To extend the beneficial reuse of waterworks sludge, nano-SiO2 was considered as an additive to improve the engineering properties of waterworks sludge ash cement paste. Waterworks sludge was first incinerated to ash at 800 o C. Different amounts of nano-SiO2 were added to the sludge ash and then mixed with cement to make a paste. The benefits of using sludge ash as a replacement for cement were assessed. Tests of the fresh paste properties (including flowability and setting time), hardened paste properties (compressive strength), and microstructure analysis (SEM) were performed to assess the influences of the different proportions of nano-SiO2 and sludge ash added to the paste. Both added admixtures could reduce the flowability and shorten the setting time of the cement paste. Furthermore, the cement paste became denser, more uniform, and exhibited improved early and later compressive strengths after the addition of nano-SiO2. However, because of the late development of pozzolanic effects, the compressive strengths of the cement paste were reduced with increased amounts of ash replaced. This study indicates that, because Portland cement is well compatible with nano-SiO2, the performance of silicate cement is improved and the strength of the hardened cement paste is raised, especially for the early strength.