The effect of NaOH concentration on the mechanical and physical properties of alkali activated fly ash-based artificial lightweight aggregate

Natural aggregate is a non-renewable material that takes up a large portion of a concrete mixture. Replacing this material with artificial lightweight aggregate (LWA) that is generated from an industrial by-product such as fly ash, will reduce the impact on the environment. Alkali-activation proved to provide better properties to the LWA compared to the use of the fly ash in a blend with Portland cement. In this research, the NaOH concentration of the activator was varied (4 M, 6 M or 8 M) in order to obtain the optimum for producing LWA. Sodium silicate was combined with the sodium hydroxide in a weight ratio of 1.5. The LWA were produced by adding the activator to the fly ash in a pan granulator and the particles of 2–4 mm diameter were used to replace the fine natural aggregates in the mortar. The mechanical and physical properties of the produced LWA were investigated, such as water absorption, density, porosity, particle size distribution, crushing resistance, and mineralogy, as well as the compressive strength of the resulting mortar. The results showed that increasing the concentration of NaOH up to 8 M had no significant effect on the density, water absorption, particle size distribution, mineralogy and the crushing strength of resulting LWA. A significant decrease in the amount of macropores occurs when the concentration of NaOH is increased from 4 M to 6 M. The maximum compressive strength of 64 N/mm² at 28 days could be achieved by mortar containing LWA with 8 M NaOH. The properties of fly ash-based LWA in this research were comparable with LWA generated from commercial expanded clay.