*Lightweight Aggregate with Inclusion of Geopolymer*

The inclusion of geopolymer in lightweight aggregate has become more concerning due to its advantages in improving the properties of lightweight aggregate. The strength of activated fly ash-based artificial lightweight aggregate by inclusion of geopolymer is comparable to that of commercialized expanded clay lightweight aggregate [22]. In addition, the inclusion of geopolymer in lightweight aggregate produced from fluidized bed combustion (FBC) fly ashes and mine tailings showed excellent mechanical properties in mortar and concrete as compared to the application of commercialized aggregate (LECA) [28]. The inclusion of geopolymer in the lightweight aggregate manufactured from recycled silt and palm oil fuel ash meets the demand for high-strength lightweight concrete and can be utilized for lightweight construction or insulating concrete [23]. In addition, the inclusion of geopolymer in lightweight aggregate that is produced from the combination

of fly ash and silica fume can be used for heavy-duty floors due to its high strength [29]. Therefore, the inclusion of geopolymer in lightweight aggregate brings advantages in the construction field, especially in the structural components. However, the use of geopolymer in lightweight aggregate is still limited, and more research is needed to identify lightweight aggregate properties.

#### **4. Manufacturing of Lightweight Aggregate**

The manufacturing process of artificial aggregate consists of three stages, which are the mixing of raw materials, pelletization, and hardening. In the first stage, which is mixing, the well-proportioned ingredients are mixed until the mixture achieves consistency. In a disc-based pelletizer machine, the mixture of the raw materials undergoes the pelletization process by the agglomeration of the fine particles using a suitable binder. Some previous studies used pozzolanic materials as binders, such as metakaolin and bentonite [5,26,27]. Meanwhile, alkaline activators are commonly used as binders for the production of geopolymer aggregate [18,27–29]. Depending on the angle of the disc, the speed of the pelletizer, and the moisture content, the appropriate size of pellets will be collected in the disc. The hardening of the fresh pellets can be accomplished by using sintering, cold bonding, or autoclaving in order to gain the strength of the aggregate. The flow chart of the production of lightweight aggregates can be illustrated as in Figure 1. Meanwhile, the current research on lightweight aggregate is summarized as in Table 1.

**Figure 1.** Flow chart of producing lightweight aggregate.




**Table 1.** *Cont.*

In the study of Punlert et al. (2017) [39], lightweight concrete was manufactured using fly ash lightweight aggregates instead of coarse aggregates, resulting in a much lower density and good strength compared to conventional concrete. Furthermore, when sintered at around 1100 ◦C, lightweight aggregate made from sewage sludge and river sediment achieved high density, low water absorption, and high strength. However, the existence of air voids in fly ash lightweight aggregates, which are crucial for absorbency, leads to difficulty in producing lightweight aggregate concrete, especially in the mix design, which requires further work to enhance the properties [40]. For this purpose, additional binders or additives are introduced as one of the alternatives towards improving the properties of lightweight aggregate.

From the previous results, the salt additives (NaCl) resulted in less viscosity and produced wider internal pores, which allowed the production of ultralight aggregates. However, the use of Na2CO3 as an additive, which is low cost and low corrosion hazard, allows the creation of ultra-lightweight aggregates [19]. In the study reported by Ren et al. [31], addition of coke particles in the manufacturing of lightweight aggregate will help to reduce the apparent density of the aggregate produced. The use of styrene butadiene rubber (SBR) improves the microstructure of lightweight aggregate, thus improving the aggregate's mechanical properties [35]. In addition, the inclusion of waste glass powder causes the pozzolanic material to inflate, resulting in a more efficient lightweight aggregate by enhancing the porosity of lightweight aggregate and decreasing water absorption [30].

In addition, pozzolanic materials with high SiO2, Al2O3, and CaO content have a high potential to be utilized in producing artificial aggregates with the addition of an alkaline activator [23]. The usage of alkaline activator as an additive for pozzolanic materials will help to influence the formation of C-S-H binding gel and sodium aluminosilicate hydrates during the geopolymerization process [41]. The formation of geopolymer aggregate by mixing pozzolanic material with alkaline activator will decrease the porosity of aggregate and improve the strength due to the extra C-S-H and calcium reaction during the reaction process alongside the denser microstructure produced [33]. Furthermore, the NaOH molarity will affect the strength of the geopolymer aggregate. For instance, low sodium hydroxide content leads to improper dissolution of fly ash, thus causing the inter-particle spaces of the participating gels to not be entirely filled [35]. It is critical to investigate the optimization of mix designation for each kind of material utilized in the manufacturing of lightweight aggregate-based geopolymer.

To summarize, recent research has shown that fly ash is the common material that had been chosen to produce the lightweight aggregate due to their excellent properties. In the future, alternative pozzolanic materials should be studied to establish their suitability in the manufacturing of lightweight aggregate. In addition, it showed that additional additives will improve the properties of lightweight aggregate in terms of specific gravity, strength, and water absorption. Furthermore, the use of geopolymer in lightweight aggregate has been shown to increase the porosity and strength of the aggregate, making it a viable option for lightweight aggregate manufacturing. According to Table 1, various methods can be used to manufacture lightweight aggregate, which are sintering, cold bonding, and autoclaving, which have been reported previously. The sintering and cold bonding methods are the methods that have been used wisely due to the excellent properties of aggregate. Despite the fact that the sintering technique consumes a lot of energy, the quality of the lightweight aggregate generated is excellent, with good strength at a low density. Because of the considerable energy required in the sintering procedure, cold bonding has grown popular because it does not require any additional sintering or heating. This process can create various grade aggregates, depending on the density of the aggregate produced. Generally, aggregate with high density will have a high strength. As a result, the use of a foaming agent may be necessary to make the aggregate lighter. There is relatively limited research on autoclaving methods in the current literature, necessitating more inquiry to assess the possible application of autoclaving methods in the manufacturing of lightweight aggregate. The strength and water absorption capabilities of lightweight aggregate generated by the autoclaving technique were good, and the incorporation of geopolymer improved the properties. As a result, greater research into autoclaving procedures is needed, particularly in terms of simplifying the process so that it can be commercialized.
