Towards low-carbon deep soil mixing: Alkali activation of fly ash and slags

¹ School of Qilu Transportation, Shandong University, Jinan- 250002, China
² Xinjiang Research Institute of Shandong University, Changji- 831100, China

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Deep Soil Mixing (DSM) is widely used to improve soft and problematic soils, traditionally relying on cement or lime as binders. However, their high carbon footprint and durability limitations under aggressive environments highlight the need for sustainable alternatives. This study evaluates alkali-activated binders produced from industrial byproducts: fly ash (FA), granulated blast furnace slag (GBFS), and calcium carbide slag (CCS), for stabilizing silty clay. Sodium hydroxide (NH) and sodium silicate (SS) were used as the primary activators, while red mud was incorporated as a minor supplementary alkaline additive. Binder slurries were prepared with a fixed Na2O content of 8% and an NH/SS ratio of 1.5, while the water-to-binder ratio was varied from 0.6 to 1.2. Slurry workability was characterized using a Marsh funnel test, and the mechanical performance of stabilized soils was evaluated through unconfined compressive strength tests after 7 and 28 days of curing. Results show that FA-rich mixtures exhibit limited early strength, whereas higher GBFS and CCS contents promote C-A-S-H gel formation, matrix densification, and significantly enhanced strength. An optimal FA:GBFS:CCS ratio of 2:5:3 achieved favorable slurry properties and delivered 20–30% higher strength than cement-stabilized soil, demonstrating the potential of this low-carbon binder for sustainable DSM practice.