The Effect of NaOH Concentration on the Mechanical and Physical Properties of Alkali Activated Fly Ash-Based Artificial Lightweight Aggregate

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 significantly reduce the impact on the environment. Alkali-activation is a promising technique to generate LWA as it proofed to provide better properties to the LWA in comparison to the use of a Portland cement — fly ash binder. However, only limited studies can be found on applying this technique to produce LWA. Furthermore, the concentration of alkali activators such as NaOH, is an important parameter that needs to be investigated in order to produce LWA with excellent properties. Thus, in this research the NaOH concentration was varied (4 M, 6 M or 8 M) in order to obtain the optimum for producing LWA. Later on, the produced LWA with a diameter of 2-4 mm was used to replace the fine aggregates in the mortar. The mechanical and physical properties of the produced LWA were investigated, such as water absorption, density, particle size distribution, crushing resistance, and mineralogy and the compressive strength of the resulting mortar. The results showed that a maximum compressive strength of 64 N/mm2 at 28 days could be achieved by mortar containing LWA 8M. Increasing the concentration of NaOH up to 8M had no significant effect on die water absorption value, particle distribution, density and crushing resistance of LWA. The result from Quantitative X-Ray Diffraction (QXRD) revealed that there is also no significant difference in the quantity of amorphous phase formed in the LWA with increasing NaOH concentration. This finding suggests that alkali-activated fly ash LWA can in the future be synthesized with a lower concentration of NaOH starting from 4M.