Thermal stability of cement systems with active mineral additives

Introduction. Thermal corrosion of cement stone is a serious issue in public utilities and other structures operated under conditions of elevated temperature and humidity. This type of corrosion is thoroughly studied by experts in plug-back work, but it is less researched in construction materials science. Due to the fact that technologies of plugging and construction works have significant differences, further research in this field is necessary.

Materials and methods. The study used fly ash from the Smolensk Hydroelectric Power Station and granulated blast furnace slag from the Novolipetsk Steel Plant in dosage of 30 %. Portland cement CEM I 42.5 N from CJSC “Oskolcement” (GOST 31108–2020) was used as the binder. The compressive and flexural strength of the specimens was determined
using a PGM-100MG4 hydraulic press. Hydration products were analyzed using an ARL 9900 Work Station X-ray fluorescence spectrometer, an STA 449 F1 Jupiter NETZSCH synchro nous thermal analyzer, and the microstructure of the cement stone was examined using a Tescan Mira 3 scanning electron microscope.

Results. It was found that active mineral additives of fly ash and slag increase the thermal stability coefficient of the cement stone from 0.47 to 0.69 (slag) and 0.72 (fly ash) after 12 months of testing. Significant differences between hydration products under normal and thermal-humidy conditions were revealed through the combined use of X-ray phase and derivatographic analysis with electron microscopy studies. The stone structure under prolonged curing in thermal-humidy conditions is complex and heterogeneous, with well-crystallized calcium hydrosilicates of varying basicity forming alongside tobermorite gel.

Conclusions. The addition of active mineral additives, such as fly ash and granulated blast furnace slag, contributes to the increased thermal stability of cement stone. Under elevated temperature and humidity, the formation of low-basicity hydrosilicates is intensified, which mitigates the difference between the solubility of the contact zones and isolated particles, thereby enhancing the thermal stability of the system.

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