Microstructural evolution and mechanical performance of goethite-rich laterite-based geopolymers: Effects of mineral phases addition

Abstract

Iron-rich laterite without thermal treatment is increasingly gaining attention as a sustainable, widely available, low-carbon precursor for geopolymer production, owing to its semi-structured disorder. Despite their utilisation potential, the effects of fine powders on formation in alkali-activated systems remain limited, particularly regarding mechanical and microstructural behaviour. The present study investigated the influence of incorporating different mineral additives, namely fine quartz sand (≤ 80 µm) (SD), basalt (BA), and volcanic ash (VA), on the phase evolution, microstructural characteristics, and mechanical performance of laterite-based geopolymer composites. Laterite (LA) was replaced with 20% or 40% (in mass) with these materials, to produce alkali-activated binder samples, which were subjected to ambient curing at 28 °C or oven–curing at 80 °C. The results showed that the incorporation of VA and SD significantly enhanced polycondensation to a greater extent compared to those made using BA. The observed improvement is attributed to increased formation of sodium alumino-ferro-silicate hydrate (N-A(Fe)-H) phases, resulting from enhanced dissolution of the amorphous fractions of the LA and aggregates, as well as improved binder–aggregate interfacial interactions. The flexural strength of ambient-cured samples increased from 6.30 MPa to 7.50 MPa and 14.0 MPa upon inclusion of 20% SD and 40% VA, respectively, while it decreased to 5.07 MPa with the incorporation of 40% BA. However, the reverse effect was observed in the sample oven-cured at 80 °C, where the flexural strength decreased from 8.20 to 6.76 MPa, 7.72–6.75 MPa, and 9.70–6.76 MPa, respectively, with an increased content of SD, BA, and VA up to 40 wt%. Finally, the bond strength at the LA/VA interface is higher than that of LA/BA and LA/SD, reflecting the higher reactivity of the VA in the alkaline environment of these composites.