Mechanical properties and permeability evolution mechanism of collapsible loess stabilized by fly ash–cement–slag blend composite system in Qinghai Lianghua area

Abstract

As the predominant binder in collapsible loess reinforcement, cement is a major source of greenhouse gas emissions, accounting for 8% of global anthropogenic CO ₂ ; this necessitates an urgent transition toward sustainable alternatives. This study quantitatively demonstrates that the industrial by‐products fly ash (FA) and cement–slag blend can serve as effective cement replacements while enhancing both mechanical and hydraulic performance. Through standardized column tests on collapsible loess specimens (binder contents: 6%–12%, curing period: 1–28 days), cement–slag blend consistently outperformed both cement and FA across several critical metrics: (1) Cement–slag blend–treated samples achieved a 28‐day unconfined compressive strength (UCS) up to 750 kPa, surpassing FA counterparts by 15% and California bearing ratio (CBR) values exceeding 90. (2) Cement–slag blend uniquely exhibited superior strength without the typical brittleness, as evidenced by stress–strain curves maintaining post‐peak ductility. (3) Cement–slag blend reduced hydraulic conductivity to 2.8 × 10 ⁻⁷  m/s, approximately 38% lower than that of FA (4.54 × 10 ⁻⁷  m/s), due to the formation of a denser calcium silicate hydrate (C–S–H) microstructure. Mechanistically, the 7‐day performance inflection point observed exclusively in cement–slag blend specimens is attributed to their high CaO (41.6 wt%) and Al ₂ O ₃ (12.4 wt%) content, which facilitate faster pozzolanic kinetics. In contrast, FA exhibited delayed reactivity stemming from a reliance on externally sourced Ca ^{2 +} . These findings establish a threshold‐driven activation model, advancing fundamental understanding of soil stabilization beyond prior linear hydration models. Practically, our data provides validated mix design guidelines: Specimens with 6% cement–slag blend content met subgrade construction standards (CBR > 90, UCS > 750 kPa) while reducing carbon footprint by 48% compared to conventional cement stabilization, offering actionable pathways for infrastructure decarbonization through scientific waste valorization.