*5.2. Water Absorption*

Water absorption provides an indication of the internal aggregate structure. Higher water absorption of aggregates indicates the large number of pores in nature and usually gives drawbacks to the aggregates. For instance, expanded perlite powder (EPP) was noted as being high in porosity, thus causing the water absorption to increase from 33% to 52% when the replacement of fly ash increased to 30% [53]. Meanwhile, when sintering is applied, increasing the sintering temperature was found to decrease water absorption of all aggregates due to the increment in the fusion of material, which led to less water surface permeability [48]. In the study conducted by Sun et al. (2021) [65], it was found that the sintered aggregate made up of red mud and municipal solid waste incineration bottom ash with the temperature increased from 1010 ◦C to 1090 ◦C caused the porosity of the aggregate to increase and the water absorption to decrease significantly until 1070 ◦C. Furthermore, Liu et al. (2018) [66] also found that lower water absorption can be obtained when the lightweight aggregate was sintered at a temperature of around 1100 ◦C. Lightweight aggregate made up of drill cuttings containing synthetic-based mud, when sintered at 1180 ◦C, had water absorption of 3.6% [24]. In addition, the water absorption of metakaolin artificial lightweight aggregate increases at the sintering temperature over 900 ◦C. The pores formed during the sintering process were found to be closed pores, thus causing a reduction in permeability to water [67].

In addition, the addition of the waste glass powder to lightweight aggregates was found to significantly reduce water absorption from 7.73% to 0.5% [30]. Meanwhile, due to the porosity of the hydrated calcium silicate, the quartz tailing aggregate (QTA) also possessed high water absorption, which varies from 13.77% to 21.93% [38]. In another study, the water absorption was reduced from 12.1% to 8.58% when styrene–butadiene rubber (SBR) was added from 1% to 3% to the lightweight aggregate, which proved the minimizing of voids when the SBR increased in the pellets [35]. Furthermore, it was found that the lightweight aggregate produced from different ratio palm oil fuel ash and silt causes high water absorption of 32.2% when 90% of clay is used [8]. This is due to the high pozzolanic reaction rate within the mixture, thus causing higher water absorption through the capillary of silt. Moreover, water absorption for aggregates incorporated with 10% cement was lowered by 13.97% due to a stronger hydration reaction and a denser microstructure with more C-S-H and CH products, thus leading to an increase in the strength of the aggregate [68].

In addition, the utilization of alkaline activator as a binder for the production of lightweight aggregate was found to increase the water absorption from 22% to 23% [22]. Meanwhile, the geopolymer lightweight aggregate sintered using microwave radiation had water absorption of 18.98%, as it was affected by the high density of the aggregate [29]. In another study, it was found that the pores in the fly ash geopolymer aggregate were reduced after the geopolymerization process, thus giving a denser microstructure, which resulted in lower water absorption at 10.05% [32]. Meanwhile, increasing the Na2SiO3/NaOH ratio from 1.5 to 4.0 caused the water absorption values for geopolymer lightweight aggregate to steadily increase from 15.2% to 19% due to the foaming activity of Na2SiO3 [58]. Furthermore, the metakaolin geopolymer aggregate sintered at 600 ◦C will have lower

water absorption as the greater the sintering temperature, the more voids created, and, hence, the higher the water absorption of lightweight aggregate [69]. Moreover, the fly ash geopolymer aggregates had higher water absorption when curing at 80 ◦C due to the water in the aggregates participating in the geopolymerization process, which improves the strength of the pellets [64].

From Table 3, the water absorption for artificial lightweight aggregate was proven to be higher than that of the natural aggregate, which can be explained by the effect of porosity due to pore formation. In addition, the water absorption of the lightweight aggregate can be affected by influence factors, including the type of materials used, the type of curing, and the type of binder used. The sintering method demonstrated that, when the temperature rises, the water absorption of lightweight aggregate decreases because it contains closed pores. The majority of research has indicated that the cold bonding procedure will have higher water absorption and will require additional treatment to eliminate this problem. Furthermore, adding additives to the lightweight aggregate will aid in water absorption reduction. The addition of geopolymer to lightweight aggregate will increase water absorption significantly. Water absorption will be reduced by the presence of a vitrified shell around the artificial lightweight aggregate. As a result, the water absorption of lightweight aggregate has an impact on mechanical qualities and should be assessed before using it in concrete.


**Table 3.** Previous studies on water absorption of lightweight aggregate.
