**7. Conclusions**

Based on the review, the cold bonding method and the autoclaving method can be considered as alternative ways to produce the lightweight aggregate due to the comparable properties to the sintering method. However, the aggregate produced from the cold bonding method and the autoclaving method are still limited and require further exploration in order to be commercialized as a sintering method. Other than features such as water absorption and specific gravity, the curing days for aggregate generated should be researched further for the cold bonding process, as this approach requires the same curing days as OPC. The construction sector will accept shorter curing days since they are more practical to commercialize. The performance of the aggregate produced by the autoclave process is good; however, the low temperature during autoclave pressure may need to be researched more in the future to lower production costs and, thus, make it a viable alternative to commercially available good-grade aggregate.

Further, lightweight aggregate having a density of less than 2.0 can be utilized extensively in lightweight concrete to lower the structure's dead load. The specific gravity of lightweight aggregate will be mostly affected by the type of material and binder used. In addition, the artificial lightweight aggregate showed lower water absorption than normal aggregate. The water absorption of lightweight aggregate is increased when geopolymer is used in the manufacturing process, and it can be further improved by utilizing additional treatments, such as vacuum impregnation or coating. Furthermore, due to the fact that the pozzolanic activity with the inclusion of geopolymer in the lightweight aggregate is higher, mechanical properties of lightweight aggregate are improvable. Moreover, the morphology of lightweight aggregate can help to determine the microstructure of lightweight aggregate. Lightweight aggregate with a denser microstructure and lower porosity was found to be more resistant to crack penetration and have higher strength. The properties of lightweight

aggregate can be improved by a greater distribution of internal aggregate holes and fewer continuous pores observed on the microstructure.

On the other hand, the interfacial zone (ITZ) between coarse aggregate and paste is a significant factor that will affect the compressive strength of the concrete. Thus, more research into the bonding mechanism between aggregate and cement matrix is critical. The compressive strength of lightweight aggregate concrete can be increased by adding additives, such as synthetic fibers. Higher compressive strength can be obtained by using lightweight aggregate in order to construct lightweight structural components to protect it from earthquake resistance. Additionally, lightweight aggregate with geopolymer demonstrates early strength development in lightweight concrete. The low thermal conductivity of lightweight aggregate concrete can be applied in the construction field as a thermal insulation material. This can assist the building to be more comfortable while using less energy. Furthermore, the application of lightweight aggregate in the concrete will produce excellent quality structural concrete, which can reduce defects when applied in the construction field. Therefore, the creation of lightweight aggregate can be a unique material that can be used in a variety of applications to minimize the usage of natural aggregate and bring benefits to the environment, such as reducing the dead load of the structure, enhancing thermal insulation properties, and reducing CO2 emissions.

**Funding:** This research was funded by Ministry of Higher Education Malaysia, Fundamental Research Grant Scheme (FRGS), and grant number of 9003-00747 FRGS/1/2019/TK06/UNIMAP/02/1. Authors gratefully acknowledge technical and financial support from King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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