This research work represents updated results of cohesive soil strength improvement with mineral wool fly ash. In the investigations, these materials were used: Portland cement CEM I 42.5 R, fly ash obtained from a mineral wool production process, sand and clay. Mixtures were prepared as follows: dry mixing of Portland cement and fly ash; dry mixing of sand and clay; adding water into Portland cement and fly ash; adding sand and clay mixture into already prepared Portland cement and fly ash

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... ension. The content of fly ash replacing Portland cement varied from 0% to 40%, and the content of sand mixture varied from 20% to 60%. After 24 hours, investigated samples were taken out from cylinder forms and kept in a desiccator with a humidity of 90% and at 20 °C temperature. Uniaxial compressive strength of the samples was determined after 548 days and compared to previous research results obtained after 7, 28 and 183 days. The most predictable compressive strength is for samples, which composition is 100% cement and 0% fly ash. In these samples, the highest compressive strength was obtained, comparing them to the other investigated samples. Compressive strength change is minimal for samples with a 10–30% amount of fly ash. The most significant decrease in compressive strength was obtained for samples with a 40% fly ash after 183 days. Nonetheless, the compressive strength of these samples increased after 548 days and is almost the same as for samples with 100% Portland cement.