Feasibility of Alkali-Activated Low-Calcium Fly Ash as a Binder for Deep Soil Mixing

This study investigates the suitability of low-calcium fly ash (FA) for stabilizing expansive soil (ES) using the in-situ deep soil mixing (DSM) technique. The primary objective of the study was to control the swell-shrink behavior of ES and achieve higher strength characteristics. Owing to the presence of a low amount of calcium oxide (CaO) in Class-F fly ash (FA), an alkali environment is required to produce pozzolanic reactions involving silica and alumina. In this study, the alkali environment in the FA was created by adding a 50:50 ratio of sodium hydroxide (NaOH) and liquid sodium silicate (Na2SiO3) solution, referred to as liquid alkali activator (LAA). The properties of stabilized ES were investigated through Atterberg limits, one-dimensional swell tests, linear shrinkage tests, consolidation tests, and unconfined compressive strength tests. Further, scanning electron microscopy images and X-ray diffraction tests were performed on stabilized samples to understand the microstructural and physicochemical reaction mechanisms. The tests were conducted by varying the binder ratio (LAA/FA) from 0 to 1.5 for curing periods of 7, 14, and 28 days. The results showed that the binder ratio LAA/FA=1.5 effectively controlled the swell-shrink behavior of ES. A 10-fold improvement in UCS was observed for LAA/FA=1.50 at a 28-day curing period. Linear shrinkage strains and swell potential of the ES were reduced by 97% and 83%, respectively, with LAA/FA=1.5 at 28 days of curing. Overall, LAA/FA=1.5 is considered an optimum binder ratio to prepare DSM columns. (C) 2021 American Society of Civil Engineers.

Automated summary

This study investigates the suitability of low-calcium fly ash (FA) for stabilizing expansive soil (ES) using the in-situ deep soil mixing (DSM) technique. The results showed that the addition of a liquid alkali activator (LAA) consisting of sodium hydroxide (NaOH) and liquid sodium silicate (Na2SiO3) solution effectively controlled the swell-shrink behavior of ES and resulted in higher strength characteristics. A binder ratio of LAA/FA=1.5 was identified as the optimum ratio for preparing DSM columns.