Effects of sewage sludge ash on the performance of cement paste and its water film thickness

Among the various methods for treating sewage sludge from municipal sewage treatment plants, incineration stands out as an environmentally friendly and safe disposal option. As a result, a substantial amount of sewage sludge ashes (SSA) is generated as the ultimate by-product. Some published literature suggests that cement pastes incorporating a small amount of SSA demonstrate comparable strength. However, inconsistent results on the workability of fresh pastes containing SSA are shown. Thus, this study aims to investigate the impact of SSA on the workability and compressive strength of cement paste, as well as explore the influence of water film thickness (WFT) on cement paste with SSA. A total of 30 cement paste samples were made, each with varying SSA contents and different water/cementitious materials (W/CM) ratios. The experiment results show that the increase of SSA content leads to the decrease of flowability and compressive strength, but an increase in packing density and cohesiveness. In addition, the WFT and the flow rate of these samples can be related by a single function, and the WFT is proportional to the flow rate. However, the paste’s flow spread, cohesiveness, and compressive strength are influenced by the WFT and the SSA content.