Approximately 300,000 tons of municipal solid waste incineration bottom ash (BA) and 600,000
tons of fayalite slag (FS) are generated annually in Finland from metallurgical and incineration
processes, with the majority of them disposed of in landfills or used in low-value applications.
This study investigated the potential upcycling of FS and BA as sand replacements in cementbased
mortars to avoid landfilling, conserve natural resources, and ensure efficient use of industrial
residues. Standard sand (SS) was used as the main fine aggregate. The effect of replacing
SS partly or wholly with either FS or BA was investigated through workability, compressive
strength, ultrasonic pulse velocity (UPV), scanning electron microscope analysis, capillary water
absorption, alkali–silica reaction (ASR), freeze-thaw cycles in water, and combined sodium sulfate
and sodium chloride solution exposure. The aggregates’ leaching results were below the
values stipulated by Finnish and EU regulations. Partial or full replacement of SS with either FS or
BA resulted in lower workability. Full replacement of SS with FS resulted in comparable properties
to the reference mix in terms of compressive strength, UPV, and capillary water absorption.
Meanwhile, partial replacement of SS with FS resulted in higher compressive strength and UPV
but reduced water absorption. In contrast, partial or full replacement of SS with BA resulted in
lower compressive strength and UPV, as well as increased water absorption compared with the
reference mix. All samples remained stable after exposure to freeze-thaw cycles in water. However,
only the reference samples and samples containing 50% replacement of SS with either FS or
BA were stable after exposure to freeze-thaw cycles in a combined sulfate and chloride solution,
whereas those containing 100% FS or BA were completely degraded. Of all the aggregates, only
FS satisfied the 14-day ASR requirements according to the ASTM C1260 standard, achieving a low
expansion rate of 0.009%.
