The paper summarises information gained within the verification of high-temperature fly ashes after the denitrification process. There was an examination of characteristics of fly ashes from various heat sources as well as possibilities of using these types of fly ashes to various types of building applications. An effect on final properties of a building material produced in laboratory or experimental conditions was observed. Positive results were achieved within the verification of using fly ash to fly ash-cement mixtures. A quality of prepared suspensions was equal to the practically verified one prepared from a fly ash from a powerplant without SNCR. These mixtures after 28 days achieved compressive strengths 8 MPa. Significant increase of strengths in the course of time was recorded. After 90 days compressive strengths achieved values 13-16 MPa and after 1 year even 17-20 MPa. Flexural strengths after 28 days were around 1.5 MPa. This parameter also showed a tendency to increase in time. Fly ash after denitrification was effective within a preparation of repairing materials based on epoxy resin. For these materials, 50% substitution of natural aggregate with fly ash after denitrification process was proven. Final strength characteristics of the new prepared material outperformed a reference mixture, containing only natural fine-grained aggregate/filler. A using of fly ash as a compound of filler of alkali-activated systems was also tested. Fly ash after SNCR in the amount of 20 to 60% in combination with fine-milled blast furnace slag and sodium water glass was tested. These materials achieved compressive strengths 50–75 MPa and flexural strengths 6–10 MPa.
The interfacial transition zone is one of the weakest areas in concrete. Microcracks and new phases often form in this zone, affecting the resulting concrete’s mechanical properties. In this research, concrete in which Portland cement was partially replaced by powdered limestone and/or metakaolin was cured for 1 and 28 days and then mechanical properties were tested. The interfacial transition zone was studied by scanning electron microscopy and selected phases were analyzed by electron microprobe. The addition of a small amount of limestone or metakaolin increases the strength of the concrete. The strength significantly decreases with a content of 10% or more supplementary cementitious materials (SCMs) in concrete. The microcrack’s thickness at the binder-aggregate interface reaches up to 0.8 μm. In concretes with limestone, rhombohedral calcite crystals up to 0.5 μm in size accumulate in the interfacial zone. This accumulation can significantly reduce the strength of the concrete when the amount of limestone is high. Zeolites with a composition corresponding to Mg-mazzite are formed at the interface of cement paste, quartz and albite, partially replaced by sericite.
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