*6.1. Mechanical Properties*

The utilization of lightweight aggregate also affects the significant properties of the concrete produced, including mechanical properties, thermal conductivity, and ultrasonic pulse velocity. The mechanical properties of lightweight aggregate concrete determine the suitability of artificial lightweight aggregate used in the concrete. The interfacial zone (ITZ) between the coarse aggregate and paste is a critical factor that will affect the mechanical properties of the concrete [80].

Compressive strength is the ability of the structure to resist compression, and this is the main design variable for engineers. The compressive strength of mortar containing fly ash aggregate will increase slowly at the beginning of the hardening stage, but it will increase rapidly after 14 days. In addition, the heavier the mortar, the higher the compressive strength. For instance, mortar of fly ash with an 8 M concentration of NaOH shows the heaviest bulk density with the highest compressive strength [22]. The compressive strength for quartz tailings aggregate (QTA) concrete reaches 74 MPa, which is considered high strength concrete. This is because the cement binders are readily penetrated to form a later mechanical interlocking structure around the aggregates to strengthen the bonding between aggregate and cement paste since the shell of QTA is a porous, fibrous, and needle-flake tobermorite structure [38]. The increasing amount of styrene-butadiene rubber (SBR) in pellets will cause an increase in the compressive strength of SBR modified lightweight aggregate (SLWA) concrete. This is due to the microstructure of the SLWA, which generates a strong bond between the aggregate and the cement paste [35]. Due to the impact on the restriction of the spread of cracking occurring, the beneficial effect of the addition of fibres to the lightweight aggregate cement mix gives a higher compressive strength (over 40 MPa) and can be used as a structural application in the construction field [81]. Table 5 shows the compressive strength of lightweight aggregate concrete reported by past researches. According to Table 5, it can be concluded that the artificial lightweight aggregate concrete had achieved the compressive strength for structure concrete which is 17 MPa based on ACI 318M-14.

In general, the concrete that used the autoclaved quartz tailing lightweight aggregate had a compressive strength of up to 74 MPa and alkali-activated fly-ash-based artificial lightweight aggregate achieved 64 MPa of compressive strength at 28 days. In addition, when lightweight aggregate has been used in concrete, it achieved early strength compared to conventional concrete. The additives added to the lightweight aggregate enhanced the aggregate's strength, which will also increase the lightweight concrete's strength. The use of lightweight aggregate in concrete met the minimal compressive strength requirements for the use of structural components. As a result, artificial lightweight aggregate has been an innovative material that can be used to manufacture lightweight structural components that have been highlighted in the recent construction field to minimize the structure's dead load and protect it from earthquakes.


**Table 5.** Previous studies on compressive strength of lightweight aggregate concrete.
