Challenges and opportunities related to the use of sewage sludge ash in cement-based building materials – A review

Due to the increasingly demanding legal regulations, regarding both sewage sludge (SS) and solid residues obtained during its thermal utilization processes, new solutions to limit their negative impact on the environment are sought. One approach that eliminates the risk of sewage sludge ash (SSA) is its use in cement building materials. The article contains a detailed literature review regarding the use of SSA in mortars and concretes, where it is defined as the addition or substitute of the main binder. The issues of the impact of SSA quality (CaO content, the share of pozzolanic oxides, their ratio) on cement products are discussed in details. In addition to the description of the mechanical properties of building materials, the SSA impact on parameters such as workability, shrinkage of products and setting time is also described. It can be stated that using SSA in building industry is one of the most ecological approaches of SSA managing. Due to the toxic nature of SSA, data on the leaching of heavy metals from hardened mortar or concrete samples are presented to emphasize the contaminants immobilization effectiveness by cement products. Based on the analysis of the data presented in the article, solutions that can reduce the negative consequences of including SSA in building materials have been identified. However, due to the significant differences in SS thermal utilization technologic and SS characteristics, every SSA should be thoroughly characterized before implementing the most suitable stabilization/solidification methods.