Influence of Concrete Waste and Fly Ash Additions on the Mechanical and Antimicrobial Properties of Portland Cement Mortars

Construction and demolition activities generate over one-third of all waste produced within the European Union, with the largest fraction being mineral materials, and concrete representing up to 90% of this volume. In this context, the recycling of this type of waste is an important research topic with growing scientific and industrial relevance. While numerous studies have examined the influence of recycled concrete and other industrial waste on the technical performance of Portland cement-based composites, the antimicrobial resistance of these composites remains largely unexplored. Therefore, in this study we evaluate the effects of three different waste materials on the key properties of Portland cement mortar, as well as on its antimicrobial resistance; the investigated waste materials were fly ash (produced in thermal power plants), recycled concrete fines resulted from the mechanical processing of concrete waste generated in construction and demolition activities, as well as dried concrete slurry (a byproduct of concrete batching plants). The partial replacement of Portland cement with these concrete wastes slightly increased the mortar’s workability (up to 4.6%). However, it also led to an 11–12% reduction in compressive strength after 28 days of hardening. After 60 days of curing, the antimicrobial properties of these mortars were evaluated by assessing their effect on planktonic microbial growth and their anti-adherent capacity against the most common pathogenic strains (S. aureus, E. coli, P. aeruginosa, C. albicans, and C. parapsilosis). Antimicrobial assays were performed at two different concentrations of microbial suspensions, and the mortars exhibited significant antibiofilm properties against all strains, especially against E. coli. The study identified mortar formulations in which partial replacement of cement with construction, demolition, and industrial waste materials resulted in compressive strength and antimicrobial resistance comparable to those of conventional reference mortars. These findings highlight the potential to integrate recycled waste into Portland cement-based materials, supporting both structural integrity and microbial resistance and advancing sustainable construction practices.