Concentrated zero-valent iron (ZVI) slurries (CIS) are commonly encountered during the water treatment using ZVI, in the processes such as delivery, settling, and pumping of the ZVI slurry; about the CIS, knowledge about their rheology is critical to the operation of these processes, but little is known about this aspect so far. Herein, microrheology of the CIS of four different ZVI is studied using diffuse wave spectroscopy (DWS). Results show that the CIS of two iron nanoparticles solidify immediately after their separation from water (<3 h), with a fast growth of macroscopic viscosity and a rapid weakening of the fluidity; the CIS of the microscale ZVI (mZVI) is nearly pure viscous or liquid-like and solidifies slower than the nanoparticle CIS; CIS of the floc modified ZVI (mZVI(floc)) is viscoelastic and remains stable in rheology over an extended period up to four days. Such differences in CIS rheology are explained by the coarse surface of the nanoparticle aggregates, the smooth surface of mZVI particles, the intertwined floc coating of mZVI(floc) particles, and the corresponding disparity in surface friction between the particles or aggregates. The solidification of CIS is explained by the binding effect of the oxidation products of the ZVI-water reaction. This study provides information about CIS rheology and offers guidance for process operations in the water treatment using ZVI. This study also shows that the DWS technique is informative in characterizing CIS and the method may be equally applicable for studying other concentrated particle slurries in water treatment.