Calcite powders were prepared from black marble waste by milling and alkaline hydrothermal processes. Crystal modifiers including two sulfates and an organophosphate chelator were used to study their effects on the crystallinity and particle morphology of calcite powders. Compared with control experiments without crystal modifiers, the Na2SO4 modifier drastically decreased the crystallinity of calcite powder, while the organophosphate modifier distinctly decreased the brightness and crystallite size of the calcite powder. Besides the rhombohedral crystallites, the calcite powders with sulfate modifiers contained a few flocculent agglomerates and nanosheets, which were assigned to amorphous/vaterite phases and portlandite phase, respectively. The organophosphate modifier induced spherical nanoaystals instead of the rhombohedral crystallites. With hydrothermal treatment performed for 4 hat 240 degrees C, nanosized calcite powder (88.3 nm) with a powder brightness of 94.3% was achieved at 1% organophosphate modifier. The eco-friendly technique enables the scalable recycling of marble wastes into nanosized calcite powders. (C) 2020 Elsevier B.V. All rights reserved.