Ground granulated blast-furnace slag (BFS) and silica fume (SF) were used to enhance properties of G-Oil Well cements under hydrothermal conditions (1.2 MPa, 200 degrees C). Pore structure development depending on phase composition was studied. Structure depletion due to transformation of C-S-H to alpha-C2SH and C6S2H3 in referential samples was avoided when more than 20 wt.% of cement was replaced. C-S-H with C/S ratio similar to 1 and C-A-S-H phases were formed reflecting the amount of BFS and SF. Carbonation and formation of calcite from vaterite increased total porosity. Transformation through aragonite had the opposite effect and increased the gel pores portion. (C) 2017 Elsevier Ltd. All rights reserved.