Durability of industrial by-products combined with cement-solidified sludge subjected to immersion and cyclic wetting-drying in various fluids

The utilization of industrial by-products (IBPs) in cement (OPC)-solidified sludge has been extensively studied as an approach to enhance the mechanical performance, but the presence of salt in groundwater and wetting-drying action would compromise the mass and mechanical behavior of such solidified sludge. Unfortunately, there is a paucity of research on the single or coupled impacts of these two phenomena on IBP-OPC-solidified sludge. This paper aims to address the research gap by exploring the durability of IBP-OPC-solidified sludge in distilled water, sulfate solution and chloride solution through the full immersion tests (IT) and cyclic wetting-drying (WD) tests. Samples were prepared where collected sludge was mixed with OPC and various types of IBPs (including fly ash, slag, calcium carbide residue and silica fume). The resulting unconfined compressive strength, mass loss, and visual changes of samples were evaluated after IT and WD tests. Furthermore, scanning electron microscopy, physisorption experiments, and X-ray diffraction tests were conducted to examine the microstructure, pore evolution and mineralogical characteristics, respectively. Grey correlation model analysis was used to quantify the relationship between components inside IBPs and strength. The results indicate that the use of IBPs is effective in minimizing the mass loss and strength degradation of OPC-solidified samples, with the exception of calcium carbide residue in a sulfate environment. The use of IBPs can promote the formation of C-(A)-S-H gels and increase the amount of hydrated products through pozzolanic reactions and reactions between salt and aluminosilicate phases. Furthermore, the pore volume of solidified samples tends to be reduced, creating a denser matrix with more smaller pores that contribute to excellent mechanical properties and durability. Overall, the findings suggest that IBP-OPC has the potential for promising applications in the high performance of solidified sludge in severe conditions, such as coastal and offshore engineering.