*5.3. Mechanical Properties*

In terms of mechanical properties, fly ash with expanded perlite powder (EPP) has a higher crushing strength due to increasing pozzolanic activity [53]. Sintered sediment lightweight aggregate with a bulk density of 859 kg/m<sup>3</sup> had the highest crushing strength of 13.4 MPa. This occurrence proves that the aggregate strength increases with increasing bulk density [47]. The crushing strength decreased as the silt content increased due to the binding failure of palm oil fuel ash (POFA) with the silt content [8]. Meanwhile, the fly-ash metakaolin binder aggregate showed high crushing strength when curing under high temperatures [59]. For instance, when the sintering temperature exceeds 900 ◦C, the sintered aggregate made up of metakaolin and alkaline activator has high aggregate impact value, which cause decreasing aggregate strength due to the increasing amounts of pore space in the aggregate [67]. Moreover, combination of fly ash and clay with 10% of sodium carbonate and sintering temperature of 1220 ◦C leads to a higher pellet strength of 4.25 MPa [31].

In addition, regardless of methods applied, the cold-bonded fly ash aggregate, sintered fly ash aggregate, as well as autoclaved aggregates were found to have a high impact value of 9.56%, 10.2%, and 11.46%, respectively [71]. According to research of Kamal and Mishra (2020) [60], the addition of binder is noted as effective due to the binder's role, which is to wrap the pellets, therefore causing the voids to have better resistance to compression. Meanwhile, the addition of styrene–butadiene rubber (SBR) to lightweight aggregate leads to a lower impact value, which makes the aggregate stronger [35]. The impact resistance of cement-based fly ash aggregate was enhanced by the cement content because of the increased hydration reaction. In addition, the curing temperature will also increase the impact resistance of artificial aggregate [33]. The high porosity of phosphogypsum-based cold-bonded aggregates with 90% of phosphogypsum accounts for high water absorption with 13.6%, as it holds fewer binders and allows it to absorb more water [72]. The inclusion of cement enhanced the pellet strength from 1 MPa to 2.3 MPa when compared to the pellet strength of the cold-bonded lightweight aggregate made with only concrete slurry waste and fine incinerator bottom ash [36].

In addition, the lightweight aggregate with the lowest water absorption has better sustainability towards the impact of the load. In a previous study, it was discovered that increasing the maximum amount of fly ash replacement with 10% cement or 5% calcium hydroxide increased cold-bonded aggregate strength with decreasing water absorption [70]. On the other hand, curing at higher temperatures causes the impact value of artificial lightweight aggregate to improve by 12.5% to 14.75% and the crushing strength by 28.2% to 39.7% [64]. According to a study by Rehman et al. (2020) [68], the aggregate with the lowest water absorption of 12.5% had the lowest aggregate impact value of 22.12%, thus proving the stronger microstructure and lower porosity had led to high resistance to crack penetration and increasing strength. Meanwhile, according to Ghosh (2018) [73], the autoclaved aggregate made by using fly ash and cement can be used to replace the gravel as the crushing value and impact value due to the similar value.

From the previous studies in Table 4, it is shown that the mechanical properties of lightweight aggregate mainly depend on the type of material used. Furthermore, adding additives to the lightweight aggregate can aid to improve its strength. The majority of the researchers concluded that adding geopolymer to the aggregate leads to an improvement in strength performance. The method of curing, on the other hand, will have an impact on the strength of the lightweight aggregate, as a higher curing temperature will result in greater strength. The mechanical properties of the lightweight aggregate can be affected by the microstructure of the lightweight aggregate.


**Table 4.** Previous studies on mechanical properties of lightweight aggregate.

*5.4. Morphology*

The morphology of lightweight aggregate can be observed through scanning electron microscopy (SEM). The artificial lightweight aggregate that was made from calcining coal ash and dredged soil was observed through SEM, and the morphology can be shown as in Figure 2. The observation of voids from the morphology proved that the formation of voids contributes to lower specific density and loose bulk density.

**Figure 2.** SEM of artificial lightweight aggregate made up of calcining coal ash and dredged soil [77].

Figure 3 shows SEM images of sintered lightweight aggregate at 1180 ◦C. The increasing amount of waste glass powder in the sample allows the tiny voids to be filled up, thus causing less porosity of the sintered lightweight aggregate with increasing particle density [30]. Besides that, excess gas may be created when using the sintering method. The formation of pores will occur continuously when the temperature applied is too high [78]. In addition, for the sintering method, the lightweight aggregate sintered at 1100 ◦C had a smooth and thick surface with solitary and round pores with widths ranging from 10 to 20 μm which minimizes porosity. This will lead to enhanced densification by creating samples with minimal water absorption and high compressive strength [66]. As shown in Figure 4, the pores in the aggregates LWA1 and LWA2 are significantly larger because organic substance components derived from sewage sludge release gases that aid in the development of pores and thus form a porous aggregate structure [18].

In addition, alkaline activators such as sodium hydroxide and sodium silicate have been used previously as liquid precursors and mixed with aluminosilicate materials such as fly ash and rice husk ash to create a cold bonded lightweight aggregate known as geopolymer aggregate, and the result can be depicted as in Figure 5. The denser matrix that is shown in Figure 5 for geopolymer aggregate with the solid-to-liquid (fly ash/alkaline activator) ratio of 3.0 results in a lower AIV value, where the strength of the geopolymer aggregate is higher. The geopolymer aggregates indicated that excellent solidification of the fly ash with alkaline activators occurred by geopolymerization reaction as the alkaline activator dissolved most of the fly ash particles [79]. Furthermore, the SEM of quarry tailings autoclaved aggregate in Figure 6 showed that high stream curing had a denser structure, which increased the strength alongside with reduced water absorption of the aggregate [37].

**Figure 3.** SEM of sintered lightweight aggregate samples with 10% and 15% of water glass content at 1180 ◦C [30].

**Figure 4.** SEM of sintered lightweight aggregate made up of clay and sewage sludge at 1100 ◦C and 1150 ◦C [18].

**Figure 5.** SEM of geopolymer aggregate at solid/liquid ratio of 2.0, 2.5, 3.0, and 3.5 [76].

**Figure 6.** SEM of autoclaved lightweight aggregate with different curing pressure: (**a**,**b**) P0.50, (**c**,**d**) P1.00, (**e**,**f**) P1.25 [37].

From the morphology of lightweight aggregate, the pore of the lightweight aggregate can be observed through SEM. The formation of pores observed from the morphology is significant towards proving the increment and decrement in some properties of aggregates, such as specific gravity and density. Based on the previous study, aggregate that was made up of fly ash through geopolymerization showed the highest distribution of pores. The connected pores lead to higher water absorption than disconnected pores due to the ability to absorb more water into the aggregate. Meanwhile, the denser structure that was observed through SEM can provide good properties of lightweight aggregate, which can bring benefits in the application of concrete.
