recycled WG as a raw material for such applications has the potential to be very beneficial. As a result, the WG recycling rate can be increased, thereby reducing the need for landfill space. Ting et al. [126] evaluated the properties of natural sand and recycled WG aggregate mortar 3D printing applications. The mixes were subjected to rheological and mechanical characterizations to compare the various aggregates' effects. The rheology results demonstrated the benefit of recycling WG in CBMs for 3D printing. The fact that the fresh material has a lower plastic viscosity and dynamic yield stress show that the recycled WG mix has better flow characteristics than the natural sand mix. This was most likely due to excess water in the recycled WG mix because of the WG particles' reduced capacity of water absorption than the natural sand, as observed in another study [114]. Other possible explanations include the smooth surface of recycled WG particles related to the natural sand particles [127]. However, the CS, STS, and flexural strength of recycled WG mixes were significantly less than those of natural sand mixes. Numerous publications have attributed these findings to the low adhesive strength amongst WG particles and the surrounding matrix at ITZ [48,114].

#### *7.2. Geopolimerization*

Cement is the primary constituent of concrete, acting as a binder for the aggregate particles contained in any concrete mixture. However, producing cement consumes a significant amount of energy and CO2 emission. To address this issue, most studies and research have focused on developing a more environmentally friendly alternative binder. Geopolymer is a suitable alternative to ordinary concrete. Geopolymer is a novel binder that is currently being developed. This is a type of binder that was developed to replace cement in the manufacture of concrete. The objective is to develop sustainable and environmentally friendly concrete that does not contain cement as a binder. The geopolymer contains a binder that is rich in silica and alumina. WG is an amorphous material, and its chemical composition has been listed in Table 6. Recently, WG powder has been investigated as a potential source of alumina for geopolymer production [128].

Geopolymers derived from WG are a relatively new field of study. According to a study, WG-based geopolymer achieved comparable CS to fly ash-based geopolymer, and the mechanical properties were highly dependent on the WG particle size, curing conditions, and alkali solution concentration [128]. Novais et al. [129] conducted a study to partially replace metakaolin with WG powder in the production of geopolymer and studied its influence on the mechanical properties. The results indicated that adding 12.5% WG increased CS by nearly 46%, whereas a further increase in the amount of WG has the opposite effect when compared to the pure metakaolin-based geopolymer. Additionally, the results demonstrated the enormous influence of curing conditions on the CS of geopolymers containing WG. The CS loss was reduced considerably when geopolymers with a high WG content were cured at ambient conditions rather than in sealed bags. This allows for the addition of up to 37.5% WG without compromising CS.

#### **8. Conclusions and Future Recommendations**

#### *8.1. Conclusions*

In this study, a scientometric review was performed on the utilization of waste glass (WG) in concrete for sustainable construction, along with a comprehensive discussion. Scientometric analysis was carried out to evaluate the relevant study fields, publication trend of articles, most contributing sources, keywords co-occurrence, most cited articles and authors, and active countries contributing to the field of WG utilization in concrete for sustainable construction. Moreover, the sustainable aspects of WG utilization in construction materials were reviewed, and the influence of WG on the performance of cement-based materials (CBMs) was assessed. Particularly, the effect of WG on workability, compressive strength, split-tensile strength, flexural strength, microstructure, and durability of the resulting composite was evaluated. The following conclusions have been made:


reacting with glass. By substituting coarser WG for natural aggregates, the mechanical strength may be reduced. The influence of WG depends on both particle size and the proportion of the replacement.

• The addition of WG can help improve the microstructure and reduce the permeability of CBMs, thereby increasing their resistance to sulphate attack and freeze-thaw and eventually enhancing the durability of composites. However, glass may impair the resistance to carbonation because of Ca(OH)2 consumption by glass.

Appropriate WG selection is critical to the success of applications. It is suggested that the amount, size, and type of WG used in CBMs are appropriate for achieving adequate MPs and durability, reliant on the anticipated applications.

#### *8.2. Future Recommendations*

This review demonstrated the importance of developing a complete knowledge of the impacts of WG on the MPs of CBMs in order to ensure the long-term viability and durability of structures. The subsequent research needs can be found as a result of the foregoing discussions:


**Author Contributions:** D.Q.: conceptualization, methodology, investigation, formal analysis, visualization, and writing-original draft preparation. Y.H.: conceptualization, methodology, investigation formal analysis, writing-reviewing and editing, and supervision. X.L.: supervision, resources, funding acquisition, and writing-reviewing and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** Jilin Provincial Department of Science and Technology Project (20180201027SF).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was sponsored in part by Jilin Provincial Department of Science and Technology Project (20180201027SF).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

