Study on mechanical properties and microstructure of fly-ash-based geopolymer for solidifying waste mud

Abstract

The chemical solidification of construction waste mud presents a strategy for mitigating environmental concerns as well as addressing problematic geotechnical engineering properties. The present study explores the efficacy of utilizing fly ash (FA)-based geopolymer, activated by calcium carbide residue (CCR) and sodium silicate solution (NS), for waste mud solidification. An unconfined compressive strength (UCS) test was conducted to ascertain the optimal CCR/NS ratio. Supplementary tests were executed to examine factors such as water content, pH level, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and natural dehydration. According to the UCS results, an optimal 28-day strength of 2.2 MPa was observed when employing a CCR/NS ratio of 6:4. A notable reduction in the water content of the mud was achieved in the short term, with the most effective results observed at a pH level of 12. Microstructural analyses indicated that calcium silicate hydrate (CSH) and aluminosilicate-Na hydrate (NASH) acted as the primary cementitious products, filling pores and creating bonds with particles to facilitate strength development. Variations in crack morphology and water content during the natural dehydration process were also analyzed for both solidified and untreated mud. The findings suggest that solidified mud holds promise for application in subgrade engineering projects, potentially diverting significant volumes of waste mud and industrial by-products such as CCR and FA away from landfills.