Fly ash-GGBS blended geopolymers for expansive soil stabilization: A critical review of evidence, limitations, and pathways to practice

Expansive clays continue to challenge geotechnical practice, while cement- and lime-based stabilization carries a significant carbon burden. This review evaluates fly ash (FA)-ground granulated blast-furnace slag (GGBS) geopolymers as low-carbon soil stabilizers, synthesizing evidence on compaction behavior, strength development, microstructural mechanisms, durability, and environmental performance. Across the analyzed studies, FA increases optimum moisture content (OMC) by 2–8% due to its water demand, whereas GGBS raises maximum dry density (MDD) by 0.5–1.4 kN/m 3 and accelerates early strength. Blended FA-GGBS systems deliver substantial gains in unconfined compressive strength (UCS), with values reaching 2.27–9.5 MPa at 28 days, representing 4- to 9-fold improvements over untreated soils. California Bearing Ratio (CBR) values increase dramatically from 1.5% to 3% in untreated soils to 9–416% in geopolymer-treated specimens, meeting subgrade and liner design targets. FA-GGBS geopolymers present a credible pathway to performance-based, lower-carbon ground improvement, with CO₂ reductions approaching 90% compared to Portland cement. Critical gaps persist in leachability testing, field validation, and long-term environmental impact assessment.