Environmental performance of alkali-activated binders for ground improvement

Abstract

The use of alkali-activated binders (AAB) with industrial waste materials such as fly ash (FA) and ground granulated blast-furnace slag (GGBFS) as precursors for ground improvement has recently gained popularity as an alternative to Portland cement. This study assesses the life cycle environmental impacts of AABs, after scaling up the process from laboratory to field application, compared to conventional cement-based binders. Several terrestrial, marine, and climatic impact categories were evaluated from cradle to construction, covering the extraction of resources, processing, and manufacturing of alkali activators and other materials, transportation, and construction. The study investigated different AAB mixes including two precursors, FA and GGBFS, and two alkali activators, sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). The comparative analysis demonstrated that, under the tested conditions and methods, the production of both alkali activators is the key environmental hotspot throughout all life cycle impact categories. The soil treated with AABs with FA as precursor and NaOH as an activator outperformed cement in four out of the ten evaluated impact categories, namely global warming, tropospheric ozone formation, fine particulate-matter formation, and terrestrial acidification. Standard normalization and aggregation of all impact categories have confirmed the significant improvement of the soil treated with this AAB mix compared to cement. Moreover, the sensitivity analysis corroborated the superiority of this AAB mix over cement under varying transportation distances and compressive strengths. Furthermore, an eco-efficiency analysis was carried out, combining the incurred costs with the environmental footprint, and revealed that this AAB was the most eco-efficient binder out of the examined mixes. Overall, the findings suggest that, to improve the environmental performance of AABs in ground improvement applications, the manufacturing of activators must utilize sustainable sources of energy for electricity production. Moreover, the reduction of activators, or use of sustainable alternatives, in AABs is crucial, especially when low compressive strength is required.