Influence Mechanism of Fulvic Acid on the Strength of Cement-Solidified Dredged Sludge

Abstract

Cement solidification was a widely used method to broaden the resource utilization of dredged sludge. However, the organic matter in sludge limit the application of cement solidification. The fulic acid (FA) was used to simulate the natural organic matter in sludge. With the increase in FA content, the sludge gradually changed from low-liquid-limit clay to high-liquid-limit clay. The unconfined compression test showed that the strength of cement-solidified dredged sludge (CDS) decreased with FA content. The influence mechanism of the FA on cement solidification was revealed by a water content test, a hydration heat test, scanning electron microscopy, and thermogravimetric analysis. FA hindered the conversion of pore water to combined water and reduced the hydration heat inside CDS. The FA in sludge weakened the internal bond within CDS by hindering the development of C-S-H gel from spheroidal to flake. At the same time, the final amount of hydrates such as C-S-H, C-A-H and AFt also decreased in the CDS containing FA. The weakening mechanism of FA on the strength of CDS can be attributed to three aspects: (1) FA adsorbed on the surface of cement minerals and hindered the contact between cement minerals and pore water; (2) acidic FA reduced the pH of the pore liquid in CDS; (3) the carboxyl and hydroxyl functional groups of FA adsorbed calcium ions in pore liquid through ion coordination.