This investigation aims to study the physico-chemical and surface characteristics of some composite cement pastes. Granulated slag (GBFS) was substituted with fired drinking water sludge (FDWTS) with 5, 10 and 15 wt% to prepare composite cements. The hydration characteristics of composite cement pastes were studied by the determination of portlandite and chemically combined water contents at different curing ages of hydration as well as the phase composition of hydration products. The surface properties were studied using the nitrogen adsorption technique. The results showed that the chemically combined water content increased by using FDWTS instead of GBFS. The free portlandite content increased up to 7 days and then decreased up to 90 days. Compressive strength increases with FDWTS up to 5% and then decreases with its increase up to 15% by weight. The specific surface areas (SBET, m2/g) and pore volumes (VP, ml/g) increased with FDWTS content in composite cement pastes.