This paper discusses the behaviour of iron-rich sludge, a problematic waste product of potable water treatment, minewater/groundwater management and hydrometallurgical processing. The work confirmed the highly hydrous and largely amorphous nature of the sludge and the significant differences that occur in reactivity (rate of acid leaching) and density (densification) depending variously upon the mode of sludge formation, pre-conditioning and storage. A structural study using a combination of FT-IR/ATR, XRD and TG/DTA correlated decreases in reactivity with ageing (i.e., with increases in particle size and/or degree of crystallinity) and increases in density with expulsion of weakly bonded water. The work underpinned studies on recycling of water treatment sludge with ion exchange resins and accelerated densification during sludge management.