*4.1. Sintering*

Sintering is a process that consumes high energy to produce artificial aggregate with enhanced properties. As reported by Sun et al. (2021) [34], raw materials with a high amount of SiO2 and Al2O3 commonly use sintering. When the pellets in the disc pelletizer

are shaped, the pellets will dry for a day before undergoing the sintering process at a temperature of between 1180 ◦C and 1200 ◦C [42,43]. In some cases, some of the pellets are fused at a temperature above 1200 ◦C for optimum properties [21,44]. Most of the previous research used a similar drying method prior to sintering [45,46]. Chen et al. [47] also reported a similar method where the pellets undergo a drying stage, followed by preheating at 500 ◦C expanding temperature of a temperature between 1100 ◦C and 1150 ◦C for the sintering process.

Meanwhile, according to Grygo and Pranevich [48], the aggregate produced through the sintering process is lighter and has high strength performance. The sintering process is a popular application for mass manufacture of lightweight aggregates that does not require a long-term curing process [49]. Lytag, Pollytag, LECA, and liapour are some of the commercially sintered lightweight aggregates around the world. The factory that manufactured LECA has three lightweight aggregate production lines with a total capacity of 750,000 cubic meters per year [50]. Sintered artificial lightweight aggregate is one of the possible materials to make concrete lighter than the standard aggregate concrete [5]. In addition, Tian et al. (2021) [51] state that sintering aggregates with the help of geopolymerization reactions can have higher aggregate strength and low density. However, the sintering process involves a high level of energy during pelletization, which results in higher manufacturing costs [44]. Aside from that, the sintering process generates a large amount of pollutants, which will cause environmental problems. The sintering method needed a lot of energy regardless of its potential engineering properties with respective mix design applied [52].

In short, the lightweight aggregate produced at the temperature of 1200 ◦C provides the best properties of the aggregate. To acquire the best features of lightweight aggregate, the suggested sintering temperature for metakaolin is 900 ◦C, 1100 ◦C for sewage sludge and river sediment, and 900 ◦C for fly ash. As a result, materials such as metakaolin and fly ash are more advantageous due to the energy savings at the lowest sintering temperature required to manufacture lightweight aggregate. The sintering method will also be able to produce lightweight aggregate in a shorter time, at which it is suitable to be used to replace natural aggregate. Nevertheless, the sintering method will require high energy during the production, and this will increase the price of the production. The usage of sintered lightweight aggregate in the construction field will increase the overall cost of construction. As a result, new approaches, such as cold bonding, are being studied to address the problems that the sintering method encountered.
