An innovative approach to fly ash-based geopolymer concrete mix design: Utilizing 100 % recycled aggregates

Abstract

Concrete is a widely used construction material globally, known for its ability to incorporate substantial quantities of industrial by-products such as fly ash (FA) and construction and demolition (C&D) debris. These by-products, originating from thermal power plants and construction activities, often pose environmental challenges when disposed of improperly. To address this issue, recycled aggregate geopolymer concrete (RAGPC) has emerged as a promising solution. However, the lack of a suitable mix design method has hindered the structural application of RAGPC. This study aims to introduce a novel mix design method utilizing 100 % recycled aggregate (RA) combined with FA through geopolymer technology, offering a simpler and more rational approach than previous methodologies. The proposed approach was validated through various concrete tests, utilizing a sodium silicate to sodium hydroxide ratio of 1.5 and a concentration of sodium hydroxide of 16 M as alkaline activation content (AAC), with an AAC to binder (B) ratio ranging from 0.3 to 0.8. Results indicate that incorporating FA and RA into concrete, coupled with elevated curing at 60 °C for 24 h for RAGPC hardening, yields compressive strengths (CS) ranging from approximately 14 to 35 MPa after a 28-day curing period. SEM and XRD tests were also conducted to analyse the polymerization processes, revealing the significant positive influence of FA on RAGPC performance. The findings suggest that fly ash has a superior synergistic influence on recycled aggregate geopolymer concrete performance. Moreover, due to its excellent resistance to chloride ingress and seawater corrosion, geopolymer concrete is well-suited for marine applications, including the construction of ports, harbours, offshore platforms, and coastal protection structures.