Utilization of Alkali-Activated Fly Ash for Construction of Deep Mixed Columns in Loose Sands

Abstract

Results of optimization of an alternative alkali-activated binder (AAB) system based on two industrial waste products found in abundance in Australia, namely fly ash (FA) and granulated blast furnace slag (S), are presented. Early strength development of low-calcium alkali-activated precursors has typically been found to be insufficient for field applications or highly dependent on impractical curing conditions. S can be used effectively to initiate FA bond development without requiring high-temperature curing conditions. The AAB presented was found to exhibit comparable 7- and 28-day strength to cement treated samples, in addition to providing a reduction in vertical shrinkage. AAB exhibited a strong sensitivity to the total water content in the mix; however, acceptable unconfined strengths were achieved for samples with water contents up to 40%. High application rates (p = percentage of dry precursor to total mixture weight) exhibited optimum strength at close to the optimum water content of the sand, whereas lower application rates (p≤8–12% w.t) were found to show better mechanical strength on the dry side of the optimum water content of the sand. Alkali-activated blends of FA and S, particularly at a S∶FA ratio of 50∶50, can provide a viable alternative to cement-based binders for the stabilization of granular soils in deep soil mixing projects.