Enhancing recycled aggregate-based geopolymer concrete: The effect of metakaolin on performance and microstructure

• With a 100% incorporation rate of RAs, the workability of geopolymer concrete remains within slump class S4. • The coexistence of CASH and NASH gels improved the compressive strength of geopolymer concretes containing 100% RAs by approximately 27%. • XRD analysis revealed an increase in mullite content attributed to partially reacted metakaolin rather than newly formed geopolymer gel. In a circular-economy context, incorporating recycled aggregates (RAs) into geopolymer concretes (GPC) offers a promising alternative to conventional concrete, although the high porosity and weak interfacial transition zone (ITZ) of RAs often deteriorate mechanical and durability performance. This study investigates whether metakaolin (MK) addition can compensate for these negative effects by promoting the simultaneous formation of calcium-aluminosilicate hydrate (CASH) and sodium-aluminosilicate hydrate (NASH) gels in granulated blast-furnace slag (GBFS) based geopolymers containing up to 100% RAs. Nine mixtures were prepared with two MK contents (0% and 30%) and four RAs incorporation levels (0%, 50%, 75%, and 100%). Slump, porosity, capillary absorption, TGA, XRD, SEM, and compressive strength tests were performed and compared to the reference concrete. Although RAs reduced workability and increased porosity and water absorption, all GPC mixtures remained in slump class S4. MK addition significantly enhanced geopolymerization, especially in highly porous mixtures with 100% RAs, as confirmed by increased mass loss in TGA and by XRD showing a rise in mullite content mainly related to residual or partially reacted metakaolin phases from 0–1% to 17.5–19.7 wt%. This microstructural densification improved the ITZ and resulted in a 27% increase in compressive strength for GPC with 100% RAs.