Strength and stiffness development of one-part fly ash-bottom ash geopolymer stabilized soft clay

Abstract One‑part geopolymers—pre‑blending solid precursors with solid activators—circumvent on‑site handling of corrosive alkaline solutions, offering a viable route for soft soil stabilization. This study evaluates this approach using fly ash (FA) and bottom ash (BA) to stabilize a soft clay (LL = 38.8%), with FA:BA ratios of 100:0, 70:30, and 50:50; binder contents of 20–50%; initial water contents of 0.7LL–1.3LL; and curing times of 7–90 days. BA incorporation accelerates early strength but retards long‑term gain: the logarithmic coefficient declines from 0.436 (pure FA) to 0.397 (50% BA), quantifying the trade‑off between transient Ca 2+ ‑driven gelation and sustained N–A–S–H polycondensation. The FA70BA30 blend at 50% binder and 1.0LL attains a 28‑day UCS of 0.59 MPa, meeting the 0.5–0.8 MPa excavation‑support standard. Two predictive correlations are established: a linear UCS–1/ R relation for mix design, and an exponential UCS– V s relation for field quality control; the exponent increases systematically from 1.166 to 5.594 with BA content, indicating that stiffness in BA‑rich matrices becomes governed by interparticle packing rather than gel continuity. XRD and SEM–EDS reveal that gehlenite dissolution in BA releases Ca 2+ to form hybrid (N,C)–A–S–H gels, whereas abundant inert residues deplete alkali and suppress sustained N–A–S–H polycondensation. At equal binder content, FA70BA30 exhibits superior post‑28‑day strength gain (+ 44.1% vs. + 11.1% for OPC), coupled with 73% lower CO 2 footprint and ~ 14% lower material cost. The FA:BA ratio thus serves as a key design parameter for balancing early strength and long‑term performance, positioning one‑part FA–BA geopolymers as a low‑carbon, cost‑competitive alternative for soft clay stabilization.