In this work, a porous calcium silicate hydrate (CSH) humidity control material was prepared by hydrothermal synthesis with opoka (ROP) and slaked lime. The microstructures and humidity control properties of the prepared sample were characterized and analyzed in detail. Results show that CSH possesses plenty of mesoporous structures with the pore size range of 3-20 nm, which are superior to ROP. The maximum moisture adsorption capacity of CSH is about 2-3 times of ROP. The superior humidity control performance of CSH can be attributed to the increase of specific surface area and pore volume and the more reasonable and uniform pore size distribution. Moreover, CSH also exhibits good reusability within three cycles of adsorption/desorption. Furthermore, the synthesized CSH was added to an interior wall coating to test its humidity control performance in practical building materials. The result indicated that the moisture adsorption capacity of the coating with CSH in 24 h at high humidity can reach 110 g/m(2), and the moisture desorption capacity can also reach 70 g/m(2). The as received CSH shows excellent humidity control performance and can be used as a smart indoor humidity control material for various construction applications. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.