Mechanical properties and microstructural characteristics of seawater-mixed sintered sludge cement paste

Due to the easy availability and pozzolanic activity, dredged sludge is often used to prepare sintered sludge cement (SSC) to alleviate the emission reduction pressure in the construction industry. However, the significant decrease in mechanical properties caused by the incorporation of large volume SSA has become the primary factor limiting its application. Herein, this study innovatively proposes a method of using seawater mixing to overcome the above issue and systematically reveals the enhancement mechanism, based on the microstructure characteristics characterized by XRD, TG, SEM, EDS and MIP. The results show that seawater can improve compressive strength, flexural strength, and microhardness by up to 47.10 %, 21.08 %, and 31.6 %, respectively. Meanwhile, with the increase of sintered sludge ash (SSA), the improvement of macro-micro mechanical properties by seawater shows an upward trend. The enhanced alkaline environment caused by mixing cement clinker with seawater accelerates the pozzolanic reaction of SSA, resulting in a maximum increase of 17.78 % in chemically bound water content. This compensates for the reduced hydrate generated by cement hydration in quantity and increases the optimal content of SSA in cement paste from 10 % to 20 %. In addition, the chloride ions and sulfate ions promote the transformation of Hemicabonate, Monosulfate and Monocabonate into Friedel's salt, effectively repairing the visible structural defects caused by the introduction of SSA, which is reflected at the micro scale as a decrease in porosity and a rise in gel pores.